Alkene radical cations, kinetics

Keywords Alkene radical cations Ion pairs Kinetics Stereochemical memory effects Tandem reactions... 

Relatively few kinetic data are available for the carbon-carbon bond forming reactions of alkene radical cations. Nevertheless, rate constants for the cyclization illustrated in Scheme 9, with generation of the alkene radical cation by the fragmentation method, have been measured. These cyclization rate constants are significantly faster than those of the corresponding neutral radicals [89]. 

The regiochemistry of nucleophilic addition to alkene radical cations is a function of the nucleophile and of the reaction conditions. Thus, water adds to the methoxyethene radical cation predominantly at the unsubstituted carbon (Scheme 3) to give the ff-hydroxy-a-methoxyethyl radical. This kinetic adduct is rearranged to the thermodynamic regioisomer under conditions of reversible addition [33]. The addition of alcohols, like that of water, is complicated by the reversible nature of the addition, unless the product dis-tonic radical cation is rapidly deprotonated. This feature of the addition of protic nucleophiles has been studied and discussed by Arnold [5] and Newcomb [84,86] and their coworkers. 

The mechanistic aspects of aromatic and alkene radical cation reactions have been reviewed. A second review article covers the structure and properties of hydrocarbon radical cations, as revealed by low-temperature ESR and IR spectroscopy. A review of the reactivity of trivalent phosphorus radical cations has appeared which discusses ionic and SET processes and their kinetics. " The structure and reactivity of distonic radical cations have been reviewed, including experimental and calculated heats of formation, structures, reactivity, and mechanisms. ... 

The one-electron oxidation of cyclohexenes by S04 in aqueous solution has been studied from kinetic and stereochemical standpoints [49]. It was found that the alkene oxidation proceeds by an addition-elimination mechanism with 804" adding to the C=C double bond in the first step followed by C -OSOs" heterolysis to give a solvent-separated alkene radical cation-sulfate ion pair (SnI mechanism) (Scheme 9). 

KINETICS AND MECHANISMS FOR THE REACTIONS OF ALKENE RADICAL CATIONS ... 

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. 

Pulse radiolysis has been used to generate alkene radical cations for kinetic studies. " In this case ionizing radiation produces radical cations of the solvent which, in the case of alkanes and chloroalkanes, have lifetimes that are sufficient to be scavenged by mM concentrations of an appropriate donor. The method has been used to generate styrene radical cations in nonpolar solvents, as indicated in Eq. 13 (RH = cyclohex-ane). . . It has also been used in polar solvents, in which case the alkene is oxidized by a strong transient oxidant such as SO "orTP produced by pulse radiolysis. ... 

As noted above, most kinetic studies of alkene radical cations in solution have focused on aryl-substituted systems since they have convenient optical propenies and have been extensively studied by other techniques. Radical cations are frequently identified on the basis of their characteristic UV-visible absorptions and the comparison of their spectra to those obtained in matrices at low temperature." However, a number of other diagnostic tests are also commonly employed to identify these intermediates. For example, their kinetic behavior as a function of solvent nucleophilicity or added nucleophiles is analogous to that of other electrophilic species. Thus, reaction with nucleophiles such as azide and halide ions provides support for the assignment of a transient to a radical cation, although it will not serve to eliminate a carbocation intermediate. More useful in the latter respect is the method of generation of the transient since PET does not in general lead to the formation of carbocations. Quenching of the observed transient with a more easily oxidized... 

The following three sections discuss recent time-resolved experiments on inter- and intramolecular cycloadditions of aryl-alkene radical cations. These studies address some of the mechanistic issues raised by the earlier studies and also provide kinetic data for the cycloadditions of a number of aryl and diaryl-alkene radical cations. Such kinetic data are essential for the development of this chemistry as a useful synthetic strategy and as a mechanistic probe for radical cation chemistry. 

Despite the demonstrated utility of alkene radical cation cycloadditions, little kinetic data for these reactions are currently available. However, two recent studies have provided rate constants for the initial step in the cyclobutanation or Diels-Alder reactions of a number of styrene radical cations.Previous work by Bauld had shown that the rrradical cation reacts with a variety of alkenes to generate either cyclobutane or Diels—Alder adducts (Eqs. 23, 24) 110 j, g [jnetic data for the styrene radical cation cycloadditions, in combination with the dimerization results discussed above, provide a detailed assessment of the effects of radical cation and alkene structure on dimerization and cross addition reactions. 

