In a wittig reaction, how does phosphorus enable the [2+2] cycloaddition?

Question

I understand that forming the P=O drives the Wittig reaction forward but I am unable to comprehend why it helps initially when there is an overlap of the HOMO and LUMO? Does it lower the energy gap between the HOMO and LUMO?

Answer 1

There is a very informative article on the Wittig Reaction in Wikipedia. The driving force for formation of the four membered ring includes the formation of a quite stable P-O bond.

If you are worried about the fact that this is a [2+2] cycloaddition, which you would expect to be forbidden, I would suggest (though this is out of my area) that the re-hybridisation of the phosphorus atom may be relevant, since the use of a d-orbital introduces an additional nodal plane.

Answer 2

Are you serious? Because it wants it!!! WAAAAAAAANNNTS IIIIITTTTTTT!