Catalytic Transfer Hydrogenation with Terdentate CNN Ruthenium Complexes: The Influence of the Base

The catalytic activity of the terdentate complex [RuCl(CNN)(dppb)] ( A ) [dppb=Ph 2 P(CH 2 ) 4 PPh 2 ; H CNN=6‐(4′‐methylphenyl)‐2‐pyridylmethylamine] in the transfer hydrogenation of acetophenone ( S ) with 2‐propanol has been found to be dependent on the base concentration. The limit rate has been observed when NaO i Pr is used in high excess ( A /base molar ratio > 10). The amino‐isopropoxide species [Ru(O i Pr)(CNN)(dppb)] ( B ), which forms by reaction of A with sodium isopropoxide via displacement of the chloride, is catalytically active. The rate of conversion of acetophenone obeys second‐order kinetics v= k [ S ][ B ] with the rate constants in the range 218±8 (40 °C) to 3000±70  M −1  s −1 (80 °C). The activation parameters, evaluated from the Eyring equation are Δ H ≠ =14.0±0.2 kcal mol −1 and Δ S ≠ =−3.2 ±0.5 eu. In a pre‐equilibrium reaction with 2‐propanol complex B gives the cationic species [Ru(CNN)(dppb)(HO i Pr)] + [O i Pr] − ( C ) with K ≈2×10 −5   M . The hydride species [RuH(CNN)(dppb)] ( H ), which forms from B via β‐hydrogen elimination process, catalyzes the reduction of S and, importantly, its activity increases by addition of base. The catalytic behavior of the hydride H has been compared to that of the system A /NaO i Pr (1:1 molar ratio) and indicates that the two systems are equivalent.