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Photochemistry of adamantyl derivatives of hydroxybiphenyls : ESIPT vs. photosolvolysis

Quinone methides (QM) are important intermediates in chemistry and biology that react with DNA and amino acids. One method of QM formation involves excited state intramolecular proton transfer (ESIPT) of certain phenol derivatives [1]. We reported on efficient photochemical formation of adamantyl QMs that are characterized by longer lifetimes than their unsubstituted analogues [2]. These QMs were formed from (2-hydoxyadamantan-2-yl)phenols. The excitation (S1) led to the formal (H2O assisted) proton transfer (PT) from the phenol to the alcohol coupled with dehydration, giving rise to QMs. Incorporation of the bulky adamantyl groups made these QMs more stable and selective in the reactions with nucleophiles. We were particularly interested in the QMs that can selectively react with amine nucleophiles, since these may find applications in biological systems. In continuation of that research we substituted hydoxybiphenyl systems with 2-hydroxyadamantan-2-yl moiety [3]. These derivatives can undergo photosolvolysis by the mechanism described above. In addition, these derivatives can undergo ESIPT recently discovered in the Wan’s group. In the singlet excited state of 2-phenylphenol the phenolic OH becomes sufficiently acidic to protonate the carbon atom of the adjacent aromatic ring [1]. The presentation aims to address competition between the ESIPT to carbon atoms and the formal PT to the oxygen of the benzyladamantyl alcohol, both processes giving rise to QMs. The mechanisms of the photochemical reactions were investigated by preparative irradiations, fluorescence and laser flash photolysis.

ESIPT; Quinone methides; 2-hydroxyadamantan-2-yl moiety; hydoxybiphenyl

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