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Acceleration of Reductive Elimination of [Ar‐Pd‐C sp 3 ] by a Phosphine/Electron‐Deficient Olefin Ligand: A Kinetic Investigation
Author(s) -
Zhang Heng,
Luo Xiancai,
Wongkhan Kittiya,
Duan Hui,
Li Qiang,
Zhu Lizheng,
Wang Jian,
Batsanov Andrei S.,
Howard Judith A. K.,
Marder Todd B.,
Lei Aiwen
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200802209
Subject(s) - chemistry , phosphine , reductive elimination , ligand (biochemistry) , negishi coupling , catalysis , medicinal chemistry , olefin fiber , palladium , turnover number , homogeneous catalysis , coupling reaction , reagent , moiety , reaction rate constant , photochemistry , stereochemistry , organic chemistry , kinetics , biochemistry , physics , receptor , quantum mechanics
Dramatic rate enhancement of reductive elimination of [Ar‐Pd‐C sp 3] 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 sp 3] were determined to be greater than 0.3 s −1 , which is about four or five orders of magnitude greater than values reported previously.The kinetics of the reductive elimination step of a C sp 3C sp 2Negishi 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 sp 3] 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 CC 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.