Lifetimes and UV-visible absorption spectra of benzyl, phenethyl, and cumyl carbocations and corresponding vinyl cations. A laser flash photolysis study

Abstract

Benzyl (4-MeO, 4-Me, and 4-methoxy-1-naphthylmethyl), phenethyl (4-Me2N, 4-MeO, 3,4-(MeO)2, 4-Me, 3-Me, 4-F, 3-MeO, 2,6-Me2, parent, and 4-methoxy-1-naphthylethyl) and cumyl (4-Me2N, 4-MeO, 4-Me, parent) cations have been studied by laser flash photolysis (LFP) in 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). In most cases styrene or α-methylstyrene precursors were employed for the phenethyl and cumyl ions, the intermediate being obtained by solvent protonation of the excited state. Benzyl cations were generated by photoheterolysis of trimethylammonium and chloride precursors. While a 4-MeO substituent provides sufficient stabilization to permit observation of cations in TFE, cations with less stabilizing substituents usually require the less nucleophilic HFIP. Even in this solvent, the parent benzyl cation is too short-lived (lifetime <20 ns) to be observed. When generated in HFIP, phenethyl cations can be seen to react with unphotolyzed styrene, giving rise to dimer cations that are observed to grow in as the initial phenethyl cation decays. The dimer cations, in common with the oligomer cations seen in cationic styrene polymerization, have a λmax 15-20 nm higher than the monomer and react with both solvent and styrene several orders of magnitude more slowly. This stabilization relative to the phenethyl may reflect an interaction with the aryl group present at the gamma-carbon. Cations 4-MeOC6H4C+(R)-CH3 (R = Me, Et, i-Pr, t-Bu, cyclopropyl, C6H5, 4-MeOC6H4) were generated in TFE via the photoprotonation route. The alkyl series shows that steric effects are important in the decay reaction. The cation with R = cyclopropyl is a factor of 1.5 less reactive than the cation where R = phenyl. Several vinyl cations have also been generated by photoprotonation of phenylacetylenes. ArC+=CH2 has a reactivity very similar to that of its analog ArC+H-CH3, the vinyl cation being slightly (factors of 2-5) shorter-lived. For the various series of cations, including vinyl, substituents in the aryl ring have a consistent effect on the λmax, a shift to higher wavelength relative to hydrogen of 15 nm for 4-Me, 30 nm for 4-MeO, and 50 nm for 4-Me2N.Key words: photogenerated carbocations, carbocation lifetime, styrene, photoprotonation.

Carbocationic n‐endo‐trig Cyclizations

“…The resulting UV/Vis absorptions at λ max =340–360 nm agreed with the previously reported7c,d UV/Vis spectra for 1 a . When the benzyl cations 1 a – e were generated in the presence of a high excess of the π‐systems 2 a – d (Scheme ), exponential decays of the absorbances of the carbocations were observed from which the first‐order rate constants k obs were obtained.…”

“…[c] Second‐order rate constants for the reactions of 1 d with 2 a – d from the slope k obs versus [ 2 ] at high nucleophile concentrations, see Equations (2)–(6). [d] Reference 7d reports 3.9×10 5 s −1 .…”

Photochemical Formation of Anthracene Quinone Methide Derivatives

“…LFP experiments for 2 were also conducted in 2,2,2-trifluoroethanol (TFE), in which electrophilic species such as QMs 64,34,36 and carbocations 65,66 However, the spectra did not differ from those measured in aqueous CH3CN. Contrary to 1, formation of QM2 and the corresponding carbocation could not be time-resolved.…”

Photogeneration of carbocations: applications in physical organic chemistry and the design of suitable precursors

“…Since the late 1980s, a large number of studies have employed nanosecond laser flash photolysis to study the generation and the decay kinetics of carbocations . Some of the substrates that were successfully employed for the photogeneration of carbocations are shown in Chart (b) . Valuable information about the formation of carbocationic intermediates in photoreactions has also been derived by the analysis of the products …”