Utilization of N,N-dialkylcarbamic acid derived from secondary amines and supercritical carbon dioxide: stereoselective synthesis of Z alkenyl carbamates with a CO2-soluble ruthenium-P(OC2H5)3 catalyst.

Reversible transformation of diethylamine (1) and supercritical carbon dioxide (scCO(2)) into N,N-diethylcarbamic acid (2) was confirmed by direct acquisition of (1)H and (13)C{(1)H} NMR spectra. The equilibrium between 1+CO(2) and 2 is strongly influenced by conditions of the supercritical state. Low temperature favors formation of carbamic acid, whereas high temperature causes decarboxylation. On the basis of the spectroscopic results of carbamic acid formation under scCO(2) conditions, the ruthenium-catalyzed formation of alkenyl carbamates from terminal alkynes, 1, and carbon dioxide was investigated to demonstrate the useful transformation of elusive carbamic acids. Selectivity toward the CO(2)-fixation products over enynes obtained by dimerization of the alkyne substrates was improved by the use of scCO(2) as a reaction medium. In particular, a CO(2)-soluble ruthenium complex, trans-[RuCl(2){P(OC(2)H(5))(3)}(4)], was found to be effective in affording Z alkenyl carbamates with high stereoselectivity.