Tetraethynylethenes: Fully cross‐conjugated π‐electron chromophores and molecular scaffolds for all‐carbon networks and carbon‐rich nanomaterials

The preparation of tetraethynylethene (3,4‐diethynylhex‐3‐ene‐1,5‐diyne) 1 as well as of a great diversity of differentially mono‐, di‐, and triprotected derivatives by newly developed synthetic routes is described. These fully cross‐conjugated molecules are versatile building blocks and precursors to two‐dimensional all‐C networks and novel C‐rich nanoarchitecture with unusual structural and electronic properties, such as perethynylated expanded radialenes, or molecular wires and polymers with the novel polytriacetylene backbone. A key step in all of these routes was the Corey‐Fuchs dibromoolefination of aldehydes and ketones. Dibromoolefination of silyl‐protected penta‐1,4‐diyn‐3‐ones yielded the corresponding dibromomethylidene derivatives which, by twofold Pd‐catalyzed alkyne coupling, were transformed into tetraethynylethene derivatives. In routes to tetraethynylethenes with free cis‐or trans‐enediyne moieties, dibromoolefination of aldehyde groups produced geminal dibromoethenes which, upon elimination/metallation with LDA followed by quenching with H⁺ or other electrophiles, yielded free or substituted ethynyl groups in high yields. Tetra‐ and triprotected tetraethynylethenes are rather stable compounds that could be isolated in pure form, whereas derivatives with two or more free CH termini were only stable in dilute solution and polymerized rapidly in pure form. A trans‐bis(triisopropylsilyl)‐protected derivative represented an exception and could be isolated as stable crystals. X‐Ray analysis revealed that the two bulky (i‐Pr)₃Si groups isolate the reactive chromophores from one another in the crystal and prevent intermolecular reactions. The structures of several tetraethynylethenes were revealed in high‐quality X‐ray crystal structures.