28 Dec 2016

Understanding the Mechanism of Grubbs-Catalyzed Olefin Metathesis

Literature Discussion

Submitted by Richard Lord, Grand Valley State University
Categories
Description: 

A literature discussion has been developed for two courses: (i) a more basic set of questions appropriate for a sophomore level course or, possibly, a one semester upper level course that does not spend much time on organometallics, and (ii) an in-depth, in- and out-of-class set of assignments appropriate for an organometallics unit or course. Both sets of questions explore the mechanism of olefin metathesis in first- and second-generation Grubbs catalysts using a variety of spectroscopic kinetic techniques that were presented in the paper Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 6543-6554 (doi: 10.1021/ja010624k).

Learning Goals: 

A student should be able to do the following after completing this LO:

Identify and discuss the importance of an olefin metathesis reaction.

Distinguish between Fischer- and Schrock-type carbenes and count electrons in complexes featuring these ligands.

Predict Ia vs. Id mechanisms based on electron counting.

Discuss why 31P and 1H NMR, and UV-Vis, were appropriate spectroscopic techniques for measuring kinetics in these systems.

Explain how variations in the catalyst (halide, carbene substituent, phosphine substituent, phosphine vs. NHC, etc.) affect ligand exchange.

Describe why the second-generation catalysts outperform the first-generation catalysts based on olefin binding vs. phosphine loss (in contrast to the historical reasons for their design), and why trans- effect arguments do not apply to the Grubbs system.

 

 

Implementation Notes: 

Two of us ran portions of these (they were not complete) in Fall 2016. Both of us noted that students were confused by the NHC representation in the manuscript because the authors assume that the reader knows there is no H on the carbene carbon, yet one would predict a H there based on the line structure formalism.

It is important to point out to students that the work here represents only the first 1-2 steps of the overall mechanism of olefin metathesis. There is a question in both the basic and advanced exercises that has students analyze the metallacyclobutane intermediate, but this species is formed later in the mechanism than any of the work presented here.

Time Required: 
~1 class period
Creative Commons License: 
Creative Commons Licence

Comments

I tried running the "advanced" version in my 200 level class that has a number of senior students due to a curriculum change (our 400 level course only runs every other year now). The pre-class activity was a success. The in-class activity appears to take far more time than we (the authors) anticipated. Only one group of seven from both the senior and sophomore student groups managed to finish question 8 completely in a 50 minute class. This was partially due to good discussion from the groups around questions 1-5. If you are going to use the LO as it is written, I recommend two class periods (~100 minutes).

Asking the students to pre-read some of the specific sections from the paper may allow this to fit into a single class period. I plan to try this the next time I teach our 400 level course.

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