Generation and Reactions of a Benzodehydrotropylium Ion–Co₂(CO)₆ Complex

Abstract

A series of 7-methylenedehydrobenzo[7]annulen-5-ol hexacarbonyldicobalt complexes were generated by Hosomi–Sakurai reactions of allylsilanes containing o-alkynylarylaldehyde-Co₂(CO)₆ complexes. One of the cyclization products was converted into its corresponding dihydrobenzo[7]annulen-7-ol hexacarbonyldicobalt complex, an immediate precursor to a benzodehydrotropylium–Co₂(CO)₆. The cation was generated in situ and reacted with four nucleophiles, and its aromatic stabilization was determined by computational methods.

Introduction

The propargylium cation–Co₂(CO)₆ complex (1, Figure 1) has become one of the most important species in metal-stabilized carbocation chemistry, by virtue of its excellent stability and reactivity that is both good and predictable; its transformations are normally known as Nicholas reactions.¹ Among its noteworthy features is the fact that the alkynedicobalt unit contributes sufficiently to the overall carbocation picture such that simple substitution at the propargylic center gives very limited change in carbocation stability, and in many cases cations that would be difficult to generate in the absence of the Co₂(CO)₆ unit are generated readily in its presence.^2,3 Furthermore, the bending of alkynyl carbon bond angles to ca. 140° allows the Nicholas reaction to work well in medium-ring-size-generating reactions. As a result, and in conjunction with reliable reductive decomplexation protocols, the ready preparation and manipulation of cycloheptynedicobalt complexes has been exploited extensively in total synthesis.^4,5

Figure 1.

Propargyldicobalt cations and the target precursor.

In addition to purely synthetic ends, the presence of an adjacent alkynedicobalt unit has been shown to have significant effects on the properties of nominally aromatic or antiaromatic carbocations.³ The development of the aforementioned multiple cycloheptynedicobalt preparation methods has allowed our group to make contributions to the study of metal complexes of the putatively aromatic 1,2-dehydrotropylium ion (henceforth referred to simply as a dehydrotropylium ion). We have reported the generation, trapping, and evaluation of a dehydrotropylium ion −Co₂(CO)₆ complex (2) and found a reduced level of aromatic stabilization in that system, approximately 25% of that of a tropylium ion, employing NICS(1) calculations and homodesmotic reactions as the primary measures.^6,7 By contrast, a dehydrotropylium ion itself only has been proposed as a mass spectral fragment of 2-acylbenzofurans and benzothiophene,⁸ and neither has it been prepared synthetically nor has had its properties investigated.⁹ The η²-platinum(0), palladium(0), and zirconium(II) complexes of a dehydrotropylium ion or related cations have been prepared by the Jones group and show no to modest C–C bond alternation.⁷ Dehydrotropone–Co₂(CO)₄ (dppm) shows considerable bond alternation.¹⁰

In the tropylium ion system, the effect of ring fusion to another aromatic ring has been reported to have contradictory effects, with benzo-fused systems less stable¹¹ but with azulene and heteroaromatic systems giving enhanced stability.¹² A benzodehydrotropylium ion has been proposed solely as a mass spectral fragment ion, without any insight on its properties.⁸ Dehydrobenzotropone–Co₂(CO)₄ (dppm) complexes have not been converted to tropylium ion derivatives.¹⁰ Given the limitations of the above information, we were interested in developing methods for access to the benzo-fused analogue of the dehydrotropylium ion–Co₂(CO)₆ complex (3a), to assess the viability of the cation itself.

Results and Discussion

We envisioned that a benzodehydrotropylium cation–Co₂(CO)₆ complex (3), specifically as methyl-substituted 3b, would be most readily obtained through benzocycloheptynol complex 4, which in turn would be prepared in several steps from 2-ethynylbenzaldehyde (5a) (Scheme 1). 2-Ethynylbenzaldehyde (5a), itself available by a literature Sonogashira-desilylation protocol from 2-bromobenzaldehyde via 6a,^13,14 was subjected to a Sonogashira reaction with 2-bromoallyltrimethylsilane to give enyne 7a (71% yield, Table 1). Upon treatment with Co₂(CO)₈, complex 8a formed in 80% yield. Generation of the cycloheptynol complex could then be accomplished by subjecting 8a to an equimolar amount of BF₃-OEt₂ at 0 °C in CH₂Cl₂ (0.016 M in 8a) for 20 min; the resulting Hosomi–Sakurai reaction gave exo-methylene benzocycloheptynol–Co₂(CO)₆ complex 9a in 75% yield (Scheme 2). The identity of 9a was evident from the ¹H resonances of the exo-methylene function (5.86 ppm and 5.70), the benzylic carbinol methine (4.98 ppm), and the diastereotopic methylene function (3.11 and 2.88 ppm).

Scheme 1. Preparation of Hosomi–Sakurai Precursors.

(a) K₂CO₃, MeOH, or n-Bu₄NF, THF; (b) H₂C=CH(Br)–CH₂SiMe₃, Pd(PPh₃)₄, CuI, i-Pr₂NH–THF; (c) Co₂(CO)₈, 0 °C, Et₂O.

Table 1. Hosomi–Sakurai Precursor Preparation.

	7 (%)	8 (%)
a	71	80
b	73	75
c	66	85
d	63	77
e	72	a

^aYield determined after a two-step complexation/cyclization process.

Scheme 2. Hosomi–Sakurai Reactions.

While other reaction fates and other ring size preparations are known,^15,16 this is the first example of a cycloheptynoldicobalt complex preparation by a Hosomi–Sakurai reaction. As a result, we wished to explore the generality of this method for benzocycloheptynedicobalt complex preparation and investigated this protocol on a series of related compounds, including those derived from 2-ethynyl-5-methoxybenzaldehyde (5b), 3-ethynyl-2-furancarboxaldehyde (5c), 3-ethynyl-2-thiophenecarboxaldehyde (5d), and 3-ethynyl-1-methyl-1H-indole-2-carboxaldehyde (5e). The Sonogashira reaction of 5b–e with 2-bromoallyltrimethylsilane afforded 7b–e in a straightforward fashion (7b, 73%; 7c, 66%; 7d, 63%; 7e, 72%) (Table 1). The reaction of 7b–7d with Co₂(CO)₈ formed the hexacarbonyldicobalt complexes 8b (75% yield), 8c (85%), and 8d (77%) in good yield. Conversely, alkyne 7e gave an alkyne complex (8e), which had limited stability to chromatography; consequently, 8e was carried forward to the subsequent cyclization reaction without rigorous purification.

