A Retrosynthetic Approach for Photocatalysis

Photoredox Catalysis by 21-Thiaporphyrins: A Green and Efficient Approach for C-N Borylation and C-H Arylation

Photoredox catalysis provides a green and sustainable alternative for C-H activation of organic molecules that eludes harsh conditions and use of transition metals. The photocatalytic C-N borylation and C-H arylation mostly depend on the ruthenium and iridium complexes or eosin Y and the use of porphyrin catalysts is still in infancy. A series of novel 21-thiaporphyrins (A2B2 and A3B type) were synthesized having carbazole/phenothiazine moieties at their meso-positions and screened as catalysts for C-N borylation and C-H arylation. This paper demonstrates the 21-thiaporphyrin catalyzed C-N borylation and het-arylation of anilines under visible light. The method utilizes only 0.1 mol % of 21-thiaporphyrin catalyst under blue light for the direct C-N borylation and het-arylation reactions. A variety of substituted anilines were used as source for expensive and unstable aryl diazonium salts in the reactions. The heterobiaryls and aryl boronic esters were obtained in decent yields (up to 88 %). Versatility of the 21-thiaporphyrin catalyst was tested by thiolation and selenylation of anilines under similar conditions. Mechanistic insight was obtained from DFT studies, suggesting that 21-thiaporphyrin undergo an oxidative quenching pathway. The photoredox process catalyzed by 21-thiaporphyrins offers a mild, efficient and metal-free alternative for the formation of C-C, C-S, and C-Se bonds in aryl compounds; it can also be extended to borylation reaction.

Visible Light Photoredox-Catalyzed Decarboxylative Alkylation of 3-Aryl-Oxetanes and Azetidines via Benzylic Tertiary Radicals and Implications of Benzylic Radical Stability

Four-membered heterocycles offer exciting potential as small polar motifs in medicinal chemistry but require further methods for incorporation. Photoredox catalysis is a powerful method for the mild generation of alkyl radicals for C-C bond formation. The effect of ring strain on radical reactivity is not well understood, with no studies that address this question systematically. Examples of reactions that involve benzylic radicals are rare, and their reactivity is challenging to harness. This work develops a radical functionalization of benzylic oxetanes and azetidines using visible light photoredox catalysis to prepare 3-aryl-3-alkyl substituted derivatives and assesses the influence of ring strain and heterosubstitution on the reactivity of small-ring radicals. 3-Aryl-3-carboxylic acid oxetanes and azetidines are suitable precursors to tertiary benzylic oxetane/azetidine radicals which undergo conjugate addition into activated alkenes. We compare the reactivity of oxetane radicals to other benzylic systems. Computational studies indicate that Giese additions of unstrained benzylic radicals into acrylates are reversible and result in low yields and radical dimerization. Benzylic radicals as part of a strained ring, however, are less stable and more π-delocalized, decreasing dimer and increasing Giese product formation. Oxetanes show high product yields due to ring strain and Bent's rule rendering the Giese addition irreversible.

Visible-Light-Promoted Dual Photoredox/Nickel-Catalyzed Chemoselective Reduction of Secondary and Tertiary Amides with Hydrosilanes in the Presence of an Ester

We report a one-step procedure to selectively reduce secondary and tert-amides to their corresponding amine derivatives in the presence of an ester. This was achieved via the synergistic combination of a photoredox, a nickel catalytic system, and phenyl silane as a reductant in the presence of blue light-emitting diode light (455 nm) at room temperature. Further, this mild light-promoted dual metallaphotoredox catalytic system was also successful in selectively reducing a lactam to the cyclic amines, without affecting the ester moiety present in the molecules.

Organic photoredox catalyzed dealkylation/acylation of tertiary amines to access amides

A mild metal-free C-N bond activation strategy for the direct conversion of inert tertiary amines with acyl chlorides into tertiary amides via organic photoredox catalysis is presented. In this protocol, a novel organic photocatalyst (Cz-NI-Ph) that showed excellent catalytic performance during C-N bond cleavage is developed. Moreover, this reaction features green and mild conditions, broad substrate scope, and readily available raw materials.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Strategies toward the Difunctionalizations of Enamide Derivatives for Synthesizing α,β-Substituted Amines

