The hydroboration of (trimethylsilyl)ethyne with dialkylboranes and its application to the syntheses of (E)-1-(trimethylsilyl)alk-1-enes and 2-(trimethylsilyl)alk-1-enes

Abstract

The reaction of (trimethylsilyl)ethyne 1 with a stoicheiometric amount of dialkylborane 2 proceeds to the monohydroboration stage, giving a mixture of regioisomers, (E)-[2-(trimethylsilyl)-ethenyl]dialkylborane 3 and [1-(trimethylsilyl)ethenyl]dialkylborane 4. In the hydroboration with bulky dialkylboranes, derived from internal alkenes, the former predominates. Successive treatment of the mixture with methyllithium and benzenesulphenyl chloride exclusively affords highly pure (E)-2-alkyl-1-(trimethylsilyl)ethene 6 whose alkyl group migrates from the boron atom, while in the hydroboration with less bulky dialkylboranes, derived from terminal alkenes, the latter predominates. Successive treatment of the mixture with aq. NaOH and iodine exclusively affords highly pure 2-(trimethylsilyl)alk-1-ene 7 whose alkyl group migrates from the boron atom.