Olefin Metathesis Involving Ruthenium Enoic Carbene Complexes

Olefin metathesis has become a valuable reaction in organic synthesis, as has been demonstrated by its frequent use as the key bond constructions for total syntheses of many natural products. With the recent discovery of highly active catalyst 1, trisubstituted and functionalized alkenes have been synthesized efficiently by cross-metathesis (CM), further expanding the substrate scope for this reaction. With these successes in hand, unprecedented metathesis reactions were explored. There have been no previous reports of the dimerization of α,β-unsaturated carbonyl compounds by a metathesis mechanism. Molybdenum- and tungsten-based catalysts form metallocyclobutane with acrylates, but they are inactive due to carbonyl oxygen chelation. Our group reported the synthesis of ester carbene 4 by a nonmetathesis route and showed that 4 was extremely reactive. In fact, ester carbene 4 was the first carbene to ring-open cyclohexene but did not react in a catalytic fashion.4 The nontrivial synthesis, lack of stability, and the ineffective catalytic activity of ester carbene 4 has limited its uses in organic synthesis.