Modifications of substituents on the chelating oxygen atom in stereoretentive Hoveyda–Grubbs-type complexes and their influence on catalytic properties

Abstract

One of the main drawbacks of stereoretentive ruthenium catalysts that allow the synthesis of olefins with a defined double bond geometry is their limited stability in the presence of oxygen. To remove the bottleneck that inhibits their widespread use, we prepared a series of Hoveyda–Grubbs-type complexes with a modified ether moiety in the benzylidene substituent. The yields obtained in air remain lower than those obtained under an inert atmosphere. Unexpectedly, however, O-benzyl catalyst Ru14b typically led to better yields with high selectivities compared to the benchmark OiPr catalyst Ru4. A parallel DFT study confirmed that the four-coordinate Ru(IV)-oxo product reported earlier by Fogg results from the oxygen attack on the Ru–C bond and the elimination of aldehyde from a metalladioxetane-like transition state.