Steric effects in the <sup>17</sup>O NMR spectroscopy of aromatic methyl ethers

Wysocki, Marian; Jardon, P.W.; Mains, Gilbert J.; Eisenbraun, E. J.; Boykin, David W.

doi:10.1002/mrc.1260250412

Cited by 30 publications

(6 citation statements)

References 17 publications

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“…Furthermore, H-4 and H-5 are deshielded, relative to additivity expectations, by nearly as much as H-3 and H-6 in DMB. The simple explanation is that one methoxy group hinders the IT electron donation of the other somewhat, leading to less shielding of the ring protons than calculated from additivity; then the methyl protons in DMB should be more shielded in DMB than in anisole, as should the 170 nuclei, as observed (16,24,29). Consequently, the 'H NMR chemical shifts of DMB are consistent with other NMR data.…”

Section: Conformational Deductions From 6~('~'3ch3) ~N D ' J ( '~C supporting

confidence: 74%

“…It is stated that solvation or packing forces in condensed media can overcome these energy differences (1 l), perhaps accounting for the long-range protonyoton coupling constants (14), 13C NMR chemical shifts, and 3~ spin-lattice relaxation times (15) for DMB in solution, all of which are interpreted as consistent with a predominantly planar conformer. However, "0 NMR chemical shifts, measured at 348 K in acetonitrile solution (16), indicate that the methoxy groups of DMB rotateGout of the benzene plane by an amount larger than does the methoxy group in 2-methylanisole.…”

Section: Introductionmentioning

confidence: 97%

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¹H and ¹³C NMR studies of the conformational mobility of 1,2-dimethoxybenzene in solution

Schaefer¹,

Sebastian²,

Lemire³

et al. 1990

Can. J. Chem.

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. Can. J. Chem. 68, 1393 (1990). IH NMR and 13C NMR spectral data are presented for 1,2-dimetho~~benzene-a,a'-~~C, as well as for a number of other anisole derivatives. For the title molecule, the coupling constants between the I3C nucleus in the side chain and the para ring proton or 13C nucleus in the benzene ring show that the expectation value of sin2 8 is very near 0.2 at 300 K, where 8 is the angle by which the methoxy groups twist out of the aromatic plane. This value of (sin2 8) is much larger than that of 0.05 for anisole in solution, emphasizing the greater conformational mobility of the methoxy groups in 1,2-dimethoxybenzene (DMB). Long-range coupling constants between the methyl and ring protons in DMB are also discussed and compared with those in anisole and some of its derivatives. The preponderance of conformations with large values of 8, thought to occur in the vapor, disappears in solution at ambient temperatures. Under these conditions, the molecule is perhaps best described as preferring a planar conformation in which, however, the methoxy groups undergo excursions in 8 whose average value is near 25".Key words: 1,2-dimethoxybenzene, conformational behaviour, internal mobility, long-range coupling constants, I H and I3C NMR. Introduction The conformational behaviour of 1,2-dimethoxybenzene (DMB), veratrole, has been studied extensively with somewhat mixed results. Early STO-3G MO computations, with geometries based on calculations for anisole, have the 0°,900 (see 1)

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Section: Conformational Deductions From 6~('~'3ch3) ~N D ' J ( '~C supporting

confidence: 74%

Section: Introductionmentioning

confidence: 97%

¹H and ¹³C NMR studies of the conformational mobility of 1,2-dimethoxybenzene in solution

Schaefer¹,

Sebastian²,

Lemire³

et al. 1990

Can. J. Chem.

