Laser-based depletion zone photoreaction: Selective synthesis of [2+2]-crossdimers of coumarin and 5-fluorouracil

Generally the homodimerization in an intermolecular, photochemical [2+2]-cycloaddition is the dominant reaction over the crossdimerization. Using the example of Coumarin with a 10-fold excess of 5-fluorouracil, just a 2:1 crossdimer-to-homodimer (CD/HD) ratio is obtained in a conventional Rayonet reactor. Here we present an efficient and versatile procedure for the sensitizer-free synthesis of intermolecular [2+2]-cycloaddition crossdimers in solution. Applying a monochromatic pulsed laser system as the light source first enables a highly wavelength selective excitation of only one substrate. Second, the high energy pulses establish a depletion zone inside the reaction vessel, where the concentration of the absorbing substrate in its ground state is very low. This causes the homodimerization to be completely suppressed, while favoring the crossdimerization reaction by increasing the probability to meet a non-absorbing cross-component. In our example the CD/HD-ratio shifts from less than 2 to more than 8. The depletion zone photoreaction is altogether useful for the synthesis of crossdimers.