Enamide and enecarbamate derivatives containing a nucleophilic center at the β-position from their nitrogen atom as well as a latent electrophilic site at their α-position are interesting motifs in organic chemistry. This dual reactivity─analogous that of the enamines─enables difunctionalization and increased structural complexity. Furthermore, an electron-withdrawing group on nitrogen drastically increases their stability. In that respect, enamides and enecarbamates are excellent partners for multicomponent transformations, and our research primarily focuses on these compounds in particular.Difunctionalization generally occurs through the nucleophilic addition of the enecarbamate on an electrophile to form iminium, which can subsequently react with a nucleophilic species. Although potent, such an approach is highly challenging due to the low stability of the intermediate iminium, leading to undesired hydrolysis or oligomerization. Epimerization, competitivity, and compatibility issues between the reaction partners are additional hindrances to developing these methodologies. To overcome these limitations, we described many complementary strategies.To control the enantioselectivity of these transformations, chiral phosphoric acids were found to be particularly well-suited to activate multiple reactants due to the formation of a hydrogen bonds network, allowing for an organized transition state in a chiral pocket. Interestingly, when deprotonated as phosphates, they can also play the role of ligands for Lewis acidic metals.To avoid iminium oligomerization, we successfully used stabilized α-arylated enamides. However, this approach was restricted to a simple nucleophilic addition at the β-position. To achieve the difunctionalizations of α-unsubstituted derivatives, we explored reversibly linked nucleophile and electrophile to address their compatibility problem. Alternatively, we devised a sequential methodology for resolving the stability issue of the N-acyl iminium based on its intermediate trapping using a temporary nucleophile (alcohol or thiol). Interestingly, the trapping agent could further be displaced by the desired final α-substituent under Lewis acidic or photocatalytic activation. This led us to design new chiral and bifunctional phosphoric acid catalysts bearing chromophores to merge asymmetric organocatalysis and photochemistry.These photocatalysis studies incited us to focus on radical processes to manage original functionalizations that would not be feasible otherwise. β-Alkylation and β-trifluoromethylation of enecarbamates via visible-light-promoted atom transfer radical additions were successfully performed. As β-allylations remained unattainable with the precedent methods, we eventually turned our attention to cerium(IV)-mediated oxidative single electron transfers. It allowed for singly occupied molecular orbital activation of these substrates to elicit their umpolung reactivity.Thus, the functionalization of enecarbamate derivatives appears as a valid synthetic strategy for obtaining important building blocks for agrochemical, pharmaceutical, and cosmetic industries, including diamines, haloamines, aminotryptamines, and less accessible trifluoromethylated or allylic compounds.

Ion-Pairing Catalysis in Stereoselective, Light-Induced Transformations

With the rapid development of photoredox catalysis, numerous concepts for asymmetric induction were successfully and broadly adapted from polar two-electron transformations to radical chemistry. While this applies to organocatalysis or transition metal chemistry, asymmetric ion-pairing catalysis remains a niche application within light-driven reactions today. This perspective gives an overview of recent examples, strategies, and their application in stereoselective transformations at the interface of ion-pairing and photo(redox) catalysis.

Tuning Deazaflavins Towards Highly Potent Reducing Photocatalysts Guided by Mechanistic Understanding – Enhancement of the Key Step by the Internal Heavy Atom Effect

Deazaflavins are well suited for reductive chemistry acting via a consecutive photo‐induced electron transfer, in which their triplet state and semiquinone – the latter is formed from the former after electron transfer from a sacrificial electron donor – are key intermediates. Guided by mechanistic investigations aiming to increase intersystem crossing by the internal heavy atom effect and optimising the concentration conditions to avoid unproductive excited singlet reactions, we synthesised 5‐aryldeazaflavins with Br or Cl substituents on different structural positions via a three‐component reaction. Bromination of the deazaisoalloxazine core leads to almost 100 % triplet yield but causes photo‐instability and enhances unproductive side reactions. Bromine on the 5‐phenyl group in ortho position does not affect the photostability, increases the triplet yield, and allows its efficient usage in the photocatalytic dehalogenation of bromo‐ and chloroarenes with electron‐donating methoxy and alkyl groups even under aerobic conditions. Reductive powers comparable to lithium are achieved.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

A nickel/organoboron catalyzed metallaphotoredox platform for C(sp2)–P and C(sp2)–S bond construction

A boron-based organic photocatalyst has been applied in metallaphotoredox catalyzed C–P and C–S bond construction reactions.

