9 Jun 2017

Catalysis: Iron-catalyzed Arylation of Alkyl Halides

Lab Experiment

Submitted by Bradley Wile, Ohio Northern University
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Description: 

This LO describes a laboratory experiment in which students generate (in situ) an iron catalyst for the arylation of alkyl halides (Kumada coupling). Students pool data from the class to discern what features lead to successful catalyst systems. GC-MS or GC-FID may be used to quantify the catalytic performance of each system, and results may be expressed as % conversion, with TON/TOF values. Students gain experience proposing reasonable coordination complexes that may be formed from the catalyst precursors, and searching the literature/databases for related compounds/systems. Students write a full report, and address the questions listed in the handout. 

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File Laboratory Handout230.63 KB
Learning Goals: 

To explore the use of transition metal complexes as homogeneous catalysts for an organic transformation. As a group, to identify the possible influence of spectator ligands on catalytic performance by pooling data. Specific technical and educational objectives are:

  • Students will be able to operate the inert atmosphere glovebox and block reactor to conduct a (mostly) air-free reaction
  • Students will be able to analyze gas chromatographic (GC) data to determine the percent conversion, TON, and TOF for a catalytic system
  • Students will be able to use data from in situ experiments to propose a structure for an active catalyst
  • Students will be able to draw conclusions about the relationship between steric and electronic parameters of ligands and catalyst performance 
Equipment needs: 

GC-MS or GC-FID (other detectors may be used, but I have not quantified using these).

I run this using a vial in an Al heating block, but an oil or water bath would work just as well.

Reagents and solvents are listed in the instructor notes (with CAS numbers).

Implementation Notes: 

I created this experiment as a way to introduce catalysis without the need to rationally prepare new compounds for each group. I would like to use the wisdom of the group to find out what tweaks are needed to adapt this experiment to other institutions/courses. If you would like to conduct this experiment with your students, it would be great if we could compile data from your group using the Google Spreadsheet link embedded in the instructor notes. Ultimately, I would like to publish this experiment in JCE or similar if it is well received. Although this experiment has only been run once using this organic transformation, a previous version using a hydrosilylation reaction was successfully employed for five years previous to this.

In the lecture, students will have covered the formation of coordination complexes, as well as types of ligands, electron counting, etc. I use catalysis as a motivation and starting point for several of my discussions throughout the semester, so while they will not have discussed this explicitly, they will have all of the components mentioned in this experiment. Typically, I will discuss catalysis and turnover one or two lectures after the in lab portion of this experiment, so students are already grappling with TON/TOF calculations. I do not cover GC interpretation explicitly in lecture, since all of our students will have spent a little time on this in the organic chemistry majors lab. Most questions can be handled with an impromptu discussion about data interpretation. 

DISCLAIMER - The nature of the experiment (novel catalysts generated in situ) leads to some unpredictability. If you (or your students) aren't up for an element of surprise in your catalytic data, this may not be for you. I pitch the experiment to the students this way, and offer to conduct a different (but probably less exciting) experiment if they are concerned about the potential for poor/no catalytic activity. So far no one has taken me up on this offer.

Time Required: 
~1 laboratory period (3 hours) + 20 minutes for GC setup and data processing
Evaluation
Evaluation Methods: 

Students completed a full lab report for this activity, in which they described their results, including specific responses to the questions in the handout. This was evaluated using the rubric in the instructor notes. The report is graded out of a total of 50 points. 

Evaluation Results: 

Overall, students did well. The grade range for most students is 40/50 to 50/50. This rubric and set of questions is the result of iterating a similar experiment using a different organic transformation. 

Students had some unexpected proposals for the structural details that might explain catalytic activity (or lack thereof). In most cases, I was fine with these being incorrect (or less likely to be correct), if they carried this logic through their response to question 3. If the students propose a new ligand that will address their hypothesis, great! We can go in the lab and test these with the next group. If a student proposes a reasonably priced ligand, I generally buy it for next year's group to try out. 

Question 4 (chirality) prompted some vague responses. I need a more detailed prompt and discussion about this point. I think it is relevant since the word "chirality" immediately grabs the attention of many organic chemists and students who are interested in organic aspects of catalysis.

If I am pressed for time to discuss TON/TOF in class, students will generally ask more questions about how to calculate these values. I may incorporate Sibrina Collins' activities about TON/TOF in future (see above links) to address these in greater detail in the lecture course. 

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