PHOTOCHEMISTRY
Novel reactions led by flow photochemistry
Andrew Mansfield , flow chemistry leader at Syrris , summarises the latest areas in which flow photochemistry has been applied
Chemists have been performing reactions initiated by heat or a chemical reagent for many centuries . Advances in technology combined with the rediscovery of ‘ reagentless ’ activation methods , such as using light in photochemistry , have changed the way researchers devise their synthetic routes towards novel molecules . Photochemistry has gained a lot of recognition over the last 20 years . This growing interest has led to the development of innovative , modular flow photochemistry systems that allow users to efficiently switch between chemistries .
Historical background
The fundamental aim of photochemistry in synthesis is to selectively activate molecules for promoting novel chemical
Figure 1 - Photooxygenation set-up in flow to make a key cannabidiol intermediate
transformations that are greener under mild conditions . Photochemistry is widely used in academia and in industries like pharmaceuticals and agrochemicals , for the synthesis of complex natural products and molecules that are inaccessible by thermal processes . Traditionally , photons from a broad spectrum of wavelengths – natural
Figure 2 - Automated flow system for a light-mediated Negishi reaction light from the sun and mercury or xenon lamps – served as a powerful tool to access singlet and triplet states in photochemical reactions , such as cycloadditions , cyclisations , rearrangements and oxygenations . 1-4 However , these light sources required filters to narrow the wavelength bandwidth . Over-irradiation of the reaction mixture often led to decomposition of starting materials and products , resulting in the formation of by-products via undesired reaction pathways . The development of single – or nearly monochromatic – wavelength LEDs over the last 15 years has offered an even more economical and environmentally friendly alternative to broad-spectrum lamps for activating molecules or functional groups of interest . Not all organic molecules are photoactive , so the field has moved on to include the use of valuable organic and transition metal photocatalysts like [ Ru ( bpy ) 3 ] 2 + to aid the desired process . 5 Photo-induced reactions can therefore be tuned to make and break bonds in a unique fashion , using a discrete wavelength . This both
54 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981