Observation of Stereotopic Group Recognition in Chiral Borate Complexes in Solution

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