A-electron withdrawing group

Other substituent, which does not participate in the stabilization, can be considered as a spectator substituent (Scheme 3) [34]. This stabilization system is particularly efficient with an amino group which is not only a 71-donating substituent but also a a-electron withdrawing group. This stabilization mode provides a large choice of substituents allowing an easy functionalization of the corresponding stable mono-aminocarbenes. In fact, several types of acyclic and cyclic amino carbenes have been already synthesized [8, 9]. 

Effects a and d can apply in all three mechanisms, though steric effects are greatest for the E2 mechanism. Effect b does not apply in the El mechanism, and effect c does not apply in the ElcB mechanism. Groups such as Ar and C=C increase the rate by any mechanism, whether they are a or p (effect a). Electron-withdrawing groups increase the acidity when in the p position, but have little effect in the a position unless they also conjugate with the double bond. Thus Br, Cl, CN, Ts, N02, CN, and SR in the p position all increase the rate of E2 eliminations. 

A = electron-withdrawing group D = electron-donating group... 

Exceptions to the carbophilic addition of heteronucleophiles to thiocarbonyl compounds were demonstrated with thiols and thioaldehydes [160] or dithioesters [36] bearing an a-electron withdrawing group, which undergo selective thiophilic addition. 

The more stable position for a electron-withdrawing group, such as carbomethoxy, is at C-3 of the starting cyclohexadienyl complexes, and terminal addition is again observed. Even the weak nncleophile, allylsilane, reacts at room temperature at the less-hindered terminal position (equation 86). 

If the acidity of the a hydrogen of iminium intermediates is increased, the a-deprotonation will be smoothly carried out by the aid of a weak base avoiding the undesired dealkylation. For this purpose, an electron-withdrawing substituent (EWG) is required on the a carbon of iminium salts N-alkylation of imines with the alkyl halides bearing an a-electron-withdrawing group... 

The feasibility of the use of acylphosphonates as a carbonyl group radical acceptor have been demonstrated. Radical cyclization of the acylphosphonate (418) in the presence of hexamethylditin gave the cyclopentanone (419) in 91% yield. Additionally, various electrophilic alkyl radicals from activated olefins bearing a-electron-withdrawing groups smoothly reacted with alkenyl acylphosphonates (420). Furthermore, similar results were obtained with al-kynylarylphosphonates (421) (Scheme 99). " ... 

Reactions of BFa and epoxides bearing an a-electron-withdrawing group (EWG) have been extensively studied. Often the course of such reactions can be predicted by assuming formation of a carbenium ion P to the EWG (avoidance of positive charge adjacent to the EWG dipole). When this occurs, facile migration of the EWG may take place. 

Artamkina, G.A., Tarasenko, E.A., Lukashev, N.V., and Beletskaya, LP, Synthesis of perhaloaromatic diethyl methylphosphonates containing a-electron-withdrawing group. Tetrahedron Lett., 39, 901,... 

Perhaps the most successful approach for promoting the 47r participation of 1-oxabutadiene systems in intermolecular Diels-Alder reactions employs a,/8-unsaturated carbonyl compounds substituted with an additional C-3 (a) electron-withdrawing group. The addition of the C-3 electron-withdrawing substituent increases the electron-deficient character of the oxabutadiene system, decreases the LUM0oxabutadiene, and as expected, enhances the observed [4 + 2] cycloaddition rate and regioselec-... 

An effective approach to promoting the 4ir participation of oxabutadiene systems in Diels-Alder reactions employs the intramolecular cycloaddition reactions of a,/3-unsaturated carbonyl compounds.27 If this intramolecular variant of the oxabutadiene [4 + 2] cycloaddition is combined with the use or generation of a,j8-unsaturated carbonyl compounds bearing an additional C-3 (a) electron-withdrawing group, exceptionally effective room temperature, regio-, diastereo-, and enantioselective [4 + 2] cycloadditions are observed.134-138 Table 7-II details the intramolecular [4 + 2] cycloadditions of a,j3-unsaturated carbonyl compounds.27,131-145... 

Active methylene A methylene group with hydrogen atoms rendered acidic due to the presence of an adjacent (a) electron withdrawing group, such as a carbonyl group. 

In another example, the a-electron-withdrawing group was nitro [110] Scheme 67 shows how various 2-(pyrrolidin-l-yl)thiophenes were thus generated. 

Some of the specific unsaturated substrates suffering from the above shortcomings, which also leverage many commercial applications as well as various synthetic transformations useful to research, are allyl chloride, allyl alcohol derivatives, and a number of compounds with a electron-withdrawing group at the allylic position. Also handicapped are many 1,3-dienes, and other dienes where the desirable terminal olefinic product either cannot be obtained (butadiene, for instance), or suffers partial isomerization to the internal olefin which then is not useful for the targeted application. 

Living radical polymerization [32] would be expected to occur only when activated halides are used in conjunction with arenediazoates. Suitably activated halides contain at least one a-electron-withdrawing group, such as an ester, acid, ketone, or nitrile group ... 

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