A-Imines, heteroaromatic

Heteroaromatic A-imines undergo 1,3-dipolar cycloaddition reactions across the A-imine nitrogen atom and the a-position of the ring. With nitriles, triazolopyridines are formed by dehydrogenation of... 

A common method of preparation of heteroaromatic A-imines is the deprotonation of the corresponding A-aminoimmonium salts (12). Most of the pyridine A-imines synthesized have been obtained by this method. The strength of base necessary for deprotonation depends on the substituent R in the salts (12) (see Section III,F). For instance, dilute aqueous alkali is sufficient for A-acyl-, Ar-sulfonyl-, and A-carbamoylimines, but concentrated alkali must be used for unsubstituted A-imines. Unsubstituted pyridine A-imines have only been detected by spectroscopic methods in solution (see Section III,B, and III,D). 

Amino-1,2,4-triazoles (50) or their derivatives are usually starting material for the synthesis. They are readily available and already contain the exocyclic N-N bond required for heteroaromatic A-imines. 4-Amino-1,2,4-triazoles can be quaternized by alkyl halides or tosylates at the N-l atom to give the salt 52.79-82 (Scheme 5) The orientation of quatemization is proved by the reactions in Scheme 6 for the example of the quaternary acylamino salts 52. Quaternary salts of the type 52 can also be prepared by reaction of 1,3,4-oxadiazolium salts (51)83 with aryl hydrazines84 and from aryl hydrazine hydrohalides and orthoesters.86 With alkali, the 1-alkyl-s-triazole-4-imines8(7-82 86 can be obtained in the normal manner from these salts (Scheme 5). The free A-imines are all stable except the A-unsubstituted compound itself.87 Recently, other structures were tentatively reported 88 for the deprotonation products of analogous quaternary salts (52) with hydrazine. 

Systematic investigations of the mass spectrometric fragmentation of heteroaromatic A-imines show a difference between the behavior of five-and six-membered ring compounds. 

As previously pointed out, heteroaromatic A-imines contain the structural element of an azomethine imine. A typical reaction of this class of compound is 1,3-dipolar cycloaddition.121139 In heteroaromatic A-imines, such reactions are difficult since they involve loss of the aromaticity of the heterocyclic ring. Factors essential to the success or failure of a 1,3-dipolar reaction are the aromatic character of the parent heterocycle, the electron effect of the substituent R, and the nature of the dipolarophile. 

Heteroaromatic A-imines are photochemically active compounds (see Table III) On irradiation they generally yield products of (a) N-N bond cleavage, (b) ring enlargement, (c) rearrangement. However, the available data do not permit definition of the conditions necessary for selective reactions. Apparently unimportant changes in the substituents and the solvent may lead to different results. Therefore, the photochemical reactions will be discussed separately for the individual A-imines. 

Seven-membered rings are featured in three of the chapters of the present volume, namely, the benzazepines (S. Kasparek), 1,5-benzodiazepines (D. M. G. Lloyd and H. P. Cleghorn), and 2,3-dihydro-l,4-diazepines (D. M. G. Lloyd, H. P. Cleghorn, and D. R. Marshall). Recent advances in oxazole chemistry are described by R. Lakhan and B. Ternai, and H.-J. Timpe surveys the heteroaromatic A-imines. The final chapter is a review of aromaticity (M. J. Cook, A. R. Katritzky, and P. Linda), and it concentrates on the heterocyclic aspects of this controversial subject. 

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