The enyne complexes 8b–e were subjected to reaction with BF₃-OEt₂ under conditions analogous to the 8a–9a cyclization reaction. In each of the cases, the fused bicyclic exo-methylene cycloheptynol complex could be formed in reasonable to excellent yield, with the methoxy-substituted 9b (5 min, 72% yield) being formed somewhat more rapidly than 9a and the 5,7-systems 9c (59% yield) and 9d (92% yield) forming somewhat more slowly (ca. 1 h) than 9a. Compound 9e was formed, also over 1 h, in 44% yield, for the two-step process and based on the amount of metal-free alkyne 7e. Each of 9b–9e gave characteristic ¹H NMR spectral resonances closely analogous to those for 9a; for instance, in the case of 9d, the resonances for the exo-methylene appeared at 5.85 and 5.70 ppm, the carbinol methine at 5.05 ppm, and the diastereotopic methylene at 3.00 and 2.87 ppm. Despite the more modest yield for 9e, these results demonstrate that this Hosomi–Sakurai-reaction-based protocol is an effective general method for the preparation of exo-methylene-substituted cycloheptynol–Co₂(CO)₆ complexes.

With the goal of ultimate generation of the dehydrotropylium ion–Co₂(CO)₆ complex, compound 9a was selected for further manipulation. A Swern oxidation¹⁷ afforded ketone 10 with minimal decomposition (89% yield) (Scheme 3), as evidenced by the appearance of a shifted CH₂ singlet at 3.76 ppm in the ¹H NMR spectrum and ketone carbonyl ¹³C NMR spectral resonance at 198.3 ppm. While this material could also be obtained with MnO₂ oxidation, the yields were inferior (28%, 43% by recovered starting material). With the ketone function in place, the exo-methylene group of 10 could be induced to isomerize into conjugation with the carbonyl by a catalytic amount of H₂SO₄, giving enone 11 in 88% yield. The formation of this conjugated ketone was evidenced by the disappearance of the exo-methylene function of 10 in the ¹H NMR spectrum and the appearance of a single vinyl proton resonance at 6.68 ppm and a methyl group at 2.43 ppm. It is noteworthy that the analogous Co₂(CO)₄–dppm complex, 12, has been prepared previously by a carbonylative Heck reaction,¹⁰ but this process is not tolerant for the Co₂(CO)₆ complex. Finally, reduction of 11 with DIBAL-H gave a crude reaction product whose ¹H NMR spectral resonances were consistent with 4, but this material underwent prompt isomerization on silica gel; following chromatographic purification, 13 was isolated in 68% yield, along with a small amount of the elimination product 14 (29%). The appearance of two vinyl protons at 6.39 and 6.15 ppm (each J = 12.5 Hz) in the ¹H NMR spectrum was the most distinctive evidence for this isomerization product. Since 13 was equally serviceable relative to 4 as a precursor to the dehydrotropylium ion–Co₂(CO)₆ complex, compound 13 was considered appropriate for further study (Scheme 4).

Scheme 3. Preparation of a Dehydrotropylium Ion Complex Precursor.

(a) CH₃S(O)CH₃, ClC(O)C(O)Cl, CH₂Cl₂, −78 °C; then 9a, Et₃N, −78 to −20 °C, 89% (b) H₂SO₄, CH₂Cl₂, 0 °C, 88%; (c) DIBAL-H, THF, −78 °C; (d) silica gel, 68% (13) + 29% (14).

Scheme 4. Nicholas Reactions of Cation Precursor 13.

The reactivity of 13 was investigated with a series of nucleophiles, chosen as being representatives of the groups of nucleophiles commonly incorporated in Lewis-acid-mediated reactions of metal-stabilized carbocations and in view of their relative nucleophilicities as measured on the Mayr N scale.¹⁸ The reaction of 13, BF₃-OEt₂, and acetophenone trimethylsilyl enol ether (N = 6.22) gave 15 (94% yield) as a 9:1 regioisomeric mixture of 15γ and 15α (Table 2). N-Methylpyrrole (N = 5.85) gave condensation product 16 (81% yield) with only the γ-regioisomer apparent (16γ:16α > 30:1). Methallyltrimethylsilane (N = 4.41) afforded 17 (89% yield) as a 13:1 mixture of 17γ and 17α, whereas allyltrimethylsilane itself (N = 1.68) afforded 18 (82% yield) as a similar regioisomeric mixture (18γ:18α = 7.5:1). The major isomers (15–18γ) were distinguished in the ¹H NMR spectra by the single endocyclic vinyl proton (5.85 ppm in 15γ), whereas the minor isomer (15α, 16–18α), when present, was distinguished by the two doublets or AB pattern for the two endocyclic vinyl protons (6.39 ppm, and 6.30, AB, J = 12.3 Hz in 15α). In addition, resonances for the incorporated nucleophile [i.e., 5.79 (m), 5.13 ppm (d, J = 17.1 Hz,), 5.09 (d, J = 10.2 Hz) for the vinyl protons of the allyl unit of 18γ] were diagnostic. Attempts to incorporate less reactive nucleophiles, with N < 1 (i.e., thiophene, N = −1.01), resulted in the formation of elimination product 14, whereas radical homocoupling products were not seen in any of these reactions, in contrast to analogous reactions on dehydrotropylium–Co₂(CO)₆ ion (2) precursors.⁶

Table 2. Nicholas Reactions of 13.

Nu (Mayr N value)	product, yield (%)	γ:α
acetophenone TMS enol ether (6.22)	15, 94	9.0:1
N-methylpyrrole (5.82)	16, 81	>30:1
methallyltrimethylsilane (4.41)	17, 89	13:1
allyltrimethylsilane (1.68)	18, 82	7.5:1

From these results, it is clear that the benzodehydrotropylium ion–Co₂(CO)₆3b is generated readily. In previous work, Nicholas reactions of cations that are allylic in addition to being propargylic to an alkyne–Co₂(CO)₆ group have been shown to favor attacks at the γ-site [i.e., the terminus remote to the alkyne–Co₂(CO)₆ unit] over the α-site for the majority of nucleophiles (with a greater selectivity for lower reactivity nucleophiles), for reasons that are not entirely well-understood. This observation has been found to apply to both acyclic¹⁹ and cyclic systems.²⁰ Here, this situation is much less straightforward, as the γ-site of 3b is benzylic, whereas the α-site is substantially more hindered than in the literature cases.

With the isolation of the benzodehydrotropylium ion 3b complex not readily available, due to the elimination pathway, the structural aspects of 3b were addressed computationally. The putative aromaticity of 3b was also addressed in terms of geometric, ring current, and energetic factors, namely, HOMA values,²¹ NICS(1) values,²² and appropriate homodesmotic reactions,²³ for comparison to 1 and to tropylium ion. At the B3LYP/6-311+G(d,p) + ZPVE level, the minimized structure of 3b reveals a 7-membered ring that is nearly planar (Figure 2), with dihedral angles in the 7-membered ring averaging 8.4° and no individual dihedral angle in the ring being >18.5° (see the Supporting Information). Very modest bond alternation is apparent, with bond lengths ranging from 1.385 to 1.437 Ǻ, excluding the formal triple bond. Within the harmonic oscillator model of aromaticity, these bond lengths correspond to a HOMA of 0.636 (EN = 0.256, GEO = 0.108). While this figure is coincident with molecules of substantial aromaticity, it is lower than the dehydrotropylium ion–Co₂(CO)₆ complex 0.950 (EN = 0.044, GEO = 0.007). However, this earlier work^6a also demonstrated that HOMA was a poor measure of aromaticity in these charged systems in comparison with other methods.

