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Mechanism of the Pd‐catalyzed Decarboxylative Allylation of α‐Imino Esters: Decarboxylation via Free Carboxylate Ion
Author(s) -
Li Zhe,
Jiang YuanYe,
Yeagley Andrew A.,
Bour James P.,
Liu Lei,
Chruma Jason J.,
Fu Yao
Publication year - 2012
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.201201425
Subject(s) - decarboxylation , chemistry , carboxylate , ligand (biochemistry) , medicinal chemistry , oxidative decarboxylation , catalysis , reaction mechanism , reductive elimination , regioselectivity , cationic polymerization , stereochemistry , polymer chemistry , organic chemistry , biochemistry , receptor
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.