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

Irradiation of 2‐phenyl‐1‐naphthol (6) in CH3CN/D2O (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 1La energy surface via a conical intersection with the S0 state, delivering 7. In aqueous solvent, clusters with H2O are formed wherein proton transfer (PT) to solvent and a H2O‐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. Irradiation of 2‐phenyl‐1‐naphthol in CH3CN/D2O (3:1) leads to very efficient incorporation of deuterium at the ortho‐positions of the adjacent phenyl ring along with minor incorporation at the naphthalene positions 5 and 8. These findings are explained by excited state intramolecular proton transfer (ESIPT) from the phenol OH to the corresponding carbon atoms (see figure).