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J Org Chem. 2012 Apr 6;77(7):3501-7. doi: 10.1021/jo300290v. Epub 2012 Mar 20.

Probing for a leaving group effect on the generation and reactivity of phenyl cations.

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PhotoGreenLab, Department of Chemistry, University of Pavia, Viale Taramelli 12, 27100 Pavia, Italy.


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 = MeSO(3) and CF(3)SO(3), 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 > CF(3)SO(3) > MeSO(3) (Φ = 0.50 to 0.16 in CF(3)CH(2)OH and lower values in MeCN-H(2)O). 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).


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