Sunday, March 22, 2015

A Case Study of the Mechanism of Alcohol-Mediated Morita Baylis- Hillman Reactions. The Importance of Experimental Observations.

R. Erik Plata, and Daniel A. Singleton Journal of the American Chemical Society 2015, ASAP.
Contributed by +Jan Jensen

This excellent and unusually well-written paper has already been discussed quite a bit including a blog post at In the Pipeline, (including a lively discussion in the comments section), a Reddit thread, and a C&EN highlight, not to mention a lot of twittering. Here are my 2 cents.

To summarize: the authors picked a somewhat challenging (by their own admission) reaction and show that published computational studies as well as their own calculations (using standard approaches) yield mechanistic conclusions that in many cases are "not even wrong" as Pauli once said.

The study offers some important lessons and some of the lessons are quite fundamental and, judging by the literature, often needed.

* Don't ignore the experimental data.  If your calculations predict an overall increase in standard free energy for a reaction that is observed to proceed, there's something wrong.  The same goes for calculated free energy barriers that suggest reaction rates on the order of years.

* Life is often complicated. Don't just pick the simplest or computationally most tractable reaction mechanism.

* Thermodynamics requires just as much attention as the electronic structure theory. ZPE or enthalpy is not a substitute for the free energy (especially for bimolecular reactions) and the thermodynamic terms should not be corrected in an ad hoc fashion. Oh, and use the correct standard state.

* If you want to compare your computed enthalpies and entropies to experiments, you must include the entropy enthalpy of solvation in your calculations.

* The paper suggests that the largest error comes from the enthalpy of solvation due to the implicit solvent model.  You may have to include explicit solvent molecules to get accurate results.

This work is licensed under a Creative Commons Attribution 4.0 International License.