DBU-Promoted Nucleophilic Activation of Carbonic Acid Diesters

The reactivity of carbonic acid diesters in the presence of the amidine base DBU (1,8-diazabicyclo[5.4.0] undec-7-ene) has been investigated for the first time. Organic carbonates can be activated by DBU through the formation of N-alkoxy-carbonyl ketene aminal 2 as the ultimate product. The latter species may form through deprotonation of the corresponding N-alkoxycarbonyl-amidinium cation 1(+) by the amidine base. We have for the first time isolated and characterized, both in the solid state (X-ray crystal structure determination, IR) and in solution (NMR), a few 1(+) chloride salts and studied their reactivity towards the organic base. The reactivity of both 1(+) and 2 with methanol has also been explored. Ketene aminal 2 behaves as a CO2R carrier, as it can selectively transfer the alkoxycarbonyl group to the alcohol and regenerate the amidine base.