The Hydroboration of 3-Chloro-1-trimethylsilyl-1-propyne with Dialkylboranes and Its Application to the Syntheses of (E)-3-Trimethylsilyl-2-alkenes and 3-Trimethylsilyl-1-alkenes

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Abstract
The reaction of 3-chloro-1-trimethylsilyl-1-propyne with a stoichiometric amount of dialkylborane proceeds to the monohydroboration stage, giving a mixture of regioisomers, (Z)-dialkyl(3-chloro-1-trimethylsilyl-1-propenyl)borane and (Z)-dialkyl[1-chloromethyl-2-(trimethylsilyl)vinyl]borane. The hydroboration mixture is protonolyzed with acetic acid to give (Z)-3-chloro-1-trimethylsilyl-1-propene in a quantitative yield. Treatment of the hydroboration mixture with aqueous sodium hydroxide gives a good yield of highly pure (E)-3-trimethylsilyl-2-alkene whose alkyl group migrates from the boron atom. On the other hand, successive treatment of the hydroboration mixture with methyllithium and acetic acid gives a good yield of 3-trimethylsilyl-1-alkene whose alkyl group migrates from the boron atom. The above procedures provide convenient syntheses of (E)-3-trimethylsilyl-2-alkenes and 3-trimethylsilyl-1-alkenes. It is proposed that both products are afforded through the corresponding allylboranes derived from (Z)-dialkyl(3-chloro-1-trimethylsilyl-1-propenyl)borane, respectively. Thus, the former is formed by hydrolysis of alkyl[1-alkyl-1-(trimethylsilyl)allyl]hydroxyborane, while the latter is formed by protonolysis of alkylmethyl[3-alkyl-3-(trimethylsilyl)allyl]borane.