Scandium(III) Triflate as a Lewis Acid Catalyst of Oxime Ligation

Imine-forming reactions are widely applicable in bioconjugation owing to their high chemoselectivity. The ligation of a ketone or aldehyde with an aminooxy functional group to form a physiologically stable oxime bond is often used to link complex and precious biomolecules. Although the reaction proceeds modestly in acidic solution, the abundance of protonated carbonyl species at pH 7 limits its utility in many biological applications. The use of nucleophilic aryl amines, such as aniline or a phenylenediamine, allows a high population of protonated Schiff base to undergo transimination to the oxime product. Although this method affords significant enhancements at low pH, reactions can still be sluggish at neutral pH, especially with ketones such as acetophenone that are commonly used in bioconjugation. Here, we employ scandium(iii) trifluromethanesulfonate (triflate) (Sc(OTf)3), a uniquely water-stable Lewis acid, as a co-catalyst with ortho-phenylenediamine in the oxime ligation to yield up to an order of magnitude rate enhancement over the catalysts when applied individually.