Benzocyclobutene-o-xylylene valence tautomerization: Oxygen and sulfur analogs.

Gas phase pyrolysis of o-hydroxybenzyl alcohol at 750 degrees C gave o-quinonemethide, which, on isolation, formed a mixture of the corresponding dimer and trimer. However, pyrolysis of o-hydroxy-[1-(4-pentenyl)]benzyl alcohol permitted intramolecular trapping of the intermediate o-quinonemethide, giving 3,4-trimethylene-3,4-dihydrobenzochroman. In contrast, gas phase pyrolysis of o-mercaptobenzyl alcohol readily gave benzo[b]thiete in good yield. The pyrolysis makes this interesting highly strained molecule readily available for study and synthetic application. Preliminary results showed that benzo[b]thiete readily undergoes thermal dimerization, ring-opening reactions with nucleophiles, and a variety of Diels-Alder reactions.