Montmorillonites pinacol rearrangements

The pinacol rearrangement of simple methyl- and phenyl-substituted diols in the presence of different montmorillonites was examined by Gutierrez and Ruiz-Hitzky [33,37-39]. The transformations were performed in dry media without solvent, with the reacting diols intercalated into the interlamellar space of the montmorillonites. High catalyst/diol ratios (> 5) and mild reaction conditions were usually used. 

Rapid activation by microwave heating was also studied in the montmorillonite-catalyzed pinacol rearrangement [37,41]. Reactions performed with microwave heating are characterized by very short reaction tlmes-pinacol produced the corresponding ketone selectively In the presence of a variety of M" -montmorillonites (23-99% yield in 15 min) [37], whereas diphenylacetaldehyde was formed from hydrobenzoin over KIO and KSF clay via selective phenyl migration (near quantitative yield in 5 min) [41],... 

Scheme 6.25 Pinacol-pinacolone rearrangement on AI3+-montmorillonite K 10 clay.

Heating 2,3-dimethyl-2,3-diol, pinacol (24), at 80°C in the presence of Nafion-H gave a 92% yield of pinacolone (methyl-tert-butyl ketone) (25) (Eqn. 22.18). Similar yields were obtained with other ditertiary vicinal diols. Reaction of pinacol with either La-HY or H-ZSM-5 or aluminum exchanged montmorillonite 2 gave both pinacolone (25) and 2,3-dimethylbutadiene (26), resulting from the dehydration of the diol, in nearly equal quantities. Only a small amount of dehydration was observed when the rearrangement was run over... 

Shinde, A. B., Shrigadi, N. B., Bhat, R. P., Samant, S. D. Pinacol-Pinacolone Rearrangement on FeCIs Modified Montmorillonite K10. 

Scheme 8.39. Pinacol-pinacolone rearrangement on montmorillonite KIO clay.

Microwave activation of organic reactions on clay-adsorbed compounds Rearrangement of pinacol to pinacolone Intercalated in montmorillonite Gutierrez et al. (137)... 

Gutierrez et al. (1989) have reported a solventless pinacol-pinacolone rearrangement using microwave irradiation. The process involves the irradiation of the gem-diols with AP+montmorillonite KIO clay for 15 min to afford the rearrangement product in excellent yields (98-99%). These results are compared to conventional heating in an oil bath, where the reaction times are too long (15 hr). 

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