N-Hydroxyphthalimide (NHPI) is widely used as a versatile N-oxyl catalyst. Herein, we disclose that this compound rearranges smoothly in superacids such as CF3SO3H (triflic acid, TfOH) or FSO3H–SbF5 with ring expansion to give 1H-benzod1,2oxazine-1,4(3H)-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′-dimethylspiroisoindoline-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.
Genaev et al. (Thu,) studied this question.