333382-71-1Relevant academic research and scientific papers
Stereochemistry of imine reduction by a hydroxycyclopentadienyl ruthenium hydride
Casey, Charles P.,Bikzhanova, Galina A.,Guzei, Ilia A.
, p. 2286 - 2293 (2007/10/03)
The stereochemistry of hydrogen transfer from [2,5-Ph2-3,4- Tol2(η5-C4COD)]Ru(CO)2D to N-aryl imines to give amine complexes was shown to be mostly trans stereospecific. Stereospecific hydrogen transfer is proposed to generate an amine and a coordinatively unsaturated ruthenium intermediate in close proximity. Coordination of the amine is proposed to occur faster than lone pair inversion of the amine. In contrast, hydrogen transfer to N-alkyl imines is stereorandom. It is proposed that stereochemistry is lost in part due to the reversibility of the hydrogen transfer being faster than amine coordination.
Reduction of imines by hydroxycyclopentadienyl ruthenium hydride: Intramolecular trapping evidence for hydride and proton transfer outside the coordination sphere of the metal
Casey, Charles P.,Bikzhanova, Galina A.,Cui, Qiang,Guzei, Ilia A.
, p. 14062 - 14071 (2007/10/03)
Reduction of imines by [2,5-Ph2-3,4-Tol2(η 5-C4COH)]Ru(CO)2H (2) produces kinetically stable ruthenium amine complexes. Reduction of an imine by 2 in the presence of an external amine trap gives only the complex of the newly generated amine. Reaction of 2 with H2N-p-C6H4N=CHPh (11), which contains an intramolecular amine trap, gave a 1:1 mixture of [2,5-Ph 2-3,4-Tol2(η4-C4CO)](CO) 2RuNH(CH2Ph)(C6H4-p-NH2) (8), formed by coordination of the newly generated amine to the ruthenium center, and [2,5-Ph2-3,4-Tol2(η4-C 4CO)](CO)2RuNH2C6H 4-p-NHCH2Ph (9), formed by coordination of the amine already present in the substrate. These results require transfer of hydrogen to the imine outside the coordination sphere of the metal to give a coordinatively unsaturated intermediate that can be trapped inside the initial solvent cage. Amine diffusion from the solvent cage must be much slower than coordination to the metal center. Mechanisms requiring prior coordination of the substrate to ruthenium would have led only to 8 and can be eliminated.
Isomerization and deuterium scrambling evidence for a change in the rate-limiting step during imine hydrogenation by Shvo's hydroxycyclopentadienyl ruthenium hydride
Casey, Charles P.,Johnson, Jeffrey B.
, p. 1883 - 1894 (2007/10/03)
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, k RuH/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.
Hydrogen transfer to carbonyls and imines from a hydroxycyclopentadienyl ruthenium hydride: Evidence for concerted hydride and proton transfer
Casey,Singer,Powell,Hayashi,Kavana
, p. 1090 - 1100 (2007/10/03)
Reaction of { [2,5-Ph2-3,4-Tol2(η5-C 4CO)]2H}Ru2(CO)4(μ-H) (6) with H2 formed [2,5-Ph2-3,4-Tol2(η5-C 4COH)Ru(CO)2H] (8), the active species in catalytic carbonyl reductions developed by Shvo. Kinetic studies of the reduction of PhCHO by 8 in THF at - 10 °C showed second-order kinetics with ΔH? = 12.0 kcal mol-1 and ΔS? = -28 eu. The rate of reduction was not accelerated by CF3CO2H, and was not inhibited by CO. Selective deuteration of the RuH and OH positions in 8 gave individual kinetic isotope effects kRuH/kRuD = 1.5 ± 0.2 and kOH/kOD = 2.2 ± 0.1 for PhCHO reduction at 0 °C. Simultaneous deuteration of both positions in 8 gave a combined kinetic isotope effect of kORuH/kODRuD = 3.6 ± 0.3. [2,5-Ph2-3,4-Tol2(η5-C 4COSiEt3) Ru(CO)2H] (12) and NEt4+[2,5-Ph2-3,4-Tol 2(η4-C4CO) Ru(CO)2H]- (13) were unreactive toward PhCHO under conditions where facile PhCHO reduction by 8 occurred. PhCOMe was reduced by 8 30 times slower than PhCHO; MeN=CHPh was reduced by 8 26 times faster than PhCHO. Cyclohexene was reduced to cyclohexane by 8 at 80 °C only in the presence of H2. Concerted transfer of a proton from OH and hydride from Ru of 8 to carbonyls and imines is proposed.
