Kinetics of the stereoselective deuteration of malonate hydrogens in some bis(malonato)cobalt(III) compounds and the reversal of stereoselectivity induced by the solution pH.
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The deuteration rates at malonate methylene groups have been measured for several [Co(mal)<SUB>2</SUB>(N)<SUB>2</SUB>]<SUP>−</SUP> ions over the pD range of 2 to 9 at 36.4 °C, where mal=malonate ion, (N)<SUB>2</SUB>=ethylenediamine, <I>cis</I>-(NH<SUB>3</SUB>)<SUB>2</SUB>, 1,3-propanediamine, <I>N</I>,<I>N</I>′-dimethylethylenediamine, <I>cis</I>-(pyridine)<SUB>2</SUB>, and 1,10-phenanthroline. The malonate deuteration is acid-catalysed at pD less than about 4 and base-catalysed at higher pD in all the compounds. In the aminocontaining compounds (the first four compounds), the reaction proceeds stereoselectively in both high and low pD regions. Though the degree of the stereoselectivity depends upon the compound, the fast-exchanging hydrogen is, for all the amino-containing compounds, the one which is adjacent to the coordinating nitrogen atom. The mechanisms of acid- and base-catalysed deuteration, as well as the origin of the stereoselectivity, are discussed. For the ethylenediamine and <I>cis</I>-(NH<SUB>3</SUB>)<SUB>2</SUB> compounds, reversal of the stereoselectivity takes place at around pD=8 and 9, respectively, and the fast-exchanging hydrogen becomes the one farthest apart from the coordinating nitrogen. Concomitant with this reversal, both malonate and amine exchange rates fall together. Thus, the rate of hydrogen-deuterium exchange at the NH<SUB>3</SUB> groups does not show the usual first-order dependence upon the OD<SUP>−</SUP> concentration. These observations are explained by a mechanism in which equally reactive malonate and amine hydrogens compete for OD<SUP>−</SUP> catalyst.
- 公益社団法人 日本化学会の論文
著者
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YONEDA Hayami
Department of Chemistry, Faculty of Science, Hiroshima University
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Sakaguchi Ushio
Department of Chemistry, Faculty of Science, Hiroshima University
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Morito Kaiji
Department of Chemistry, Faculty of Science, Hiroshima University
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