Alkylammonium carbamate

Abstract—The nature of the product of the reaction between an aminated silane and carbon dioxide was re-examined with the aid of simple model compounds, several amines, and several aminosilanes. Since the reaction products previously proposed include the amine bicarbonate and a carbamate derived from the amine, ammonium bicarbonate and ammonium carbamate were studied as models for the anions. Carbon dioxide adducts of neat model amines were prepared and studied. Results from a variety of techniques are summarized. Among the most useful was Fourier transform infrared (FTIR) spectroscopy of fluorolube mulls. FTIR spectra were distinctive and assignments characteristic of the two species were extracted from the spectral data. Comparisons of these assignments with the products of the reaction between carbon dioxide and various amines were made. The results indicate that alkylammonium carbamates are the principal product. Nuclear magnetic resonance (NMR) spectra in D20 indicated much dissociation and were not helpful in defining the products. 

Keywords Silane coupling agents alkylammonium carbamates aminosilane-carbon dioxide salts amine-carbon dioxide salts. 

A few comments about the possible consequences of the formation of alkylammonium carbamates on the behavior of aminosilanes as coupling agents are appropriate at this point. 

The fixation of C02 by amines, which has been recognized for a long time, can either take place directly or it can be mediated by metal or nonmetal species to provide a carbamate group, RR NC02 , which is bound either ionic or covalently to an electrophilic center [1-5]. The direct interaction of C02 with primary or secondary amines can afford carbamic acids or alkylammonium carbamates (Equations 6.1 and 6.2) [1, 2], In the presence of metals, metal salts (Equation 6.3) [3j, k] or metal-complexes, metal carbamates can be obtained [3, 4] ... 

With respect to carbamic acids, alkylammonium carbamates, (RR NH2)02CNRR ... 

R = H, alkyl), are relahvely more stable compounds, and have been recognized for a long time. Under anhydrous conditions, while aliphatic tertiary amines do not absorb C02 [25], aliphahc primary and secondary amines easily react with C02 to yield alkylammonium carbamates which, in a few cases, have been fully characterized in the solid state using X-ray diffraction (XRD) [2, 18, 26]. For this,... 

Scheme 6.1 Chem ical stability of alkylammonium carbamates. ...

Scheme 6.2 Mechanism of formation of alkylammonium carbamates (according to Danckwerts [28a]).

In solution, under an inert gas atmosphere (N2), alkylammonium carbamates (RNH3)02CNHR (R = Bz, allyl, Cy) exhibit modest stability, even at ambient temperature. They readily form carbamic acids RNHC02H, which can either decar-boxylate or undergo self-association with the formation of dimers (Scheme 6.1) [27]. 

Scheme 6.2 illustrates the pathway proposed by Danckwerts for the conversion of amines and C02 into alkylammonium carbamates [28a]. The mechanism implies the intermediate formation of a zwitterion species which, by deprotonation, converts into the carbamate product. This reaction pathway has been doubted by Crooks, who proposed a termolecular single-step mechanism [28b], Borowiak and coworkers [28c] have modeled the reaction of dimethylamine with C02 in the presence of a second molecule of amine, obtaining an activation enthalpy (40 kj moT1) which was in good agreement with the experimental value. The reaction product was described as the H-bond-stabilized adduct of dimethylcarbamic acid with dimethylamine. 

Yoshida et al. were the first to report the synthesis of carbamate esters by the direct reaction of aliphatic amines, C02 and alkyl halides [47]. The process involved the O-alkylation of intermediate alkylammonium carbamate salt, and required relatively, severe conditions (333-393 K 4MPa C02), long reaction times (1-2 days) and an excess of amine (2.5 equiv.) with respect to the alkylating agent. The method was shown to be effective only with secondary aliphatic amines which, however, were converted into organic carbamates in low to moderate yield and with modest selectivity because of significant side-formation of N-alkylation products. 

Scheme 6.8 Interaction of alkylammonium carbamate ion pair with dimethyl carbonate (DMC).

The synthons of oxiranes have also been used in this respect. For example, the reaction of C02 with a-bromoacylophenones in the presence of aliphatic primary amines, in methanol, afforded 3-alkyl-4-hydroxyoxazolidin-2-one derivatives under mild conditions [83a]. However, neither oc-bromoacetophenone nor a-chloroacetophenone afforded any carbamate product, and no urethanes were obtained with aromatic or aliphatic secondary amines. The proposed mechanism involved, as the first step, the formation of a 3-alkyl-2-methoxy-2-phenyloxirane intermediate, which reacted with alkylammonium carbamate to give the oxazo-lidone product (Scheme 6.16). This synthetic protocol was successfully applied to the synthesis of bis(oxazolidin-2-one) derivatives by reactions of 2-methoxy-3,3-dimethyl-2-phenyloxirane or a-bromoisobutyrophenone with C02 and aliphatic a,G)-diamines [83b],... 

We have written the carbamate alkylammonium structure as an aid in comparing assignments of observed absorptions. 

All of these compounds, whether alkylammonium, metal and p-block carbamates, are potential carriers or sources of carbamic groups, and can be used in the synthesis of organic carbamates [6] if the transfer of the RR NC02 moiety to a suitable organic substrate is accomplished [6, 7]. Moreover, alkylammonium, metal and p-block carbamates, as well as carbamic acid esters, are also potential... 

Aresta and Quaranta studied the reactivity of alkylammonium N-alkylcarbamates (RNH3)02CNHR towards a different acylating substrate, such as dimethyl carbonate (DMC) [62a, b]. Carbamate salts (RNH3)02CNHR (R = benzyl, allyl, cyclohexyl), prepared in situ from aliphatic primary amines and C02, reacted with DMC to afford N-alkyl methylcarbamates (Equation 6.6). The reaction requires mild conditions (343-363 K 0.1 MPa C02 pressure) and can be carried out in DMC used as solvent and reagent. At 363 K, carbamate esters were obtained in satisfactory yield (45-92%) with high selectivity, as side products such as ureas, N,N-dialkylcarbamate esters, and alkylated amines were formed in very small amounts. 

Based on our observations and literature precedent, the following pathways were postulated to explain the catalytic effect of CO2 on amidation reactions. Reaction of the amine with CO2 would lead to the alkylammonium N-aUcyl carbamate, 23. Nucleophilic attack of the oxygen center of carbamate 23 on the imidazolide would give intermediate 24, which upon extrusion of CO2 would lead to the amide (Scheme 4.7, pathway a ). Alternately, the nitrogen center of tautomer 25 may attack the carbonyl of the imidazolide to directly give the amide (Scheme 4.7, pathway b ). 

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