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

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 was 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 tetrasubstituted enol ethers by iodination followed
by Suzuki coupling. The synthesis of a seven-membered ring compound
containing a phenol ether moiety was accomplished using a sequential
process that included the present stereoselective carboindation.