Vinyl Tris(trimethylsilyl)silanes: Substrates for Hiyama Coupling

Abstract

The oxidative treatment of vinyl tris(trimethylsilyl)silanes with hydrogen peroxide in aqueous sodium hydroxide in tetrahydrofuran generates reactive silanol or siloxane species that undergo Pd-catalyzed cross-couplings with aryl, heterocyclic and alkenyl halides in the presence of Pd(PPh₃)₄ and tetrabutylammonium fluoride. Hydrogen peroxide and base are necessary for the coupling to occur while activation of the silanes with fluoride is not required. The conjugated and unconjugated tris(trimethylsilyl)silanes serve as good cross-coupling substrates. The (E)-silanes undergo coupling with retention of stereochemistry while coupling of (Z)-silanes occurred with lower stereoselectivity to produce an E/Z mixture of products.

Introduction

Pd-catalyzed cross-coupling reactions are a powerful method for the formation of carbon-carbon bonds under conditions that are compatible with a broad range of functional groups.¹ Among these methods, couplings between organometallics derived from Group 14 metals and various electrophiles are well developed. Thus, the Stille reaction has been widely applied in modern synthetic organic chemistry.² Furthermore, the Hiyama coupling has received increased attention due to the lower toxicity of organosilicon compounds.³ Procedures for coupling with organogermanes have also been reported.^4,5

Hiyama and Hatanaka reported on the positive effect that fluoride ion imparts on the nucleophilicity of certain substituents bonded to trialkylsilanes and attributed this enhancement to the formation of pentacoordinated silicon species.⁶ This strategy has opened up the door for the wide application of structurally diverse silanes including: (i) heteroatom functionalized precursors such as halosilanes, oxysilanes, silanols and polysiloxanes and (ii) all-carbon substituted silicon species such as phenyl-, benzyl-, 2-thienyl-, 2-pyridyldimethylsilanes.³

Denmark et al. developed Pd-catalyzed coupling reactions employing siletanes (alkenylsilacyclobutanes) in the presence of TBAF and Pd(0) such as Pd₂(dba)₃.^7a The siletanes were found to be converted in situ into reactive alkenyl(propyl)silanol and disiloxane species via the ring opening.^7b When the alkenylsilanols were synthesized independently, they were also found to undergo coupling.⁸ Hiyama, Mori and coworkers independently found that silver(I) oxide is an excellent activator for the coupling of alkenylsilanols with aryl iodides.⁹ Generally, the organosilanols can couple by two mechanistically distinct processes. One is operative under basic conditions and requires only formation of the silanolates while the second pathway involves the use of a fluoride activator that can form an active fluorosiliconate.¹⁰

Recent developments in Si-cross-coupling include the application of triallyl(aryl)silanes for the synthesis of biaryls,^11a a palladium-phosphinous acid-catalyzed and NaOH-promoted coupling of arylsiloxanes in water,^11b application of alkenyl- and aryl[2-(hydroxymethyl)phenyl]dimethylsilanes as a reusable silicon based precursor for the fluoride-free couplings via intramolecular activation,^11c vinylation of aryl bromides using an inexpensive vinylpolysiloxane,^11d application of (2-pyridyl)allyldimethylsilanes as novel pyridyl transfer,^11e and fluoride-free methodologies.11f,g Herein, we report application of various tris(trimethylsilyl)silanes as “masked” substrates for the cross-coupling reactions with aryl halides and alkenyl bromides which occurred under oxidative conditions in the presence of aqueous sodium hydroxide with or without tetrabutylammonium fluoride.

Results and Discussion

The (E)-vinyl tris(trimethylsilyl)silanes (TTMS-silanes) 2a,c-f were prepared by stereoselective radical-mediated silyldesulfonylations^5a of the corresponding (E)-vinyl sulfones 1 with (TMS)₃SiH (Scheme 1). The cyclohexylidene silane 2g was similarly prepared. The (Z)-vinyl silanes 4a-d were synthesized in 80-92% yield by the radical hydrosilylation¹² of the corresponding alkynes 3 with (TMS)₃SiH. Attempted hydrosilylation of 1-alkynes 3a-c with (TMS)₃SiH in the presence of Rh(COD)₂BF₄/PPh₃/NaI or RhCl(PPh₃)₃/NaI catalyst systems¹³ produced E isomers 2a-c in high yields but complete control of stereochemistry was generally difficult since Z isomers 4a-c were also formed (~5-20%). Thus, hydrosilylation of 3b gave 2b/4b (E/Z, ~9:1) mixture, which was purified to afford 2b (82%). Isomerization (20 h, 60 °C) of the (Z)-silanes 4a and 4c with 0.5 equivalent of (TMS)₃SiH in the presence of Rh catalyst¹³ also afforded the (E)-silanes 2a (78%) and 2c (89%). Prolonged heating (54 h) of 4b in the presence of (TMS)₃SiH/RhCl(PPh₃)₃/NaI effected quantitative conversion of 4b into 2b.