Figure 2.

Optimized structure of 3b. Selected by lengths [Å]: C5–C6 1.364, C6–C7 1.403, C7–C8 1.415, C8–C9 1.385, C9–C4a 1.436, C4a–C9a 1.437.

The NICS(1) values of 3b were calculated to evaluate aromaticity by ring current criteria (see the Supporting Information). These calculations resulted in two different values for the 7-membered ring, with (arbitrarily) above the ring top face giving a value of −4.88 and the opposite face yielding a value of −2.03. A strict averaging of these numbers gives a value of −3.46, somewhat higher than the dehydrotropylium–Co₂(CO)₆ cation (1) (−2.92) and approximately one-third of that of the tropylium ion itself (−10.5 with the identical functional and basis set). Nevertheless, it is apparent that the proximity of each Co(CO)₃ unit is likely affecting these numbers, and they should be taken with caution.

After careful consideration of potential homodesmotic measures of aromaticity, the equation in Scheme 5 was chosen as the most appropriate reaction, given the nature of 3b as both a propargyldicobalt cation and as a benzylic cation. Employing DFT calculations (B3LYP/6-311+G(d,p) + ZPVE), cation 3b was evaluated as being aromatic by 2.7 kcal mol^–1 (see the Supporting Information). Given the apparent issues with the HOMA and NICS(1) calculations, we consider this the most reliable of the measures of the aromaticity of 3b. This value is nearly identical (≤0.1 kcal mol^–1 less) to that resulting from the dehydrotropylium ion complex 2 (2.8 kcal mol^–1) and ca. 23% of the value arrived at for the tropylium ion itself (11.6 kcal mol^–1).^6a

Scheme 5. Homodesmotic-Reaction-Based Evaluation of 3b.

In summary, we have prepared the alcohol precursor to a dehydrobenzotropylium ion–Co₂(CO)₆ by a Hosomi–Sakurai reaction and carried out subsequent manipulation of the exo-methylene benzocycloheptynol complex into a benzocycloheptenynol complex. The Hosomi–Sakurai reaction to generate the benzocycloheptyne–Co₂(CO)₆ ring system has been demonstrated to have some generality. The dehydrobenzotropylium ion–Co₂(CO)₆ complex 3b may be generated in solution and reacts with nucleophiles of Mayr nucleophilicities of N > 1. The cation itself is modestly aromatic, having approximately the same aromatic stabilization as dehydrotropylium ion−Co₂(CO)₆ and roughly one quarter of that of tropylium ion.

Experimental Section

General Considerations²⁵

Reagents were obtained from commercial sources unless otherwise stated. Reactions were conducted under an inert atmosphere (N₂) using glassware dried in an oven (110 °C, >1 h). The solvent for each reaction was acquired from a solvent purification system (Innovative Technologies). BF₃-OEt₂ was distilled prior to use and stored under an inert atmosphere (N₂). Reactions were subject to a “conventional workup” by partitioning the reaction mixture between an aqueous phase and a diethyl ether or dichloromethane phase, combining the organic phases, followed by drying (MgSO₄), filtration, and concentration of the organic phase. Flash chromatography was performed according to the method of Still.²⁶ High-resolution mass spectrometry (HRMS) results were obtained via a direct insertion probe-electron ionization method (70 eV) on a GCT time-of-flight (ToF) mass spectrometer at the McMaster Regional Centre for Mass Spectrometry and in the University of Windsor Mass Spectrometry lab with a ToF mass spectrometer using the atmospheric solids analysis probe (ASAP) and a corona discharge to facilitate ionization. Elemental analysis was performed by Guelph Chemical Laboratories, Guelph, ON (Canada). ¹H NMR spectra were recorded on 300 or 500 MHz spectrometers. Chemical shifts (δ) are reported in parts per million (ppm), relative to the 7.26 ppm resonance for the residual CHCl₃ in CDCl₃, unless otherwise indicated. Coupling constants are reported in Hertz (Hz). ¹³C NMR data were obtained at either 75 or 125 MHz. Infrared spectra (IR) were recorded on a FT-IR spectrophotometer using KBr plates. 2-Bromo-3-(trimethylsilyl)-1-propene and²⁴ alkynes 6a,¹⁴5a,¹⁴6b,¹⁴5b,¹⁴6c,²⁷5c,¹³6d,²⁷5d,²⁷ and 6e(28) were prepared by literature methods.

3-Ethynyl-1-methyl-1H-indole-2-carbaldehyde (5e)

To a solution of 6e (0. 741 g, 2.90 mmol) in anhydrous THF (25 mL) was added a solution of TBAF (1.0 M in THF, 2.9 mL, 1 equiv) at room temperature. The solution was stirred for 5 min, and a solution of aqueous NH₄Cl (sat) was added. The mixture was subjected to conventional extractive workup using diethyl ether to give the material satisfactory for further use. Compound 5e was isolated as a yellow solid (0.5122 g, 95% yield), mp 122–123 °C. IR (KBr) ν_max 3284, 2102, 1669, 1476 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 10.21 (s, 1H), 7.84 (dt, J = 1.0, 8.0 Hz, 1H), 7.47 (apparent td, J = 1.0, 8.0 Hz, 1H), 7.40 (apparent dt, J = 1.0, 8.5 Hz, 1H), 7.26 (td, J = 1.0, 7.5 Hz, 1H), 4.09 (s, 3H), 3.53 (s, 1H); ¹H NMR (500 MHz, CD₂Cl₂) δ 10.20 (s, 1H), 7.82 (d, J = 8.1 Hz, 1H), 7.49–7.43 (m, 2H), 7.26 (td, J = 7.0, 1.0 Hz, 1H), 4.07 (s, 3H), 3.61 (s, 1H); ¹³C NMR (75 MHz, CDCl₃) δ 182.3, 139.3, 136.5, 128.0, 127.8, 122.1, 121.9, 110.6, 110.3, 84.7, 74.8, 31.8; HRMS m/e for C₁₂H₉NO [M⁺] calcd 183.0684, found 183.0684.

2-(3-((Trimethylsilyl)methyl)but-3-en-1-yn-1-yl)benzaldehyde (7a)

General Method I: To a solution of 2-ethynylbenzaldehyde (6a, 0.543 g, 4.18 mmol) and 2-bromo-3-(trimethylsilyl)-1-propene (0.807 g, 4.18 mmol) in a degassed (5:1) mixture of diisopropylamine/THF (40 mL) were added Pd(PPh₃)₄ (0.241 g, 5 mol %) and CuI (0.080 g, 10 mol %). The solution was stirred for 2 h at room temperature. An aqueous solution of NH₄Cl (sat) was added, and the mixture was subjected to a conventional extractive workup using diethyl ether. The crude material was subjected to flash chromatography (25:1 petroleum ether/Et₂O) to give product 7a (0.719 g, 71%) as a yellow oil; IR (KBr) ν_max 2927, 1700, 1599 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 10.53 (d, J = 0.5 Hz, 1H), 7.91 (m, 1H), 7.53 (m, 2H), 7.41 (m, 1H), 5.37 (d, J = 2.0 Hz, 1H), 5.18 (d, J = 1.5 Hz, 1H), 1.79 (d, J = 0.5 Hz, 2H), 0.11 (s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ 192.0, 135.9, 133.9, 133.2, 128.6, 128.4, 127.3, 127.2, 120.8, 98.7, 83.8, 28.1, −1.5; HRMS m/e for C₁₅H₁₈OSi [M⁺] calcd 242.1127, found 242.1130.

