title: Conjugate addition vs Heck reaction: a theoretical study on competitive coupling catalyzed by isoelectronic metal (Pd(II) and Rh(I)).
abstract: Density functional theory studies have been carried out to investigate the mechanism of the Pd(II)(bpy)- and Rh(I)(bpy)-catalyzed conjugate additions and their competitive Heck reactions involving α,β-unsaturated carbonyl compounds. The critical steps of the mechanism are insertion and termination. The insertion step favors 1,2-addition of the vinyl-coordinated species to generate a stable C-bound enolate intermediate, which then may isomerize to either an oxa-π-allyl species or an O-bound enolate. The termination step involves a competition between β-hydride elimination, leading to a Heck reaction product, and protonolysis reaction that gives a conjugate addition product. These two pathways are competitive in the Pd(II)-catalyzed reaction, while a preference for protonolysis has been found in the Rh(I)-catalyzed reaction. The calculations are in good agreement with the experimental observations. The potential energy surface and the rate-determining step of the β-hydride elimination are similar for both Pd(II)- and Rh(I)-catalyzed processes. The rate-determining steps of the Pd(II)- and Rh(I)-catalyzed protonolysis are different. Introduction of an N- or P-ligand significantly stabilizes the protonolysis transition state via the O-bound enolate or oxa-π-allyl complex intermediate, resulting in a reduced free energy of activation. However, the barrier of the β-hydride elimination is less sensitive to ligands. For the Rh(I)-catalyzed reaction, protonolysis is calculated to be more favorable than the β-hydride elimination for all investigated N and P ligands due to the significant ligand stabilization to the protonolysis transition state. For the Pd(II)-catalyzed reaction, the complex with monodentate pyridine ligands prefers the Heck-type product through β-hydride elimination, while the complex with bidentate N and P ligands favors the protonolysis. The theoretical finding suggests the possibility to control the selectivity between the conjugate addition and the Heck reaction by using proper ligands.
authors.affiliation: Shanghai-HongKong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Ling Ling Road, Shanghai, 200032, China.
authors.forename: Qian
authors.initials: Q
authors.lastname: Peng
authors.forename: Hong
authors.initials: H
authors.lastname: Yan
authors.forename: Xinhao
authors.initials: X
authors.lastname: Zhang
authors.forename: Yun-Dong
authors.initials: YD
authors.lastname: Wu
mesh_terms: D000447:Aldehydes / Q000138:chemical synthesis* / Q000737:chemistry; D002384:Catalysis; D009942:Organometallic Compounds / Q000737:chemistry*; D010165:Palladium / Q000737:chemistry*; D011789:Quantum Theory; D012238:Rhodium / Q000737:chemistry*
chemical_list: D000447:Aldehydes; D009942:Organometallic Compounds; D010165:Palladium; D012238:Rhodium