Phenonium ions, generation

Treatment of aryl-substituted alkenes with hypervalent iodine compounds can lead to the formation of phenyliodinated intermediates, which can be stabilized by the aryl substituent via the formation of phenonium ions. Subsequent nucleophilic attack might then lead to rearranged products. This behavior can be nicely seen by comparing the unsaturated carboxylic acids 78 in their reaction with (diacetoxyiodo)benzene 3. The substrate 78a without the phenyl substituent is cyclized to the phenyliodinated intermediate 79, which is then attacked by the acetate under the formation of lactone 81 [142]. Substrate 78b is, however, then stabilized by the formation of an intermediate phenonium ion 80 and attack by the acetate is accompanied by a 1,2-phenyl migration and 82 is generated, Scheme 35 [143]. 

Phenonium ions, of which 105 is the parent species, have long attracted attention, initially concentrated on kinetic and stereochemical evidence for their, generation in solvolysis reactions (equation 10), and more recently with direct observation of the ions in solution " . It has been persuasively argued that these ions are best represented as cyclopropyl-substituted carbocationic systems, with the bisected geometry best suited for TT-donation from the cyclopropyl ring. 

Spiro[2.5]octadienyl radical (146), the radical analog of the phenonium ion (105), is a very short-lived species but was detected by its visible absorption and fluorescence on generation by radical abstraction from 147 prior to facile ring-opening to 148 (equation 29). There appeared to be a modest (two-fold) acceleration compared to 1,3- or 1,4-cyclohexadiene in the rate of abstraction from 147, and this was attributed to stabilization by a favorable interaction between cyclopropyl and the adjacent semioccupied orbital in 146. Kinetic acceleration was also proposed to occur in hydrogen atom abstraction from spiro[2.n]alkanes (149). ... 

A/ -Tosylindole 87 was prepared by the metal-free C—H amination of Af-tosylaniline 86 and DDQ.The reaction is believed to proceed through a radical cation generated from a SET process and a phenonium ion intermediate. Most of the substituted indoles were isolated in very good yields and a variety of aryl, heteroaryl, halogen, and trifluoromethyl substituents were tolerated (14OL3720). 

Jenekhe and coworkers found that DDQ converts anthrazo-line 7 to heptacyclic bis(indolo[l,2-a]quinolone) 8 (equation 3) [19]. Youn and Jang used DDQ to convert Af-Ts-2-alkenylanilines into indoles via, it is proposed, a radical cation generated by SET (equation 4) [20]. In the case of p,p-disubstituted 2-alkenylanilines, the 2,3-disubstituted indole product is derived from a C-2 to C-3 aryl migration via a phenonium ion (equation 5). 

The effects of methoxy and hydroxy substituents on phenonium ions (780) generated in strong acids have been examined. Methoxy-groups in the wieta-positions destabilize the phenonium ion, whereas it is stabilized by ortho- and para-methoxy-groups, according to n.m.r. evidence. Interestingly, solutions of such ions in strong acids show weak... 

The solvolysis of trityl derivatives in strongly acidic aqueous media has also been examined. Rates of hydrolysis were correlated to Hr acidity function and theX excess acidity scale. The solvolysis and substitution reactions of 2-(4-methylphenyl)ethyl tosylate have also been studied. " In trifluoroethanobwater (50 50) solution, the corresponding phenonium ion is generated. The kinetics of azide trapping has been studied and compared with results from an analogous system 2-(4-methoxyphenyl)ethyl tosylate. Owing to electronic effects, the methyl-substituted phenonium ion is a more reactive electrophilic species. 

---