Pincer-type Heck catalysts and mechanisms based on Pd(IV) intermediates: a computational study.

Pincer-type palladium complexes are among the most active Heck catalysts. Due to their exceptionally high thermal stability and the fact that they contain Pd(II) centers, controversial Pd(II)/Pd(IV) cycles have been often proposed as potential catalytic mechanisms. However, pincer-type Pd(IV) intermediates have never been experimentally observed, and computational studies to support the proposed Pd(II)/Pd(IV) mechanisms with pincer-type catalysts have never been carried out. In this computational study the feasibility of potential catalytic cycles involving Pd(IV) intermediates was explored. Density functional calculations were performed on experimentally applied aminophosphine-, phosphine-, and phosphite-based pincer-type Heck catalysts with styrene and phenyl bromide as substrates and (E)-stilbene as coupling product. The potential-energy surfaces were calculated in dimethylformamide (DMF) as solvent and demonstrate that Pd(II)/Pd(IV) mechanisms are thermally accessible and thus a true alternative to formation of palladium nanoparticles. Initial reaction steps of the lowest energy path of the catalytic cycle of the Heck reaction include dissociation of the chloride ligands from the neutral pincer complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Cl)] [X=NH, R=piperidinyl (1 a); X=O, R=piperidinyl (1 b); X=O, R=iPr (1 c); X=CH(2), R=iPr (1 d)] to yield cationic, three-coordinate, T-shaped 14e(-) palladium intermediates of type {2,6-C(6)H(3)(XPR(2))(2)}Pd (2). An alternative reaction path to generate complexes of type 2 (relevant for electron-poor pincer complexes) includes initial coordination of styrene to 1 to yield styrene adducts [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Cl)(CH(2)=CHPh)] (4) and consecutive dissociation of the chloride ligand to yield cationic square-planar styrene complexes {2,6-C(6)H(3)(XPR(2))(2)}Pd(CH(2)=CHPh) (6) and styrene. Cationic styrene adducts of type 6 were additionally found to be the resting states of the catalytic reaction. However, oxidative addition of phenyl bromide to 2 result in pentacoordinate Pd(IV) complexes of type {2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(C(6)H(5)) (11), which subsequently coordinate styrene (in trans position relative to the phenyl unit of the pincer cores) to yield hexacoordinate phenyl styrene complexes {2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(C(6)H(5))(CH(2)=CHPh) (12). Migration of the phenyl ligand to the olefinic bond gives cationic, pentacoordinate phenylethenyl complexes {2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(CHPhCH(2)Ph) (13). Subsequent beta-hydride elimination induces direct HBr liberation to yield cationic, square-planar (E)-stilbene complexes with general formula {2,6-C(6)H(3)(XPR(2))(2)}Pd(CHPh=CHPh) (14). Subsequent liberation of (E)-stilbene closes the catalytic cycle.