5-Methoxy-2-(3-((trimethylsilyl)methyl)but-3-en-1-yn-1-yl)benzaldehyde (7b)

2-Ethynyl-5-methoxybenzaldehyde (6b, 0.147 g, 0.918 mmol, 1.1 equiv) and 2-bromo-3-(trimethylsilyl)-1-propene (0.1616 g, 0.836 mmol) were subjected to the conditions of General Method I. Flash chromatography (5:1 petroleum ether/Et₂O) gave product 7b (0.166 g, 73%) as a yellow oil; IR (KBr) ν_max 2955, 2200, 1690, 1601, 1493 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 10.49 (s, 1H), 7.45 (d, J = 8.5 Hz, 1H), 7.39 (d, J = 3.0 Hz, 1H), 7.10 (dd, J = 3.0, 8.5 Hz, 1H), 5.33 (s, 1H), 5.14 (s, 1H), 3.86 (s, 3H), 1.78 (s, 2H), 0.11 (s, 9H); ¹³C NMR (125 MHz, CD₂Cl₂) δ 191.3, 159.8, 137.2, 134.5, 132.0, 128.6, 121.3, 119.6, 109.8, 96.9, 83.6, 55.7, 27.9, −2.0; HRMS m/e for C₁₆H₂₀O₂Si [M⁺] calcd 272.1233, found 272.1239.

3-(3-((Trimethylsilyl)methyl)but-3-en-1-yn-1-yl)furan-2-carbaldehyde (7c)

3-Ethynylfuran-2-carbaldehyde (6c, 0.0737 g, 0.614 mmol) and 2-bromo-3-(trimethylsilyl)-1-propene (0.1185 g, 0.613 mmol) were subjected to the conditions of General Method I. Flash chromatography (5:1 petroleum ether/Et₂O) gave compound 7c (0.0939 g, 66% yield) as a yellow oil; IR (KBr) ν_max 2956, 1683, 1420, 1250 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 9.77 (d, J = 1.0 Hz, 1H), 7.61 (apparent dd, J = 1.5, 2.8 Hz, 1H), 6.58 (d, J = 3.0 Hz, 1H), 5.38 (d, J = 1.5 Hz, 1H), 5.21 (d, J = 1.5 Hz, 1H), 1.77 (d, J = 0.5 Hz, 2H), 0.10 (s, 9H); ¹³C NMR (125 MHz, CD₂Cl₂) δ 176.2, 153.0, 148.0, 128.6, 121.4, 120.0, 115.5, 99.8, 77.5, 28.1, −1.7; HRMS m/e for C₁₃H₁₆O₂Si [M⁺] calcd 232.0920; found: 232.0925.

3-(3-((Trimethylsilyl)methyl)but-3-en-1-yn-1-yl)thiophene-2-carbaldehyde (7d)

3-Ethynylthiophene-2-carbaldehyde (6d, 0.1425 g, 1.05 mmol) and 2-bromo-3-(trimethylsilyl)-1-propene (0.2035 g, 1.05 mmol) were subjected to the conditions of General Method I, with the exception of the reaction mixture being stirred for 2 h at 50 °C. Flash chromatography (2:1 petroleum ether/ether) afforded compound 7d (0.165 g, 63% yield) as a yellow oil; IR (KBr) ν_max 2955, 2925, 2195, 1668, 1418, 1248 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 10.12 (d, J = 1.0 Hz, 1H), 7.66 (dd, J = 1.0, 5.0 Hz, 1H), 7.15 (d, J = 5.0 Hz, 1H), 5.38 (d, J = 1.5 Hz, 1H), 5.20 (d, J = 1.5 Hz, 1H), 1.78 (s, 2H), 0.11 (s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ 183.1, 143.4, 134.0, 131.6, 128.2, 121.3, 110.3, 98.3, 80.5, 28.1, −1.5; HRMS m/e for C₁₃H₁₆OSSi [M⁺] calcd 248.0691, found 248.0673.

1-Methyl-3-(3-((trimethylsilyl)methyl)but-3-en-1-yn-1-yl)-1H-indole-2-carbaldehyde (7e)

3-Ethynyl-1-methyl-1H-indole-2-carbaldehyde (6e, 0.512 g, 2.79 mmol) and 2-bromo-3-(trimethylsilyl)-1-propene (0.539 g, 2.79 mmol) were subjected to the conditions of General Method I, with the exception of the reaction mixture being stirred for 30 min at room temperature. Flash chromatography (5:1 petroleum ether/Et₂O) gave product 7e (0.595 g, 72%) as a yellow oil; IR (KBr) ν_max 2954, 2200, 1668, 1476 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 10.18 (s, 1H), 7.83 (d, J = 8.0 Hz, 1H), 7.46 (td, J = 1.0, 7.5 Hz, 1H), 7.37 (d, J = 8.5 Hz, 1H), 7.24 (m, 1H), 5.39 (d, J = 2.0 Hz, 1H), 5.17 (s, 1H), 4.08 (s, 3H), 1.84 (s, 2H), 0.14 (s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ 182.4, 139.6, 135.4, 128.8, 127.8, 127.7, 122.3, 121.6, 119.7, 112.2, 110.5, 99.3, 79.2, 31.8, 28.3, −1.5; HRMS m/e for C₁₈H₂₁NOSi [M⁺] calcd 295.1392, found 295.1384.

Hexacarbonyl[μ-η⁴-(2-(3-((trimethylsilyl)methyl)but-3-en-1-yn-1-yl)benzaldehyde)]dicobalt (8a)

General Method II: To a solution of 7a (0.132 g, 0.545 mmol) in anhydrous Et₂O at 0 °C was added an unweighed amount of dicobalt octacarbonyl (excess). The solution was stirred for 1.5 h at 0 °C and was subsequently concentrated under reduced pressure at 0 °C. The crude brown-colored material was added to the top of a plug of silica and washed with hexanes, followed by diethyl ether. The concentration of the Et₂O washings afforded compound (8a) as a dark-brown oil (0.230 g, 80% yield); IR (KBr) ν_max 2925, 2852, 2092, 2055, 2022, 1693, 1593 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 10.37 (s, 1H), 7.94 (d, J = 7.8 Hz, 1H), 7.68 (d, J = 7.5 Hz, 1H), 7.61 (td, J = 1.25, 7.5 Hz, 1H), 7.46 (t, J = 7.7 Hz, 1H), 5.48 (d, J = 1.0 Hz, 1H), 5.35 (d, J = 1.0 Hz, 1H), 1.72 (d, J = 1.0 Hz, 2H), 0.10 (s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ 199.0, 191.1, 143.7, 141.5, 134.7, 133.5, 132.6, 128.2, 117.3, 101.9, 88.6, 26.5, −0.8; HRMS m/e for C₂₁H₁₈Co₂O₇Si [M⁺ – 2CO] calcd 471.9587, found 471.9604.

