Radical cations reactions with alkenes

Electron transfer from the alkene leads to a radical cation that can undergo coupling (Scheme la). The radical cation can also react with the nucleophilic heteroatom of a reagent to afford addition or substitution products (Scheme lb). Adducts can be likewise obtained by oxidation of the nucleophile to a radical that undergoes radical addition. Reactions between alkenes and nucleophiles can be realized too with chemical oxidants that are regenerated at the anode (mediators) (see Chapter 15). Finally, cycloadditions between alkenes can be initiated by a catalytic anodic electron transfer. These principal reaction modes are subsequently illustrated by selected conversions. 

Cycloaddition Reactions with Alkenes Olefins can react with electrogenerated radicals, cationic species or dienophiles. 

Vinylindote radical-cations, for example that derived from 64, take part in a Diels-Alder reaction with alkenes. Subsequent oxidation of the initial product with loss of two protons and dimethylamine gives the pyrido[l,2a]indoie. Reaction is achieved either by direct electrochemical oxidation or by photochemical electron... 

RBSctions of Radical Anions With Radicals. The coupling of arene or alkene radical anions with radicals is an important reaction, and one that has significant synthetic potential. For example, radicals formed by nucleophilic capture of radical cations couple with the acceptor radical anion, resulting in (net) aromatic substitution. Thus, the l-methoxy-3-phenylpropyl radical (113 R = H) couples with dicyanobenzene radical anion loss of cyanide ion then generates the substitution product 132.2 + ... 

There are a number of other mechanisms by which alkenes can undergo photochemical f2 + 2) cycloaddition, one of which works well for electron-rich alkenes and electron-acceptor sensitizers. The pathway is through the radical cation of the alkene, which attacks a second, ground-state alkene molecule and then cydizes and accepts an electron to give the product cyclobutane. Typical of this group of reactions is the formation of 1,2-dialkoxycydobutanes from alkoxy-ethylenes with drcyanonaphthalene as sensitizer 12.78). 

Electron impact mass spectrometry of the cyclobutanedione (24) gives rise to dimethylcarbene radical cation.35 Appearance energy measurements and ah initio calculations indicated that the radical cation lies 84 kJ mol 1 above the propene radical cation and is separated from it by a barrier of 35 kJ mol-1. Diarylcarbene radical cations have been generated by double flash photolysis of diaryldiazomethanes in the presence of a quinolinium salt (by photo-induced electron transfer followed by photo-initiated loss of N2).36 Absolute rate constants for reactions with alkenes showed the radicals to be highly electrophilic. In contrast to many other cation radicals, they also showed significant radicophihc properties. 

The distinction between electrophilic and electron-transfer mechanisms of addition reactions to vinyl double bonds of ArX—CH=CH2 (X = S, O, Se) has been achieved by studying substituent effects. Specifically, the effects of meta and para substituents on the rates of electrophilic additions correlated with Hammett radical cations correlates with statistical tests. The ofclcctrophilicj/o-1 (FT) dichotomy is in accord with the conventional paradigm for cr/cr+ correlations and further support has been found by ah initio calculations. Interestingly, the application of this criterion to the reactions of aryl vinyl sulfides and ethers with tetracyanoethylene indicates that cyclobutanes are formed via direct electrophilic addition to the electron-rich alkene rather than via an electron-transfer mechanism.12... 

Two-laser two-photon results revealed photoisomerization of the cation E,E-11 to its stereoisomer Z,E-11, which undergoes thermal reversion with a lifetime of 3.5 ps at room temperature. Absolute rate constants for reaction of styrene, 4-methylstyrene, 4-methoxystyrene and /i-methyl-4-methoxystyrene radical cation with a series of alkanes, dienes and enol ethers are measured by Laser flash photolysis [208]. The addition reactions are sensitive to steric and electronic effects on both the radical cation and the alkene or diene. Reactivity of radical cations follows the general trend of 4-H > 4-CH3 > 4-CH3O > 4-CH30-jff-CH3, while the effect of alkyl substitution on the relative reactivity of alkenes toward styrene radical cations may be summarized as 1,2-dialkyl < 2-alkyl < trialkyl < 2,2-dialkyl < tetraalkyl. 

