Diazine oxide

In a similar way, oxaziridines open to nitrones and oxadiaziridines give diazine oxides <1970JOC2482> (see also Section 3.5.2.2). 

On heating 264 to its decomposition temperature (differently from ene/diazine oxides), [3+2] cycloaddition to oxatetrazolidine 265 was not found (Equation 33) <1998TL2303>. 

There are no general methods for the preparation of 1,2,6-oxadiazines, and 2,1,3-benzoxadiazines appear to be unknown. Mention of the 1,2,6-oxadiazine system was made in regard to diazine/diazine oxide thermolysis (Section 9.07.9.1.3), but the ring system was not prepared. 

Note The NMR spectra of cinnoline 1- and 2-oxides have been measured for comparison with those of related diazine oxides. 3-Methoxy-4-(o-nitroanilino)cmnoline 1-oxide (106) underwent deoxygenation to give 3-methoxy-4-(o-nitroanilino)cinnoline (107) (PCI3, CHCI3, reflux. 

The cleavage of fused pyrazines represents an important method of synthesis of substituted pyrazines, particularly pyrazinecarboxylic acids. Pyrazine-2,3-dicarboxylic acid is usually prepared by the permanganate oxidation of either quinoxalines or phenazines. The pyrazine ring resembles the pyridine ring in its stability rather than the other diazines, pyridazine and pyrimidine. Fused systems such as pteridines may easily be converted under either acidic or basic conditions into pyrazine derivatives (Scheme 75). 

Oxidation of A-aminoazetidines (19), deoxygenation of A-nitrosoazetidines (20) and direct deamination of azetidines (21) with difluoroamine leads to cyclopropanes (23) by extrusion of nitrogen from a diazine intermediate (22) (63JA97). A further interesting ring contraction occurs in the Ag" catalysed solvolysis of the A-chloroazetidine (24), which appears to involve the intermediate cation (2S) (7ITLI09). 

Aspects of benzonitrile oxide cycloaddition to all three parent diazines have been investigated <96T6421>, and the stereochemistry of resultant biscycloadducts determined <96JCR(S)220>. 

The ylide 44, prepared from the corresponding diazine and tetracyanoethylene oxide, rearranges in methanol the give the 1,3-diazepine 45 <96TL1587>. The x-ray geometry for 45 is reported. 

As with other diazines, addition of benzonitrile oxide to pyridazines results in the formation of angular-fused tricyclic products C1995T11855, 1996T6421>. 

Numerous examples of rearrangement in related systems have been reported. Oxazepines 77, for example, have been isolated as the major products of irradiation in benzene of 9-cyano- and 9-chloroacridine 10-oxides (78) and are again believed to arise via oxaziridines 7965 products arising by further irradiation have also been described.66 Diazine N-oxides... 

A study of the a-arylation of diazine mono iV-oxides, under Heck-like conditions, also gave emphasis to pyrazines but a number of examples using pyrimidines and pyridazines were also described (Scheme 1). A wide range of aryl chlorides, bromides and iodides was used and the products were easily deoxygenated by catalytic reduction. An interesting feature was the use of a copper additive, which was only required for the pyrimidine reactions, to give a very substantial improvement in yield <06AG(I)7781>. 

Other methods for the generation of cation-radicals should also be mentioned. Mesoionic 5,5 -azinobis(l,3-diphenyltetrazole) and related mesoionic compounds give cation-radicals on oxidation by lead tetraacetate. The reaction proceeds in the presence of sodium tetrafluoroborate. The tetra-fluoroborate cation-radical salt is stable and can be purified by column chromatography on silica gel and stored in air for several months without appreciable decomposition (Araki et al. 1999). The cation-radicals of A,A -dimethyldiazines are produced by usual oxidation of the parent diazine... 

Electrophilic attack of a metal complex on one of the nitrogen atoms of 1,2-diazines has been reported to occur in the mechanism of new metal mediated methods to prepare C-N bonds. Pyrrolo-fused pyridazines and phthalazines for instance were synthesized via attack of the 1,2-diazine on a palladacyclobutane intermediate 34 formed via oxidative addition of an alkylidenecyclopropane to Pd(PPh3)2 (Equation 7) <2004JOC3202>. 

Diazine A -oxides can be regioselectively formed with H2O2 in formic acid as exemplified by the reaction of 2-chlorobenzo[/]cinnolines, 2-chloro-5,6-dihydrobenzo[/]cinnolines, and 3-chloro-9//-indeno[2,l-f]pyridazines <2000AP341>. This area was extensively discussed in CHEC(1984) <1984CHEC(2)1>. 

Oxidation of alkylthio- (or arylthio-)l,2-diazines to the corresponding sulfmyl and sulfonyl derivatives has been described in CHEC(1984) and GHEC-II(1996) <1984CHEC(2)1, 1996CHEC-II(6)1>. 

Dialkylphosphine oxides are another class of highly efficient ligands for coupling reactions using Pd or Ni (equation 85). Electron-poor fluoroazines and -diazines react already in the presence of NiClafdppp) , while hindered and electron-rich P(Bu-t)3 and (t-Bu)2P(S)H can be used for the coupling of unactivated aryl chlorides in the presence of Ni. 

A-phenyl-A -[4-chlorophenyl]diimide A -oxide. The term diazine has also been used instead of diimide [6],... 

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