Oxidation of veratryl alcohol by molecular oxygen in aqueous solution catalyzed by cobalt salen-type complexes:: the effect of reaction conditions

A series of Schiff-base salen-type cobalt complexes Co(salen) (1) [salen = N,N'-bis(salicylidene)ethylenediamine], Co(alpha-CH(3)salen) (2) [alpha-CH(3)salen = N,N'-bis(alpha-methylsalicylidene)ethylenediamine], Co(4-OHsalen) (3) [4-OHsalen = N,N'-bis(4-hydroxosalicylidene)ethylenediamine], Co(sulfosalen) (4) [sulfosalen = N,N'-bis(5-sulfonatosalicylidene)ethylenediamine], Co(acacen) (5) [acacen = N,N'-bis(acetylacetone)ethylenediamine] and Co(N-Me-salpr) (6) [N-Me-salpr = bis(salicylideniminato-3-propyl)methylamine] were studied as catalysts for dioxygen activation in the oxidation of 3,4-dimethoxy benzylalcohol (veratryl alcohol) in basic aqueous conditions. The oxidation occurs selectively at benzylic position as in all experiments veratraldehyde was the only observed product. The highest catalytic activity was reached with the unsubstituted complex (1), which was used for further studies on the effect of temperature, pressure, time and pH. The effect of pH was noted to be critical and the reaction proceeds up to 72 h if the pH is kept at about 12.5. The need for higher temperatures (over 60 degreesC) was also confirmed. Reactivity of the catalyst increase linearly with increased O-2 pressure, the best turnover numbers (TON = moles of veratryl aldehyde produced per mole of complex) reached in similar conditions were 58 for ambient pressure and 103 for oxygen pressure of 10 bar. The catalytic activity can be further enhanced if the catalyst: substrate ratio is increased. Turnover numbers as high as 330 were detected with catalyst: substrate ratio 1:5950 at ambient pressure. (C) 2003 Elsevier Science B.V. All rights reserved.