CATALYTIC CYCLOPHANES .9. A THIAZOLIO-CYCLOPHANE AS MODEL FOR PYRUVATE OXIDASE AND ONE-POT SYNTHESIS OF AROMATIC ESTERS BY ELECTROCHEMICAL OXIDATION OF ALDEHYDES MEDIATED BY BIS(COENZYME) CATALYSIS

Macrocycle 3 with a hydrophobic cavity and an appended thiazolium ring is prepared following a novel synthetic sequence to monofunctionalize d cyclophanes. Although the thiazolium ring of 3 prefers to be located in the cyclophane cavity, it could be displaced with low energetic co sts by benzene and naphthalene guests which form stable 1:1 inclusion complexes with 3 in protic solvents. Initial rate studies show that 3 is a pyruvate-oxidase mimic and catalyzes the oxidation of aromatic al dehydes to carboxylic acids in aqueous solution. Cyclophane 3 also cat alyzes the conversion of aromatic aldehydes to the corresponding ester s in alcoholic solvents. The supramolecular catalyst 3 exhibits enzyme -like saturation kinetics, large turnover numbers, as well as high rea ction and substrate selectivity, and it is far superior to the non-mac rocyclic catalysts 4 and 5 which lack a substrate binding site. Follow ing cyclic voltammetric investigations of the redox behavior and stabi lity of thiazolium ions, a new one-pot electrochemical synthesis of ar omatic esters is developed: Aromatic esters are prepared efficiently b y indirect electrochemical oxidation of the corresponding aldehydes in alcoholic solvents, mediated by two coenzymes, the thiazolium ions 3 or 5 and flavin 21. At the extraordinarily low working electrode poten tial of -300 mV (vs. Ag/AgCl), high yields of the esters are obtained with high current efficiencies and high turnovers of the catalysts whi ch are stable under the reaction conditions. The origin of the substra te and reaction selectivity, which is particularly pronounced in the s upramolecular reactions catalyzed by 3, is analyzed.