Reductive Bergman-Type Cyclizations of Cross-Conjugated Enediynes to Fulvene and Fulvalene Anions: The Role of the Substituent

Various cross-conjugated enediynes undergo “Bergman-type” cycloaromatizations upon reduction
with potassium metal, generating anions of fulvenes and fulvalene derivatives. This new anionic cyclization
is considerably more facile than the classic Bergman cyclization with linear enediynes, creating highly reactive
diradicals at −78 °C. Not all cross-conjugated enediynes yield cyclized dianions upon reduction; some
give uncyclized, Y-shaped, cross-conjugated dianions, while others apparently yield radical−anions that
either dimerize or persist as monomers. One system yields both a cyclized and an uncyclized dianion. The
substituents are thus shown to be a critical factor in determining the outcome of the reduction. Cyclization
occurs within a specific “window of opportunity” that is governed by the substituents.