Access to N -Hydroxyisoindolinones by Superacid-Induced Reactons of N -Hydroxyphthalimide and their Catalytic Activity in Fluorene Oxidation

N -Hydroxyphthalimide (NHPI) is widely used as a versatile N-oxyl catalyst. Herein, we disclose that this compound rearranges smoothly in superacids such as CF 3 SO 3 H (triflic acid, TfOH) or FSO 3 H–SbF 5 with ring expansion to give 1 H -benzo[d][1,2]oxazine-1,4(3 H )-dione. However, when condensed with di- p -tolyl ether in TfOH or underwent ionic hydrogenation with cyclohexane in the presence of aluminum chloride, NHPI retains its skeleton, yielding 2-hydroxy-2′,7′-dimethylspiro[isoindoline-1,9′-xanthen]-3-one and N -hydroxyisoindolinone, respectively. The catalytic properties of the obtained compounds were tested in the oxidation of fluorene to fluorenone with sodium chlorite, where the superior catalytic activity of N -hydroxyisoindolinone was demonstrated compared to that of both the starting NHPI and the resulting spiro derivative.