Ammonia elimination from amines

In Robinson s now well-known suggestions, regarding the processes by which alkaloids may be produced in plants, two main reactions are used j the aldol condensation and the similar condensation of carbinol-amines, resulting from the combination of an aldehyde or ketone with ammonia or an amine, and containing the group. C(OH). N., with substances in which the group, CH. CO. is present. By these reactions it is possible to form the alkaloid skeleton, and the further necessary changes postulated include oxidations or reductions and elimination of water for the formation of an aromatic nucleus or of an ethylene derivative. 

Conversion of Amides into Carboxylic Acids Hydrolysis Amides undergo hydrolysis to yield carboxylic acids plus ammonia or an amine on heating in either aqueous acid or aqueous base. The conditions required for amide hydrolysis are more severe than those required for the hydrolysis of add chlorides or esters but the mechanisms are similar. Acidic hydrolysis reaction occurs by nucleophilic addition of water to the protonated amide, followed by transfer of a proton from oxygen to nitrogen to make the nitrogen a better leaving group and subsequent elimination. The steps are reversible, with the equilibrium shifted toward product by protonation of NH3 in the final step. 

The balance between those molecules which are protonated and those which are not has emerged as an inportant function in determining the course of any reaction occurring within the Interlamellar region. One carefully studied reaction is that of the elimination of ammonia from amines (21), a reaction proceeding by two sinple steps... 

From a simplified scheme of reduction of the amide function it can be seen that the first stage is formation of an intermediate with oxygen and nitrogen atoms linked to an sp carbon. Such compounds tend to regenerate the original sp system by elimination of ammonia or an amine. Thus an aldehyde is formed and may be isolated, or reduced to an alcohol. Alternatively the product is an amine resulting from direct hydrogenolysis of the sp intermediate. 

A remaining question about the elimination process on a solid surface is the geometric structure of the adsorbed molecule during reaction. If the elimination is a concerted reaction, the acidic function of the catalyst surface (which binds the amine part of the reactant) and the basic function (which abstracts the p -hydrogen atom) should act simultaneously. In that case, it seems as if the reacting amine can assume only a configuration close to the syn configuration shown in Fig. 7. However, syn eliminations are less common than trans eliminations, and we have shown that the elimination of ammonia from amines occurs primarily via trans elimination [F. Rota, V. Ranade, and R. Prins, J. Catal. 200 (2001), to be published]. This problem was realized even... 

Whereas metal hydride is eliminated from the o--complex intermediates (cf. 309) in Chichibabin aminations, which results in the subsequent reaction of the metal hydride with ammonia or amines and the evolution of hydrogen, the o--complex intermediates can also be oxidized to the corresponding amino-N-heterocycles. Thus, 5-azacinnoline (320) can be aminated with primary and secondary aliphatic or benzyl amines in the presence of potassium hydroxide/potassium ferricyanide to give the corresponding amino derivatives 321 in up to 65% yield. The... 

Benzofuroxan (5) reacts under mild conditions with an enamine or with an aldehyde or ketone in the presence of ammonia or an amine to form mono- or disubstituted quinoxaline dioxides (151). Dark red dihydro-quinoxaline dioxides (e.g., 152) may be formed when an enamine is used which does not possess an a-hydrogen atom for elimination.381 2-Amino-substituted quinoxaline dioxides have been produced using (a) an ynamine [providing tertiary amines (153)],382 (b) chloroacetaldehyde and a primary amine [producing secondary amines (154)],9 and (c) some nitriles, e.g., malononitrile (to give the primary amine 155).382 The last reaction, however, is by no means a general one, since benzimidazoles are also produced from these substrates (see below). l-Hydroxyquinoxalin-2-one 4-oxides (156) can also be formed in several ways using (a) an a-substituted /1-ketoester (157)... 

Second Stage Dissociation of the tetrahedral intermediate Just as steps 1-3 corresponded to addition of water to the carbonyl group, steps 4-6 correspond to elimination of ammonia or an amine from TI and restoration of the carbonyl group. 

Primary amines and thiols undergo similar elimination with Al(III)-exchanged montmorillonites. Primary amines are converted at 473 K to di-alk-l-ylamine by loss of ammonia. Likewise, hydrogen sulfide is eliminated from thiols to form di-alk-l-yl thioethers. ... 

Similarly, ionized alcohols and ethers containing a chain of at least three contiguous carbon atoms attached at one end to the oxygen atom frequently expel water or the alcohol derived from the smaller alkyl group76-80. However, the corresponding ionized amines rarely eliminate ammonia or small alkylamines in great abundance. This contrast reflects energetic factors. Water and small alcohols are extremely stable molecules (AHf = —240 and —190 kJ mol-1, respectively, for water and methanol), but ammonia and methylamine are not particularly stable (AHf = —20 and —25 kJmol-1, respectively)82,83. Moreover,... 

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