Carbon dioxide-amine reaction mechanism

More recently Hand et al. (ref. 9) have studied the decomposition reaction of N-chloro-a-amino acid anions in neutral aqueous solution, where the main reaction products are carbon dioxide, chloride ion and imines (which hydrolyze rapidly to amine and carbonyl products). They found that the reaction rate constant of decarboxylation was independent of pH, so they ruled out a proton assisted decarboxylation mechanism, and the one proposed consists of a concerted decarboxylation. For N-bromoamino acids decomposition in the pH interval 9-11 a similar concerted mechanism was proposed by Antelo et al. (ref. 10), where the formation of a nitrenium ion (ref. 11) can be ruled out because it is not consistent with the experimental results. Antelo et al. have also established that when the decomposition reaction takes place at pH < 9, the disproportionation reaction of the N-Br-amino acid becomes important, and the decomposition goes through the N,N-dibromoamino acid. This reaction is also important for N-chloroamino compounds but at more acidic pH values, because the disproportionation reaction... 

Miscellaneous Reactions.—A full report has appeared of the reactions of carbon dioxide and carbon disulphide with tervalent phosphorus aryl esters and amines the products are ureas and thioureas, respectively.74 The suggested mechanism, previously invoked for similar reactions of carboxylic acids, involves the N-phosphonium salt (97). 

Direct condensation of propargyl alcohols 125 (Fig. 5.31), carbon dioxide, and propylamine can be realized by Ru3(CO)i2 catalysis at 80 °C under CO2 pressure of 50 kg/cm. The reaction mechanism is rationalized as shown below. When diethylamine is used in place of a primary amine, 2-oxoalkyl WA -diethylcarba-mates are isolated in moderate yields. 

In aprotic media a l-(acyloxycarbonyl)imidazole such as 16 is formed primarily which reacts to the acylimidazole and carbon dioxide. Imidazole now serves as a good leaving group and so the previously synthesized amine 6 could be added and the desired amide was formed via the usual addition elimination mechanism. One of the advantages of using this more expensive way of activation is the possibility to run the nitro reduction, acid activation and acylation in the same solvent (ethyl acetate) thus all three reactions could be telescoped into a single step during production. 

Procedures are reported for the synthesis of ethyleneurea from ethylene glycol or ethanolamine either with urea or with ammonia and carbon dioxide. Pressure and other variables affecting the reactions are discussed and a mechanism is suggested by which amination of the carbon skeleton takes place. 

A very similar reaction is decarboxylation. Starting from the same imine we could lose carbon dioxide instead of a proton by a very similar mechanism. Reprotonation and imine transfer releases the amine corresponding to the original amino acid. The enzymes catalysing these reactions are R called decarboxylases. 

The mechanism of CDI-mediated acylation of amines is well understood. The first step involves a partial protonation of the basic imidazole-nitrogen, protonated A-acetylimidazole has a p a of 3.6,f l leading to an activated species which is then attacked by the carboxylate. The resulting mixed anhydride extrudes carbon dioxide giving rise to A-acylimidazole which on treatment with an amine compound leads to the desired anoide (Scheme 1). An important advantage of this method over the carbodiimide method is that the byproducts carbon dioxide and imidazole are readily and quantitatively separated from the reaction product by simple washing procedures. 

Takuwa and his coworkers have demonstrated that the irradiation of the aromatic carbonyl compounds (120) in the presence of the stannanes (121) affords the unsaturated alcohols (122) as the principal products. An electron transfer mechanism is proposed. Electron transfer is also involved in the reaction of amines with alkenes such as the phenylethylenes (123). The electron transfer in this instance affords an alkenyl radical anion the presence of which has been demonstrated by a variety of techniques. A further reaction has been uncovered in the photoreaction of, for example, the alkene (123a) with iV, AT-diethylani lino in the presence of carbon dioxide. This treatment affords the three carboxylated derivatives (124), (125), and (126) by trapping of the radical anion by carbon dioxide. Similar carboxylation was demonstrated for (123b) and biphenyleno. The influence of the amine on the yield of product was studied. ... 

The mechanism of the Eschweiler-Clarke reaction proceeds via the formation of an imine, followed by reduction by formic acid. That the methylation is attributable to the formaldehyde and the reduction to the formic acid has been confirmed using 14C-labeled isomers of each in a series of studies.5 Thus, the amine reacts with formaldehyde to produce an imine, and this is then reduced with the loss of carbon dioxide by formic acid. In the case of primary amines this process is then repeated to produce a tertiary N.iV-dimethyl amine. 

The Fixation of Carbon Dioxide and Nitrogen.—As reduced carbon compounds are convenient fuels, the possibility of achieving the reductive fixation of COa in vitro is appealing, if remote. There are very few data on photochemical reactions involving carbon dioxide, for it has no low-lying excited states and has not historically been of much interest to the photochemist. However, what appears to be the first example of photofixation of COa in a non-biological system has been briefly reported.18 Photoirradiation (with a high-pressure mercury lamp) of phenanthrene in the presence of an amine and C02 in a polar solvent (MeaSO or HCONMea) yielded 9,10-dihydrophenanthrene-9-carboxylic acid, in unspecified quantum yield. The mechanism appears to involve formation of COaT by electron transfer from the photoexcited amine, followed by attack of COaT on position 9 of phenanthrene. Similar reductive carboxylation of anthracene, pyrene, naphthalene, and biphenyl was observed. 

---