Hexacarbonyl[μ-η⁴-(5-methoxy-2-(3-((trimethylsilyl)methyl)but-3-en-1-yn-1-yl) benzaldehyde)]dicobalt (8b)

A solution of 7b (0.155 g, 0.57 mmol) in anhydrous Et₂O (40 mL) was subjected to General Method II to afford product 8b (0.238 g, 75% yield) as a dark-brown oil; IR (KBr) ν_max 2959, 2090, 2055, 1689, 1599, 1481 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 10.36 (s, 1H), 7.60 (d, J = 8.5 Hz, 1H), 7.43 (d, J = 2.5 Hz, 1H), 7.18 (dd, J = 2.5, 8.5 Hz, 1H), 5.48 (s, 1H), 5.34 (s, 1H), 3.89 (s, 3H), 1.71 (s, 2H), 0.10 (s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ 199.2, 190.9, 159.7, 143.8, 135.2, 133.8, 133.5, 122.5, 117.3, 110.6, 101.6, 89.1, 55.8, 26.4, −0.8; HRMS m/e for C₂₂H₂₀Co₂O₈Si [M⁺ – 4CO] calcd 445.9795, found 445.9785.

Hexacarbonyl[μ-η⁴-(3-(3-((trimethylsilyl)methyl)but-3-en-1-yn-1-yl)furan-2-carbaldehyde)]dicobalt (8c)

A solution of 7c (0.0305 g, 0.131 mmol) in anhydrous Et₂O (35 mL) was subjected to General Method II to afford product 8c (0.058 g, 85% yield) as a dark-brown oil; IR (KBr) ν_max 2958, 2094, 2056, 2026, 1679, 1477, 1251 cm^–1; ¹H NMR (300 MHz, CDCl₃) δ 9.84 (d, J = 0.6 Hz, 1H), 7.65 (apparent dd, J = 0.6, 1.8 Hz, 1H), 6.64 (d, J = 1.8 Hz, 1H), 5.46 (d, J = 0.9 Hz, 1H), 5.29 (d, J = 0.9 Hz, 1H), 1.78 (d, J = 0.6 Hz, 2H), 0.10 (s, 9H); ¹³C NMR (125 MHz, CD₂Cl₂) δ 199.0, 176.2, 147.9, 146.6, 143.5, 136.1, 116.5, 115.6, 100.1, 77.9, 26.0, −1.3; HRMS m/e for C₁₉H₁₆Co₂O₈Si [M⁺ – CO] calcd 489.9329, found 489.9312.

Hexacarbonyl[μ-η⁴-(3-(3-((trimethylsilyl)methyl)but-3-en-1-yn-1-yl)thiophene-2-carbaldehyde)]dicobalt (8d)

A solution of 7d (0.0165 g, 0.066 mmol) in anhydrous Et₂O (15 mL) was subjected to General Method II to afford product 8d (0.0273 g, 77% yield) as a dark-brown oil; IR (KBr) ν_max 2957, 2925, 2855, 2092, 2056, 2026, 1721, 1664, 1461 cm^–1; ¹H NMR (300 MHz, CDCl₃) δ 10.09 (d, J = 1.2 Hz, 1H), 7.72 (dd, J = 5.0, 1.2 Hz, 1H), 7.23 (d, J = 5.0 Hz, 1H), 5.49 (d, J = 0.7 Hz, 1H), 5.34 (m, 1H), 1.77 (d, J = 0.9 Hz, 2H), 0.12 (s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ 198.7, 182.4, 147.2, 143.4, 137.1, 134.7, 132.9, 117.0, 99.8, 81.4, 26.2, −1.0; HRMS m/e for C₁₉H₁₆Co₂O₇SSi [M⁺ – CO] calcd 505.9101, found 505.9100.

Hexacarbonyl[μ-(8,9-dehydro-7-methylene-6,7-dihydro-5H-benzo[7]annulen-5-ol)]dicobalt (9a)

General Method III: To a solution of 8a (0.2177 g, 0.412 mmol) in anhydrous CH₂Cl₂ (25 mL) at 0 °C was added BF₃-OEt₂ (0.051 mL, 0.41 mmol, 1.0 equiv). After stirring for 20 min at 0 °C, aqueous NaHCO₃ (sat) was added to the mixture, which was then subjected to a conventional extractive workup using CH₂Cl₂. The resulting residue was subjected to flash chromatography (2:1 petroleum ether/Et₂O) to afford alcohol (9a) as a dark-brown solid (0.1415 g, 75% yield), mp 91–93 °C; IR (KBr) ν_max 3438, 2090, 2055, 2022, 1633 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 7.78 (d, J = 7.5 Hz, 1H), 7.41 (m, 1H), 7.31–7.36 (m, 2H), 5.87 (s, 1H), 5.71 (s, 1H), 4.99 (dd, J = 4.0, 7.5 Hz, 1H), 3.12 (d of 1/2 AB, J = 8.0, 14.5 Hz, 1H), 2.90 (1/2 AB, J = 14.5 Hz, 1H), 1.82 (d, J = 4.0 Hz, 1H); ¹³C NMR (75 MHz, CDCl₃) δ 199.2, 143.0, 141.3, 135.8, 134.3, 129.4, 129.2, 128.5, 123.4, 88.4, 88.0, 73.3, 41.3; HRMS m/e for C₁₈H₁₀Co₂O₇ [M⁺] calcd 455.9090, found 455.9085. Anal. calcd for C₁₈H₁₀Co₂O₇: C, 47.40; H, 2.21; found: C, 47.16; H, 1.95.

Hexacarbonyl[μ-(8,9-dehydro-3-methoxy-7-methylene-6,7-dihydro-5H-benzo[7]annulen-5-ol)]dicobalt (9b)

Following General Method III, a solution of compound 8b (0.1445 g, 0.259 mmol) in anhydrous CH₂Cl₂ (30 mL) was treated with BF₃-OEt₂ (33 μL, 0.26 mmol, 1.0 equiv). The reaction was conducted for 5 min (with monitoring by TLC). The crude material was subjected to flash chromatography (5:1 petroleum ether/Et₂O) to afford compound 9b (0.0901 g, 72% yield) as a dark-brown solid, mp 76–78 °C; IR (KBr) ν_max 3413, 2923, 2088, 2051, 2021 cm^–1; ¹H NMR (300 MHz, CDCl₃) δ 7.69 (d, J = 8.4 Hz, 1H), 6.92 (dd, J = 2.7, 8.4 Hz, 1H), 6.89 (d, J = 2.7 Hz, 1H), 5.84 (s, 1H), 5.67 (d, J = 0.9 Hz, 1H), 4.91 (dd, J = 4.5, 7.5 Hz, 1H), 3.85 (s, 3H), 3.09 (d of 1/2 AB, J = 7.8, 14.5 Hz, 1H), 2.88 (1/2 AB, J = 14.5 Hz, 1H), 1.84 (d, J = 4.5 Hz, 1H); ¹³C NMR (75 MHz, CDCl₃) δ 199.4, 159.9, 143.1, 135.9, 127.4, 123.1, 114.7, 114.5, 88.8, 88.6, 73.3, 55.5, 41.7; HRMS m/e for C₁₉H₁₂Co₂O₈ [M⁺] calcd 485.9196, found 485.9202.

