1:30, Tu – Fluffy

3:30, Tu – Lauren

11:00, W – Harry

1:30, W – Stephen

3:30, W – Alex

The exam will be held in my office unless another room proves more convenient.

There most certainly are errors in these schemes, but they are fiendishly subtle. I strongly recommend that you build models of these transition states. Only by building models was I able to look reliably at “9” with the correct configurations at each chiral C and rotate the molecule so that I could see what product 9 would make. Read the rest of this entry »

I’ve had a chance to look at the drawings and labels that you compiled for paper 5. I think everyone did a really good job here. I don’t have any complaints, but, as I worked my way through your paperwork, a couple of ideas occurred to me that might prove useful to you ahead of next week’s exam.

Read the rest of this entry »

We tend to rely on expert authors to be … experts. But even experts are human beings so, to be fair, we must expect an occasional “expert” mistake. I want you to take a close look at some of the structural formulas in Paper 5 (OL, 2007, 9(22), 4653) and see if any mistakes have been made. I’m not promising that there are any. You will have to decide for yourself.

  • First, examine formulas 9, 10 and 7 in Scheme 1. If the transition state geometry is really the one shown in 9, will 10 and 7 be obtained? If the Scheme contains geometrical inconsistencies, accept the stereochemistry of 7 as correct and work backwards. What should 10 look like? What should 9 look like?
  • Second, examine the formulas of 13 and 14 in Scheme 2. This transformation is more complicated because 13 contains a chiral center. Draw a chair transition state that is consistent with this transformation.  Can 9 account for this transformation?

Due Monday. Suggestion: if you are having trouble visualizing these structures, use models. You should be able to figure out how to build a basic Claisen transition state using SPARTAN and you can decorate it from there.

HW #4 raised two sets of issues that deserve comment.

First, a couple of reminders about FMO analysis:

  • identify both FMO interactions and then identify the dominant one
  • regioselectivity depends on transition state geometry, which depends on orbital overlap, so you need to give me orbital coefficients for the important FMO (note: I didn’t list the coefficients in the answer key, but I looked at them when positioning the * symbols for problem #1)
  • the HOMO and LUMO are not always the “FMO”

Wait, aren’t the HOMO and LUMO always the frontier MOs by definition? Yes, they are always the FMO, but adhering to that definition is unwise if either the HOMO or LUMO has the wrong shape. Always use the “frontier” MO that will guide the reaction. If the reaction is pericyclic, you must pay attention to the kinds of bonds that are being broken and find the orbitals that correspond to these bonds. If you’re breaking a π bond, you need to find a π-type MO. All of you fell into this “trap” on problems #1 and #3.

Second, one of you pointed out that, if the reactants in problem #2 (3a, 3b, 3c) are always at equilibrium, the Curtin-Hammett principle will apply. Yes! I totally forgot about that when I wrote my answer key. I am going to go home and eat crow (while thinking admiring thoughts about the intelligence of Reedies). Well done!

Follow

Get every new post delivered to your Inbox.