DFT Investigation of Hydrogen Atom Abstraction from NHC-Boranes by Methyl, Ethyl and Cyanomethyl Radicals-Composition and Correlation Analysis of Kinetic Barriers.

Understanding the hydrogen atom abstraction (HAA) reactions of N-heterocyclic carbene (NHC)-boranes is essential for extending the practical applications of boron chemistry. In this study, density functional theory (DFT) computations were performed for the HAA reactions of a series of NHC-boranes attacked by •CH2CN, Me• and Et• radicals. Using the computed data, we investigated the correlations of the activation and free energy barriers with their components, including the intrinsic barrier, the thermal contribution of the thermodynamic reaction energy to the kinetic barriers, the activation Gibbs free energy correction and the activation zero-point vibrational energy correction. Furthermore, to describe the dependence of the activation and free energy barriers on the thermodynamic reaction energy or reaction Gibbs free energy, we used a three-variable linear model, which was demonstrated to be more precise than the two-variable Evans-Polanyi linear free energy model and more succinct than the three-variable Marcus-theory-based nonlinear HAA model. The present work provides not only a more thorough understanding of the compositions of the barriers to the HAA reactions of NHC-boranes and the HAA reactivities of the substrates but also fresh insights into the suitability of various models for describing the relationships between the kinetic and thermodynamic physical quantities.

Does the Neophyl-like Rearrangement Play a Decisive Role in Intramolecular Cyclization of Iminyl Radicals? A Combined Quantum Chemistry and Numerical Simulation Investigation of the Cyclization Mechanism and Product Distributions of Bicyclic 2-Allyl-2-methyl-2,3-dihydro-1 H-inden-1-iminyl Radical and Several Iminyl Model Compounds.

In this study, we performed a theoretical investigation of the intramolecular cyclization of bicyclic 2-allyl-2-methyl-2,3-dihydro-1 H-inden-1-iminyl radical 1 along with several iminyl model compounds. The results were used to comparatively evaluate the reaction mechanism suggested previously, in which the neophyl-like rearrangement was deemed to play a decisive role. The present computation and numerical simulation identify the experimentally observed endo product in the high-temperature cyclization of 1. The product results from a kinetically controlled endo cyclization-reduction pathway involving an initial reversible 5- exo ring-closure/ring-opening process, not via 5- exo cyclization/neophyl-like rearrangement/ endo-radical reduction pathway as proposed previously. Considering many available theoretical and experimental results, the neophyl-like rearrangement seems to play only a minor role in the intramolecular cyclization of N- and C-centered radicals. The structural effect of cyclized radical intermediates of bicyclic 1 leads to a lower thermodynamic reaction energy of exo cyclization than of endo cyclization, which together with the temperature effect should be responsible for the formation of the dominant endo product in the high-temperature region. Additionally, this investigation provided further insight into the cyclization of 1 and compounds structurally similar to 1; that is, control of endo- or exo-regioselective products is readily available by regulating the reaction temperature.

DFT investigation of hydrogen atom-abstraction reactions of NHC-boranes by various carbon-centered radicals: barriers and correlation analyses.

In this study, we employed a quantum-mechanical computational method to investigate the hydrogen-atom abstraction reactions of two nitrogen heterocyclic carbene boranes (NHC-boranes), NHC-BH3 and NHC-BH2CN, by a series of carbon-centered radicals bearing various substituents. We explored the degree of correlation of the activation and free energy barriers to their components. Furthermore, we also investigated the effects of the radical and substituent sizes, nucleophilicity/electrophilicity indices, and the spin density distribution of the radical reactants on the three fundamental barriers and the thermal contribution of the reaction energy to the kinetic barrier. Using the generated data, we assessed the abilities of the various radical reactants to abstract the hydrogen atom from NHC-boranes. Further, we performed a similar analysis after dividing those radical reactants into four groups, which were classified based on the dominant factor affecting their electronic density distribution, which involves the inductive effect, conjugation, hyperconjugation, and the feedback of lone-pair electrons. The results and conclusions of this investigation not only provide insight into the relationships between some of the key kinetic and thermodynamic parameters, which is useful for understanding the dynamics of such hydrogen-abstraction reactions, but also provide information for selecting suitable radical reactants for further experimental investigations.