Valence-bond studies of contact nuclear spin-spin coupling. III. .pi.-Electron coupling in aromatic and cyclic unsaturated hydrocarbons

Seven methyl derivatives of the 3-and 4-fluoropyridines are synthesized and their p.m.r. spectra are analyzed. The nuclear spin-spin coupling constants are compared with previous results for the four methyl derivatives of 2-fluoropyridine. A model in which the nitrogen atom polarizes primarily the a electron system of the ring, leaving the x electron contribution to the coupling mechanism relatively unaffected, qualitatively accounts for the large majority of the coupling constants. For example, the coupling over six bonds between methyl protons and a fluorine nucleus, Jy3, is the same whether the fluorine atom or the methyl group is placed ortho to the nitrogen atom and is little different from its value in p-fluorotoluene. The model is consistent with significant a electron contributidns to long-range couplings over four and five bonds from methyl protons to fluorine nuclei or ring protons. Evidence is adduced for resonance structures in which fluorine conjugates with nitrogen or with ring carbon atoms. An earlier suggestion, that hyperconjugation of the methyl group with nitrogen is necessary to the interpretation of the observed couplings, is dropped. Instead, a substantial polarization of the a electron core near C-2 and -6 is invoked but apparently does not extend appreciably beyond these atoms in the ring.Sept derives mtthyles des fluoro-3 et 4 pyridines sont synthetises et leurs spectres r.m.n. analyses. Les constantes de couplage spin-spin sont comparees aux resultats antkrieurs obtenus pour les quatre derives methylks de la fluoro-2 pyridine. Un modile dans lequel l'atome d'azote polarise principalement le systeme electronique cr du cycle laissant relativement inchangee la contribution electronique x au mecanisme du couplage, explique qualitativement la grande majorite des constantes de couplage. Par exemple, le couplage a travers six liaisons entre les protons du methyle et le noyau de fluor, Jy3, est le mdme si le fluor ou le methyle est plack en ortho par rapport a I'azote et diffire peu de la valeur obtenue dans le p-fluorotoluine. Le modile est compatible avec les contributions importantes des electrons cr aux couplages a longue distance a travers quatre et cinq liaisons, a partir des protons du methyle au noyau du fluor ou au protons du cycle. Une preuve de structures resonantes dans lesquelles le fluor se conjugue a I'azote ou aux atomes de carbone du cycle est apportke. La suggestion premiere selon laquelle I'hyperconjugaison du groupe mtthyle avec I'azote est necessaire a I'interpretation des couplages observes, n'est pas retenue. A la place, une polarisation substantielle des klectrons a proches du C-2 et du -6 est avancee mais ne s'etend pas apparemment au-deli de ces derniers de f a~o n notable.

Section: ( I ) the Methyl Proton -Ring Protonsupporting

confidence: 82%

Sigma and Pi Electron Contributions to Long-range Spin–Spin Coupling Constants in the Methyl Derivatives of the Fluoropyridines

Rowbotham¹,

Schaefer²

1972

“…(2) Long-range Couplings and Conformarional Information The long 6J,H.CH and mechanism portional to -range couplings over six bonds, 6JpH-CF, depend on a a-x electron (16)(17)(18)(19) and apparently are prosin2 8, where 8 is the angle defined in 6 to 8. Thus, in toluene and p-xylene,…”

Section: Results and Discussion ( I ) Spectral Analysismentioning

confidence: 99%

Nuclear magnetic resonance and molecular orbital studies of the conformational preferences of the fluoromethyl group in some benzylfluoride derivatives

Schaefer¹,

Rowbotham²,

Parr³

et al. 1976

The proton magnetic resonance spectra of some benzylfluoride derivatives yield long-range spin–spin coupling constants between ring protons and protons or fluorine nuclei in the fluoromethyl group. In conjunction with the eigenfunctions for a hindered twofold rotor, the couplings over six bonds are used to show that in 3,5-dichlorobenzylfluoride in solution the C—F bond prefers the benzene plane by 260 ± 50 cal/mol; in close agreement with ab initia and MINDO/3 molecular orbital calculations. The latter method suggests that in a conformation in which the C—F bond lies in a plane perpendicular to the benzene ring, the C – C – F angle reduces to 107.2° and the C – C – H angles become 116.1°, perhaps due to increased conjugation of the C—F bond or fluorine atom with the π electrons of the ring. The observed barrier is presumably a delicate balance between steric interactions, hyperconjugation or p–p conjugation effects, and dipole–dipole interactions between polarized bonds.

“…In the study of long-range spin-spin coupling constants, their mechanisms, and their application to conformational problems (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16), it has been shown ( 5 ) that 6~:.CH varies as sin2 0 in toluene derivatives. Here 0 is the angle by which the a C-H bond twists out of the benzene plane.…”

Section: Introductionmentioning

confidence: 99%

The conformational dependence of in some 4-fluorophenyl derivatives of methane, ethene, and cyclohexane

Schaefer¹,

Peeling²,

Penner³

et al. 1985

The spin-spin coupling over six bonds between '"F and "C nuclei on the sidechain in thirteen 4-fluorophenyl derivatives appears to be mediated by a a-.rr mechanism. Its magnitude depends somewhat on the hybridization state of the carbon atom canying the coupled nucleus, as well as on the electronegativity of substituents attached to this carbon atom. A consistent behaviour of this coupling is observed if its value is assumed to be proportional to sin2 0, where 0 is the angle by which the bond canying the coupled carbon nucleus twists out of the ring plane. However, in 4-fluorostyrene 'J%" is a -rr electron coupling in the planar form, so that its magnitude decreases as the vinyl group twists out of the benzene plane. The a--rr contribution to this coupling is smaller than the -rr component. IntroductionIn the study of long-range spin-spin coupling constants, their mechanisms, and their application to conformational problems (1-16), it has been shown ( 5 ) that 6~:.CH varies as sin2 0 in toluene derivatives. Here 0 is the angle by which the a C-H bond twists out of the benzene plane. The coupling proceeds via a u-.rr mechanism and involves the sidechain proton and the para ring proton. Similarly, S~;,CH, the coupling between the sidechain proton and the para carbon-13 nucleus also varies as sin2 0 (17,18), implying that this parameter allows an extension (8) of the J method (I) as a conformational