Photogeneration of 1,5-naphthoquinone methides via excited-state (formal) intramolecular proton transfer (ESIPT) and photodehydration of 1-naphthol derivatives in aqueous solution

Abstract

The photochemistry of naphthols 1, 2, 4, 5 and 9, and phenol 10 has been studied in aqueous solution with the primary aim of exploring the viability of such compounds for naphthoquinone and quinone methide photogeneration, along the lines already demonstrated by our group for phenol derivatives. 1-Naphthol (1) is known to be substantially more acidic than 2-naphthol (2) in the singlet excited state (pKa* = 0.4 and 2.8, respectively) and it was expected that this difference in excited-state acidity might be manifested in higher reactivity of 1-naphthol derivatives for photochemical reactions requiring excited-state naphtholate ions, such as quinone methide formation. Our results show that three types of naphthoquinone methides (26a, 26b, and 27) are readily photogenerated in aqueous solution by irradiation of 1-naphthols. Photolysis of the parent 1-naphthol (1) in neutral aqueous solution gave 1,5-naphthoquinone methide 26a as well as the non-Kekulé 1,8-naphthoquinone methide 26b, both via the process of excited-state (formal) intramolecular proton transfer (ESIPT), based on the observation of deuterium exchange at the 5- and 8-positions, respectively, on photolysis in D2O–CH3CN. A transient assignable to the 1,5-naphthoquinone methide 26a was observed in laser flash photolysis experiments. The isomeric 2-naphthol (2) was unreactive under similar conditions. The more conjugated 1,5-naphthoquinone methide 27 was formed efficiently via photodehydroxylation of 4; isomeric 5 was unreactive. The efficient photosolvolytic reaction observed for 4 opens the way to design related naphthol systems for application as photoreleasable protecting groups by virtue of the long-wavelength absorption of the naphthalene chromophore.Key words: photosolvolysis, excited-state intramolecular proton transfer, quinone methide, photorelease, photoprotonation.

Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

“…73 m -Hydroxyphenacyl acetate and a 5-hydroxy-1-naphthacyl analog were unreactive under photo-Favorskii conditions. 105 …”

Photo-physical properties of 1-hydroxy-2-naphthaldehyde: A combined fluorescence spectroscopy and quantum chemical calculations

“…The fluorescence quantum yields of the PT species of HN12 in solvents of different polarities are measured relative to recrystallised a-naphthol (U f = 0.174 in CYC) as the secondary standard (Table 1) and calculated on the basis of the following equation [36,37]:…”

Excited State Intramolecular Proton Transfer (ESIPT) from Phenol to Carbon in Selected Phenylnaphthols and Naphthylphenols

“…31 In addition, solvent-assisted formal transfer of the OH proton to the distal carbon atom at the position 5 takes place with a quantum efficiency of 0.11, as evidenced by Dexchange experiments. 31,32 Photolyses of naphthols 1, 7 and 8 and phenols 5 and 6 in CH 3 CN−D 2 O at c(D 2 O) = 14 M also gave rise to D-incorporation. Interestingly, the efficiency of the deuteration at lower D 2 O concentration (c(D2O) = 0.3 M) for 1 and 5 was 10−20% higher than at c(D 2 O) = 14 M. The finding is in accordance with direct ESIPT mechanism of D-exchange in these derivatives that is partly perturbed on addition of more H 2 O (D 2 O).…”