Shanghai-HongKong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Ling Ling Road, Shanghai, 200032, China.
J Org Chem. 2012 Sep 7;77(17):7487-96. doi: 10.1021/jo301319j. Epub 2012 Aug 21.
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.
运用密度泛函理论研究了钯（II）（bpy）和铑（I）（bpy）催化的共轭加成及其涉及α，β-不饱和羰基化合物的竞争性 Heck 反应的反应机理。该机理的关键步骤是插入和终止。插入步骤有利于乙烯配位物种与生成稳定的 C 键合烯醇化物中间体的 1,2-加成，然后该中间体可能异构化为氧代-π-烯丙基物种或 O 键合烯醇化物。终止步骤涉及β-氢消除导致 Heck 反应产物和质子化反应生成共轭加成产物之间的竞争。这两条途径在钯（II）催化的反应中是竞争性的，而在铑（I）催化的反应中发现质子化反应具有优势。计算结果与实验观察结果吻合良好。对于钯（II）和铑（I）催化的过程，β-氢消除的势能面和速率决定步骤相似。钯（II）和铑（I）催化的质子化的速率决定步骤不同。通过 O 键合烯醇化物或氧代-π-烯丙基配合物中间体，引入 N 或 P 配体可以显著稳定质子化过渡态，从而降低活化自由能。然而，β-氢消除的势垒对配体的敏感性较低。对于铑（I）催化的反应，由于对质子化过渡态的显著配体稳定作用，对于所有研究的 N 和 P 配体，质子化均被计算为比β-氢消除更有利。对于钯（II）催化的反应，带有单齿吡啶配体的配合物通过β-氢消除生成 Heck 型产物，而带有双齿 N 和 P 配体的配合物则有利于质子化。理论发现表明，通过使用适当的配体，有可能控制共轭加成和 Heck 反应之间的选择性。
