The preference profile in ruthenium tetroxide oxidations

Abstract

The preference profile of Ru-VIII-generated in a catalytic cycle, maintained by periodate in carbon tetrachloride : acetonitrile : water has been examined from a practical vantage using tyrosine, phenylalanine and lysine as primary substrates. Other factors such as pH, acetonitrile versatility, transport of oxidized ruthenium species across the layers and hydrophobic alignment, influence the course of the reaction. Aryl oxidation, which takes place at the organic interface, is strongly influenced by ring perturbation (pOH-C6H4CH2CH,++; PhCH2CH,+, PhCH2O,+; PhCH2OCONH(Z),-; PhCH2OCO,-; pOH-C6H4CH2CO,+; PhCH2CO,-; PhCO,-). In the case of tyrosine, the preference profile switches from ring oxidation at pH 3 to alpha-amino group oxidation at pH 6 and 9, whilst with phenylalanine, the amino group is exclusively oxidized even at pH 3. With lysine, the reasonable differences in pK(2) between the alpha-amino group (8.95) and the omega-amino unit (10.53), elicit sharp preferences. At pH 3 as well as at 6, the a-amino group is selectively oxidized leading to glutaric acid mono-amide, a finding supported by studies with N-alpha and N-omega protected lysines. Lysine and arginine side-chains are found largely unaffected by the reagent at pH 3 and 6. The findings have been rationalized on the basis of an integrated mechanism. The work has endeavoured to reconcile seemingly conflicting reports in the literature and to project the reagent for selective modifications in synthesis.