Synthesis of semibullvalene derivatives via Co2(CO)8-mediated cyclodimerization of 1,4-dilithio-1,3-butadienes

Abstract

Co₂(CO)₈-mediated cyclodimerization of 1,2,3,4-tetrasubstituted 1,4-dilithio-1,3-butadienes readily afforded octa-substituted semibullvalenes. The X-ray crystal structure of 1,2,5,6-tetraethyl-3,4,7,8-tetraphenyl semibullvalene was determined to show an unsymmetrical, localized structure, which is in sharp contrast with the C₂ symmetrical structure of its tetramethyl analogue. In addition, this Co₂(CO)₈-mediated cyclodimerization was found to be dependent on the substitution pattern of 1,4-dilithio-1,3-butadienes. The 1,4-diphenyl dilithio compound gave its corresponding cyclooctatetraene derivative, while the dilithio reagent without substituents at its 1,4-positions afforded a cyclopentadienone [4 + 2] dimer.