Oxidative deprotection of oximes, phenylhydrazones and semicarbazones using pyridinium chlorochromate in catalytic amount with t-butyl hydroperoxide and in the solid state on montmorillonite K-10 clay support under microwaveirradiation

Pyridinium chlorochromate (PCC) and other oxochromium(vt) reagents have been extensively employed in excess of the stoichiometric amount (2 equivalents or more) for efficient cleavage of oximes to carbonyl compounds. The aim of this study is to replace excess use of toxic chromium(VI) reagents and develop cleaner environment-friendly general methods of cleavage of imine derivatives such as oximes, phenylhydrazones and semicarbazones using PCC in catalytic or stoichiometric amount. Two approaches of greening the deprotection process with PCC are conceived of which the first one is based on the catalytic use of the reagent (0.1 equiv.) in combination with an excess of 70% t-butyl hydroperoxide as the cooxidant. Mild selective regeneration of carbonyl compounds from oximes, phenylhydrazones and semicarbazones has been accomplished in good-to-excellent yields (70-98%) by this method. With focus on rate enhancement coupled with efficiency of cleavage, an alternative microwave-assisted solid-state solventless protocol of deprotection employing a stoichiometric amount of PCC dispersed on montmorillonite K-10 clay is also evaluated. It offers an expeditious efficient (74-98% yields) general route of cleavage of these procarbonyl compounds. Absence of overoxidation, particularly for oxidation-prone aryl aldehydes, is the key advantage of both these methods.