Microfluidic Visible‐Light Paternò–Büchi Reaction of Oxindole Enol Ethers

A Paternò-Büchi Reaction of Aromatics with Quinones under Visible Light Irradiation

Reported herein is a Paternò-Büchi reaction of aromatic double bonds with quinones under visible light irradiation. The reactions of aromatics with quinones exposed to blue LED irradiation yielded oxetanes at -78 °C, which was attributed to both the activation of double bonds in aromatics and the stabilization of oxetanes by thiadiazole, oxadiazole, or selenadiazole groups. The addition of Cu(OTf)2 to the reaction system at room temperature resulted in the formation of diaryl ethers via the copper-catalyzed ring opening of oxetanes in situ. Notably, the substrate scope was extended to general aromatics.

Unveiling the impact of the light-source and steric factors on [2+2] heterocycloaddition reactions

Information gained from in-depth mechanistic investigations can be used to control the selectivity of reactions, leading to the expansion of the generality of synthetic processes and the discovery of new reactivity. Here, we investigate the mechanism of light-driven [2+2] heterocycloadditions (Paternò-Büchi reactions) between indoles and ketones to develop insight into these processes. Using ground-state UV-Vis absorption and transient absorption spectroscopy (TAS), together with DFT calculations, we found that the reactions can proceed via an exciplex or electron-donor-acceptor (EDA) complex, which are key intermediates in determining the stereoselectivity of the reactions. We used this discovery to control the diastereoselectivity of the reactions, gaining access to previously inaccessible diastereoisomeric variants. When moving from 370 to 456 nm irradiation, the EDA complex is increasingly favoured, and the diastereomeric ratio (d.r.) of the product moves from >99:<1 to 47:53. In contrast, switching from methyl to ⁱpropyl substitution favours the exciplex intermediate, reversing the d.r. from 89:11 to 16:84. Our study shows how light and steric parameters can be rationally used to control the diastereoselectivity of photoreactions, creating mechanistic pathways to previously inaccessible stereochemical variants.

Mechanisms and Synthetic Strategies in Visible Light-Driven [2+2]-Heterocycloadditions

The synthesis of four membered heterocycles usually requires multi-step procedures and prefunctionalized reactants. A straightforward alternative is the photochemical [2+2]-heterocycloaddition between an alkene and a carbonyl derivative, conventionally based on the photoexcitation of this latter. However, this approach is limited by the absorption profile of the carbonyl, requiring in most of the cases the use of high-energy UV-light, that often results in undesired side reactions and/or the degradation of the reaction components. The development of new and milder visible light-driven [2+2]-heterocycloadditions is, therefore, highly desirable. In this Review, we highlight the most relevant achievements in the development of [2+2]-heterocycloadditions promoted by visible light, with a particular emphasis on the involved reaction mechanisms. The open challenges will also be discussed, suggesting new possible evolutions, and stimulating new methodological developments in the field.

Synthesis of functionalized spirocyclic oxetanes through Paternò-Büchi reactions of cyclic ketones and maleic acid derivatives

A telescoped three-step sequence to functionalised spirocyclic oxetanes is reported, involving Paternò-Büchi reactions between maleic acid derivatives and cyclic ketones. p-Xylene suppresses the competing alkene dimerization that has plagued previous work, allowing access to 35 novel spirocyclic oxetanes that cannot be prepared using existing methodologies, and which represent versatile intermediates for further elaboration.

Energy-Transfer-Enabled Dearomative Cycloaddition Reactions of Indoles/Pyrroles via Excited-State Aromatics

Exploring the enormous chemical space through an expedient building-up of molecular diversity is an important goal of organic chemistry. The development of synthetic methods toward molecules with unprecedented structural motifs lays the foundation for wide applications ranging from pharmaceutical chemistry to materials science. In this regard, the dearomatization of arenes has been recognized as a unique strategy since it provides novel retrosynthetic disconnections for various spiro or fused polycyclic molecules with increased saturation and stereoisomerism. However, inherent thermodynamic challenges are associated with dearomatization processes. The disruption of the aromaticity of arene substrates usually requires large energy inputs, which makes harsh conditions necessary for many ground-state dearomatization reactions. Therefore, further expansion of the scope of dearomatization reactions remains a major problem not fully solved in organic chemistry.The past decade has witnessed tremendous progress on photocatalytic reactions under visible light. Particularly, reactions via an energy transfer mechanism have unlocked new opportunities for dearomatization reactions. Mediated by appropriately chosen photosensitizers, aromatic substrates can be excited. This kind of precise energy input might make feasible some dearomatization reactions that are otherwise unfavorable under thermal conditions because of the significant energy increases of the substrates. Nevertheless, the lifetimes of key intermediates in energy-transfer-enabled reactions, such as excited-state aromatics and downstream biradical species, are quite short. How to regulate the reactivities of these transient intermediates to achieve exclusive selectivity toward a certain reaction pathway among many possibilities is a crucial issue to be addressed.Since 2019, our group has reported a series of visible-light-induced dearomative cycloaddition reactions for indole and pyrrole derivatives. It was found that the aromatic units in substrates can be excited under the irradiation of visible light in the presence of a suitable photosensitizer. These excited aromatics readily undergo various [m + n] cycloaddition reactions with appropriately tethered unsaturated functionalities including alkenes, alkynes, N-alkoxy oximes, (hetero)arenes, and vinylcyclopropanes. The reactions yield polycyclic indolines and pyrrolines with highly strained small- and/or medium-sized rings embedded, some of which possess unique bridge- or cagelike topologies. Systematic mechanistic studies confirmed the involvement of an energy transfer process. Density functional theory (DFT) calculations revealed the correlation between the substrate structure and the excitation efficiency, which accelerated the optimization of the reaction parameters. Meanwhile, DFT calculations demonstrated the competition between kinetically and thermodynamically controlled pathways for the open-shell singlet biradical intermediates, which allowed the complete switches from [2 + 2] cycloaddition to 1,5-hydrogen atom transfer in reactions with N-alkoxy oximes and to [4 + 2] cycloaddition in reactions with naphthalene. Furthermore, ab initio molecular dynamics (AIMD) simulations uncovered post-spin crossing dynamic effects, which determine the regioselectivity for the open-shell singlet biradical recombination step in the reactions of pyrrole-derived vinylcyclopropanes.An increasing number of scientists have joined in the research on visible-light-induced dearomative cycloaddition reactions and contributed to more elegant examples in this area. The visible-light-induced dearomatization reaction via energy transfer mechanism, although still in its infancy, has exhibited great potential in the synthesis of molecules that can hardly be accessed by other methods. We believe that future development will further push the boundary of organic chemistry and find applications in the synthesis of functional molecules and related disciplines.