The data on cycloadditions of alkene radical cations indicate that dimerization will usually compete efficiently with cross additions and demonstrate the necessity for obtaining detailed kinetic data in order to design appropriate synthetic methods based on radical cation chemistry. The mechanistic data obtained from both time-resolved and steady-state experiments demonstrate the complexity of cycloaddition chemistry. This may be a particular limitation in the use of cycloaddition reactions in the design of mechanistic probes for assessing whether a particular reaction involves radical cation intermediates. The results also highlight the importance of using both product studies and the kinetic and mechanistic data obtained from time-resolved methods to develop a detailed understanding of the reactions of radical cations. 

Homer JH, Bagnol L, Newcomb M. Kinetics of radical heterolysis reactions forming alkene radical cations. J Am Chem Soc. 2004 126 14979-14987. 

The reaction of a series of substituted styrene radical cations with anions has recently been studied in detail by laser flash photolysis. Representative kinetic data are summarized in Tables 3 and 4 and demonstrate that most of the anions studied react with styrene radical cations with diffusion controlled rate constants. These reactions can involve either addition to the p-carbon to give a benzyl radical (Eq, 15) as discussed above or electron transfer to regenerate the precursor alkene plus the oxidized nucleophile (NU , Eq. 16). Transient absorption spectra have been used to distinguish between these two possibilities. For example, reaction of the radical cation with either bromide or chloride leads to the formation of a transient that is identified... 

The data for the reactions of four substituted styrene radical cations with selected dienes are summarized in Table 8. - As discussed above for the reaction of styrene radical cations with nucleophiles, the interpretation of these data is complicated by the possibility that two competing reactions are responsible for the observed quenching of the radical cation. One of these is electron transfer from the alkene to the styrene radical cation to generate the neutral styrene and the radical cation of the alkene (Eq. 29). In this case, the quenching rate constant is that for electron transfer, and does not provide any information on the kinetics for the initial addition, although the secondary radical calion/neutral pair may in some cases lead to adduct formation. The other reaction is addition of the alkene to the radical cation to generate an adduct radical cation that is the precursor of the final cyclobutanation and Diels-Alder products (Eq. 30). 

The effects of alkyl substitution in substituted ethenes decrease in the order 1,2-dialkyl < 2-alkyl < 2,2-dialkyl - trialkyl < tetraalkyl. This indicates that both electronic and steric factors are important in determining the nucleophilicity of the alkene toward radical cations. For example, alkyl substitution increases the nucleophilicity of the alkene, thus rendering 2,2-dialkyl and tetraalkylalkenes more reactive toward styrene radical cations than less-substituted alkenes such as 1-hexene. On the other hand, the kinetic acceleration that results fiom electronic effects of additional alkyl groups is offset by steric hindrance, as demonstrated by the low reactivity of 1,2-dialkyl-... 

The kinetic data discussed above demonstrate the effects of varying the structure of both the styrene radical cation and the alkene on the initial step in the cycloaddition reaction. However, the transient experiments do not provide any evidence that would permit one to distinguish between a concerted or stepwise mechanism. The kinetic data obtained for additions to a range of alkenes do show considerable similarities to those reported for the addition of carbenium ions to the same substrates. For example, rate constants for the addition of the bis(4-methyl-phenyl)methyi cation to a series of ring-substituted styrenes also correlate with the Hammett a and a parameters with p and p values of-5.2 and -5.0, respectively." The latter reactions are thought to proceed via a partially bridged transition slate and might, therefore, be expected to show similarities to concerted... 

The combined data in Tables 7-9 for the additions of styrene radical cations to their neutral precursors (dimerizations) and to other alkenes lead to a potentially important conclusion with respect to the design of cross-addition reactions. These data indicate that dimerization rate constants are frequently several orders of magnitude greater than the rate constants for cross addition. The absolute rate constants for the two reactions can be used to adjust the concentrations of the neutral styrene that leads to the radical cation and the alkene in order to maximize the yield of the cross-addition product. The kinetic and mechanistic data obtained for these reactions thus provides the basis for the development of synthetic strategies that utilize radical cation chemistry. 

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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