The photoenol tautomer of 5‐methyl‐1, 4‐naphthoquinone

Abstract

Abstract Irradiation of pale yellow 5‐methyl‐1, 4‐naphthoquinone ( 1 , Scheme 1 ) yields the blue photoenol 4‐hydroxy‐5‐methylidene‐1(5 H )‐naphthalenone (2) which is stable at 77 K. At room temperature the enol retautomerizes to starting material, the reaction rate being strongly dependent on the hydrogen‐bond‐acceptor basicity of the solvent. The enol is trapped in the presence of acid by protonation at the remaining carbonyl oxygen atom and subsequent electrophilic reaction of the exocylclic methylene group.