Mechanistic insights and origin of chemoselectivity for S–O bond cleavage in dinitrobenzenesulfonic carbamates

Abstract

The NHC-catalyzed reactions of aldehydes with dinitrobenzenesulfonic carbamates unprecedentedly cleave the S–O bond but not the N–O bond, which provides elegant access to the synthesis of hydroxylamine. In this work, the mechanism and the origin of chemoselectivity as well as the substituent effects of the reactions were investigated in detail by density functional theory (DFT). The computational results show that the reaction is initiated by the coordination of the NHC catalyst to aldehydes forming a zwitterionic intermediate, which can subsequently be oxidized by dinitrobenzenesulfonic carbamates via proton transfer and S–O bond cleavage accompanied by electron transfer. Then, the conjugation of the dissociated “O” synthon from dinitrobenzenesulfonic carbamate with the oxidized intermediate followed by catalyst elimination can lead to the experimentally observed O-acyl hydroxylamine carbonyl ester product. The second reaction step associated with the deprotonation along with S–O bond cleavage is rate-determining, and the carbamate parts were negatively-charged in the corresponding transition states because of electron transfer, which reasonably explains why electron deficiency is important for the occurrence of the reaction. Moreover, we also considered the N–O bond cleavage, which was demonstrated to require more energy. Activation/strain analysis and NCI analysis reveal that more effective electrostatic interactions as well as π–π interactions in combination with C–H⋯O hydrogen bond interactions in the S–O bond cleavage transition state contribute to the observed chemoselectivity. This work not only clarifies the detailed reaction mechanism for the NHC-catalyzed reactions of aldehydes with dinitrobenzenesulfonic carbamates for the synthesis of hydroxylamine, but also provides a reasonable model to predict the chemoselectivity and reaction outcomes for different substrates.