# Converting a cis-alkene to trans-alkene

I was thinking about simple method to convert a cis-alkene alkene like cis-2-butene to trans-2-butene.

I came up with this method (the method can also be used to convert trans alkene to cis alkene):

• First use a peracid like mCPBA to convert the cis alkene to an epoxide:

• then I used the wittig reagent $\ce{PPh_3}$ to get a betaine. For the $\ce{- O-}$ to be able to attack $\ce{PPh3}$, they should be on same side. So rotation about $\ce{C-C}$ bond:

• Finally, the leaving of $\ce{POPh3}$:

I would like to know if my method is correct and if there are any other methods to bring about the conversion.

• In cases of cis alkenes heating should suffice, the reverse way is actually quite interesting. – Martin - マーチン Dec 16 '15 at 10:09
• This method will convert trans to cis as well right? – Aditya Dev Dec 16 '15 at 12:59
• Maybe. The theory is sound I think, not so sure about the praxis. We would have to dig into the literature. – Martin - マーチン Dec 16 '15 at 13:29
• I went through Jerry March's book. This is from page 1374: books.google.co.in/… – Aditya Dev Dec 16 '15 at 13:33

Your method looks fine.There is another simple method(with three steps though).
1.Convert the the cis alkene to a dibromo derivative using bromine;
2.Use two equivalents of NaNH2 to get an alkyne (elimination);
3.Selective reduction to trans alkene by using sodium in liquid ammonia.
However this requires that Hydrogen be present as a substituent on the double bond, on both carbons.

• I know that method. Do you have anything else to add about my method? – Aditya Dev Dec 16 '15 at 13:01

For E/Z trisubstituted alkenes capable of E2 elimination the following sequence may apply. Add the elements of water in a syn, anti-Markovnikov fashion to the double bond. Remove the elements of water as p-toluenesulfonic acid via anti E2 elimination. The cycle below illustrates this point.

There are two other methods which I know, if you were to take a cis-1,2-diaryl alkene and expose it to UV light then you will excite the alkene into a 1,2-diradical. The C-C single bond in this is free to rotate.

It will then randomly form a 1:1 mixture of the cis and trans isomers. As the photophysical properties of the two isomers are different (exctinction coefficent) it is possible using light of a given wavelength to convert any mixture of cis / trans into a mixture with a given cis / trans ratio. A photostationary mixture. By changing the wavelength the ratio in the mixture changes.

Another method would be to react the cis alkene with a metal hydride which has a vaccent coordination site such as RuHCl(CO)(PPh3)2. Then do a beta hydride elimination to form an alkene and the metal hydride again. This relates to the way in which trans fats are formed during the hydrogenation of plant oils (lipids with cis alkenes)