Triflic Acid-Mediated\nCondensation of Phthalimide\nwith Diaryl Ethers as a Route to Spiro-Isoindolinones: Mechanistic\nInsights and Related Reactions

Phthalimide and <i>N</i>-phenylphthalimide\nsmoothly condense\nwith di-<i>p</i>-tolyl ether in triflic acid (CF<sub>3</sub>SO<sub>3</sub>H, TfOH) to obtain the corresponding spiro[isoindoline-1,9′-xanthen]-3-ones.\nStructural analogs of phthalimide, such as phthalic anhydride and\n1,3-indandione (but not saccharin), show similar reactivity. In contrast,\nN-(tetrafluoropyridin-4-yl)phthalimide reacts with DTE by an alternative\npathway, yielding isobenzofuran dispiro derivative. The mechanistic\naspects of these reactions are discussed on the basis of in situ NMR\nand theoretical (DFT) studies, providing insights on the key intermediacy\nof O,O-diprotonated forms of the starting compounds.