Lead oxide esterification catalyst

The resurgence of interest in NHC as catalysts in the last 6 years led several chemists to reinvestigate external oxidants for the azolium-catalyzed esterification of aldehydes. Using cyanide, which shares many similarities to azolium salts in its reactions with aldehyde, Corey had shown in 1968 that the combination of an aldehyde and sodium cyanide in the presence of Mn02 and an alcohol led to esters. Scheldt executed this work with triazolium carbene as a promoter, leading to similar results (Scheme 14.9). Other oxidants including TEMPO radicals (2,2,6,6-tetramethylpiperidine 1-oxyl) and azobenzene" were also found suitable for NHC-catalyzed oxidative esterifications. 

Habid and Malek49 who studied the activity of metal derivatives in the catalyzed esterification of aromatic carboxylic acids with aliphatic glycols found a reaction order of 0.5 relative to the catalyst for Ti(OBu)4, tin(II) oxalate and lead(II) oxide. As we have already mentioned in connection with other examples, it appears that the activation enthalpies of the esterifications carried out in the presence of Ti, Sn and Pb derivatives are very close to those reported by Hartman et al.207,208 for the acid-catalyzed esterification of benzoic and substituted benzoic acids with cyclohexanol. These enthalpies also approach those reported by Matsuzaki and Mitani268 for the esterification of benzoic acids with 1,2-ethanediol in the absence of a catalyst. On the other hand, when activation entropies are considered, a difference exists between the esterification of benzoic acid with 1,2-ethanediol catalyzed by Ti, Sn and Pb derivatives and the non-catalyzed reaction268. Thus, activation enthalpies are nearly the same for metal ion-catalyzed and non-catalyzed reactions whereas the activation entropy of the metal ion-catalyzed reaction is much lower than that of the non-catalyzed reaction. 

As expected, we found that the oxidation of 4-methylphenol in acetic acid medium in the presence of Pd-Sn / C catalyst leads to 4-hydroxybenzylic acetate with a good selectivity (Table 2, entry 2). The study of the reactionnal intermediates in such a medium shows the difficulty to oxidize the acetate into 4-hydroxy-benzaldehyde under these conditions (Table 2, entry 5) while the esterification of alcohol by acetic acid is complete (Table 2, entry 3). 

The trialdehyde 38 was obtained in four steps in 60-65% overall yield from trimesic acid (34, Scheme 3). Esterification of 34 with 1-propanol in excess, by refluxing with hydrogen chloride catalyst, leads to triester 35 in quantitative yield. Hydrogenation of 35 in acetic acid solvent (Pt catalyst) yields pure cij,ds-cyclohexane-l,3,5-tricarboxylate ester 36, also in quantitative yield. Reduction of ester 36 with lithium aluminum hydri in tetrahydrofuran solvent produces c ,cis-l,3,5-tris(hydroxymethyl)cyclo-hexane cis,cis-37) in 90-95% yields. Swern oxidation of triol 37 led to cij,c -l,3,5-triformylcyclohexane 38 in 70% yield. The stereochemistry of 38, as well as that of precursors 36 and 37, was established as ca,cis in each case by high resolution H NMR. 

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