2.3- Dihydro-l,4-diazepines and

A novel preparation of the 1,4-diazepine system (174) has been reported through isomerization of the Schiff base (175).186,187 This same type of diazepine (176) was prepared by the reaction of acetylacetone and meso-l,2-diphenylethylenediamine.188 Spectroscopic studies indicated that it was in the form shown. The NMR spectra of these 2,3-dihydro-l,4-diazepines and their cations have been discussed.187 The Schiff base of benzaldehyde and 1,2,3-triaminocyclopropane also isomerizes to give 174 (R = C6H5, R = C6H6CH=N).186 187,189 The mass spectrometric fragmentation of compounds of the type 176 has been reported.180a... 

Lloyd, D., McNab, H., 2,3-Dihydro-l,4-diazepines and 2,3-Dihydro-1,4-diazepinium Salts, 56,1. 

Throughout this article, the terms dihydrodiazepine and dihydro-diazepinium are used exclusively to refer to 2,3-dihydro-l,4-diazepines (1) and their monocations (2), respectively. Spectroscopic data show that the dihydrodiazepines normally exist in the conjugated form (1) rather than in the tautomeric bisimino form (3). 

Perhaps the most consistently satisfying and enjoyable feature of the chemistry of 2,3-dihydro-1,4-diazepines and 2,3-dihydro-l,4-diazepinium salts has been their ability to provide, quite consistently, unexpected results and products. There is every hope that these compounds will continue to maintain this tradition and provide interesting new facets in their chemistry in the future. 

Volume 56 of Advances in Heterocyclic Chemistry consists of four chapters. Drs. Douglas Lloyd and Hamish McNab from Scotland have provided an updated account of the chemistry of 2,3-dihydro-l,4-diazepines, a subject which has not been specifically reviewed for eighteen years and one with which these authors have been intimately concerned. 

In several synthetic studies, cyclopropane derivatives were used as synthones or building elements for ring enlargement steps, e.g. reaction of enamines with cyclopropenone [65], synthesis of 2,3-dihydro-l,4-diazepine by thermal isomerization of 1,2-diamino-cyclopropanes [32] [66], and preparation of 3-amino-fulvenes from methylencyclopropenes with alkynamines [67]. 

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. 

The 1,2- and 1,4-diazepines are of great importance, especially the 1,4-diazepine derivatives 2,3-dihydro-l,4-diazepine 4, 1,5-benzodiazepine 5 and 1,4-benzodiazepine 6 ... 

Dihydro-l,4-diazepines 4 are formed from ethylenediamine and y -diketones by acid-catalysed cyclocondensation ... 

Brisander, M., Harris, S.G., Lloyd, D. et al. (1998) Diazepines. Part 30. A comparison between the extent of delocalization of electrons in a vinamidine and its protonated form. Crystal and molecular structure of two 2,3-dihydro-l,4-diazepines. Journal of Chemical Research, Synopses, 2, 72-73. 

Electron impact mass spectrometry of a series of 2,3-diaryl-2,3-dihydro-l,4-diazepines (16) showed an intense molecular ion peak and in all the cases studied the initial fragmentation step was the same, namely cleavage of the N(l)—C(2) bond with the loss of an ArCH=N radical <65CB3479>. The other major peaks arose by cleavage of the initially formed cation fragment (Scheme 1). 

Dihydro-l,4-diazepines will perform 1,3-dipolar cycloaddition reactions with nitrile oxides and nitrile imines but it is only the imine bonds in the diazepine that react. Thus, 2,3-dihydro-5,7-diphenyl-1-methyl-17/-1,4-diazepine (64 R = Me) only gave mono adducts (65) with arylnitrile... 

Ethylenediamine (340) and its iV-substituted analogues with 1,3-dialdehydes or -diketones give 2,3-dihydro-1,4-diazepines, e.g. (341) (78H(ll)S50). l,4-Diazepin-5-ones, e.g. (342), can be readily prepared by the reactions of 1,2-diamines with (3-keto esters (67AHC(8)2l, p. 57). Similarly reactions with malonic acids and esters give 1,4-diazepine-5,7-diones (343) (67AHC(8)2l, pp. 55,69,68CRV747, p. 78i). 

---