Synthesis of 2,5-Diaryl-1,5-Dienes from Allylic Bromides Using Visible-Light Photoredox Catalysis

Abstract

Visible-light photoreductive coupling of 2-arylallyl bromides in the presence of the photocatalyst Ru(bpy)₃(PF₆)₂, a Hantzsch ester, and i-Pr₂NEt gives 2,5-diaryl-1,5-dienes in high yield. This method avoids the use of stoichiometric metal reductants and is compatible with the presence of halogen, alkyl, electron-donating and electron-withdrawing substituents on the aromatic ring.

INTRODUCTION

The importance of visible-light photocatalysis in organic synthesis has increased substantially in recent years, as it is a sustainable and green method and offers unique opportunities for controlling selectivity.^1,2 In the area of free radical reactions, visible-light photocatalysis is allowing a variety of such processes to be accomplished without the use of stoichiometric adjuvants such as tin reagents. A variety of precursors such as halides,^1d,1f,3 selenosulfonates,⁴ sulfonium⁵ and sulfonyl⁶ derivatives, diazonium salts,⁷ carboxylic acids,⁸ (N-acyloxy)phthalimides,⁹ N-phthalimidoyl oxalates,¹⁰ enamines,¹¹ and other α-amino substituted compounds¹² can be used to generate carbon radicals under these conditions. In a recent study of the reaction of tertiary radicals generated from N-(acyloxy)phthalimides under visible-light photoredox conditions with allylic halides as acceptors,^9b we observed the formation of substantial amounts of 2,5-diphenylhexa-1,5-diene as a side product in attempted couplings with α-(bromomethyl)styrene. Inasmuch as the formation of 1,5-dienes by reductive coupling of allylic halides is typically accomplished using stoichiometric metal reductants (Wurtz couplings),¹³ and no fully satisfactory method appears to be available for the synthesis of 2,5-diaryl-1,5-dienes,¹⁴ we decided to explore the utility of visible-light photoredox catalysis for preparing such dienes. The outcome of these investigations, which led to a general, high-yielding method for preparing 2,5-diaryl-1,5-dienes by utilizing allylic bromides as radical precursors for the first time in photoredox catalysis, is reported herein.

RESULTS AND DISCUSSION

Salient results of our initial optimization of the synthesis of 2,5-diphenylhexa-1,5-diene (1a) from α-(bromomethyl)styrene (2a) are summarized in Table 1. By simple omission of the N-(acyloxy)phthalimide radical precursor, diene 1a was obtained in 78% yield under the conditions employed in our earlier cross-coupling studies (entry 1).^9b Control experiments showed that the photocatalyst, Ru(bpy)₃(PF₆)₂, light, and Hünig’s base (i-Pr₂NEt) are essential (entries 2–4). In the absence of the Hantzsch ester (diethyl 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate, 3), diene 1a was formed in low yield (24%), with 62% of bromide 2a being recovered (entry 5). Subsequent optimization reactions revealed that the product yield and conversion was lower when the catalyst loading was decreased to 0.5 mol% and unchanged when increased from 1.0 to 1.5 mol% (entries 6 and 7). One equiv of Hantzsch ester 3 and 2 equiv of Hünig’s base appeared optimal (entries 8–12). Dichloromethane was preferred over MeCN or THF as the reaction solvent, and increasing the starting concentration of bromide 2a to 0.4 M was also beneficial (entries 13–16). The reaction was nearly complete after 6 h at room temperature with conversion not increasing further after 18 h (entries 17–19). Finally, combining the optimum reaction parameters led to full conversion of allylic bromide 2a and clean formation of 2,5-diphenylhexa-1,5-diene (1a) in about 80% yield at scales up to 2 mmol (entry 20).

Table 1.

Optimization and control experiments for the coupling of α-(bromomethyl)styrene (2a) to form 2,5-diphenylhexa-1,5-diene (1a).

Entry	Modification	Yield of 1a (%)a	Recovery of 2a (%)a
1	–	78	4
2	no Ru(bpy)3(PF6)2	ND	92
3	no light	ND	65
4	no i-Pr2NEt	6	69
5	no Hantzsch ester 3	24	62
6	Ru(bpy)3(PF6)2 (0.5 mol%)	47	35
7	Ru(bpy)3(PF6)2 (1.5 mol%)	79	<2
8	Hantzsch ester 3 (0.5 equiv)	57	25
9	Hantzsch ester 3 (0.75 equiv)	71	11
10	i-Pr2NEt (0.5 equiv)	43	35
11	i-Pr2NEt (1.0 equiv)	73	7
12	i-Pr2NEt (2.0 equiv)	82	2
13	MeCN (0.2 M)	61	28
14	THF (0.2 M)	52	38
15	CH2Cl2 (0.1 M)	69	16
16	CH2Cl2 (0.4 M)	80	ND
17	2 h	48	43
18	6 h	72	13
19	24 h	76	6
20	i-Pr2NEt (2.0 equiv) CH2Cl2 (0.4 M)	81,80b,78c	ND

^aIsolated yield of 1a and recovery of 2a after silica gel chromatography, 0.2 mmol scale.

^b1.0 mmol scale.

^c2.0 mmol scale. ND = not detected.

To investigate the scope of this synthesis of 2,5-diaryl-1,5-dienes, a broad selection of α-(bromomethyl)styrenes 2 was prepared from commercially available acetophenones by the two-step sequence illustrated in Scheme 1. Whereas the initial Wittig methylenation was easily accomplished,¹⁵ the allylic bromination step required some optimization (see Supporting Information for details).¹⁶

Scheme 1.

Synthesis of α-(bromomethyl)styrenes 2 from acetophenones.

With a reliable access to various substituted α-(bromomethyl)styrenes 2 in hand, the scope of the visible-light photoredox catalyzed formation of the 2,5-diaryl-1,5-dienes 1 was explored (Table 2). Substrates containing halogen substituents at the meta or para position gave diene products 1b–1d in yields in excess of 80% (entries 2–4), comparable to the yield realized with the unsubstituted precursor (entry 1). A fluoro substituent at the ortho position prevented full conversion of the allylic bromide, resulting in a 68% yield of 1e (entry 5). In this case, increasing the reaction time led to higher conversions of 2e; however, the yields of 1e remained in the same range. Similar results were observed in the synthesis of dienes 1f–1h containing tolyl substituents, with ortho substitution leading to lower conversion and lower yield (entries 6–8). Both strong electron-donating (OMe) and electron-withdrawing (CN) substituents were tolerated, giving the corresponding diene products 1i–1m in yields of 60–70% (entries 9–13). The presence of an ortho substituent again resulted in much lower conversion and yield of diene product 1k. However in this case, increasing the catalyst loading to 1.5 mol% and the reaction time to 48 h, resulted in diene 1k being formed in 67% yield (entry 11). Substrates containing nitro substituents were partially consumed under our standard reaction conditions, however no 1,5-diene products were formed.¹⁷ Overall, this visible-light photoredox catalyzed Wurtz-type coupling protocol offers – in contrast to other literature-known methods¹⁴ – a mild, catalytic and broadly tolerant access to 2,5-diarylhexa-1,5-dienes from easily accessible starting materials in one step and high yields.

Table 2.

Scope of the visible-light photoredox catalyzed synthesis of 2,5-diaryl-1,5-dienes 1.

Entry	R	Product (Yield in %)a
1	H (2a)	1a (81%, 80%b)
2	4-Cl (2b)	1b (80%)
3	4-Br (2c)	1c (82%)
4	3-Br (2d)	1d (84%, 79%b)
5	2-F (2e)	1e (66%)c
6	4-Me (2f)	1f (79%)d
7	3-Me (2g)	1g (85%)d
8	2-Me (2h)	1h (71%e, 69%f)
9	4-OMe (2i)	1i (59%)
10	3-OMe (2j)	1j (72%)
11	2-OMe (2k)	1k (67%)g
12	4-CN (2l)	1l (70%)
13	3-CN (2m)	1m (70%)d

^aIsolated yield of 1 after silica gel chromatography (average of two experiments).

^b1.0 mmol scale.

^c26% recovered SM 2e (89% brsm).

^dCH₂Cl₂ (0.2 M).

^e29% recovered SM 2h (quant. brsm).

^f1.0 mmol scale, 25% recovered SM 2h (91% brsm).

^gRu(bpy)₃(PF₆)₂ (1.5 mol%), 42 h reaction time, 18% recovered SM 2k (81% brsm).

