Metal-Ammonia Reduction of Triptycene and Related Benzobarrelene Derivatives

This paper describes the metal-ammonia reduction of several bicyclic compounds including dibenzobarrelene, triptycene, and symmetrical and unsymmetrical benztriptycenes. Although previous studies indicate that triptycene undergoes ring opening on reaction with potassium in THF, successful reduction in ammonia has been accomplished for all of these compounds with selection of proper conditions. Triptycene exhibits rather unusual regioselectivity in that the major product is the 2, 4a-dihydro isomer with the expected 1, 4-dihydro isomer produced in lesser amounts. This unanticipated result is discussed in terms of π-π conjugation in the intermediate radical anion (or dianion). As might be expected, the reduction of isomeric benztriptycenes appears to be dominated by the naphthalene moiety and results in both inner-ring and outer-ring reduction products. Furthermore, 1, 2-benztriptycene shows a novel metal effect in that lithium produces ring cleavage whereas the use of sodium provides “normal” reduction products.