[CrimpJiggler]

On an exam question recently, I had a question which took me by surprise. It asked me what product I'd get if I combine an alkene with a ketene in the presence of light. I know how this reaction would occur thermally, and I know why it occurs (i.e. the 90 degree orbital overlap thing) but I don't know what happens if you throw light into the equation. I just guessed and said that the oxygen of the ketene is where the excitation occurs, therefore the alkene forms a bond with the oxygen and one with the sp carbon (as opposed to the thermal reaction which forms two carbon-carbon bonds).

[AlphaScent]

This explains it better than I can.

http://en.wikipedia.org/wiki/Ketene_cycloaddition

[clarkstill]

If the [2+2] cycloaddition were just that it would be thermally disallowed, and so it goes via the orbital interaction you alluded to (a [2+2+2]).  However, photochemical [2+2] cycloadditions are symmetry allowed, and would work for 2 simple alkenes, never mind a ketene.  I think you get exactly the same products as in the thermal reaction, but by a different orbital interaction.  I guess you could also get the oxa-[2+2], but the enol ether product looks much less happy...