ON THE MECHANISM OF THE RUTHENIUM-CATALYZED RECONSTITUTIVE CONDENSATION OF ALLYLIC ALCOHOLS AND TERMINAL ALKYNES

The ruthenium complex (eta-5-C5H5)(PPh3)2RuCl (1) catalyzes the addition of allylic alcohols to terminal alkynes, yielding beta,gamma-unsaturated ketones. The intermediacy of a ruthenium vinylidene complex is indicated by the synthesis of this proposed intermediate and the demonstration of the same reaction profile as with catalyst 1. Loss of terminal deuterium in labeled alkynes supports this conclusion. Ligand substitution studies demonstrate the necessity of phosphine loss and precoordination of the allylic alcohol. Deuterium labeling of allyl alcohol demonstrates that the two allylic termini do not become equivalent and that the olefin geometry does not scramble. In contrast to these observations, 3-buten-2-ol shows complete regioselectivity in the condensation but randomization of olefin geometry as determined by deuterium labeling. A cohesive mechanistic rationale accommodates these seemingly disparate observations.