REACTIVITIES OF RADICAL CATIONS - CHARACTERIZATION OF STYRENE RADICAL CATIONS AND MEASUREMENTS OF THEIR REACTIVITY TOWARD NUCLEOPHILES

A variety of substituted styrene radical cations (2) have been generated by 266- or 308-nm photoionization of the parent olefin in polar solvents or by electron-transfer quenching of triplet chloranil. The rate constant for decay of most of the radical cations is limited by a combination of nucleophilic addition of solvent and addition to the parent olefin. The latter rate constants are in the range of 10(9) M-1 s-1 for most of the radical cations without beta-methyl substitutents. The reactivity of styrene radical cations toward nucleophiles such as alcohols, azide, and halides has also been examined. In acetonitrile or trifluoroethanol, azide, chloride, and bromide react with approximately diffusion-controlled rate constants (approximately 10(10) M-1 s-1). However, the rate constants for addition of methanol and other alcohols are substantially slower and show the expected steric and electronic effects with variations in para-substituents, methyl substitution of the olefin, or modification of the alcohol structure. Transient absorption spectra demonstrate that the reaction with alcohols and halide ions involves addition to the beta-position to generate the corresponding benzylic radical, in agreement with literature product studies for some of these systems. There is no evidence for competing electron transfer in cases where this would be exoergonic. Although the selectivity of anionic nucleophiles toward styrene radical cations is low in acetonitrile or water, most of these nucleophiles react much less rapidly in 4:1 aqueous acetonitrile. In some cases, decreases of 4 orders of magnitude are observed. The results agree well with data for nucleophilic addition to carbocations and are attributed to hydrogen-bonding of the nucleophile in the aqueous solvents.