EWG group

Alkyl radicals generated by reduction of organomercury compounds can also add to alkenes having EWG groups. Radicals are generated by reduction of the organomercurial by NaBH4 or a similar reductant. These techniques have been... 

For electron withdrawing substituted methylenecyclopropanes (R = EWG) the stabilization of the negative charge by the EWG group as in TS-A justifies the high regioselectivity observed. 

For X=OH and R=EWG group or alkyl, the Mitsunobu reaction (10) is evidently the most convenient procedure for the synthesis of the corresponding nitronates (35a) (Scheme 3.34). 

The use of strongly stabilized nucleophiles, for example, of [(EtO)2P(0)CHX] Li+ 58a-d, where X is a powerful EWG group, such as Me02C, CN, S02Me, or P(O) (OEt)2, in the conjugated addition with a-nitroolefins gives rise to more complex processes (201b) (Scheme 3.58). 

The reactions of ammonia or primary amines with five-membered cyclic nitronates containing the EWG -group at the C-5 atom involve deoxygenation of the nitronate fragment, aromatization of the ring, and amidation of the ester... 

On the whole, the introduction of alkyl substituents at the a-carbon atom leads to a sharp decrease in the rate of [3 + 2]-cycloaddition, that is, nitronates derived from primary AN react much faster than nitronates derived from analogous secondary AN. The introduction of functional EWG groups leads to a substantial increase in the rate of [3 + 2]-cycloaddition. It can also be noted that... 

Interesting results were obtained in the reaction of strong nucleophiles with isoxazolidines (235) containing two EWG groups at the C-3 atom (Scheme 3.157, Eqs. 2 and 3). If R=NC>2, stabilization of the anionic intermediate A is... 

Earlier, [3+ 2]-cycloaddition reactions of nitronates have been described in terms of the FMO theory. For example, French researchers studied reactions of olefins containing EWG groups with nitronates by the FMO—INDO method (248, 338b, 419). Recently, more modem methods have been used for calculations of FMO and the potential energy surfaces for several analogous reactions (87, 399,... 

N,C Elimination The reactions of standard BENAs with bases were considered in the previous section. As a rule, these reactions proceed at the silicon atom of the nitroso acetal fragment. However, if a EWG-group is adjacent to the y-C atom of BENA, the ally lie proton (Hy) at this carbon atom becomes so labile that it can be eliminated already in the presence of bases at room temperature (504), thus initiating the transformation of such BENA into conjugated en oximes (Scheme 3.227). 

In Sections 3.5.3. and 3.5.4, it was demonstrated that silylation of AN sometimes afford conjugated nitrosoalkenes as unstable intermediates with various reactivities (503). The generation of scarcely known a-nitroso alkenes, containing EWG groups in the p position, by elimination of trimethylsilanol from the corresponding SENAs is of particular interest (see Scheme 3.224). 

Control experiments in which carbon monoxide was omitted employing benzylamine as the nucleophile and CQzMe as the EWG group failed to give any Michael adduct under conditions where the cascade proceeded normally in the presence of CO. Hence, in cases so far studied, we believe path A is operational and that the amide is deprotonated to provide an anionic nucleophile in the final step. 

The reaction tolerates a wide variety of EWG groups (ester, amide, nitrile, ketone, sulphonyl, 2-pyridyl etc), and amines (aliphatic, aryl,... 

The acidity of the propargylic proton of the starting compound 18 allows the equilibration with the allene 19 induced by bases such as tertiary amines or alcoholates (Scheme 7.4). Such prototropic rearrangements furnish the title compounds 19 with at least one proton at the terminal carbon atom, often in good yields. The EWG group involves carboxylic acids [33], esters [34], ketones [35, 36], isonitriles [37], sul-fones [38], sulfoxides [39, 40] and phosphonates [41], The oxidation of easily accessi-... 

The EDG and EWG groups must be directly attached to the diene or dienophile. If another carbon atom is between the group and the diene or dienophile, the group doesn t count. 

Intramolecular Coupling of Electrophores Linked Through the EWG Groups... 

It is clear that the considerable limitations noted by Verboom and Rein-houdt were largely due to the constraints imposed by the benzene or enami-nic ring. In Viehe s work on acyclic enamine reactions, the broad range of enamine bases, the ease of cyclization even in the absence of a-EWG groups, and the variation in product stereochemistry, all would appear to be the result of removing these constraints. Clearly, much remains to be done to exploit these observations fully. 

Hybridization Electronegativity Estimation of pKg Most Acidic Hydrogen Crosscheck Carbanion Stabilization from a HOMO-LUMO Perspective Electron-Withdrawing Groups Stabilize Anions by Delocalization the More Acidic (Lower pKg) the CHs-ewg Is, the Better the ewg Group... 

These two electron sinks are the least common of the six that make up the set of multiple-bonded electron sinks with no attached leaving group. They behave very similarly to the conjugate acceptors previously discussed. The electronegative C-ewg group replaces the electronegative Y in accepting the electron flow from the nucleophile. 

Reactions of substituted cumyl benzoates in 50 50 trifluoroethanol-water show no effect of [NaNj] on the rate of reaction between 0 and 0.5M for either EWG or ERG substituents. The product ratio, however, as shown in the figure, is highly dependent on the cumyl substituent. ERG substituents favor azide formation, whereas EWG groups result in more solvent capture. Eormulate a reaction mechanism that is consistent with these observations. 

The effect of the bond dipole associated with EWG groups can also be expressed in terms of its interaction with the cationic ct complex. The atoms with the highest coefficient of the LUMO i/tj are the most positive. The unfavorable interaction of the bond dipole will therefore be greatest at these positions. This effect operates with substituents such as carbonyl, cyano, and nitro. With alkoxy and amino substituents, the unfavorable dipole interaction is outweighed by the stabilizing delocalization effect of the electron pair donation. 

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