Solution pH-induced reversal of stereoselectivity. Deuteration of malonate methylenes in some bis(malonato) cobalt (III) complexes

Deuteration of some bis(malonate)cobalt(III) complexes takes place stereoselectively in heavy water. The malonate portion of the HNMR spectrum of K[Co(mal)2(en)] in acidified D2O (pD 3.23) was measured at intervals after dissolution. Initially, one set of an expected AB quartet was observed because of the two malonate protons; (H/sub N/) is adjacent to the NH2 group in any conformation of the malonate--cobalt ring while the other (H/sub O/) is adjacent to the malonate oxygen. The low-field resonance pair was assigned to the H/sub N/ and the high-field pair to H/sub O/. As time passes, the singlet appears and grows in intensity. The singlet originates from the species CDHO produced by deuteration of only H/sub N/, because the chemical shift of this singlet is that of H/sub O/. The intensity of the singlet diminishes as the remaining H/sub O/ is deuterated. The whole spectral change with time can be expressed as CH/sub N/H/sub O/ → CDH/sub O/ → CDD. The rate of deuteration of H/sub N/ is much faster than that of H/sub O/. The novel feature of this process is the reversal of the stereoselectivity accompanying the pH change when the deuteration takes place in basic solution (pD 8.09). Natural logarithms of NMR intensities of the AB quartet and of the H/sub N/ or H/sub O/ singlet were plotted against time and from these plots pseudo first-order rate constants k/sub AB/ and k/sub HN/ or k/sub HO/ were obtained on the assumption that k/sub AB/ = k/sub HN/ + k/sub HO/