Secondary amines, reaction with carbon dioxide

The secondary reduction of the terminal radical by Sml2 generates samarium alkyl species which are suitable for classical organometallic reactions, e.g. protonation, acylation, reactions with carbon dioxide, disulfides, diselenides, or the Eschenmoser salt. A broad variety of products is available (hydroxy-substituted alkanes, esters, carboxylic acids, thioethers, selenoethers, tertiary amines) by use of the double-redox four-step (reduction-radical reaction-reduction-anion reaction) route (Scheme 20) [73]. 

In the presence of ruthenium complexes, primary amines react with carbon dioxide at 120-140 °C to give Af,A -disubstituted symmetrical ureas Ruthenium complexes also catalyze the reaction of secondary amines with alkynes and carbon dioxide to give vinyl carbamates". ... 

The reaction of a vinyl ether with carbon dioxide and a secondary amine gives a carbamic ester (246). 

For example, polymers having hydroxyl end groups can be prepared by reaction of polymer lithium with epoxides, aldehydes, and ketones III-113). Carboxylated polymers result when living polymers are treated with carbon dioxide (///) or anhydrides (114). When sulfur (115, 116), cyclic sulfides (117), or disulfides (118) are added to lithium macromolecules, thiol-substituted polymers are produced. Chlorine-terminus polymers have reportedly been prepared from polymer lithium and chlorine (1/9). Although lithium polymers react with primary and secondary amines to produce unsubstituted polymers (120), tertiary amines can be introduced by use of p-(dimethylamino)benzaldehyde (121). 

Ammonium carbamates are readily and reversibly produced on reaction of secondary amines with carbon dioxide. In the presence of a ruthenium catalyst precursors such as Ru3(CO)12 [3], (arene)RuCl2(PR3) [4] or Ru(methallyl)2(dppe) [5] (dppe=bis(diphenylphosphino)ethane) complexes, the three-component combination of a secondary amine, a terminal alkyne, and carbon dioxide selectively provides vinylcarbamates resulting from addition of carbamate to the terminal carbon of the triple bond (Scheme 2). 

The measurement of circular dichroism is an especially sensitive and excellent method for studying the reactions of 34. A freshly prepared solution of 34 at pH 7.8 contains no 33 or 22 after several hours, a Cotton effect is observable, and it attains a maximum after two days. Thus, the equilibrium 34 33 has been established. The position of the equilibrium is dependent on the pH at pH 7.8, the content of 33 is 3—6%, on the assumption that 33 has a molar extinction coefficient of circular dichroism similar to that found for other cyclic azomethines. On treatment of the solution with carbon dioxide to pH 6.8, the Cotton effect disappears immediately, because the acid-catalyzed Amadori rearrangement of 33(= 16) —> 22 is accelerated. Compound 22 is, however, a secondary amine, and is a stronger base than 33 and 34, so that the continued formation of 22 causes ftie pH to rise. Thereby, the acid-catalyzed Amadori rearrangement is slowed down so much that the equilibrium 33 34, with its high Cotton... 

A fourth type of product, a carbamate RNHCOOR , can be obtained from primary or secondary amines, if these are treated with CO, O2, and an alcohol R OH in the presence of a catalyst. " Primary amines react with dimethyl carbonate in supercritical CO2 (see p. 414) to give a carbamate. " Carbamates can also be obtained from nitroso compounds, by treatment with CO, R OH, Pd(OAc)2, and Cu(OAc)2, " and from nitro compounds. " When allylic amines (R2C=CHRCHRNR 2) are treated with CO and a palladium-phosphine catalyst, the CO inserts to produce the p,y-unsa-turated amides (R2C=CHRCHRCONR 2) in good yields. " Ring-expanded lactams are obtained from cyclic amines via a similar reaction (see also, 16-22). Silyloxy carbamates (RNHC02SiR 3) can be prepared by the reaction of a primary amine with carbon dioxide and triethylamine, followed by reaction with triisopropylsilyl triflate and tetrabutylammonium fluoride. ... 

It is the control of the reactivity of this carbamate anion which has allowed us to generate in quantitative yields either urethanes or isocyanates directly. By understanding those factors which promote the nucleophilicity of the carbamate oxygen, we have discovered how to generate urethanes directly from either primaiy or secondary amines (Activated Carbon Dioxide Chemistry-I (ACDC-I) (12, 13, 15-18). A key aspect of this chemistry was our discovery that carbamate anions can serve as excellent nucleophiles, when a suitable tertiary amine base is utilized to generate the carbamate anion in the reaction of carbon dioxide with a primary or secondary amine (equation 4). By proper choice of the tertiary amine co-base, which in general does not react with CO2 under the mild conditions employed. 

The reaction of CO2 with organic amines (RR R"N, ArNH2 R = H or aliphatic moiety, Ar = aromatic moiety) is of great interest as the aliphatic members have been used for up-taking CO2 from a gas stream or for synthetic purposes (3.8). The reactivity of amines with CO2 depends on the nature of the organic moiety they bear and the reaction conditions. In fact, under strictly anhydrous conditions, although tertiary aliphatic amines do not absorb CO2, aliphatic primary and secondary amines, as well as ammonia itself, react quickly with carbon dioxide to form the corresponding ammonium carbamates (3.8) which, in a few cases, have been fully characterized in the sohd state by X-ray diffraction [10, 11]. 

Uncontaminated solutions of tertiary amines such as MDEA are generally not corrosive whatever the acid gas. According to API 945, solutions of most amines are not corrosive if the ratio of hydrogen sulfide to carbon dioxide is above roughly S/9S, because the corrosion reaction leads to formation of a protective sulfide film (API, 1990). The most corrosive combinations appear to be those of primary or secondary amines with carbon dioxide. 

Although the nucleophilic addition of secondary amines to thiirene dioxides can be interpreted as following the same mechanistic pathway, the reaction was found to be second order in amine119 (which is typical for the addition of amines to olefins in appropriate solvents13 2 133), and the addition is syn. As a result, mechanisms with a cyclic-concerted addition across the carbon-carbon bond, or a stepwise addition involving two molecules of amine per one molecule of thiirene dioxide, have been proposed. 

Exxon s Flexsorb SE solvents achieve high hydrogen sulfide selectivity by virtue of their molecular structure. These solvents are sterically hindered secondary amines. A bulky molecule is used to shield the available hydrogen radical on the nitrogen atom and prevent the insertion of carbon dioxide. The reaction with hydrogen sulfide is not sensitive to the amine s structure, so the steric hinderance affords higher hydrogen sulfide selectivity. 

Reaction with Nitrogen Nucleophiles. The acid-catalyzed reaction of primary, secondary, and tertiary amines with ethyleneimine yields asymmetrically substituted ethylenediamines (71). Steric effects dominate basicity in the relative reactivity of various amines in the ring-opening reaction with ethyleneimine (72). The use of carbon dioxide as catalyst in the aminoethylation of aliphatic amines, for which a patent application has been filed (73), has two advantages. First, the corrosive salts produced when mineral acids are used as catalysts (74,75) are no longer formed, and second, the reaction proceeds with good yields under atmospheric pressure. 

Urethanes. Methyl carbamates (1) can be prepared from primary or secondary amines, alkyl halides, and carbon dioxide in a reaction promoted by copper(I) /-butoxide (equation I). The ligand t-butyl isocyanide can be replaced with tri-n-butylphosphine. Copper(I) f-butoxide is more effective than other copper salts. In the case of diethylamine, the intermediates a and b were isolated and b was converted to the methyl carbamate in 86% yield. 

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