A variety of substituted styrene radical cations (2) have been generated by 266or 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 109 M-1 s_1 for most of the radical cations without /3-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 diffusioncontrolled rate constants (~1010 M-1 s_l). 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 /8-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.(1) Issued as NRCC-35250.(2) Fox, . A.; Chanon, M., Eds. Photoinduced Electron Transfer,

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Absolute rate constants for the reactions of styrene, 4-methylstyrene, 4-methoxystyrene, and β-methyl-4-methoxystyrene radical cations with a series of alkenes, dienes, and enol ethers have been measured by laser flash photolysis. The measured rate constants correspond to either addition or electron transfer reactions, with the latter predominating when the oxidation potential of the alkene is lower than that of the styrene. The measured rate constants for the diene additions provide some of the first absolute kinetic data for the initial step in the syntheticallyuseful radical cation mediated Diels-Alder reaction. The addition reactions are sensitive to steric and electronic effects on both the radical cation and the alkene or diene. For example, the reactivity of the radical cations follows the general trend of 4-H > 4-CH 3 > 4-CH 3 O > 4-CH 3 O-β-CH 3 . The effects of alkyl substitution on the relative reactivity of alkenes toward styrene radical cations may be summarized as 1,2-dialkyl < 2-alkyl < trialkyl e 2,2dialkyl < tetraalkyl. The addition of the 4-methoxystyrene radical cation to a series of ring-substituted styrenes gives a reasonable Hammett correlation with a F value of -5. Thus, the addition of radical cations to a variety of alkenes and dienes follows similar trends to those observed for the addition of other electrophiles, such as diarylcarbenium ions. The results are consistent with previous suggestions of a concerted pathway for radical cation mediated cycloaddition reactions, although direct spectroscopic evidence for the initial product radical cation is obtained only for the additions to substituted styrenes.

The addition of the 4-methoxystyrene radical cation to neutral 4-methoxystyrene and the cleavage reactions of the 1,2-bis(4-methoxyphenyl)cyclobutane radical cation in acetonitrile have been studied by product analysis and by nanosecond and picosecond transient absorption spectroscopy. The main product upon radical cation mediated dimerization of 4-methoxystyrene using chloranil as the electron transfer sensitizer is a substituted dihydronaphthalene, 1,2-dihydro-7-methoxy-l-(4'-methoxyphenyl)naphthalene, at low concentrations of 4-methoxystyrene. At higher concentrations, the main product is 1,2-bis(4-methoxyphenyl)cyclobutane. Cleavage of the cyclobutane radical cation is found to give 4-methoxystyrene and the dihydronaphthalene in a 1:3 ratio. In the time-resolved experiments, the radical cations are generated from 4-methoxystyrene and 1,2-bis(4-methoxyphenyl)cyclobutane by 266-nm photoionization or by 355-nm photoinduced electron transfer using triplet chloranil as the sensitizer. A transient with an absorption maximum at 500 nm is observed as a short-lived intermediate in both the radical cation mediated 4-methoxystyrene dimerization and the cyclobutane radical cation cleavage experiments. Spectroscopic and kinetic considerations lead to the conclusion that this transient is a substituted hexatriene radical cation produced as an intermediate in the conversion of the 1,2-bis(4-methoxyphenyl)cyclobutane radical cation to thedihydronaphthalene. Observed rate constants for the decay of the 4-methoxystyrene radical cation measured at 4-methoxystyrene concentrations of 0,0001 to 0.01 5 M increase in a linear fashion with concentration, while at concentrations >0.2 M, the observed rate constant is found to be independent of concentration. Analysis of the kinetic data according to a rate law derived from a concerted [2 + 11 mechanism leads to the following rate constants: kl = 1.4 X lo9 M-I s-l for the addition reaction, k-l = 8 X 107 s-I for the cycloreversion reaction, k2 = 2.5 X lo8 s-l for the rearrangement of the cyclobutane radical cation, and kj = 1.5 X 1Olo M-l s-l for the reduction of the cyclobutane radical cation by neutral 4-methoxystyrene. Monitoring the kinetics of the reactions of the cyclobutane radical cation gives the same values for the rate constants k2 and kl.

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4 3 R)SiCI, P Scheme 1. Synthesis of la-c (Ar = aryl).excess dichlorosilane (Sa-c) to provide l a in 31 %, l b in 12 Yo, and lc in 15 % yield, respectively. The spectral data of these compounds (Table 1) are consistent with the proposed structures. Particularly revealing are the 29Si-NMR spectra,

Flash photolysis of phenyldiazoacetic acid and the methyl, n-butyl, and isobutyl esters of benzoylformic acid in aqueous solution generated a transient species that was identified, through product determination and the form of acid−base catalysis plus solvent isotope effects on its decay, as the enol of mandelic acid, 1. When benzoylformate esters were the flash photolysis substrates, the enol was formed by hydration of phenylhydroxyketene, 5, itself generated by Norrish type II photoelimination of the esters, and when phenyldiazoacetic acid was the substrate, the enol was formed by hydration of phenylcarboxycarbene, 6, produced by dediazotization of the diazo acid. Rates of enolization of mandelic acid were also determined, by monitoring the incorporation of deuterium into its α-position from a D2O solvent, and combination of these with rates of ketonization gave the keto−enol equilibrium constant, pK E = 16.19. The acidity constant of the enol ionizing as an oxygen acid was determined as well, p = 6.39, and combination of that with K E led to the ionization constant of the keto form of mandelic acid ionizing as a carbon acid, p = 22.57. (These acidity constants are concentration quotients, applicable at ionic strength = 0.10 M.) These results are compared with other keto−enol systems, and their bearing on the enzymatic racemization of mandelic acid is discussed.

Rates of hydrogen ion catalyzed enolization of acetone and ketonization of acetone enol were measured over a range of temperatures in water and in acetonitrile solution. The data give AH* = 20.0 ± 0.1 kcal mol™1, AS* = -12.1 ± 0.3 cal K'1 mol'1, and AH* = 20.1 ± 0.5 kcal mol'1, AS* = -5.8 ± 1.7 cal K™1 mol'1, for enolization in water and acetonitrile, respectively, and AH* = 9.7 ± 0.4 kcal mol'1, AS* = -8.6 ± 1.4 cal K™1 mol'1, and AH* = 11.4 ± 0.2 kcal mol™1, AS* = 1.6 ± 0.8 cal K™1 mol'1, for ketonization in water and acetonitrile, respectively. These values lead to AH0 = 10.3 ± 0.4 kcal mol"1, AS°= -3.5 ± 1.5 cal K'1 mol'1, for the keto-enol equilibrium in water, and AH0 = 8.7 ± 0.6 kcal mol'1, AS°= -7.4 ± 1.9 cal K™1 mol'1, for the equilibrium in acetonitrile. This is the first determination of thermodynamic parameters for a simple ketone-enol equilibrium in solution; the results are remarkably similar to the thermodynamic parameters for this reaction in the gas phase. A mechanism involving acid catalysis of the bromination of acetone enol by TV-bromosuccinimide, the process used to monitor enolization in acetonitrile solution, is ruled out.