Hyperconjugative electron release

In piperidine the electron lone-pair can occupy either an axial or an equatorial position in 1-methylpiperidine the axial orientation (lb) is favoured by 99 1 over the equatorial (la). PE spectra and ab initio calculations on methylpiperidines indicate that axial 2-methyl substituents lower the amine lone-pair ionization potential by about 0.26 eV, while equatorial 2-methyl substituents as well as methyl groups on carbon atoms 3 and 4 lower the lone-pair IP by less than 0.1 eV63. This establishes the mechanism of stabilization of the amine radical cation as hyperconjugative electron release, which is larger for CC bonds than for CH bonds. The anti-periplanar orientation of the nitrogen lone-pair and the vicinal C—Me bond (lc) is much more favourable for this type of interaction than the synclinal orientation (Id). 

The mechanism in Scheme 8 was proposed for the oxidation reaction. In the first step, the Cu(II) salt, which is formed in the autooxidation of cuprous chloride, forms a complex with the amine. This is followed by a rate-determining electron transfer from the amine to the Cu(II) species giving Cu(I) and an aminium radical. The subsequent steps were considered to be fast. The authors accounted for the secondary hydrogen-deuterium kinetic isotope effect by suggesting that there was hyperconjugative electron release to the aminium ion nitrogen that forms in the slow step of the reaction. 

The authors also reasoned that constraint of the C-Si bond to the nodal plane of the n system would lead to diminished CH2Si hyperconjugative electron release. In agreement with this is the observed shift of only 0.4 ppm for C(6) in 2-silatetralin relative to C(6) in tetralin, further, C(6) in 6-fluorotetralin and 6-fluoro-2-silatetralin are almost within experimental error. Finally, assuming a constant ortho effect for fluorine, a shielding of 0.9 ppm was measured for C(6) in 7-fluoro-2-silatetralin relative to C(7) in 6-fluorotetralin. 

Table VIII shows that the chemical shift of C(4) in benzyltrimethylstannane is shifted upfield by 2.4 ppm compared with C(4) in neopentylbenzene demonstrating the hyperconjugative electron releasing ability of the CH2Sn(CH3)s moiety.

Ge(CH3)3CioH6F (Table 24, Nos. 24 to 26). and F substituent chemical shifts have been evaluated relative to the unsubstituted fluorohydrocarbon and used to determine the order of hyperconjugative electron release of M(CH3)3CH2 groups where M = Si, Ge, Sn, and Pb [47]. 

Diphenylethane 22 is more reactive towards chlorosulfonic acid than diphenyl-methane 18, probably as a result of increased hyperconjugative electron release from the ethylene bridge bond. When diphenylethane 22 was heated with a large excess of chlorosulfonic acid (20 equivalents) at 100 °C (4 hours), the cyclized product 24 was isolated in excellent (90%) yield (Equation 8). The cyclization to the seven-membered ring sulfone 24 was more easily achieved than the analogous conversion of diphenylmethane 18 to the six-membered ring sulfone 21... 

The concept of hyperconjugation arose from the discovery of apparently anomalous electron-release patterns for alkyl groups. By the field effect alone, the order of electron release for simple alkyl groups connected to an unsaturated system is fert-butyl > isopropyl > ethyl > methyl, and this order is observed in many phenomena. Thus, the dipole moments in the gas phase of PhCHa, PhC2Hs, PhCH(CHa)2, and PhC(CHa)a are, respectively, 0.37, 0.58, 0.65 and 0.700. ... 

The effectiveness of equatorial vs. axial bystanders at promoting the 1,2-H shift (MeO > Me > Ph) may be related to their ability to stabilize the partial positive charge that arises at the migration origin during the 1,2-H shift. In this scenario, the lone pairs of the MeO group are superior to the hyperconjugative and inductive properties of Me, whereas the conformationally dependent h-electron release of Ph is the least effective. 

Hyperconjugation also predicts that the R3SiCH2 group will be electron releasing at the ortho and para positions of an aromatic ring, and activating to electrophilic aromatic substitution, as shown in equation 4. 

Anchimeric Assistance—a rate enhancing effect caused by electron release from a neighbouring bond or group with some movement of nuclei (purely hyperconjugative stabilization is thus excluded) ... 

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