The likely mechanism for the formation of 2,5-diaryl-1,5-dienes under the conditions we report is summarized in Scheme 2.¹⁸ After visible-light photoexcitation of the catalyst Ru(bpy)₃²⁺, the corresponding excited state Ru(bpy)₃^2+* is quenched by a stoichiometric reductant (Hantzsch ester 3 or i-Pr₂NEt). Single-electron transfer from the resulting Ru(I)-species to the allylic bromide 2 generates allylic radical A with loss of bromide anion. Dimerization of A would yield the 2,5-diaryl-1,5-diene product 1. Alternatively, addition of allylic radical A to allylic bromide 2, followed by loss of bromide radical from B, or single-electron reduction of B followed by expulsion of bromide anion, would yield product 1.^9b

Scheme 2.

Proposed mechanism for the Ru(bpy)₃²⁺-catalyzed generation of allylic radicals from the corresponding bromides and possible reaction pathways.

To explore whether product formation occurs preferentially via radical dimerization or an addition-fragmentation mechanism, we examined the reaction of the unsymmetrical allylic precursors (1-bromobut-2-en-2-yl)benzene (4) and (3-bromobut-1-en-2-yl)benzene (5) (Scheme 3). Both allylic bromides would generate the same delocalized radical, but should exhibit different reactivity towards allylic substitution, with addition of a radical to the exomethylene double bond of 5 being faster than addition to allylic isomer 4 having a trisubstituted double bond. As an addition-fragmentation mechanism would be inconsistent with the formation of diene 8 containing two disubstituted double bonds in equal amounts from both bromide precursors, dimerization of allylic radicals is undoubtedly the major pathway. The somewhat higher yield of diene 6 harboring two trisubstituted double bonds from allylic precursor 5 suggests that an addition-fragmentation pathway occurs to a limited extent, a conclusion that was exploited later in the reaction depicted in Scheme 4.

Scheme 3.

Investigation of the reaction pathways leading to the observed Wurtz-type coupling products.^a

^aIsolated yield after silica gel chromatography (average of two experiments).

Scheme 4.

Synthesis of non-symmetrically substituted 2,5-diaryl-1,5-dienes by reaction via the addition-fragmentation pathway.

After investigating the behavior of various substituted α-(bromomethyl)styrene derivatives in visible-light photocatalyzed coupling, allylic bromides with substitution other than an aryl moiety in the 2-postion were tested (Table 3). Methyl 2-(bromomethyl)acrylate (9) gave diene product 10 in moderate yield. However, 2-bromoallyl bromide (11) delivered dibromodiene 12 in low yield only, whereas attempted coupling of 2-methylallyl bromide gave no 1,5-diene product. In all reactions summarized in Table 3, no starting material was recovered after a reaction time of 18 h.¹⁹ Attempted extension of the method to 2-aryl-substituted allylic chlorides was briefly examined; however, only starting material was recovered in all cases. Even the more strongly reducing photocatalyst Ir(ppy)₃ did not convert α-(chloromethyl)styrene to diene product 1.

Table 3.

Visible-light-mediated photocatalytic Wurtz-type coupling of various allylic bromides.^a

Starting Material	Product (Yield in %)b
	ND

^aReaction conditions from Table 2.

^bIsolated yield after silica gel chromatography.

ND = not detected.

In order to access non-symmetrically substituted 2,5-diaryl-1,5-dienes, reaction conditions that would favor the addition-fragmentation pathway over radical dimerization were examined. The investigations were conducted with 4-chloro-α-(bromomethyl)styrene (2b) as the radical precursor and α-(chloromethyl)styrene (14) as the acceptor (Scheme 4). Employing 14 in excess (5 equiv) under otherwise identical standard conditions led to a 1:1 mixture of dienes 1b and 15 (67% overall yield). However, the addition of the bromide precursor 2b over 7 h using a syringe pump, which ensures a low concentration of allylic radicals in solution, and increasing the catalyst and reagent loading gave exclusively the unsymmetrical 2,5-diaryl-1,5-diene 15 in 60% yield.

CONCLUSION

2,5-Diaryl-1,5-hexadienes are formed in high yields at room temperature by reductive coupling of 2-arylallyl bromides in the presence 1 mol% of the commercially available photocatalyst Ru(bpy)₃(PF₆)_2, Hantzsch ester 3, i-Pr₂NEt, and visible light. This attractive method avoids the use of stoichiometric metals and is believed to proceed largely via dimerization of photogenerated allylic radical intermediates. By slightly adapting the reaction conditions the addition-fragmentation pathway is favored that grants access to non-symmetrically substituted 2,5-diaryl-1,5-dienes.

EXPERIMENTAL SECTION

Materials and Methods

Unless stated otherwise, reactions were conducted in oven-dried glassware under an atmosphere of argon using anhydrous solvents (either freshly distilled or passed through activated alumina columns). For all photoredox-catalyzed coupling reactions, CH₂Cl₂ was sparged with argon for 5 minutes prior to use. All commercially obtained reagents were used as received. Ru(bpy)₃(PF₆)₂ and other photocatalysts were obtained from Sigma Aldrich. Methyl vinyl ketone (MVK), 2,3-dibromopropene (11) and TMS-Cl were distilled prior to use. The reaction components Hantzsch ester 3²⁰, methyl 2-(bromomethyl)acrylate (9)²¹ and α-(chloromethyl)styrene (14)²² were prepared according to literature procedures. Usually one representative coupling reaction and yield of the product is described in detail; isolated yields reported in the Results and Discussion section are the average yields obtained from duplicate experiments. Reaction temperatures were controlled using a temperature modulator, and unless stated otherwise, reactions were performed at room temperature (rt, approximately 23 °C). Thin-layer chromatography (TLC) was conducted with silica gel 60 F254 pre-coated plates, (0.25 mm) and visualized by exposure to UV light (254 nm) or by anisaldehyde, ceric ammonium molybdate, iodine, and potassium permanganate staining. Silica gel 60 (particle size 0.040–0.063 mm) was used for flash column chromatography. ¹H NMR spectra were recorded at 500 or 600 MHz and are reported relative to deuterated solvent signals. Data for ¹H NMR spectra are reported as follows: chemical shift (δ ppm), multiplicity, coupling constant (Hz) and integration. ¹³C NMR spectra were recorded at 125 or 150 MHz. Data for ¹³C NMR spectra are reported in terms of chemical shift. IR spectra were recorded on a FT-IR spectrometer and are reported in terms of frequency of absorption (cm⁻¹). High-resolution mass spectra were obtained with a LCT spectrometer. Blue LEDs (30 cm, 1 watt) were purchased from http://www.creativelightings.com (product code CL-FRS5050-12WP-12V) and powered by 8 AA batteries.

Preparation of α-Methylstyrene Derivatives via Wittig Olefination (Scheme 1)

General Procedure for the Wittig Olefination of Acetophenone Derivatives.¹⁵

A mixture of methyltriphenylphosphonium bromide (12.0–15.0 mmol, 4.32–5.40 g, 1.2–1.5 equiv) in dry THF (18.3 mL) under argon atmosphere was cooled to 0 °C in an ice bath. Then n-BuLi (2.5 M solution in hexanes, 12.0–15.0 mmol, 4.8–6.0 mL, 1.2–1.5 equiv) was added slowly over 10–15 minutes under stirring. After the resulting orange mixture was maintained at 0 °C for 1 hour, a solution of the acetophenone derivative (10.0 mmol, 1.0 equiv) in dry THF (7.0 mL) was added dropwise over 10–15 minutes at 0 °C. The reaction was allowed to warm up to room temperature, stirred overnight and finally quenched with a saturated aqueous solution of sodium chloride (75 mL). The resulting mixture was extracted with pentane (3 × 100 mL) or diethylether (3 × 100 mL). The combined organic phases were washed with a saturated aqueous solution of sodium chloride, dried over sodium sulfate and concentrated under reduced pressure. The crude residue was purified by silica gel chromatography.

