Aniline derivatives reaction with acetaldehyde

The arylation of acetaldehyde enolate by iodobenzene17-128 or p-iodoaniline73 failed, and the reduced products, benzene or aniline were obtained. In the reaction of aldehyde enolates with o-substituted derivatives, however, arylation does occur and in these cases subsequent cyclization reactions usually take place (see Section 2.2.4.2). The special effect of the o-substituents in facilitating reactions with aldehyde enolates is shown by the rare example of simple substitution (i.e. without concomitant cyclization) in equation (34).71... 

In a reaction which is mechanistically related to the Skraup reaction an a,/ -unsaturated carbonyl compound, generated by way of an acid-catalysed aldol condensation, reacts with a primary aromatic amine in the presence of acid to yield a quinoline derivative (Doebner-Miller reaction). For example, when aniline is heated with paraldehyde (which depolymerises to acetaldehyde during the reaction) in the presence of hydrochloric acid the final product is 2-methyl-quinoline (101) (quinaldine, Expt 8.40). Retrosynthetic analysis for the 1,2-dihydroquinoline reveals crotonaldedhyde as the unsaturated carbonyl component which is in turn formed from acetaldehyde (see Section 5.18.2, p. 799). 

Reaction of a primary aromatic amine (such as aniline) with a carhonyl compound such as acetaldehyde, in the presence of acid, also generates a 2-alkylquinoline in the Doebner-Miller reaction. Reaction of aniline with acetaldehyde gave 262, but a subsequent aldol condensation with the imine derived from acetaldehyde (sec. 9.4.F.i) led to 263.1 Friedel-Crafts cyclization was followed by loss of aniline to give 264, and aromatization gave 2-methylquinoline (265). 

Quinoline was discovered in coal tar by Friedlieb Ferdinand Runge in 1834 it is present in concentrations of approximately 0.3%. Quinoline is recovered by extraction with sulfuric acid from the methylnaphthalene fraction of coal tar, followed by springing with ammonia and rectification of the crude base mixture. Quinoline can be synthesized by the Skraup method, by the reaction of aniline with glycerol (or acrolein produced from glycerol) and catalytic gas-phase reaction of aniline with acetaldehyde. Since the supply of the tar-derived material has been adequate for a long time, synthetic production is not warrented. 

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