title: Mechanism of the Pd-catalyzed decarboxylative allylation of α-imino esters: decarboxylation via free carboxylate ion.
abstract: The Pd-catalyzed decarboxylative allylation of α-(diphenylmethylene)imino esters (1) or allyl diphenylglycinate imines (2) is an efficient method to construct new C(sp(3))-C(sp(3)) bonds. The detailed mechanism of this reaction was studied by theoretical calculations [ONIOM(B3LYP/LANL2DZ+p:PM6)] combined with experimental observations. The overall catalytic cycle was found to consist of three steps: oxidative addition, decarboxylation, and reductive allylation. The oxidative addition of 1 to [(dba)Pd(PPh(3))(2)] (dba = dibenzylideneacetone) produces an allylpalladium cation and a carboxylate anion with a low activation barrier of +9.1 kcal mol(-1). The following rate-determining decarboxylation proceeds via a solvent-exposed α-imino carboxylate anion rather than an O-ligated allylpalladium carboxylate with an activation barrier of +22.7 kcal mol(-1). The 2-azaallyl anion generated by this decarboxylation attacks the face of the allyl ligand opposite to the Pd center in an outer-sphere process to produce major product 3, with a lower activation barrier than that of the minor product 4. A positive linear Hammett correlation [ρ = 1.10 for the PPh(3) ligand] with the observed regioselectivity (3 versus 4) supports an outer-sphere pathway for the allylation step. When Pd combined with the bis(diphenylphosphino)butane (dppb) ligand is employed as a catalyst, the decarboxylation still proceeds via the free carboxylate anion without direct assistance of the cationic Pd center. Consistent with experimental observations, electron-withdrawing substituents on 2 were calculated to have lower activation barriers for decarboxylation and, thus, accelerate the overall reaction rates.
authors.affiliation: Department of Applied Chemistry, China Agricultural University, Beijing 100193, PR China.
authors.forename: Zhe
authors.initials: Z
authors.lastname: Li
authors.forename: Yuan-Ye
authors.initials: YY
authors.lastname: Jiang
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Yeagley
authors.forename: James P
authors.initials: JP
authors.lastname: Bour
authors.forename: Lei
authors.initials: L
authors.lastname: Liu
authors.forename: Jason J
authors.initials: JJ
authors.lastname: Chruma
authors.forename: Yao
authors.initials: Y
authors.lastname: Fu