Observation of stereotopic group recognition in chiral borate complexes insolution

The borate ester of 1a and 2e exhibits complete quaternization or the boron center. An equilibrium is observed between the two diastereomeric complexe s 3 and 4 formed by coordination to the boron center of either amino group of 2e. At -78 degrees C, this equilibrium is slow on the NMR timescale and a diastereomeric ratio of about 3:1 is observed. At room temperature, a rap id equilibration is observed resulting in two NMR resonances (both H-1 and C-13) for the dimethylamino groups of 2e in 3/4. Comparison with the NMR ch aracteristics of the analogous berate complexes of Ih and Ij and of 2e and 2d shows that these resonances each correspond to both methyl groups of a s ingle dimethylamino group. The chemical shift difference occurs solely from the difference between the time fractions that the dimethylamino groups ar e coordinated to the boron center, and forms, therefore, a direct measure o f 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(la)(ae) allowed the more rapid evaluation of the e nantiotopic group recognition in many other berate complexes, which range f rom 57:43 to 67:33 at room temperature. The recognition of the (after compl exation) diastereotopic dimethylamino groups of 2e in the berate complex B( la)(Se) was utilized in a consecutive methylation reaction, which yielded e nantiomerically enriched ammonium salt 7. The enantiomeric excess proved to be identical to the diastereomeric excess observed in the preceding recogn ition event. Strong evidence is presented that complex 3 is indeed the majo r diastereomer present in a solution of 3/4, as expected from molecular mod els.