Dense carbon dioxide has been utilised for the formylation of various cyclic and aliphatic amines, including pyrrolidine, piperidine, piperazine, aniline, morpholine, propylamine and (x-methylbenzylamine, in the presence of hydrogen and RuCl2(dppe)2 (dppe = 1,2-bis(diphenylphosphino)ethane) as catalyst. Formylations were performed at 100-120degreesC and in the pressure range from 185 to 225 bar in a stirred autoclave. Chemoselectivity was nearly 100% with the exception of piperazine, where the mono- and diformylated product was formed. All amines could be formylated with a turnover frequency exceeding significantly that reported for the same reaction in the literature. In situ monitoring of the phase behaviour revealed that formation of solid carbarnate can strongly affect the efficiency of the formylation. Formation of solid carbamate which slows down the reaction rate could be suppressed by addition of small amounts of water to the reaction mixture. The formylation using CO2 as Cl-building block and RUCl2(dppe)2 as catalyst offers an interesting environmentally benign alternative to conventional formylation based on carbon monoxide, formic acid or formic acid derivatives. (C) 2003 Elsevier B.V All rights reserved.