944583-29-3Relevant articles and documents
Probing for a leaving group effect on the generation and reactivity of phenyl cations
Abitelli, Erika,Protti, Stefano,Fagnoni, Maurizio,Albini, Angelo
scheme or table, p. 3501 - 3507 (2012/06/15)
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).
The β effect of silicon in phenyl cations
Dichiarante, Valentina,Salvaneschi, Andrea,Protti, Stefano,Dondi, Daniele,Fagnoni, Maurizio,Albini, Angelo
, p. 15919 - 15926 (2008/09/20)
Irradiation of chloroanisoles, phenols, and N,N-dimethylanilines bearing a trimethylsilyl (TMS) group in the ortho position with respect to the chlorine atom caused photoheterolysis of the Ar-Cl bond and formation of the corresponding ortho-trimethylsilylphenyl cations in the triplet state. The β effect of silicon on these intermediates has been studied by comparing the resulting chemistry in alcoholic solvents with that of the silicon-free analogues and by computational analysis (at the UB3LYP/6-311+G(2d,p) level in MeOH). TMS groups little affect the photophysics and the photocleavage of the starting phenyl chlorides, while stabilizing the phenyl cations, both in the triplet (ca. 4 kcal/mol per group) and, dramatically, in the singlet state (9 kcal/mol). As a result, although triplet phenyl cations are the first formed species, intersystem crossing to the more stable singlets is favored with chloroanisoles and phenols. Indeed, with these compounds, solvent addition to give aryl ethers (from the singlet) competed efficiently with reduction or arylation (from the triplet). In the case of the silylated 4-chloro-N,N- dimethylaniline, the triplet cation remained in the ground state and trapping by π nucleophiles remained efficient, though slowed by the steric bulk of the TMS group. In alcohols, the silyl group was eliminated via a photoinduced protiodesilylation during the irradiation. Thus, the silyl group could be considered as a directing, photoremovable group that allowed shifting to the singlet phenyl cation chemistry and was smoothly eliminated in the same one-pot procedure.
