Formaldehyde imines trimerization

The propensity of formaldehyde imines (22) to trimerize to give hexahydro-l,3,S-triazines (24) has limited their synthetic utility. Sekiya and cowoikers and Bestmann et alP have shown that iV, -bis(tri-methylsilyl)methoxymethylamine (75) is a useful synthetic equivalent of formaldehyde imine. Treatment... 

In contrast to the behavior of alkylamines231 or trimethylsilylmethylamine232, the imine formed from the reaction of BSMA with aqueous formaldehyde does not trimerize... 

Ammonia reacts with formaldehyde to give hexamethylenetetramine (3.13) and with acetaldehyde to give a trimer (3.14). Other more steric-ally hindered imines remain as the monoadduct and do not add a. second molecule of amine. These imines are readily reduced by, for example, sodium cyanoborohydride to form amines or they may be hydrolysed to regenerate the carbonyl compound. 

Primary amines react with ketones and aldehydes to form imines. In the case of formaldehyde (R = H), these products are typically cyclic trimers. 

Reaction of trimer 28 of the imine obtained from formaldehyde and the appropriate arylglyci-nate with the acid chloride-pyridine complex 27 gives 87% of the cycloadduct 29 in 50% de71. Reaction of azidoketene with imine 30, obtained from benzaldehyde and alanine tm-butyl ester, gives a 1 1 mixture of ds-J -lactams 31 and 3272. 

Monoimines derived from most aldehydes and ammonia undergo a related, extremely rapid trimerization to produce 2,4,6-trisubstituted 1,3,5-hexahydrotriazines. These reactions also proceed by additions of an imine 1,2-dipole to itself (Refs. 89,90). The reaction of amines with aldehydes to form N,N, N -tiisubstituted-1,3,5-hexahydrotriazineshasaIso been observed amines include anilines and benzylamine. However, with most amines the reaction is limited to reactions with formaldehyde (Refs. 113-116). Only methylamine and allylamine have been observed to produce... 

The reaction of aldehydes and ketones with ammonia (NH3) itself is shown as the first item in Table 9.4. Despite the apparent simplicity of the process shown in Scheme 9.65, the reaction of aldehydes and ketones with this nucleophile (base) often produces a complex mixture of products because the addition process does not stop after the first step. For example, methanal (formaldehyde, H2C=0) yields hexamethylenetetramine (Table 9.4) and ethanal (acetaldehyde, H3CCHO) provides the corresponding C-methylated trimer. However, diarylketones (At20=0) often give stable imines (Equation 9.51). 

An objection to the above proposals for the formation of network structures by the interaction of urea and formaldehyde is that they involve the participation of imido hydrogen atoms, which are known to be very unreactive towards methylol groups. In order to avoid this criticism, it has been suggested [4] that acidification of methylolureas yields methylene-imines which trimerize to give substituted trimethylene triamine compounds which then polymerize to a network structure by interaction of methylol and amine groups ... 

Addition products of aldehydes and ketones with ammonia and amines (hemiaminals) are unstable compounds that are then stabilised by a number of subsequent reactions (Figure 8.28). Dehydration of addition products yields imines. Other possible products are enamines, amines containing a double bond linkage -CH=CH—NH- (Figure 8.29), cycHc trimers (symmetrical 1,3,5-triazines) and various coloured polymers, generally of unknown structures. Reaction of the addition products with another amine molecule gives rise to aminals, reaction with another molecule of the carbonyl compounds yields a,a -dihydroxyamines. For example, in addition to 1,3,5-triazines, it is mostly polymers that are formed from aliphatic aldehydes and ammonia or amines. The reaction of formaldehyde with ammonia yields hexamethylenetetramine. 

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