Studies on the mechanism of intermolecular enyne metathesis: Kinetic method and alkyne substituent effects

The kinetics of enyne metathesis were studied by IR spectroscopy for a variety of alkyne-alkene combinations. The rate law was determined for alkyne-ethylene and alkyne-1-hexene cross metathesis. In the cases examined, greater substitution on the alkyne accelerates the rate of metathesis, and chelation by propargylic esters was ruled out through rate comparison with hydrocarbon alkynes. The findings are discussed in terms of an alkylidene-first reaction mechanism, phosphine-bound ruthenium carbene resting states, and the rate-determining turnover of vinyl carbene intermediates (for alkyne-1-hexene metatheses).