Chemistry 324 - Spring 2008

The instructions for this assignment can be found here.

To summarize, you were asked to predict the regiochemistry of the following cycloaddition using FMO (note: the left and right-hand products will be referred to as meta and ortho, respectively):

Then you were asked to compare the FMO predictions to those based on calculated energy barriers. Finally, you were asked to look at other properties of the transition states (geometry, electron distribution) that might support the overall picture. My results follow …

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Todd and Melissa have requested Wed, April 30, 10 am as the date and time for our wrap-up breakfast. Does this work for everyone else? If I don’t hear otherwise, I will assume it does.

I will drive my van so that we can load everyone into it.

A number of you have stopped by to talk about the exam’s problems, or perhaps I should say, problems you’re having with the exam.

In order to minimize any inequities that might arise from me giving information to some students and not others, here is a summary of major items that I have communicated:

Problem #1

1. I mis-lettered the parts of problem #1. It starts with part b instead of part a. Oh well. Please follow the lettering in the exam.
2. C-Li bonds are drawn as covalent in the figures. This is convenient for the artist (me), but for purposes of drawing and analyzing pericyclic reactions, you may find it more useful to view this as an ionic bond.
3. Other aspects of the formulas might also be adjusted to make them more convenient for drawing and analyzing pericyclic reactions. (Just one of several possible hints: there is more than one way to draw a benzene ring…)

Problem #2

1. I have given you incomplete information about stereochemistry. I don’t intend to change that. Do the best you can. Perhaps more than one outcome is possible. Perhaps not.

Overall

1. Although this is a take-home exam, it is not meant to be a marathon exam. It was constructed as a take-home so that I wouldn’t have to sacrifice class time for the exam, and you would be able to do some modeling. I really figured that the non-modeling portion of the exam could be completed in an hour or less.

Download the description of the final project.

When it comes time to write, you may also be interested in my excerpts from the ACS Style Guide. (Also posted on the Miscellaneous page under “Journal articles - described“)

Exam #2 (2 page PDF). Due Monday, April 28, at 3 pm in my mailbox or my office (slip your exam under my door).

Notice that you must also email models to me for one problem so that I can examine them.

The article on CuAAC (copper-catalyzed azide-alkyne cycloadditions) can be found in issue #1, volume #40 of Aldrichimica Acta.

• Aldrichimica Acta home page
• Sortable index of all Aldrichimica Acta articles

Click chemistry even has its own Wikipedia page. Click here.

We aren’t tracking very well with the online schedule at this point. Here is what I had planned at the start of the semester:

• M 4/14 - Click 3+2 cycloadditions, distribute paper #8
• W 4/16 - Thermal 3+2 cycloadditions, collect HW #8
• F 4/18 - discuss paper #8, distribute exam #2, distribute final project instructions
• M 4/21 - 3+2 modeling session

I would still like to cover these topics, but we’ve lost a day because I never built student evaluations into my schedule, so I am going to catch up by eliminating paper #8. Here is my current plan for the remainder of the semester:

• M 4/14 - Photochemical 2+2 & singlet oxygen
• W 4/16 - Click 3+2 cycloadditions, collect HW #8
• F 4/18 - Thermal 3+2 cycloadditions, distribute exam #2, distribute final project instructions
• M 4/21 - 3+2 modeling session

After M 4/21, I will also change my office hours to 10-11 instead of 11-11:30 because we won’t have any more scheduled classes. However, I would still like to take all of you to breakfast before the end of the semester.

Here are some links for Friday’s lecture. My computer was goofy and slow so you might want to look these materials over more carefully on your own.

Jablonski diagrams

• Lecture animation
• Another diagram. This one correctly represents internal conversion and intersystem crossing as processes that (more or less) conserve energy.
• Wikipedia also has a short and useful description of internal conversion that contains some material on biochemical systems that I was unaware of

(Hanovia) Photochemical reactor

• Lecture image
• Another version that allows magnetic stirring (power supply and stand also shown; Sigma-Aldrich). Click on image for larger image.

One of you wrote “this sucks” next to your NMR assignments. I have to agree. A complete explanation of the 1H and 13C spectra is beyond me.

On other hand, while I wish I had chosen a more accessible set of data, these spectra are typical. Mother Nature is rarely kind. We just have to deal with that.

Here is my quick take on the NMR data along with some subtle relationships that should not be overlooked …

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Today’s paper contains a number of interesting issues. Rather than take several days to create one big long post, I am going to slice my ideas into small chunks and invite your comments.

Here’s one issue: diastereoselectivity in the reactions of 8 with dienes (Table I). Since this reaction involves three distinct steps, there are multiple points at which to stop and ponder selectivity.

• Conversion of 8 (diazo ester) into a metal carbenoid
• Cycloaddition of carbenoid and diene making a divinylcyclopropane
• Tautomerization of divinylcyclopropane to benzofuran

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