Synthesis, X-ray structure, and properties of a tetrabenzannelated 1,2,4,5-cyclophane.

Parylene is the most frequently used material in the protective encapsulation of modern electronic components and medical implants.[1] This high-performance polymer is produced by the pyrolytic decomposition of [2.2]paracyclophane.[1c] Another high-performance organic material with even stronger C C bonds would be produced if another highly strained, all-aromatic cyclophane could be pyrolyzed, thus resulting in a TMsuperparylene∫. However, contrary to the mode of pyrolytic decomposition of [2.2]paracyclophane, where scission of the Csp3 Csp3 bond is the important first step leading to a p-xylylene monomer, in the case of a molecule such as 1 (Scheme 2), cleavage of a biaryl bond would produce a very reactive diradical monomer. Angle and bond strain in organic molecules and their effect on properties also continue to be an active field of research.[2] Over the last five decades, a substantial number of chemists have prepared many fascinating, strained saturated and unsaturated molecules.[2,3] The most notable of the strained unsaturated molecules are those of the fullerene C60 and the cyclophane families.[5] In the former, the hexagons are essentially cyclohexatrienes[6] and in the latter, the hexagons, while considerably distorted, still retain their benzenoid character. Since the first synthesis of [2.2]paracyclophane diene by Dewhirst and Cram,[7] a variety of [2.2]paracyclophanes with unsaturated or benzannelated bridges have been synthesized.[8] The influence of the bridges on the transannular benzene interactions and the geometry of the strained cyclophanes has been widely investigated.[9] To date, only a few unsaturated bridged and benzannelated cyclophanes are known,[9] but no benzannelated [2n]cyclophane with more than two bridges (n> 2) has been reported.[10] One would expect that, as the number of o-phenylene bridges increased, the total strain would also increase. To prepare a superparylene and to test the effect of benzo bridges in place of the alkyl bridges of cyclophanes one needs a rapid, reasonably high-yield synthetic entry. A priori, based on existing cyclophane synthetic methodology, the preparation of a symmetrical tetrabenzannelated [2n]cyclophane tetraene would appear to be rather difficult and lengthy. However, careful consideration of the molecular symmetry of the target revealed that the synthesis could be easily achieved. Herein, we describe the synthesis, X-ray structure, and some of the properties of the symmetrically benzannelated [24]cyclophane tetraene 1. In future publications we will report on the results of its pyrolytic decomposition. In Scheme 1 we depict the retrosynthetic analysis with a rather unusual disconnection leading to two dibenzocyclooctadiene-diynes and four methine units. As shown, the latter can originate from a meso-ionic precursor.