Photochemistry of 1,1′-bi-2-naphthol (BINOL) — ESIPT is responsible for photoracemization and photocyclization

<jats:p> The photochemistry of 1,1′-bi-2-naphthol (BINOL, 5) has been studied in aqueous solution and found to undergo rapid deuterium incorporation at the 4 and 5 positions (in D<jats:sub>2</jats:sub>O-CH<jats:sub>3</jats:sub>CN). All data is consistent with exchange arising via a formal excited state intramolecular proton transfer (ESIPT) from the naphtholic OH to the 4 and 5 positions of the other ring to give quinine methides (QMs) 8 and 9, respectively, both of which subsequently revert to starting material. Photolysis of enantiomerically pure (+)-5 in D<jats:sub>2</jats:sub>O-CH<jats:sub>3</jats:sub>CN resulted in racemization concurrent with deuterium incorporation. This is strong evidence to indicate that photoracemization of BINOL is a direct result of ESIPT, in keeping with the invocation of planar QM intermediates. Prolonged irradiation also gave a ring-closed product that is assigned as dihydrobenzoxanthene 7, based on NMR and UV–vis data, and in analogy to known reactions of similar biaryl systems initiated by ESIPT. The formation of 7 is believed to arise via initial ESIPT from the naphtholic OH to the 7 position of the other naphthol ring generating an o-quinone methide intermediate that subsequently undergoes exclusive electrocyclic ring closure to give 7. The deuterium exchange and photocyclization reach maximum quantum efficiency at ~8 mol/L water (in CH<jats:sub>3</jats:sub>CN). A “water relay” mechanism for ESIPT is proposed that is consistent with the need for water in the photochemical deuterium exchange, racemization, and formation of 7. The photostability and photoracemization of other related BINOL asymmetric catalysts in water should be a concern based on the reported results herein.Key words: BINOL, ESIPT, photoprotonation, photoracemization, photocyclization, quinone methide. </jats:p>