Analysis of Intramolecular Dynamic Processes in Enantiomeric Diaryl Atropisomers and Related Derivatives by ²H NMR Spectroscopy in Polypeptide Liquid Crystals

Abstract

We demonstrate the analytical potential of 2H-{1H} NMR spectroscopy in weakly ordering, chiral lyotropic liquid crystals made of poly(gamma-benzyl-L-glutamate) (PBLG) dissolved in chloroform or dichloromethane for investigating the intramolecular dynamic processes of four deuterated diaryls (derivatives of 1-(4'-methylphenyl)naphthalene). When the rotation of the aryl groups about the sp(2)--sp(2) bond is sufficiently slow relative to the NMR timescale, the method allows the spectral discrimination of enantiomeric atropisomers or enantiotopic directions in the prochiral derivatives. The effect of the position of substituents on the phenyl group on the conformational dynamics of these compounds has been examined as well as the nature of the organic co-solvent. When coalescence phenomena are observed, simulation of the experimental 2H-{1H} lineshapes using a formalism tailored for two deuterons undergoing mutual exchange allows the rate constants and the activation parameters for the internal rotation processes to be calculated. Experimental values of DeltaH(not equal) have been compared with data evaluated by molecular modelling calculations and the activation parameters are discussed for the various compounds. It is shown that these polypeptide mesophases have no significant impact on the interconversion dynamics of these compounds. In contrast with the nematic thermotropic phases, Haller's equation cannot be used to predict the evolution of the quadrupolar splittings (Deltanu(Q) values), and hence the order parameters, versus T in the PBLG mesophases. For these particular lyotropic systems, it is shown that an exponential function of the form Deltanu(Q)[Hz]=C x exp (-E/RT[K]) provides excellent agreement between the experimental and expected Deltanu(Q) values. Analysis of the results reported in this work suggests that orientation and chiral discrimination phenomena in these lyotropic solvents could be treated separately because they would involve different interaction mechanisms.

Analytical contribution of deuterium 2D‐NMR in oriented media to 2H/1H isotopic characterization: the case of vanillin

“…Thus, a study of the variation of RQCs versus T (over 50 K) in the case sample S‐1 (see Figure B) showed that 2 H splitting can be reduced by a factor varying from 2 ( 2 H a,c ) to 4 ( 2 H b ). More interestingly, we can see that the variation in 2 H splitting in the range of T explored can be fitted with a decreasing exponential function of the form Δν Q (Hz) = A exp (– E /k T ) , as already reported for the PBLG phases . As these sites belong to the same rigid bloc (aromatic ring), the variations of orientational behavior versus T are expected to be very similar, leading to a rather small variation of their differences.…”

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

“…No observables are expected for symmetrical rotors (Figure 2-I), but owing to the asymmetry of the environment, aryl group rotationc an be quantified in solid aromatic polymers [24] and in oriented polypeptide solvents where orientation ande nantiotopic discrimination involve different interaction mechanisms. [25] Considering the two degenerate processes shown in Figure2-I, we note that such phenomena are silent in NMR spectroscopy unless an additional symmetry-breaking element is introduced, for example, ani sotope label or ab ranching point.C lassical atropisomerismo f2 ,2'-derivatives of biphenyl (2,S cheme2)w as rationalised by Sternhell and co-workers, [26] who showed that the barrier to rotationi nb iphenyls is gov- . Twot rigonal rotors:exchange cross peaks willbeo bserved when X, Yand Zchange their environment(their colouri mitating magnetic "memory"): I) no cross peaks;I I) cross peak X$X;III) cross peaks X$X, X$X, X$X;IV) cross peaks X$X, Y$Y;V)multiple cross peaks X$X$X, three Y$Y;VI) cross peaks X$X, Y$Y, Z$Z (the same applies to A/B/C, reciprocally).…”

“…No observables are expected for symmetrical rotors (Figure ‐I), but owing to the asymmetry of the environment, aryl group rotation can be quantified in solid aromatic polymers and in oriented polypeptide solvents where orientation and enantiotopic discrimination involve different interaction mechanisms …”

Correlation 2D‐NMR experiments involving both 13C and 2H isotopes in oriented media: methodological developments and analytical applications

“…Recently, 2 H NMR using chiral anisotropic solvents has been proposed as an efficient and versatile technique for stereochemical analysis . Thus, this technique has been successfully applied: (i) discriminate enantiomers, including isotopic ones, and enantiotopic elements in prochiral molecules, (ii) to measure enantiomeric excesses ( ee ) associated with asymmetric synthesis or biosynthetic pathways, (iii) to determine the relative and absolute configuration of small molecules, (iv) to study conformational exchange, and (v) to measure the isotopic profile of natural products …”