Oxazetes

Intramolecular ring closure by valence isomerization to a carbon in the 3-position is not common, but known l,l-di-t-butyl-2-nitrosoethylene, stable at room temperature, cyclizes at 220 °C to the stable 4,4-di-f-butyl(4iT)oxazete (75AG(E)70). 

Nitronate(47a) is not the only oxazete derivative. For example, sterically hindered nitroalkenes (42b-d) can be prepared by nitration and halogenation of readily available allenes (48). Compounds (42b-d) are rather smoothly isomerized into the corresponding four-membered cyclic nitronates (47b-d) by the first-order reaction equation (168). Storage of nitronate (47c) is accompanied by its slow transformation into acid chloride (47e) from which amide (47f) can be easily synthesized. 

It appears that the radical cation of nitroethylene in the Xsec time frame rearranges to ionized nitrosoacetaldehyde via the 4//-oxazet N-oxide61. The informative part of the spectra corresponds to the ions originating from CHO and H2CO losses from the molecular ion61. 

Several 4i/-l,2-oxazete structures have been assigned from spectral and chemical data. The IR band at 1585-1595 cm-1 is most characteristic, although 13C, NMR and MS have been carefully analyzed (80TL2025, 75AG(E)69). 

One example of decomposition of an oxazete shows identical behavior (Scheme 8) (80TL2025). 

The one case reported for hydrolysis of an oxazete gives an oxime ester however, it does not appear to be a synthetically useful transformation (Scheme 23) (80TL2025). 

The little known 4/7-1,2-oxazete N-oxide ring has also been prepared by cyclization (75AG(E)69), as has the parent l,2-di-r-butyl-l,2-oxazete and several derivatives (Scheme 66) (75AG(E)70). 

Geometrical parameters of 4H-1,2-oxazete 11 selected bond lengths (A) and bond angles at the HF/6-311G level... 

Oxazete-fV-oxide 15 has been postulated as the main intermediate in the mechanism of the unimolecular degradation of nitroethylene 14. The calculated barrier of 205.6 kj mol-1, for the reaction shown in Equation (1), at the B3LYP/6-311+- -G(df,p) level, is close to the experimental energy found in the gas-phase nitroethylene degradation (192.05 kj mol-1) <2001MC163>. 

In the study of the potential energy surface [B3LYP/6-311G+-h(d,p)] for the reaction of the ethynyl radical HC2 with nitric oxides NO and N02, Peeters and co-workers reported that a cyclic 1,2-oxazete intermediate such as 17 may be involved. In the reaction of HC2 and NO, the four-membered cyclic structure 17, precursor of HCN and CO, could reasonably be formed from 16 by means of a ring-closing reaction having an activation barrier of 18 kcal mol-1 (Scheme 2) <1998CPL91>. 

The pyrolysates obtained at 700-1100 K from 1,1,2-trichloronitroso-ethane 32 and having the ionic peaks at m/z 30 and m/z 62 were determined to be formaldehyde and cyanogen chloride by microwave spectroscopy the above fragments were supposed to be produced by cleavage of 4/7-1,2-oxazete 33, generated by intramolecular cyclization of 1-chloro-l-nitroso-ethene CH2 = C(Cl)-NO (Scheme 8) <2000MI177>. 

A futher possibility exists. It has been shown that nitrosoalkenes decompose, with formation of HCN, via the formation of an oxazete and retro-cyclisation31 (Scheme 6). 

H-Oxazet 4,4-Di-tert.-butyl-3-nitromethyl- -2-oxid E14b, 913... 

H-l,2-Oxazet 3-(Brom-nitro-methyl)-4,4-di-tert.-butyl- -2-oxid El4b, 914 (aus 1,3-Dinitro-Verb.)... 

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