Coordinated carbenes from electron-rich olefins on RuHCl(PPr3i)(2)

Dehydrohalogenation of RuH2Cl2L2 (L = PPr3i) gives (RuHClL2)(2), shown to b e a halide-bridged dimer by X-ray crystallography; the fluoride analog is a lso a dimer. (RuHClL2)(2) reacts with N-2, pyridine and C2H4 (L') to give R uHCIL'L-2, but with vinyl ether and vinyl amides, H2C=CH(E) [E = OR, NRC(O) R'] such olefin binding is followed by isomerization to the heteroatom-subs tituted carbene complex L2HClRu=C(CH3)(E), The reaction mechanism for such rearrangement is established by DFT(B3PW91) computations, for C2H4 as olefi n (where it is found to be endothermic), and the structures of intermediate s are calculated for H2C=C(H)(OCH3) and for cyclic and acyclic amide-substi tuted olefins. It is found, both experimentally and computationally, that t he amide oxygen is bonded to Ru with a calculated bond energy of approximat ely 9 kcal mol(-1) for an acyclic model. Less electron-rich vinyl amides or amines form eta(2)-olefin complexes, but do not isomerize to carbene compl exes. Calculated Delta E values for selected "competition" reactions reveal that donation by both Ru and the heteroatom-substituted X are necessary to make the carbene complex L2HClRu=C(X)(CH3) more stable than the olefin com plex L2HClRu(eta(2)-H2C=CHX). This originates in part from a diminished end othermicity of the olefin -->, carbene transformation when the sp(2) carbon bears a pi-donor substituent. The importance of a hydride on Ru in furnish ing a mechanism for this isomerization is discussed. The compositional char acteristics of Schrock and Fischer carbenes are detailed, it is suggested t hat reactivity will not be uniquely determined by these characteristics, an d these new carbenes RuHCl[C(X)CH3]L-2 are contrasted to Schrock and Fische r carbenes.