Flow chemistry involves the use of channels or tubing to conduct a reaction in a continuous stream rather than in a flask Flow equipment provides chemists with unique control over reaction parameters enhancing reactivity or in some cases enabling new reactions This relatively young technology has received a remarkable amount of attention in the past decade with many reports on what can be done in flow Until recently, however, the question, “Should we do this in flow?” has merely been an afterthought This review introduces readers to the basic principles and fundamentals of flow chemistry and critically discusses recent flow chemistry accounts

The Hitchhiker's Guide to Flow Chemistry

Continuous-flow photochemistry in microreactors receives a lot of attention from researchers in academia and industry as this technology provides reduced reaction times, higher selectivities, straightforward scalability, and the possibility to safely use hazardous intermediates and gaseous reactants. In this review, an up-to-date overview is given of photochemical transformations in continuous-flow reactors, including applications in organic synthesis, material science, and water treatment. In addition, the advantages of continuous-flow photochemistry are pointed out and a thorough comparison with batch processing is presented.

Applications of Continuous-Flow Photochemistry in Organic Synthesis, Material Science, and Water Treatment

The [2 + 2] photocycloaddition is undisputedly the most important and most frequently used photochemical reaction. In this review, it is attempted to cover all recent aspects of [2 + 2] photocycloaddition chemistry with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated separately in sections 2 and 4, whereas the vast majority of photocycloaddition reactions which occur by direct excitation or sensitization are divided within section 3 into individual subsections according to the photochemically excited olefin.

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.5b00723

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2+2] Photocycloaddition Reactions

http://pubs.acs.org/doi/pdf/10.1021/acs.oprd.5b00325

Flow Chemistry: Recent Developments in the Synthesis of Pharmaceutical Products

Photomediated controlled radical polymerization

The use of flow photochemistry and its apparent superiority over batch has been reported by a number of groups in recent years. To rigorously determine whether flow does indeed have an advantage over batch, a broad range of synthetic photochemical transformations were optimized in both reactor modes and their yields and productivities compared. Surprisingly, yields were essentially identical in all comparative cases. Even more revealing was the observation that the productivity of flow reactors varied very little to that of their batch counterparts when the key reaction parameters were matched. Those with a single layer of fluorinated ethylene propylene (FEP) had an average productivity 20% lower than that of batch, whereas three-layer reactors were 20% more productive. Finally, the utility of flow chemistry was demonstrated in the scale-up of the ring-opening reaction of a potentially explosive [1.1.1] propellane with butane-2,3-dione.

/pdf/batch-versus-flow-photochemistry-a-revealing-comparison-of-1ubl8tnkt3.pdf

Batch versus Flow Photochemistry: A Revealing Comparison of Yield and Productivity

The photoactivated evolution of a series of enantiomerically pure 5-oxymethyl-2(5H)-furanones has been investigated. The observed intramolecular photoreactions have proven to be a straightforward entry to diverse and stereochemically rich fragment-molecules, most of which contain the privileged tetrahydropyran (THP) scaffold. The formation of the THP involves a 1,5-hydrogen atom transfer process, leading to a diradical intermediate that recombines to form a new σ C-C bond. These reactions take place under both sensitized and nonsensitized conditions, and they are highly stereoselective. When the substrate contains an allyl residue, the intramolecular [2 + 2] cycloaddition leading to cyclobutanes competes advantageously. When the substrate contains a THP residue, the cyclization involves the concomitant formation of [6,6]-spiroketals with nonanomeric relationships.

Guillaume Lejeune

Papers

Batch versus Flow Photochemistry: A Revealing Comparison of Yield and Productivity

Intramolecular Photoreactions of (5S)-5-Oxymethyl-2(5H)-furanones as a Tool for the Stereoselective Generation of Diverse Polycyclic Scaffolds.