Isolation and structural characterization of stable carbamic–carbonic anhydrides: an experimental and computational study

Carbamic–carbonic anhydrides are elusive species that have been only indirectly detected under controlled conditions. This functional group is transiently formed during the reaction of secondary amines with anhydrides in the presence of nucleophilic catalysts such as 4-(dimethylamino)pyridine. In this work, the synthesis and isolation of two carbamic–carbonic anhydrides are reported, including the first-ever solid-state structure of this functional group. The remarkable stability of these chiral carbamic–carbonic anhydrides allowed their study by NMR, HRMS, FTIR-ATR, and thermal analysis techniques (DSC and TGA). A thorough analysis of the bonding situation by computational studies hints that the origin of this unusual stability relies on n → σ* stabilizing orbital interactions hampering the occurrence of decarboxylation.