New Approaches to Olefin Cross-Metathesis

Abstract

New methodology for the selective cross-metathesis (CM) of terminal olefins employing ruthenium benzylidene 1 is described.1 CM with symmetric internal olefins was found to provide a useful means for homologating terminal olefins to protected allylic alcohols, amines, and esters. Due to the limited commercial availability of symmetric internal olefins, a two-step CM procedure was developed in which terminal olefins were first homodimerized prior to the CM reaction. Terminal olefins with allylic methyl substituents were observed to provide CM products in diminished yield albeit with markedly improved trans-selectivity. Reaction rates were measured for CM reactions utilizing butenediol and allyl alcohol derivatives, and the results demonstrated distinct advantages in reaction rate and stereoselectivity for reactions employing the disubstituted olefins. In the course of studies of substrates with allylic oxygen substituents, a new CM application was discovered involving the metathesis of acrolein acetal derivatives with terminal olefins. Acrolein acetals, including asymmetric variants derived from tartaric acid, proved to be exceptionally robust and trans-selective CM substrates. In related work, a pinacol-derived vinyl boronate was also found to be a reactive CM partner, providing a novel means for converting terminal olefins into precursors for the Suzuki coupling reaction.