The Mechanism of the Ru-Assisted C−C Bond Cleavage of Terminal Alkynes by Water

Abstract

The hydration of phenylacetylene in the presence of the complex mer,trans-(PNP)RuCl2(PPh3) in THF at 60 °C leads to the cleavage of the C−C triple bond with formation of the carbonyl complex fac,cis-(PNP)RuCl2(CO) and toluene [PNP = CH3CH2CH2N(CH2CH2PPh2)2]. A study under different experimental conditions, the use of model and isotope labeling experiments, and the detection of several intermediates, taken altogether, show that the C−C bond cleavage reaction comprises a number of steps, among which the most relevant to the mechanism are 1-alkyne to vinylidene tautomerism, conversion of a vinylidene ligand to hydroxycarbene by intramolecular attack of water, deprotonation of hydroxycarbene to σ-acyl, deinsertion of CO from the acyl ligand, and hydrocarbon elimination by protonation of the metal-alkyl moiety. The following intermediate species have been isolated and characterized: the vinylidene fac,cis-(PNP)RuCl2{CC(H)Ph}, the (aquo)(σ-alkynyl) complex fac-(PNP)RuCl(C⋮CPh)(OH2), and the (benzyl)carbonyl mer-(PNP)RuCl(η1-CH2Ph)(CO). Other intermediates such as the σ-acyl mer-(PNP)RuCl(η1-COCH2Ph)(CO) have been intercepted by addition of appropriate reagents, while the independent synthesis of the aminocarbene complex fac,cis-(PNP)RuCl2{C(NC5H10)(CH2Ph)} and its reaction with water have provided evidence for the intermediacy of a hydroxycarbene species in the C−C bond cleavage reaction.