The Effect of Oxygen Lone Paris on 1J(13CH) Values in 1,3-Dioxanes.

Bock, Karl Walter; Wiebe, L. I.; Lindström, Björn; Enzell, Curt R.; Swahn, Carl‐Gunnar

doi:10.3891/acta.chem.scand.27-2676

Cited by 34 publications

(17 citation statements)

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“…A similar difference occurs for a gauche and a trans orientation of the lone-pair with respect to the CH bond, as illustrated in 1,3-dioxanes (XXXIIa) 99 the lone-pair, associated with the effect of OH, appears to be smallest in the oxime XXXVIIIf where the C-l-C-2-N angle is largest) and XXXIX-XLII. Two different lone-pair effects exist in some of the examples given, one due to a lone-pair on one of the coupled nuclei and another from the adjacent atom.…”

Section: Xxxl B (All -Trans)mentioning

confidence: 84%

Effect of electron lone‐pairs on nuclear spin–spin coupling constants

Gil

Philipsborn

1989

Magnetic Reson in Chemistry

220

107

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The nature and types of lone-pair effects on nuclear spin coupling constants are reviewed in the context of a localized bond description of molecular electronic structure. Emphasis is placed on the importance of residual delocalization involving the otherwise lone-pair orbital, in terms of which the effect of an X lone-pair, when compared with an isoelectronic Y-H or X-H+ group or with an X-R group, and its orientational dependence can be interpreted. One-, two-and three-bond coupling constants are considered and the importance of lone-pair effects for configurational and conformational information is stressed and illustrated. They also serve for a better understanding of substituent inductive effects on coupling constants.A large collection of illustrative examples are presented, with particular attention paid to couplings involving H, C, N, F and P nuclei, organized in a systematic manner into nine categories. The signs of the lone-pair effects on the reduced coupling constants are found to be independent of the actual nuclei under study in the same category. This is taken as an indication that the electron lone-pairs mainly affect the Fermi contact contribution to the coupling and, accordingly, an interpretation is given in terms of simple sum-over-states models. In addition, symmetry-based relationship are established involving the sign of lone-pair effects in coupling constants between nuclei which are a different number of bonds apart.KEY WORDS NMR Multinuclear spin coupling Lone-pair effects Substituent effects Stereochemistry MO Sum-over-states models Electron delocalization CONTENTS

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Section: Xxxl B (All -Trans)mentioning

confidence: 84%

Effect of electron lone‐pairs on nuclear spin–spin coupling constants

Gil

Philipsborn

1989

Magnetic Reson in Chemistry

220

107

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“…Specifically, for the methylenic C-H bonds adjacent to oxygen in a six- membered ring, the axial C-H bond coupling constant is generally smaller by 8-10 Hz than 1 J(CH eq ) [60]. As an example, for cis-4,6-dimethyl-1,3-dioxane 1 J(C 2 H ax ) = 157.4 Hz < 1 J(C 2 H eq ) = 167.5 Hz [63] (159.0 and 171.1 Hz our calculated values). With reference to the formal 1,3-dioxane segment -O 11 -C 10 (H,C 9 )-OR in 3-5, the orientation of the C 10 -H bond is axial in the a-derivatives, whereas it is equatorial in the b-derivatives.…”

Section: Nmr Parametersmentioning

confidence: 96%

A theoretical and experimental study on the molecular and electronic structures of artemisinin and related drug molecules

Galasso¹,

Kovač²,

Modelli³

2007

Chemical Physics

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“…We report here a study of the 'H and 13C NMR spectra of some 1,3-dioxanes 4 and related compounds, in a search for further evidence for this interaction. Bock and Wiebe4 reported the NMR spectra of a series of 1,3-dioxanes.…”

Section: ' Jc(5)h(seq)*mentioning

confidence: 99%

NMR parameters for 1,3-dioxanes: evidence for a homoanomeric interaction

Cai¹,

Davies²,

Schiesser³

1994

J. Chem. Soc., Perkin Trans. 2

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~~ ~'H and 13C NMR spectra have been recorded for a series of 1.3-dioxanes, 2-0x0-I ,3,2-dioxathianes, 1,3,2-dioxaphosphorinanes and 2-thioxo-l,3,2-dioxaphosphorinanes, which are held in a chair conformation. In all of them, the spectral parameters are compatible with the equatorial C(5)-H reversed Perlin Effect), and 3JH(4)H(5es) < 3JH(4qq)H( 5al)]. This is the reverse of what IS usually observed in cyclohexanes, or at the 2-position in tetrahydropyrans, where the sequence is ascribed to an n-+cT* (anomeric) interaction between the or-oxygen and the axial C(5)-H bond. It is suggested that this reversal may be due to an n--+c* (homoanomeric) interaction between the P-oxygen and the equatorial C( 5)-H bond, through a W-arrangement of orbitals. This interpretation is supported by ab initio 6-31 G" calculations on the 1,3-dioxane molecule, which show that the equatorial C( 5)-H bond is weaker and longer than the axial C( 5)-H bond.bond being weaker and longer than the axial C( 5)-H bond [e.g. 1JC(5)H(5eq) < lJ9( 5)"(5ax) (a Chemical shifts and coupling constants in 'H and 13C NMR spectra have been used extensively for determining the axial/equatorial location of protons and substituents in cyclohexane and heterocyclohexanes. ' In cyclohexanes, an equatorial proton shows a chemical shift 0.1-0.7 ppm downfield of an axial proton. Values of 3JH(ax)H(ax) are typically 8-10 Hz, and of 3JH(ax)H(eq) and 3JH(eq)H(eq) are 2-3 Hz, and in cyclohexane, lJCH(ax) is 122.4 Hz, less than lJCH(eq) which is 126.4 H z . ~ In methylcyclohexanes, an axial methyl group (6, 17.5-18.9) is always upfield of an equatorial one (6, 23).3Similar rules are accepted to apply to heterocyclohexanes containing first row atoms.4-Thus in P-D-glucopyranose, the axial proton at C(l) shows lJCH(ax) 160 Hz, and in a-Dglucopyranose the corresponding equatorial proton shows lJCH(eq) 169 H z 8 This order of the one-bond coupling constants ( ' J C H ( ~~) < ' J c H ( ~~) )which was first described by Perlin and Casu in 1969,' has been referred to by Wolfe' as the Perlin effect. At the I-position in pyranoses, it has been ascribed to the anomeric interaction l o between the axially directed nonbonding electron pair on oxygen and the lJCH(eq) at all positions around the ring; this has been suggested to result from a dominant

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The Effect of Oxygen Lone Paris on 1J(13CH) Values in 1,3-Dioxanes.

Cited by 34 publications

References 0 publications

Effect of electron lone‐pairs on nuclear spin–spin coupling constants

Effect of electron lone‐pairs on nuclear spin–spin coupling constants

A theoretical and experimental study on the molecular and electronic structures of artemisinin and related drug molecules

NMR parameters for 1,3-dioxanes: evidence for a homoanomeric interaction

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