UV-induced intramolecular hydrogen-atom migration of 4-chlororesorcinol and 4,6-dichlororesorcinol in low-temperature argon matrices

UV-induced photoreactions of 4-chlororesorcinol (4-chloro-1,3-benzenediol; denoted as Cl-RN) and 4,6-dichlororesorcinol (4,6-dichloro-1,3-benzenediol; denoted as Cl-RN-Cl) in low-temperature argon matrices have been investigated by infrared (IR) spectroscopy and density-functional-theory (DFT) calculation. In the photolysis of Cl-RN, elimination of HCl occurred to produce a ketocarbene, which immediately changed to a five-membered ring ketene (3-hydroxy-2,4-cyclopentadiene-1-ylidenemethanone; denoted as HO-CPYM) by Wolff rearrangement. In addition, dissociation of CO from the CCO part of HO-CPYM occurred to produce 2,4-cyclopentadienone (CPDN) by intramolecular hydrogen-atom migration from the OH group to the carbene atom. Similarly, in the photolysis of Cl-RN-Cl, HO-CPYM-Cl was produced by elimination of HCl and Wolff rearrangement, which changed to Cl-CPDN by intramolecular hydrogen-atom migration. The conformations around the CO bonds of the reactants, Cl-RN and Cl-RN-Cl, and the intermediates, HO-CPYM and HO-CPYM-Cl, are determined by comparison of the observed IR spectra with the corresponding calculated spectral patterns obtained by the DFT method.

► Photoreactions of 4-chlororesorcinol and 4,6-dichlororesorcinol were investigated.
► A five-membered ring ketene and 2,4-cyclopentadienone were identified as intermediates.
► Conformations around the CO bonds of reactants and intermediates were determined.
► Isomerization due to intramolecular hydrogen-atom migration was examined.