Ruthenium carbenes supported on mesoporous silicas as highly active and selective hybrid catalysts for olefin metathesis reactions under continuous flow

Abstract

In the search for a highly active and selective heterogenized metathesis catalyst, we systematically varied the pore geometry and size of various silica-based mesoporous (i.e., MCM-41, MCM-48, and SBA-15) and microporous (ZSM-5 and MWW) versus macroporous materials (D11-10 and Aerosil 200), besides other process parameters (temperature, dilution, and mean residence time). The activity and, especially, selectivity of such "linker-free" supports for ruthenium metathesis catalysts were evaluated in the cyclodimerization of cis-cyclooctene to form 1,9-cyclohexadecadiene, a valuable intermediate in the flavor and fragrance industry. The optimized material showed not only exceptionally high selectivity to the valuable product, but also turned out to be a truly heterogeneous catalyst with superior activity relative to the unsupported homogeneous complex. In the ring-opening/ring-closing metathesis (RO-RCM; see picture) of cis-cyclooctene, mesoporous silicas (i.e., MCM-41, MCM-48, SBA-15) are preferred supports for homogeneous ruthenium carbenes (see picture). The optimized "linker-free" support showed not only exceptionally high selectivity toward 1,9-cyclohexadecadiene, a valuable intermediate in the flavor and fragrance industry, but is also a truly heterogeneous catalyst with superior activity relative to the unsupported homogeneous complex. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.