Enediyne Isomers of Tetraethynylethene
journal contributionposted on 04.08.2006, 00:00 by Nathan P. Bowling, Robert J. McMahon
Three isomers of tetraethynylethene (1, C10H4) have been prepared by palladium-catalyzed Negishi coupling of a trimethylsilylbutadiynyl zinc reagent with a bromoalkene, followed by mild deprotection with potassium carbonate in methanol. The unsubstituted enynes, 3-ethynyloct-3-ene-1,5,7-triyne (2), trans-dec-5-ene-1,3,7,9-tetrayne (3), and cis-dec-5-ene-1,3,7,9-tetrayne (4), exhibit modest stability at −20 °C but decompose rapidly at room temperature. Electronic absorption spectra of 2, 3, and 4 reveal a characteristic vibronic progression at 260−320 nm. Spectral features at shorter wavelength discriminate among the isomers, and permit the assignment of 2 and 3 as apparent dimerization products of triplet carbene H−C⋮C−C̈−C⋮C−H in matrices at low temperature. Computed relative energies of these C10H4 isomers (MP2/6-31G*) are 1 (14.0 kcal/mol), 2 (6.8 kcal/mol), 3 (0.0 kcal/mol), and 4 (1.0 kcal/mol).