Hydrogen Abstraction by Alkoxyl Radicals: Computational Studies of Thermodynamic and Polarity Effects on Reactivities and Selectivities

Abstract

Density functional theory calculations (ωB97X-D) are reported for the reactions of methoxy, tert-butoxy, trichloroethoxy, and trifluoroethoxy radicals with a series of 26 C−H bonds in different environments characteristic of a variety of hydrocarbons and substituted derivatives. The variations in activation barriers are analyzed with modified Evans−Polanyi treatments to account for polarity and unsaturation effects. The treatments by Roberts and Steel and by Mayer have inspired the development of a simple treatment involving the thermodynamics of reactions, the difference between the reactant radical and product radical electronegativities, and the absence or presence of α-unsaturation. The three-parameter equation (ΔH ⧧ = 0.52ΔH rxn (1 − d) − 0.35Δχ AB 2 + 10.0, where d = 0.44 when there is α-unsaturation to the reacting C−H bond), correlates well with quantum mechanically computed barriers and shows the quantitative importance of the thermodynamics of reactions (dictated by the reactant and the product bond dissociation energies) and polar effects.

Exploring the Reaction Mechanism of C–H Oxidation by Copper–Salen Complexes

“…In the second phase of the mechanism, LCuOH can abstract a hydrogen atom from C 3 H 6 or C 3 H 7 –CC–Ph to form a radical plus LCuOH 2 . It should be noted that alkoxy radicals, such as OMe and O t Bu, can also abstract hydrogen atoms with low barriers . However, it is likely that these radicals may be “captured” by the copper salen in solution.…”

“…It should be noted that alkoxy radicals, such as OMe and O t Bu, can also abstract hydrogen atoms with low barriers. 56 However, it is likely that these radicals may be "captured" by the copper salen in solution. Two distinct transition states were located for the abstraction of a hydrogen LCuOH investigated here except that the dianion ligand is tridentate instead of tetradentate as is the case for the salen ligands (I, II).…”

Rapid and Accurate Estimation of Activation Free Energy in Hydrogen Atom Transfer-Based C–H Activation Reactions: From Empirical Model to Artificial Neural Networks

“… 49 Our recent work simplified Roberts’ equation with reaction energy, radical electronegativity differences, and unsaturation and led to good results for the HAT reactions of 26 sp 3 C–H bonds by alkoxyl radicals, with R 2 = 0.89 and mean absolute error (MAE) and root mean square error (RMSE) values of 0.9 and 1.1 kcal/mol, respectively (compared with the DFT calculations). 50 …”

“…49 Our recent work simplified Roberts' equation with reaction energy, radical electronegativity differences, and unsaturation and led to good results for the HAT reactions of 26 sp 3 C−H bonds by alkoxyl radicals, with R 2 = 0.89 and mean absolute error (MAE) and root mean square error (RMSE) values of 0.9 and 1.1 kcal/mol, respectively (compared with the DFT calculations). 50 On the other hand, the significant advancement of machine learning (ML) technology provides a new strategy for solving various chemical problems. The artificial neural network (ANN), as one of the most popular ML methods, is a datadriven adaptive method.…”

“…52,53 While our study was ongoing, Hong et al have provided an ML approach for reactivity prediction in photoredox-mediated HAT catalysis. 54 In the present study, we aim to (1) apply the ANN model to the HAT-based sp 3 C−H activation to achieve the goal of rapid and accurate prediction of the activation free energy, (2) assess the performance of the simplified Roberts' equation proposed in our recent study, 50 and (3) compare the performance between MLR and ANN with the same physical variables. The substrate C−H bonds have been expanded (Figure 1).…”