1.3.5- Oxadiazinium salts

As mentioned above, the polar 1,4-cycloaddition of N-acyliminium ions 14 and 18 to olefins gives 5,6-dihydro-4H-l,3-oxazinium salts 16 (Section fI,A,l), or forms the 1,3-oxazinium salts with acetylenes (Section III, A). The same ions 89 and 91 add nitriles to furnish 1,3,5-oxadiazinium salts 90 and 92, little investigated until now (65CB334 88ZOR230). Salts 92 are unstable and can dissociate at elevated temperatures with nitrile liberation. Therefore, they may serve as original reservoirs for active jV-acyliminium cations. 

The bands at 1640 and 1725 cm" in the IR spectra of 1,3,5-oxadiazinium salt (92) correspond to vibrations of O —C—N and C=N fragments, respectively (89ZOR2416). Unfortunately, the IR spectral characteristics are often not identified (i.e., the bands are simply listed) or are contradictory (84KGS953 93KGS537, 93KGS542). 

Pyrimidones (472) react with hydrazine hydrate at 130-140°C to give the 1,2,4-triazole (473) (83H(20)1243). 1,3,5-Oxadiazinium salts (475) with hydroxylamine give the 1,2,4-oxadiazoles (474) and with hydrazines form the 1,2,4-triazole derivatives (476). The substituents in these cationic species are usually aryl, restricting the appeal of these ring interconversions (65CB334,67CB3736). 

Benzoyl chloride condenses with benzonitrile or aryl cyanates in the presence of aluminum trichloride to give 1,3,5-oxadiazinium salts (Scheme 58) (67CB3736). 

It is known that A-chloromethyl benzamide 234 (i.e. derivative 216 of formaldehyde) reacts with benzonitrile in the presence of tin tetrachloride to yield 4//-1,3,5-oxadiazinium salt 235, the hydrolysis of which gives bis-amide 236131 (equation 67). 

In amidines transamination is possible,especially useful for such purposes are imidoylimid-azolides, e.g. (341 equation 172). In some cases it is usefiil to prepare amidines by ring opening of suitable heterocycles, which can be achieved by treatment with amines or other nucleophilic (basic) compounds. Examples are the synthesis of amidines firom 1,3,5-oxadiazinium salts (342), 3-amino-l,2-benzisothiazoles (343), 2-ethoxycarbonyl-3,l-benzoxazin-4(4//) one (344), 1,2,5-oxa-diazolo[3,4]pyrimidine 1-oxides (pyrimidofuroxans 345) and l,2,4,6-thiatriazenium-5-olate 1,1-dioxides (345) as shown in Scheme 58. 

This ring system is represented by the 1,3,5-oxadiazinium salts, which are diaza analogs of pyrylium salts. The parent compound is not known, nor are simple alkyl derivatives, but triaryl-and bis(dialkylamino)oxadiazinium salts are stable compounds. The salts contain a complex counterion, such as pentachlorostannate, hexachloroantimonate, tetrachloroferrate, tetrafluo-roborate or perchlorate. 

The reaction of dicarboxylic acid dichlorides with aryl cyanides in the presence of antimony(V) chloride gives his(1,3,5-oxadiazinium) salts 9a-l whose colors range from yellow to brown.67 The stability of the salts increases with the distance of the heterocyclic rings from each other. 

The more interesting reactions of 1,3,5-oxadiazinium salts with nucleophiles result in the formation of heterocyclic compounds. The nucleophile attacks C2 of the oxadiazinium ring ring opening and recyclization follow. Thus concentrated aqueous ammonia affords 1,3,5-tri-azines I.57... 

Oxadiazinium salt 1 is converted into the corresponding 1,3,5-thiadiazinium salt 2 by the action of hydrogen sulfide in acetonitrile in the presence of triethylamine.100... 

Ammonolysis of 1,3,5-oxadiazinium salts gives 1,3,5-triazines. Stable oxadiazinium salts can be formed by reaction of an aryl nitrile with an aromatic acid chloride and Lewis acid (AICI3, SbCls, FeCls, or SnCl4). Electron-withdrawing groups in the aromatic rings render the salts unstable, and they are difficult to isolate. The salts (221) were isolated and converted with ammonia into the triazines (222) (Scheme 68) <86MI 6l2-02>. 

Trimerization of nitriles, isocyanates, isothiocyanates, imidates, and carbodiimides all lead to symmetrical 2,4,6-trisubstituted 1,3,5-triazines (see Section 6.12.9.5). The use of lanthanide trifluoromethanesulfonate and ammonia as cocatalysts is claimed as a big improvement. The trisaminal of 2,4,6-triformyl-l,3,5-triazine is also useful for further derivatization to unusual structures (see Section 6.12.7.1). Treatment of a 1 1 pyridine/conc. ammonia solution of an aromatic aldehyde with excess Fremy s salt is another development. Separation of the amide coproduct was claimed to be easy. The synthesis fails with aliphatic aldehydes (see Section 6.12.9.5.4). Aminolysis of 2,4,6-triaryl-1,3,5-oxadiazinium salts gives symmetrical 1,3,5-triazines but the reactions are limited in that electron-withdrawing groups in the aromatic rings lead to instability and difficulty in separation of products (see Section 6.12.10.4). 

Great interest has been shown in the synthesis of 1,3,5-oxadiazinium salts (209) by the reaction of an aryl carbonitrile (2 equivs.) with an acyl halide (1 equiv.) in the presence of a Lewis acid, generally antimony pentachloride (Scheme 32). Stannic chloride, aluminum chloride and ferric chloride have also been employed but are less effective catalysts <1892CB2266, 56CB209, 65CB334, 67CB3736, 86MI 618-01>. 

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>. 

Lithiated 1,3-diazabutadiene 78 is known to react with 1,3,5-oxadiazinium salt 76d to furnish the oligonitrile 79 (Equation 12) <1994AGE2337>. 

Antimony pentachloride reacts with a-chloro isocyanates to form l-oxa-3-azabutatriene salts, R2C=N+=C=0 SbCle, 26, which undergo [2+2+2] cycloaddition reactions with disubstituted cyanamides to give 1,3,5-oxadiazinium salts 27. ... 

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