CYCLOPENTENONE FORMATION VIA HYDROGEN ACTIVATION IN THE REACTIONS OF CHROMIUM CARBENE COMPLEXES WITH ALKYNES

The reactions of alkyl chromium carbene complexes with alkynes have been found to give cyclopentenones. Mechanisms are proposed to account for the formation of these products that involve metal hydride intermediates. As bas been previously reported for tungsten, molybdenum alkyl complexes have been found to give 1,3-dienes rather than cyclopentenones. The difference between chromium and molybdenum and tungsten may be that a metal hydride intermediate can re-add to an olefin in the case of chromium rather than undergo reductive elimination. A mechanism for the formation of cyclopentenones involving a free vinylketene was ruled out on the basis of an experiment in which the free vinylketene was generated via thermolysis of a cyclobutenone and found not to give a cyclopentenone product but rather an intramolecular [2 + 2] cyloadduct.