The Use of Clay Catalysts

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. 

In the transformation of pinacol both activity and selectivity were found to depend on the acidity (expressed as ionic potential, = cation charge/ionic radius) of the metal ions present in M -montmorillonites (batch reactor, 373 K, 1 h) [39]. The highest activity was that of Cr and Al (ionic potentials 4.35 and 6.00, respectively). Selectivity was also shown to increase in the sequence Na Ca Cu La Cr Al . Thus, ions with low acidic character (Na and Ca ) induce increased diene formation (38% and 19%, respectively). 

M -montmorillonites also resulted in the selective formation of 3,3-diphenyl- 

Pillared layered clays (PILC) containing different hydrated cations in the interlayer space are also selective catalysts. Pinacol and 2,3-diphenyl-2,3-butanediol have been studied in the presence of Al-PILC, Zr-PILC, and Cr-PILC [38], Zr-PILC, with the lowest acidity, gave rise to the largest amount of diene from pinacol (32 % and 22 %, depending on the method of preparation). The rearrangement was selective in the presence of a mixed-metal pillared layered clay (Fe Aln-PILC, sealed tube, 398 K, 1 h, catalyst/diol = 5) [40], 

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], 

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