Intramolecular trapping of an intermediate in the reduction of imines by a hydroxycyclopentadienyl ruthenium hydride : Support for a concerted outer sphere mechanism

Reduction of imines by [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2H (1) produces kinetically stable ruthenium amine complexes. Reduction of an imine possessing an intramolecular amine was studied to distinguish between inner sphere and outer sphere mechanisms. 1,4-Bn15NH(c-C6H10)NBn (12) was reduced by 1 in toluene-d8 to give 85% of [2,5-Ph2-3,4-Tol2(η4-C4CO)](CO)2RuNHBn(c-C6H10)15NHBn (16-RuN,15N), resulting from coordination of the newly formed amine to the ruthenium center, and 15% of trapping product [2,5-Ph2-3,4-Tol2(η4-C4CO)](CO)2Ru15NHBn(c-C6H10)NHBn (16-Ru15N,N), resulting from coordination of the intramolecular trapping amine. These results provide support for an outer sphere transfer of hydrogen to the imine to generate a coordinatively unsaturated intermediate, which can be trapped by the intramolecular amine. An opposing mechanism, requiring coordination of the imine nitrogen to ruthenium prior to hydrogen transfer, cannot readily explain the observation of the trapping product 16-Ru15N,N.