Abstract
The chemical fixation of CO2 into organic compounds has drawn much attention as an alternative for reducing its emittion into atmosphere. Especially, its oxygen-retaining fixations are of great advantage over oxygeneliminating ones such as methanation in respect of the energy consumption and the usefulness of products. The author and his co-workers have found that ruthenium complexes such as rutheniumcarbonyl and ruthenium chloride catalyze the reaction of CO2, amines and alkynes including acetylene (Scheme 1) to give vinyl carbamate derivatives (2, 3, 4, 7, 8). The similar reaction of primamines with acetylene (eq.5) gave 1, 3-dialkylurea derivatives (9) at lower temperatures than the reaction without catalysis which requires ca.180 °C.
Although the attempt of synthesizing carbob nates from CO2, alkohol, and alkynes in the presence of ruthenium complex/ t-amine catalyst system was unsuccessful (eq.6), the previously known reaction of CO2 with 2-propynols catalyzed by copper salts (eq.7) has been found to proceed at much lower temper ature and to give linear diesters of carbonic acid (11, 12) in addition to the previously synthesized cyclic carbonate (14c) using this catalytic system (Scheme 2). When sec-amines or prim-amine was used instead of t-amine, carbamates (16) or 2 (3H) -oxazolones (17) were formed respectively (Scheme 3). Furthermore, the presence of prim-alcohols has led to the formation of carbonates (18) in the reaction of 2propynols and CO2 catalyzed by the combination of ruthenium complex and amine (Scheme 4).
The reaction mechanism for the addition of carbamic acid to alkynes may include the nucle ophilic attack of carbamate anion to alkyne which should be activated by coordinating to ruthenium (Scheme 5). The intramolecular addition of alkyl carbonate anion is postulated for the reaction of 2-propynols with CO2( S cheme 6). The further reactions occur according to the active hydrogen compounds presen t in the reaction media.
