Electron-releasing effects

Substitution of hydrogen by methyl results in a slight rate increase as a result of the electron-releasing effect of the methyl group. A r-butyl substituent produces a large rate decrease because the steric effect is dominant. 

At the second stage of chlorine substitution in the tetramers there is a greater statistical probability for the incoming nucleophile to attack the phosphorus adjacent to =P(C1)(NHR), viz. P4 or P8, rather than the remote phosphorus, viz. P6 (Fig. 9). However, this statistical effect is countered by the electron releasing effect of the substituent already present on P2, which tends to deactivate P2 as well as P4 and P8 towards further nucleophilic substitution. It is observed that reactive amines such as dimethylamine (94) or ethylamine (95) react with N4P4C18 and... 

Due to electron withdrawing effect of nitro group and electron releasing effect of methoxy group. 11.20 (i) Hydration of Propene. 

Group IV substituents, especially the trimethylsilyl group, apparently enhance the electron affinity of aromatic systems. The effect is particularly noticeable in aniline derivatives. The strong electron-releasing effect of the amino group decreases the electron affinity of the aniline derivatives and hinders reduction to the radical anions. Nitroanilines may be reduced to radical anions (65). The only other aniline radical anions that have been reported bear silyl substituents either at nitrogen (62) or on the ring (83, 85, 86). 

Thus, strong shieldings are observed for fi carbons of enol ethers and alkynyl ethers, as shown for 1,1 -dimethoxyethene (54.7 ppm) and ethoxyethyne (23.4 ppm) in Table 4.26. In 1-alkoxy-l,3-butadienes, transmission of the electron releasing effect along the conjugated double bonds affects alternate carbons similarly, shielding the carbons in / and <5 position as illustrated for l-ethoxy-2-methyl-l,3-butadiene. 

Carbonyl carbon-13 shifts of aldehydes, ketones and carboxylic acids, including all derivatives, occur between 150 and 220 ppm [281]. Within this range, carboxy carbons are shielded (150-180 ppm) relative to carbonyl carbons in aldehydes and ketones (190-220 ppm). This is attributed to an electron releasing effect of the additional hetero... 

Carbon-13 chemical shifts of representative aldehydes [284] and ketones [285-288] are collected in Tables 4.27 and 4.28. Inspection of the data shows that a, / , and y effects are up to 7, 2, and — 1 ppm, respectively. These increments are significantly smaller compared with those reported for alkyl carbons. Obviously, the electron releasing effect of alkyl groups (( +(-/-effect) slightly attenuates positive polarization of the carbonyl carbons. 

Carbon-13 NMR spectra of the H2C On series, such as deltic, squaric, croconic and rhodizonic acids, obtained in anhydrous solvents [304] display carbonyl shifts similar to those reported for quinones (Table 4.33). Considerable shielding of the carbonyl carbon of deltic acid diethyl ester is not only attributed to the three-membered ring but also to an electron releasing effect of the ethoxy groups. 

Polarization of double bonds due to the electron releasing effect of thioalkyl groups in thioenol ethers is much weaker than that reported for enol ethers (Table 4.26), as can be verified for the (E) and (Z) isomers of methyl propenyl sulfide in Table 4.40 [326, 327]. 

Shieldings observed for the o- and p- N-phenyl ring carbons in cyclic guanidines are attributed to a ( + )-M electron releasing effect of the imino nitrogen. This indicates an N-phenylimino rather than the tautomeric N-phenylamino compound [348] (bottom of page 241). 

The lone-pair electrons of bridgehead nitrogens in indolizine and its aza analogs [458] are delocalized, as concluded from carbon-13 shifts and in accordance with CNDO calculations All ring carbons of the parent indolizine except C-5 and C-9 (Table 4.67) are shielded (99-120 ppm) due to the (+ )-M electron releasing effect of the bridgehead nitrogen. 

The ionic nature of the catalyst, which produces isotactic polymer, falls within a rather narrow range of cationic character. This requirement depends on the electron releasing effect of the aromatic ring at the propagating end of the double bond. 

Shima, Smid and Szwarc (56) studied the effect of the methyl substitution in the polymerization of butadiene, isoprene and dimethyl-butadiene. They showed that the electron-donating methyl group decreased the rate of polymerization catalysed by polystyrylsodium. This same electron releasing effect of the methyl is seen, since the 3.4-structure, not 1.2-structure, is produced predominantly from isoprene. This results from the anionic propagation mechanism of the alkali metal alkyl catalysed polymerization of dienes which produced 1.2 and 3.4-structures. 

The very strong electron-releasing effect of the silatranyl group causes a significant reduction in the redox potential corresponding to the reversible Fen/Fem transition of l-silatranyl-l -(trimethoxysilyl)ferrocene 52 (AE1/2 = —0.19 V) and 1,1 -bis(silatranyl)ferrocene 53 (AE1/2 = —0.47 V) with respect to that of ferrocene ( 1/2 = +0.40 V)279. 

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