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Low Catalyst Loading in Ring-Closing Metathesis Reactions

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Abstract

An efficient procedure is described for ring-closing metathesis reactions. A conversion of 95 % for diethyl diallylmalonate in dilute solution could be achieved within a few minutes, reaching TOF=4173 min−1, with very low loading of commercially available Ru catalysts that contained unsaturated NHC ligands. In general, only 50 to 250 ppm of the catalyst is required to achieve near-quantitative conversion into a broad variety of 5–16-membered heterocyclic compounds. The practicality of this procedure was illustrated in the synthesis of 5–8-membered N-tert-butoxycarbonyl (N-Boc)- and N-para-toluenesulfonyl (N-Ts)-protected cyclic amines and 9–16-membered lactones. The synthesis of macrocyclic proline-based lactams required slightly higher catalyst loadings. Along with monocyclic products, oligomeric byproducts, mostly cyclodimers, were isolated and characterized.

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