1.2- Diazetidines, formation

Simple aliphatic imines are often considered as being photounreactive. One of the few reported examples of a 1,3-diazetidine formation concerns an imine of difluoroacetone (4.30)433). 

The order of redox potentials for oxidation of (4) (F>C1 Br) has been reported and found most consistent with a detectable resonance contribution through the fluoro substituent being one carbon atom more removed from the double bond) is consistent with the Whiffer effect (er-hyperconjugative destabilization proceeding through two pathways is more than double the same effect through one pathway), in consonance with the AMI prediction. The facial selectivity of epoxidation and diazetidine formation from (4) proved to be in... 

A recent review on four-membered heterocycles formed from imino-phosphoranes concentrates on the preparation and the reactivity of 2,4-diimino-l,3-diazetidine and related compounds (93JPR305). As an example, the synthesis via bisiminophosphorane 85 is described in Scheme 42. The bisiminophosphorane has both a heteroaryl and a styryl site. From a mechanistic view, the reaction of the bisiminophosphorane proceeds with aryl isocyanate formation via an aza-Wittig mechanism. Intermediate car-bodiimide formation (86) occurs directly on the iminophosphorane moiety... 

Formation of Four-membered from Six-membered Heterocycles 5.15.3.8.1 1,2-Diazetidine derivatives... 

The aziridinofullerenes and azafulleroids obtained by Mattay et al. were subjected to further reactions to yield fullerene adducts of the second generation. As a model compound 4-methyl-l,2,4-triazoline-3,5-dione (NMTAD) 187, which is one of the most reactive dienophiles for Diels-Alder reactions, was selected. Sheridan et al. studied the photoinduced [4 + 2] cycloadditions of NMTAD to arenes [295], With alkenes, however, NMTAD reacts in photoinduced cycloadditions in a [2 + 2] fashion under formation of diazetidines [296], Both the sulfonyl-azafulleroids (185) and aziridinofullerene (186) were irradiated at 420 nm to give both the same [2 + 2] cycloadduct 186 (Scheme 71) [291], This indicates that at a certain stage rearrangements must be involved. 

The enthalpies of formation of 1,2- and 1,3-diazetidines have been determined from the calculated energies based on the reactions represented by the Equations (II) and (III) using different methods (see Table 12). For each molecule the predicted A//fS were fairly consistent from method to method. 

Table 12 Calculated enthalpies of formation AHfS fords- and trans-1,2-diazetidines, and cis- and frans-1,3-diazetidine (in kJ mol 1)...

In the case of 1,2-diazetidines, in which two nitrogens are bonded to each other, there was a marked difference in the enthalpy of formation between the as- and /ram-isomer. As expected, the more sterically hindered f/r-isomer had a slightly higher enthalpy of formation, about 20-30 kj mol-1. Similarly, for 1,3-diazetidines, although the f/r-isomer had a slightly higher enthalpy of formation than the tram-isomer, the difference (1-3 kj mol-1) is probably within the expected error limits of the method. 

Prinzbach and co-workers <2000EJ0743> carried out photochemical and thermal transformations of several diazetine derivatives in a strained cyclic system. They reported the formation of diazetidine 80 by photolysis of diazenelene 79 in acetonitrile. 

Auricchio et al. have proposed the formation of 1,2-diazetidine-l,2-dioxide as a possible intermediate in the thermal isomerization of furoxanes and benzofuranoxane derivatives (Scheme 34 and 35) <1997T17407>. 

Furin and co-workers prepared several fluorinated diazetines by the reaction of perfluoro compound 131 with isopropyl amine, /-butyl amine, 2-amino-6-bromobenzothiazole, and 2-amino-1-methyl imidazole in THF in the presence of triethylamine (Equation 32) <2000RJ0109>. The same group also reported the formation of several diazetidines 132 and 133 by the reaction of perfluoro compound 131 with 2-nitro-, 4-nitro-, or pentafluoro-aniline in the presence of triethylamine (Scheme 15, Section 2.13.7.2) <2000JFC(104)263>. 

It is expected that perfluoroaniline or nitroaniline first reacts on the C=N bond at the carbon center by the formation of a C-N bond followed by cyclization to furnish 1,3-diazetidines. 

Beckert and co-workers reported the synthesis of 1,2-diazetidine derivatives 246 by the reaction of bis-imidoyl choloride 244 with amidrazone 245 <2002H(57)1257>. The product underwent decomposition during the isolation. Thus, they carried out the isolation at lower temperature to furnish 246a (38%) and 246b (48%). On the other hand, reaction of hydrazine 247 with bis-imidoyl chloride 244 gave 1,2-diazetidines derivatives 248 (Scheme 37). Some of the structures were confirmed by X-ray crystallography (Section 2.13.3.1) The reaction is expected to proceed via the formation of a C-N bond with the displacement of a chlorine atom. 

The formation of diazetidines by [2+2] cycloaddition can be achieved by a thermal or photoinduced reaction. The reaction may proceed stepwise, either by dipolar or radical intermediates. A large number of 1,2-diazetidine derivatives have been prepared by [2+2] cycloaddition of an alkene to an azo compound. A large number of syntheses involving these types of fragments have been complied by Richter and Ulrich <1983HC(42)443> and also by Timberlake and Elder <1984CHEC(7)449>. 

In addition to a series of previously well-known reactions of 2-amino-l,3,4-oxadiazoles such as the formation of SchifFs bases or phthalamido derivatives,48-69,81,93 it has been possible to convert 2-amino-5-phenyl-1,3,4-oxadiazole by addition of isocyanate via the corresponding urea into (5-phenyl-l,3,4-oxadiazol-2-yl)diazetidine-diones (45).128... 

Diazetidine-2,4-diimines, carbodiimide dimers, formation, reactions, 59, 133, 148 ... 

Nitrogen substituted carbodiimides are usually not stable in the carbodiimide configuration. Some derivatives can be distilled under vacuum, but on standing they undergo slow dimerization reactions. In one case of N-heterocyclic carbodiimides the 1,3-diazetidine diimine dimers are obtained instead of the monomers. Semiempirical calculations on the formation of 1,3-diazetidine diimine dimers of H2N—N=C=NMe confirm the Z,Z and E,E configurations of the dimers obtained. N-nitrosubstituted carbodiimides are more stable. 

Reaction of aryl vinyl ethers, however, invariably gives mixtures of diazetidines and oxadiazines. The ratio is mainly dependent on the nature of the substituent on the phenyl group and, in part, on the nature of the alkyl group of the dialkyl diazenedicarboxylate and the solvent substituents which enhance the nucleophilicity of the alkene and the electrophilicity of the diazene, and polar solvents that favor a polar mechanism, result in concurrent formation of the diazetidine as the predominant product and vice versa15, l< Similarly, diazetidines 7 and oxadiazines 8 are formed in comparable yields in the reactions of vinyl sulfides with dimethyl diazenedicarboxylate, the ratio is principally dependent on the nature of the sulfur substituent (aryl, alkyl)17. 

Acyclic enamines give diazetidines with dimethyl diazenedicarboxylate, e.g., formation of 9. when hydrogen abstraction and ene reaction can be suppressed18, but from the less nucleophilic A -vinylpyrrolidinone the oxadiazine 10 is obtained16. 

Th only general method for the formation of the diazetidine ring ie the cyoloaridition of azo compound and unsatuTated oom-pounde with a highly reactive double bond. bi aU but one ease the olefin used has been a keteae, and the products are therefore diazetidi-nonee (CXXIII, Eq. 70) the reaction ia analogous to the preparatioti of /Sdactams from hetenes and azomethines. 

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