Metallomicelles made of Ni(II) complexes of lipophilic 2-pyridineketoximes as powerful catalysts of the cleavage of carboxylic acid esters

A series of 2-pyridineketoximes With paraffinic chains of different lengths (CH(3), la; C(8)H(17), Ib; C(13)H(27), Ic) has been synthesized, and their complexes with Cu(II), Co(II), Zn(II), and Ni(II) have been investigated in the cleavage of p-nitrophenyl esters of carboxylic and phosphoric acids in water (la) or in comicelles with CTABr (Ib,c). While the Co(II) and Cu(II) complexes are ineffective in promoting the cleavage of acetate (PNPA) and hexanoate (PNPH) esters, the Zn(II) and especially the Ni(II) complexes strongly accelerate the cleavage of such substrates. With the latter metal ion, the effective species is a complex with a 2:1 ligand/metal ion stoichiometry and a pK(a) of the oximic hydroxyl of approximately 5 and 3.7 in the absence and in the presence of CTABr, respectively. Strong evidence has been obtained concerning the mode of action which involves nucleophilic attack of the oximate function on the carbonyl carbon of the ester to give, as transient intermediate, the acylated oxime; its Ni(II)-mediated hydrolysis by water has also been investigated to better define the whole catalytic cycle. The system displays its highest efficiency at low pH values and in comicelles of CTABr: in the presence of 1b/Ni(II)/CTABr at pH 4 the observed rate enhancements are of over 5 orders of magnitude in the cleavage of PNPH and the system is truly catalytic with a sizable turnover rate. In comparison with the high reactivity toward carboxylic esters, the metal ion complexes of the ketoximes investigated are surprisingly ineffective in promoting the cleavage of p-nitrophenyl esters of diphenylphosphoric acid (PNPDPP).