Hexacarbonyl[μ-(7,8-dehydro-6-methylene-6H-cyclohepta[b]furan-8-ol)]dicobalt (9c)

A solution of compound 8c (0.0410 g, 0.079 mmol) in anhydrous CH₂Cl₂ (15 mL) was treated with BF₃-OEt₂ (9.8 μL, 0.079 mmol, 1.0 equiv) as described in General Method III. The reaction was conducted for 1 h (with monitoring by TLC). The crude material was subjected to flash chromatography (2:1 petroleum ether/Et₂O) to afford compound 9c (0.0208 g, 59% yield) as a viscous brown oil; IR (KBr) ν_max 3390, 2094, 2056, 2026 cm^–1; ¹H NMR (300 MHz, CDCl₃) δ 7.44 (d, J = 1.9 Hz, 1H), 6.63 (d, J = 1.9 Hz, 1H), 5.86 (s, 1H), 5.71 (s, 1H), 5.03 (m, 1H), 3.00 (d of 1/2 AB, J = 6.3, 14.1 Hz, 1H), 2.78 (1/2 AB, J = 14.1 Hz, 1H), 1.93 (d, J = 7.2 Hz, 1H); ¹³C NMR (125 MHz, CDCl₃) δ 199.0, 152.1, 143.7, 141.6, 124.8, 118.6, 112.7, 91.0, 79.7, 65.1, 39.6; HRMS m/e for C₁₆H₈Co₂O₈ [M⁺ – 2CO] calcd 389.8985, found 389.8989.

Hexacarbonyl[μ-(7,8-dehydro-6-methylene-6H-cyclohepta[b]thiophen-8-ol)]dicobalt (9d)

A solution of compound 8d (0.0165 g, 0.031 mmol) in anhydrous CH₂Cl₂ (8 mL) was treated with BF₃-OEt₂ (3.5 μL, 0.031 mmol, 1.0 equiv) as described in General Method III. The reaction was conducted for 1 h (with monitoring by TLC). The crude material was subjected to flash chromatography (2:1 petroleum ether/Et₂O) to afford compound 9d (0.0131 g, 92% yield) as a viscous brown oil; IR (KBr) ν_max 3398, 2926, 2855, 2090, 2055, 2022, 1715, 1625 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 7.29 (d, J = 5.5 Hz, 1H), 7.25 (d, J = 5.0 Hz, 1H), 5.85 (s, 1H), 5.70 (s, 1H), 5.05 (dd, J = 6.5, 8.2 Hz, 1H), 3.00 (d of 1/2 AB, J =8.2, 14.0 Hz, 1H), 2.87 (1/2 AB, J = 14.0 Hz, 1H), 2.04 (d, J = 6.5 Hz, 1H); ¹³C NMR (125 MHz, CDCl₃) δ 198.9, 143.5, 142.3, 133.4, 130.9, 125.7, 124.1, 89.3, 82.1, 68.2, 41.7; HRMS m/e for C₁₆H₈Co₂O₇S [M⁺] calcd 461.8655, found 461.8652.

Hexacarbonyl[μ-(9,10-dehydro-5-methyl-8-methylene-5,6,7,8-tetrahydrocyclo-hepta[b]indol-6-ol)]dicobalt (9e)

A solution of 7e (0.0415 g, 0.141 mmol) in CH₂Cl₂ (30 mL) was subjected to General Method II. Following removal of volatiles under reduced pressure, the residue containing 8e was dissolved in anhydrous CH₂Cl₂ (15 mL) and treated with BF₃-OEt₂ (18 μL, 0.14 mmol, 1.0 equiv) as described in Method III. The reaction was conducted for 1 h (with monitoring by TLC). The crude material was subjected to flash chromatography (2:1 petroleum ether/Et₂O) to afford compound 9e (0.0315 g, 44% yield) as a black solid, mp > 250 °C; IR (KBr) ν_max 3561, 2921, 2086, 2036, 2000, 1720 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 7.87 (d, J = 8.0 Hz, 1H), 7.38 (d, J = 8.0 Hz, 1H), 7.34 (td, J = 1.0, 7.0 Hz, 1H), 7.27 (td, J = 1.0, 7.0 Hz, 1H), 5.91 (s, 1H), 5.71 (d, J = 1.5 Hz, 1H), 5.22 (m, 1H), 3.84 (s, 3H), 3.15 (d of 1/2 AB, J = 5.5, 14.0 Hz, 1H), 2.90 (1/2 AB, J = 14.0 Hz, 1H), 1.70 (d, J = 10.0 Hz, 1H); ¹³C NMR (75 MHz, CDCl₃) δ 199.9, 142.1, 138.3, 137.7, 128.4, 123.7, 123.3, 121.0, 119.8, 110.1, 109.6, 92.1, 83.0, 63.5, 40.3, 29.6; HRMS m/e for C₂₁H₁₃Co₂NO₇ [M⁺] calcd 508.9356, found 508.9352.

Hexacarbonyl[μ-(8,9-dehydro-7-methylene-6,7-dihydro-5H-benzo[7]annulen-5-one)]dicobalt (10)

To a solution of oxalyl chloride (60 μL, 0.69 mmol) in CH₂Cl₂ (15 mL) at −78 °C was added DMSO (60 μL, 0.84 mmol). After 30 min, a solution of 9a (0.1130 g. 0.248 mmol) in CH₂Cl₂ (4 mL) was added. After stirring at −78°C for 45 min, Et₃N (0.30 mL, 2.2 mmol) was added, and the solution was gradually allowed to warm to −20 °C over 45 min. Addition of NH₄Cl(aq) and a conventional extractive workup (CH₂Cl₂), followed by flash chromatography (10:1 petroleum ether:Et₂O), gave 10 (0.1005 g, 89%) as a red-brown oil; IR (KBr) ν_max 2926, 2095, 2057, 2026, 1682, 1596 cm^–1; ¹H NMR (500 MHz, CDCl₃) δ 7.75 (d, J = 7.5 Hz, 1H), 7.68 (dd, J = 1.5, 7.7 Hz, 1H), 7.56 (td, J = 1.5, 7.7 Hz, 1H), 7.41 (td, J = 1.0, 7.7 Hz, 1H), 5.77 (s, 1H), 5.75 (s, 1H), 3.76 (s, 2H); ¹³C NMR (125 MHz, CD₂Cl₂) δ 198.7, 198.3, 139.1, 138.6, 136.8, 133.4, 132.6, 128.9, 128.4, 122.3, 87.8, 87.4, 51.9; HRMS m/e for C₁₈H₈Co₂O₇ [M⁺] calcd 453.8934, found 453.8914.

