Hyperconjugative stabilizing effect

In fact, the hyperconjugative stabilizing effect is the net result of the destahiUzing interaction of the occupied anionic C2p and the filled Oc-nu orbitals and the stabilizing interaction of the C2p with the 0 c-Nu orbital. A complete study of the stereochemistry in nucleophilic vinylic substitution reactions has been reported by Apeloig Y, Rappoport Z (1979) J. Am. Chem. Soc. 101 5095-5098. 

Methylindole has a p/sTa of -4.6 and it is therefore a weaker base than indole itself this unusual effect has been ascribed in part to the decreased hyperconjugative stabilization of the conjugate acid (38) by the one hydrogen at position 3 compared with the two hydrogens at position 3 in the 3//-indolium ion (39). 

The pA of 1,3-dithiane is 36.5 (Cs" ion pair in THF). The value for 2-phenyl-1,3-dithiane is 30.5. There are several factors which can contribute to the anion-stabilizing effect of sulfur substituents. Bond dipole effects contribute but carmot be the dominant factor because oxygen substituents do not have a comparable stabilizing effect. Polarizability of sulfur can also stabilize the carbanion. Delocalization can be described as involving 3d orbitals on sulfur or hyperconjugation with the a orbital of the C—S bond. MO calculations favor the latter interpretation. An experimental study of the rates of deprotonation of phenylthionitromethane indicates that sulfur polarizability is a major factor. Whatever the structural basis is, there is no question that thio substituents enhance... 

The anomeric configuration is set in the reductive lithiation step, which proceeds via a radical intermediate. Hyperconjugative stabilization favors axial disposition of the intermediate radical, which after another single electron reduction leads to a configurationally stable a-alkoxylithium intermediate. Protonation thus provides the j9-anomer. The authors were unable to determine the stereoselectivity of the alkylation step, due to difficulty with isolation. However, deuterium labeling studies pointed to the intervention of an equatorially disposed a-alkoxylithium 7 (thermodynamically favored due to the reverse anomeric effect) which undergoes alkylation with retention of configuration (Eq. 2). 

Concerning steric factors, 43 is attacked in the most hindered position ( inverse effect of substitution ) likewise, 39 is attacked at the most hindered carbon. Obviously, the transition states for the formation of 44 or 50 show limited sensitivity to the degree of substitution, and the relief of ring strain is a more significant factor than the steric hindrance in the transition state. On the other hand, steric factors are important in systems such as P-phellandrene radical cation 40 which is attacked at the xo-methylene carbon (most easily accessible), or the tricyclane radical cation 56 which is attacked at the less hindered 3° carbon further removed from the dimethyl-substituted bridge (approach a). Both reactions also benefit Irom the formation of the most highly substituted, hyperconjugatively stabilized free radicals. 

The experimental dependency of the /J-silyl effect on 0 in solvolysis reactions is sketched in Figure 764. Obviously, it differs from that anticipated for a k mechanism with rate-determining formation of siliconium ion or from the cosine-squared function expected for the pure hyperconjugative stabilization model. Apparently, the /J-silyl effect is operative in the solvolysis of both the syn- and rmt/ -periplanar conformations. The rate acceleration in the latter might be ascribed to a more favourable geometry for the (T-anchimeric assistance. 

The /3-silyl effect is attenuated in a-ferrocenyI-substituted vinyl cations because the strong electron-donating effect of the a-ferrocenyl group renders the cation centre less electron-deficient and thus lowers the demand for hyperconjugative stabilization by substituents at the /1-carbon149. 

RA, unpublished data). By comparison, the large and unfavorable effect of benzoannelation on the stability of the benzenonium ion as reflected in its small negative pATR value (-2.3) compared with the larger negative values for the 1-naphthalenonium ion ( 8.0) and 9-phenanthrenonium ion (-11.6) is surprising (cf. Table 2, p. 44). The order is explained, however, if it reflects the relative magnitudes of the hyperconjugative stabilization of the ions, which in turn depends on the aromaticity of their no-bond resonance structures. 

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