Understanding the Ligand Influence in the Multistep Reaction of Diazoalkanes with Palladium Complexes Leading to Carbene-Aryl Coupling.

Abstract

The reaction of diphosphino aryl complexes [Pd(C₆F₅)(L-L)(NCMe)](BF₄) (L-L = dppe, dppp, dppb) with diazoalkanes N₂CHR (R = -CH=CHPh, Ph) leads to η³-allyl or η³-benzyl palladium derivatives that are the organometallic products resulting from carbene-aryl coupling. The experimental trend shows that the reaction is favored for dppe > dppp > dppb. It involves several consecutive steps, i.e., diazoalkane coordination, nitrogen extrusion to give a Pd-carbene, and migratory insertion, which are experimentally inseparable, but they can be studied with the help of DFT calculations. The bulkiness and bite angle of the ligand exert a large influence in the relative rate of the steps involved in the reaction, and we have found that carbene formation by N₂ extrusion is the step with the largest barrier for dppe. In contrast, the coordination of the diazoalkane is the most energy-demanding step for the larger dppp and dppb diphosphines. Thus, ligand substitution controls the rate, an important elemental step rarely considered in mechanistic studies of carbene cross coupling reactions. Since diazoalkanes are the most common carbene precursors, either directly or generated from hydrazones, the choice of ligand can be very important to facilitate the entrance of the carbene precursor in the catalytic cycle.