Montmorillonite clay Beckmann rearrangements

A simple montmorillonite K 10 clay surface is one among numerous acidic supports that have been explored for the Beckmann rearrangement of oximes (Scheme 6.27) [54]. However, the conditions are not adaptable for the aldoximes that are readily dehydrated to the corresponding nitriles under solventless conditions. Zinc chloride has been used in the above rearrangement for benzaldehyde and 2-hydroxyacetophe-none, the later being adapted for the synthesis of benzoxazoles. 

Rare-earth exchanged [Ce ", La ", Sm"" and RE (RE = La/Ce/Pr/Nd)] Na-Y zeolites, K-10 montmorillonite clay and amorphous silica-alumina have also been employed as solid acid catalysts for the vapour-phase Beckmann rearrangement of salicylaldoxime 245 to benzoxazole 248 (equation 74) and of cinnamaldoxime to isoquinoline . Under appropriate reaction conditions on zeolites, salicyl aldoxime 245 undergoes E-Z isomerization followed by Beckmann rearrangement and leads to the formation of benzoxazole 248 as the major product. Fragmentation product 247 and primary amide 246 are formed as minor compounds. When catalysts with both Br0nsted and Lewis acidity were used, a correlation between the amount of Br0nsted acid sites and benzoxazole 248 yields was observed. 

Salicylaldoximes 38 and 40 underwent isomerization followed by Beckmann rearrangement at 200 °C in the presence of K-10 montmorillonite clay and silica-alumina to afford o-hydroxybenzamide 39 and benzoxazole 41, respectively. ... 

Cinnamaldoxime 65 in the presence of different H-zeolites, K-10 montmorillonite clay, amorphous SiOT-AhOa and y-alumina underwent the Beckmann rearrangement via the migration of the awft-styiyl moiety to electron-deficient nitrogen followed by an intramolecular cyclization to afford the isoquinoline 66 as the major product. Cinnamonitrile 67 and cinnamaldehyde were obtained as minor products. 

Bosch et al. (1995) have also achieved the Beckmann rearrangement of ketox-imes with montmorillonite KIO clay in dry media in good yields. 

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