Primary amines Schiffs base formation

CHC(=0)Me, BuNH2, MeCN, reflux, 24 h 16% change in regioselectivity and preservation from deacylation may perhaps be explained by the possibility for Schiff base formation of the synthon and product, respectively, in the presence of the primary amine] 158 a plausible reason for the formation of mono- instead of di-lV-oxides was advanced.158... 

Aldehydes and ketones can react with primary and secondary amines to form Schiff bases, a dehydration reaction yielding an imine (Reaction 45). However, Schiff base formation is a relatively labile, reversible interaction that is readily cleaved in aqueous solution by hydrolysis. The formation of Schiff bases is enhanced at alkaline pH values, but they are still not stable enough to use for crosslinking applications unless they are reduced by reductive amination (see below). 

In enzymic decarboxylations the mechanistic pathway seems to involve Schiff base formation between an —NH2 from a lysine residue and a C=0 of the keto acid.52 Likewise, with small-molecule primary amines, catalysis of decarboxylation of /3-ketoacids53-58 has been ascribed to a Schiff base intermediate. The overall reaction for oxalacetate is... 

Naphthalene- and anthracene-derived phenols did, however, almost uniformly precipitate (Table VI). In natural materials (not grapes or wines) which contain them they would be included in the formaldehyde precipitable group. Several primary amines capable of SchifFs base formation reacted with formaldehyde to lose their F-C oxidizability, but only the resorcinol analog, 3-aminophenol, precipitated (Table VIII). Sulfite also reacted but did not precipitate with formaldehyde, and the F-C oxidizability was suppressed (Table IX). The resorcinol derivative, 2,4-dimethoxycinnamic acid, formed a precipitate with formaldehyde, but it did not react appreciably in the F-C assay. 

Melt poly condensation is also the most popular method for other thermotropic condensation polymers, including the polyazomethines where the reaction between aromatic aldehydes or ketones and primary amines with elimination of water leads to azomethine (Schiffs base) formation 48). 

Benzodiazocine 264 was prepared through a 4-component Ugi reaction including a primary amine tethered to a BOC-protected internal amino nucleophile, followed by a postcondensation base-catalyzed cyclization. Thus, 2 equiv of aldehyde 262 were employed to promote Schiff base formation and a one-pot, double scavenging protocol with immobilized tosylhydrazine and di-isopropylethylamine removed both the excess aldehyde and any unreacted acid 261. The intermediate 263 was then subjected to treatment with TFA, followed by proton scavenging with resin bound morpholine, to promote cyclization to afford the eight-membered ring (Scheme 47) <2001TL4963>. 

Schiff Base Formation. The Schiff base reaction (Scheme 11.1), in which primary amines (1 and 2) react with aldehydes (3 and 4) to give imi-nes (5 and 6), was chosen as a primary study of the microreaction unit and its coupling with MALDI-TOF-MS, because it is a straightforward reaction and products are obtained in high yields. The pH control required for the Schiff base formation29 turned out to be not necessary under lab-on-a-chip conditions as a consequence of the high surface-to-volume ratio that characterizes microreactors.36... 

An improved aldehydic component for Schiff base formation, and hence amine-to-carbonyl transformation, has been described already. The major fragmentation path in the photolysis of phenacyl derivatives of amines is Norrish Type II elimination, providing a further means of conversion of primary amines into carbonyl compounds (Scheme 126). 

The deamination of primary amines such as phenylethylamine by Escherichia coli (Cooper et al. 1992) and Klebsiella oxytoca (Flacisalihoglu et al. 1997) is carried out by an oxidase. This contains copper and topaquinone (TPQ), which is produced from tyrosine by dioxygenation. TPQ is reduced to an aminoquinol that in the form of a Cu(l) radical reacts with O2 to form H2O2, Cu(ll), and the imine. The mechanism has been elucidated (Wihnot et al. 1999), and involves formation of a Schiff base followed by hydrolysis in reactions that are formally analogous to those involved in pyridoxal-mediated transamination. 

The reductive alkylation of a primary amine with ketone leads to the formation of a stable imine. In the presence of hydrogen and a hydrogenation catalyst, the imine is reduced to a secondary amine. Similarly, a diamine reacts stepwise to form dialkylated secondary amines. However, several side reactions are possible for these reactions as outlined by Greenfield (12). The general scheme depicting the reaction between primary amine or diamine to yield secondary amine through a Schiff base is shown in Figure 17.1. 

The formation of diimine systems by Schiff -base-type condensation of suitable aldehydes and primary amines has been widely applied. Those reported are mostly strong field systems and their relevance to the spin crossover field is generally in systems of the kind [Fe(diimine)2(NCS)2]. The effect of the incorporation of substituents likely to hinder coordination has been studied. Robinson and Busch noted a fundamental difference at room temperature in the electronic properties of the [Fe N6]2+ derivatives of 2-pyridi-nalmethylhydrazone and 2-pyridinal-dimethylhydrazone, those of the former being low spin and those of the latter high spin [49]. The temperature-dependence of the magnetism of the latter complex was not reported but may well be of interest. However, spin crossover [Fe(diimine)3]2+ systems have been characterised for systems where the incorporation of appropriate substituents has reduced the ligand field. 

Formation of compounds like 115 seems to have occurred in the similar way as it was established for anilines and other primary amines [167]. The initial step of this reaction is treatment of aldehyde with aminozole giving Schiff base 116. Further, nucleophilic attack of imine carbon by mercapto moiety of the acid leads to the intermediate 117 and its subsequent cyclization via gem-diol 118 yields target heterocycles 115 (Scheme 55). 

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