Mechanistic Insights and Origin of Chemoselectivity for S–O bond Cleavage in Dinitrobenzenesulfonic Carbamates
The NHC-catalyzed reactions of aldehydes with dinitrobenzenesulfonic carbamates unprecedentedly cleave the S–O bond but not N–O bond, which provide an elegant access to the synthesis of hydroxylamine. In this work, the mechanism and the origin of chemoselectivity as well as the substituent effects for the reactions were investigated in detail by density functional theory (DFT). Computational results show that the reaction is initiated with the coordination of NHC catalyst to aldehydes forming 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 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 explain why the electron deficiency is important to the occurrence of the reaction. Moreover, we also considered the N–O bond cleavage, which was demonstrated to require more energy. Activation/strain analyses and NCI analyses reveal that the more effective electrostatic interactions as well as π-π interaction in combination with C–H∙∙∙O hydrogen bond interactions in the S–O bond cleavage transition state contribute to the observed chemoselectivity. The work not only clarifies the detailed reaction mechanism for the NHC-catalyzed reactions of aldehydes with dinitrobenzenesulfonic carbamates for synthesis of hydroxylamine, but also provides a reasonable model to predict the chemoselectivity and reaction outcomes for different substrates.