Acridizinium cations

Although it is possible to have a polar cycloaddition with a cation containing only carbon and hydrogen, the majority of those which have been found to undergo 1,4-cycloaddition are aromatic quaternary salts. The acridizinium cation (1) used in the first polar cycloaddition reaction of a quaternary salt has been used in the largest number of polar cycloaddition studies to date. The acridizinium ion (1) is particularly suitable for such a study since it is easily prepared, is stable (permitting sealed... 

Table I records the results obtained in the preparation of 30 cycloaddition products from the acridizinium cation. As was demonstrated by Fields, Regan, and Dignan, even preparative experiments done at different temperatures and in different solvents are adequate to prove the inverse electron demand character of the reaction. Nucleophilic alkenes, like ketene diethyl acetal, reacted in minutes at room temperature while the strongly electrophilic alkene, tetracyanoethylene, failed to react under any conditions.

Cationic additions of cyclopropenes are illustrated by the rapid and highly stereoselective addition of cyclopropene and its 1-methyl derivative to the 6,11-positions of the acridizinium cation. A marked preference (80-90 %) is shown for the regioisomer 168 (R = Me) which results from electrophilic attack of the cation on 1-methylcyclopropene such that charge separation is maximized. 

In the same way, cyclopropene and 1-methylcyclopropene add to acridizinium cations 2 with stereoselectivity in the latter case which corresponds to initial electrophilic attack of the cation on the cyclopropene, i.e. the methyl group is predominantly at Cl2. ... 

In spite of the known tendency of norbornene and related systems to undergo rearrangements of the Wagner-Meerwein type during eleetro-philic addition, no such rearrangement was observed when norbornene underwent cycloaddition with the acridizinium or the. A/ -methylenium benzamide cation. As Schmidt correctly pointed out, this lack of rearrangement is an argument for a concerted reaction. Alternatively, if the cycloaddition is nonsynchronous, the time interval between step 1 and step 2 must be very short. 

Compounds with bridgehead nitrogen atoms between two six-membered rings are necessarily cationic and are therefore very unreac-tive. The positions in these molecules that are best conjugated with the positive nitrogen [e.g., the 4- and 8-positions in acridizinium (11.11)] will be the most strongly deactivated. 

The quaternary salts of A -heterocycles are readily reduced and often give reversible one-electron reduction waves, as, for example, with pyridium salts [122]. Dimers have been obtained by this pathway e.g. from acridizinium and benzothiazolium salts [116]. Such processes are particularly important both with biological substrates (e.g. NAD is reported to give essentially 4,4 -dimers on reduction) [123] and in the monoelectronic reduction of di-cations such as dipyridinium and a variety of diaza polycyclic aromatic di-cations [124]. 

Reduction of acridizinium ion (106) and substituted acridizinium ions in MeCN or DMF gives a dimer ( 80%) [306]. Although not confirmed experimentally, the most likely positions for coupling are indicated. The results of LSV measurements were in agreement with rate-determining dimerization of neutral radicals, and for 106 a lower limit for the rate constant of 10 M s was obtained by CV measurements [306]. The dimer could be quantitatively reoxidized to the substrate cations either electrochemically (at a potential 0.5 V anodic relative to the initial reduction peak) or by action of oxygen [306]. 

The ideas described above were applied in Ref. 6 to the analysis of experimental data on complex conductivity of TCNQ salts with asymmetric cations - quinolinium /Qn/ and acridizinium /Adz/. It was supposed that the structural disorder caused by randomly oriented cations produced the localization, and the temperature dependence of 6 was attributed to the effect of phonons on the localization. 

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