Identifying stereoisomers of the asymmetric hydrogenation catalyst [Me-BPE-Rh(COD)]+BF4 -

The performance of a catalyst used in asymmetric synthesis is likely to be dependent upon its stereoisomeric purity. An impurity was detectable by (31)P NMR in early development batches of the asymmetric hydrogenation catalyst [(S,S)-Me-BPE-Rh(COD)](+)BF(4) (-) made from the ligand bis((2S,5S)-2,5-dimethylphospholano)ethane [(S,S)-Me-BPE]. Its identity as a stereoisomer with one chiral and one meso-phospholane ring was deduced by comparison of the (31)P NMR spectra and GC traces of the ligand with a deliberately synthesized mixture of isomers. Interestingly, the impurity corresponded to a trans-meso isomer formed by thermal (200 degrees C) pyramidal inversion at phosphorus of the initially synthesized cis-meso-phospholane when the ligand was purified by distillation. Low levels of this trans-meso/chiral impurity do not significantly impair the enantioselectivity of the rhodium complex as an asymmetric hydrogenation catalyst, but high levels of stereochemical impurities resulted in a loss of both enantioselectivity and activity. Therefore it is indeed important to establish that a catalyst used in asymmetric catalysis is sufficiently stereoisomerically pure. Owing to strict control of the stereochemical purity of the key hexane-2,5-diol intermediate, the impurity is not detected in production batches.