Scheme 1. Stereoselective synthesis of (E)- and (Z)-vinyl tris(trimethylsilyl)silanes (2 and 4)^a.

^a Reagents and conditions: (a) (Me₃Si)₃SiH/AIBN/toluene or benzene (85 °C; oil-bath); (b) (Me₃Si)₃SiH/RhCl(PPh₃)₃ or Rh(COD)₂BF₄/PPh₃/NaI/neat/Δ.

We started by examining different coupling conditions. Thus, attempted coupling of (Z)-vinyl silane 4b with iodobenzene under typical coupling conditions employed for organosilanes⁶ [e.g., TBAF/Pd(0)/THF] yielded the desired product 5b in less than 5% (Table 1, entry 1). However, we found that fluoride-free treatment of 4b with H₂O₂/NaOH^5a in aqueous THF for 15 min. followed by addition of iodobenzene and Pd(PPh₃)₄ provided (E/Z)-5b in 61% yield (entry 2). When NaOH was replaced with KOSiMe₃ base¹⁴ the yield did not improve, although the reaction mixture became homogeneous with the combination of KOSiMe₃/H₂O₂ (entry 3). TBAF was found to be an excellent activator under aqueous (H₂O₂/NaOH) and “anhydrous” (H₂O₂/KOSiMe₃) oxidative conditions furnishing coupling product 5b in 90% and 81% yield, respectively (entries 4 and 5). Both peroxide and base were necessary since only 18% conversion to 5b was obtained without NaOH or KOSiMe₃ (entry 6) and almost no product was formed without peroxide (entry 7). Other fluoride sources such as NaF and CsF did not show improvement over TBAF (entries 8 and 9). Anhydrous oxidative conditions [t-BuOOH/KH/Pd(0)] with or without TBAF did not give satisfactory results (entry 10), although such conditions worked well for the coupling of analogous TTMS-germanes.^5b These results indicate that the peroxide and base are necessary for the coupling of TTMS-silanes to occur while fluoride facilitates this conversion (vide infra Table 2 and 3 vs Table 4).

Table 1.

Effect of Reaction Parameters on the Cross-Coupling of TTMS-Silanes

entry	peroxide	base	fluoride	yielda (%)	E/Zb
1	none	none	TBAF	<5c	5/95
2	H2O2	NaOH	none	61d,e,f	15/85
3	H2O2	KOSiMe3	none	60	25/75
4	H2O2	NaOH	TBAF	90	3/97
5	H2O2	KOSiMe3	TBAF	81	2/98
6	H2O2	none	TBAF	18c	67/33
7	none	KOSiMe3g	TBAF	<2c	n/a
8	H2O2	NaOH	NaF	60	40/60
9	H2O2	NaOH	CsF	12c	60/40
10	t-BuOOH	KH	noneh	15c	75/25

^aIsolated yields (combined for both isomers of 5b). Couplings were performed on 0.1 mmol scale of silane (0.03 mM).

^bDetermined by GC-MS [with internal standard of (E)- and (Z)- stilbenes] and/or ¹H NMR of the crude reaction mixture.

^cBased on GC-MS.

^dPd₂(dba)₃ also gave 5b (52%; E/Z, 25:75).

^eAttempted couplings with H₂O₂ (without NaOH) or with NaOH (without H₂O₂) failed to give 5b.

^fCoupling with bromobenzene instead of iodobenzene gave 5b (40%; E/Z, 20:80).

^gReaction with NaOH instead of KOSiMe₃ was also unsuccessful.

^hCoupling in the presence of TBAF gave^c 5b in ~2% yield.

Table 2.

