NBO Orbital Interaction Analysis for the Ambiphilic Metal–Ligand Activation/Concerted Metalation Deprotonation (AMLA/CMD) Mechanism Involved in the Cyclopalladation Reaction of N,N‑Dimethylbenzylamine with Palladium Acetate

Natural bond orbital (NBO) analysis obtained from density functional theory (DFT) calculations on the intermediates and transition states in the ambiphilic metal–ligand activation/concerted metalation deprotonation (AMLA/CMD) mechanism for the cyclopalladation reaction of N,N-dimethylbenzylamine with palladium acetate shows the agostic, syndetic (π or σ-electron density from the ring that assists the agostic donation), and backbonding orbital overlaps involved. The analysis shows that these components are absent for the anagostic intermediate but progressively increase in going from the anagostic/agostic transition state to the agostic intermediate and then to the agostic/cyclopalladate transition state. For the all-important agostic/cyclopalladate transition state, agostic donation is very large [NBO E(2) total, 152.3 kcal mol–1], the syndetic π-donations total over half of this at 106.3 kcal mol–1 with the overlap forming in close proximity to the carbon where palladation occurs, and there is the emergence of σ-agostic donation (3.2 kcal mol–1). In comparison to the agostic intermediate, Pd to C–Hσ* back bonding increases marginally and CO lone pair donation to this antibonding orbital increases significantly. The total back-donations have an E(2) value of 72.7 kcal mol–1, which is nearly half the magnitude of the agostic donation and provides a significant influence on the lengthening of the C–H bond.