Studies on the solvent dependence of the carbamic acid formation from ω-(1-naphthyl)alkylamines and carbon dioxide

Abstract

Carbamic acid formation from amine and carbon dioxide in a variety of solvents was investigated by measuring NMR ( 1H, 13C, HMBC) and IR spectra in situ. Bubbling of CO 2 through solutions of naphthylalkylamines 1– 3 in DMSO, DMF or pyridine (protophilic, highly dipolar, aprotic solvent) resulted in complete conversion of the amines to the corresponding carbamic acids 4– 6 . In dioxane (protophilic, dipolar, aprotic solvent), the carbamic acid and a small amount of the ammonium carbamate were formed. By contrast, in MeCN (protophobic, dipolar, aprotic solvent), in benzene or CHCl 3 (apolar, aprotic solvent), or in 2-PrOH or MeOH (dipolar, amphiprotic solvent), ammonium carbamates 7– 9 rather than 4– 6 were formed, although the ammonium bicarbonates/carbonates were competitively formed in MeOH. The ammonium carbamates precipitated in many cases and hence they could be separated. The selective generation of the undissociated carbamic acids in preference to the ammonium carbamates in protophilic, dipolar, aprotic solvents (DMSO, DMF, pyridine, and dioxane) is rationalized by considering the acid–base equilibria between the amines 1– 3 and the carbamic acids 4– 6 in nonaqueous media. The obtained selectivity is likely due to the larger p K a values for 4– 6 than the amines 1– 3 in these solvents. Interestingly, the fluorescence intensities for 1– 3 were dramatically enhanced (4–50 times) in DMSO or DMF upon introduction of CO 2, while they were not altered very much in dioxane, MeCN, benzene, CHCl 3, 2-PrOH, and MeOH, except small to medium increases (1.3–3 times) for 1 in dioxane, MeCN, 2-PrOH and MeOH. As a whole, the solvent effects observed in these fluorescence studies are consistent with those observed in the above NMR and IR studies. Finally, methoxycarbonylation of amine 3 into the methyl carbamate was successfully accomplished by using (trimethylsilyl)diazomethane in the presence of CO 2.