Roots: From carbenes to allenes and coordination polymers Ever present never twice the same

The purpose of this Feature Article is to demonstrate that recognizing the similarities in different areas of chemistry allows the prediction of potential results in related fields. For instance, during our investigations of 2,2-diethoxyvinylidene-triphenylphosphorane we became interested in 2,2-diethoxydiazoethene. In order to obtain diazoethenes, we studied vinyl-diazonium salts and geminal vinyldiazides as precursors. In the course of these investigations, we realized their synthetic potential to produce, via substituentdependent 1,5-, 3,5-, or 1,5ʹ-cyclization, a whole variety of heterocycles. However, more importantly, we became familiar with the chemistry of carbenes, which prompted an investigation of the carbene-like character of push–pull substituted allenes. Due to the ambiphilicity of their central carbon atom, they readily dimerized. Consequently, our strong interests in push–pull substituted allenes drew our attention to tetradonor substituted allenes, and as a result, we employed tetraethoxyallene as a synthetic equivalent to the fictitious malonic ester 1,1-/1,3-dianion synthon. This concept led to the synthesis of heterocumulenes and to the transallenation reaction to give allenecarboxanilides, which was further developed as a cumuhomologation for the synthesis of butatrienes via haloallenes from propargyl alcohols. The Diels–Alder reaction and intramolecular domino cyclization of multi-functional allenecarboxanilides yielded complex fused heteroarenes. Finally, the 1,5-cyclization of vinyl-azides, reported earlier, provided tetrazolylidene ligands, triggering our interest in supramolecular coordination chemistry, for example, the synthesis of one-, twoand three-dimensional coordination polymers.