DESIGN AND DEVELOPMENT OF NEW AEROBIC BORON HECK REACTIONS

Abstract

Transition-metal catalyzed cross-coupling has, in many ways, revolutionized synthetic chemistry by providing streamlined syntheses using highly modular building blocks. Despite the widespread application of the Suzuki and Buchwald-Hartwig cross-coupling reactions, utilization of more contemporary cross-coupling protocols is narrower. Progress in applied research using transition- metal catalyzed reactions is often hampered by the necessity for unwieldy substrate syntheses. Seeking to tackle this problem, we have developed two novel C-C bond forming reactions in the context of the oxidative boron Heck reaction that converts simple starting materials into highly useful products which are directly applicable to many powerful synthetic protocols.

Firstly, an aerobic boron Heck reaction has been developed, which facilitates the conversion of feedstock olefins, namely, ethylene and propylene to styrenes and b-methylstyrenes using abundant (hetero)aryl boronic acids. The relevance to complex molecule synthesis is highlighted by a two-step synthesis of the pharmaceutical phenprocoumon.

Secondly, an aerobic boron Heck reaction of cyclobutene has been developed using abundant (hetero)aryl boronic acids. Instead of forming the classical Heck product (3-aryl-cyclobutene) a linear 1-aryl-1,3-diene is generated with complete chemo-, regio- and stereo-selectivity. 1-Aryl- 1,3-dienes have found myriad applications in modern transition-metal catalyzed cross-couplings however the substrates are often prepared by poorly efficient Wittig reactions. Both computational and mechanistic experimental data support a mechanism that proceeds by a classical Heck reaction

followed by pericyclic ring opening mechanism for C-C bond cleavage. 1-Aryl-1,3,5-trienes were similarly prepared using alkenyl boronic acids.