4-Chloro-α-methylstyrene

Following the general procedure, title compound was synthesized from 4-chloroacetophenone (10.0 mmol, 1.55 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided 4-chloro-α-methylstyrene (9.51 mmol, 1.45 g, 95%) as a colorless oil. R_f 0.77 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.39 (d, J = 8.7, 2H), 7.29 (d, J = 8.7, 2H), 5.36 (s, 1H), 5.11–5.09 (m, 1H), 2.14–2.13 (m, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 142.3, 139.8, 133.3, 128.5, 126.9, 113.1, 21.9. Characterization data obtained matched those previously reported in literature.²²

4-Bromo-α-methylstyrene

Following the general procedure, title compound was synthesized from 4-bromoacetophenone (10.0 mmol, 1.99 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided 4-bromo-α-methylstyrene (8.82 mmol, 1.74 g, 88%) as a colorless oil. R_f 0.76 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.45 (d, J = 8.7, 2H), 7.33 (d, J = 8.6, 2H), 5.36 (s, 1H), 5.11–5.10 (m, 1H), 2.13–2.12 (m, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 142.3, 140.2, 131.4, 127.3, 121.5, 113.2, 21.8. Characterization data obtained matched those previously reported in literature.^23,24

3-Bromo-α-methylstyrene

Following the general procedure, title compound was synthesized from 3-bromoacetophenone (10.0 mmol, 1.99 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided 3-bromo-α-methylstyrene (9.53 mmol, 1.88 g, 95%) as a colorless oil. R_f 0.78 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.60 (t, J = 1.8, 1H), 7.41–7.38 (m, 2H), 7.20 (t, J = 7.9, 1H), 5.38 (s, 1H), 5.14–5.12 (m, 1H), 2.13 (s, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 143.5, 142.1, 130.4, 129.9, 128.8, 124.2, 122.6, 113.8, 21.8. Characterization data obtained matched those previously reported in literature.²⁵

2-Fluoro-α-methylstyrene

Following the general procedure, title compound was synthesized from 2-fluoroacetophenone (10.0 mmol, 1.38 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided 2-fluoro-α-methylstyrene (8.84 mmol, 1.20 g, 88%) as a colorless oil. R_f 0.67 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.31 (dt, J = 7.8, 0.6, 1H), 7.25–7.21 (m, 1H), 7.10 (t, J = 7.5, 1H), 7.04 (dd, J = 11.1, 8.3, 1H), 5.24–5.22 (m, 2H), 2.16–2.14 (m, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 160.1 (d, J = 248.1), 140.3 (d, J = 0.8), 130.4 (d, J = 13.6), 129.5 (d, J = 4.4), 128.8 (d, J = 8.4), 124.0 (d, J = 3.5), 116.7 (d, J = 4.0), 116.0 (d, J = 23.0), 23.2 (d, J = 3.4); IR (thin film): 3083, 2974, 2924, 2855, 1633, 1573, 1489, 1448, 1216, 1092 cm⁻¹; HRMS-CI (m/z) [M]⁺ calculated for C₉H₉F 136.0688, found 136.0687.

4-Methyl-α-methylstyrene

Following the general procedure, title compound was synthesized from 4-methylacetophenone (10.0 mmol, 1.34 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided 4-methyl-α-methylstyrene (8.76 mmol, 1.16 g, 88%) as a colorless oil. R_f 0.69 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.38 (d, J = 8.2, 2H), 7.15 (d, J = 7.9, 2H), 5.35 (s, 1H), 5.05–5.04 (m, 1H), 2.36 (s, 3H), 2.16–2.14 (m, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 143.2, 138.5, 137.3, 129.0, 125.5, 111.7, 22.0, 21.2. Characterization data obtained matched those previously reported in literature.^22,23

3-Methyl-α-methylstyrene

Following the general procedure, title compound was synthesized from 3-methylacetophenone (10.0 mmol, 1.34 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided 3-methyl-α-methylstyrene (8.62 mmol, 1.14 g, 86%) as a colorless oil. R_f 0.69 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.30 (d, J = 8.9, 2H), 7.24 (t, J = 7.4, 1H), 7.11 (d, J = 7.6, 1H), 5.38–5.37 (m, 1H), 5.09–5.08 (m, 1H), 2.39 (s, 3H), 2.18–2.16 (m, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 143.5, 141.4, 137.8, 128.3, 128.2, 126.4, 122.8, 112.4, 22.0, 21.7. Characterization data obtained matched those previously reported in literature.²⁶

2-Methyl-α-methylstyrene

Following the general procedure, title compound was synthesized from 2-methylacetophenone (10.0 mmol, 1.34 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided 2-methyl-α-methylstyrene (8.28 mmol, 1.09 g, 83%) as a colorless oil. R_f 0.50 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.19–7.11 (m, 4H), 5.21–5.19 (m, 1H), 4.86–4.85 (m, 1H), 2.33 (s, 3H), 2.05 (s, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 146.0, 144.0, 134.6, 130.2, 128.0, 126.9, 125.7, 114.8, 24.5, 20.0. Characterization data obtained matched those previously reported in literature.²²

4-Methoxy-α-methylstyrene

Following the general procedure, title compound was synthesized from 4-methoxyacetophenone (10.0 mmol, 1.50 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided 4-methoxy-α-methylstyrene (8.61 mmol, 1.28 g, 86%) as a colorless oil. R_f 0.21 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.43 (d, J = 8.9, 2H), 6.88 (d, J = 8.9, 2H), 5.31–5.30 (m, 1H), 5.02–5.00 (m, 1H), 3.83 (s, 3H), 2.16–2.14 (m, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 159.2, 142.7, 133.9, 126.7, 113.7, 110.8, 55.4, 22.1. Characterization data obtained matched those previously reported in literature.^23,24,26

3-Methoxy-α-methylstyrene

Following the general procedure, title compound was synthesized from 3-methoxyacetophenone (10.0 mmol, 1.50 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided 3-methoxy-α-methylstyrene (9.05 mmol, 1.34 g, 90%) as a colorless oil. R_f 0.25 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.26 (t, J = 8.0, 1H), 7.08 (d, J = 7.9, 1H), 7.02–7.01 (m, 1H), 6.83 (dd, J = 8.2, 2.3, 1H), 5.38 (s, 1H), 5.11–5.09 (m, 1H), 3.83 (s, 3H), 2.15 (s, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 159.6, 143.3, 142.9, 129.3, 118.2, 112.8, 112.7, 111.6, 55.4, 22.0. Characterization data obtained matched those previously reported in literature.^23,24

2-Methoxy-α-methylstyrene

Following the general procedure, title compound was synthesized from 2-methoxyacetophenone (10.0 mmol, 1.50 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided 2-methoxy-α-methylstyrene (7.98 mmol, 1.18 g, 80%) as a colorless oil. R_f 0.32 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.28–7.24 (m, 1H), 7.21 (dd, J = 7.5, 1.8, 1H), 6.94 (dt, J = 7.4, 1.0, 1H), 6.90 (d, J = 8.2, 1H), 5.18–5.16 (m, 1H), 5.09–5.07 (m, 1H), 3.85 (s, 3H), 2.15–2.13 (m, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 156.7, 144.5, 132.9, 129.5, 128.4, 120.6, 115.2, 110.9, 55.5, 23.3. Characterization data obtained matched those previously reported in literature.²⁴

4-Cyano-α-methylstyrene

Following the general procedure, title compound was synthesized from 4-acetylbenzonitrile (10.0 mmol, 1.45 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with diethyl ether and purification by silica gel chromatography (10% diethyl ether/hexanes) provided 4-cyano-α-methylstyrene (4.66 mmol, 667 mg, 47%) as a yellow oil. R_f 0.52 (10% ethyl acetate/hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.61 (d, J = 8.5, 2H), 7.54 (d, J = 8.5, 2H), 5.47 (s, 1H), 5.25–5.24 (m, 1H), 2.16 (s, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 145.8, 141.9, 132.2, 126.2, 119.1, 115.8, 111.0, 21.6. Characterization data obtained matched those previously reported in literature.^27,28

3-Cyano-α-methylstyrene

Following the general procedure, title compound was synthesized from 3-acetylbenzonitrile (10.0 mmol, 1.45 g, 1.0 equiv), methyltriphenylphosphonium bromide (12.0 mmol, 4.32 g, 1.2 equiv) and n-BuLi (2.5 M solution in hexanes, 12.0 mmol, 4.8 mL, 1.2 equiv). Extraction with diethyl ether and purification by silica gel chromatography (10% diethyl ether/hexanes) provided 3-cyano-α-methylstyrene (6.47 mmol, 926 mg, 65%) as a colorless oil. R_f 0.55 (10% ethyl acetate/hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.72 (s, 1H), 7.68 (dd, J = 8.0, 1.0, 1H), 7.55 (dd, J = 8.0, 1.0, 1H), 7.43 (t, J = 7.8, 1H), 5.42 (s, 1H), 5.21–5.19 (m, 1H), 2.16–2.14 (m, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 142.5, 141.4, 130.9, 129.9, 129.3, 129.2, 119.1, 114.8, 112.6, 21.6; IR (thin film): 3089, 2976, 2947, 2921, 2359, 2230, 1630, 1596, 1575, 1481, 1441, 1377, 1192 cm⁻¹; HRMS-ESI (m/z) [M + Na]⁺ calculated for C₁₀H₉NNa 166.0633, found 166.0641.

