Dynamic π-Bonding of Imidazolyl Substituent in a Formally 16-Electron Cp*Ru(κ2-P,N)+ Catalyst Allows Dramatic Rate Increases in (E)-Selective Monoisomerization of Alkenes

Alkene isomerization can be an atom-economical approach to generating a wide range of alkene intermediates for synthesis, but fully equilibrated mixtures of disubstituted internal alkenes typically contain significant amounts of the positional as well as geometric (E and Z) isomers. Most classical catalyst systems for alkene isomerization struggle to kinetically control either positional or E/Z isomerism. We report coordinatively unsaturated, formally 16-electron Cp*Ru catalyst 5, which facilitates simultaneous regio- and stereoselective isomerization of linear 1-alkenes to their internal analogues, providing consistent yields of (E)-2-alkenes greater than 95%. Because nitrile-free catalyst 5 is more than 400 times faster than previously published nitrile-containing analogues 2 + 2a, very reasonable 0.1-0.5 mol % loadings of 5 complete ambient temperature reactions within 15 min to 4 h. UV-vis, NMR, and computational studies depict the imidazolyl fragment on the phosphine as a hemilabile, four-electron donor in kappa(2)-P,N coordination. For the first time, we show direct experimental evidence that the PN ligand has accepted a proton from the substrate by characterizing the intermediate Cp*Ru[eta(3)-allyl][kappa(1)-P)P-N+H], which highlights the essential role of the bifunctional ligand in promoting rapid and selective alkene isomerizations. Moreover, kinetic studies and computations reveal the role of alkene binding in selectivity of unsaturated catalyst 5.