Hydrogen Transfer to Carbonyls and Imines from a Hydroxycyclopentadienyl Ruthenium Hydride:  Evidence for Concerted Hydride and Proton Transfer

Abstract

Reaction of ([2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)CO)](2)H)Ru(2)(CO)(4)(mu-H) (6) with H(2) formed [2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)COH)Ru(CO)(2)H] (8), the active species in catalytic carbonyl reductions developed by Shvo. Kinetic studies of the reduction of PhCHO by 8 in THF at -10 degrees C showed second-order kinetics with Delta H(double dagger) = 12.0 kcal mol(-1) and Delta S(double dagger) = -28 eu. The rate of reduction was not accelerated by CF(3)CO(2)H, and was not inhibited by CO. Selective deuteration of the RuH and OH positions in 8 gave individual kinetic isotope effects k(RuH)/k(RuD) = 1.5 +/- 0.2 and k(OH)/k(OD) = 2.2 +/- 0.1 for PhCHO reduction at 0 degrees C. Simultaneous deuteration of both positions in 8 gave a combined kinetic isotope effect of k(OHRuH)/k(ODRuD) = 3.6 +/- 0.3. [2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)COSiEt(3))Ru(CO)(2)H] (12) and NEt(4)(+)[2,5-Ph(2)-3,4-Tol(2)(eta(4)-C(4)CO)Ru(CO)(2)H](-) (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 degrees C only in the presence of H(2.) Concerted transfer of a proton from OH and hydride from Ru of 8 to carbonyls and imines is proposed.

Mechanism of Pd(NHC)-Catalyzed Transfer Hydrogenation of Alkynes

“…They showed that the transfer of hydrogens may take place in a concerted fashion or it may involve two separate hydrogen transfer steps with approximately equal barriers, and that these processes can be distinguished. 25 For a concerted reaction in which both hydrogens are transferred in a single step, the kinetic isotope effect for the doubly labeled material should be equal to the product of the two individual isotope effects. In our case, KIE CHOH/CHOD * KIE CHOH/CDOH = 5.95 (sd=1.7), which is appreciably larger than the experimentally observed KIE CHOH/CDOD of 3.60.…”

Catalytic dehydrogenation of 1,2- and 1,3-diols

“…The deuterated substrates 1,2-butanediol-d2 and 2,2',6,6'-cyclohexanone-d4 were prepared by previously reported procedures [12,13]. The Casey-Shvo catalyst (1) [10] and the Rh(dppp)2Cl catalyst (2, dppp = 1,3-bis(diphenylphosphino)propane) [14] were synthesized according to literature procedures.…”

“…Prior research has shown 3 that the Casey/Shvo catalyst achieves higher dehydrogenation rates with primary alcohols than with secondary alcohols [10], thus, it seems likely that the major product of the dehydrogenation reactions will be an α-hydroxyaldehyde (α-HA), formed by dehydrogenation at the primary alcohol (eq. 1) [10,11].…”

“…Prior research has shown 3 that the Casey/Shvo catalyst achieves higher dehydrogenation rates with primary alcohols than with secondary alcohols [10], thus, it seems likely that the major product of the dehydrogenation reactions will be an α-hydroxyaldehyde (α-HA), formed by dehydrogenation at the primary alcohol (eq. 1) [10,11]. However, since dehydrogenations with this catalyst are reversible [10], it is likely the flux of products will lead to the more stable α-hydroxyketone (α-HK, eq.…”

“…1) [10,11]. However, since dehydrogenations with this catalyst are reversible [10], it is likely the flux of products will lead to the more stable α-hydroxyketone (α-HK, eq. 2).…”