Polar addition electron-withdrawing

The rates of addition to the unsaturated 1- and 1,1-disubsituted olefins are thought to be mainly determined by polar factors. Electron-withdrawing substituents will facilitate the addition of nucleophilic species, while electron-donating substituents will enhance the addition of electrophilic species. The addition of an initiating free radical to a monomer is called the initiation step, which is the first step of a chain reaction or propagation that ends through a termination reaction, in which two radicals interact in a mutually destructive reaction to form covalent bonds and cease propagation. 

For olefins with Ji-substitucnts, whether electron-withdrawing or electron-donating, both the HOMO and LUMO have the higher coefficient 021 the carbon atom remote from the substituent. A predominance of tail addition is expected as a consequence. However, for non-conjugated substituents, or those with lone pairs (e.g. the halo-olefins), the HOMO and LUMO are polarized in opposite directions. This may result in head addition being preferred in the case of a nucleophilic radical interacting with such an olefin. Thus, the data for attack of alkyl and fluoroalkyl radicals on the fluoro-olefins (Table 1.2) have been rationalized in terms of FMO theory.16 Where the radical and olefin both have near neutral philicity, the situation is less clear.21... 

Additionally, it has been noted that Tetralin operates via hydride transfer, at least in its reduction of quinones. Thus it has been shown that Tetralin readily donates hydrogen to electron-poor systems, such as quinones at 50°-160°C. The reaction is accelerated by electron-withdrawing substituents on the H-acceptor and polar solvents, and is unaffected by free radical initiators (6). These observations are consistent with hydride transfer, as is the more recent finding of a tritium isotope effect for the reaction (7). 

The allenes 1 directly connected with an electron-withdrawing substituent have been used successfully as synthetic building blocks for more than four decades (see Scheme 7.1). The polarization of the C=C double bonds by the acceptor substituent allows a wide and very useful range of subsequent reactions, for example nucleophilic additions, cycloadditions and miscellaneous syntheses of heterocydes. 

Fields et alf and Schmidt made closely parallel observations concerning polar cycloaddition of ethylenes substituted at the a-position by an electron-withdrawing group and having no substituent at the jS-position. In both cases the product observed was that to be expected if the electrophile had added to the j3-carbon atom. Since it is clear that the normal ground-state polarization of acrylonitrile (127) and methyl methacrylate (128) should tend to destabilize the cation produced by j8-addition, it was concluded that the orientation of polar cycloadditions could not be predicted by the rules of electrophilic addition and that this apparent anomaly pointed toward a more concerted type of cycloaddition reaction. 

In connection with the substituent effects, the kinetic stability of benzyne is suggested to be increased by electron withdrawal (-/) and decreased by electron release (+/).73 However, the inference cannot be extrapolated to selectivity of substituted arynes in general. For example, in additions involving competition between phenyllithium and lithium piperidide, the methyl substituents (+/) on benzyne increase its selectivity, whereas methoxy groups (-/) decrease it (Scheme 6). On the other hand, in reactions of car-banions derived from acetonitrile in alkylamine solvents both +/ and -/ benzyne substituents lower selectivity and cause predominant amination. Thus, the method was found unsuitable for preparation of many substituted benzyl nitriles.74 In symmetrically disubstituted arynes there is partial cancellation of polarization, and in fact acceptable yields of acetonitrile adducts could be obtained from 3,6-dimethoxy-benzyne.75 The selectivity of substituted arynes varies with the set of nucleophiles in the competition and no comprehensive theory or simple generalization is available on this point. 

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