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 degrees 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 degrees 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.