The Coupling of Vinyl (Z)-TTMS-Silanes

entry	silane	R1X	product	yielda (%)	E/Zb
1	4a	PhBr	5a	82	40/60
2	4a	2-iodopyridine	9a	60	2/98
3	4b	PhI	5b	90	3/97
4	4b	PhBr	5b	86	30/70
5	4b	PhCl	5b	<5c	n/a
6	4b	PhOTf	5b	<5c	n/a
7	4b	PhCH=CHBrd	6b	72	44/56e
8	4b	(CH3)2C=CHBr	7b	87	4/96
9	4b	iodothiophenef	8b	75	15/85
10	4b	(4)BuPhI	10b	61	24/76
11	4b	1-iodonaphthaleneg	11b	73	15/85
12	4c	PhBr	5c	97	9/91
13	4d	PhBr	5d	70	55/45

^aIsolated yields (combined for the E/Z isomers). Couplings were performed on 0.1-1.0 mmol scale of silanes (0.03 mM). Pd(PPh₃)₄ (10% mol).

^bDetermined by GC-MS and/or ¹H NMR of the crude reaction mixture.

^cGC-MS.

^dE/Z = 88:12.

^e1E ,3E/1Z ,3E.

^f2-Iodo- 5-methylthiophene.

^gCoupling with 1-bromonaphthalene gave 11b (51%, E/Z = 27:73).

Table 3.

The Coupling of Vinyl (E)-TTMS-Silanes

entry	silane	R2X	producta (E)	yieldb (%)
1	2a	PhI	5a	83
2	2a	PhBr	5a	67
3	2a	(4)CH3OPhI	5c	79
4	2a	(4)CF3PhI	5d	90
5	2a	(CH3)2C=CHBr	7a	85
6	2a	iodothiophenec	8a	58
7	2a	2-iodopyridine	9a	70
8	2b	PhI	5b	72
9	2b	iodothiophenec	8b	66
10	2b	(4)BuPhI	10b	59
11	2b	1-bromonaphthalene	11b	48
12	2b	1-iodonaphthalene	11b	70
13	2c	PhBr	5c	63
14	2c	PhCH=CHBrd	6ce	60
15	2d	PhBr	5d	86
16	2e	PhI	5e	85
17	2e	PhBr	5e	72
18	2f	PhI	5f	70
19	2f	PhBr	5f	78
20	2g	PhI	5g	84
21	2g	PhBr	5g	65
22	2g	(CH3)2C=CHBr	7g	50
23	2g	iodothiophenec	8g	83

^aOnly E-isomers were detected (¹H NMR, GC-MS) except for the g series where stereochemistry is not relevant. Couplings were performed on 0.1-0.5 mmol scale of silanes (0.03 mM). Pd(PPh₃)₄ (10% mol).

^bIsolated yields.

^c2-Iodo-5-methylthiophene.

^dE/Z = 88:12.

^e1E/3E.

Table 4.

Fluoride-Free Coupling of the Vinyl TTMS-Silanes

entry	silane	R1X	product	Yield a (%)	E/Zbb
1	2a	PhI	5a	75c	100/0
2	2a	PhBr	5a	50	100/0
3	2b	iodothiophened	8b	52	100/0
4	2b	1-iodonaphthalene	11b	46	100/0
5	2c	PhCH=CHBre	6c	45	100/0f
6	2e	PhI	5e	56	100/0
7	2g	PhI	5g	65	n/a
8	2g	(CH3)2C=CHBr	7g	42	n/a
9	4a	PhBr	5a	55	25/75
10	4b	PhI	5b	61g	15/85
11	4b	PhBr	5b	40h	20/80
12	4b	1-bromonaphthalene	11b	30	17/83

^aIsolated yields. Couplings were performed on 0.1 mmol scale of silanes (0.03 mM). Pd(PPh₃)₄ (10% mol).

^bDetermined by ¹H NMR and/or GC-MS of the crude reaction mixture.

^cWith KOSiMe₃ instead of NaOH yield was 60% (E/Z, 100:0).

^d2-Iodo-5-methylthiophene.

^eE/Z, 88:12.

^fOnly E,E isomer was observed.

^gWith KOSiMe₃ instead of NaOH yield was 60% (E/Z, 25:75).

^hWith KOSiMe₃ instead of NaOH yield was 48% (E/Z, 10:90).

