Mechanistic Study of Hydrogen Transfer to Imines from a Hydroxycyclopentadienyl Ruthenium Hydride. Experimental Support for a Mechanism Involving Coordination of Imine to Ruthenium Prior to Hydrogen Transfer

Reaction of [2,3,4,5-Ph4(η5-C4COH)Ru(CO)2H] (2) with different imines afforded ruthenium amine
complexes at low temperatures. At higher temperatures in the presence of 2, the complexes decomposed
to give [Ru2(CO)4(μ-H)(C4Ph4COHOCC4Ph4)] (1) and free amine. Electron-rich imines gave ruthenium amine
complexes with 2 at a lower temperature than did electron-deficient imines. The negligible deuterium isotope
effect (kRuHOH/kRuDOD = 1.05) observed in the reaction of 2 with N-phenyl[1-(4-methoxyphenyl)ethylidene]amine (12) shows that neither hydride (RuH) nor proton (OH) is transferred to the imine in the
rate-determining step. In the dehydrogenation of N-phenyl-1-phenylethylamine (4) to the corresponding
imine 8 by [2,3,4,5-Ph4(η4-C4CO)Ru(CO)2] (A), the kinetic isotope effects observed support a stepwise
hydrogen transfer where the isotope effect for C−H cleavage (kCHNH/kCDNH = 3.24) is equal to the combined
(C−H, N−H) isotope effect (kCHNH/kCDND = 3.26). Hydrogenation of N-methyl(1-phenylethylidene)amine
(14) by 2 in the presence of the external amine trap N-methyl-1-(4-methoxyphenyl)ethylamine (16) afforded
90−100% of complex [2,3,4,5-Ph4(η4-C4CO)]Ru(CO)2NH(CH3)(CHPhCH3) (15), which is the complex
between ruthenium and the amine newly generated from the imine. At −80 °C the reaction of hydride 2
with 4-BnNHC6H9NPh (18), with an internal amine trap, only afforded [2,3,4,5-Ph4(η4-C4CO)](CO)2RuNH(Ph)(C6H10-4-NHBn) (19), where the ruthenium binds to the amine originating from the imine, showing that
neither complex A nor the diamine is formed. Above −8 °C complex 19 rearranged to the thermodynamically
more stable [Ph4(η4-C4CO)](CO)2RuNH(Bn)(C6H10-4-NHPh) (20). These results are consistent with an inner
sphere mechanism in which the substrate coordinates to ruthenium prior to hydrogen transfer and are
difficult to explain with the outer sphere pathway previously proposed.