Regio- and Stereoselective Carboindation of Internal Alkynyl Ethers with Organosilicon or -stannane Nucleophiles.

Abstract

We achieved regio- and stereoselective carboindation of terminal and internal alkynyl ethers using InI3 and organosilicon or -stannane nucleophiles to synthesize (Z)-β-alkoxyalkenylindiums. The carbometalation regio- and stereoselectively proceeded in anti-addition fashion, which was confirmed by X-ray diffraction analysis of (Z)-β-alkoxyalkenylindium products. Theoretical calculation on the carboindation of alkynyl ethers to elucidate the effect of an alkoxy group were conducted in parallel with calculations on a carbon analogue of the alkynyl ether. Reaction profiles and computational data of carboindation suggest that the alkoxy group enhances the interaction between InI3 and an alkyne moiety and reduces the activation energy. Many types of carbon nucleophiles such as silyl ketene acetals, silyl ketene imines, a silyl cyanide, an alkynyl stannane, and an allylic stannane were applicable to the present reaction system to give highly functionalized metalated enol ethers (β-alkoxyalkenylindiums). The prepared β-alkoxyalkenylindiums were transformed to various functionalized tetra-substituted enol ethers by iodination followed by Suzuki coupling. The synthesis of a 7-membered ring compound containing a phenol ether moiety was accomplished using a sequential process that included the present stereoselective carboindation.