Amide Bond Formation via Aerobic Photooxidative Coupling of Aldehydes with Amines Catalyzed by a Riboflavin Derivative

We report an effective, operationally simple, and environmentally friendly system for the synthesis of tertiary amides by the oxidative coupling of aromatic or aliphatic aldehydes with amines mediated by riboflavin tetraacetate (RFTA), an inexpensive organic photocatalyst, and visible light using oxygen as the sole oxidant. The method is based on the oxidative power of an excited flavin catalyst and the relatively low oxidation potential of the hemiaminal formed by amine to aldehyde addition.

Strategic Use of Visible-Light Photoredox Catalysis in Natural Product Synthesis

Recent progress in the development of photocatalytic reactions promoted by visible light is leading to a renaissance in the use of photochemistry in the construction of structurally elaborate organic molecules. Because of the rich functionality found in natural products, studies in natural product total synthesis provide useful insights into functional group compatibility of these new photocatalytic methods as well as their impact on synthetic strategy. In this review, we examine total syntheses published through the end of 2020 that employ a visible-light photoredox catalytic step. To assist someone interested in employing the photocatalytic steps discussed, the review is organized largely by the nature of the bond formed in the photocatalytic step.

Scalability of photochemical reactions in continuous flow mode

Continuous flow photochemistry as a field has witnessed an increasing popularity over the last decade in both academia and industry. Key drivers for this development are safety, practicality as well as the ability to rapidly access complex chemical structures. Continuous flow reactors, whether home-built or from commercial suppliers, additionally allow for creating valuable target compounds in a reproducible and automatable manner. Recent years have furthermore seen the advent of new energy efficient LED lamps that in combination with innovative reactor designs provide a powerful means to increasing both the practicality and productivity of modern photochemical flow reactors. In this review article we wish to highlight key achievements pertaining to the scalability of such continuous photochemical processes.

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The Future of Retrosynthesis and Synthetic Planning: Algorithmic, Humanistic or the Interplay?

The practice of deploying and teaching retrosynthesis is on the cusp of considerable change, which in turn forces practitioners and educators to contemplate whether this impending change will advance or erode the efficiency and elegance of organic synthesis in the future. A short treatise is presented herein that covers the concept of retrosynthesis, along with exemplified methods and theories, and an attempt to comprehend the impact of artificial intelligence in an era when freely and commercially available retrosynthetic and forward synthesis planning programs are increasingly prevalent. Will the computer ever compete with human retrosynthetic design and the art of organic synthesis?

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

Ni-Catalyzed Reductive and Merged Photocatalytic Cross-Coupling Reactions toward sp3/sp2-Functionalized Isoquinolones: Creating Diversity at C-6 and C-7 to Address Bioactive Analogues

Naturally occurring isoquinolones have gained considerable attention over the years for their bioactive properties. While the late-stage introduction of various functionalities at certain positions, namely, C-3, C-4, and C-8, has been widely documented, the straightforward introduction of challenging sp3 carbon-linked acyclic aminoalkyl or aza- and oxacyclic appendages at C-6 and C-7 remains largely underexplored. Interest in 6-substituted azacyclic analogues has recently garnered attention in connection with derivatives exhibiting anticancer activity. Reported here is the first application of the versatile and recently emerging field of Ni-catalyzed reductive cross-coupling reactions to the synthesis of 6- and 7- hetero(cyclo)alkyl-substituted isoquinolones. In a second and complementary approach, a new set of C-6- and C-7-substituted positional isomers of hetero(cyclo)alkyl appendages were obtained from the merging of photocatalytic and Ni-catalyzed coupling reactions. In both cases, 6- and 7-bromo isoquinolones served as dual-purpose reacting partners with readily available tosylates and carboxylic acids, respectively.

Dual aminoquinolate diarylboron and nickel catalysed metallaphotoredox platform for carbon-oxygen bond construction

Herein, aminoquinolate diarylboron complexes are utilized as photocatalysts in dual Ni/photoredox catalyzed carbon-oxygen construction reactions. Via this unified metallaphotoredox platform, diverse (hetero)aryl halides can be conveniently coupled with acids, alcohols and water. This method features operational simplicity, broad substrate scope and good compatibility with functional groups.