Photolytic Generation of Benzhydryl Cations and Radicals from Quaternary Phosphonium Salts: How Highly Reactive Carbocations Survive Their First Nanoseconds

Abstract

UV irradiation (266 or 280 nm) of benzhydryl triarylphosphonium salts Ar(2)CH-PAr(3)(+)X(-) yields benzhydryl cations Ar(2)CH(+) and/or benzhydryl radicals Ar(2)CH(•). 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 (Ar(2)CH(+)), the photo-nucleofuge (PPh(3) or P(p-Cl-C(6)H(4))(3)), the counterion (X(-) = BF(4)(-), SbF(6)(-), Cl(-), or Br(-)), and the solvent (CH(2)Cl(2) or CH(3)CN) were investigated. Photogeneration of carbocations from Ar(2)CH-PAr(3)(+)BF(4)(-) or -SbF(6)(-) 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(-) = BF(4)(-) or SbF(6)(-)), while photolyses of phosphonium halides Ar(2)CH-PPh(3)(+)X(-) (X(-) = Cl(-) or Br(-)) in CH(2)Cl(2) yield benzhydryl radicals Ar(2)CH(•) due to photo-electron transfer in the excited phosphonium halide ion pair. At low concentrations in CH(3)CN, 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 CH(3)CN or CF(3)CH(2)OH. Efficient photo-generation of the most reactive benzhydryl cations (3,5-F(2)-C(6)H(3))(2)CH(+) and (4-(CF(3))-C(6)H(4))(2)CH(+) was only achieved using the photo-leaving group P(p-Cl-C(6)H(4))(3) and the counter-anion SbF(6)(-) in CH(2)Cl(2). 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 PAr(3), 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(2)CH(+) for the purpose of studying their electrophilic reactivities.

Wavepacket Splitting in the First 100 fs Determines the Products from the Bond Cleavage of Diphenylmethylchloride

“…These two changes can be modeled with the identical stretched exponential function with τ SE = 11.1 ps. The changeover is due to electron transfer within the radical pair that leads to the ion pair [5,6]. The electron transfer is fastest for the pairs with the closest distance and eventually stops because the near neighbors are used up and the other pairs diffuse apart.…”

Ambident Reactivities of Formaldehyde N,N‐Dialkylhydrazones

“…[10] For the fast reactions (k > 10 6 L mol À1 s À1 ), laser flash photolytic techniques were employed: [12] The benzhydrylium ions 2 were generated by laser flash irradiation (7 ns pulse, 266 nm, 40-60 mJ pulse À1 ) of the corresponding benzhydryltriphenylphosphonium tetrafluoroborates 2-PPh 3 BF 4 in the presence of the nucleophiles 1 (Scheme 3). The intermediate benzhydrylium ions 2 were identified by their UV/Vis spectra.…”

Ambidente Reaktivität von Formaldehyd‐N,N‐dialkylhydrazonen

“…) wurden Laserblitz-photolytische Techniken eingesetzt: [12] Die Benzhydryliumionen 2 wurden dabei durch Bestrahlung der entsprechenden Benzhydryltriphenylphosphonium-tetrafluoroborate 2-PPh 3 BF 4 in Gegenwart der Nucleophile 1 mit einem Laserblitz (7-ns-Puls, 266 nm, 40-60 mJ Puls À1 ) generiert (Schema 3). Die intermediär gebildeten Benzhydryliumionen 2 wurden durch ihre UV/Vis-Spektren identifiziert.…”