5
$\begingroup$

The way I know to tell whether an alkene is E or Z -is to look at the coupling constant of the protons across the doble bond. Large value (16 Hz) indicates an (E)-alkene where as smaller values (11 Hz) usually indicate a (Z)-alkene.

In my project I've been doing a cross metathesis between two alkenes which gives me a tri-substituted alkene, and I cannot observe the coupling constant across this double bond. The reaction gives a mixture of products which I think are the (E)- and (Z)-isomers. I'm able to separate them on HPLC but they do not crystallise.

Is there another method of determining this? A lab member suggested I could derivatise the alkene but I don't think this would provide any more useful information.

$\endgroup$
  • 1
    $\begingroup$ I think you are more likely to get a helpful answer if you show us what the structure of the alkene looks like. $\endgroup$ – Zhe Jun 28 '17 at 17:45
  • 3
    $\begingroup$ Also, what about NOESY to check if which hydrogens are spatially close on the mono-substituted side to others on the di-substituted side. $\endgroup$ – Zhe Jun 28 '17 at 17:47
  • $\begingroup$ Zhe I don't know whether I'm permitted to share my chemistry on this forum but the alkene is trisubstituted. On one side there is a ch2 and in the other side a CHOH $\endgroup$ – James Dealon Jun 28 '17 at 18:06
  • 1
    $\begingroup$ Moot point now, but you can obscure the functional groups that are further out, but any handles close to the alkene will help if you're looking for suggestions with derivatization. For example, in the answer, you might show that you have methylene and hydrogen on one side and TMS and methylene on the other. $\endgroup$ – Zhe Jun 28 '17 at 18:40
8
$\begingroup$

As @Zhe points out, its not possibly to definitively answer your questions without knowing the structure of the olefin, as its important what is around the olefin, as well as just how many protons are attacked to the olefin.

If you read many of the original Grubbs' papers (and indeed any papers using a Grubbs' metathesis to make tri-substituted alkenes), the common method to determine the geometry is to use the nuclear Overhauser effect, or nOe for short.

The nOe identifies interactions through space (as opposed to standard NMR where we're looking at the interactions through bonds. By recording the nOe, one can essentially 'measure' how close two protons are, which in this case would allow you to distinguish E and Z.

enter image description here

Practically speaking, there are several different NMR experiments that one can run to observe the nOe.

  • The 1D nOe (1D-NOESY) difference experiment involves irradiation of a particular signal (for instance the one alkene proton), to allow measurement of what other protons are proximal in space. This experiment has the advantage of being quick to run, but in order to gain useful information, you have to run several of them (irradiate the alkene proton, then run a different 1D-NOESY in order to make sure that the proton that the alkene proton is 'seeing' also 'sees' the alkene proton
  • The 2D nOe (2D-NOESY) experiment shows all nOe enhancements in a molecule, this is incredibly useful and often used in structure elucidation, but can be slow to run, especially if the amount of sample available is limited.
$\endgroup$
  • $\begingroup$ How can you tell me to run this using a Bruker NMR machine ? I don't see the option on the drop down list there is only 1H 13C 19F COSY HSQC-Dept and something called dqfcosy $\endgroup$ – James Dealon Jun 28 '17 at 18:04
  • 3
    $\begingroup$ I suspect you're talking about an open-access NMR machine where you just leave your sample in the carousel and wait for an email. The nOe experiments require a bit more setup, so usually need running manually. I have no idea what the situation is at your lab, but either the lab members or the NMR department can likely sort this out (the NOESY experiments are present on all Bruker machines by default so there should be no problems) $\endgroup$ – NotEvans. Jun 28 '17 at 18:13
  • $\begingroup$ As an example, FYI, see the MIT guideline on running a 1D nOe here $\endgroup$ – NotEvans. Jun 28 '17 at 18:17
  • $\begingroup$ @JamesDealon if you use IconNMR, you're only getting a restricted list of preconfigured experiments. There are something like 20 different NOESY sequences included with Bruker Topspin, which ones you can run is the decision of your NMR facility. This is something you just have to ask your NMR facility manager, if there's no NOESY in the list in IconNMR, you can't run one without help of an administrator. $\endgroup$ – Mad Scientist Jun 28 '17 at 21:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.