University:
Amherst College
Course:
CHEM 221 | Organic Chemistry I
Academic year:

2022
Views:

391

Pages:

3
Author:

Brennen Smith

Lecture outline Aromatic transition state theory (cont'd) Electrocyclic reactions. These involve a net exchange of one π-bond for one σ-bond. (Two examples were shown on the previous page.) Formation of a new σ-bond between the ends of a conjugated π-system to form a ring requires the end p-orbitals to rotate — either in the same direction (conrotation) or in opposite directions (disrotation). Similarly, in the reverse reaction, ring opening, might occur by conrotation or by disrotation. X Y X Y via disrotatory process ? or X (both racemic) Y via conrotatory process The triene above is E,E. What would be the results of dis- and of conrotatory ring closure starting with the E,Z-triene? What about the Z,Z-triene? It turns out that for a 6-e– electrocyclic process disrotation is favored. To understand why we need to look at the arrays of interacting orbitals in the transition states for the two processes. The 4-e– process generally goes in the ring opening direction, but we'll analyze the ring closure for consistency with the example above. Which is favored in this case? X X Y Y via disrotatory process ? or X (both racemic) Y via conrotatory process For the 4-e– electrocyclic process conrotation is favored. Here are the corresponding orbital arrays. Orbital arrays that have one phase change have the same topology as a Möbius strip. 4ne Möbius arrays are aromatic; 4n+2 -e– Möbius arrays are antiaromatic. – Predict the mode of ring-opening (con- or disrotatory) for the following cations, and add the methyl groups to the products. Aromatic transition state theory also applies nicely to sigmatropic shifts. Experimentally, it is found that [1,3] shifts of H do not occur, but [1,5] shifts do. [1,7] shifts do not occur — unless the H can be transferred to the opposite face of the π-system! [1,3] H [1,5] H [1,7] H Sigmatropic shifts of C are even more interesting because the C can migrate with retention or inversion of configuration. [1,3] shifts of C occur with inversion of the configuration of the migrating C; [1,5] shifts occur with retention.

Lecture Outline Aromatic Transition State Theory

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