Radical Additions to Alkenes

Kinetics is used to investigate mechanisms of radical additions to alkenes. Outside the solvent cage, the initiator-derived radicals may undergo the desired bimolecular reaction with the substrate, or side reactions. When the substrate is an alkene, the exothermic intermolecular addition of the reactive radical (R ) to the double bond results in the formation of two new single carbon-carbon bonds in place of the double bond. This reaction represents conversion of an initiator into a propagating radical in radical polymerisations, and is becoming increasingly important in a number of synthetically useful intermolecular small molecule reactions. The addition of R to monosubstituted and 1,1-disubstituted alkenes is nearly always at the unsubstituted carbon atom (tail addition), and thus is normally not affected by the individual steric demand of the alkene substituents. Equation 10.4 is the expression for the rate of addition (R ) of R to an alkene where [M] is the monomeric alkene concentration  

The temperature dependence of the rate constants of radical addition (k ) is described by the Arrhenius equation (Section 10.2). At a given temperature, rate variations due to the effects of radical and substrate substituents are due to differences in the Arrhenius parameters, the frequency factor, A , and activation energy for addition, . For polyatomic radicals, A values span a narrow range of one to two orders of magnitude [6.5 log (A /dm3 mol-1 s-1) 8.5] [2], which implies that large variations in fcj are mainly due to variations in the activation energies, E. This is illustrated by the rate constants and Arrhenius parameters for the addition to ethene of methyl and halogen-substituted methyl radicals shown in Table 10.1. 

Radical addition to alkenes is strongly influenced by polar effects, and reductive radical additions are nearly always anti-Markovnikov. Resonance stabilisation of the unpaired electron in R can decrease the rate of addition, while the stabilisation of the adduct radical 

Non-activated double bonds, e.g. in the allylic disulfide 1 (Fig. 10.2) in which there are no substituents in conjugation with the double bond, require high initiator concentrations in order to achieve reasonable polymerisation rates. This indicates that competition between addition of initiator radicals (R = 2-cyanoisopropyl from AIBN) to the double bond of 1 and bimolecular side reactions (e.g. bimolecular initiator radical-initiator radical reactions outside the solvent cage with rate = 2A t[R ]2 where k, is the second-order rate constant) cannot be neglected. To quantify this effect, [R ] was evaluated using the quadratic Equation 10.5 describing the steady-state approximation for R (i.e. the balance between the radical production and reaction). In Equation 10.5, [M]0 is the initial monomer concentration, k is as in Equation 10.4 (and approximately equal to the value for the addition of the cyanoisopropyl radical to 1-butene) [3] and k, = 109 dm3 mol 1 s l / is assumed to be 0.5, which is typical for azo-initiators (Section 10.2). The value of 11, for the cyanoisopropyl radicals and 1 was estimated to be less than Rpr (Equation 10.3) by factors of 0.59, 0.79 and 0.96 at 50, 60 and 70°C, respectively, at the monomer and initiator concentrations used in benzene [5]  

A comparative time-resolved CW EPR and FT EPR investigation has been carried out for the addition of 2-hydroxy-2-propyl radicals to acrylate and methacrylate monomers by Weber et al. The 2-hydroxy-2-propyl radicals 

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The reaction of perfluoroalkyl iodides with electron donor nucleophiles such as sodium arene and alkane sulfinates in aprotic solvents results in radical addition to alkenes initiated by an electron-transfer process The additions can be carried out at room temperature, with high yields obtained for strained olefins [4 (equations 3-5)... 

Se-phenyl areneselenosulfonates (24) undergo facile free-radical addition to alkenes to produce / -phenylseleno sulfones (25) in excellent yield86,87 (see Scheme 7). The addition occurs regiospecifically and affords anti-Markovnikov products contrary to the analogous boron trifluoride catalyzed reaction which produces exclusively Markovnikov and highly stereospecific products86 (equation 37). Reaction 36 has been shown to have the radical... 

Radical addition to alkenes has been used in cyclizations in aqueous media. Oshima and co-worker studied triethylborane-induced atom-transfer radical cyclization of iodoacetals and iodoacetates in water.121 Radical cyclization of the iodoacetal proceeded smoothly both in aqueous methanol and in water. Atom-transfer radical cyclization of allyl iodoacetate is much more efficient in water than in benzene or hexane. For instance, treatment of allyl iodoacetate with triethylborane in benzene or hexane at room temperature did not yield the desired lactone. In contrast, the compound cyclized much more smoothly in water and yielded the corresponding y-lactone in high yield (Eq. 3.31). 

Influence of a TT-Bond Adjacent to the Reaction Center on the Activation Energy of the Radical Addition to Alkenes [40-48]... 

Radical additions to alkenes and aromatic systems are well known reactions. The trapping in this manner of radicals obtained by reduction of the aliphatic carbonyl function has proved to be a versatile electrochemical route for the formation of carbon-carbon bonds. Such reactions are most frequently carried out in protic solvents so that the reactive species is a o-radical formed by protonation of the carbonyl radical-anion. Tlie cyclization step must be fast in order to compete with further reduction of the radical to a carbanion at the electrode surface followed by protonation. Cyclization can be favoured and further reduction disfavoured by a... 

Additions. Homolytic bimolecular addition reactions of carbon-centered radicals to unsaturated groups have been studied in detail because these are the reactions of synthesis and polymerization. Within this group, radical additions to substituted alkenes are by far the best understood. An excellent compilation of rate constants for carbon radical additions to alkenes is recommended for many specihc kinetic values. ... 

Rates of radical additions to alkenes are controlled mainly by the enthalpy of the reaction, which is the origin of regioselectivity in additions to unsymmetrical systems, with polar effects superimposed when there is a favorable match between the electrophilic or nucleophilic character of the radical and that of the radico-phile. For example, in the addition of an alkyl radical to methyl acrylate (2), the nucleophilic alkyl radical interacts favorably with the resonance structure 3. Polar effects are apparent in the representative rate constants shown in Figure 4.14 for additions of carbon radicals to terminal alkenes. Addition of the electron-deficient or electrophilic rert-butoxycarbonylmethyl radical to the electron-deficient molecule methyl acrylate is 10 times as fast as addition of... 

H. Eischer and L. Radom, Factors controlling the addition of carbon-centered radicals to alkenes—an experimental and theoritical perspective, Angew. Chem., Int. Ed. Engl. 2001, 40, 1340 (rate constants for radical additions to alkenes). 

Fluorinated radicals play a significant role in synthetic organo-fluorine chemistry, for example, in electrophilic radical addition to alkenes, single-electron transfer reactions (SET), telomerization of fluoroalkenes with perfluoroalkyl iodides, polymerization to fluoropolymers and copolymers, and thermal, photochemical and radiation destruction of fluorocarbons. Furthermore, such free radicals are of interest for studying structures, reaction kinetics and ESR spectroscopic parameters.38... 

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