The protocols established above (Table 1, entries 2-5) have proven to be general for the coupling of a range of alkenyl, aryl and heterocyclic iodides and bromides with an array of vinyl TTMS-silanes. Thus, treatment of the conjugated silane (Z)-4a with H₂O₂/H₂O (30%, 3 equiv.) and NaOH (3 equiv.)/H₂O in THF followed by addition of bromobenzene, Pd(PPh₃)₄ and TBAF gave stilbene 5a (82%, Table 2, entry 1). Analogously, (Z)-4b coupled with iodobenzene and bromobenzene to give p-methylstilbene 5b in 90% and 86% yield, respectively (entries 3 and 4). Less reactive electrophiles such as chlorobenzene and aryl triflate¹⁵ failed to give the desired coupling products (entries 5 and 6). The substituent on the phenyl ring in (Z)-silanes 4a-d (p-MeO, p-Me, H, p-CF₃) effects coupling reactions with bromobenzene, increasing both the yields (from 70% to 97%) and steroselectivity outcome (E/Z from 55:45 to 9:91) as the substituent changed from an electron-withdrawing group to an electron-donating group (entries 1, 4, 12 and 13). TTMS-silanes also coupled with both π-deficient (entry 2) and electron-rich heterocyclic halides (entry 9) as well as with iodonaphthalene (entry 11).

Not only aryl halides but also vinyl halides can be coupled with vinyl TTMS-silanes. Thus, oxidative treatment of (Z)-4b with β-bromostyrene produced the diene 6b (72%, entry 7). The aliphatic 1-bromo-2-methyl-1-propene coupled with (Z)-4b yielding 7b in high yield and high stereoselectivity (entry 8).

The (E)-TTMS-silanes 2a-f underwent coupling with retention of stereochemistry. Thus, coupling of conjugated silanes 2a-d with aryl, heterocyclic, and aliphatic bromides or iodides (H₂O₂/NaOH/Pd(0)/TBAF) provided products stereoselectively in good to excellent yields (48-90%, Table 3, entries 1-15). The electron-deficient aryl iodides gave to some extent higher yields than the electron rich aryl iodides in the reactions with silane 2a (entries 1, 3 and 4).

The nonconjugated (E)-vinyl silanes 2e-f also undergo coupling with iodobenzene and bromobenzene under oxidative conditions in the presence of aqueous NaOH and TBAF to give the desired E alkenes 5e-f (70-85%; entries 16-19). The vinyl silane 2g, derived from cyclohexanone, successfully coupled with aryl (entries 20 and 21), alkenyl (entry 22) and heterocyclic halides (entry 23) to produce trisubstituted alkenes.

Although TBAF promotes couplings of TTMS-silanes in the presence of H₂O₂/base, we found that fluoride activation of TTMS-silanes was not necessary for the cross-coupling to occur. Thus, oxidative treatment (H₂O₂/NaOH or KOSiMe₃) of the conjugated silane (E)-2a with bromo- and iodobenzene also produced (E)- stilbene (entries 1 and 2, Table 4). Other conjugated and nonconjugated (E)- and (Z)-silanes coupled with aryl, heterocyclic and aliphatic alkenyl halides (entries 3-12). Again coupling of (Z)-silanes occurred with lower stereoselectivity to produce E/Z mixture (entries 9-12). It is noteworthy that TBAF containing reactions are, however, generally higher yielding and more stereoselective than the fluoride-free reactions.

The lack of stereoselectivity for the coupling of (Z)-silanes probably results from the isomerization of the vinyl siloxane intermediates derived from (Z)-TTMS-silanes under the coupling conditions. Isomerization¹⁶ of the products was excluded based on the following experiments: (i) no isomerization of the (Z)-stilbene 5a was observed when (Z)-5a was refluxed in THF in the presence of H₂O₂/NaOH or TMSOK with or without Pd(0) and/or TBAF, (ii) coupling of the (Z)-silane 4b under typical conditions [H₂O₂/NaOH/Pd(0)THF/with or without TBAF] with phenyl iodide or bromide in the presence of 0.25 or 1.0 equiv. of the (Z)-stilbene 5a produced product 5b as the E/Z mixture [see Table 2 (entries 3 and 4) and Table 4 (entries 10 and 11)], while isomerization of the (Z)-stilbene 5a into E isomer was not observed (GC/MS).

A “side-by-side” comparison of the coupling of (Z)-4b with 1-iodonaphthalene [1 h (30%, E/Z 0:100); 3 h (58%, E/Z 3:97)] and 1-bromonaphthalene [1 h (22%, E/Z 5:95); 3 h (35%, E/Z 13:87)] showed that product 11b is formed at different pace (Table 2, entry 11). It appears that coupling with the aryl iodides is faster and occurs with a higher degree of stereoretention than with the corresponding aryl bromides (see also Table 2, entry 3 vs. 4; Table 4, entry 10 vs. 11). Longer stirring of the silanes 2 and 4 with H₂O₂/NaOH (45 min. vs. 15 min.) prior to the addition of the aryl halide and the catalyst resulted in no improvement of yield and stereoselectivity.

