Abstract-The proton magnetic resonance spectra of P-pinene pinocarvone and the cis-and trans-pinocarveols have been completely assigned at 220 and 300 MHz. On the basis of the proton-proton spin-spin couplings derived, conformations have been deduced for these molecules with greater certainty than has hitherto been possible. Pinocarvone is Y-shaped, while in all the other compounds the conformation is intermediate between a Y-shape and a bridged chair, with the C, atom bent away from the gem dimethyl groups. These conformations are discussed in terms of the steric interactions in these systems and are compared with related molecules.THE DEDUCTION of the precise conformations in solution by NMR of a-pinene and some derivatives was given in the previous part of this series.l It was therefore of interest to investigate similarly the conformation of /3-pinene, particularly in view of the need to interpret conformationally the recent results of the kinetic study of the hydration of a-and P-pinene reported by some of US.^ Complete spectral analyses have also been given for similar s y~t e m s . 3 *~*~ P-pinene itself (1) presents a formidable spectral analysis problem, and to aid in the interpretation, the cis-and trans-pinocarveols (2) and (3) and pinocarvone (4) were also studied. The related system nopinone (5) had previously been studied by US,^ while ( 5 ) R, = H (6) R1 = CH3 brief details of the PMR spectra of (2) and (3) have been given by other workers,, although no complete analysis was presented.
Abstract-The NMR spectra of 1 , I -dichloro-2,2-difluoroethane (l), 1 ,l-dibromo-2,2-difluoroethane (2), meso and dl 1.2-dichloro-1,2-difluoroethane (3) and 1,1,2,2-tetrachloroethane (4) have been analysed in a number of solvents. The 19F spectrum of 3 in Lbornyl acetate at 56.4 and 94.1 MHz allows an unambiguous identification of the meso and dl isomers. The spectra of the dand I isomers consist of two AA'XX' spectra with a small chemical shift difference between the d and I forms, whilst that of the meso form is an apparent AA'XX' spectrum at 56.4 MHz but an ABXX' spectrum at 94.1 MHz, the '*F nuclei in this isomer being anisochronous in this solvent. The observed solvent and temperature dependence of the couplings of 1, 2, meso 3 and 4 when combined with the calculated solvation energies, allow the determination of the rotamer energies and couplings in these molecules. The rotamer energy differences (E, -Et) in the liquid and vapour states are 0.6 and -0.2 kcal/mol (1); 0.4 and -0.5 kcal/mol (2); 0.9 and 0.2 kcal/mol meso (3) and -0.1 and -0.8 kcaljmol (4). The 3J(HH), 3J(HF) and 3J(FF) couplings for the distinct rotamers are considered together with those of similarly constituted molecules. The general agreement demonstrates that the solvation theory may be applied to multisubstituted ethanes without any basic modifications. The trans oriented HH couplings show a linear substituent electronegativity dependence, which differs appreciably from that obtained for disubstituted ethanes, however. Thegauche couplings show the influence of dihedral angle variations as well as substituent electronegativity. The rotamer 3J(FF) couplings in meso 3 are -38.2 Hz (J,) and -17.4 Hz ( J J .
Abstract-The 56.4 MHz 18F spectrum of a mixture of mesu and dl 1,2-difluoro-l,2 dichloroethane in ~-bornyl acetate resolved the 18F resonances of the separate d and E stereoisomers, thus identifying the dl spectrum, but not those of the meso isomer.These observations are considered with other examples of the use of chiral solvents to distinguish meso and dl isomers.THE RECENT observation by Kainosho et a1.l that chiral solvents and chiral shift reagents can be used to distinguish meso from d or I diastereoisomers, prompted us to record some observations on a similar theme, which complement but differ significantly from those given in Ref. 1.We were interested in the spectral analysis and rotational isomerism of 1,2 dichloro-1 ,2-difluoroethane (1). This compoundis supplied* as a roughly equal mixture of meso and dl isomers which proved impossible to separate by any physical method. The AA'XX' spectra of the isomers could, however, be analysed separately, because although the IH spectra of the isomers were almost coincident the 19F spectra were well separated, particularly in polar solvents (Fig. 1). Unfortunately all the couplings are very similar for the two isomers in any solvent ( Fig. 1) and no unambiguous differentiation of the meso or dl isomers was possible.The 19F spectrum of the mixture in optically active L-bornyl acetate (Fig. l), however, allowed an immediate differentiation.We noted that: (a) one set of peaks is separated into doublets and the other is unchanged; (b) this splitting is a chemical shift-not a coupling, as is evident from the increased splitting at 94.1 MHz; (c) there is no effect on the IH spectrum; and (d) three other chiral solvents, viz. fenchol, fenchone and carvone, did not resolve the spectra.We ascribe that set which is resolved in the L-bornyl acetate to the d (and 1) forms, i.e. the chemical shifts of the d and 1 isomers differ slightly in the chiral solvent. Thus, this is an exact extrapolation of the resolution of a single d (or 1) stereoisomer in a chiral s o l~e n t .~*~ In all three forms the chemical shifts of the two diastereotopic I9F nuclei are identical.This observation and interpretation at first sight appears to differ significantly from that of Kainosho et a1.l They found for example that the methine protons of meso-dimethyl 2,3-diaminosuccinate are anisochronous and give rise to an A13 pattern, and further state that the observation of coupling clearly distinguishes the meso from the d or Istereoisomers, since by internal comparison the methine hydrogens of the enantiomers are identical in chiral media. They obtained similar results using a chiral shift reagent on a mixture of cis and trans 2,3-butylene oxide in which the ring protons of the meso isomer are split into an AB pattern and those of the d and I isomers separate but do not split.One way to visualise the process conceptually is to imagine a bond formed between the chiral shift reagent (or solvent) and the solute. This would give for example in the 2,3-butylene oxide case, in which of course there is bon...
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