Hexacarbonyl[μ-(8,9-dehydro-7-methyl-5H-benzo[7]annulen-5-one)]dicobalt (11)

To a solution of 10 (80.9 mg, 0.20 mmol) in anhydrous CH₂Cl₂ (10 mL) at 0 °C was added H₂SO₄ (3 drops in 2 mL of anhydrous CH₂Cl₂) in a dropwise fashion over a period of 20 min. The solution was stirred for 2 h at 0 °C, after which water was added, and the mixture was subjected to a conventional extractive workup using dichloromethane. The crude material was subjected to flash chromatography (5:1 petroleum ether/Et₂O) to afford 11 (70.8 mg, 88% yield) as a brown solid, mp 77–79 °C; IR (KBr) ν_max 2918, 2095, 2057, 2031, 1730, 1605, 1582 cm^–1; ¹H NMR (300 MHz, CDCl₃) δ 8.25 (dd, J = 1.2, 8.1 Hz, 1H), 7.87 (dd, J = 1.2, 7.6 Hz, 1H), 7.66 (td, J = 1.2, 7.5 Hz, 1H), 7.53 (td, J = 1.2, 7.5 Hz, 1H), 6.67 (d, J = 1.2 Hz, 1H), 2.43 (d, J = 1.2 Hz, 3H); ¹³C NMR (75 MHz, CD₂Cl₂) δ 198.5, 189.3, 149.5, 137.8, 136.7, 133.6, 133.5, 132.4, 131.4, 129.1, 85.4, 83.7, 23.6; HRMS m/e for C₁₈H₈Co₂O₇ [M⁺] calcd 453.8934, found 453.8941.

Hexacarbonyl[μ-(8,9-dehydro-7-methyl-7H-benzo[7]annulen-7-ol)]dicobalt (13)

To a solution of compound 11 (52 mg, 0.11 mmol) in anhydrous CH₂Cl₂ (7 mL) at −78 °C was added DIBAL-H (0.46 mL of a 1.0 M solution in THF, 0.46 mmol, 4 equiv) in a dropwise manner. The solution was stirred for 1 h at −78 °C. An aqueous solution of NH₄Cl (sat) was added, and the reaction mixture was subjected to a conventional extractive workup using dichloromethane. Following removal of the volatiles under reduced pressure, flash chromatography (10:1 petroleum ether/Et₂O) afforded elimination product 14 (14.0 mg, 29% yield) followed by alcohols 13 and 4 (inseparable by chromatography). The isolated mixture containing (13) and (4) was dissolved in hexanes, and silica gel was added. After stirring for 1 h, the silica was removed by filtration and the collected filtrate was concentrated under reduced pressure to afford alcohol (13) (35.5 mg, 68% yield from ketone 11). Compound 13; a red-brown solid; mp 80–81 °C (dec); IR (KBr) ν_max 3448, 2924, 2853, 2092, 2055, 2024, 1723 cm^–1; ¹H NMR (300 MHz, CDCl₃) δ 7.67 (m, 1H), 7.30–7.40 (m, 2H), 7.20 (m, 1H), 6.39 (d, J = 12.5 Hz, 1H), 6.15 (d, J = 12.5 Hz, 1H), 2.31 (s, 1H), 1.55 (s, 3H); ¹³C NMR (75 MHz, CDCl₃) δ 199.2, 139.8, 135.5, 132.0, 133.0, 131.81, 131.79, 128.9, 128.7, 128.2, 107.1, 86.5, 73.7, 32.0; HRMS m/e for C₁₈H₁₀Co₂O₇ [M⁺ – 2CO] calcd 399.9192, found 399.9196. Compound 14; a red-brown viscous oil; IR (KBr) ν_max 3016, 2086, 2033, 2017, 2000, 1624 cm^–1; 1H NMR (500 MHz, CDCl₃) δ 7.70 (m, 1H), 7.27–7.31 (m, 2H), 7.15 (m, 1H), 6.37 (1/2 AB, J = 12.6 Hz, 1H), 6.34 (1/2 AB, J = 12.6 Hz, 1H), 5.71 (s, 1H), 5.63 (s, 1H); ¹³C NMR (125 MHz) 198.7, 143.8, 137.0, 133.13, 133.10, 133.05, 130.9, 130.1, 128.9, 128.6, 121.6, 88.2, 87.3; HRMS (ASAP) m/e for C₁₈H₈Co₂O₆ [M⁺ + H] calcd 438.9063, found 438.9059. Resonances for 4 could be observed in the ¹H NMR spectra of the crude reaction product at δ 7.74 (dd, J = 1.0, 7.4, Hz, 1H), 7.54 (d, J = 7.6, 1H), 7.44 (apparent dt, J = 1.2, 7.5 Hz, 1H), 7.40 (apparent dt, J = 1.1, 7.4, 1H), 6.05 (m, 1H), 5.12 (br s, 1H), 2.21 (s, 3H), 1.97 (d, J = 4.3, 1H).

General Method IV: To a solution of (13) in anhydrous dichloromethane at 0 °C was added an excess amount of the nucleophile (5–8 equiv). BF₃-OEt₂ (3 equiv) subsequently was added dropwise while maintaining the temperature at 0 °C. The solution was stirred for 30 min (with monitoring by TLC). Aqueous NaHCO₃ (sat) was added, followed by a conventional extractive workup using dichloromethane. Flash chromatography (100:1 petroleum ether/Et₂O) afforded the final product.

Hexacarbonyl[μ-(8,9-dehydro-2-(7-methyl-5H-benzo[7]annulen-5-yl)-1 phenylethanone)]dicobalt (15γ and 15α)

A solution of alcohol compound 13 (68.0 mg, 0.149 mmol) in CH₂Cl₂ (10 mL) was subjected to General Method IV with trimethyl((1-phenylvinyl)oxy)silane (143 mg, 0.745 mmol, 5 equiv) and BF₃-OEt₂ (57 μL, 0.45 mmol, 3 equiv), followed by flash chromatography (20:1 petroleum ether: Et₂O), to afford a 9.0:1 mixture of 15γ and 15α (0.0780 g, 94% yield) as a red-brown viscous oil; IR (KBr) ν_max 2923, 2853, 2088, 2050, 2017, 1688, 1449 cm^–1; ¹H NMR (300 MHz, CDCl₃) (major product, 15γ) δ 7.96 (d, J = 7.4 Hz, 2H), 7.73 (m, 1H), 7.58 (m, 1H), 7.42–7.49 (m, 2H), 7.28–7.36 (m, 2H), 5.85 (dd, J = 5.9, 1.4 Hz, 1H), 4.05 (m, 1H), 3.5d (dd, J = 16.8, 8.6 Hz, 1H), 3.46 (d, J = 16.8, 4.5 Hz, 1H), 2.14 (s, 3H); resonances from the minor product (15α) could be observed at 7.88 (d, J = 7.5 Hz, 1H), 7.67 (m, 1H), 6.36 (1/2 AB, J = 12.3 Hz, 1H), 6.30 (1/2 AB, J = 12.2 Hz, 1H), 3.40 (d, J = 16.3 Hz, 1H), 3.26 (d, J = 16.3 Hz, 1H), 1.66 (s, 3H); ¹³C NMR (75 MHz, CDCl₃) (major product, 15γ) δ 199.6, 197.6, 138.5, 137.1, 136.7, 135.1, 133.4, 133.2, 130.5, 128.9, 128.6, 128.0, 127.4, 126.8, 91.8, 88.6, 44.1, 40.2, 23.3; resonances from the minor isomer could be observed at 197.1, 140.4, 135.5, 131.4, 131.3, 129.4, 128.4, 51.8, 42.5, 28.8; HRMS m/e for C₂₆H₁₆Co₂O₇ [M⁺ – 6CO] calcd 389.9865, found 389.9844.

