Ethylenes acceptor-substituted

Alkenes without allylic H atoms—such as ethylene, acceptor-substituted ethylenes, and styrene—can be alkenylated and arylated by Heck reactions in a clearly predictable fashion. These alkenes can be alkenylated to provide 1,3-dienes, a,/3,y,S-unsaturated carbonyl compounds (Figures 13.27 and 13.28), a,j8,y,8-unsaturated carboxyl compounds (Figures 13.27 and 13.28), as well as aryl-substituted 1,3-dienes. Moreover, the same alkenes can be arylated to give styrenes, a./l-unsaturated /3-arylated carbonyl compounds, or a./l-unsaturated /3-arylated carboxyl compounds (Figure 13.26) and stilbenes (Figure 13.29). 

The application of ethylene in Heck reactions often shows different activities from other alkenes, because of Wacker-type side reactions. It was found, however, that iodo- and acceptor-substituted bromoarenes are cleanly converted in aqueous media to the corresponding styrenes utilizing a palladium-TPPMS complex [13], Furthermore, high purity o- and p-vinyltoluenes were prepared on a large scale (in... 

Substituents modulate the electron-donor or acceptor strengths depending upon their electron-releasing or electron-withdrawing properties as evaluated by the Hammett a+ (or a) constant(s).62 For example, the multiple substitution of methyl groups on ethylene leads to the strong re-donor 2,3-dimethyl-2-... 

Cyano-substituted ethylenes react in a different way with aliphatic ketones. The orientation of photochemical cycloaddition (4.661 is the opposite of that found for electron-rich alkenes, and the reaction is highly stereoselective (4.69) in the early stages. These processes involve the formation and subsequent decay of an excited complex (exciplex) from the (n,n ) singlet state of the ketone and the alkene. Aryl ketones undergo intersystem crossing so efficiently that such a singlet-state reaction is rarely observed, but the reaction of a benzoate ester with an electron-rich alkene 14.70 rnay well be of this type, with the ester acting as electron-acceptor rather than electron-donor. 

Takaki reports that ketone enolates add to dimethylstyryl sulfonium perchlorate (155) or methyl styryl sulfone (156) in a Robinson-type annulation sequence to afford the corresponding 3-hydroxythiadecalin (157) or 5-dioxide (158), respectively subsequent reductive desulfonation of (158) affords diene (159).131 However, additions to acceptor (155) suffer from competing cyclopropanation which is dependent on the electrophilicity of the carbonyl group and the ring size of the ketone (Scheme 61). As an aside, DeLucchi reports that l,l-bis(benzenesulfonyl)ethylene (160) adds to ketones at the more substituted a-carbon under neutral conditions in refluxing acetonitrile (equation 18).132... 

Amines can act as reductants for excited states of other electron acceptors, too. Again, two possibilities exist (1) amine and acceptor form a CT (2) the complex formation takes place either with the excited state of the amine or with that of the electron acceptor (see discussions of ketone-amine combinations). Examples for the former principle are such combinations as DMA-nitrobenzene [105], triphenylamine-tetracyano ethylene, 4-nitrophenole, 4-aminochlorobenzene [106], tributylamine-tetrachloromethane [107], DMA-substituted chloroacetic adds [108, 109],... 

In analogy to the cycloaddition of electron-rich alkenes with electron-deficient olefins , cycloadditions between donor-acceptor pairs of cyclopropanes and unsaturated compounds have been obtained. They include for instance the cycloaddition of substituted cyclopropanes with ethylene-tetracarbonitrile (TCNE), (equation 23 ). [10]Paracyclophadienes have been obtained from the reaction of tetrasubstituted 1,2-dicyclopropylethylenes with TCNQ. ... 

The reaction of carbon disulfide with 1,2-alkylene diamines (I) yields N-(2-aminoethyl) dithiocarbamic acids (II) which split off hydrogen sulfide thermally to give imidazolidine-2-thiones (III) (Hofmann-process). The simplest example, the reaction of carbon disulfide with ethylenediamine, is described in Organic Synthesis (5). The reaction is general for N,N -dialkyl-, N,N -diaryI, as well as for N,N -bis-(arylakyl) ethylene diamines. The rate of reaction is determined by the basicity of the diamine. Electron-donor substituents in the para-position of N-aromatically substituted ethylene diamines accelerate dithiocarbamate formation electron-acceptor groups retard it. 

As expected, introduction of a second amino group on C in enamines with acceptors on lowers the barriers to C=C rotation. This can be ascribed to a better stabilization of the transition state, in which the carbocationic part assumes the character of an amidinium ion. Some typical barriers for l,l-bis(dimethylamino)ethylenes with acceptor groups in position 2 (24) are shown in Table 6. It is observed that, with a pair of good acceptors, the barriers become too low to be accessible with the NMR technique. With weaker acceptors, the order of the barriers follows the expected capacity of the acceptor groups to stabilize the carbanionic part of the transition state. Kessler has measured the C=C barriers in a number of p-substituted l,l-bis(dimethylamino)-2-cyano-2-phenylethenes (24a) and found that the logarithms of the estimated rate constants at 25 °C correlated well with the a values for the para substituents. Similar correlations with opposite signs for p and lower sensitivity were found for the C—N and C—aryl rotations. 

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