But-1-en-2-ylbenzene

Following the general procedure, title compound was synthesized from propiophenone (10.0 mmol, 1.34 g, 1.0 equiv), methyltriphenylphosphonium bromide (15.0 mmol, 5.40 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided but-1-en-2-ylbenzene (8.06 mmol, 1.07 g, 81%) as a colorless oil. R_f 0.75 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.42 (d, J = 7.6, 2H), 7.33 (t, J = 7.5, 2H), 7.26 (t, J = 7.3, 1H), 5.28 (s, 1H), 5.07–5.05 (m, 1H), 2.52 (q, J = 7.4, 2H), 1.11 (t, J = 7.4, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 150.2, 141.7, 128.4, 127.4, 126.1, 111.1, 28.2, 13.1. Characterization data obtained matched those previously reported in literature.²⁹

But-2-en-2-ylbenzene

Following the general procedure, title compound was synthesized from acetophenone (10.0 mmol, 1.20 g, 1.0 equiv), ethyltriphenylphosphonium bromide (15.0 mmol, 5.57 g, 1.5 equiv) and n-BuLi (2.5 M solution in hexanes, 15.0 mmol, 6.0 mL, 1.5 equiv). Extraction with pentane and purification by silica gel chromatography (100% pentane) provided a 1.7:1 mixture of E/Z isomers of but-2-en-2-ylbenzene (7.30 mmol, 966 mg, 73%) as a colorless oil. R_f 0.72 (100% hexanes); ¹H NMR (500 MHz, CDCl₃, mixture of isomers = 0.6:0.4): δ 7.40–7.29 (m, 3.4H), 7.25–7.20 (m, 1.6H), 5.88 (dq, J = 6.9, 1.1, 0.6H), 5.58 (dq, J = 6.8, 1.1, 0.4H), 2.05 (s, 3.0H), 1.82 (d, J = 6.9, 1.8H), 1.63–1.59 (m, 1.2H); ¹³C NMR (125 MHz, CDCl₃, mixture of isomers): δ 144.2, 142.0, 136.9, 135.6, 128.3, 128.2, 128.1, 126.6, 126.5, 125.7, 122.6, 121.7, 25.5, 15.6, 15.0, 14.5. Characterization data obtained matched those previously reported in literature.³⁰

Preparation of α-(Bromomethyl)styrene Derivatives via Allylic Bromination (Scheme 1)

General Procedure for the Allylic Bromination of α-Methylstyrene Derivatives.¹⁶

To a mixture of the α-methylstyrene derivative (0.50 mmol, 1.0 equiv) and TMS-Cl (0.50–50 μmol, 1–6 μL, 0.1–10 mol%) in dry CH₂Cl₂/THF (4:1, 1.5 mL) under argon atmosphere, NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (0.50–50 μmol, 0.3–31 mg, 0.1–10 mol%) were added in one portion. After stirring for one hour, the mixture was concentrated under reduced pressure. The resulting residue was filtered three times with pentane or diethyl ether and the combined filtrates were concentrated under reduced pressure. The crude product mixture was then purified by silica gel chromatography.

α-(Bromomethyl)styrene (2a)

Following the general procedure, 2a was synthesized from commercially available α-methylstyrene (0.50 mmol, 65 μL, 1.0 equiv), TMS-Cl (5.0 μmol, 1 μL, 1 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (5.0 μmol, 3 mg, 1 mol%). Filtration with pentane and purification by silica gel chromatography (100% pentane) provided 2a (0.22 mmol, 43 mg, 43%) as a colorless oil. R_f 0.41 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.52–7.49 (m, 2H), 7.41–7.37 (m, 2H), 7.36–7.32 (m, 1H), 5.57 (s, 1H), 5.50 (s, 1H), 4.40 (s, 2H); ¹³C NMR (125 MHz, CDCl₃): δ 144.4, 137.7, 128.7, 128.4, 126.2, 117.2, 34.4. Characterization data obtained for 2a matched those previously reported in literature.²²

4-Chloro-α-(bromomethyl)styrene (2b)

Following the general procedure, 2b was synthesized from 4-chloro-α-methylstyrene (0.50 mmol, 76 mg, 1.0 equiv), TMS-Cl (5.0 μmol, 1 μL, 1 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (5.0 μmol, 3 mg, 1 mol%). Filtration with pentane and purification by silica gel chromatography (100% pentane) provided 2b (0.26 mmol, 60 mg, 52%) as a colorless oil. R_f 0.40 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.43 (d, J = 8.8, 2H), 7.35 (d, J = 8.8, 2H), 5.54 (s, 1H), 5.50 (s, 1H), 4.35 (s, 2H); ¹³C NMR (125 MHz, CDCl₃): δ 143.3, 136.1, 134.3, 128.8, 127.6, 117.8, 34.0. Characterization data obtained for 2b matched those previously reported in literature.²²

4-Bromo-α-(bromomethyl)styrene (2c)

Following the general procedure, 2c was synthesized from 4-bromo-α-methylstyrene (0.50 mmol, 94 mg, 1.0 equiv), TMS-Cl (5.0 μmol, 1 μL, 1 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (5.0 μmol, 3 mg, 1 mol%). Filtration with pentane and purification by silica gel chromatography (100% pentane) provided 2c (0.25 mmol, 69 mg, 50%) as a colorless oil. R_f 0.41 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.50 (d, J = 8.5, 2H), 7.36 (d, J = 8.6, 2H), 5.55 (s, 1H), 5.51 (s, 1H), 4.35 (s, 2H); ¹³C NMR (125 MHz, CDCl₃): δ 143.4, 136.6, 131.8, 127.9, 122.5, 117.8, 33.9; IR (thin film): 2359, 2340, 1682, 1588, 1490, 1394, 1276, 1211, 1072, 1008 cm⁻¹; HRMS-CI (m/z) [M]⁺ calculated for C₉H₈Br₂ 273.8993, found 273.8994.

3-Bromo-α-(bromomethyl)styrene (2d)

Following the general procedure, 2d was synthesized from 3-bromo-α-methylstyrene (0.50 mmol, 94 mg, 1.0 equiv), TMS-Cl (25 μmol, 3 μL, 5 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (25 μmol, 16 mg, 5 mol%). Filtration with pentane and purification by silica gel chromatography (100% pentane) provided 2d (0.29 mmol, 81 mg, 59%) as a colorless oil. R_f 0.42 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.64–7.62 (m, 1H), 7.48–7.45 (m, 1H), 7.42–7.39 (m, 1H), 7.27–7.23 (m, 1H), 5.55 (s, 1H), 5.26 (s, 1H), 4.34 (s, 2H); ¹³C NMR (125 MHz, CDCl₃): δ 143.3, 139.9, 131.4, 130.2, 129.4, 124.9, 122.9, 118.5, 33.8; IR (thin film): 3062, 2969, 2359, 1623, 1591, 1556, 1476, 1395, 1297, 1210, 1070 cm⁻¹; HRMS-CI (m/z) [M]⁺ calculated for C₉H₈Br₂ 273.8993, found 273.8984.

2-Fluoro-α-(bromomethyl)styrene (2e)

Following the general procedure, 2e was synthesized from 2-fluoro-α-methylstyrene (0.50 mmol, 76 mg, 1.0 equiv), TMS-Cl (25 μmol, 3 μL, 5 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (25 μmol, 16 mg, 5 mol%). Filtration with pentane and purification by silica gel chromatography (100% pentane) provided 2e (0.24 mmol, 51 mg, 48%) as a colorless oil. R_f 0.46 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.37–7.29 (m, 2H), 7.15 (dt, J = 7.5, 0.8, 1H), 7.10–7.05 (m, 1H), 5.61 (s, 1H), 5.42 (s, 1H), 4.39 (s, 2H); ¹³C NMR (125 MHz, CDCl₃): δ 160.0 (d, J = 247.9), 141.4, 130.6 (d, J = 4.0), 130.0 (d, J = 8.5), 126.6 (d, J = 13.8), 124.3 (d, J = 3.6), 120.8 (d, J = 2.7), 116.0 (d, J = 22.6), 35.5 (d, J = 5.3). Characterization data obtained for 2e matched those previously reported in literature.³¹

4-Methyl-α-(bromomethyl)styrene (2f)

