Tertiary amines, from reductive alkylation amination

The reduction is usually effected catalytically in ethanol solution using hydrogen under pressure in the presence of Raney nickel. As in the reduction of nitriles (Section 5.16.1, p. 771), which also involves the intermediate imine, ammonia or the amines should be present in considerable excess to minimise the occurrence of undesirable side reactions leading to the formation of secondary and tertiary amines. These arise from the further reaction of the carbonyl compound with the initially formed amine product. Selected experimental conditions for these reductive alkylation procedures have been well reviewed.210 Sodium borohydride has also been used as an in situ reducing agent and is particularly effective with mixtures of primary amines and aliphatic aldehydes and ketones.211... 

Even if the imine may not be isolated, the transient species may sometimes be trapped by reaction with a suitable nucleophile. This is the basis of the reductive amination reaction in which an amine is formed from the reaction of ammonia with a carbonyl compound in the presence of a reducing agent such as sodium borohydride or formate. Use of a primary or secondary amine results in the specific formation of secondary or tertiary amines respectively (Fig. 5-45). This synthetic method allows the preparation of high yields of amines, in contrast to the unselective and uncontrollable reaction of alkylating agents with amines. A specific example involving the preparation of a-phenylethylamine from acetophenone is presented in Fig. 5-46. 

Acylation-reduction converts ammonia to a primary amine, a primary amine to a secondary amine, or a secondary amine to a tertiary amine. These reactions are quite general, with one restriction The added alkyl group is always 1° because the carbon bonded to nitrogen is derived from the carbonyl group of the amide, reduced to a methylene (—CH2—) group. 

Reductive alkylation of ammonia may proceed under mild conditions over nickel catalysts. In examples using Raney Ni, temperatures ranging from 40 to 150°C and hydrogen pressures of 2-15 MPa have been used to obtain satisfactory results.3,4 In general, the reductive alkylation of ammonia with carbonyl compounds may produce primary, secondary, and tertiary amines, as well as an alcohol, a simple hydrogenation product of the carbonyl compound (Scheme 6.1). The selectivity to respective amine depends primarily on the molar ratio of the carbonyl compound to ammonia, although the nature of catalyst and structure of the carbonyl compound are also important factors for the selectivity. As an example, the reaction of benzaldehyde in the presence of 1 equiv of ammonia in ethanol over Raney Ni gave benzylamine in an 89.4% yield while with 0.5 molar equivalent of ammonia dibenzylamine was obtained in an 80.8% yield (eq. 6.1).4... 

Aliphatic and aromatic amines react with nitrous acid to form N-nitroso derivatives. For example, dimethylamine hydrochloride on treatment with sodium nitrite and hydrochloric acid is converted to nitrosodimethyl amine in 90% yield. In like manner, N-nitrosomethylaniline is synthesized from N-methylaniline in 93% yield. The ready formation of these derivatives and the easy reconversion to the amine by reduction affords an advantageous procedure for separating secondary amines from primary and tertiary amines, as shown in the synthesis of N-ethyl-m-toluidine and other N-alkyl derivatives by the alkylation of w-toluidine. ... 

Several methods are available for the preparation of tertiary amines from fatty acids. One of the oldest is the classical Leuckart reaction (32), which uses formaldehyde and formic acid for the reductive alkylation of a secondary amine. 

Several alkyltrimethylammonium salts were also examined in this study 53b The predominant reaction is reductive cleavage to give the alkane and tertiary amine. In the discussion of the mechanism of the reaction, it was suggested 53> that methyl and other primary alkyl groups cleave from nitrogen as carbanions while secondary and tertiary alkyl groups cleave as free radicals (see Sect. VI. C. for further discussion of radical cleavage reactions). 

Direct replacement of an amino group by hydrogen is possible only in certain cases. Such reductive fissions are particularly well known for tertiary amines and quaternary ammonium compounds and occur especially readily with allyl- and benzyl-substituted amines. In the last-mentioned cases the benzyl group is removed as toluene, and this reaction route is used for preparation of secondary from primary aliphatic amines the primary amine is first condensed with benzaldehyde, and the resulting Schiff base is reduced to the alkylbenzylamine this is converted by alkylation into the dialkylbenzyl-amine, from which finally the benzyl group is removed by catalytic hydrogenation 544... 

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