Abstract
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.
Highlights
Over the past two decades, boron chemistry has reemerged as one of the most interesting fields in organic and inorganic chemistry
The selected N-heterocyclic carbene (NHC)-boranes, which investigated reactions are shown in Scheme
CH2 CN and Me are similar to and lower than, respectively, those with Et. These results indicate that the intrinsic, activation and free energy barriers for the hydrogen atom abstraction (HAA) reactions of a given NHC-borane with different radicals can only be estimated using the electrophilicity indices of the attacking radicals if these indices are sufficiently different
Summary
Over the past two decades, boron chemistry has reemerged as one of the most interesting fields in organic and inorganic chemistry. For the HAA reactions of NHC-boranes, such an approach can aid in understanding the compositions of barriers and the effect of the kinetic and thermodynamic components on their. In the field of physical organic chemistry, the correlation between the free energy barrier and the reaction Gibbs free energy remains of particular interest because the former is the key kinetic physical quantity for determining the reaction rate constant and the latter is the thermodynamic driving force of the reaction. A three-variable linear model (see: Supplementary Materials) was suggested to describe the relationship between the kinetic and thermodynamic quantities for the HAA reactions of NHC-boranes This model was shown to be more intuitive than the Marcus-theory-based nonlinear HAA model [37] and more accurate than the two-variable Evans–Polanyi linear model [34,35]. We hope that the present work will aid in improving experimental design and provide a deeper insight into physical-organic models with the aim of obtaining a better understanding of the physical quantities that affect reactivity
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