Electronic Effect-Guided, Palladium-Catalyzed Regioselective B-H Activation and Multistep Diarylation of o-Carboranes with Aryl Iodides.

Density functional theory calculations at IDSCRF-B3LYP/DZVP computational level were conducted on palladium-catalyzed regioselective B-H activation and diarylation of o-carboranes with aryl iodides in solution. Computational results indicate that this reaction follows a multistep mechanism and needs to get over several transition states before the final B(4,5)-diarylated o-carborane derivatives are formed. B-H activation, oxidation addition, and successive reduction of the Pd(II) catalyst involving a Pd(II)-Pd(IV)-Pd(II) catalytic cycle has been confirmed, in which AgOAc plays a crucial role. Electron-donating group on the cage carbon of o-carboranes is verified to be beneficial for its B-H activation and diarylation, while steric hindrance between the aryl and o-carboranyl groups retards it. Natural population analysis and Gibbs free energetic results predict consistent regioselectivities with experiments and manifest the pivotal role of electronic effect in controlling regioselective B-H activation of o-carboranes. These results are expected to shed some light on further improvement of experimental conditions and better controlling of regioselectivities.

The pivotal role of electronics in preferred alkene over alkyne Ni-carboryne insertions and absolute regioselectivities.

The in situ formation mechanisms of active Ni-carboryne species (COM1) and subsequent alkene/alkyne Ni-C bond insertion priorities, as well as relevant cycloaddition regioselectivities and kinetics, were investigated using the IDSCRF-B3LYP density functional theory (DFT) method, and all atoms were equitably treated at the DGDZVP level. The results reveal the o-carborane species to be energetically hedged into a four-step path (barrier heights 5.3, 19.7, 18.4 and 0.3 kcal mol-1, respectively) prior to being transferred into the active Ni-carboryne species (COM1) with the assistance of nBuLi and NiCl2(PPh3)2 at room temperature. In direct agreement with empirical trends, alkene insertion into Ni-C bonds on COM1 is exclusively favoured over the competing alkyne insertion. Electronic structure analyses of the corresponding transition structures showed that the preference of alkenes to alkynes is due to different bonding characteristics during this insertion process, namely, back donation for alkenes but donation for alkyne insertion, as evidenced by molecular graphics and NBO charge distributions. Subsequent alkyne additions (i.e. post alkene insertion) arise as the rate-determining step (RDS) for each of the five different reactions (a-e) explored. The solution free-energy barriers of these RDSs (30.5-38.5 kcal mol-1) were in quantitative agreement with their corresponding experimental yields, evidencing the reliability of the DFT results to reproduce chemical phenomena and energetic trends in real Ni-catalysed carboryne-alkene/alkyne cycloadditions.

Multi-Pathway Consequent Chemoselectivities of CpRuCl(PPh3 )2 /MeI-Catalysed Norbornadiene Alkyne Cycloadditions.

Chemoselectivities of five experimentally realised CpRuCl(PPh3 )2 /MeI-catalysed couplings of 7-azabenzo-norbornadienes with selected alkynes were successfully resolved from multiple reaction pathway models. Density functional theory calculations showed the following mechanistic succession to be energetically plausible: (1) CpRuI catalyst activation; (2) formation of crucial metallacyclopentene intermediate; (3) cyclobutene product (P2) elimination (ΔGRel(RDS) ≈11.9-17.6 kcal mol-1 ). Alternative formation of dihydrobenzoindole products (P1) by isomerisation to azametalla-cyclohexene followed by subsequent CpRuI release was much less favourable (ΔGRel(RDS) ≈26.5-29.8 kcal mol-1 ). Emergent stereoselectivities were in close agreement with experimental results for reactions a, b, e. Consequent investigations employing dispersion corrections similarly support the empirical findings of P1 dominating in reactions c and d through P2→P1 product transformations as being probable (ΔG≈25.3-30.1 kcal mol-1 ).