The effect of substituents on geminal proton–proton coupling constants. III

Abstract

RAJ CAPOOR. Can. J. Chem. 55,2642Chem. 55, (1977.The effect of substituents at the 3-position in a series of N-methyl 5,6-dihydro-7H,l2H-dibenzo [c,f]azocines on the geminal coupling constants of the C-12 methylene protons has been determined. The slope of the Hammett plot of 'J us. o has been found to be + 0.20. The orientation of the methylene protons with respect to the n orbitals of the benzene ring bearing the substituent is such that no hyperconjugative effect should be present. The value of +0.20 is in contrast to a previously measured slope of -1.9 for compounds having a geometry ideal for hyperconjugative effects and substantiates the predictions of theoretical MO calculations. As a result, the reliability of this conformational dependence of p for use in conformational analysis has been strengthened.A comparison of the data for the azocines with those in the literature indicates the difference between the minimum and maximum effects of a phenyl substituent on a geminal coupling constant of an attached methylene group is 5.5 Hz. We have previously shown (l,2) that the effect of para substituents on the geminal coupling constants of the methylene protons in several series of benzylic systems was proportional to the Hammett substituent constants and that the magnitude of change in '5, represented by p, the slope of the Hammett plot, depended upon the conformation of the methylene protons with respect to the adjacent aromatic ring. This angular dependence was attributed to the fact that hyperconjugation of a benzene ring with the 'NRCC No. 15949. 'Present address: Defence Research Establishment Ottawa, Ottawa, Canada. adjacent methylene group causes a negative shift3 in J which is itself angularly dependent. Both valence bond (VB) calculations (6) and molecular orbital (MO) calculations (7) predicted a maximum effect on 'J when + = 0" and no effect when + = 90", where + is defined using the conformers shown in Fig. 1. However, in the calculations, there was a discrepancy between the predictions of the VB and MO methods in the region of + = 90". The former predicted a 3Since the geminal coupling constants in benzylic systems have been shown to be negative (3), it is most convenient to describe the effect of, for example, the phenyl group in toluene ('J = -14.8 HZ (4)) US. methane ('J = -12.4 (5)) as causing a negative shift in 'J.Can. J. Chem. Downloaded from www.nrcresearchpress.com by 54.191.190.102 on 05/11/18For personal use only.

Stereodynamics and Conformational Chirality of the Atropisomers of Ditolyl Anthrones and Anthraquinone

“…The observed difference between these two J gem -couplings (24.0 − 20.7 = 3.3 Hz for ϕ = 0° and 23°, respectively) is consistent with the differences reported in ref (e.g., 23.6 − 18.1 = 5.5 Hz for ϕ = 0° and 45°, respectively)…”

“…In the case of the calculated planar structure of 1-syn, this dihedral angle is exactly 0°, thus this coupling reaches its maximum value. 14 In the case of 2, the bent geometry of the central ring causes the dihedral angle φ to become about 23°(according to DFT calculations), thus reducing the value of the J-coupling, as experimentally observed (i.e., -20.7 and -24.0 Hz in 2 and 1, respectively). 15 (ii) In the case of 2, the low-field hydrogen signal of the ABsystem displays a small additional 4 J coupling (0.95 Hz) with the two hydrogens in positions 4 and 5 of the anthrone ring, whereas the high-field hydrogen signal does not display such a coupling (Figure S-1 of the Supporting Information).…”

“…(i) The large geminal coupling has been reported to be a function of the dihedral angle ϕ, defined as the angle between the π orbital and an imaginary line passing through both protons of the methylene group. In the case of the calculated planar structure of 1 - syn , this dihedral angle is exactly 0°, thus this coupling reaches its maximum value .…”

“…(i) The large geminal coupling has been reported to be a function of the dihedral angle ϕ, defined as the angle between the π orbital and an imaginary line passing through both protons of the methylene group. In the case of the calculated planar structure of 1 - syn , this dihedral angle is exactly 0°, thus this coupling reaches its maximum value . In the case of 2 , the bent geometry of the central ring causes the dihedral angle ϕ to become about 23° (according to DFT calculations), thus reducing the value of the J -coupling, as experimentally observed (i.e., −20.7 and −24.0 Hz in 2 and 1 , respectively)…”

Conformational study of a series of 6-substituted 5,6,7,12-tetrahydrodibenzo[a,d]-cyclooctenes by nuclear magnetic resonance spectroscopy

“…Recently we have shown that the hyperconjugative effect of a phenyl substituent on the geminal coupling constant of an attached methylene group varies from 0-5.5 Hz (19). Specifically the coupling coilstant is 5.5 Hz more negative when the rt orbitals of the ring and a line drawn through the two protons are parallel (projected angle + = 0") rather than perpendicular (+ = 90").…”