This short review describes paths to superphanes with metal-stabilized cyclobutadiene and cyclopentadienone rings in a one-pot and stepwise fashion. The stepwise synthesis also led to a new class of belt-like systems with large rings composed of CpCo-stabilized cyclobutadiene rings connected with propane chains. Finally, our research yielded the first beltenes composed of alternating four-and eight-membered conjugated rings.A superphane is a phane in which all hydrogen atoms of the cyclic conjugated rings are tethered by bridges. This definition goes back to Boekelheide and Hopf [1]. So far, a number of superphanes with benzene rings, heterocycles, and metal-stabilized π-systems have been prepared [2]. Our affair with superphanes started with the reaction of cyclodeca-1,6-diyne 1 with CpCoL 2 , which afforded in a onepot reaction the superphane 2 (Scheme 1) [3]. This reaction was first extended to other cyclic alkynes such as cyclotetradeca-1,8-diyne [4] and cyclooctadeca-1,10-diyne [5]. The superphanes proved to be excellent starting materials for bridged cage hydrocarbons [6] and model compounds to study the interactions between two metal centers separated by chains of different lengths [7]. This encouraged us to design also a path for a stepwise synthesis of superphanes with four-and five-membered [8-10] rings as summarized in Scheme 2. The resulting superphanes are stable at room temperature, and their molecular structures were investigated. The intermediates 5-7, which were cyclized in an intramolecular reaction to 2 and 8-11, stimulated us to look for higher oligomers.

A superphane consisting of one thiophene ring and one CpCo-stabilized cyclobutadiene ring, 8, was prepared in an eight-step synthesis from 5-cyclodecynol protected with tertbutyldimethylsilyl, 13. The first key step in this sequence was the reaction of 13 with zirconocene and subsequently with S 2 Cl 2 to yield the tricyclic thiophene derivative 12. The second key step was the ring closure of 11 with CpCo(COD) to give 8. X-ray structural investigations of 11 and 8 confirmed the structural assignments. Evidence for the close proximity of the CpCo-stabilized cyclobutadiene and the thiophene rings is also given by the electronic absorption spectrum of 8 and the oxidation potential (-0.07 V).

Superphanes consisting of one thiophene ring and various substituted RCo-stabilized cyclobutadiene (CBD) rings (R = Cp, C5H4(CH3), C5(CH3)5, C5H4(Si(CH3)3), C5H4(COCH3), 8−12) and one superphane with one thiophene ring and one CpCo-stabilized cyclopentadienone ring (13) were synthesized. The starting point was a tricyclic diyne with a central thiophene ring and two condensed 4-cyclodecynes, 19. Heating of 19 with RCo(COD) yielded 8−12, whereas the heating of 19 with CpCo(CO)2 gave 13. X-ray structural investigation of 8−13 afforded the geometrical parameters in the solid state. Cyclic voltammetry showed a strong interaction between the thiophene ring and the CpCo-stabilized cyclobutadiene part. This interaction was substantiated by comparison between the CV data of 8−12 and model systems containing only the CpCo(CBD) part.

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