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

Transmetalation between Pd(Br)(PhA)(PCyp3)2 (Ph = phenyl, Cyp = cyclopentyl) and Cu(CaHMePhB)(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 CaHMePhB group. In its transition state, the CaHMePhB group has almost planar structure around the Ca atom. That planar geometry is stabilized by conjugation between the π* orbital of the PhB and the 2p orbital of the Ca. 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 CaHMeEt group, transmetalation occurs in a retention manner. In the planar CaHMeEt group of the inversion transition state, the Ca 2p orbital cannot find a conjugation partner because of the absence of π-electron system in the CaHMeEt. Transmetalation of CaHMe(CH═CH2) occurs in a retention manner because the vinyl π* is less effective for the conjugation with the Ca 2p because of its higher orbital energy than the Ph π*. The introduction of electron-withdrawing substituent on the PhB is favorable for inversion transmetalation. These results suggest that the stereochemistry of the Ca atom in transmetalation can be controlled by electronic effect of the CaHMeR (R = phenyl, vinyl, or alkyl) and sizes of the substituent and ligand.