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Probing for a Leaving Group Effect on the Generation and Reactivity of Phenyl Cations

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journal contribution
posted on 06.04.2012, 00:00 by Erika Abitelli, Stefano Protti, Maurizio Fagnoni, Angelo Albini
Phenyl cations are smoothly generated by the photoheterolytic cleavage of an Ar–LG bond (LG = leaving group). With the aim of evaluating the scope of the method, a series of 4-methoxy-2-(trimethylsilyl)­phenyl derivatives (sulfonic, LG = MeSO3 and CF3SO3, phosphate, LG = (EtO)2(O)­PO esters and the corresponding chloride) have been compared as probes for evaluating the leaving group ability. The photocleavage was a general reaction, with the somewhat surprising order (EtO)2(O)­PO ∼ Cl > CF3SO3 > MeSO3 (Φ = 0.50 to 0.16 in CF3CH2OH and lower values in MeCN–H2O). The ensuing reactions did not depend on the LGs but only on the structure of the phenyl cation (the silyl group tuned the triplet to singlet intersystem crossing and the electrophilicity) and on the medium (formation of a complex with water slowed the electrophilic reactions).