Solar photochemistry: optimisation of the photo Friedel-Crafts acylation of naphthoquinones

A practical and robust photo Friedel Crafts acylation of naphthoquinones is described. Although the reaction proceeds slowly in sunlight, the optimised conditions offer a substantial improvement to those already reported, by the utilisation of a more reliable and practical 'sun-mimicking' light source, a less hazardous solvent system (trifluorotoluene) and faster reaction times. Using these conditions, the reaction scope has been expanded to include functionalised aldehyde and naphthoquinone substrates, affording the desired photo-products in acceptable to excellent yields (17-81%). Factors influencing the regiochemistry of the photo Friedel Crafts reaction on unsymmetrical naphthoquinones have also been investigated.