Electron-withdrawing character

The utihty of these adhesives arises from the electron-withdrawing character of the groups adjacent to the polymerizable double bond, which accounts for both the extremely high reactivity or cure rate and thek polar nature, which enables the polymers to adhere tenaciously to many diverse substrates. 

Cycloaddition of 2-nitrosopyridine 48 with nitrile oxides can give either di-A -oxides such as 49 or 3-mono-A -oxides such as 50 (93JHC287). In general, greater electron withdrawing character in the aromatic substituent appears to favor formation of the di-A -oxides. Sulfur ylides such as compound 51 are obtained from aryl isothiocyanates and l-amino-2-methylthiopyridinium iodides (84JCS(P1)1891) nitrogen ylides can be obtained from a similar reaction (86H(24)3363). 

Just as with cefpimazole, cefepime (54) has a quaternary nitrogen substituent at C-3 whose electron withdrawing character and excellent nucleofugic properties increases the reactivity of the... 

Because of the strongly electron-withdrawing character of the Cr(CO)5 unit, the reaction with alkynes to hydroquinone and phenol derivatives [35-37] (Dotz reaction) is possible according to Scheme 6 (see also Chap. 4 Chromium -templated Benzannulation Reactions ). 

The limitations of the system with regard to substrates and oxidants was attributed to the strong electron-withdrawing character of the perfluorinated chains and the lower steric hindrance in the position adjacent to phenols, in marked contrast to the ferf-butyl groups present in Jacobsen s catalyst, hi view of this, a second generation of fluorinated salen ligands le and If was... 

The low-valent ferrate [Fe(CO)3(NO)] 76 or Hieber anion was discovered some 50 years ago by Hieber and Beutner [43, 44] in order to extend the Hieber base reaction [45,46], in which iron pentacarbonyl 78 reacts with alkaline bases to form the [Fe(CO)4] anion [47, 48]. Compared to its homoleptic analogue, the Hieber anion is more stable because the electron-withdrawing character of the nitrosyl ligand stabilizes the negative charge at the iron atom. 

Phenazine leucos are generally more reactive and more susceptible to air oxidation than the thiazines and oxazines. Incorporation of electron-withdrawing groups on the acyl substituent at the 10-position of the leuco dye can provide a substantial improvement in the thermal and light stability of the leuco form and it is found that in general the stronger the electron-withdrawing character of the acyl substituents the more stable the leuco is.18... 

That charge-transfer effects are not involved follows from the fact that the rate of triplet decay in perfluorobenzene is larger than that in benzene. If the benzophenone triplet were to act as acceptor and the benzene derivative as donor in a charge-transfer complex, the substitution of perfluorobenzene for benzene should render this type of process much less probable due to the strongly electron-withdrawing character of the fluorine atoms. 

The greater the electron-withdrawing character of substituents in ring A, the greener the hue of brightening. 

As a consequence of the more electron withdrawing character of the boratabenzene ligand, anionic borabenzene complexes are greatly stabilized as compared to their Cp counterparts. The bis (boratabenzene) metal complexes 32 of V (62), Cr (62), Fe (below -10°C) (62), and Co (173) show fully reversible one-electron reductions at easily accessible cathodic potentials. 

Table 27.1 indicates that the overall local hardness increases with decreasing electron-withdrawing character of the substituents. Consequently, a larger repulsion between the tt-electron clouds of the two stacked rings is expected, yielding a smaller contribution to the electrostatic component of the stabilization energy AEeiec as revealed also in the table. 

Consequently, the relative contribution of xenodeborylation to fluorine addition across the C=C double bond and the ratio of the [BF4] to [XCF=CFBF3] anions depends on the electron-withdrawing character of X (acidity of the borane) and the actual ratio of XeF2 (fluoride donor) to XCF=CFBF2 (fluoride acceptor). 

In the Diels-Alder reaction between an unsymmetrical diene and dienophile, up to eight stereoisomers can be formed (March, 1992a). It is known that the regioselectivity of the Diels-Alder reaction can be biased so that only the four ori/io-adducts are produced (Fig. 21) through increasing the electron-withdrawing character of the substituent on the dienophile (Danishefsky and Hershenson, 1979). However, stereochemical control of the Diels-Alder reaction to yield the disfavoured exo-products in enantiomerically pure form has proved to be very difficult. 

The competing effects of the electron-releasing ability of the non-complexing benzene ring against the electron-withdrawing character of the substituents (acetyl, nitrile) are illustrated in Fig. 9. 

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