The borate ester of 1a and 2e exhibits complete therefore, a direct measure of the ratio 3/4. This ratio amounts to 63:37 at room temperature, which was confirmed quaternization of the boron center. An equilibrium is observed between the two diastereomeric complexes 3 and by a protonation titration experiment. The elucidation of these NMR characteristics of B(1a)(2e) allowed the more 4 formed by coordination to the boron center of either amino group of 2e. At -78Β°C, this equilibrium is slow on the NMR rapid evaluation of the enantiotopic group recognition in many other borate complexes, which range from 57:43 to timescale and a diastereomeric ratio of about 3:1 is observed. At room temperature, a rapid equilibration is observed 67:33 at room temperature. The recognition of the (after complexation) diastereotopic dimethylamino groups of 2e in resulting in two NMR resonances (both 1 H and 13 C) for the dimethylamino groups of 2e in 3/4. Comparison with the the borate complex B(1a)(2e) was utilized in a consecutive methylation reaction, which yielded enantiomerically NMR characteristics of the analogous borate complexes of 1h and 1j and of 2c and 2d shows that these resonances each enriched ammonium salt 7. The enantiomeric excess proved to be identical to the diastereomeric excess observed in the correspond to both methyl groups of a single dimethylamino group. The chemical shift difference occurs solely from the preceding recognition event. Strong evidence is presented that complex 3 is indeed the major diastereomer present in difference between the time fractions that the dimethylamino groups are coordinated to the boron center, and forms, a solution of 3/4, as expected from molecular models.
timescale was observed, a ratio of 7:1 between the two dia-[a] University of Twente, Supramolecular Chemistry and Technology strong guest-complexing nature of the compounds, the rec-P. O. Box 217, 7500 AE Enschede, The Netherlands