Azomethine imines dimerization

Compounds 332a, b possess an azomethine imine structure and should undergo cycloaddition reactions.4208 Although dimerization of 332b has not been observed,335 dipolarophiles yield cycloadducts 444 and Michael adducts 445.420a The structure of the product obtained from methyl propiolate, an unsymmetrical acetylene, has been studied by NMR and shown to be 446.420b... 

The reaction of aldehydes with disubstituted hydrazines gives, via the addition product (380), the azomethine imine intermediate (381), which dimerizes to the hexasubstituted hexahydrotetrazine (377 Scheme 21). 

Dimerization of the latter to a dihydrotetrazine comprises the major reaction pathway, however photolysis of the dihydrotetrazine to benzonitrile and 3,5-diphenyl-1,2,4-triazoIe also occurs. The overall reaction therefore resembles thermolysis but is more selective. Photolysis of 2,5-diphenyltetrazole in the presence of dipolarophiles methyl fumarate and dicyclopentadiene gives 1,3 dipolar addition products supporting the intermediacy of the azomethine imine. 

The non-classical system (182) has been synthesized, starting from 4,5-bis(chloromethyl)-l,2,5-thiadiazole, and characterized by the isolation of its dimer and adducts. The more complex but stable non-classical thiophens (183)—(185) have been prepared, starting from azomethine imine ylides, generated in situ from 1-aminopyridinium, 1-aminoquinolinium, or... 

Similarly, heating of methyl 3-acyl-l-(diphenylmethyleneamino)-4,5-dioxo-4,5-dihydro-l//-pyrrole-2-carboxylates 96 gave the corresponding dimethyl 2,8-diacyl-3,9-dioxo-5,5,ll,ll-tetraphenyl-3,5,9,lTtetrahydrodipyrazolo[l,2- 7 l, 2 -i [l,2,4,5]tetrazine-l,7-dicarboxylates 99 in good yields. The proposed mechanism involves thermal decarbon-ylation to give the ketene intermediate 97, which cyclizes into the azomethine imine 98, followed by dimerization (Scheme 24) <2004T5319>. 

Methyl (naphthylideneamino)acetate undergoes dimerization to produce a mixture of two diastereomeric imidazolidines when treated with Mg(C104)2 or C0CI2 (89). Other imines can also be used as acceptor molecules (Scheme 11.12). With the exception of azomethine ylides incorporating sodium and titanium ions, other N-metalated ylides undergo highly endo-selective cycloadditions with a,p-unsaturated... 

Upon pjo-olysis (315 C) bicyclic triazolines like (210) afford various mixtures of imines (211) and bicyclic aziridines (212), depending on the size of the condensed ring (see Table 19) (68T2757). The aziridines formed can be considered as potential azomethine ylides, and can also dimerize under the conditions of thermoljreis (79BSF(2)633>. The preparative aspect of these extrusion reactions is discussed in Section 4.11.6. 

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