Peralkynylated Buta-1,2,3-Trienes: Exceptionally Low Rotational Barriers of Cumulenic CC Bonds in the Range of Those of Peptide CN Bonds
✍ Scribed by Audrey Auffrant; Bernhard Jaun; Peter D. Jarowski; Kendall N. Houk; François Diederich
- Publisher
- John Wiley and Sons
- Year
- 2004
- Tongue
- English
- Weight
- 155 KB
- Volume
- 10
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
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.