The Photochemical Activity of a Halogen-Bonded Complex Enables the Microfluidic Light-Driven Alkylation of Phenols

A mild light-driven protocol for the direct alkylation of phenols is reported. The process is driven by the photochemical activity of a halogen-bonded complex formed upon complexation of the in situ generated electron-rich phenolate anion with the α-iodosulfone. The reaction proceeds rapidly (10 min) under microfluidic conditions, delivering a wide variety of ortho-alkylated products (27 examples, up to 97% yield, >20:1 regioselectivity, on a gram scale), including densely functionalized bioactive phenol derivatives

Catalyst-free [2 + 2] photocycloadditions between benzils and olefins under visible light

The catalyst-free [2 + 2] photocycloaddition between benzils and simple olefins is reported. The adoption of visible light proved essential for the transformation, as shorter wavelengths led to uncontrolled decomposition. When cyclic olefins were used, the reaction occurred smoothly to afford the expected oxetanes regio- and stereoselectively after 24 h of irradiation. In contrast, in the case of acyclic olefins, longer reaction times were typically required and small amounts (ca. 20%) of [4 + 2] photocycloadducts and by-products deriving from competitive hydrogen atom abstraction were observed. The selectivity of the transformation could be consistently improved by decreasing the reaction temperature, thus restoring the desired [2 + 2] reactivity. An overall mechanistic picture is also offered based on the chemical and photophysical quenching experiments and the stereochemical output is rationalized based on Griesbeck models.

Radical α-Trifluoromethoxylation of Ketones under Batch and Flow Conditions by Means of Organic Photoredox Catalysis

The first light-driven method for the α-trifluoromethoxylation of ketones is reported. Enol carbonates react with N-trifluoromethoxy-4-cyano-pyridinium, using the photoredox catalyst 4-CzIPN under 456 nm irradiation, affording the α-trifluoromethoxy ketones in ≤50% isolated yield and complete chemoselectivity. As shown by 29 examples, the reaction is general and proceeds very rapidly under batch (1 h) and flow conditions (2 min). Diverse product manipulations demonstrate the synthetic potential of the disclosed method in accessing elusive trifluoromethoxylated bioactive ingredients.

Unlocking the Synthetic Potential of Light-Excited Aryl Ketones: Applications in Direct Photochemistry and Photoredox Catalysis

In this Account, we summarize the contributions of our group to the field of photochemistry and photocatalysis. Our work deals with the development of novel synthetic methods based on the exploitation of photoexcited aryl ketones. The application of new technologies, such as microfluidic photoreactors (MFPs), has enhanced the synthetic performance and scalability of several photochemical methods, e.g., Paternò–Büchi and photoenolization/Diels–Alder processes, while opening the way to unprecedented reactivity. In addition, careful mechanistic analysis of the developed methods has been instrumental in disclosing a new family of powerful organic photocatalysts that can mediate several thermodynamically extreme photoredox processes.

1 Introduction

1.1 Shining Light on Aryl Ketones: From the Historical Background to Recent Synthetic Applications

1.2 Preliminary Mechanistic Considerations

2 Synthetic Transformations Driven by Triplet State Benzophenones

3 Synthetic Transformations Driven by Triplet State o-Alkyl-Substituted Benzophenones

4 The Evolution of Aryl-Ketone-Derived Products: Applications in Organophotoredox Catalysis

5 Conclusions and Future Directions

Photosensitized Intramolecular [2+2] Cycloaddition of 1H-Pyrrolo[2,3-b]pyridines Enabled by the Assistance of Lewis Acids

The [2+2] photocycloaddition of alkenyl-tethered 1H-pyrrolo[2,3-b]pyridine derivatives sensitized with 3',4'-dimethoxyacetophenone under irradiation by a high-pressure mercury lamp through Pyrex glass was dramatically accelerated by the addition of Lewis acids, preferably Mg(OTf)₂, to give the products stereoselectively in high yields. The reaction without a Lewis acid gave only small amounts of the [2+2] cycloaddition products. Conformational fixation of the substrates by coordination with a Lewis acid was presumed to facilitate the cycloaddition.