Cyclobutene - Fused Aromatic Molecules

The Diels-Alder cycloaddition of dimethyl-1 ,2-cyclobutenedicarboxylate to a 1,3-diene results in the formation of an adduct which can subsequently be decarboxylated and dehydrogenated to provide a cyclobutene-fused benzenoid system. The use of · appropriate dienes has led to the preparation of a series of paraand metabis-annelated benzenes. Variations in the 1H and C NMR and in the UV spectra are found to be a function of the size of the annelated rings as well as their relative orientation. A similar approach has been employed in a preparation of the very elusive tricyclobutabenzene which may r epresent the missing link between 1,5,9-cyclododecatriyne and 6-radialene. Tricyclobutabenzene is a stable crystalline material (mp 141-142 °C) whose physical properties are quite consistent with higher homologs. An X-ray structure of its perfluoro analog shows no evidence for any bond alternation. A double-barreled application of the Diels-Alder approach has led to the synthesis of naphtho[b,e]dicyclobutene. The physical properties of this molecule are also compared to higher homologs.