Aerobic Alcohol Oxidation Catalyzed by Polyester-Based Pd(II)Macrocomplexes

Palladium(II) compounds bearing either mono- or bi-dentate nitrogen ligands are suitable to catalyze the aerobic oxidation of alcohols to ketones or aldehydes in the homogeneous phase since water or hydrogen peroxide are formed as the only side products. Among the former palladium-based catalysts, Uemura’s catalytic system [i.e., trans-[Pd(OAc)2(pyridine)2] (OAc = acetate)], characterized by its versatility to oxidize structurally different alcohols, has found wide application. The major concern with this latter catalytic system refers to the stabilization of palladium in its catalytically active oxidation state [i.e., Pd(II)], avoiding thus the formation of Pd(II) aggregates and the successive precipitation of palladium black, which is almost inactive in aerobic oxidation reactions performed in toluene. Hence, aerobic alcohol oxidation reactions were peformed either with a Pd(OAc)2 to pyridine ratio of 1:4 (i.e., the ‘‘optimized Uemura’’ catalytic conditions) or in the presence of sterically hindered three-substituted pyridine derivatives. We present a different approach to recycle the catalytic system, which consists in the coordination of Pd(OAc)2 by 4- pyridinemethylene-end-capped poly(l-lactide) and poly(caprolactone) (PCL), which are soluble under the applied catalytic conditions (i.e., toluene, 70 C). Upon addition of n-pentane or methanol the polymer-anchored catalyst precipitates, and thus, can be separated from reaction solution. This soluble polymer-anchored catalyst approach combines the advantage of homogeneous and heterogeneous catalysis.