A comparative study of inter- and intramolecular C–H aminations: mechanism and site selectivity

Abstract

The mechanism and site selectivity of inter-and intramolecular C–H bond aminations have been studied using the M06L method. The similarities and differences between inter- and intramolecular aminations are discussed in detail, with an emphasis on the correlation between reaction pathway and site selectivity. Our calculations show that, for both inter- and intramolecular aminations, the triplet stepwise pathway is of H-atom transfer character, while the singlet concerted pathway is of hydride-transfer character. More importantly, for intermolecular C–H bond amination, the triplet stepwise pathway is favored over the singlet one. Conversely, in the intramolecular case, the singlet concerted pathway is dominant. This inverse preference for the singlet–triplet pathway for inter- and intramolecular aminations is an important contributing factor to their inverse site selectivity. The calculated benzylic-to-tertiary (B : T) C–H amination ratios for the inter- and intramolecular aminations are 13 : 1 and 1 : 17, respectively, which are in agreement with experimental results of 7 : 1 and 1 : 7. Specifically, intermolecular amination favors oxidation of the secondary benzylic C–H bond over the tertiary C–H bond, whereas the intramolecular case shows a preference for tertiary C–H amination. This inverse site selectivity can be mainly attributed to the fact that inter- and intramolecular aminations proceed according to the different state pathways mentioned previously, where the enthalpic and entropic contribution to free energy differ. In addition, there is a higher energy barrier for the cleavage of the C–H bond in the intermolecular amination than in the intramolecular case, due to a much larger negative entropy of activation for the former. These results provide some valuable information for the design and deconstruction of highly selective and highly efficient C–H amination systems.