Dependence of hyperconjugative spin-spin coupling upon electron demand

Abstract

It is found that the geminal coupling constants of the methyl protons in a series of para-substituted toluenes are quite insensitive to the electron-withdrawing nature of the substituents, which range from an a carbanion to an a carbonium ion. The long-range coupling constants between the methyl and o-hydrogens are likewise insensitive to a similar range of substituents. It is concluded that hyperconjugation involving the methyl groups is not strongly dependent upon electron demand, unlike chemical Baker-Nathan effects, though the conclusion must be tempered with the reservation that the exact relationships between the magnitudes of the coupling constants and the magnitude of hyperconjugative stabilization are not yet known.A common but imperfectly understood chemical observation is the Baker-Nathan reactivity order, in which alkyl groups appear to become better electrondonating groups as the number of «-hydrogens is increased.1 11•2 Thus, the Baker-Nathan order of electron release is Me > -Bu, the opposite of the accepted inductive order. It is commonly but by no means universally found that the Baker-Nathan order prevails in situations of large electron demand upon the alkyl group, while the inductive order is found where the electron demand is small.1•2 The Baker-Nathan effect is frequently explained as due to carbon-hydrogen hyperconjugation, which it is supposed should increase as electron demand increases.1 Another school of thought holds that the Baker-Nathan effect is a steric effect, usually involving solvent.2An entirely different manifestation of carbonhydrogen hyperconjugation is found in certain nuclear spin-spin coupling constants. It now seems well established that the long-range coupling constants between an aromatic methyl group and the ring protons arise primarily through a 7r-coupling mechanism, i.e., through carbon-hydrogen hyperconjugation.3-11 It is also now evident that the geminal coupling constants of methyl or methylene protons are enhanced (i.e., given larger negative values) by adjacent 7r-electron systems, and that such enhancement has the geometric dependence expected if hyperconjugation were involved and is opposite in sign to the effect produced by substituents which withdraw electrons inductively.12-17 A molecular orbital study by Pople and Bothner-By17 predicts (1) E. Berliner, Tetrahedron, 5, 202 (1959).

Photochemical Equilibration/Isomerization of p-, m-, and o-Methylbenzonitrile

“…Photolysis of 2,6-Dideuterio-4-methylbenzonitrile (11). This known deuterated isomer was synthesized by the pathway shown in eq 4 with the idea that the deuterium labeling might provide important information on the mechanism for the phototranspositions of the methylbenzonitriles. For the para isomer, there are four possible meta bondings which lead to a prefulvene biradical and then, possibly, to a benzvalene.…”

Signs of methyl–methyl proton spin–spin coupling constants in xylene derivatives (interbenzylic coupling)

“…A substantial measure of agreement between theory (5-7) and experiment (1)(2)(3)(4)(8)(9)(10)(11) has led to the conclusion that the signs and magnitudes of the ortho and para benzylic coupling constants are primarily determined by n-electron contributions, transmitted to the benzylic proton by o -7 c exchange polarization. For the meta coupling constant, although agreement between theory and experiment is reasonable for an unsubstituted methyl group (1-7), experimental results indicate substantial o-electron contributions, at least in situations where a benzylic proton is constrained to lie in the nodal plane of the n-electron system (8,12,13).…”

Long-range proton spin-spin coupling

“…A surprising feature of the benzylic coupling constants is their insensitivity to substituent effects. [188][189][190][191] For example, in 2-bromo-5-chlorotoluene,188 the benzylic coupling constants differ at most by 0.1 Hz from the values in toluene.13 On this basis it appears13 that the mechanisms responsible for aromatic coupling and benzylic coupling may be mutually exclusive. Since it appears that the benzylic coupling constants can be adequately discussed in term of -electron mechanisms, this provides a further argument that substituent effects are most important in the -electron framework.…”