Hi

Mechanistic studies using isotopic labeling experiments are a very powerful technique, but this is also a very broad topic. First, I think it is important to stress the difference between "simple" isotopic labeling and the study of kinetic isotope effects. The former will typically just inform you where the labelled atom ends up in the product (or the intermediates if you can detect and monitor them), whereas the latter will give you more information, usually regarding the rate determining step of the reaction. For a short and especially informative review on kinetic isotope effects I recommend the following paper by J. Hartwig: Angew. Chem. Int. Ed. 2012, 51, 3066-3072.

Concerning the synthesis of the labelled compounds needed for these studies, there are several ways to approach this, and this depends heavily on your particular system. Supposing you just want to label a particular atom to monitor where it goes, it is important to have a mechanistic hypothesis beforehand, so you can design carefully the experiment so that it gives you useful information. Once you have selected the right atom(s) to label, you need to synthesise the molecule, since they are not typically commercially available (check Sigma-Aldrich website for a wide variety of isotopically labelled building blocks). Basically you need to introduce a isotopically labelled building block so that the atom you want ends up labelled in the target compound. For the introduction of a deuterium atom in the place of a hydrogen, you typically have more synthetic flexibility. A useful method (especially for aromatic C-H bonds) is to prepare the analogous compound with a Br in the desired position and then generate the Grignard or organolithium reagent that is subsequently quenched with D2O. There are obviously many other methods. For instance, it sometimes can be useful to deuterate the substrate directly from the non-deuterated compound by catalytic C-H deuteration (see for instance: J. Am. Chem. Soc., 2006, 128, 3974–3979), although the selectivity becomes an issue with these procedures. Hydrogenation with deuterium gas (D2) can also be a useful technique if your substrates are adequate.

Finally, when performing these experiments is very important to make sure that the reaction you are studying does not provoke H/D scrambling or (partial or total)loss of the isotopic labeling, since that will hinder or simply make impossible to obtain valid and reliable data.

Hope this helps. Good luck.

Kind regards,

Martí

Hi Martí Garçon ,

thank you very much for your reply. As you said this need a detailed discussion. Hope we will get a good way to approach this through discussion.