Lodovico Lunazzi

The structure and conformation of 4,4′-dichlorobiphenyl determined by N.M.R. of oriented molecules in nematic solvents

The NMR-spectrum of 4,4′-dichlorobiphenyl oriented in the nematic phase of M.B.B.A. is analysed. The phenyl ring geometry is determined. The inter-ring ortho H-H distance, the intermolecular potential due to the liquid crystal and the angle of minimal energy are derived on the basis of suggested non-bonded interaction and conjugation potentials.

Rotational barriers of biphenyls having heavy heteroatoms as ortho-substituents: experimental and theoretical determination of steric effects

The free energies of activation for the aryl-aryl rotation of 17 biphenyl derivatives, bearing a heavy heteroatom (S, Se, Te, P, Si, Sn) as ortho substituent, have been measured by variable temperature NMR. These numbers, so called B values, represent a meaningful measure of the steric hindrance exerted by the selected substituents. DFT computations match quite satisfactorily the experimental barriers and the ground state geometries as well (determined, in two cases, by X-ray diffraction). The present values extend the available list of B values and thus provide an enlarged basis for the compilation of the space requirements of standard substituents, based solely on experimental determinations.

The biphenyl-monitored effective size of unsaturated functional or fluorinated ortho substituents.

The size of a series of typical substituents has been probed by dynamic NMR measurements of the barriers to aryl-aryl rotation of the corresponding biphenyls. The resulting B values are meaningful because only mono-ortho substituted compounds were investigated and thus the results are not compromised by the non-additivity of multiple steric effects. On the basis of the chosen model system ethynyl and cyano groups were found to be slightly smaller than a phenyl ring. In contrast, vinyl and, in particular, formyl groups proved to be larger than phenyl. The latter difference is due to the loss of conjugation forces at the planar transition state. Alpha-Hydroxyhexafluoroisopropyl is slightly more bulky than tert-butyl. Pentafluorophenyl and trifluoromethoxy exhibit nearly the same effective size as phenyl and methoxy, respectively. Trifluoromethyl is somewhat smaller than isopropyl.

Stereomutation of axially chiral aryl coumarins.

Coumarins substituted by an aryl group in position 4 display restricted rotation about the Ar-C4 bond, giving rise to conformational or configurational enantiomers (atropisomers) when such a restricted motion leads to a C(1) symmetry. The dynamics of the stereomutation processes of these axial enantiomers was monitored by dynamic NMR, dynamic enantioselective HPLC, or racemization kinetics, depending on the activation energies involved. These results were further supported by DFT computations. In the two cases where the enantiomers were sufficiently long living as to be physically separated at ambient temperature, the absolute configuration was determined by means of a theoretical simulation of their electronic circular dichroism spectra (ECD).

The effect of exocyclic conjugation on the inversion of a saturated six-membered ring. A dynamic NMR study of N-substituted morpholines.

Abstract The effect of exocyclic conjugation on the inversion of a saturated six-membered ring is shown in a dynamic nmr study of a series of N-substituted morpholines where the substituents have similar shapes but different conjugating ability. As conjugation increases the ring inversion barrier decreases in contrast to previous observations of an unsaturated ring with conjugating substituents.

The Torsional Barriers of 2-Hydroxy- and 2-Fluorobiphenyl: Small but Measurable

By making use of a novel diastereotopicity probe, namely C(CF(3))(2)OH, it has been possible to measure by very low temperature (19)F NMR spectroscopy the elusive aryl-aryl rotation barriers of biphenyls bearing an OH or F group in one ortho position. The experimental values (5.4 and 4.4 kcal mol(-1), respectively) are matched by those from ab initio calculations (5.3 and 4.3 kcal mol(-1), respectively).

Lanthanide induced shift as a tool for isomer assignment in esters of unsaturated fatty acids

Abstract Addition of Yb(fod) 3 to methyl oleate ( cis ) and methyl elaidate ( trans ) shifts the carboxylic lines of their 13 C-NMR spectra to different extents; that of the cis isomer less than that of the trans isomer, as is to be expected. On the same theoretical ground it can be anticipated that the opposite will occur for C-17: an effect that has been confirmed experimentally. The method is thus proposed as a means of aiding in the assignment of the cis and trans configuration in esters of fatty acids with one double bond.

The electron spin resonance spectrum of the 2,4,6-triphenylpyryl radical

A high resolution E.S.R. spectrum of the 2,4,6-triphenylpyryl radical is presented. By investigating some deuterium substituted derivatives and with the help of simulated spectra, all hyperfine coupling constants were derived. Also an attempt to rationalize the electron-proton coupling constants in terms of twisting angles of the phenyl rings out of the heterocyclic plane was made.

Evidence of restricted rotation in the phenyl thiaxanthyl radical

The 9-phenyl thiaxanthyl radical has been obtained from the corresponding cation. A complete analysis of its E.S.R. spectrum has been achieved by using some deuterated derivatives as well as computer simulated spectra. The unusual ratio of the ortho/para hyperfine splittings (✠) has been interpreted as due to the occurrence of restricted rotation of the phenyl ring. Molecular orbital calculations indicate that the experimental results may be interpreted by assuming the phenyl ring to be twisted from the heterocyclic plane by an angle of about 72°. No significant differences were observed between the values of the spin density distribution calculated with standard p and d models.

Stereomutations of Conformational Atropisomers of Hindered 1,2‐Diaryltetrahydropyrimidines

The barriers required to interconvert the conformational enantiomers (atropisomers) of three 2-(o-halophenyl)-1-mesityl-1,4,5,6-tetrahydropyrimidines (the ortho-halogen substituents being I, Br, Cl) have been measured by low-temperature 1H NMR spectroscopy. In addition, the barrier for the inversion of the heterocyclic six-membered ring has been determined by monitoring the 13C NMR spectra at even lower temperatures. When the mesityl substituent is replaced by a 2,3-dimethylphenyl group, two stereogenic axes are created, generating two diastereomeric conformers. These were identified by low-temperature NMR as existing in a 10:1 population ratio, with a 11.5 kcal·mol−1 interconversion barrier. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)