[AlphaScent]

So my PI and had a fun back and forth on whether a reaction is termed stereospecific or stereoselective.

I am doing the Shi epoxidation.  It is known that trans (E) double bonds react with higher e.e. than do cis.

But I am doing a enyne and I have 95% E and 5% Z.  The GC is showing no change to the Z but the E isomer completely reacted forming the epoxide in what I hope is a high e.e.

Now the epoxidation itself is stereoselective giving what I hope is 95% e.e., but what is called if the particular isomer is favored over the other.  In this case the E over the Z.  Would that not be called stereospecific?

Stereospecific: A reaction in which stereochemically different molecules react differently.

[hollytara]

That is not quite right....

In a stereospecific reaction, different stereochemical forms of the reactant lead to different stereochemical forms of the product. 

The Shi is supposed to be concerted, so the Z (cis) ought to give the cis epoxide, and the E (trans) ought to give the trans epoxide.  That would be a stereospecific reaction.  The difference in rate between the two reactions is not really part of what makes it stereospecific. 

[AlphaScent]

Thanks!!  I was kind of leaning that way when I thought about this again.  Thanks for your input!

[clarkstill]

It is possible for a reaction to be both stereoselective and stereospecific.

For example, the Shi epoxidation is stereospecific in the sense that both new bonds to oxygen will be syn-to each other (due to the concertedness that Hollytara decsribed), and as a result the (E)-alkene gives the trans epoxide and the (Z) gives the cis. However, the preference for the formation of one enantiomer or the other is an example of stereoselectivity.

You might say the reaction is diastereo-specific and enantio-selective.

- I havent added anything to what Hollytara has said, just worded it slightly differenty. To her go the spoils