A novel weakly diatropic dehydroannulene containing the cyclobutadiene(cyclopentadienyl)cobalt unit. An organometallic dehydroannulene that obeys Huckel's rule

This contribution describes the synthesis of novel dehydrobenzannulenes containing CpCo-stabilized cyclobutadiene complexes. These dehydrobenzannulenes were made in two different ways. The first access involves a shotgun approach in which 1,2-diethynyl-3,4-bistrimethylsilylcyclobutadiene(cyclopentadienyl)cobalt and 1,2-diiodobenzene are reacted under Heck-type conditions utilizing (PPh3)(2)PdCl2 as catalyst. The formation of a dimeric dehydrobenzannulene containing two benzene rings, two cyclobutadiene units and four alkyne units - even though in low yield - was observed. The second approach to organometallic dehydroannulenes involves the construction of a precursor by coupling 1,2-diethynyl-3,4-bistrimethylsilylcyclobutadiene(cyclopentadienyl)cobalt to 1-bromo-2-trimethylsilylethynylbenzene under Pd catalysis, deprotection and Vogtle coupling of the formed precursor to furnish the corresponding monomeric cycle in high yield. This cycle has been characterized by X-ray single-crystal analysis. (C) 2000 Elsevier Science S.A. All rights reserved.