Following the general procedure, 2f was synthesized from 4-methyl-α-methylstyrene (0.50 mmol, 76 mg, 1.0 equiv), TMS-Cl (0.50 μmol, 1 μL, 0.1 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (0.50μmol, 0.3 mg, 0.1 mol%). Filtration with pentane and purification by silica gel chromatography (100% pentane) provided 2f (0.12 mmol, 25 mg, 24%) as a colorless oil. R_f 0.38 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.39 (d, J = 8.2, 2H), 7.19 (d, J = 7.9, 2H), 5.53 (s, 1H), 5.44 (s, 1H), 4.38 (s, 2H), 2.37 (s, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 144.2, 138.3, 134.7, 129.4, 126.1, 116.5, 34.5, 21.3. Characterization data obtained for 2f matched those previously reported in literature.²²

3-Methyl-α-(bromomethyl)styrene (2g)

Following the general procedure, 2g was synthesized from 3-methyl-α-methylstyrene (0.50 mmol, 76 mg, 1.0 equiv), TMS-Cl (5.0 μmol, 1 μL, 1 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (5.0 μmol, 3 mg, 1 mol%). Filtration with pentane and purification by silica gel chromatography (100% pentane) provided 2g (0.22 mmol, 45 mg, 43%) as a colorless oil. R_f 0.35 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.31–7.25 (m, 3H), 7.16 (d, J = 6.6, 1H), 5.54 (s, 1H), 5.48 (s, 1H), 4.38 (s, 2H), 2.39 (s, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 144.5, 138.2, 137.7, 129.2, 128.5, 127.0, 123.3, 117.2, 34.5, 21.7; IR (thin film): 3031, 2921, 2361, 2341, 1602, 1489, 1442, 1278, 1210 cm⁻¹; HRMS-CI (m/z) [M]⁺ calculated for C₁₀H₁₁Br 210.0044, found 210.0046.

2-Methyl-α-(bromomethyl)styrene (2h)

Following the general procedure, 2h was synthesized from 2-methyl-α-methylstyrene (0.50 mmol, 76 mg, 1.0 equiv), TMS-Cl (5.0 μmol, 1 μL, 1 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (5.0 μmol, 3 mg, 1 mol%). Filtration with pentane and purification by silica gel chromatography (100% pentane) provided 2h (0.28 mmol, 58 mg, 55%) as a colorless oil. R_f 0.52 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.25–7.16 (m, 3H), 5.64 (s, 1H), 5.15–5.14 (m, 1H), 4.25 (s, 2H), 2.34 (s, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 145.1, 139.4, 135.4, 130.4, 128.9, 127.9, 125.7, 119.6, 37.0, 19.9. Characterization data obtained for 2h matched those previously reported in literature.²²

4-Methoxy-α-(bromomethyl)styrene (2i)

A mixture of 4-methoxy-α-methylstyrene (2.00 mmol, 296 mg, 1.0 equiv), NBS (1.23 mmol, 220 mg, 0.62 equiv) and benzoyl peroxide (0.10 mmol, 24 mg, 5 mol%) in CCl₄ (18 mL) was refluxed at 78 °C under argon atmosphere. After three hours, a second portion of NBS (1.23 mmol, 220 mg, 0.62 equiv) and benzoyl peroxide (0.10 mmol, 24 mg, 5 mol%) was added and the reaction mixture was refluxed for three additional hours. After cooling to room temperature, the resulting precipitate was removed by filtration and the filtrate was concentrated under reduced pressure. Purification of the crude product by silica gel chromatography (2% diethyl ether/hexanes) provided 2i (0.91 mmol, 207 mg, 46%) as a colorless oil. R_f 0.37 (5% ethyl acetate/hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.45 (d, J = 8.8, 2H), 6.91 (d, J = 8.8, 2H), 5.48 (s, 1H), 5.40 (s, 1H), 4.37 (s, 2H), 3.83 (s, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 159.8, 143.7, 130.1, 127.7, 115.6, 114.0, 55.4, 34.6. Characterization data obtained for 2i matched those previously reported in literature.³²

3-Methoxy-α-(bromomethyl)styrene (2j)

Following the general procedure, 2j was synthesized from 3-methoxy-α-methylstyrene (0.50 mmol, 74 mg, 1.0 equiv), TMS-Cl (5.0 μmol, 1 μL, 1 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (5.0 μmol, 3 mg, 1 mol%). Filtration with diethyl ether and purification by silica gel chromatography (2% diethyl ether/hexanes) provided 2j (0.22 mmol, 49 mg, 43%) as a colorless oil. R_f 0.53 (5% ethyl acetate/hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.30 (t, J = 8.0, 1H), 7.08 (d, J = 7.7, 1H), 7.04–7.02 (m, 1H), 6.88 (dd, J = 8.2, 2.5, 1H), 5.56 (s, 1H), 5.49 (s, 1H), 4.37 (s, 2H), 3.84 (s, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 159.8, 144.3, 139.3, 129.6, 118.7, 117.6, 113.6, 112.3, 55.4, 34.3; IR (thin film): 2957, 2833, 2364, 1599, 1577, 1490, 1453, 1427, 1323, 1288, 1237, 1211, 1046 cm⁻¹; HRMS-CI (m/z) [M]⁺ calculated for C₁₀H₁₁BrO 225.9993, found 225.9983.

2-Methoxy-α-(bromomethyl)styrene (2k)

A mixture of 2-methoxy-α-methylstyrene (2.00 mmol, 296 mg, 1.0 equiv), NBS (1.23 mmol, 220 mg, 0.62 equiv) and benzoyl peroxide (0.10 mmol, 24 mg, 5 mol%) in CCl₄ (18 mL) was refluxed at 78 °C under argon atmosphere. After three hours, a second portion of NBS (1.23 mmol, 220 mg, 0.62 equiv) and benzoyl peroxide (0.10 mmol, 24 mg, 5 mol%) was added and the reaction mixture was refluxed for three additional hours. After cooling to room temperature, the resulting precipitate was removed by filtration and the filtrate was concentrated under reduced pressure. Purification of the crude product by silica gel chromatography (2% diethyl ether/hexanes) provided 2k (0.90 mmol, 204 mg, 45%) as a colorless oil. R_f 0.50 (5% ethyl acetate/hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.36–7.32 (m, 1H), 7.27 (dd, J = 7.5, 1.8, 1H), 6.99 (dt, J = 7.4, 0.8, 1H), 6.92 (d, J = 8.4, 1H), 5.53 (s, 1H), 5.29–5.28 (m, 1H), 4.49 (s, 2H), 3.85 (s, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 156.6, 145.1, 131.0, 129.5, 128.3, 120.7, 119.3, 110.7, 55.5, 36.1; IR (thin film): 2936, 2834, 1626, 1598, 1490, 1461, 1434, 1242, 1211, 1026 cm⁻¹; HRMS-CI (m/z) [M + NH₄]⁺ calculated for C₁₀H₁₁BrONH₄ 244.0337, found 244.0335.

4-Cyano-α-(bromomethyl)styrene (2l)

Following the general procedure, 2l was synthesized from 4-cyano-α-methylstyrene (0.50 mmol, 72 mg, 1.0 equiv), TMS-Cl (50 μmol, 6 μL, 10 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (50 μmol, 31 mg, 10 mol%). Filtration with diethyl ether and purification by silica gel chromatography (10% diethyl ether/hexanes) provided 2l (0.26 mmol, 58 mg, 52%) as a colorless solid. R_f 0.34 (10% ethyl acetate/hexanes); mp: 58–59 °C; ¹H NMR (500 MHz, CDCl₃): δ 7.67 (d, J = 8.2, 2H), 7.59 (d, J = 8.2, 2H), 5.65 (s, 1H), 5.63 (s, 1H), 4.36 (s, 2H); ¹³C NMR (125 MHz, CDCl₃): δ 143.1, 142.2, 132.5, 127.0, 120.0, 118.7, 112.0, 33.2; IR (thin film): 2921, 2360, 2227, 1606, 1506, 1447, 1403, 1212 cm⁻¹; HRMS-ESI (m/z) [M + Na]⁺ calculated for C₁₀H₈NBrNa 243.9738, found 243.9729.