Hexacarbonyl[μ-(8,9-dehydro-1-methyl-3-(7-methyl-5H-benzo[7]annulen-5-yl)-1H-pyrrole)]dicobalt (16γ)

A solution of alcohol compound 13 (10.2 mg, 0.0224 mmol) in CH₂Cl₂ (5 mL) was subjected to General Method IV with N-methylpyrrole (9.1 mg, 0.11 mmol, 5 equiv), resulting in the isolation of 16γ (9.4 mg, 81% yield) as a viscous brown oil; IR (KBr) ν_max 3434, 2918, 2090, 2051, 2021, 1641 cm^–1; ¹H NMR (300 MHz, CD₂Cl₂) δ 7.71 (dd, J = 1.5, 7.5 Hz, 1H), 7.28 (m, 1H), 7.21 (td, J = 1.8, 7.5 Hz, 1H), 6.64 (apparent t, J = 2.1 Hz, 1H), 6.34 (m, 1H), 6.30 (m, 1H), 6.25 (m, 1H), 6.19 (td, obscured, 1H), 4.38 (d, J = 2.1 Hz, 1H), 3.20 (s, 3H), 2.21 (t, J = 2.8 Hz, 3H); ¹³C NMR (75 MHz, CD₂Cl₂) δ 200.4, 199.1, 139.7, 137.1, 136.3, 132.8, 132.7, 130.2, 128.7, 127.2, 126.5, 122.0, 108.1, 106.9, 92.8, 89.3, 41.8, 33.5, 22.8; HRMS m/e for C₂₃H₁₅Co₂NO₆ [M⁺ – 2CO] calcd 462.9665, found 462.9662.

Hexacarbonyl[μ-(8,9-dehydro-7-methyl-5-(2-methylallyl)-5H-benzo[7]annulene)]dicobalt (17γ and 17α)

A solution of alcohol compound 13 (61.5 mg, 0.135 mmol) in CH₂Cl₂ (10 mL) was subjected to General Method IV with methallyltrimethylsilane (0.12 mL, 0.67 mmol, 5 equiv) and BF₃-OEt₂ (51 μL, 0.40 mmol) to afford a (13:1) mixture of 17γ and 17α (59.1 mg, 89% yield 17γ + 17α) as a viscous red-brown oil; IR (KBr) ν_max 2956, 2922, 2852, 2094, 2055, 2011, 1601, 1463 cm^–1; ¹H NMR (300 MHz, CDCl₃) (major product, 17γ) δ 7.70 (dd, J = 7.3, 1.8 Hz, 1H), 7.29–7.35 (m, 2H), 7.21 (d, J = 7.4 Hz, 1H), 5.74 (dd, J = 4.7, 1.4 Hz, 1H), 4.88 (s, 1H), 4.79 (s, 1H), 3.26 (m 1H), 2.58–2.73 (m, 2H), 2.16 (s, 3H), 1.70 (s, 3H); resonances from the minor product (17α) could be detected at 7.65 (m, 1H), 7.15 (m, 1H), 6.39 (d, J = 12.1 Hz, 1H), 5.97 (d, J = 12.1 Hz, 1H), 4.98 (s, 1H), 4.83 (s, 1H), 2.42 (d, J = 12.5 Hz, 1H), 2.22 (d, J = 12.5 Hz, 1H), 1.86 (s, 3H), 1.48 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) (major product, 17γ) δ 200.1, 199.4, 143.0, 139.4, 137.5, 135.3, 133.1, 133.0, 131.1, 128.5, 127.0, 125.9, 113.2, 92.7, 89.4, 42.2, 40.8, 23.0, 22.2; resonances from the minor product could be observed at 142.2, 141.0, 131.4, 129.6, 128.3, 127.9, 116.1, 53.2, 43.2, 29.0, 25.2; HRMS m/e for C₂₂H₁₆Co₂O₆ [M⁺ – 2CO] calcd 437.9713, found 437.9720.

Hexacarbonyl[μ-(8,9-dehydro-5-allyl-7-methyl-5H-benzo[7]annulene)]dicobalt (18γ and 18α)

A solution of alcohol compound 13 (11.4 mg, 0.0250 mmol) in CH₂Cl₂ (7 mL) was subjected to General Method IV with allyltrimethylsilane (22.8 mg, 0.201 mmol, 8 equiv) to afford a (7.5:1) mixture of 18γ and 18α (9.9 mg, 82% yield 18γ + 18α) as a viscous brown oil; IR (KBr) ν_max 2924, 2854, 2088, 2048, 2015, 1445, 1102 cm^–1; ¹H NMR (500 MHz, CDCl₃) (major product, 18γ) δ 7.68 (d, J = 7.5 Hz, 1H), 7.30–7.35 (m, 2H), 7.20 (d, J = 7.5 Hz, 1H), 5.79 (m, 1H), 5.75 (d, J = 5.3 Hz, 1H), 5.13 (d, J = 17.1 Hz, 1H), 5.09 (d, J = 10.2 Hz, 1H), 3.15 (m, 1H), 2.63–2.72 (m, 2H), 2.14 (s, 3H); resonances from the minor product (18α) could be detected at 7.63 (m, 1H), 7.14 (m, 1H), 6.39 (d, J = 12.2 Hz, 1H), 5.95 (m, 1H), 5.88 (d, J = 12.2 Hz, 1H), 2.47 (m, 1H), 2.35 (m, 1H), 1.40 (s, 3H); ¹³C NMR (75 MHz, CDCl₃) (major product, 18γ) δ 200.0, 199.5, 139.2, 137.3, 136.4, 135.4, 132.9, 131.4, 128.5, 127.0, 126.1 117.0, 92.7, 89.3, 43.5, 38.2, 22.9; resonances from the minor product could be observed at 140.2, 133.9, 129.7, 127.9, 118.7, 49.4, 29.2; HRMS m/e for C₂₁H₁₄Co₂O₆ [M⁺] calcd 479.9454, found 479.9440.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b02390.

• ¹H and ¹³C NMR data of all new compounds; final coordinates for computationally determined structures; magnetic shielding tensors for 3b (PDF)

Author Present Address

^† Department of Chemistry, Brandon University, Brandon, Manitoba R7A 6A9, Canada (E.A.C.B.).

The authors declare no competing financial interest.