Peralkynylated Buta‐1,2,3‐Trienes: Exceptionally Low Rotational Barriers of Cumulenic CC Bonds in the Range of Those of Peptide CN Bonds

A variety of 1,1,4,4‐tetraal kynylbutatrienes and 1,4‐dialkynylbutatrienes was synthezized by dimerization of the corresponding gem‐dibromoolefins. Both 1 H and 13 C NMR spectroscopy indicated that the di‐ and tetraalkynylated butatrienes are formed as a mixture of cis and trans isomers. Variable temperature NMR studies evidenced a facile cis–trans isomerization, thus preventing the separation of these isomers by gravity or high‐performance liquid chromatography (HPLC). For 1,1,4,4‐tetraalkynylbutatrienes, the activation barrier Δ G ≠ was measured by magnetization transfer to be around 20 kcal mol −1 , in the range of the barrier for internal rotation about a peptide bond. Unlike the tetraalkynylated [3]cumulenes, 1,4‐dialkynylbutatrienes are more difficult to isomerize and could, in one case, be obtained isomerically pure. Based on experimental data, the rotational barrier Δ G ≠ for 1,4‐dialkynylbutatrienes is estimated to be around 25 kcal mol −1 . The hypothesis of a stabilizing effect of the four alkynyl substituents on the proposed but‐2‐yne‐1,4‐diyl singlet diradical transition state of this cis–trans isomerization is further supported by a computational study.