2-Azaallenium ions

A series of 2-azaallenium ions have been prepared and characterized including X-ray crystal structures.709,710 Charge delocalization and, consequently, structural features depend significantly on substituents. In many cases, the contribution of the allenium ion form 399a is more significant. 

A simple approach to the 2-azaallenium ions 361, based on the reaction of carbonyl compounds with chlorocarbonyl isocyanates 360, was described152. No mechanism for this reaction was suggested, but the scheme displayed in equation 101 can apply based on the considerations stated above86. 

The rearrangements of the second type also proceed with the participation of the internal nucleophile but the latter was formerly the electrophilic moiety 356 which was added to carbonyl oxygen atom to form the carbenium center on the carbonyl carbon atom ". 356 contains the heteroatom having the n-electrons, and the intramolecular nucleophilic attack of the carbenium center proceeds according to the general scheme shown in equation 98. A number of reactions, namely the conversion of ketones into chlorocarbenium ions 357 (equation 99) as well as reaction of carbonyl compounds with l-oxa-2-azaallenium ions 358 behaving like acyl cations giving 359 (equation 100) can be classified under this second type. 

The ethylene bromonium and 1-bromoethyl cations and their neutral and anionic counterparts have been the subject of a tandem mass spectrometric study of dissociation and gas-phase redox reactions. IR and Raman studies of the bioactive bromonium cation (19), as its hydrogensulfate salt, agree with the results of an X-ray structure determination, and theoretical calculations are also in agreement, except for the details of the NO2 groups. The azaallenium ion (22) is an intermediate in the photolysis of (20) (21) and (22) could both be seen. Flash photolysis of (23) leads to (24), (25), and (26), all of which could be trapped by nucleophiles (27) was not an intermediate. NMR lineshape analysis of the spectmm of (28) leads to reaction rate constants of formation for both the intimate ion pair (29) and the solvent-separated ion pair (30). ... 

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... 

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