Acceleration of Reductive Elimination of [Ar‐Pd‐C] by a Phosphine/Electron‐Deficient Olefin Ligand: A Kinetic Investigation

Abstract

Abstract Dramatic rate enhancement of reductive elimination of [Ar‐Pd‐C ] was observed in the presence of a phosphine/electron‐deficient olefin ligand. Through systematic kinetic investigations of the Negishi coupling of ethyl 2‐iodobenzoate with alkylzinc chlorides (see scheme), the rate constants for reductive elimination of [Ar‐Pd‐C ] were determined to be greater than 0.3 s −1 , which is about four or five orders of magnitude greater than values reported previously. magnified image The kinetics of the reductive elimination step of a C C Negishi cross‐coupling catalyzed by a 1:1 complex 2 of palladium and the phosphine/electron‐deficient olefin ligand ( E )‐3‐(2‐diphenylphosphanylphenyl)‐1‐phenyl‐propenone ( 1 ) was studied. Complex 2 is an exceptionally efficient and highly selective catalyst for Negishi cross‐coupling reactions involving primary and secondary alkylzinc reagents bearing β‐hydrogen atoms. Turnover numbers (TONs) as high as 10 5 and turnover frequencies (TOFs) as high as 1000 s −1 were observed. The reactions occurred rapidly and selectively even at 0 °C. The fact that the reaction was first order in [Pd] is consistent with homogeneous catalysis by Pd complexes rather than by Pd nanoparticles (NPs). Through systematic kinetic investigations of the Negishi coupling of ethyl 2‐iodobenzoate with alkylzinc chlorides, the rate constants for reductive elimination of [Ar‐Pd‐C ] were determined to be >0.3 s −1 , which is about 4 or 5 orders of magnitude greater than the values previously reported for [Pd(dppbz)] and [Pd(PPh 3 ) 2 ] systems (dppbz=1,2‐bis(diphenylphosphino)benzene). The use of a 2:1 ratio of 1 :Pd resulted in reduced catalytic activity and selectivity, presumably because the olefin moiety could no longer assist in the reductive elimination step. Importantly, hydrogenation of the CC double bond in ligand 1 generated a saturated ligand ( 1 H 2 ), which was not only less effective than 1 , but also gave rise to substantial amount of ethylbenzoate formed by competing β‐hydride elimination. Thus, the π‐accepting olefin moiety in 1 must enhance reductive elimination rates, and, consequently, inhibit formation of byproducts resulting from β‐hydride elimination.