3-Cyano-α-(bromomethyl)styrene (2m)

Following the general procedure, 2m was synthesized from 3-cyano-α-methylstyrene (0.50 mmol, 72 mg, 1.0 equiv), TMS-Cl (50 μmol, 6 μL, 10 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (50 μmol, 31 mg, 10 mol%). Filtration with diethyl ether and purification by silica gel chromatography (10% diethyl ether/hexanes) provided 2m (0.21 mmol, 47 mg, 43%) as a colorless solid. R_f 0.33 (10% ethyl acetate/hexanes); mp: 61–63 °C; ¹H NMR (500 MHz, CDCl₃): δ 7.76 (s, 1H), 7.71 (d, J = 8.0, 1H), 7.62 (d, J = 7.7, 1H), 7.49 (t, J = 7.8, 1H), 5.59 (s, 2H), 4.35 (s, 2H); ¹³C NMR (125 MHz, CDCl₃): δ 142.6, 139.0, 131.7, 130.6, 130.0, 129.5, 119.3, 118.7, 112.9, 33.3; IR (thin film): 2920, 2360, 2340, 2229, 1573, 1481, 1277, 1260, 1212 cm⁻¹; HRMS-ESI (m/z) [M + Na]⁺ calculated for C₁₀H₈NBrNa 243.9738, found 243.9746.

(Z)-(1-Bromobut-2-en-2-yl)benzene (4)

Following the general procedure, 4 was synthesized from but-1-en-2-ylbenzene (0.50 mmol, 66 mg, 1.0 equiv), TMS-Cl (12.5 μmol, 2 μL, 2.5 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (12.5 μmol, 8 mg, 2.5 mol%). Filtration with pentane and purification by silica gel chromatography (100% pentane) provided 4 (0.30 mmol, 62 mg, 59%) as a colorless oil. R_f 0.32 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.44 (d, J = 7.9, 2H), 7.35 (t, J = 7.6, 2H), 7.29 (d, J = 7.4, 1H), 6.09 (q, J = 7.1, 1H), 4.40 (s, 2H), 1.92 (d, J = 7.1, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 140.6, 137.0, 129.5, 128.6, 127.5, 125.9, 29.3, 14.6; IR (thin film): 3027, 2977, 2918, 1600, 1493, 1446, 1207 cm⁻¹; HRMS-CI (m/z) [M + NH₄]⁺ calculated for C₁₀H₁₁BrNH₄ 228.0388, found 228.0383.

(3-Bromobut-1-en-2-yl)benzene (5)

Following the general procedure, 5 was synthesized from but-2-en-2-ylbenzene (0.50 mmol, 66 mg, 1.0 equiv), TMS-Cl (12.5 μmol, 2 μL, 2.5 mol%), NBS (0.60 mmol, 106 mg, 1.2 equiv) and Yb(OTf)₃ (12.5 μmol, 8 mg, 2.5 mol%). Filtration with pentane and purification by silica gel chromatography (100% pentane) provided 5 (0.35 mmol, 74 mg, 70%) as a colorless oil. R_f 0.38 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.45 (d, J = 7.0, 2H), 7.39–7.31 (m, 3H), 5.53 (s, 1H), 5.40 (s, 1H), 5.12 (q, J = 6.7, 1H), 1.94 (d, J = 6.8, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 150.4, 139.9, 128.5, 128.1, 127.0, 115.3, 48.9, 24.9; IR (thin film): 3056, 2975, 2925, 2362, 1624, 1574, 1494, 1443, 1374, 1172, 1071 cm⁻¹; HRMS-CI (m/z) [M]⁺ calculated for C₁₀H₁₁Br 210.0044, found 210.0034.

Visible-Light Photoredox Catalyzed Wurtz-Type Coupling Reactions (Tables 1, 2, 3 and Scheme 3)

General Procedure for the Wurtz-type coupling reaction of substituted allylic bromides

A 1-dram vial was charged with Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv) and a magnetic stir bar under argon. After sequential addition of CH₂Cl₂ (0.5 mL, sparged with argon for five minutes), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and the substituted allylic bromide (0.20 mmol, 1.0 equiv), the vial was capped and placed in the center of a 30 cm-loop of blue LEDs. After stirring for 18 hours, the reaction mixture was concentrated under reduced pressure. The crude product was then purified by silica gel chromatography.

2,5-Diphenylhexa-1,5-diene (1a)

Following the general procedure, 1a was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and α-(bromomethyl)styrene (2a, 0.20 mmol, 39 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). Purification by silica gel chromatography (100% pentane) provided 1a (81 μmol, 19 mg, 81%) as a colorless solid.

Under the same conditions, coupling product 1a (94 mg, 80%) was obtained from Ru(bpy)₃(PF₆)₂ (0.01 mmol, 8.5 mg, 1 mol%), Hantzsch ester 3 (1.00 mmol, 255 mg, 1.0 equiv), i-Pr₂NEt (2.00 mmol, 0.32 mL, 2.0 equiv) and α-(bromomethyl)styrene (2a, 1.00 mmol, 197 mg, 1.0 equiv) in CH₂Cl₂ (2.5 mL). In the same fashion, product 1a (184 mg, 78%) was obtained from Ru(bpy)₃(PF₆)₂ (0.02 mmol, 17 mg, 1 mol%), Hantzsch ester 3 (2.00 mmol, 510 mg, 1.0 equiv), i-Pr₂NEt (4.00 mmol, 0.63 mL, 2.0 equiv) and α-(bromomethyl)styrene (2a, 2.00 mmol, 394 mg, 1.0 equiv) in CH₂Cl₂ (5 mL).

R_f 0.37 (100% hexanes); ¹H NMR (500 MHz, CDCl₃)

δ 7.42 (d, J = 7.5, 4H), 7.35 (t, J = 7.4, 4H), 7.32–7.28 (m, 2H), 5.31 (d, J = 1.2, 2H), 5.07 (s, 2H), 2.68 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 148.1, 141.2, 128.4, 127.5, 126.3, 112.7, 34.4. Characterization data obtained for 1a matched those previously reported in literature.³³

2,5-Bis-(4-chlorophenyl)hexa-1,5-diene (1b)

Following the general procedure, 1b was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 4-chloro-α-(bromomethyl)styrene (2b, 0.20 mmol, 46 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). Purification by silica gel chromatography (100% pentane) provided 1b (81 μmol, 25 mg, 81%) as a colorless solid. R_f 0.44 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.29 (s, 8H), 5.26 (s, 2H), 5.03 (s, 2H), 2.59 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 146.8, 139.5, 133.3, 128.6, 127.6, 113.5, 34.2. Characterization data obtained for 1b matched those previously reported in literature.^14k

2,5-Bis-(4-bromophenyl)hexa-1,5-diene (1c)

Following the general procedure, 1c was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 4-bromo-α-(bromomethyl)styrene (2c, 0.20 mmol, 55 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). Purification by silica gel chromatography (100% pentane) provided 1c (83 μmol, 32 mg, 83%) as a colorless solid. R_f 0.41 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.45 (d, J = 8.4, 4H), 7.23 (d, J = 8.4, 4H), 5.26 (s, 2H), 5.03 (s, 2H), 2.59 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 146.8, 140.0, 131.5, 127.9, 121.5, 113.6, 34.1. Characterization data obtained for 1c matched those previously reported in literature.^14k

2,5-Bis-(3-bromophenyl)hexa-1,5-diene (1d)

Following the general procedure, 1d was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 3-bromo-α-(bromomethyl)styrene (2d, 0.20 mmol, 55 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). Purification by silica gel chromatography (100% pentane) provided 1d (87 μmol, 34 mg, 87%) as a colorless oil.

Under the same conditions, coupling product 1d (154 mg, 79%) was obtained from Ru(bpy)₃(PF₆)₂ (0.01 mmol, 8.5 mg, 1 mol%), Hantzsch ester 3 (1.00 mmol, 255 mg, 1.0 equiv), i-Pr₂NEt (2.00 mmol, 0.32 mL, 2.0 equiv) and 3-bromo-α-(bromomethyl)styrene (2d, 1.00 mmol, 275 mg, 1.0 equiv) in CH₂Cl₂ (2.5 mL).

R_f 0.50 (100% hexanes); ¹H NMR (500 MHz, CDCl₃)

δ 7.50 (t, J = 1.8, 2H), 7.42–7.39 (m, 2H), 7.30–7.26 (m, 2H), 7.20 (t, J = 7.8, 2H), 5.28 (d, J = 1.0, 2H), 5.07 (s, 2H), 2.59 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 146.6, 143.3, 130.5, 130.0, 129.4, 124.9, 122.7, 114.2, 34.1; IR (thin film): 3081, 2942, 2859, 1806, 1626, 1589, 1556, 1473, 1407, 1291, 1068 cm⁻¹; HRMS-CI (m/z) [M + NH₄]⁺ calculated for C₁₈H₁₆Br₂NH₄ 407.9962, found 407.9958.

