title: Acceleration of reductive elimination of [Ar-Pd-C(sp3)] by a phosphine/electron-deficient olefin ligand: a kinetic investigation.
abstract: The kinetics of the reductive elimination step of a C(sp(3))-C(sp(2)) 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 beta-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 degrees 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 C=C double bond in ligand 1 generated a saturated ligand (1H(2)), which was not only less effective than 1, but also gave rise to substantial amount of ethylbenzoate formed by competing beta-hydride elimination. Thus, the pi-accepting olefin moiety in 1 must enhance reductive elimination rates, and, consequently, inhibit formation of byproducts resulting from beta-hydride elimination.
authors.affiliation: College of Chemistry and Molecular Sciences, Wuhan University, 430072, PR China.
authors.forename: Heng
authors.initials: H
authors.lastname: Zhang
authors.forename: Xiancai
authors.initials: X
authors.lastname: Luo
authors.forename: Kittiya
authors.initials: K
authors.lastname: Wongkhan
authors.forename: Hui
authors.initials: H
authors.lastname: Duan
authors.forename: Qiang
authors.initials: Q
authors.lastname: Li
authors.forename: Lizheng
authors.initials: L
authors.lastname: Zhu
authors.forename: Jian
authors.initials: J
authors.lastname: Wang
authors.forename: Andrei S
authors.initials: AS
authors.lastname: Batsanov
authors.forename: Judith A K
authors.initials: JA
authors.lastname: Howard
authors.forename: Todd B
authors.initials: TB
authors.lastname: Marder
authors.forename: Aiwen
authors.initials: A
authors.lastname: Lei