Photochemical Formation and Electrophilic Reactivities of Vinyl Cations. Influence of Substituents, Anionic Leaving Groups, Solvents, and Excitation Wavelength on Photoheterolysis and Photohomolysis of 1-(p-R-Phenyl)-2-(2,2'-biphenyldiyl)vinyl Halides

Article abstract of DOI:10.1021/j100089a027

Using product analysis and time-resolved laser flash photolysis techniques, the photochemistry at λ_exc ca. 250 and 310 nm (exc = excitation) of 1-(p-R-phenyl)-2-(2,2'-biphenyldiyl)vinyl halides (R = H, Me, MeO) in methanol and acetonitrile at room temperature was studied.The title compounds undergo photoheterolysis and photohomolysis to give vinyl cations =C⁺- (carbenium ions) and vinyl radicals =C^.-.The cation : radical ratio increases with electron-donating strength of the substituent R, H < Me < MeO, and with anionic leaving group power of the halide, F^- < Cl^- < Br^- < I^-, indicating that the cleavage of the C-X bond to yield cation and halide anion proceeds, in the rate-determining step, by heterolysis and not by homolysis followed by electron transfer in the radical pair.The cation : radical ratio is solvent dependent: e.g., for the vinyl bromide with R = MeO and with λ_exc = 308 nm, in CH2Cl2 as solvent, only radical is observed, in comparison with only cation in the much more polar solvent acetonitrile.In acetonitrile-methanol mixtures, the absolute yields of both cation and radical go through a maximum as the methanol content is increased, however, the catio : radical ratio decreases continuously with increasing .For a particular R, the quantum yield for C-X bond cleavage is higher at ca. 250 than at ca. 310 nm.Also, the cation : radical ratio is wavelength-dependent: at λ_exc ca. 310 nm there is relatively more heterolysis than at λ_exc ca. 250 nm.Rate constants for reaction of the cations with nucleophiles were determined in acetonitrile.Those for reaction of the cation with R = MeO (lifetime in acetonitrile 7 μs) with anionic nucleophiles such as the halides are at the diffusion limit in this solvent, ca. 2 * 10¹⁰ M^-1 s^-1, whereas those for reaction with water, alcohols, and cyclic ethers are of the order 10⁵-10⁶ M^-1 s^-1.The less stabilized cation (R = Me) reacts with alcohols faster by the factor ca. 100, and it decays in acetonitrile ca. 100 times more rapidly.Also, 1-(p-R-phenyl)-2-dimehtylvinyl bromides (R = H, Me, MeO) were photolyzed in acetonitrile.In the case of R = MeO, the cation p-CH3O-C6H4-C⁺=CMe2 was seen (lifetime 770 ns), and its reactivity with alcohols (k values of 10⁶-10⁷ M^-1 s^-1) and halides (k values of 10¹⁰ M^-1 s^-1) was determined.The vinyl radicals =C^.- react with O2 to yield vinylperoxyl radicals =C-O2^. which have absorption maxima at ca. 390 nm.