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 deltaG( not equal ) 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 DeltaG( not equal ) 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.