Symmetric esters by Tischtschenko reaction of aldehydes catalyzed by bi- and tridentate catalysts derived from catechol or gallol, trimethylaluminum and isopropanol

New inexpensive aluminum-based bidentate and tridentate chelates were found to be efficient catalysts for the Tischtschenko reaction of aldehydes. The conversion of n-butanal to n-butyl n-butyrate using catechol-derived catalysts at room temperature was complete (the yield of the butyrate was 99%) in two hours. High yields of symmetric esters were obtained in the case of n-alkyl and α-branched aliphatic aldehydes whereas reactivity of unsaturated aldehydes was found to be poor. Selected reactive intermediates were studied computationally at the (pBP)/DNPP level using the Spartan program. The results of computational studies indicate that in the case of the catechol-derived catalyst bidentate chelation of two aluminum atoms to an oxygen atom of aldehyde to form a structure ‘(O–Al)2OCAld.’ is less favorable than monodentate chelation to one aluminum atom activated by the other aluminum to form a structure ‘O–Al–O–Al–OCAld.’. The structure of this activated monodentate system clearly resembles more closely the transition state of the hydride-transfer step of the Tischtschenko reaction than the corresponding non-activated monodentate system ‘O–Al’+‘O–Al–OCAld.’.