Reactivity of Radical Cations. Effect of Radical Cation and Alkene Structure on the Absolute Rate Constants of Radical Cation Mediated Cycloaddition Reactions¹

Abstract

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.

SmI2(H2O)n Reduction of Electron Rich Enamines by Proton-Coupled Electron Transfer

“…However, this seems very unlikely because the difference in the rate constants for protonation of the radical anions would have to precisely balance the difference in ET equilibrium constants. For the related oxidation of these styrenes, the difference in the CV oxidation peak in MeCN is 0.56 V; 12 if the difference in reduction potentials are similar, this would imply a ~10 9 difference in K eq for ET.…”

Electron‐Transfer Cycloreversion of 2,3‐Diaryloxetanes: Influence of the Substitution and the Photosensitizer on the Regioselectivity

“…[25,32] For 3, it is known [21] that E A/A-· ϭ Ϫ0.02 V vs. SCE and E T ϭ 49 kcal/mol. The coulombic interaction is ϩ0.79 eV in benzene, Ϫ0.06 eV in acetonitrile and Ϫ0.05 eV methanol.…”

“…Taking into account the redox potentials of 4b, 2a and 2b, which are E (Dϩ·/D) ϭ 1.33 V, E (A/A-·) ϭ Ϫ0.29 V and Ϫ0.21 V vs. SCE, [32,26] respectively, BET is highly exergonic in both cases [∆G ET (4b…”

Selectivity in Radical Cation Cycloadditions

“…70 71 Later on, a concerted but nonsynchronous [2 þ 1] cycloaddition involving a "long bond intermediate" 53 þ was proposed. 9,51,72,73 . Laser flash studies of ET dimerization reactions of vinylanisole and other arylethene derivatives concluded that the rate determining step in the formation of 54 þ was the cleavage of the long bond intermediate 53 þ .…”