Synthesis of Dihydrobenzosiloles and Silacyclopentanes by Double Hydroalumination of Terminal Alkynes

Abstract

We have developed an efficient method for the synthesis of 2,3-dihydro-1H-1-benzosiloles in 19% to 90% isolated yield from 1-hydrosilyl-2-ethynylbenzenes by using two equivalents of diisobutylaluminum hydride. The reaction mechanism involves regioselective double hydroalumination of the alkyne moiety followed by cyclization to a 2-alanyldihydrobenzosilole. A silacyclopentane (silolane) was also synthesized in 97% isolated yield from the corresponding 4-silylbut-1-yne under the same reaction conditions. Although the substrate-scope study was conducted on a 0.5-mmol scale, a gram-scale reaction of (2-ethynylphenyl)(diphenyl)silane under the optimized reaction conditions successfully afforded the desired product in 94% isolated yield without loss of reactivity.

Key words

diisobutylaluminum hydride - cyclization - benzosiloles - silolanes

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• 18 Optimization of the Synthesis of Dibenzosilole 2a (Table [1]) A 1.0 M solution of DIBAL-H in hexane (0.80 mmol, 0.80 mL) was added to a reaction vessel containing 1a (0.40 mmol) in octane (0.80 mL) at rt. The mixture was stirred in an oil bath at 80 °C for 5 h, then allowed to cool to rt. 1 M aq HCl (3.0 mL) was added to the mixture, and the resulting aqueous mixture was stirred for 10 min, then extracted with hexane. The combined organic layer was washed with brine, dried (Na₂SO₄), and concentrated. The yield of 2a was estimated by using the integral values for the ¹H NMR signals with dibenzyl ether as an internal standard. In the case of Entries 4 and 8, the residues were purified by column chromatography (silica gel, hexane).
• 19 1,1-Diphenylsilolane (7) A 1.0 M solution of DIBAL-H in hexane (0.80 mmol, 0.80 mL) was added to a reaction vessel containing alkyne 6 (0.40 mmol) in octane (0.80 mL) at rt. The mixture was stirred in an oil bath at 80 °C for 5 h and then allowed to cool to rt. 1 M aq HCl (3.0 mL) was added to the mixture, and the resulting aqueous mixture was stirred for 10 min, then extracted with hexane. The combined organic layer was washed with brine, dried (Na₂SO₄), concentrated, and purified by column chromatography (silica gel, hexane) to give a Colorless liquid; yield: 97%.
• 20 1,1-Diphenyl-1H-1-benzosilole; One-Pot Procedure (Scheme [4]) A 1.0 M solution of DIBAL-H in hexane (0.80 mmol, 0.80 mL) was added to a reaction vessel containing alkyne 1a (0.40 mmol) in octane (0.80 mL) at rt. The mixture was stirred in an oil bath at 80 °C for 5 h, then allowed to cool to rt. Benzophenone (3.6 mmol) was added to the reaction vessel then the mixture was stirred in an oil bath at 80 °C for a further 24 h, and then allowed to cool to rt. 1 M aq HCl (3.0 mL) was added to the mixture, and the resulting aqueous mixture was stirred for 10 min, then extracted with hexane. The combined organic layer was washed with brine, dried (Na₂SO₄), concentrated, and purified by column chromatography (silica gel, hexane) to give a Pale yellow liquid; yield: 55%.