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

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.