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

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