Photochemical Flow Synthesis of 3‐Hydroxyazetidines

Advances in the Continuous Flow Synthesis of 3- and 4-Membered Ring Systems

Small carbo- and heterocyclic ring systems have experienced a significant increase in importance in recent years due to their relevance in modern pharmaceuticals, as building blocks for designer materials or as synthetic intermediates. This necessitated the development of new synthetic methods for the preparation of these strained ring systems focusing on effectiveness and scalability. The high ring strain of these entities as well as the use of high-energy reagents and intermediates has often challenged their synthesis. Continuous flow approaches have thus emerged as highly effective means to safely and reliably access these strained scaffolds. In this short review, key developments in this field are summarised showcasing the power of continuous flow approaches for accessing 3- and 4-membered ring systems via thermal, photo- and electrochemical processes.

Nanostructured Carbon Nitride for Continuous-Flow Trifluoromethylation of (Hetero)arenes

Efficient catalytic methods for the trifluoromethylation of (hetero)arenes are of particular importance in organic and pharmaceutical manufacturing. However, many existing protocols rely on toxic reagents and expensive or sterically hindered homogeneous catalysts. One promising alternative to conduct this transformation involves the use of carbon nitride, a non-toxic photocatalyst prepared from inexpensive precursors. Nonetheless, there is still little understanding regarding the interplay between physicochemical features of this photocatalyst and the corresponding effects on the reaction rate. In this work, we elucidate the role of carbon nitride nanostructuring on the catalytic performance, understanding the effect of surface area and band gap tuning via metal insertion. Our findings provide new insights into the structure-function relationships of the catalyst, which we exploit to design a continuous-flow process that maximizes catalyst-light interaction, facilitates catalyst reusability, and enables intensified reaction scale-up. This is particularly significant given that photocatalyzed batch protocols often face challenges during industrial exploitation. Finally, we extrapolate the rapid and simplified continuous-flow method to the synthesis of a variety of functionalized heteroaromatics, which have numerous applications in the pharmaceutical and fine chemical industries.

Visible-Light-Mediated Energy Transfer Enables the Synthesis of β-Lactams via Intramolecular Hydrogen Atom Transfer

The synthesis of 2-azetidinones (β-lactams) from simple acrylamide starting materials by visible-light-mediated energy transfer catalysis is reported. The reaction features a C(sp³ )-H functionalization via a variation of the Norrish-Yang photocyclization involving a carbon-to-carbon 1,5-hydrogen atom transfer (supported by deuterium labelling and DFT calculations) and can be used for the construction of a diverse range of β-lactam products.

Synthesis of Cycloheptatriene-Containing Azetidine Lactones

We prepared a collection of complex cycloheptatriene-containing azetidine lactones by applying two key photochemical reactions: "aza-Yang" cyclization and Buchner carbene insertion into aromatic rings. While photolysis of phenacyl amines leads to a rapid charge transfer and elimination, we found that a simple protonation of the amine enables the formation of azetidinols as single diastereomers. We provide evidence, through ultrafast spectroscopy, for the electron transfer from free amines in the excited state. Further, we characterize the aza-Yang reaction by establishing the dependence of the initial reaction rates on the rates of photon absorption. An unanticipated change in reactivity in morpholine analogues is explained through interactions with the tosylate anion. The Buchner reaction proceeds with a slight preference for one diastereomer over the other, and successful reaction requires electron-donating carbene-stabilizing substituents. Overall, 16 compounds were prepared over seven steps. Guided by an increase in structural complexity, efforts such as this one extend the reach of chemists into unexplored chemical space and provide useful quantities of new compounds for studies focused on their properties.

Unusually Chemoselective Photocyclization of 2-(Hydroxyimino)aldehydes to Cyclobutanol Oximes: Synthetic, Stereochemical, and Mechanistic Aspects

Photocyclization of carbonyl compounds (known as the Norrish-Yang reaction) to yield cyclobutanols is, in general, accompanied by fragmentation reactions. The latter are predominant in the case of aldehydes so that secondary cyclobutanols are not considered accessible via the straightforward Norrish-Yang reaction. A noteworthy exception has been reported in our laboratory, where cyclobutanols bearing a secondary alcohol function were observed upon UV light irradiation of 2-(hydroxyimino)aldehydes (HIAs). This reaction is here investigated in detail by combining synthesis, spectroscopic data, molecular dynamics, and DFT calculations. The synthetic methodology is generally applicable to a series of HIAs, affording the corresponding cyclobutanol oximes (CBOs) chemoselectively (i.e., without sizable fragmentation side-reactions), diastereoselectively (up to >99:1), and in good to excellent yields (up to 95%). CBO oxime ether derivatives can be purified and diastereomers isolated by standard column chromatography. The mechanistic and stereochemical picture of this photocyclization reaction, as well as of the postcyclization E/Z isomerization of the oxime double bond is completed.

