A Unified Approach to (E)- and (Z)-Olefins Enabled by Low-Valent Tungsten Catalysis

Abstract

Achieving regio- and stereoselective formation of products from simple chemical building blocks is one the most important roles of catalysis in organic synthesis. Repositioning an alkene functional group through catalytic alkene isomerization represents a powerful synthetic strategy for preparing valuable alkene products from comparatively simple chemical feedstocks. The utility of this approach, however, hinges on the ability to control the positional and stereoisomerism to access a single product among numerous potential isomeric byproducts. Here, a positionally selective alkene isomerization in which modulation of the ligand environment of the homogeneous tungsten catalyst grants access to either the (E)- or (Z)-stereoisomer is described. Compared to previously reported alkene isomerization methods, this reaction offers exquisite chemo- and regioselectivity from a simple, commercially available precatalyst and ligand. Preliminary mechanistic studies suggest that tungsten’s ability to adopt 7-coordinate geometry is crucial for stereoselectivity and that substrate directivity prevents over-isomerization to the conjugated alkene, as is commonly observed with other catalysts. These features allows for exclusive formation of β,γ-unsaturated carbonyl compounds that are otherwise difficult to prepare.