Acrylonitrile, radical addition

Giese and Kretzschmar7j found the rate of addition of hexenyl radicals to methyl acrylate increased 2-fold between aqueous tetrahydrofuran and aqueous ethanol, Salikhov and Fischer74 reported that the rate constant for /-butyl radical addition to acrylonitrile increased 3.6-fold between tetradecane and acetonitrile. Bednarek et al75 found that the relative reactivity of S vs MMA towards phenyl radicals was ca 20% greater in ketone solvents than it was in aromatic solvents. 

Waters61 have measured relative rates of p-toluenesulfonyl radical addition to substituted styrenes, deducing from the value of p + = — 0.50 in the Hammett plot that the sulfonyl radical has an electrophilic character (equation 21). Further indications that sulfonyl radicals are strongly electrophilic have been obtained by Takahara and coworkers62, who measured relative reactivities for the addition reactions of benzenesulfonyl radicals to various vinyl monomers and plotted rate constants versus Hammett s Alfrey-Price s e values these relative rates are spread over a wide range, for example, acrylonitrile (0.006), methyl methacrylate (0.08), styrene (1.00) and a-methylstyrene (3.21). The relative rates for the addition reaction of p-methylstyrene to styrene towards methane- and p-substituted benzenesulfonyl radicals are almost the same in accord with their type structure discussed earlier in this chapter. 

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... 

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

The rates of radical-monomer reactions are also dependent on considerations of steric hindrance. This is easily observed by considering the reactivities of di, tri-, and tetrasubstituted ethylenes in copolymerization. Table 6-5 shows the kn values for the reactions of various chloroethylenes with vinyl acetate, styrene, and acrylonitrile radicals. The effect of a second substituent on monomer reactivity is approximately additive when both substituents are in the 1- or a-position. However, a second substituent when in the 2- or (3-position of the monomer results in a decrease in reactivity due to steric hindrance between it and the radical to which it is adding. Thus 2-10-fold increases and 2-20-fold decreases in the reactivities of vinylidene chloride and 1,2-dichloroethylene, respectively, are observed compared to vinyl chloride. 

The copolymerization of furan and 2-methylfuran with dienophiles such as maleic anhydride leads to polymer structures with furan pendent functionality. Furan, 2-methylfuran, and 2,5-dimethylfuran have been copolymerized with acrylic monomers (51,52) and acrylonitrile (52,53). The furan ring of furan, 2-methylfuran, and 2,5-dimethylfuran participates as a diene in a free radical copolymerization with acrylonitrile. The initial step for furan and for 2,5-dimethylfuran is the attachment of an acrylonitrile radical at the 2-position, but for 2-methylfuran, the attack is at the-5-position. Propagation proceeds by the attack of the furan radical on an acrylonitrile molecule, to leave one olefinic bond in the structure derived from the furan ring. If this bond is in the 4,5- or 2,3-position, it may be involved in a second additional reaction by the return of the propagating chain. 

A radical addition reaction has been used to functionalize cyclopentane. The chain carrier, C1-, was generated and regenerated by a P-elimination (wocess (equation 33). Other activated alkenes. such as maleic anhydride, furanone and acrylonitrile, have also b added to cyclohexane in 15-45% yield in a reaction initialed by Bu OOH or light. ... 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

Houk and coworkers investigated the diastereofacial selectivity in radical additions of substituted cyclohexyl radicals to alkenes [13]. In this work, the force field developed by Spellmeyer and Houk was applied to intermolecular homolytic addition with success and demonstrated the added versatility of the HS model over the BS procedure which is limited to intramolecular systems. Extraordinarily accurate predictions of diastereoselectivity were made. For example, acrylonitrile is predicted to react with the 4-ter -butyl-2-methylcyclohexyl radical 28 to alford the products... 

Scheme 5. C-Glucopyranoside synthesis via anomeric radical addition to acrylonitrile...

Scheme 1. Stereoselectivity of anomeric radical addition to C=C (ratios indicated for acrylonitrile) is controlled by conformational and stereoelectronic factors [1IJ...

Base-catalysed additions of the phosphoranes (33 X = O or NH) to acrylic esters and acrylonitrile have been reported, as well as radical addition of the phosphorane (33 X = O) to vinyl ethers. ... 

Also, the electronics of the monomer greatly affect the rate of radical addition. For example, the relative rate of addition of tert-butoxy radical to various monomers has been found to correlate well with the e value of the monomers [62] (Fig. 12). For example, acrylonitrile has an electron deficient vinyl group (e = 1.2) while the styrene vinyl group is electron rich (e = — 0.8). Methyl radicals (electron rich) add two times faster to acrylonitrile than to styrene while tm-butoxy radicals (electron poor) add 57 times more rapidly to styrene. Therefore, most of the FR used for styrene polymerization tend to be of the electron deficient type (i.e. tert-butoxy, benzoyloxy, and cyanoisopropyl). 

The types of compounds that can be polymerized readily by the radical-chain mechanism are the same types that easily undergo free-radical addition reactions. Alkenes with aryl, ester, nitrile, or halide substituent groups that can stabilize the intermediate radical are most susceptible to radical polymerization. Terminal alkenes are generally more reactive toward radical-chain polymerization than more highly substituted isomers. The dominant mode of addition in radical-chain polymerization is head-to-tail. The reason for this orientation is that each successive addition of monomer takes place in such a way that the most stable possible radical intermediate is formed. For example, the addition to styrene occurs to give the phenyl-substituted radical to acrylonitrile, to give the cyano-substituted radical ... 

Suspension with large drops 10-1000 pm of insoluble monomer and catalyst suspended in water. free radical addition, styre-nic IX resins, vinyl polymers, PVC, styrene-acrylonitrile, polypropylene batch STR (1-6 kW/m mixei tip speed 6-9 m/s 3-retreated blades at 46-120 rpm heat transfer area = 1-4 tn /m depending on the volume of the reactor with small area associated with large volumes) continuous PVC... 

In fact, recent theoreticaP and experimental studies of small radical addition reactions indicate that charge separation does occur in the transition state when highly electrophilic and nucleophilic species are involved. It is also known that copolymerization of electron donor-acceptor monomer pairs are solvent sensitive, although this solvent effect has in the past been attributed to other causes, such as a Bootstrap effect (see Section 13.2.3.4). Examples of this type include the copolymerization of styrene with maleic anhydride and with acrylonitrile. Hence, in these systems, the variation in reactivity ratios with the solvent may (at least in part) be caused by the variation of the polarity of the solvent. In any case, this type of solvent effect cannot be discounted, and should thus be considered when analyzing the copolymerization data of systems involving strongly electrophilic and nucleophilic monomer pairs. 

---