CHIRAL RECOGNITION OF ETHERS BY NMR SPECTROSCOPY 空塞： h nder ding oto)edh has C symmetry as free ligand and in the transition etheral fun sIt tured out that straightforward a tion of dirn um olar amou in CDCh) and can,thereore.be ee the de .Theiether54laeenottealinaae ing to note groups of 56 is greatly retained.This contributior om erature.This is reminiscent of the behavior -Butyl phenyl ethers bind to the dirhodium comple tree igan 240K T=240±10 280K 300K Chirality DOI 10.1002/chinhas C1 symmetry as free ligand and in the transition state. iii. The overall mobility of the ligands is reduced by adduct formation; this process is similar for both ethers and can, therefore, be neglected in the estima￾tion. iv. The triether 54 loses its isotropic motion completely, whereas segmental motion of the methyl and isobutyl groups of 56 is greatly retained. This contribution reduces the activation entropy (larger negative value) of 54 much more than that of 56. Thus, one can expect that the absolute value of the acti￾vation entropy DDS# is much larger for 54 than for 56. As a consequence, the free activation energy DDG# (Gibbs￾Helmholtz equation) of 54 bcomes high enough to make coalescence phenomena visible in VT NMR spectroscopy, whereas this cannot be observed for 56 in the accessible temperature range. Another study dealt with some cycloveratrylenes and cryptophanes, hydrocarbons containing a number of etheral functions.99 It turned out that straightforward appli￾cation of the dirhodium method (equimolar amount of ligands and 52 dissolved in CDCl3) produces many signal dispersions with baseline separations so that it is easy to determine the ratio of the enantiomers with a good preci￾sion. This is exemplified in Scheme 23 for cryptophane A (59) possessing D3 symmetry. It is interesting to note that the H-7 signals are broadened to some extent already at room temperature. This is reminiscent of the behavior of the triethers 54 and 55 discussed earlier and is based on the same reason: large negative activation entropy due to the loss of symmetry and mobility. It should be men￾tioned that complexation induced shifts Dd indicate that the methoxy groups are somewhat better donors than the oxygen atoms in the glycol linkers but competition by the latter cannot be excluded.99 2-Butyl phenyl ethers bind to the dirhodium complex 52, too, as can be monitored from their complexations shifts Dd in the standard dirhodium experiment (Table 4).100 As expected, positive Dd-values at the directly attached ali￾phatic carbon C-2 indicate adduct formation but the donor Scheme 21. Temperature-dependent H-1/3/7/9/11/13 (left) and C-1,9,11 NMR signals (right) of 54 in the presence of an equimolar amount of 52. Top traces belong to the same signals in the absence of 52 at room temperature for comparison98 (the small 1 H peak at d 5 4.50 ppm is an impurity). Scheme 20. Structures of 54 and 55 (top) and 1 H NMR signal of H-10 of 54 under H-1/9/11 decoupling (bottom); left: free ligand, right: in the presence of an equimolar amount of 52 (dispersion Dm 5 6 Hz).97 CHIRAL RECOGNITION OF ETHERS BY NMR SPECTROSCOPY 61 Chirality DOI 10.1002/chir