Captodative acrylonitriles

Table 5. Initiated radical homopolymerization of captodative acrylonitriles CH2 = C(d)CN...

Table 13. Copolymerization parameters of captodative acrylonitriles H2C = C(tf)CN (MJ in their radical copolymerization with monomers (M2) other than styrene...

In a systematic study of the addition of cyclohexyl radicals to a-substi-tuted methyl acrylates, Giese (1983) has shown that the captodative-substituted example fits the linear correlation line of log with o-values as perfectly as the other cases studied. Thus, no special character of the captodative-substituted olefin is displayed. More recently, arylthiyl radicals have been added to disubstituted olefins in order to uncover a captodative effect in the rate data (Ito et aL, 1988). Even though a-A, A -dimethyl-aminoacrylonitrile reacts fastest in these additions, this observation cannot per se be interpreted as the manifestation of a captodative effect. Owing to the lack of rate data for the corresponding dicaptor- and didonor-substituted olefins, it is not possible to postulate a special captodative effect. The result confirms only that the A, A -dimethylamino-group, as expected from its a, -value, enhances the addition rate. In the sequence a-alkoxy-, a-chloro-, a-acetoxy- and a-methyl-substituted acrylonitriles, it reacts fastest. 

The radical polymerization behavior of captodative olefins such as acrylonitriles, acrylates, and acrylamides a-substituted by an electron-donating substituent is reviewed, including the initiated and spontaneous radical homo- and copolymerizations and the radical polymerizations in the presence of Lewis acids. The formation of low-molecular weight products under some experimental conditions is also reviewed. The reactivity of these olefins is analyzed in the context of the captodative theory. In spite of the unusual stabilization of the captodative radical, the reactivity pattern of these olefins in polymerization does not differ significantly from the pattern observed for other 1,1-disubstituted olefins. Classical explanations such as steric effects and aggregation of monomers are sufficient to rationalize the observations described in the literature. The spontaneous polymerization of acrylates a-substituted by an ether, a thioether, or an acylamido group can be rationalized by the Bond-Forming Initiation theory. 

To broaden our understanding of the chemical behavior of these novel monomers, it would be appropriate to try anionic polymerizations of captodative monomers. Inasmuch as sulfur is able to stabilize adjacent carbanions, a-alkylthioacrylates and -acrylonitriles should respond well to anionic initiators. Cationic polymerization of certain captodative monomers may also be of interest as alkylthio- and cyano-substituents can stabilize a cationic propagating center. 

Captodative alkenes 67 can be dialkylated, for example, by addition of iso-butyronitrile radical derived from thermal decomposition of AIBN under the same conditions as those which lead to polymerization of other acrylic alkenes. For example, a-morpholino-acrylonitrile (67, c = CN, d = N(CH2CH2)20) leads to 69, in 71% yield (Scheme 12) [4a]. With a-/-butylthio-acrylonitrile (67, c = CN, d = SC(CHj)3), the same process leads to 70 in 88% yield [7]. The adduct radical 68 is highly stabilized, and is in equilibrium with dimer 70. The reaction is quite general, and has been applied to other captodative alkenes (c = CN, COR, CO2R and d = NR2, OR, SR) together with various sorts of radical partners, derived from alkanes, alcohols, thiols, thioethers, amines, amides, ketones, aldehydes, acetals and thioacetals [44, 45]. 

Captodative alkenes also react well with other alkenes in [2-I-2] cycloaddition reactions. For example, trifluorochloroethylene 115 reacts more efficiently with a-alkythioacrylonitrile 116 than with acrylonitrile [58]. 

Captodative alkenes are also good dipolarophiles [65] and dienophiles [66]. The corresponding [3+2] and [4+2] cycloaddition reactions seem to be concerted, as usual, so that diradicals are not directly involved as such. However, the mechanism may be rather asynchronous, with the transition state having pronounced di-radicaloid character, and thereby being stabilized by the captodative effect [67]. This can explain the higher dienophilic character of a-methylthioacrylonitrile 127 compared to acrylonitrile, despite the unfavorable steric effect (Scheme 22) [68]. 

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