Scheme 5. Mechanistic picture of the crossed intramolecular [2+2] photocycloaddition catalysed by Rh complex 6: (top) most photons are absorbed by complex 6 but not by complex 8 between cationic rhodium fragment [Rh]⁺ and substrate 4a. If excited, complex 8* reacts via 1,4-diradical intermediates 9 or 9′ to product 5a. (Bottom) simplified structure of complex 8 from the side and from the top illustrating that access onto the enone double bond from the Si face (relative to C2) is preferred. The first C–C bond forming step establishes the absolute configuration in 9 or 9′.