N-Imines, heteroaromatic

The mechanism of attack of 1,3-dipolar reagents on fluoroalkenes can be considered to be either stepwise or concerted. Heteroaromatic N-imines react by a stepwise 1,3 addition to perfluoroalkenes and -alkynes to give fluorinated pyrazolo[l,5-a]pyridines [82JCS(P1)1593]. Pyridinium /-butoxycarbonylmethylide with fluoroalkenes gave pyrrolo[l,2-a]pyri-dines [86JCS(P 1) 1769] and indolizines (22) are obtained with pyridinium phenacylide [91JFC(51)407]. 

Heteroaromatic N-Imines H.-J. Timpe, Adv. Heterocycl. Chem., 1974,17, 213-255. Chemistry of the Heterocyclic N-Oxides , A. R. Katritzky and J. M. Lagowski, Academic Press, New York, 1971. 

Tamura, Y., Ikeda, M., Advances in the Chemistry of Heteroaromatic N-Imines and N-Aminoazonium Salts, 29, 71. 

Heteroaromatic N-imines and N-aminoazonium salts 82AHC(29)71. Hetarylcarboranes 83KGS579. 

The ultraviolet (UV) absorption spectra of the heteroaromatic N-imines are dependent upon many factors, such as the nature of the parent heterocycles, substituents on the imino nitrogen, and solvent used. Usually pyridine N-acylimines show two intense absorption maxima at 230-235 and 330-360 nm in aprotic solvents such as dioxane, benzene, or chloroform, the latter band of which disappears by addition of acid.13,20,21,75,77 In protic solvents such as alcohol or water a hypsochromic shift and a hypochromic effect of the long-wavelength absorption are observed for example, pyridine N-benzoylimine shows a maximum at 352 nm (e 9700) in dioxane but at 313 nm (e 4650) in ethanol. 13,21 The long-wavelength absorption of pyridine N-ethoxycarbonylimine has been ascribed to a n-n transition on the basis of a Pariser-Parr-Pople (PPP) calculation and measurements of the absorption spectra of preoriented pyridine N-imines in polarized UV light.125... 

Heteroaromatic N-imines and IV-aminoazonium salts show a variety of reactivities, depending on the nature of the heteroaromatic ring and the substituents on the imino or amino nitrogen. The most important types of the reactions are (i) reactions with electrophiles at the imino or amino nitrogen, (ii) reactions with nucleophiles on the heteroaromatic ring, (iii) 1,3-dipolar cycloaddition, (in) 671-electrocyclic reaction of 1,5-dipoles (mainly thermal reaction), (n) 47t-electrocyclic reaction (mainly photochemical reaction), and (vi) N—N bond cleavage (by thermolysis, photolysis, oxidation, and reduction). 

Since the JV-imines are generally unstable, the reaction is usually carried out by treating the JV-aminoazonium salts with base in the presence of 1,3-dipolarophiles. The versatility of this reaction is demonstrated by application to a wide range of the six-membered heteroaromatic N-imines which include substituted pyridine JV-imines,187 di-JV-imines of 2,2 -bipyridyl and... 

The major reaction of five-membered heteroaromatic N-imines (1-alkyl-l, 2,4-triazole 4-acylimines254 and 4-arylimines,255 2-phenylbenzotriazole 1-cyanoimine,97 thiazole JV-acetylimine,239 and 1-alkybenzimidazole 3-acylimines122) is N—N bond fission, with a few exceptions. In the case of 1-methylbenzimidazole JV-ethoxycarbony limine, a 1,2-migration of the ethoxycarbonylamino group has been observed.122... 

Volume 29 consists of six contributions. Three of these deal with specific groups of compounds and update previous contributions in this publication on the same type of compounds. Thus, R. K. Smalley, who writes on The Chemistry of Indoxazenes and Anthranils 1966-1979, updates the article by Wunsch and Boulton published in 1967 in Volume 8. Raymond Bonnett and Stephanie A. North, writing on The Chemistry of the Isoindoles, update the contribution by White and Mann, published in Volume 10, in 1969, and Richard M. Scrowston, with his review entitled, Recent Advances in the Chemistry of Benzo[fe]-thiophenes, updates that by himself and Iddon in Volume II, published in 1970. The chemistry of Furoxans and Benzofuroxans is covered by A. Gasco and A. J. Boulton. This and the subject of Mononuclear Heterocyclic Rearrangements reviewed by Michele Ruccia, Nicolb Vivona, and Domenico Spinelli, are both topics close to the hearts of the Editors of this volume. Finally, Yasumitsu Tamura and Masazumi Ikeda have summarized Advances in the Chemistry of Heteroaromatic N-Imines and N-Aminoazonium Salts. Although this subject was reviewed as recently as 1974, in Volume 17 of Advances, its growth since then has been so dramatic that this further review is very timely. 

Unlike some aliphatic amine N-imines,6 heteroaromatic N-imines are always alkylated at the nitrogen atom of the imino group,11 1214 34 37 72 - 86 - 87 [Eq. (10)] although mesomerism also affords other points of attack (see Section III,A). The alkylation of unsubstituted pyridine N-imines with polynitrohalobenzenes yields N-(polynitroarylimines) (see Section II, A,4). 

The earlier reviews described a number of synthetic approaches to this system. The most generally useful, however, were observed to be those starting with A -aminopyridinium derivatives and constructing the five-membered ring. The chemistry of A -aminoazonium salts, and heteroaromatic N-imines in general, has been reviewed <81AHC(29)73>. Syntheses reported since the publication of the first edition continue to reflect the above-mentioned observation. 

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