2,5-Bis-(2-fluorophenyl)hexa-1,5-diene (1e)

Following the general procedure, 1e was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 2-fluoro-α-(bromomethyl)styrene (2e, 0.20 mmol, 43 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). Purification by silica gel chromatography (100% pentane) provided 1e (66 μmol, 18 mg, 66%) as a colorless oil. R_f 0.45 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.25–7.18 (m, 4H), 7.08 (dt, J = 7.5, 1.1, 2H), 7.02 (ddd, J = 10.7, 8.3, 0.9, 2H), 5.17 (s, 2H), 5.15 (s, 2H), 2.58 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 160.0 (d, J = 247.3), 144.4, 130.3 (d, J = 4.4), 129.8 (d, J = 14.3), 128.9 (d, J = 8.4), 124.0 (d, J = 3.5), 116.3 (d, J = 2.3), 115.8 (d, J = 22.9), 35.0 (d, J = 3.2); IR (thin film): 3082, 2933, 2860, 1799, 1631, 1573, 1487, 1447, 1214, 1090, 1033 cm⁻¹; HRMS-CI (m/z) [M + H]⁺ calculated for C₁₈H₁₆F₂H 271.1298, found 271.1292.

2,5-Bis-(4-methylphenyl)hexa-1,5-diene (1f)

Following the general procedure, 1f was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 4-methyl-α-(bromomethyl)styrene (2f, 0.20 mmol, 42 mg, 1.0 equiv) in CH₂Cl₂ (1.0 mL). Purification by silica gel chromatography (100% pentane) provided 1f (82 μmol, 22 mg, 82%) as a colorless solid. R_f 0.19 (100% hexanes); mp: 56–57 °C; ¹H NMR (500 MHz, CDCl₃): δ 7.29 (d, J = 8.1, 4H), 7.14 (d, J = 7.9, 4H), 5.25 (d, J = 1.3, 2H), 5.00 (s, 2H), 2.62 (s, 4H), 2.35 (s, 6H); ¹³C NMR (125 MHz, CDCl₃): δ 148.0, 138.3, 137.2, 129.1, 126.1, 111.8, 34.5, 21.3; IR (thin film): 3082, 3023, 2921, 2861, 1624, 1512, 1453 cm⁻¹; HRMS-CI (m/z) [M + H]⁺ calculated for C₂₀H₂₂H 263.1800, found 263.1804.

2,5-Bis-(3-methylphenyl)hexa-1,5-diene (1g)

Following the general procedure, 1g was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 3-methyl-α-(bromomethyl)styrene (2g, 0.20 mmol, 42 mg, 1.0 equiv) in CH₂Cl₂ (1.0 mL). Purification by silica gel chromatography (100% pentane) provided 1g (88 μmol, 23 mg, 88%) as a colorless oil. R_f 0.36 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.24–7.18 (m, 6H), 7.10 (d, J = 6.9, 2H), 5.27 (d, J = 1.4, 2H), 5.04 (s, 2H), 2.63 (s, 4H), 2.36 (s, 6H); ¹³C NMR (125 MHz, CDCl₃): δ 148.3, 141.2, 137.9, 128.3, 128.2, 127.0, 123.4, 112.5, 34.5, 21.7; IR (thin film): 3035, 2919, 2859, 2360, 1787, 1626, 1600, 1575, 1486, 1452 cm⁻¹; HRMS-CI (m/z) [M + H]⁺ calculated for C₂₀H₂₂H 263.1800, found 263.1792.

2,5-Bis-(2-methylphenyl)hexa-1,5-diene (1h)

Following the general procedure, 1h was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 2-methyl-α-(bromomethyl)styrene (2h, 0.20 mmol, 42 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). Purification by silica gel chromatography (100% pentane) provided 1h (72 μmol, 19 mg, 72%) as a colorless oil.

Under the same conditions, coupling product 1h (90 mg, 69%) was obtained from Ru(bpy)₃(PF₆)₂ (0.01 mmol, 8.5 mg, 1 mol%), Hantzsch ester 3 (1.00 mmol, 255 mg, 1.0 equiv), i-Pr₂NEt (2.00 mmol, 0.32 mL, 2.0 equiv) and 2-methyl-α-(bromomethyl)styrene (2h, 1.00 mmol, 210 mg, 1.0 equiv) in CH₂Cl₂ (2.5 mL).

R_f 0.39 (100% hexanes); ¹H NMR (500 MHz, CDCl₃)

δ 7.18–7.11 (m, 6H), 7.06 (d, J = 6.9, 2H), 5.20 (s, 2H), 4.89 (d, J = 1.9, 2H), 2.45 (s, 4H), 2.26 (s, 6H); ¹³C NMR (125 MHz, CDCl₃): δ 149.6, 143.0, 134.9, 130.2, 128.5, 126.9, 125.5, 114.1, 35.9, 20.0; IR (thin film): 3070, 3016, 2924, 2360, 1634, 1487, 1452, 1211, 1045 cm⁻¹; HRMS-CI (m/z) [M + H]⁺ calculated for C₂₀H₂₂H 263.1800, found 263.1804.

2,5-Bis-(4-methoxyphenyl)hexa-1,5-diene (1i)

Following the general procedure, 1i was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 4-methoxy-α-(bromomethyl)styrene (2i, 0.20 mmol, 45 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). Purification by silica gel chromatography (5% diethyl ether/hexanes) provided 1i (63 μmol, 19 mg, 63%) as a colorless solid. R_f 0.45 (10% ethyl acetate/hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.35 (d, J = 8.5, 4H), 6.87 (d, J = 8.4, 4H), 5.22 (s, 2H), 4.97 (s, 2H), 3.82 (s, 6H), 2.62 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 159.2, 147.5, 133.6, 127.3, 113.7, 111.1, 55.4, 34.5. Characterization data obtained for 1i matched those previously reported in literature.^14f

2,5-Bis-(3-methoxyphenyl)hexa-1,5-diene (1j)

Following the general procedure, 1j was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 3-methoxy-α-(bromomethyl)styrene (2j, 0.20 mmol, 45 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). Purification by silica gel chromatography (5% diethyl ether/hexanes) provided 1j (74 μmol, 22 mg, 74%) as a colorless oil. R_f 0.50 (10% ethyl acetate/hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.27–7.23 (m, 2H), 6.99 (d, J = 7.8, 2H), 6.94–6.92 (m, 2H), 6.83 (dd, J = 8.2, 2.5, 2H), 5.29 (d, J = 1.2, 1H), 5.06 (s, 2H), 3.82 (s, 6H), 2.64 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 159.7, 148.1, 142.8, 129.4, 118.6, 112.9, 112.8, 112.2, 55.3, 34.5; IR (thin film): 3078, 2934, 2833, 1602, 1575, 1487, 1460, 1427, 1286, 1231, 1047 cm⁻¹; HRMS-CI (m/z) [M + H]⁺ calculated for C₂₀H₂₂O₂H 295.1698, found 295.1700.

2,5-Bis-(2-methoxyphenyl)hexa-1,5-diene (1k)

Following the general procedure, 1k was synthesized from Ru(bpy)₃(PF₆)₂ (3.0 μmol, 3 mg, 1.5 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 2-methoxy-α-(bromomethyl)styrene (2k, 0.20 mmol, 45 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). The reaction time was extended from 18 hours to 42 hours. Purification by silica gel chromatography (5% diethyl ether/hexanes) provided 1k (69 μmol, 20 mg, 69%) as a colorless solid. R_f 0.44 (5% ethyl acetate/hexanes); mp: 98–100 °C; ¹H NMR (500 MHz, CDCl₃): δ 7.26–7.22 (m, 2H), 7.10 (dd, J = 7.4, 1.7, 2H), 6.90 (dt, J = 7.4, 1.0, 2H), 6.85 (d, J = 8.2, 2H), 5.12 (s, 2H), 5.00 (d, J = 2.0, 2H), 3.75 (s, 6H), 2.55 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 156.6, 148.9, 132.2, 130.4, 128.4, 120.5, 114.2, 110.7, 55.5, 35.0; IR (thin film): 3072, 2944, 2837, 1633, 1597, 1489, 1458, 1435, 1239, 1024 cm⁻¹; HRMS-CI (m/z) [M]⁺ calculated for C₂₀H₂₂O₂ 294.1620, found 294.1625.

