Studies on the solvent dependence of the carbamic acid formation from ω-(1-naphthyl)alkylamines and carbon dioxide
Probably due to pKa(6)>pKa(3) in protophilic dipolar aprotic solvents, the conversion 3→6 is nearly complete while the formation of 9 is minimal.
Introduction
Suitable amines can reversibly trap carbon dioxide through carbamic acids, which are important intermediates in view of their synthetic, industrial and biological applications.1, 2, 3, 4, 5 In an attempt to accomplish new methods for irreversible fixation of carbon dioxide, we have been pursuing basic research on the reactivity of carbon dioxide with amines bearing an aromatic group as a chromophore and fluorophore. Mechanistic studies of amine–CO2 reactions in organic solvents are scarce unlike in aqueous solutions, although carbamic acids have been known for a long time to be readily generated from a primary or secondary aliphatic amine RR′NH.1, 2, 3, 4, 5 Because of their transient nature, however, their isolation in the free acid form RR′NCO2H is very difficult.† They are usually obtained as an ammonium carbamate salt [RR′NCO2−][+H2NRR′]. Thus, bubbling of CO2 gas through a solution of suitable amine in a variety of solvents such as chloroform (CHCl3), methylene chloride, acetonitrile (MeCN), benzene, toluene, ethyl ether, tetrahydrofuran (THF), dioxane, and methanol (MeOH) (anhydrous), generally results in precipitation of the corresponding ammonium carbamate salt as a white solid. By contrast, we6 and others7 have recently found that in a particular solvent like dimethyl sulfoxide (DMSO) and N, N-dimethylformamide (DMF), where the carbamate salts are well soluble, the carboxylation of a primary aralkylamine into the carbamic acid RNHCO2H proceeds up to the complete (∼100%) conversion. We have already studied a number of primary aralkylamines: for instance, ω-(1-naphthyl)alkylamines (ω=1–3) 1–3, ω-(9-anthryl)alkylamines (ω=1–3), ω-phenylalkylamines (ω=1 or 2), tryptamines, and catecholamines.6 Their N-carboxylation in DMSO by CO2 bubbling proceeded smoothly to ∼100% conversion. However, arylamines such as aniline and its derivatives did not undergo substantial N-carboxylation under similar conditions, although indoline and p-anisidine were N-carboxylated to a low conversion (22 and 11%, respectively: Chart 1).‡
Since amine, carbamic acid, and ammonium carbamate are rapidly equilibrated in the solution phase (Eqs. (1), (2)),2, 5, 8 we assumed that in the dissolving solvents (DMSO and DMF) all the free aralkylamine could finally be converted to the carbamic acid by excess CO2. We now report that the observed complete transformation of the amine into the carbamic acid is not only due to the dissolving power of the solvent, but also is a consequence of the solvent effect on the acid–base equilibrium in Eq. 2. We investigated 1–3, especially 3-(1-naphthyl)propylamine (3), because its ammonium carbamate 9 has been found to have relatively large solubility in various solvents, an advantageous property for spectroscopic studies. The solvents employed are (deuterated) DMSO, DMF, pyridine (Py), dioxane, MeCN, benzene, CHCl3, 2-propanol (2-PrOH), and MeOH. The carboxylation experiments were carried out by passing a large excess of CO2 gas through a solution of amine (0.13–0.15 M for NMR and IR experiments and ∼6×10−5 M for absorption and fluorescence measurements) at room temperature for 0.5–1 h. Analyses for the products were performed in situ by measurements of 1H and 13C NMR including 2D NMR (COSY, NOESY, HMQC, and HMBC), IR, and absorption and fluorescence spectra.
R=alkyl or aralkyl, R′=H, alkyl or aralkyl.
Section snippets
Results and discussion
The NMR measurements in situ after bubbling of CO2 through a solution of 3 in DMSO, DMF or Py revealed that 3-(1-naphthyl)propylcarbamic acid (6) was formed quantitatively in complete conversion (e.g., see Fig. 1 for the DMSO case; also see Fig. 2(a)–(c)). In dioxane, formation of the predominant product 6 (see Fig. 2(d)) was accompanied by precipitation of a small amount of ammonium carbamate 9.§
Conclusion
On the basis of the NMR and IR studies coupled with the fluorescence study, we conclude that bubbling of CO2 through the solutions of naphthylalkylamines 1–3 in protophilic, highly dipolar, aprotic solvent (DMSO, DMF or pyridine) produces quantitatively the undissociated carbamic acids 4–6. In dioxane (protophilic, dipolar, aprotic solvent), the carbamic acid and a minor amount of the ammonium carbamate are formed. By contrast, in MeCN (protophobic, dipolar, aprotic solvent), benzene or CHCl3
General
1H and 13C NMR spectra were measured on a JEOL EX-270J, GSX-270, or AL-300 spectrometer. Measurements of 2D NMR were carried out with JEOL JUM-A400. Mass, IR, and absorption spectra were recorded on JEOL JMS-HX 110A, SHIMADZU FTIR-8400, and SHIMADZU UV-2400PC spectrometers, respectively. Fluorescence spectra were gathered with a SHIMADZU RF-5300PC spectrometer and excited at the absorption maximum (280–285 nm). Melting points were measured on a YANACO MP-S3 microscopic hot-stage and are
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Nippon Kagaku Kaishi
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