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

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[SiMo12O40], H-3[PMo12O40], H-4[SiW12O40] and H-3[PW12O40]). 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[PW12O40] results in a highly stable catalyst, due to the strong interaction between the acid and the support. A similar stabilization of H-4[SiMo12O40] 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(N)2 mechanism.