Very Efficient Generation of Quinone Methides through Excited State Intramolecular Proton Transfer to a Carbon Atom

Abstract

Irradiation of 2-phenyl-1-naphthol (6) in CH(3) CN/D(2) O (3:1) leads to very efficient incorporation of deuterium at the ortho-positions of the adjacent phenyl ring (overall Φ=0.73±0.07), along with minor incorporation at the naphthalene positions 5 and 8. These finding are explained by excited state intramolecular proton transfer (ESIPT) from the phenolic OH group to the corresponding carbon atoms, the main pathway giving rise to quinone methide (QM) 7, which has been characterized by LFP (τ≈20 ns; 460 nm). The ESIPT reaction paths have been explored with the second order approximate coupled cluster (CC2) method. In nonprotic solvents the ESIPT from the naphthol O-H to the ortho-position of the phenyl ring proceeds in a barrierless manner along the (1) L(a) energy surface via a conical intersection with the S(0) state, delivering 7. In aqueous solvent, clusters with H(2) O are formed wherein proton transfer (PT) to solvent and a H(2) O-mediated relay mechanism gives rise to naphtholates and QMs. The results are compared with 2-phenylphenol (3) that also undergoes barrierless ESIPT giving a QM via a conical intersection. However, due to an unfavorable conformation in the ground state, the quantum efficiency for ESIPT of 3 is significantly lower (Φ for D-exchange=0.041). These results show that ESIPT from phenol to carbon need not be an intrinsically inefficient process.