The photodimerisation of coumarin

The photochemical dimerisation of coumarin can be rationalised on the basis of simple ab initio calculations. The direct reaction, that occurs through the first excited singlet state, gives the syn head-to-head dimer. This dimer can be obtained if the reaction occurs via a frontier orbitals control and if the reaction is concerted. The sensitised reaction gives the anti head-to-head dimer: this product can be obtained by assuming that the frontier orbitals control of the reaction is able to explain the observed regiochemical behaviour. The stereochemical behaviour can be explained on the basis of the formation of the most stable biradical intermediate.