Flow Photochemistry as a Tool in Organic Synthesis

Abstract

Photochemical transformations of molecular building blocks have become an important and widely recognized research field in the past decade. Detailed and deep understanding of novel photochemical catalysts and reaction concepts with visible light as the energy source has enabled a broad application portfolio for synthetic organic chemistry. In parallel, continuous‐flow chemistry and microreaction technology have become the basis for thinking and doing chemistry in a novel fashion with clear focus on improved process control for higher conversion and selectivity. As can be seen by the large number of scientific publications on flow photochemistry in the recent past, both research topics have found each other as exceptionally well‐suited counterparts with high synergy by combining chemistry and technology. This review will give an overview on selected reaction classes, which represent important photochemical transformations in synthetic organic chemistry, and which benefit from mild and defined process conditions by the transfer from batch to continuous‐flow mode.

Scalability of photochemical reactions in continuous flow mode

“…Over the last decade continuous flow photochemistry has matured into a powerful field within synthetic organic chemistry. Key features that enable effective photochemical transformations in flow mode are the ability to uniformly irradiate solutions of substrates that are continuously pumped through narrow diameter tubing or microchannels [1][2][3]. High spatiotemporal control furthermore allows the effects from overirradiation to easily be minimised as in principle each molecule resides within the irradiated section of the flow reactor for the same amount of time, which can be controlled with high precision in flow mode [4].…”

Bridging Lab and Industry with Flow Electrochemistry

“…This narrow distance is proportional to the solvent/electrolyte system's resistance and is hence highly advantageous. Inside the reactor, reaction conditions, such as temperature and pressure, are controlled, and the stream of product is either carried into a second reactor or displaced into a collection unit ( Rehm, 2020 ). The amount of time reactants spend in a flow reactor is defined by residence time and is a factor of flow rate and reactor volume (residence time = flow rate × volume).…”

Direct Deoxygenative Intramolecular Acylation of Biarylcarboxylic Acids

“…Continuous flow technology has attracted more and more attention in recent years, 17 and shows some significant advantages over conventional batch reactions, including the in situ formation and direct use of reaction intermediates, convenience of temperature control, increased reaction contact area, and ease of amplification. Hence, the model reactant 1a was used to study this intramolecular cyclization reaction under continuous flow conditions.…”

“…In summary, a green, mild and simple method to access fluorenone derivatives is described. A number of substituted fluorenones 17 have been synthesized via this process, which may have potential as luminescent materials. Continuous flow chemistry 18 was successfully applied in this deoxygenative intramolecular acylation, showing the potential for scale-up and possible industrialization of the process.…”