Synthesis of benzonorbornadienes: regioselective benzyne formation.

This report details the synthesis of several benzonorbornadienes by Diels--Alder cycloaddition of cyclopentadiene derivatives with substituted benzyne intermediates, which were generated by low-temperature metal--halogen exchange of halobenzenes. General conditions were developed, allowing synthesis of most benzonorbornadienes described herein at the multigram scale with isolated yields approaching 90% in some cases. Cycloaddition of the benzyne produced by substitution of a chlorodifluorobenzene for a bromodifluorobenzene in the metal--halogen exchange reaction unexpectedly gave a different benzonorbornadiene. The benzyne, which resulted by a deprotonation pathway rather than by metal-halogen exchange, formed in a highly regioselective elimination step.