Titanium-Mediated Oxidative Arylation of Homoallylic Alcohols

Abstract

The addition of aryl groups to unactivated olefins represents a direct approach to C!C bond construction. Pd-catalyzed coupling of aryl halides with alkenes, the Heck reaction, offers a viable approach to arylation. However, terminal unactivated olefins often display low reactivity in Heck reactions and can yield mixtures of styrenes and allyl arenes owing to poor regiocontrol in the b-hydride elimination. High selectivity for styrene products has been observed with oxidative couplings between aryl boronic acids and terminal olefins. In this context, we wondered if inexpensive Group 4 transition metals could promote oxidative couplings between olefins and aryl organometallic reagents. Indeed zirconocene and titanocene catalysts as well as stoichiometric titanium(IV) reagents are capable of effecting the carbometalation of terminal alkenes with a variety of nucleophilic partners, including alkylaluminum species and alkyl Grignard reagents. However these methodologies are limited to the addition of simple alkyl groups and cyclization reactions. More recently, titanacyclopropanes and titanacyclopropenes have been shown to effect alkylation and vinylation of alkenols and alkynols. These methods involved the generation of low-valent titanium complexes through bhydride elimination/reductive elimination pathways. Therefore, we were uncertain if substrates lacking this ability, for example, aryl groups, would participate in addition reactions. Here we report an oxidative coupling of homoallylic alcohols and aryl Grignards [Eq. (1)]. This work demonstrates the ability of aryl titanium complexes to add to unactivated olefins and therefore reveals a new reaction manifold for Group 4 transition metals.