| Popis: |
The bromination of 1,5‐hexadiyne at ‐40° in chloroformic solution gives 1,2,5,6‐tetrabromo‐1,5‐hexadiene (5) with a yield of 948; at higher temperatures (e. g. ‐15°), bromination of the double bonds occurs. The stereochemistry of (5) has been studied by N. M. R. ‐spectroscopy; the ratio of cis to trans double bonds (defined with respect to the two bromine atoms) is shown to be 1/9. When (5) is treated repeatedly with N‐bromo‐succinimide (NBS) in CCl4 solution, 1,1, 2,5, 6,6‐hexabromo‐2,4‐hexadiene (7) (trans, trans or cis, cis) is formed, implying a double allylic bromine migration; this structure is assigned on the basis of U. V.‐ and N. M. R.‐ evidence, in comparison with the spectroscopic properties of 1,2,3‐tribromopropene. Treatment of (7) with zinc powder in ethanol produces mainly trans‐1,5‐hexadiyne‐3‐ene (yield 57%), together with a small amount (2,8%) of the cis‐derivative. Copyright © 1974 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim |
