Insight into the mechanism of the arylation of arenes via norbornene relay palladation through meta- to para-selectivity

A novel mechanism of the arylation of arenes via norbornene (NBE) relay palladation through meta- to para-selectivity was revealed via density functional theory (DFT) calculations. Our calculated results revealed that the reaction was initiated by a [mono-N-protected amino acid ligand (MPAA)–Pd] complex to activate at first the meta-C–H guided by the directing group (DG), and para-arylation was subsequently achieved by NBE relay palladation from meta- to para-position. Significantly, the palladium/norbornene (Pd/NBE) cooperative catalysis was catalyzed by a Pd–Ag bimetallic complex, which accounted for the experimental fact that no yield detected without Ag. The reaction pathway through para- to meta-selectivity was also investigated, while this pathway was kinetically unfavorable. The results revealed that the initial DG guided C–H site activation was the rate-determining step and played an important role in determining site-selectivity. The primary meta-activation was favorable in energy due to the less ring strain in the cyclic nitrile-coordinated C–H transition states in the meta position. Moreover, the perfect cooperation of a remote directing template and a transient mediator NBE through the alternating association with the Pd center achieved the relay through meta- to para-position. The present results provide a reasonable insight into the para-C–H arylation by the Pd/MPAA/NBE cooperative catalysis in conjunction with a precise DG and Ag(I) additive.