2,5-Bis-(4-cyanophenyl)hexa-1,5-diene (1l)

Following the general procedure, 1l was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 4-cyano-α-(bromomethyl)styrene (2l, 0.20 mmol, 44 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). Purification by silica gel chromatography (10% → 20% diethyl ether/hexanes) provided 1l (73 μmol, 21 mg, 73%) as a colorless solid. R_f 0.18 (10% ethyl acetate/hexanes); mp: 115–116 °C; ¹H NMR (500 MHz, CDCl₃): δ 7.62 (d, J = 8.4, 4H), 7.44 (d, J = 8.4, 4H), 5.37 (s, 2H), 5.15 (s, 2H), 2.63 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 146.1, 145.5, 132.4, 126.9, 118.9, 116.1, 111.2, 33.6; IR (thin film): 3065, 2917, 2852, 2360, 2341, 2226, 1623, 1604, 1504, 1401, 1128 cm⁻¹; HRMS-ESI (m/z) [M + Na]⁺ calculated for C₂₀H₁₆N₂Na 307.1211, found 307.1213.

2,5-Bis-(3-cyanophenyl)hexa-1,5-diene (1m)

Following the general procedure, 1m was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 3-cyano-α-(bromomethyl)styrene (2m, 0.20 mmol, 44 mg, 1.0 equiv) in CH₂Cl₂ (1.0 mL). Purification by silica gel chromatography (20% diethyl ether/hexanes) provided 1m (74 μmol, 21 mg, 74%) as a colorless solid. R_f 0.51 (20% ethyl acetate/hexanes); mp: 64–65 °C; ¹H NMR (500 MHz, CDCl₃): δ 7.61–7.59 (m, 2H), 7.58–7.55 (m, 4H), 7.46–7.42 (m, 2H), 5.32 (s, 2H), 5.12 (s, 2H), 2.61 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 145.6, 142.2, 131.1, 130.6, 129.9, 129.4, 118.9, 115.4, 112.7, 33.6; IR (thin film): 3082, 2921, 2851, 2360, 2229, 1627, 1574, 1480 cm⁻¹; HRMS-ESI (m/z) [M + Na]⁺ calculated for C₂₀H₁₆N₂Na 307.1211, found 307.1224.

Wurtz-type coupling of (Z)-(1-bromobut-2-en-2-yl)benzene (4)

Following the general procedure, the reaction of Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and (Z)-(1-bromobut-2-en-2-yl)benzene (4, 0.20 mmol, 42 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL) gave a crude product. Purification by silica gel chromatography (100% pentane) provided a mixture of the three regioisomers 6 (36 μmol, 10 mg, 36%), 7 (43 μmol, 11 mg, 43%) and 8 (15 μmol, 4 mg, 15%). Analytical samples were obtained by an automated flash chromatography system.

Data for 6: colorless solid; R_f 0.45 (100% hexanes); ¹H NMR (500 MHz, CDCl₃)

δ 7.31–7.28 (m, 8H), 7.24–7.20 (m, 2H), 5.72 (q, J = 6.9, 2H), 2.54 (s, 4H), 1.67 (d, J = 6.9, 6H); ¹³C NMR (125 MHz, CDCl₃): δ 143.1, 140.5, 128.3, 126.6, 126.4, 123.4, 28.1, 14.2. Characterization data obtained for 6 matched those previously reported in literature.³⁴

Data for 7: colorless oil; R_f 0.50 (100% hexanes); ¹H NMR (500 MHz, CDCl₃)

δ 7.30–7.19 (m, 10H), 5.75 (q, J = 6.8, 1H), 5.17 (d, J = 0.9, 1H), 5.08–5.07 (m, 1H), 2.70–2.64 (m, 2H), 2.50–2.45 (m, 1H), 1.77 (d, J = 6.9, 3H), 1.07 (d, J = 6.9, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 155.1, 143.3, 142.9, 140.1, 128.3, 128.2, 127.3, 126.9, 126.7, 126.6, 124.6, 111.0, 36.4, 35.7, 18.6, 14.6; IR (thin film): 3022, 3961, 2925, 2360, 1493, 1442, 1371, 1028 cm⁻¹; HRMS-CI (m/z) [M + H]⁺ calculated for C₂₀H₂₂H 263.1800, found 263.1797.

Data for 8 (1:1 mixture of diastereomers): colorless oil; R_f 0.53 (100% hexanes); ¹H NMR (500 MHz, CDCl₃, mixture of diastereomers)

δ 7.34–7.24 (m, 16H), 7.20–7.16 (m, 4H), 5.20–5.19 (m, 2H), 5.19 (s, 2H), 5.06 (s, 2H), 5.01–5.00 (m, 2H), 2.79–2.72 (m, 4H), 1.18 (d, J = 6.2, 6H), 0.94 (d, J = 6.8, 6H). Characterization data obtained for 8 matched those previously reported in literature.³⁴

Wurtz-type coupling of (3-bromobut-1-en-2-yl)benzene (5)

Following the general procedure, the reaction of Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and (3-bromobut-1-en-2-yl)benzene (5, 0.20 mmol, 42 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL) gave a crude product. Purification by silica gel chromatography (100% pentane) provided the three regioisomers 6 (45 μmol, 12 mg, 45%), 7 (39 μmol, 10 mg, 39%) and 8 (14 μmol, 4 mg, 14%). Characterization data obtained for 6, 7 and 8 matched those reported above.

Dimethyl 2,5-dimethylenehexanedioate (10)

Following the general procedure, 10 was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and methyl 2-(bromomethyl)acrylate² (9, 0.20 mmol, 36 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). Purification by silica gel chromatography (10% diethyl ether/hexanes) provided 10 (57 μmol, 11 mg, 57%) as a colorless oil. R_f 0.37 (10% ethyl acetate/hexanes); ¹H NMR (500 MHz, CDCl₃): δ 6.17 (d, J = 1.3, 2H), 5.54 (s, 2H), 3.76 (s, 6H), 2.50 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 167.6, 139.6, 125.7, 52.0, 31.0; IR (thin film): 2923, 2851, 2361, 1721, 1632, 1439, 1203, 1141 cm⁻¹; HRMS-CI (m/z) [M + H]⁺ calculated for C₁₀H₁₄O₄H 199.0970, found 199.0976.

2,5-Dibromohexa-1,5-diene (12)

Following the general procedure, 12 was synthesized from Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 1 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 1.0 equiv), i-Pr₂NEt (0.40 mmol, 63 μL, 2.0 equiv) and 2,3-dibromopropene (11, 0.20 mmol, 40 mg, 1.0 equiv) in CH₂Cl₂ (0.5 mL). Purification by silica gel chromatography (100% pentane) provided 12 (15 μmol, 4 mg, 15%) as a colorless oil. R_f 0.78 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 5.64 (d, J = 1.7, 2H), 5.45 (d, J = 1.8, 2H), 2.68 (s, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 132.0, 118.3, 39.9. Characterization data obtained for 12 matched those previously reported in literature.³⁵

Visible-Light Photoredox Catalyzed Coupling to Activated Olefins (Scheme 4)

2-(4-Chlorophenyl)-5-phenylhexa-1,5-diene (15)

A 1-dram vial was charged with Ru(bpy)₃(PF₆)₂ (2.0 μmol, 2 mg, 2 mol%), Hantzsch ester 3 (0.20 mmol, 51 mg, 2.0 equiv) and a magnetic stir bar under argon. After sequential addition of CH₂Cl₂ (0.5 mL, sparged with argon for five minutes), i-Pr₂NEt (0.40 mmol, 63 μL, 4.0 equiv) and α-(chloromethyl)styrene (14, 0.50 mmol, 76 mg, 5.0 equiv), the vial was placed in the center of a 30 cm-loop of blue LEDs. A solution of 4-chloro-α-(bromomethyl)styrene (2b, 0.10 mmol, 23 mg, 1.0 equiv) in CH₂Cl₂ (1.0 mL, sparged with argon for five minutes) was added over a time period of seven hours with a syringe pump. After additional stirring for 11 hours, the reaction mixture was concentrated under reduced pressure. Purification by silica gel chromatography (100% pentane) provided 15 (60 μmol, 16 mg, 60%) as a colorless oil. R_f 0.31 (100% hexanes); ¹H NMR (500 MHz, CDCl₃): δ 7.39–7.27 (m, 9H), 5.27 (d, J = 1.1, 1H), 5.26 (d, J = 1.1, 1H), 5.05 (d, J = 0.9, 1H), 5.03 (d, J = 0.9, 1H), 2.66–2.58 (m, 4H); ¹³C NMR (125 MHz, CDCl₃): δ 148.0, 147.0, 141.1, 139.6, 133.3, 128.6, 128.5, 128.4, 127.6, 127.5, 126.3, 113.3, 112.9, 34.3, 34.2; IR (thin film): 3081, 2939, 2361, 1626, 1492, 1443, 1394, 1095, 1012 cm⁻¹; HRMS-CI (m/z) [M + H]⁺ calculated for C₁₈H₁₇ClH 269.1097, found 269.1090.