Michael J.T. Robinson

Conformational effects in compounds with six-membered rings—VI : Conformational equilibria in 5-alkyl-1,3-dioxane

Abstract Conformation equilibria in 5-alkyl-1,3-dioxanes have been studied by determining the equilibria between stereoisomeric pairs of 2,5-disubstituted-1,3-dioxanes in which the 2-substituent (t-butyl or p -nitrophenyl) was large enough to act as a conformational locking group; the results were independent of the 2-substituent. The more fully studied 2-t-butyl series gave enthalpy and entropy differences for the conformations of 5-methyl, 5-ethyl and 5-isopropyl-1,3-dioxanes which were smaller than those for the analogous alkylcyclohexanes, in agreement with the difference in van der Waals radii of oxygen atoms and methylene groups. cis -2,5-Di-t-butyl-1,3-dioxane has an axial 5-t-butyl group. The enthalpy and entropy differences vary with the size of the alkyl group in an irregular manner which is not consistent with the usual assumptions made in the conformational analysis of chair-chair equilibria in 6-membered rings and which shows the inadequacy of considering only free energies.

Conformational effects in compounds with 6-membered rings—XIII : Detection of twist conformers using 13C NMR chemical shifts

Abstract 13C NMR spectra of derivatives of cyclohexane, piperidine, and thian in chair and twist eonformers, and of model compounds, lead to estimates of deshielding (Δδ = 3.6 ± 0.2 ppm) for axial C Me 3 on a cyclohexane ring and shielding (Δδ = −0.2 to −0.6 ppm) for ψe-C Me 3 in twist conformers, relative to equatorial C Me 3. Ring carbon atoms are considerably shielded in twist conformers relative to chair eonformers. The value of 13C chemical shifts in the study of chair-twist equilibria is exemplified by variable temperature measurements on diastereomeric pairs of compounds (11 and 13; 38 and 50).

A precise n.m.r. method for measuring isotope effects on acid–base equilibria: effects of solvent, solvent isotopic composition, and ionic strength on hydrogen and oxygen isotope effects in formic acid

Measurement of the isotope effect, Δ, on an n.m.r. chemical shift, as a function of the extent of ionisation, is the basis of a very sensitive method illustrated with formic acid for measuring isotope effects on mobile acid–base equilibria.

The stereochemical dependence of one-bond carbon-carbon coupling constants

Abstract Spin-spin coupling constants for carbon-carbon single bonds vary with the orientation of lone pairs on adjacent nitrogen atoms and of adjacent carbonyl groups.

Conformational effects in compounds with 6-membered rings—VII : The preferred conformation of 2,4-dioxabicyclo(3,3,1)nonane

Abstract The 1,3-diozane ring in the preferred conformation of 2,4-dioxabicyclo[3.3.1]nonane II has been shown by dipole moment measurements on the derivatives III to adopt a boat form, in contrast to other heterocyclic analogues of bicyclo(3.3.1)nonane.Abstract The 1,3-diozane ring in the preferred conformation of 2,4-dioxabicyclo[3.3.1]nonane II has been shown by dipole moment measurements on the derivatives III to adopt a boat form, in contrast to other heterocyclic analogues of bicyclo(3.3.1)nonane.

Diverse origins of conformational equilibrium isotope effects for hydrogen in 1,3-dioxans

Abstract Conformational equilibrium isotope effects in derivatives of 1,3-dioxan are consistent with four distinct origins: (i) steric hindrance, (ii) the anomeric effect, (iii) orbital interactions between C-H bonds and either non-bonding electrons through three or more coplanar bonds or (C -O)σ * orbitals, and (iv) a difference in polarity between C-H and C-D.

A carbon-13 and nitrogen-15 isotopic labelling study of intermediates and by-products in the reaction of chalcone and phenylhydrazine to give 1,3,5-triphenyl-2-pyrazoline

Abstract The formation of 1,3,5-triphenyl-2-pyrazoline 7 from chalcone and phenylhydrazine via 1,4-addition followed by an allowed 5-endo-trig cyclisation has been demonstrated by 13 C and 15 N labelling in time resolved 13 C NMR spectroscopy.

Conformational effects in compounds with six-membered rings—VIII : The preferred orientations of alkyl groups in allenes and in 2-alkyl-1,3-dioxanes and -dioxolanes

Abstract The preferred orientations of ethyl and isopropyl groups in 1-alkyl-3,3-diphenylallenes and in 2-alkyl-1,3-dioxanes and -dioxolanes have been determined from the temperature dependence of the spin-spin coupling constants for appropriate vicinal protons. In the allenes a methyl group rather than a hydrogen atom eclipses a double bond but in the cyclic acetals skew interactions between methyl groups and oxygen atoms are minimised in the preferred conformations. It is concluded that the conformational preferences in simple ketones and aldehydes depend primarily on the double bonds rather than on the electronegativity of the oxygen atom of the carbonyl group.

Origins of conformational equilibrium isotope effects for hydrogen in ketones

Abstract Conformational equilibrium isotope effects for hydrogen in cyclohexanones and other ketones are consistent with (i) hyperconjugation, (ii) analogues of hyperconjugation, and (iii) a difference in polarity between C-H and C-D bonds; steric effects are relatively unimportant.

1,3-oxygen to oxygen rearrangements of group IV esters

Abstract The 1,3-oxygen to oxygen rearrangements of group IV esters of benzoic acid have been studied using 18 O isotopic substitution: the free energies of activation for the intramolecular reactions fall from ~190 kJ mol −1 for 1A , to 76 for 1B , 59 for 1E , and −1 for 1F .