JASON D. WILLIAMS, C. OLIVER KAPPE*
*Corresponding author
Center for Continuous Synthesis and Processing (CCFLOW), Research Center Pharmaceutical Engineering (RCPE), Graz, Austria.
Institute of Chemistry, University of Graz, NAWI Graz, Graz, Austria

Abstract

In recent years, a significant amount of research has focused on the development of photochemical transformations, whose eventual scale up will likely involve the use of continuous flow processing. The light-dependent behaviour of different photochemical reactions can vary to a significant extent, particularly when examining complex systems with more than one catalytic species. Based on our recent experience in flow photochemistry, this article aims to discuss the development of flow photochemical processes, with a particular focus on considerations affecting their application to larger scale processing. Four case studies are discussed, representing a range of reaction types and light sources, with focus on the key parameters affecting scalability.

INTRODUCTION
Photochemical transformations in synthetic organic chemistry have long been known, but perhaps only in the past 10 years have they been introduced to the synthetic chemist’s toolbox in a widespread manner. With the recent popularization of photoredox chemistry, more laboratories now find themselves better equipped to include photochemical transformations in their synthetic routes. Many of these transformations are, in fact, long-known reactions, (e.g. alkene [2+2], Paterno-Büchi, benzylic bromination), which can now be performed under far more mild and synthetically favourable conditions (e.g. longer wavelength irradiation, higher concentration, with enantioselectivity), thanks to developments in reactor and light-source technology, alongside improved reaction understanding. When these transformations are passed on to be performed on larger scales, it is inevitable that continuous flow will be assessed as a scalable processing method (1). Photochemical methods are poised to revolutionise fine chemical production (2) – process chemistry groups, CROs and CMOs must be prepared to develop the resulting processes, or risk being left behind.