Denmark and Tymonko have recently utilized substrates bearing two distinct silyl subunits [RSiMe₂OH vs. RSiMe₂Bn], which required complementary activations (TMSOK vs. TBAF), for the construction of unsymmetrical disubstituted 1,4-butadienes.¹⁷ TTMS-silanes can also serve as alternative organosilane substrates in Pd-catalyzed couplings. For example, TTMS-silane 2a was remained intact under typical conditions employed in the coupling of dimethylsilanols^14,17 [TMSOK(2 equiv.)/Pd₂(dba)₃/dioxane/r.t./4 h] with more than 95% of 2a being recovered after 4 h and ~85% after 24 h. This experiment demonstrated that TTMS-silanes could act as masked silanols, which require hydrogen peroxide for initiation.

It is noteworthy that under the oxidative conditions required for coupling of TTMS-silanes, the reductive self-coupling of the halides has not been observed for the fluoride promoted reactions (Table 2 and 3) and was only sporadically observed for the fluoride-free reactions (Table 4, entries 2, 5 and 9; 1-3%, GC-MS). Moreover, byproducts resulting from the oxidative homocoupling¹⁸ of the vinyl silanes 2 and 4 have not been observed. Also, although the oxidative conditions employed for generation of the active organosilane species are similar to the ones used in Tamao-Kumada and Fleming oxidation of silanes to alcohols (including vinyl silanes to aldehydes and ketones), which involve cleavage of the C-Si bond,¹⁹ we did not observe conversion of the vinyl silanes 2 and 4 to the corresponding aldehydes. Apparently, Si-Si bond cleavage takes place chemoselectively with the C-Si bond tolerating the relatively mild oxidative conditions required for coupling.²⁰

We have not yet had the opportunity to systematically investigate the mechanism(s) of the TTMS-silanes Pd-catalyzed coupling but it appears that hydrogen peroxide chemoselectively cleaves^20b the Si–Si bond(s) to generate silanol species RSi(OH)_n(SiMe₃)_3-n (n = 1, 2, or 3). Subsequently, the silanol(s) are converted by the base to a silanolate anion, which might follow the coupling mechanism suggested by Denmark et al. for the organosilanols.¹⁰ Alternatively, TTMS-silanes can be converted by hydrogen peroxide to siloxane species RSi(OSiMe₃)_n(SiMe₃)_3-n (n = 1, 2, or 3), that can be further transformed to the reactive pentacoordinate species (hypervalent silicate anion)3b,e^,10 by fluoride or base.

In order to obtain additional mechanistic insights, we examined the coupling reaction of 2a with iodobenzene by ²⁹Si NMR. Thus, treatment of 2a with hydrogen peroxide (THF-d₈/NaOH/H₂O) resulted in the appearance of new peaks at 9.82, 7.20 and 5.59 ppm, which are characteristic for the species having oxygen attached to silicon,^7b,10a, 20a,b^,21 with concurrent disappearance of the two distinctive peaks at -85.31 ppm (Si atom attached to sp² carbon) and -14.37 pm (SiMe₃) for the silicon atoms present in substrate 2a. Addition of Pd catalyst and phenyl iodide to the resulting mixture resulted in the formation of stilbene (E)-5a Moreover, when coupling of 4b with 1-iodonaphthalene under fluoride-free conditions was quenched after 2 h, the corresponding tris(trimethylsiloxy)silyl compound 12 [(4)CH₃C₆H₄CH=CHSi(OSiMe₃)₃] was isolated in 7% yield in addition to product 11b (12%). The structure of 12 was assigned based on the HRMS and NMR spectra. Subjection of 12 to TBAF promoted coupling with 1-iodonaphthalene producing product 11b but in low yield (8%; GC/MS).

