THE DEVELOPMENT OF METAL CATALYZED BENZANNULATION PROTOCOLS FOR THE SYNTHESIS OF AROMATIC BORONIC ESTERS

Abstract

A transition metal catalyzed benzannulation of alkynylboronates and dienes/diene equivalents has been developed for the synthesis of aromatic boronic esters. Alkynylboronates participate in a regioselective cycloaddition with 1,3-butadiene derivatives, under ambient conditions in the presence of a cobalt catalyst. The methodology provides a convenient and general route to access 1,2-di-substituted, 1,2,3- or 1,3,4-tri-substituted and 1,2,3,4-tetra-substituted benzene based systems incorporating a boronate moiety. The newly formed aromatic boronic esters can undergo a Suzuki cross-coupling to afford functionalized aminopyridine derivatives. In addition, alkynylboronates participate in benzannulation reactions with cyclobutenones, at room temperature under nickel catalysis. The strategy provides an easy access to highly (tetra/penta) substituted phenol boronic ester derivatives in a remarkably regioselective manner. This chemistry offers an efficient route to quinone boronic esters, as well as the opportunity to carry out benzannulation and cross-coupling reactions in one-pot with a single pre-catalyst. To extend the general strategy, an alternative sequence that employs a palladium catalyzed cyclisation process involving o-alkynylanilines is terminated by a boration reaction to furnish the corresponding indole boronic ester derivatives.