Mesoionic Oxatriazoles

CN3O 0 N N N Mesoionic 3-phenyl-l,2,3,4-oxatriazole-5-phenylimine mesoionic 3-phenyl-l,2,3,4-oxatriazol-5-one... 

Scheme 6.22 Acylation and nitrosation of 5-amenate derivatives of mesoionic 1,2,3,4-oxatriazoles.

Scheme 6.23 Ring-opening reactions of mesoionic 5-olate and 5-amenate 1,2,3,4-oxatriazoles to nitrosohydrazine derivatives.

Two types of nuclei with an oxygen and three nitrogen atoms are possible, namely 1,2,3,4-oxatriazoles (1) and 1,2,3,5-oxatriazoles (7). The neutral aromatic species have not yet been reported but 1,2,3,4-oxatriazolium salts (2) and mesoionic species (3)-(6) are known. 1,2,3,5-Oxatriazolium salts (8) and mesoionic compounds (9)-(12) are not yet known but 1,2,3,5-oxatriazolines (13) and (14) have been reported. The two types of oxatriazoles will be discussed in two separate sections. 

The synthesis of mesoionic oxatriazoles was reported as early as 1896 <1896CB2161>, but the structure was first proved in 1964 <64JCS906>. The original method of preparation is still in use, but alternative procedures have been developed. Activity in the field peaked in 1979 and in the period... 

The mesoionic compounds can be named in several ways. The systematic name for structure (4) is a 3-substituted anhydro-5-hydroxy-l,2,3,4-oxatriazolium hydroxide. Similarly, structure (5) is named a 3-substituted anhydro-5-thiolo-l,2,3,4-oxatriazolium hydroxide, and structure (6) a 3-substituted anhydro-5-amino-l,2,3,4-oxatriazolium hydroxide. Frequently used names for structure (4) are 3-substituted 5-hydroxy-1,2,3,4-oxatriazolium hydroxide inner salt Chemical Abstracts), 3-substituted l,2,3,4-oxatriazol-5-ylio oxide, or 3-substituted l,2,3,4-oxathiazolylium-5-olate. For practical purposes, the term mesoionic l,2,3,4-oxatriazol-5-one is often used. Analogously, (5) and (6) are named mesoionic l,2,3,4-oxatriazol-5-thione and mesoionic l,2,3,4-oxatriazol-5-imine, respectively. 

The mesoionic 1,2,3,4-oxatriazoles are crystalline, polar compounds which are usually recrystallized from methanol. Melting points are mid-range, frequently from 100°C to 150°C. 

Several examples of the mesoionic 3-aryl-l,2,3,4-oxatriazole-5-oxide ring system 123 have been prepared by cyclisation of the arylhydrazones 122 these in turn are prepared from aryldiazonium salts and bromonitromethane <99KGS413>. 

This chapter covers the literature from mid-1995 to mid-2007. Recent advances include the preparation of the first A2-l,2,3,4-oxatriazolines 7 by intramolecular [3+2] cycloaddition (Section 6.08.9), synthesis of mesoionic derivatives 4 using bromonitroformaldehyde iV-arylhydrazones as the starting materials (Section 6.08.9), and detailed studies of the biologically active mesoionic oxatriazoles in the context of their nitric oxide donating capabilities (Section... 

When mixed with equimolar amounts of the dirhodium complex 22, the enantiomers of mesoionic 1,2,3,4-oxatriazoles 19-21 can be differentiated by proton NMR <2003MRC315>. The negatively charged exocyclic nitrogen atom provides the binding site to the rhodium complex <2003MRC921>. 

No molecular ions appear in the electron ionization (El) spectra of mesoionic 1,2,3,4-oxatriazoles 4 and 6 or their derivatives. Instead, [M+ —30] peaks are observed due to loss of nitric oxide <1979J(P1)747, 1999CHE363>. The typical fragmentation pathway for such compounds and their analogues is summarized in Scheme 1 <1984CHEC(4)579, 1996CHEC-II(4)679>. In their fast atom bombardment (FAB) spectra, intact molecular ion peaks are observed for the A2-l,2,3,4-oxatriazolines 23 114 [M+H] and 24 142 [M+H] <2002HAC307>. 

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