Synthetic Access to Aromatic α-Haloketones

α-Haloketones play an essential role in the synthesis of complex N-, S-, O-heterocycles; of which some exhibit a remarkable biological activity. Research further illustrated that α-bromo-, α-chloro-, and α-iodoketones are key precursors for blockbuster pharmacological compounds. Over the past twenty years, substantial advances have been made in the synthesis of these industrially relevant building blocks. Efforts have focused on rendering the synthetic protocols greener, more effective and versatile. In this survey, we summarised and thoroughly evaluated the progress of the field, established in the past two decades, in terms of generality, efficacy and sustainability.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

Forgotten and forbidden chemical reactions revitalised through continuous flow technology

Continuous flow technology has played an undeniable role in enabling modern chemical synthesis, whereby a myriad of reactions can now be performed with greater efficiency, safety and control. As flow chemistry furthermore delivers more sustainable and readily scalable routes to important target structures a growing number of industrial applications are being reported. In this review we highlight the impact of flow chemistry on revitalising important chemical reactions that were either forgotten soon after their initial report as necessary improvements were not realised due to a lack of available technology, or forbidden due to unacceptable safety concerns relating to the experimental procedure. In both cases flow processing in combination with further reaction optimisation has rendered a powerful set of tools that make such transformations not only highly efficient but moreover very desirable due to a more streamlined construction of desired scaffolds. This short review highlights important contributions from academic and industrial laboratories predominantly from the last 5 years allowing the reader to gain an appreciation of the impact of flow chemistry.

Strain Release Chemistry of Photogenerated Small-Ring Intermediates

Photochemical processes, such as isomerizations and cycloadditions, have proven to be very useful in the construction of highly strained molecular frameworks. Photoinduced ring strain enables subsequent exergonic reactions which do not require the input of additional chemical energy and provides a variety of attractive synthetic options leading to complex structures. This review covers the progress achieved in the application of sequences combining excitation by ultraviolet light to form strained intermediates, which are further transformed to lower energy products in strain-release reactions. As ring strain is considerable in small ring systems, photogenerated three- and four-membered rings will be covered, mainly focusing on examples from 2000 to May 2020.

A scalable continuous photochemical process for the generation of aminopropylsulfones

An efficient continuous photochemical process is presented that delivers a series of novel γ-aminopropylsulfones via a tetrabutylammonium decatungstate (TBADT) catalysed HAT-process. Crucial to this success is the exploitation of a new high-power LED emitting at 365 nm that was found to be superior to an alternative medium-pressure Hg lamp. The resulting flow process enabled the scale-up of this transformation reaching throughputs of 20 mmol h-1 at substrate concentrations up to 500 mM. Additionally, the substrate scope of this transformation was evaluated demonstrating the straightforward incorporation of different amine substituents as well as alkyl appendages next to the sulfone moiety. It is anticipated that this methodology will allow for further exploitations of these underrepresented γ-aminopropylsulfone scaffolds in the future.

Rearrangement of 3-Hydroxyazetidines into 2-Oxazolines

A novel rearrangement sequence of 3-hydroxyazetidines via a Ritter initiated cascade provides highly substituted 2-oxazolines in high yields. The reaction conditions and substrate scope of the transformation have been studied demonstrating the generality of the process. The derived products can also be functionalized in order to undergo further intramolecular cyclization leading to a new class of macrocycle. The final cyclization step was shown to be a transformation amenable to continuous flow processing allowing for a dramatic reduction in the reaction time and simple scale-up.

Continuous Flow Photochemistry for the Preparation of Bioactive Molecules

The last decade has witnessed a remarkable development towards improved and new photochemical transformations in response to greener and more sustainable chemical synthesis needs. Additionally, the availability of modern continuous flow reactors has enabled widespread applications in view of more streamlined and custom designed flow processes. In this focused review article, we wish to evaluate the standing of the field of continuous flow photochemistry with a specific emphasis on the generation of bioactive entities, including natural products, drugs and their precursors. To this end we highlight key developments in this field that have contributed to the progress achieved to date. Dedicated sections present the variety of suitable reactor designs and set-ups available; a short discussion on the relevance of greener and more sustainable approaches; and selected key applications in the area of bioactive structures. A final section outlines remaining challenges and areas that will benefit from further developments in this fast-moving area. It is hoped that this report provides a valuable update on this important field of synthetic chemistry which may fuel developments in the future.