Origin of inversion versus retention in the oxidative addition of 3-chloro-cyclopentene to Pd(0)L(n).

The preference for syn versus anti oxidative addition of 3-chloro-cyclopentene to Pd(0)L(n) was investigated using density functional theory (L = PH3, PMe3, PF3, ethylene, maleic anhydride, pyridine, imidazol-2-ylidene). Both mono- and bis-ligation modes were studied (n = 1 and 2). The pathways were analyzed at the B2PLYP-D3/def2-TZVPP//TPSS-D3/def2-TZVP level, and an interaction/distortion analysis was performed at the ZORA-TPSS-D3/TZ2P level for elucidating the origin of the selectivity preferences. Mechanistically, the anti addition follows an S(N)2 type mechanism, whereas the syn addition has partial S(N)1 and S(N)2' character. Contrary to the traditional rationale that orbital interactions are dominant in the anti pathway, analysis of the variation of the interaction components along the intrinsic reaction coordinate shows that the syn pathway exhibits stronger overall orbital interactions. This orbital preference for the syn pathway diminishes with increasing donor capacity of the ligand. It is caused by the donation of the isolated p orbitals on the migrating chlorine atom to the PdL(n) fragment, which is lacking in the anti pathway, whereas the HOMO-LUMO overlap between the fragments is greater for the anti pathway. Electrostatically, the syn pathway is preferred for weakly donating and withdrawing ligands, whereas the anti pathway is favored with strongly donating ligands.