How to Control Inversion vs Retention Transmetalation between Pd-Phenyl and Cu-Alkyl Complexes: Theoretical Insight.

Transmetalation between Pd(Br)(Ph)(PCyp) (Ph = phenyl, Cyp = cyclopentyl) and Cu(CHMePh)(NHC) (NHC = 1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene) is an important elementary step in recently reported catalytic cross-coupling reaction by Pd/Cu cooperative system. DFT study discloses that the transmetalation occurs with inversion of the stereochemistry of the CHMePh group. In its transition state, the CHMePh group has almost planar structure around the C atom. That planar geometry is stabilized by conjugation between the π* orbital of the Ph and the 2p orbital of the C. Another important factor is activation entropy (ΔS°); retention transmetalation occurs through Br-bridging transition state, which is less flexible than that of the inversion transmetalation because of the Br-bridging structure, leading to a smaller activation entropy in the retention transition state than in the inversion transition state. For CHMeEt group, transmetalation occurs in a retention manner. In the planar CHMeEt group of the inversion transition state, the C 2p orbital cannot find a conjugation partner because of the absence of π-electron system in the CHMeEt. Transmetalation of CHMe(CH═CH) occurs in a retention manner because the vinyl π* is less effective for the conjugation with the C 2p because of its higher orbital energy than the Ph π*. The introduction of electron-withdrawing substituent on the Ph is favorable for inversion transmetalation. These results suggest that the stereochemistry of the C atom in transmetalation can be controlled by electronic effect of the CHMeR (R = phenyl, vinyl, or alkyl) and sizes of the substituent and ligand.