1.3.5- Oxadiazinium ions

With the help of a combination of selective dissolution and chromatographic separation, several of the cyclic ethers have been separated and isolated <2003OL3745>. Though not of synthetic value, gas-phase cyclization reactions of acylium ions with nitriles, forming 1,3,5-oxadiazinium ions 375 by double nitrile addition followed by cyclization, have been reported (Scheme 75) <2001MI445>. Similarly, the gas-phase reactions of acylium and thioacylium ions with isocyanates (18 Y = 0) and isothiocyanates (18 Y = S) have been reported to result in oxadiazinium (16 X = 0) and thiadiazinium (16 X = S) salts, respectively (see Scheme 2) <2005JAM1602>. 

Gas-phase synthesis of 3,4-dihydro-2,4-dioxo-277-l,3,5-oxadiazinium ions (16 X = Y = O) via cyclization of acylium (17 X = O) and thioacylium ions (17 X = S) with isocyanates (18 Y = O) and isothiocyanates (18 Y = S) has been investigated using tandem-in-space pentaquadrupole mass spectrometry (MS) <2005JAM1602>. The formation of single (19) and double (20) addition products in these reactions was observed to occur concurrently with proton transfer. The products of double addition have been observed to be favored in reactions with ethyl isocyanate, whereas the reactions with ethyl isothiocyanate formed preferentially either the single-addition product or proton-transfer products, or both. Furthermore, ab initio calculations at the Becke3LYP//6-311- -G(dp) level indicate that cyclization of the double addition products is favored and 3,4-dihydro-2,4-dioxo-277-l,3,5-oxadiazinium ions (16 X = Y = 0) are formed (Scheme 2) <2005JAM1602>. 

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 reactive cation (101) may be generated either by protonation and dehydration of the A-hydroxymethylamide (100) or by the Lewis acid-catalyzed removal of a chloride ion from an A-chloromethylamide (102). Addition of the cation (101) to benzonitrile afforded a 2,6-diphenyl-4jy-l,3,5-oxadiazinium salt (103) in 39% yield, while addition to phenylacetylene gave 2,6-diphenyl-4.ff-l,3-oxazinium hexachloro-stannate (104) in 99% yield. 

The structures of the 2-oxazolinium, 1,3,5-oxadiazinium and 3-azapyrylium salts obtained by means of electrophilic catalysis by acylium ions102,113 point to the fact that their formation proceeds not via Af,7V-bis-acyliminium ions 332 and 333, but via the TV-acyliminium ions 322 (i.e. those protonated at the nitrogen atom). In our opinion142, the acetoxy azaallenium ions 335 can be transformed to the Af-acyliminium cations 337 by a deacylation to the N-acylimines 336, followed by protonation of the latter (equation 91). Such a process is quite possible under the conditions used for... 

Mass spectral fragmentation patterns have been used to determine the structures of the adducts formed by reaction of 2,4,6-tris(dimethylamino)-l,3,5-oxadiazinium chloride (7 R = Me) with primary alkylamines <85JCS(P1)1533>. Loss of Mc2N, and H, followed by a prominent alkylisocyanate ion favors strongly the 2-amino (32) rather than the isomeric 4-amino adducts. 

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