Electron-pushing groups

Conversely, there are compounds in which nearly free rotation is possible around what are formally C=C double bonds. These compounds, called push-pull or captodative ethylenes, have two electron-withdrawing groups on one carbon and two electron-donating groups on the other (66). The contribution of di-ionic... 

Push-pull or capto-dative ethylenes can be represented by the general formula 1 where A and/or B represent electron-donating groups and X and/or Y represent electron-accepting groups. Other substituents may be aryl or alkyl groups. The conformational properties of such compounds have been the subject of much... 

To summarize, by modifying in -C-O- either the leaving group abilities of the carbon moiety (the electrofuge) (e.g., by alkyl substitution at or by ionization of OH) or those of the nitrobenzene (by substitution at the ring) it is possible to go all the way from pure addition to what appears to be pure electron transfer. The heterolysis rates increase with increasing electron push ... 

Deprotonation provides the necessary electron push to kick out the electron pair joining C(6) with the nitrobenzene oxygen. If, however, N(l) is alkylated (as with the nucleosides and nucleotides), OH catalysis is much less efficient since it now proceeds by deprotonation from N(3) (with the uracils) or from the amino group at C(4) (with the cytosines). In these cases the area of deprotonation is separated from the reaction site by a (hydroxy)methylene group which means that the increase in electron density that results from deprotonation at N(3) is transferable to the reaction site only through the carbon skeleton (inductive effect), which is of course inefficient as compared to the electron-pair donation from N(l) (mesomeric effect) [26]. Reaction 15 is a 1 1 model for the catalytic effect of OH on the heterolysis of peroxyl radicals from pyrimidine-6-yl radicals (see Sect. 2.4). 

The first isolated singlet carbenes were push-pull systems featuring both an electron-donating and an electron-withdrawing group that interact directly with the carbene center (Scheme 2). Phosphinocarbenes 6-8 are perfect in this category,... 

It has been postulated that the stability of free radicals is enhanced by the presence at the radical center of both an electron-donating and an electron-withdrawing group.163 This is called the push-putt or captodative effect (see also pp. 129). The effect arises from increased resonance, e.g. ... 

The possibility of simultaneously finding an electron-hole and an electron pair in a -system substituted by an electron-withdrawing group (NO2) and/or electron-donating group (NH2) has been examined with cis- and trans-isomers of 10 (push-pull olefins) and for non-vicinal positions40. 

Modification and functionalisation of natural polyenes, the carotenoids, is an efficient way for the molecular engineering of polyenic chains. Terminal bis-pyridinium carotenoids, termed caroviologens, represent an approach to electron conducting molecular wires (5). Fitting polyconjugated chains with an electron donor group on one end and an electron acceptor on the other end yields push-pull systems of type 1 that may be considered as polarized, unidirectional (oriented) molecular wires and also possess marked NLO properties. 

Recent examples of the rearrangement or alkynylation pathway include conversions of arylethynyl- and er -butylethynyl(phenyl)iodonium tosylates 24 and 25 to alkynylphosphonates, -selenides, and -tellurides with the appropriate anion salts in DMF (Scheme 50) [145-147], and a similar synthesis of push-pull selenides and tellurides from alkynyliodonium triflates containing electron-withdrawing groups in the alkynyl moiety [148]. 

Alkenes substituted with an electron-withdrawing (Z) and an electron-donating group (X) will be less reactive than the unsubstituted alkene if the substituents are vicinal (Z-C=C-X push-pull alkenes), but will generally become highly reactive when these two substituents are bound to the same carbon atom. The latter type of alkene (ZXC=CR2 1,1-captodative alkenes) readily dimerize to yield cyclobutanes and can react with 1,3-dienes to yield products of [2 + 2] and/or [2 + 4] cycloaddi-... 

The same is true when an electron-withdrawing group is present in beta to the amino group and great numbers of vinylogous amides, carbamates and ureas have been transformed to their ethynylogs 29 for which the term push-pull acetylenes is used (49) 90,91 A Their chemistry has been thoroughly studied and they represent the best known class of ynamines. 

In all of these reactions, the OH group provides the electronic push . In the first two reactions, the ring contracts by an alkyl migration from the secondary alcohol, while in the third it is H that migrates from the same position. 

The cycloadduct is fragmented with Me3SiBr in acetonitrile. The electrophilic silicon atom attacks the ketone and the furan oxygen atom provides the electronic push. These two groups have the 1,4 relationship necessary for a fragmentation. First of all, we shall draw the product in the same way as the starting material—this is a good tip in a complicated mechanism. The product may look odd but we can redraw it more realistically in a moment. 

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