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.
In
this study, we performed a theoretical investigation of the
intramolecular cyclization of bicyclic 2-allyl-2-methyl-2,3-dihydro-1H-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.
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