Steric influences on the stability and synthetic accessibility of tetraalkylcyclopentadienes

Abstract

The phase transfer-catalyzed addition of cyclohexyl bromide to cyclopentadiene under basic conditions yields 2,3,5,5′-tetracyclohexylcyclopentadiene as the sole tetra-substituted isomer. An X-ray crystal structure reveals that some angle distortion exists around the quaternary carbon, including a compressed C5 ring angle (99.4(5)°). The reason for the production of only the 2,3,5,5′ isomer from the seven possible for tetracyclohexylcyclopentadiene was probed with molecular mechanical and semiempirical MO calculations. The 2,3,5,5′ form was determined to be the lowest in strain energy, although not in the heat of formation. Close intramolecular H…H contacts are present in the cyclohexyl-substituted cyclopentadienes that are absent in isopropyl analogs.