How to Control Inversion vs Retention Transmetalation between Pd<sup>II</sup>–Phenyl and Cu<sup>I</sup>–Alkyl Complexes: Theoretical Insight


Transmetalation between Pd­(Br)­(Ph<sup>A</sup>)­(PCyp<sub>3</sub>)<sub>2</sub> (Ph = phenyl, Cyp = cyclopentyl) and Cu­(C<sup>a</sup>HMePh<sup>B</sup>)­(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<sup>a</sup>HMePh<sup>B</sup> group. In its transition state, the C<sup>a</sup>HMePh<sup>B</sup> group has almost planar structure around the C<sup>a</sup> atom. That planar geometry is stabilized by conjugation between the π* orbital of the Ph<sup>B</sup> and the 2p orbital of the C<sup>a</sup>. Another important factor is activation entropy (Δ<i>S</i>°<sup>‡</sup>); 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<sup>a</sup>HMeEt group, transmetalation occurs in a retention manner. In the planar C<sup>a</sup>HMeEt group of the inversion transition state, the C<sup>a</sup> 2p orbital cannot find a conjugation partner because of the absence of π-electron system in the C<sup>a</sup>HMeEt. Transmetalation of C<sup>a</sup>HMe­(CHCH<sub>2</sub>) occurs in a retention manner because the vinyl π* is less effective for the conjugation with the C<sup>a</sup> 2p because of its higher orbital energy than the Ph π*. The introduction of electron-withdrawing substituent on the Ph<sup>B</sup> is favorable for inversion transmetalation. These results suggest that the stereochemistry of the C<sup>a</sup> atom in transmetalation can be controlled by electronic effect of the C<sup>a</sup>HMeR (R = phenyl, vinyl, or alkyl) and sizes of the substituent and ligand

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