Iridium-Catalyzed Enantioselective Vinylogous and Bisvinylogous Allenylic Substitution

Abstract

Compared to the widely explored enol silanes, the applicability of their extended variants especially as bisvinylogous nucleophiles in enantioselective catalysis has been sparse. Herein, we describe the first enantioselective vinylogous and bisvinylogous allenylic substitution using silyl dienol and trienol ethers, respectively, as a nucleophile. With racemic allenylic alcohols as the electrophile, these enantioconvergent reactions are cooperatively catalyzed by an Ir(I)/(phosphoramidite,olefin) complex and Lewis acidic La(OTf)₃ and display remarkable regio- and diastereoselectivity in most cases. The ability of such extended silyl enol ethers in distant functionalization and creation of remote stereocenters is evident from the resulting γ- and ε-allenylic unsaturated ketones, bearing δ- and ζ-stereocenters, respectively, which are obtained in moderate to high yields with good to excellent enantioselectivity. The synthetic utility of these unsaturated carbonyls bearing an allene moiety is demonstrated with several transformations, including controlled reductions and stereoselective olefinations, which lead to products with desired degrees of unsaturation.