Transformation of diols in the presence of heteropoly acids under homogeneous and heterogeneous conditions

Abstract

1,2-Diols (1,2-propanediol, isomeric 2,3-butanediols and 2,3-dimethyl-2,3-butanediol) readily undergo the pinacol rearrangement to yield the corresponding carbonyl compounds (propanal, 2-butanone and 3,3-dimethyl-2-butanone, respectively) in the presence of heteropoly acids (H 4[SiMo 12O 40], H 3[PMo 12O 40], H 4[SiW 12O 40] and H 3[PW 12O 40]). The selectivity depends on the acid strength and the form of the heteropoly acid catalyst (transformation under homogeneous conditions or in the presence of heteropoly acid supported on silica). Heat treatment of supported H 3[PW 12O 40] results in a highly stable catalyst, due to the strong interaction between the acid and the support. A similar stabilization of H 4[SiMo 12O 40] could not be achieved. The highly selective formation of cyclic ethers, characteristic of the dehydration of higher diol homologues (1,4-butanediol, isomeric 2,5-hexanediols, 2,5-dimethyl-2,5-hexanediol, 1,5-pentanediol and 1,6-hexanediol) is also catalyzed by heteropoly acids. This cyclodehydration occurs by a stereospecific intramolecular S N2 mechanism.