Conformational Studies by Dynamic NMR. 62.¹ Stereomutations of Rotamers and of Conformational Enantiomers in 1,2-Diacylbenzenes

Abstract

A number of 1,2-diacylbenzenes have been investigated by dynamic NMR spectroscopy. The 1,2-formyl derivative 1 was found to exist (at −162 °C) in the nearly coplanar ZE and EE conformations (70% and 30%, respectively) that interconvert with a free energy of activation of 4.9 kcal mol-1. On the contrary, the more hindered 1,2-diisobutanoylbenzene (4) adopts a twisted conformation (as indicated by the 13C spectrum at −157 °C) which, in principle, might correspond either to a meso or to a racemic stereolabile structure. Only the racemic conformer is, however, believed to be populated, and the interconversion barrier between the RR and SS conformational enantiomers has been determined (5.6 kcal mol-1). Although the 1,2-diacetyl- and 1,2-dipropanoylbenzenes did not exhibit dynamic NMR effects in solution, they were assigned a twisted (racemic) conformation on the basis of the corresponding solid state 13C CP-MAS spectra, in that they display a 1:1 doublet signal for each pair of enantiotopic carbons. All these conclusions agree with the predictions of molecular mechanics calculations.

Dynamic NMR study of dinitrophenyl derivatives of seven‐membered cyclic ketals of pyridoxine

“…The second process is associated with a rotation of dinitrophenyl fragment around the C–O bond leading to conformational enantiomers due to the axial chirality (Fig. ) . A similar phenomenon of axial chirality of conformers of seven‐membered sulfur‐containing twist forms of spirobisdithiepins was reported (Fig.…”

Conformational Studies by Dynamic NMR. 65.1Interconversion of Stereolabile Meso and Racemic Diastereoisomers of Hindered 1,4-Diacylnaphthalenes

“…The computer simulation of the line shape was performed by a computer program based on DNMR6 routines (QCPE), and the best fit was visually judged by superimposing the plotted and experimental traces. The free energies of activation were calculated assuming a coefficient transmission of 1/2 because both the carbonyl groups have the same probability to exchange; the use of a unitary coefficient would have given Δ G ⧧ values 0.3−0.4 kcal mol -1 higher than those reported in Table . Within the errors, the Δ G ⧧ values were found to be independent of the temperature, thus suggesting negligible Δ S ⧧ values, as is often observed in conformational processes.…”

“…Hindered alkyl aryl ketones adopt nonplanar conformations − that can give rise to stereolabile enantiomers if the substituents are in such a position as to deprive the molecules of planes or center of symmetry: 1-acyl-2-methylnaphthalenes provided examples of such a situation. Naphthalenes bearing two equal acyl substituents in positions 1 and 4, as well as two methyl groups in positions 2 and 3, possess two identical chirality axes, and as a consequence, two stereolabile meso and racemic diastereoisomers (or conformers) are expected to be observed at temperatures where the Ar−CO rotation rate is rendered sufficiently slow.…”

Architecture and function of atropisomeric molecular triads

“…As one of the structurally simplest cases, 1,2-diisobutanoylbenzene (2) was found to adopt a nonplanar conformation (from the 13 C NMR data at -157°C), either syn or anti (meso or racemic in the original publication). 17 The computed steric a IUPAC Recommendations 1996 for Basic Terminology of Stereochemistry define atropisomers as "a subclass of conformers which can be isolated as separate chemical species and which arise from restricted rotation about a single bond." Note that the definition makes no reference to the conditions under which the isolation or observation of atropisomers should be carried out; that is, it makes no restriction on how high the barrier to interconversion under normal conditions should be.…”