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“…In the 4-MeO derivatives 7, the MeO group is coplanar with the plane of the aromatic ring, contrary to the situation in 20 where the 4-MeO group is forced to a nonplanar conformation about the C ar -O bond. 6 As the 4-MeO group in 20 is thus effectively isolated from conjugative interaction with the aromatic ring, the electron-attracting inductive character of the MeO group leads to an enhanced p-π conjugation in the ArO moiety, thus promoting the relative stability of the allylic ether.…”

Section: Compounds Without Ortho-substituents (1-36)mentioning

confidence: 99%

“…In both isomers of 54 the 2-MeO group is likely to have an almost orthogonal orientation with respect to the plane of the aromatic system (cf. the orientation of the 2-MeO group in 1,2,3-trimethoxybenzene 6 ). It is surprising to find that the effect of the 3-MeO group in 54 is to decrease the reaction entropy by ca.…”

Section: Di-and Trisubstituted Derivatives With a Single Ortho-substi...mentioning

confidence: 99%

Thermodynamic, spectroscopic, and density functional theory studies of allyl aryl and prop-1-enyl aryl ethers. Part 1. Thermodynamic data of isomerization

Taskinen

2001

J. Chem. Soc., Perkin Trans. 2

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A chemical equilibration study of the relative thermodynamic stabilities of seventy isomeric allyl aryl ethers (a) and (Z )-prop-1-enyl aryl ethers (b) in DMSO solution has been carried out. From the variation of the equilibrium constant with temperature the Gibbs energies, enthalpies, and entropies of isomerization at 298.15 K have been evaluated. Because of their low enthalpies, the (Z )-prop-1-enyl aryl ethers are strongly favored at equilibrium, the Gibbs energies of the a→b isomerization ranging from Ϫ12 to Ϫ23 kJ mol Ϫ1 . The entropy contribution is negligible in most reactions, but occasionally small positive values less than ϩ10 J K Ϫ1 mol Ϫ1 of the entropy of isomerization are found. The equilibration studies were also extended to involve two pairs of related isomeric ethers with a Me substituent on C(2) of the olefinic bond. The Me substituent was found to increase the relative thermodynamic stability of the allylic ethers by ca. 3.4 kJ mol Ϫ1 .

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“…(2) The negative charge on the methoxy O atom is increased (Wysocki, Jardon, Mains, Eisenbraun & Boykin, 1987) and ought to contribute to the enlargement of the torsion angle. The first consideration must override the second.…”

Section: -(26-dimethoxyphenyl)-4h-1-benzopyran-4-onementioning

confidence: 99%

Structure of 2-(2,6-dimethoxyphenyl)-4H-1-benzopyran-4-one (2',6'-dimethoxyflavone)

Wallet¹,

Gaydou²,

Baldy³

1989

Acta Crystallogr C

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Steric effects in the ¹⁷O NMR spectroscopy of aromatic methyl ethers

Cited by 30 publications

References 17 publications

¹H and ¹³C NMR studies of the conformational mobility of 1,2-dimethoxybenzene in solution

¹H and ¹³C NMR studies of the conformational mobility of 1,2-dimethoxybenzene in solution

Thermodynamic, spectroscopic, and density functional theory studies of allyl aryl and prop-1-enyl aryl ethers. Part 1. Thermodynamic data of isomerization

Structure of 2-(2,6-dimethoxyphenyl)-4H-1-benzopyran-4-one (2',6'-dimethoxyflavone)

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Steric effects in the 17O NMR spectroscopy of aromatic methyl ethers

Cited by 30 publications

References 17 publications

1H and 13C NMR studies of the conformational mobility of 1,2-dimethoxybenzene in solution

1H and 13C NMR studies of the conformational mobility of 1,2-dimethoxybenzene in solution

Thermodynamic, spectroscopic, and density functional theory studies of allyl aryl and prop-1-enyl aryl ethers. Part 1. Thermodynamic data of isomerization

Structure of 2-(2,6-dimethoxyphenyl)-4H-1-benzopyran-4-one (2',6'-dimethoxyflavone)

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Steric effects in the ¹⁷O NMR spectroscopy of aromatic methyl ethers

¹H and ¹³C NMR studies of the conformational mobility of 1,2-dimethoxybenzene in solution

¹H and ¹³C NMR studies of the conformational mobility of 1,2-dimethoxybenzene in solution