In summary, we demonstrated that the conjugated and unconjugated vinyl tris(trimethylsilyl)silanes undergo Pd-catalyzed cross-coupling with aryl, heterocyclic and alkenyl iodides and bromides under aqueous oxidative conditions in the presence of sodium hydroxide with or without fluoride activation. Contrary to (E)-silanes, which undergo coupling with retention of stereochemistry, coupling of (Z)-silanes occurred with lower stereoselectivity giving an E/Z mixture of products. The best stereoselectivity was achieved when either aryl iodides or electron-rich TTMS-silanes were used. Under the oxidative coupling conditions neither reductive self-coupling of the halides nor oxidative homocoupling of the vinyl TTMS-silanes were observed. The tris(trimethylsilyl)silanes remained intact under typical basic conditions employed in the coupling of dimethylsilanols, thus making stable and readily accessible vinyl TTMS-silanes alternative substrates (“masked” silanols) for the Hiyama coupling. Hydrogen peroxide presumably chemoselectively cleaves Si–Si bond(s) generating active silanol/siloxane species that undergo coupling in the presence of base.

Experimental Section

¹H (Me₄Si) NMR spectra at 400 or 600 MHz, ¹³C (Me₄Si) at 100.6 MHz, ¹⁹F (CCl₃F) at 376.4 MHz and ²⁹Si (Me₄Si) at 119.2 MHz were determined with solutions in CDCl₃ unless otherwise noted. Mass spectra (MS) were obtained by electron ionization (EI) technique and HRMS were acquired by AP-ESI technique. Reagent grade chemicals were used and solvents were dried by reflux over and distillation from CaH₂ under an argon atmosphere. TLC was performed on Merck kieselgel 60-F₂₅₄ and products were detected with 254 nm light or by development of color with I₂. Merck kieselgel 60 (230-400 mesh) was used for column chromatography. Purity and identity of the products (crude and/or purified) were also established using a GC/MS (EI) system with a HP 5973 mass selective detector [capillary column HP-5MS (30 m × 0.25 mm × 25 μm)]. The vinyl sulfones 1a,^5a 1c,²² 1e,^5a 1f,^5a 1g^5a and silanes 2e-g,^5a 4a ^12a were prepared as reported.

(E)-2-(4-Methoxyphenyl)-1-[tris(trimethylsilyl)silyl]ethene (2c). Procedure A

Argon was bubbled through a solution of 1c²² (E; 150 mg, 0.55 mmol) in anhydrous benzene (10 mL) for 15 min at ambient temperature. (Me₃Si)₃SiH (0.51 mL, 410 mg, 1.65 mmol) and AIBN (24 mg, 0.14 mmol) were added, and deoxygenation was continued for another 10 min. The resulting solution was refluxed at 85 °C for 4 h [additional AIBN (92 mg, 0.55 mmol) in degassed benzene (2 mL) was injected through a septum via an automatic syringe over the 4 h period or dropwise manually]. The volatiles were evaporated, and the residue was chromatographed in hexane to give 2c (134 mg, 64%) as a colorless oil: ¹H NMR δ 0.25 (s, 27H), 3.84 (s, 3H), 6.29 (d, J = 18.8 Hz, 1H), 6.87 (d, J = 18.8 Hz, 1H), 6.89 (d, J = 8.8 Hz, 2H), 7.35 (d, J = 8.8 Hz, 2H); ¹³C NMR δ 1.0, 55.4, 114.0, 119.6, 127.2, 132.4, 145.0, 159.3; MS (EI) m/z 380 (15, M⁺), 174 (100). AP-ESI-HRMS Calcd for C₁₈H₃₆ONaSi₄ (MNa⁺): 403.1735. Found: 403.1739.

Treatment of 4c (381 mg, 1 mmol) with (Me₃Si)₃SiH (0.15 mL, 124 mg, 0.5 mmol) in the presence of Rh catalyst, as described for 2a [GC-MS of the crude reaction mixture: 2c/4c (E/Z, 92:8; t_R 22.02 min, Z, t_R 22.34 min, E)], gave 2c (338 mg, 89%).