Other studies have sought to establish the scope and limitations of the photo-NOCAS process. Thus Arnold and co-workers have examined the reactions of alkenes with 1,4-dicyanobenzene (DCB). A typical result from this reaction is shown in Scheme 1. All of the products arise from the attack of the radical cation of the alkene on the DCB sensitizer with loss of the cyano function. A further study of photo-NOCAS reactivity has demonstrated that the radical cation of 2,3-dimethylbut-2-ene, formed by irradiation in the presence of DCB/biphenyl, can be trapped by fluoride ion. The resultant radical (39) reacts with the radical anion of DCB to yield the adduct (40). The radical cation of methylenecyclopro-pane (41) can be formed by irradiation in the presence of DCB as the sensitizer. The products are illustrated in Scheme 2 and, as shown, in all cases the cyclopropane ring remains intact. The diene (42) undergoes SET to dicyanoben-zene as the sensitizer with biphenyl as the co-sensitizer. In the absence of nucleophiles many products are formed such as (43) and (44) by reaction with the solvent acetonitrile or the sensitizer, respectively. In the presence of alcohols low yields of (45) and (46) are formed by reaction of the alcohol with the radical cation of the diene (42). 

Cycloaddition reactions of alkene radical cations have been the subject of a number of mechanistic studies and are potentially useful synthetic reactions. - - - Most of the initial work on radical cation mediated cycloadditions focused on the dimerization of arylalkenes. with one of the first examples being Ledwith s report of the chloranil-sensitized dimerization of M-vinylcarba-zole to generate a diarylcyclobutane. This work led to the development of the mechanism outlined in Scheme 2, in which addition of the radical cation to neutral alkene generates an acyclic 1,4-radical cation as the primary intermediate. This intermediate cyclizes to a cyclobutane radical cation that is then reduced by the neutral alkene and regenerates a second radical cation to carry the chain. 

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 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 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. 

These results provide the first detailed calibration for a series of intramolecular radical cation probes based on cycloaddition chemistry. The cyclization rate constants cover several orders of magnitude in timescale, an ideal case for using 1—3 as probes for radical cations of different lifetimes. However, the time-resolved experiments demonstrate that the application of radical cation probes, at least those based on aryl alkene cycloaddition chemistry, may be considerably less straightforward than similar experiments with free radical probes or clocks. Some of the problems that need to be addressed include the variation of products with the reaction conditions and method of radical cation generation, and the possibility of reversibility of the initial adduct formation. Furthermore, at least some radical cation reactions are quite sensitive to solvent and this may mean that calibrations for radical cation cycloadditions will have to be done in a variety of solvents. 

Several groups investigated the reactions of radical cations in liquid alkenes. In pulse radiolysis studies the radical cations show a strong absorption band around 280 nm, which is attributed to the n-n transition of the alkene monomer radical cation. Monomer radical cations dimerize with diffusion-controlled rate coefficients with the olefin molecules the dimer cations have broad absorption bands in the 600-800 nm range (Mehnert et al. 1981, 1985 Alfassi 1989). Dimerization may proceed via a hydride ion (H ) transfer in the transition-state of radical-molecule reaction (O Fig. 23.7) ... 

When the allylic cation reacts with Br to complete the electrophilic addition, reaction can occur either at Cl or at C3 because both carbons share the positive charge (Figure 14.4). Thus, a mixture of 1,2- and 1,4-addition products results. (Recall that a similar product mixture was seen for NBS bromination of alkenes in Section 10.4, a reaction that proceeds through an allylic radical.)... 

The dependence of relative rates in radical addition reactions on the nucleophilicity of the attacking radical has also been demonstrated by Minisci and coworkers (Table 7)17. The evaluation of relative rate constants was in this case based on the product analysis in reactions, in which substituted alkyl radicals were first generated by oxidative decomposition of diacyl peroxides, then added to a mixture of two alkenes, one of them the diene. The final products were obtained by oxidation of the intermediate allyl radicals to cations which were trapped with methanol. The data for the acrylonitrile-butadiene... 

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