Diazinium salts

Diazinium salts resemble pyridinium salts in their behavior. They form pseudo-bases with hydroxide ions which can disproportionate (e.g. 2-methylphthalazinium ion (199) — 2-methylphthalaz-l-one + 2-methyl-l,2-dihydrophthalazine) or undergo ring fission (e.g. 3-methylquinazolinium ion — (200). Aqueous acid converts (201) into (202), presumably by attack of a water molecule on a protonated species with subsequent intramolecular oxidative-reductive rearrangement of an intermediate carbinol base (201a) as shown. 

In the series of N-alkyl-1,4-diazinium salts 36-40, great attention was paid to the abilities of these compounds to form either mono- or diadducts. The stabilities of the c-adducts formed were varied to a great extent either by the introduction of an aza group and/or substituents into the pyrazine ring or annelation of the latter with a benzene, pyridine, or pyrimidine ring (86KGS1380). 

POLAROGRAPHIC REDUCTION POTENTIALS OF N-ALKYL-1,4-DIAZINIUM SALTS ... 

In the secondary addition of methanol to the monomethoxy adducts 42-44, their reactivities vary to the greater extent. It has been established by 1H-NMR spectroscopy that diaddition of sodium methoxide in methanol-d4 occurs at 20°C with the 1,4-diazinium salts 36e, 37a,c,f, and 40d (Scheme 40) (86KGS1380). Monosubstituted pyrazinium salts 36c,d as well as the pteri-dinium cations 40a,b do not produce any detectable quantities of the dimethoxy adducts 45. 

Starting N-alkyl-1,4-diazinium salt Monomethoxy adduct Dimethoxy adduct K (liter/mol)... 

As already noted, cycloadducts resulting from the diaddition reaction on quaternary 1,4-diazinium salts are able to dissociate to the starting materials. Sometimes the dissociation does not form the starting substrate but instead gives products different from the starting materials. For instance, dissociation... 

Thus, the diaddition reaction of bifunctional nucleophiles to alkyl-1,4-diazinium salts followed by the oxidation of cycloadducts formed provides us with a simple and effective route to preparation of polycyclic aza-aromatic compounds. 

Similarly, imines react with diazinium salt 83 to furnish the corresponding chain-elongated products 85 bearing an imine function in the oi-position (Equation 16) <1997TL5481>. 

Similar conversions of o -adducts have been observed in the series of 1,4-diazinium salts. For instance, the reaction of 7/-methylquinoxalinium ion with diethyl malonate in methanol in the presence of diethylamine results in the formation of the kinetically favored O- and N-adducts, which are gradually converted into the thermodynamically more stable C-adduct (Scheme 65) [11, 114]. 

Balan, A. M., Florea, O., Moldoveanu, C., Zbancioc, G., lurea, D., Mangalagiu, 1.1. (2009). Diazinium salts with dihydroxyacetophenone skeleton syntheses and antimicrobial activity. Eur. J. Med. Chem. 44,2275-2279. 

Diazinium, oxazinium, and thiazinium cations possess a pyridine-like nitrogen atom, but it is of very weak basicity and nucleophilicity. However, pyridazines do form diquaternary salts with very strong alkylating agents such as oxonium compounds. Electrophilic attack at ring carbon in these compounds is practically unknown. These trends are emphasized in cationic rings with an increased number of nitrogen atoms. 

In a reaction similar to that described for the preparation of 1,3,5-oxadiazinium salts (209) (Scheme 32 see Section 6.18.10.3.1.i) methyl thiocyanate in the presence of SbCh undergoes cyclocondensations with arylthiocarbonyl chlorides to give 2-aryl-4,6-bis(methylthio)-l,3,5-thia-diazinium hexachloroantimonates (221) (39 6%) <88LA729>. 

Many jr-deficient azaarrmatic compounds (pyridines, pyrimidines, pyrazines, tri-azines, pteridines, and so on) and especially their quaternary salts have a tendency to add water to give the corresponding o -adducts [1,2,11,114—117,138]. A similar process of reversible pseudobase formation due to the addition of the hydroxide ion to A-aUcylazinium [11], 1,4-diazinium [114, 138], and 1,2,4-triazinium [115-117] cations has been well documented in the literature. For instance, 3-phenyl-5,6-dicyano-l-ethylpyrazinium tetrafluoroborate easily adds water under basic conditions to give the corresponding 2-hydroxy adduct (Scheme 37) [138]. 

Also the radical species are easily formed on treatment of l-ethyl-2,3-dicyano-1,4-diazinium [188, 189] and 1-ethyl-1,2,4-triazinium salts [190] with nucleophiles, as evidenced by dimerization of pyrazinyl radicals into the corresponding dimeric structure (Scheme 63). It is worth noting that the synthetic potential of the intermediate radicals can be used as trapped with compounds bearing C-C double or triple bonds, for instance by reacting with allyl carboranes. The latter reaction is accompanied by the hydrolysis of one cyano group and results in the formation of the corresponding 2,5-diazabicyclo[2,2,2]octenes (Scheme 63) [189]. 

---