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!