(Z)-2-(4-Methylphenyl)-1-[tris(trimethylsilyl)silyl]ethene (4b). Procedure B

(Me₃Si)₃SiH (0.31 mL, 248 mg, 1 mmol) was added in one portion via a syringe to a degassed solution of 3b (0.13 mL, 116 mg, 1 mmol) in dry benzene (3 mL) at ambient temperature under nitrogen atmosphere. AIBN (83.8 mg, 0.50 mmol) was then added and the resulting solution was heated (oil bath, 85 °C) for 3 h or until the alkyne was consumed (GC). The volatiles were evaporated in vacuo and the oily residue was flash chromatographed (hexane) on silica gel to give 4b (336 mg, 92%) as a colorless oil: ¹H NMR δ 0.16 (s, 27H), 2.37 (s, 3H), 5.82 (d, J = 14.5 Hz, 1H), 7.16 (d, J = 7.8 Hz, 2H), 7.22 (d, J = 7.8 Hz, 2H), 7.40 (d, J = 14.5 Hz, 1H); ¹³C NMR δ 1.4, 21.3, 123.1, 128.1, 129.0, 137.1, 137.8, 146.6; ²⁹Si NMR δ -88.33 [s, Si(SiMe₃)₃], -11.67 [s, Si(SiMe₃)₃]; GC-MS: (t_R 22.12 min) m/z 364 (6, M⁺), 174 (100). HRMS Calcd for C₁₈H₃₆Si₄ (M⁺): 364.1894. Found: 364.1896.

(E)-1,2-Diphenylethene (5a). Procedure C

Solution of NaOH (60 mg, 1.5 mmol) and H₂O₂ (30% solution, 0.15 mL, 1.5 mmol) in deionized H₂O (1.5 mL) were added to a stirred solution of 2a (175 mg, 0.5 mmol) in THF (15 mL) at ambient temperature. After 15 minutes, iodobenzene (84 μL, 153 mg, 0.75 mmol), Pd(PPh₃)₄ (58 mg, 0.05 mmol) and tetrabutylammonium fluoride (1M/THF, 1.5 mL, 1.5 mmol) were added and the resulting brownish mixture was heated at 55 °C (oil bath) for 10 h. The volatiles were evaporated and the residue was partitioned (H₂O/CHCl₃). The aliquot of the organic layer was subjected to GC-MS and/or ¹H NMR analysis in order to establish the overall stereochemistry. The organic layer was dried (MgSO₄), evaporated and column chromatographed (hexane) to give (E)-5a (74 mg, 83%) with data identical to commercial sample: GC-MS (t_R 17.9 min, E) m/z 180 (100, M⁺).

Treatment of 2a (35 mg, 0.10 mmol) with bromobenzene (16 μL, 23.6 mg, 0.15 mmol) by procedure C gave (E)-5a (12 mg, 67%).

Analogous treatment of 2a (35 mg, 0.10 mmol) with iodobenzene (17 μL, 31 mg, 0.15 mmol) by procedure C (without TBAF) gave (E)-5a (13.5 mg, 75%).

Analogous treatment of 2a (35 mg, 0.10 mmol) with bromobenzene (16 μL, 23.6 mg, 0.15 mmol) by procedure C (without TBAF) gave (E)-5a (9 mg, 50%). Also, biphenyl (1%; e.g., 2% consumption of bromobenzene) was detected: GC-MS (t_R 11.3 min) m/z 154 (100, M⁺).

Procedure D

KOSiMe₃ (38.5 mg, 0.3 mmol) and H₂O₂ (30% solution, 31 μL, 0.30 mmol) were added to a stirred solution of 2a (35 mg, 0.10 mmol) in THF (3 mL) at ambient temperature. After 20 minutes, iodobenzene (17 μL, 31 mg, 0.15 mmol) and Pd(PPh₃)₄ (11 mg, 0.01 mmol) were added and the resulting mixture was heated at 55 °C (oil bath) for 10 h. Aqueous work-up and purification as described in procedure C gave (E)-5a (11 mg, 60%).

(E/Z)-1,2-Diphenylethene (5a)

Treatment of 4a^12a (35 mg, 0.10 mmol) with bromobenzene (16 μL, 23.6 mg, 0.15 mmol) by procedure C gave 5a (E/Z, 40:60; 15 mg, 82%) with data identical to commercial sample: GC-MS (t_R 15.1 min, Z; t_R 17.9 min, E) m/z 180 (100, M⁺). HRMS Calcd for C₁₄H₁₃ (MH⁺): 181.1073. Found: 181.1079.

Analogous treatment of 4a (35 mg, 0.10 mmol) with bromobenzene (16 μL, 23.6 mg, 0.15 mmol) by procedure C (without TBAF) gave 5a (E/Z, 25:75; 10 mg, 55%). Also, biphenyl (3%; e.g. 6% consumption of bromobenzene) was detected (GC-MS).

