A Pd(II) → Pd(0) → Pd(II) Catalytic Cycle Enables the Generation of Bis(o-carborane)s: Confirmed by Reproduced Yields and Ratios, Regioselectivities, Additive and Substituent Effects

The palladium-catalyzed generation of bis(o-carborane)s via regioselective B-B coupling was explored by using density functional theory (DFT). It is found that the reaction proceeds through sequential regioselective B4-H activation, B4'-H/B5'-H activation, and reductive elimination, yielding bis(o-carborane)s ultimately. The palladium catalyst tends to experience a Pd(II) → Pd(0) → Pd(II) rather than Pd(II) → Pd(IV) → Pd(II) catalytic cycle, in which the second B-H (B4'-H/B5'-H) activation serves as the rate-determining step (RDS). Computed RDS step's activation barriers (27.0/29.0 kcal·mol^-1, TS2a_I/TS2a'_I) consist well with experimental yields and selectivities (P1a:P2a = 57%:23%). The regioselectivity is primarily controlled by the second B-H (B4'-H/B5'-H) activation process, with the electronic effect playing a key role and steric hindrance influencing somewhat, as confirmed by the natural bond orbital (NBO) and noncovalent interaction (NCI) analyses. Computed rate-determining free-energy barriers (27.0/29.6/31.1 kcal·mol^-1) for the AgOAc/AgF/NiCl₂-co-assisted (Path a_I), AgOAc/AgF-co-assisted (Path a_II), and AgOAc-assisted (Path a_III) cases agree perfectly with corresponding experimental trends (obtained 57%/43%/12% of P1a, respectively). Theoretical predictions of substituent effects also demonstrate consistency with experimental observations. This perfect agreement between experiments and computations validates the Pd(II) → Pd(0) → Pd(II) cycle, providing crucial insights into the B-B coupling of o-carboranes and thus aiding the controllable synthesis of functional carborane materials.