The carboxylation of diamines to cyclic ureas by CO2 in the presence of an alkali metal carbonate or alkali metal bicarbonate exhibits a typical autocatalysis behavior, in which the rates of corresponding cyclic urea formations increase with time as in a S-shaped curves. The activity of the alkali metal carbonate was greatly enhanced when ethylene urea was co-present. Computational calculation results on the carboxylation of ethylenediamine in the co-presence of KHCO3 and ethylene urea suggest that ethylene urea of keto form could tautomerize into enol form with the aid of KHCO3 at least to a certain extent, thereby playing a role as a proton donor. With such transformation and the consequent favorable hydrogen bonding interaction with KHCO3, the proton accepting and donating abilities of KHCO3 can be enhanced and, as a result, the carboxylation is accelerated. (C) 2017 Elsevier B.V. All rights reserved.