Aryls cycloheptatrienyl

The photo-oxidation of the aryl-substituted cycloheptatrienes 7-(/ -methoxy-phenyl)cycloheptatriene and 7-, 1- and 3-(/ -dimethylaminophenyl)cycloheptatrienes to the corresponding radical cations in de-aerated acetonitrile solution was accomplished by electron transfer to the electronically excited acceptors 9,10-dicyanoanthracene, iV-methylquinolinium perchlorate, iV-methylacridinium perchlorate and l,T-dimethyl-4,4-bipyridinium dichloride. In the case of l- p-methoxyphenyl)cycloheptatriene (62), deprotonation of the radical cation occurs successfully, compared with back electron transfer, to give a cycloheptatrienyl radical (63) which undergoes a self-reaction forming a bitropyl. If the photooxidation is done in air-saturated acetonitrile solution containing HBF4 and one of the acceptors, the tropylium cation is formed. Back electron transfer dominates in the / -dimethylaminocycloheptatrienes and the formation of the cycloheptatrienyl radical is prevented. 

Stabilization of an organic cation occurs whenever the electron-deficient carbon atom is conjugated with aryl groups or with atoms containing unshared pairs of electrons such as oxygen, nitrogen, or sulfur (7). Examples of cations which are useful in polymerization include triphenylmethyl (I), cycloheptatrienyl (tropylium) (II), and various alkyl (III), aryl (IV), and benzopyrylium (V, VI) derivatives. 

There are some amazingly stable cations. 1,2-Dipropylcyclopropenyl cation (IV) and aryl-substituted cyclopropenyl cations (Breslow, 1957 Breslow and Yuan, 1958 Breslow and Hover, 1960 Breslow and Chang 1961), the cycloheptatrienyl cation (V) (Doering and Knox, 1954 Turner et al., 1960), and the heptamethylcyclohexadienyl cation (VI) (Doering et al., 1958) are half-formed from their alcohols and/or olefins in the very dilute acids of pH 1-4. Tricyclopropylmethyl cation (VII) (Deno et al., 1963c) is half-formed from its alcohol in 22% H2SO4. 

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