Alkene radical cations, kinetics characterization

A number of alkene radical cations have been generated in matrices at low temperature and have also been studied by ESR, CIDNP, and electrochemical methods. However, until recently very little absolute kinetic data have been available for the reactions of these important reactive intermediates in solution under conditions comparable to those used in mechanistic or synthetic studies. In a few cases, competitive kinetic techniques have been used to estimate rates for nucleophilic additions or radical cation/alkene cycloaddition reactions. In addition, pulse radiolysis has been used to provide rate constants for some of the radical cation chemistry relevant to the pho-topolymerization of styrenes. More recently, wc and others have used laser flash photolysis to generate and characterize a variety of alkene radical cations. This method has been extensively applied to the study of other reactive intermediates such as radicals, carbenes, and carbenium ions and is particularly well-suited for kinetic measurements of species that have lifetimes in the tens of nanoseconds range and up and that have at least moderate extinction coeffleients in the UV-visible region. 

This review summarizes the generation and spectroscopic characterization of alkene radical cations and kinetic and mechanistic studies of their reactions with nucleophiles and cycloaddition chemistry. Most of the data have been obtained using laser flash photolysis techniques, but comparisons with kinetic data obtained using other methods and with steady-state experiments are presented where appropriate. To date most kinetic measurements using laser Hash photolysis techniques have focused on arylalkene radical cations since these are relatively easy to generate and have spectroscopic and kinetic behavior that is commensurate with nanosecond laser flash photolysis techniques. 

Of conrse, the cyclic cation-radical formed should be less stable than the alkene cation-radical (which contains a double bond that is favorable for the spin-charge scattering). However, the cation-radical product and corresponding nentral species are generated in a concerted process. The process involves simultaneous covalent bond formation and one-electron reduction of the cyclic product (Karki et al. 1997). Similar to other branched-chain processes, the cation-radical dimerization is characterized by an activation enthalpy that is not too high. These magnitudes are below 20 kJ mol for the pair of cyclohexadiene and trani-anethole (p-MeOCgH4CH=CHCHMe, Z-form Lorenz and Bauld 1987). It is clear that the cation-radical variant of cyclodimerization differs in its admirable kinetic relief. For cyclohexadiene and tran -anethole, catalytic factors are 10 and 10, respectively (Bauld et al. 1987). 

A variety of aryl and diarylalkene radical cations have been generated in solution and characterized using transient absorption spectroscopy. Many of these are sufficiently long-lived for detailed kinetic studies of their intermolecular reactivity under conditions that are comparable to those used in mechanistic and synthetic studies. Reactions with nucleophiles typically occur by either addition or electron transfer, with the latter dominating in cases where the oxidation potential of the nucleophile is lower than that of the alkene. The data summarized herein indicate that most arylalkene radical cations are unseleclive in their additions to anionic nucleophiles in nonprotic solvents. By contrast, the additions to neutral nucleophiles such as alcohols and amines cover a range of timescales and clearly demonstrate the... 

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