Anions hyperconjugation

Hehre and co-workers (DeFrees et al., 1977, 1979a) have published both experimental and theoretical evidence in support of negative ion (anionic) hyperconjugation. These workers determined the free energies for the gas-phase hydron3 transfer equilibria (31), (32) and (33) by pulsed ion cyclotron resonance spectroscopy (Wolf et al., 1976). These equilibria, which involve the gas-phase formation of a methylamino, a methoxy and a thiomethoxy anion, all lie to the right, i.e. the formation of the isotopically light anion is favoured. These results were rationalized in terms of the MO... 

More often, however, fluorocarbanions are intermediates in reactions of fluoroalkenes,35 e.g. by addition of nucleophiles, especially fluoride. Deprotonation of hydrofluorocarbons also yields the anion. Whereas a-fluorination destabilizes the anion, /J-fluorination is essentially stabilizing. The latter effect may be assisted by so-called fluorine-nonbond resonance (negative anionic hyperconjugation) which has been discussed in diverse ways supported by some,3 but criticized by others.2 31... 

P-Fluonnation always strongly stabilizes carbamons both by induction and by negative (anionic) hyperconjugation, 7 The latter no-bond resonance has been controversial, but its importance is now well established both theoretically [133, 134 and experimentally [67] The X-ray crystal structures of salts 8 [755] and 9 [736] provide cogent evidence for negative hyperconjugation... 

This could, in the authors opinion, possibly reflect a hyperconjugation" effect of the sort suggested by Polanyi [23], which is illustrated for the ethyl molecule [22a] in Scheme 11a A stabilization of the incipient carbanion R by anionic hyperconjugation, as it was described 2 years later is presented in Scheme 11b [22b],... 

We have also developed a rich canon of names to describe our rapidly changing perception of bonding in molecules, among which the terms hyperconjugation , negative (anionic) hyperconjugation , Heitler-London (HL) Increased-Valence-Structures, and r-Donor Bonds are those that concern us. We use them to explain upheld and downheld chemical shifts in the P-NMR spectra of compounds that we could not explain by normal [Pg.10]

Negative Hyperconjugation Negative hyperconjugation, also known as anionic hyperconjugation, is essentially the complementary interaction to hyperconjugation, with opposite electron flow. Consequently, we have to denote it. r=>(T or. r=>n. 

Seubold F.H. Anionic hyperconjugation the infrared spectra of sodium alkoxides. J. Org. Chan. 

Dixon DA, Fukunaga T, Smart BE. Stmctures and stabilities of fluorinated carbanions-evidence for anionic hyperconjugation. 7. Am. Chem. Soc. 1986 108(14) 4027-4031. 

Mention was made at the beginning of this chapter that a-proton abstraction from ethylbenzene is retarded by deuteration in the jS methyl group. The 11% retardation reported by Streitwieser and Van Sickle (12) for the reaction with cyclohexylamide in cyclohexyla-mine at 50°C., amounts to AAF /n = 22 cal., part of which may be due to anionic hyperconjugation. 

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... 

Cations are by no means the only species where the effects of hyperconjugative delocalization reveal themselves in such a striking manner. Similar effects exist in neutral systems or in anions. For instance, the normal propyl anion should tend to be eclipsed (E) since in this manner the molecule would optimize the 4-electron interactions between the ethyl group t orbital and the p orbital which carries the electron pair. In the bisected conformation, where ttchs and ttchs have both been raised in energy, the four-electron, destabilizing (see Section 1.7, rule 2) p ->7r interaction is stronger than in the eclipsed conformation. At the same time the two-electron, stabilizing p ->ir interaction is weaker than in the eclipsed conformation. Both effects favor the eclipsed conformation. 

Most of the reported secondary /3-deuterium KIEs attributed to negative ion hyperconjugation are for anion or carbanion ion-pair-forming hydron... 

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