Isomerization and Deuterium Scrambling Evidence for a Change in the Rate-Limiting Step during Imine Hydrogenation by Shvo's Hydroxycyclopentadienyl Ruthenium Hydride

Abstract

Hydroxycyclopentadienyl ruthenium hydride 5 efficiently reduces imines below room temperature. Better donor substituents on nitrogen give rise to faster rates and a shift of the rate-determining step from hydrogen transfer to amine coordination. Reduction of electron-deficient N-benzilidenepentafluoroaniline (8) at 11 °C resulted in free amine and kinetic isotope effects of k_OH/k_OD = 1.61 ± 0.08, k_RuH/k_RuD = 2.05 ± 0.08, and k_RuHOH/k_RuDOD = 3.32 ± 0.14, indicative of rate-limiting concerted hydrogen transfer, a mechanism analogous to that proposed for aldehyde and ketone reduction. Reduction of electron-rich N-alkyl-substituted imine, N-isopropyl-(4-methyl)benzilidene amine (9), was accompanied by facile imine isomerization and scrambling of deuterium labels from reduction with 5−RuDOH into the N-alkyl substituent of both the amine complex and into the recovered imine. Inverse equilibrium isotope effects were observed in the reduction of N-benzilidene-tert-butylamine (11) at −48 °C (k_OH/k_OD = 0.89 ± 0.06, k_RuH/k_RuD = 0.64 ± 0.05, and k_RuHOH/k_RuDOD = 0.56 ± 0.05). These results are consistent with a mechanism involving reversible hydrogen transfer followed by rate-limiting amine coordination.