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

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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 kOH/kOD = 1.61 ± 0.08, kRuH/kRuD = 2.05 ± 0.08, and kRuHOH/kRuDOD = 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 (kOH/kOD = 0.89 ± 0.06, kRuH/kRuD = 0.64 ± 0.05, and kRuHOH/kRuDOD = 0.56 ± 0.05). These results are consistent with a mechanism involving reversible hydrogen transfer followed by rate-limiting amine coordination.

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