How to Convert a Trans Alkene into a Cis Alkene?

Suppose you need to accomplish the following transformation:

converting a trans alkene into a cis alkene

This is not the most trivial of questions as there are no single-step reactions that can realistically accomplish this transformation. Of course, there are some isomerization techniques out there, but they all favor the more thermodynamically stable molecule. And since the more thermodynamically stable double bond is in the trans-configuration, it would be hard to accomplish.

So, we need to come up with a multistep synthesis of some sort to make this transformation happen. I suggest you take a piece of paper and a pen and try to jot down a few ideas before you continue reading.




Approach 1:

So, the first thought you probably had was around using an alkyne. After all, we know that we can easily partially reduce an alkyne to yield an alkene using the Lindlar’s catalyst and H2. The alkyne sequence will look like something like this:

approach via reduction of an alkyne

Is this a good method? Well, it’s not the ideal one for two reasons. First, the elimination reaction giving alkyne is rather difficult to pull off. Yes, this is one of the few methods we discuss in the sophomore organic chemistry course as a synthesis of alkynes. However, this reaction tends to have an alkyne triple bond migrate towards the end of the molecule eventually giving a deprotonated terminal alkyne. Also, vicinal dihalides like this one really don’t eliminate efficiently to yield a C≡C bond. Another problem you may experience with this reaction is the fact that you’re using a very strong base like NaNH2. The issue here may arise not with this specific example at hand, but with other potential compounds that may be sensitive towards the bases. So, what to do? Are there any other alternatives?

Approach 2:

As the matter of fact, yes. There’s another interesting reaction that we can use. This is not a reaction we normally cover within the scope of a regular introductory organic chemistry course, however, it’s right up its alley. I’m talking about the reaction of an epoxide with a phosphine giving a Wittig intermediate. We do typically cover the Wittig reaction in the second semester organic chemistry when discussing the reactions of aldehydes and ketones. However, we rarely even mention that the Wittig intermediate can be made through other means than the interaction of the Wittig ylide with a carbonyl.

approach via reaction of an epoxide with a phosphine

The beauty of this approach is in its simplicity. The reaction of the triphenylphosphine (PPh3) is rather selective towards a good leaving group or an epoxide. So, for as long as you’re doing this reaction for a compound that doesn’t contain any primary alkyl halides, you’ll be all set. Also, it doesn’t require any follow-up: the intermediate just follows the Wittig reaction pattern giving you the desired product!

Have you come across any cool and unusual reactions in your organic chemistry course? Let me know in the comments below!

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