Theoretical study of Pd(0)-catalyzed carbohalogenation of alkenes: mechanism and origins of reactivities and selectivities in alkyl halide reductive elimination from Pd(II) species

The mechanism of Pd(0)-catalyzed carbohalogenation of alkenes has been investigated with density functional theory. The catalytic cycle involves oxidative addition of the aryl halide, alkene insertion, and a novel C(sp(3))-I reductive elimination. The C(sp(3))-I reductive elimination leads to a weakly bonded Pd-product complex, which undergoes ligand exchange via a dissociative mechanism to regenerate the catalyst. In the rate-determining reductive elimination step, bromides and chlorides have higher barriers than iodides, because the stronger Pd-Br and Pd-Cl bonds are being cleaved in these transition states. Bulky ligands, such as P(t-Bu)(3) and Q-Phos, facilitate the C(sp(3))-I reductive elimination by preventing the formation of tetracoordinated intermediates. The mechanism of the competing beta-hydrogen elimination pathway was also investigated. For reactions involving a syn-beta-hydrogen atom in the alkyl Pd(II) iodide intermediate, beta-hydrogen elimination is much more favorable, leading to Heck-type side products. Blocking beta-elimination by the choice of substrates is the main reason why this example of carboiodination works.