Construction of Terminal Conjugated Enynes: Cu-Mediated Cross-Coupling Reaction of Alkenyldialkylborane with (Trimethylsilyl)ethynyl Bromide

Abstract

The cross-coupling reaction of (E)- and (Z)-alk-1-enyldialkylborane with (trimethylsilyl)ethynyl bromide proceeds in the presence of a catalytic amount of copper(II) acetylacetonate and a base under extremely mild conditions to provide conjugated enynes with a distal carbon-carbon triple bond. Employing sodium methoxide (1 M) as the base results in not only cross-coupling but also desilylation affording both (E)- and (Z)-alk-3-en-1-ynes, exclusively, while lithium hydroxide monohydrate gives both (E)- and (Z)-1-(trimethylsilyl)alk-3-en-1-ynes with high regio- and stereoselectivity. On the other hand, the cross-coupling reaction of (Z)-1-(tri­methylsilyl)alk-1-enyldicyclohexylborane with (trimethylsilyl)ethy­-nyl bromide proceeds in the presence of a small amount of copper(I) iodide and aqueous sodium hydroxide under extremely mild conditions to afford (Z)-1,3-bis(trimethylsilyl)alk-3-en-1-yne with high regio- and stereoselectivity. In addition, treatment of the resulting product with sodium methoxide (1 M) leads to the stereoselective formation of (Z)-3-(trimethylsilyl)alk-3-en-1-yne in a one-pot synthesis.

Key words

alkenyldialkylborane - (trimethylsilyl)ethynyl bromide - cross-coupling - alk-3-en-1-yne - 1,3-bis(trimethylsilyl)alk-3-en-1-yne

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17

In the case of (Z)-4-(cyclohex-1-enyl)-1,3-bis(trimethyl­-silyl)but-3-en-1-yne(8c), isolation by column chromato­-graphy on silica gel resulted in complete geometric isomerization to give its E isomer.