Photochemical Formation and Chemistry of Long-Lived Adamantylidene-Quinone Methides and 2-Adamantyl Cations

Hydroxymethylphenols strategically substituted with the 2-hydroxy-2-adamantyl moiety, AdPh 81-0, were synthesized, and their photochemical reactivity was investigated. On excitation to the singlet excited state, AdPh 8 undergoes intramolecular proton transfer coupled with a loss of H2O giving quinone methide 8QM. The presence of 8QM has been detected by laser flash photolysis (CH3CN-H2O 1:1, tau = 0.55 s) and UV-vis spectroscopy. Singlet excited states of AdPh 9 and 10 in the presence of H2O dehydrate giving 9QM and 10QM. Photochemically formed QMs are trapped by nucleophiles giving the addition products (e.g., Phi for methanolysis of 8 is 0.55). In addition, the zwitterionic 9QM undergoes an unexpected rearrangement involving transformation of the 2-phenyl-2-adamantyl cation into the 4-phenyl-2-adamantyl cation (Phi similar to 0.03). All analogous rearrangement was observed with methoxy derivatives 9a and 10a. Zwitterionic 9QM was characterized by UP in 2,2,2-trifluoroethanol (r = 1 mu s). In TFE, in the ground state, AdPh 10 is in equilibrium with 10QM, which allowed for recording the H-1 and C-13 NMR spectra of the QM. Introduction of the adamantyl substituent into the o-hydroxymethylphenol moiety increased the quantum yield of the associated QM formation by up to 3-fold and significantly prolonged their lifetimes. Furthermore, adamantyl substituent made the study of the alkyl-substituted quinone methides easier by LFP by prolonging their lifetimes and increasing the quantum yields of formation.