(E)-1-(4-Methylphenyl)-2-phenylethene (5b)

Treatment of 2b (36.5 mg, 0.10 mmol) with iodobenzene (17 μL, 31 mg, 0.15 mmol) by procedure C gave (E)-5b (14 mg, 72%) with data as reported:^{23 1}H NMR δ 2.22 (s, 3H), 6.92 (d, J = 18.1 Hz, 1H), 6.98 (d, J = 18.1 Hz, 1H), 7.03 (d, J = 7.9 Hz, 2H), 7.13 (t, J = 7.3 Hz, 1H), 7.22 (t, J = 7.4 Hz, 2H), 7.30 (d, J = 8.1 Hz, 2H), 7.39 (d, J = 7.9 Hz, 2H), GC-MS (t_R 19.6 min) m/z 194 (100, M⁺).

(E/Z)-1-(4-Methylphenyl)-2-phenylethene (5b)

Treatment of 4b (364 mg, 1.0 mmol) with iodobenzene (0.17 mL, 306 mg, 1.5 mmol) by procedure C gave 5b^23b (E/Z, 3:97; 175 mg, 90%): GC-MS (t_R 16.8 min, Z; t_R 19.6 min, E) m/z 194 (100, M⁺). HRMS Calcd for C₁₅H₁₅ (MH⁺): 195.1174. Found: 195.1179. (Z)-5b had: ¹H NMR δ 2.20 (s, 3H), 6.43 (s, 2H), 6.92 (d, J = 7.9 Hz, 2H), 7.03 (d, J = 7.9 Hz, 2H), 7.11-7.20 (m, 5H).

Treatment of 4b (36.5 mg, 0.10 mmol) with bromobenzene (16 μL, 23.6 mg, 0.15 mmol) by procedure C gave 5b (E/Z, 30:70; 17 mg, 86%).

Analogous treatment of 4b (36.5 mg, 0.10 mmol) with iodobenzene (17 μL, 31 mg, 0.15 mmol) by procedure C (without TBAF) gave 5b (E/Z, 15:85; 12 mg, 61%). Identical coupling with bromobenzene (0.15 mmol) gave 5b (E/Z, 20:80; 8 mg, 40%).

Treatment of 4b (36.5 mg, 0.10 mmol) with iodobenzene (17 μL, 31 mg, 0.15 mmol) by procedure D gave 5b (E/Z, 25:75; 11.6 mg, 60%). Identical coupling with bromobenzene (0.15 mmol) gave 5b (E/Z, 10:90; 9 mg, 48%).

Analogous treatment of 4b (36.5 mg, 0.10 mmol) with iodobenzene (17 μL, 31 mg, 0.15 mmol) by procedure D [with addition of TBAF (0.3 mmol) as described in procedure C] gave 5b (E/Z, 2:98; 15.7 mg, 81%).

Analogous treatment of 4b (36.5 mg, 0.10 mmol) with iodobenzene by procedure C [using aqueous NaF (12.6 mg, 0.3 mmol) instead of TBAF] gave 5b (E/Z, 40:60; 11.6 mg, 60%).

Analogous treatment of 4b (36.5 mg, 0.10 mmol) with iodobenzene by procedure C [using Pd₂(dba)₃ (9.2 mg, 0.01 mmol) instead of Pd(PPh₃)₄ and without addition of TBAF] gave 5b (E/Z, 25:75; 10 mg, 52%).

(Z)-2-(4-Methylphenyl)-1-[tris(trimethylsiloxy)silyl]ethene (12)

Treatment of 4b (50 mg, 0.14 mmol) with 1-iodonaphtalene (22 μL, 35 mg, 0.14 mmol) by procedure C [without TBAF, 2 h, NaOH (5 equiv.)] and column chromatography (hexane) gave 12 (4 mg, 7%) and 11b (E/Z, 7:93; 4 mg, 12%). Compound 12 had: ¹H NMR δ 0.07 (br s, 27H), 2.36 (s, 3H), 5.50 (d, J = 15.5 Hz, 1H), 7.12 (d, J = 7.9 Hz, 2H), 7.21 (d, J = 15.5 Hz, 1H) 7.44 (d, J = 8.0 Hz, 2H); GC-MS (t_R 17.60 min) m/z 412 (6, M⁺), 175 (100); HRMS Calcd for C₁₈H₃₇O₃Si₄ (MH⁺): 413.1814. Found: 413.1823.

Supplementary Material

01. Supporting Information Available.

Experimental procedures and characterization data for compounds 1d, 2a, 2b, 2d, 4c, 4d, 5c-g, 6b, 6c, 7a, 7b, 7g, 8a, 8b, 8g, 9a, 10b, and 11b. This material will be published online alongside electronic version of the manuscript in Elsevier web products, including www.sciencedirect.com