Alkylbenzenium Ions

A valence isomerization reaction similar to that encountered with the tropylium ion has been observed when a variety of substituted benzenes are photolyzed in strong acid. The pentamethylbenzenium ion (18) photolyzed at —78° to give a single product, the pentamethylbicyclo-[3,l,0]hexenyl cation (19), in excess of 80% conversion (Childs et al., 1968). 

It is of substantial interest to note that, r.s the temperature of the reaction mixture is increased to — 33-5°, ion 19 is converted quantitatively back to 18. At that temperature the first-order rate constant for the reversion has been calculated to be 8-0 x 10 sec , which corresponds to a free enthalpy barrier AG ) of 17-4 kcaJ/mol. 

The hexamethyl- and heptamethyl-benzenium ions (20 and 21) irradiated under the same conditions gave the corresponding bicyclic [3,1,0] products (22 and 23) (Childs and Winstein, 1968). Of the two conceivable stereoisomers which can arise from 20, only one is observed. 

This has been identified as the endo isomer, i.e. the one in which the Cg methyl group is inside (Koptyug et al., 1969). As in the ease of 19, when 22 is warmed to — 34°, a clean first-order reversion to the hexa- 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a S3rmmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2 +TT2 ] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSOg at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [a2g, + ir2 ] cycloaddition, the expected 

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We have seen that carbonium ions can undergo a variety of photoreactions, affording products which often vary considerably from those obtained in the photolysis of the corresponding uncharged compounds. The predominant mode of reaction encountered would seem to be isomerization to one or more valence bond isomers, which occurs via a symmetry-allowed disrotatory electrocyclic closure, rather than a [<72a-f 7r2a] cycloaddition in the case of alkylbenzenium ions and pro-... 

When discussing isomerization of alkylbenzenes, it is useful to recollect that the alkylbenzenium ion (a complex or arenium ion) intermediates involved are identical to those of the alkylation reaction of aromatics ... 

Cycloheptatriene, Norbomadiene, Methylenecyclohexadienes (Isotoluenes) and Bicy-clof3.2.0/heptadienes. The gas-phase ion chemistry of ionized 1,3,5-cycloheptatriene is closely related to that of ionized toluene, in particular, and to that of norbomadiene and other non-aromatic ( yllx isomers. This extensive body of work will not be discussed here since a detailed review on this topic has been published by one of these authors in the context of the gas-phase chemistry of the alkylbenzene radical cations This chemistry pertains also to the well-known isomerization of the even-electron CvHv ions and to their formation from the respective parents, e.g. CyHs" " . A related, albeit chemically different held concerns protonated cycloheptatriene, i.e. the even-electron C7H9+ ions , and alkylcycloheptatrienes, which are closely related to protonated toluene and higher alkylbenzenium ions. A parallel review by one of these authors on protonated alkylbenzenes has been pubUshed, and recent investigations on protonated alkylcycloheptatrienes have highUghtened the complexity of this gas-phase ion chemistry 42 jp a minor extent, ionized and protonated fulvenes have also been investigated with respect to their interconversion to their (mainly arene-derived) isomers. 

Data on the PMR spectra of 2,4,6-alkyldimethylbenzenium ions (alkyls-CHjCHj, CHfCHj), (013)3 cyclopropyl) are given in and the proton chemical shifts of some 4-alkylbenzenium ions are presented in Table 6. A series of ethylben-zenium ions (from mono- to heptaethyl-substituted) is also described. 

Important information on the structure of arenium ions is provided by the spin-spn coupling constants of different types of protons. For the vicinal protons located in the pentadienyl part of alkylbenzenium ions the spin coupling constant is 7 9 cps Since long-range interactions for this kind of protons are usually insignificant ,... 

Table 32. Electronic Absorption Spectra of Alkylbenzenium Ions...

In 4-isobutyl-, 4-sec-butyl- and other 4-alkylbenzenium ions the formation of benzyl-ions competes with the dealkylation. For example ... 

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