Captodative olefin

An interesting alcoholysis of epoxides has been reported by Masaki and coworkers <96BCSJ195>, who examined the behavior of epoxides in the presence of a catalytic amount of the Tt-acid tetracyanoethylene (TCNE, 85) in alcoholic media. Ring-opening is very facile under these conditions, typically proceeding via normal C-2 attack, as exemplified by styrene oxide (86). Certain epoxy ethers (e.g., 89) undergo C-1 attack due to anchimeric assistance. Analysis of the reaction mixtures revealed the presence of captodative ethylenes (e.g., 85) formed in situ, whieh were shown to be aetive in eatalyzing the reaction. The proposed mode of catalysis is represented by the intermediate 87. The affinity of these captodative olefins for... 

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. 

Reactions of Captodative Olefins with Radicals Leading to Low... 

Viehe and his collaborators investigated the addition of different radicals to various captodative olefins. They first studied the reaction of an equimolar amount of azo-bis-isobutyronitrile (AIBN) with captodative olefins in benzene at 80 °C [2]. The only isolated products were the bisadducts 1 of two isobutyronitrile radicals... 

Table 1. Reaction of captodative olefins CH2=C(c)d with large initial amounts of AIBN ...

The generalization of these reactions to other captodative olefins was often successful from a preparative point of view, as demonstrated by the reported results on the addition of fBuO, MeCO [55], RS [54], and MeCONMe—CH2 [54] radicals to various captodative olefins (Table 3). Additions of (iV-methyl-iV-... 

Table 3. Addition of various radicals to captodative olefins CH2=C(c)d...

However, a careful HPLC analysis of the mixture arising from the photo-stimulated reaction between a-methoxydeoxybenzoin and methyl ot-methoxy-acrylate at room temperature demonstrated that a considerable number of products are formed during this reaction [62]. Besides the expected products, some were found to result from the addition of a captodative (methoxy)-(methoxycarb-onyl)alkyl radical to the captodative olefin itself, i.e. a propagation step. The HPLC analysis was not quantitative, but the presence of these products even at high olefin/initiator ratio (2/1) implies that a propagation step cannot be ruled out for all the above experiments only on the basis of the isolation of low-molecular weight products in good yields. As we will show the polymerizability of captodative olefins is confirmed by an analysis of the literature. 

Because many electron-donating and electron-withdrawing groups are known and their combinations can generate a very large number of captodative olefins, we restricted our literature search to radical-initiated polymerizations of substitut-... 

From these tables, it is directly evident that captodative olefins are capable of undergoing a propagation step leading to oligo- or polymerization. Some of the olefins even lead to very high molecular weight polymers. 

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