Regioselectivity in photochemical [2 + 2] cycloadditions
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص898
2025-07-23
394
Regioselectivity in photochemical [2 + 2] cycloadditions
The observed regioselectivity is shown below. If we had combined the HOMO of the alkene with the LUMO of the enone, as we should in a thermal reaction, we would expect the oppo site orientation so as to use the larger coefficients of the frontier orbitals and to maximize charge stabilization in the transition state.
But we are not doing a thermal reaction , you will see that it is the HOMO/HOMO and LUMO/LUMO interactions that now matter in the reactions of the excited state. The sizes of the coefficients in the LUMO of the alkene are the other way round to those in the HOMO. There is one electron in this pair of orbitals— in the LUMO of the enone in fact, as the enone has been excited by the light—so overlap between the two LUMOs (shown in the frame) is bonding and leads to the observed product. The easiest way to work it out quickly is to draw the product you do not expect from a normal HOMO/LUMO or curly arrow-controlled reaction.
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