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• 787-49-5 1-(chloro(phenyl)methyl)-4-(trifluoromethyl)-benzene 

Relevant academic research and scientific papers

Photolytic generation of benzhydryl cations and radicals from quaternary phosphonium salts: How highly reactive carbocations survive their first nanoseconds

UV irradiation (266 or 280 nm) of benzhydryl triarylphosphonium salts Ar2CH-PAr3+X- yields benzhydryl cations Ar2CH+ and/or benzhydryl radicals Ar 2CH?. The efficiency and mechanism of the photo-cleavage were studied by nanosecond laser flash photolysis and by ultrafast spectroscopy with a state-of-the-art femtosecond transient spectrometer. The influences of the photo-electrofuge (Ar2CH +), the photo-nucleofuge (PPh3 or P(p-Cl-C 6H4)3), the counterion (X- = BF 4-, SbF6-, Cl-, or Br-), and the solvent (CH2Cl2 or CH 3CN) were investigated. Photogeneration of carbocations from Ar 2CH-PAr3+BF4- or -SbF6- is considerably more efficient than from typical neutral precursors (e.g., benzhydryl chlorides or bromides). The photochemistry of phosphonium salts is controlled by the degree of ion pairing, which depends on the solvent and the concentration of the phosphonium salts. High yields of carbocations are obtained by photolyses of phosphonium salts with complex counterions (X- = BF4- or SbF6 -), while photolyses of phosphonium halides Ar2CH-PPh 3+X- (X- = Cl- or Br -) in CH2Cl2 yield benzhydryl radicals Ar 2CH? due to photo-electron transfer in the excited phosphonium halide ion pair. At low concentrations in CH3CN, the precursor salts are mostly unpaired, and the photo-cleavage mechanism is independent of the nature of the counter-anions. Dichloromethane is better suited for generating the more reactive benzhydryl cations than the more polar and more nucleophilic solvents CH3CN or CF3CH 2OH. Efficient photo-generation of the most reactive benzhydryl cations (3,5-F2-C6H3)2CH+ and (4-(CF3)-C6H4)2CH+ was only achieved using the photo-leaving group P(p-Cl-C6H 4)3 and the counter-anion SbF6- in CH2Cl2. The lifetimes of the photogenerated benzhydryl cations depend greatly on the decay mechanisms, which can be reactions with the solvent, with the photo-leaving group PAr3, or with the counter-anion X- of the precursor salt. However, the nature of the photo-leaving group and the counterion of the precursor phosphonium salt do not affect the rates of the reactions of the obtained benzhydryl cations toward added nucleophiles. The method presented in this work allows us to generate a wide range of donor- and acceptor-substituted benzhydryl cations Ar 2CH+ for the purpose of studying their electrophilic reactivities.

Photo-heterolysis and -homolysis of substituted diphenylmethyl halides, acetates, and phenyl ethers in acetonitrile: Characterization of diphenylmethyl cations and radicals generated by 248-nm laser flash photolysis

Para-substituted diphenylmethyl halides, acetates, and ethers RPh(R′Ph)CH-X (R, R′ = CF3 to OCH3), upon photolysis with ～250-nm light in acetonitrile solutions, undergo homolysis and heterolysis of the C-X bond to give the radicals, RPh(R′Th)CH? (abbreviated as C?), and the cations, RPh(R′Ph)CH+ (C+). Whereas the quantum yields for homolysis (0.2-0.4) are rather independent of the nature of the substituent on the benzene ring, those for heterolysis increase with increasing electron-donator strength from ≤0.07 for CF3 to 0.3 for OMe. The cation:radical ratios are also dependent on the nucleofugal properties of X. For the halides, the observed heterolysis:homolysis ratios correlate with the pKa values of the conjugate acids HX and not with the electron affinities of X?. In acetonitrile, heterolysis is much less endothermic than homolysis. Homolysis and heterolysis can also be effected indirectly by reaction with triplet acetophenone (produced by 308-nm photolysis). Unless stabilized by one or more MeO, the cations decay predominantly by reaction with acetonitrile to give nitrilium ions. However, since this reaction is reversible (shown for the benzhydryl cation), the nitrilium ion contributes only to an insignificant degree to the formation of the final (cation-derived) products, which result from reaction with trace water (main product, benzhydryl alcohol; minor, benzhydrylacetamide). The rate constants for addition of C+ to CH3CN are in the range 3.5 × 105 to 3.8 × 107 s-1 for the cations with R = R′ = Me to R = H, R′ = CF3. The rate constants for reaction of C+ with halides (ion recombination) are ～2 × 1010 M-1 s-1 (diffusion control). The radicals C? disappear by dimerization and disproportionate, for which a complete mass balance has been achieved by product analysis for the case of the benzhydryl system. At laser-pulse powers > 10 mJ electronically excited radicals, C?*, are additionally formed in many cases, via absorption of a light quantum by ground-state C?.