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It's really weird for the product is 99% Z-comformation? Please tell me why? This reaction can be found in scifinder.

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    $\begingroup$ Welcome to chem SE, try to add some attempt at the question and don't just ask for an answer. let us know your thoughts on the problem $\endgroup$ – H.Linkhorn Jun 15 '19 at 14:06
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    $\begingroup$ In regards to the question, you are adding one molecules of $\ce{Br2}$ across the triple bond to leave a double bond. Hence it is an electrophilic addition reaction $\endgroup$ – H.Linkhorn Jun 15 '19 at 14:08
  • $\begingroup$ But why the product is z-comformation is the problem as in usual the product is e-confirmation $\endgroup$ – Yuanhang He Jun 15 '19 at 15:42
  • $\begingroup$ I would assume steric hindrance will have a large factor to play in the reaction as the bromine is a large-ish thing to have on the side of the molecule when next to the carbonyl $\endgroup$ – H.Linkhorn Jun 15 '19 at 19:11

A search of SciFinder will also produce the formation of the (E)-dibromide 3. The conditions described for the formation of the (Z)-dibromide 2 are taken from a paper by Myers1 as reported earlier by Tippett.2 Rossi3 has reported the formation of the (E)-dibromide 3 using pyridinium hydrobromide perbromide.

Given the more vigorous conditions for the formation of the (Z)-isomer 2 compared to the milder conditions for the formation of the (E)-isomer 3 suggests that the former reaction is under thermodynamic control while the latter is kinetically controlled. There is the distinct possibility that bromine radicals may effect the isomerization via species 4 as may surreptitious acid.

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1) A. G. Myers, M. M. Alauddin, M. A. M. Fuhry, P. S. Dragovich, N. S. Finney, and P. M. Harrington, Tetrahedron Letters, 1989, 30, 6997.
2) R. G. Hall and S. Trippett, Tetrahedron Letters, 1982, 23, 2603.
3) F. Bellina, A. Carpita, M. De Santis and R. Rossi, Tetrahedron Letters, 1994, 35, 6913.

  • $\begingroup$ Thank you so much,you helped a lot! $\endgroup$ – Yuanhang He Jun 16 '19 at 15:35

Although the following reaction is not syn dibromination, the following reaction sequence is a example of syn dichlorination from https://www.semanticscholar.org/paper/Catalytic%2C-stereospecific-syn-dichlorination-of-Cresswell-Eey/a53c83d35d5acf0a7ac1311bd9462dedd2dc666f .

The possible mechanism is follows.enter image description here


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