Aromatization as an Impetus to Harness Ketones for Metallaphotoredox-Catalyzed Benzoylation/Benzylation of (Hetero)arenes

Construction of C-S and C-Se Bonds via Photocatalytic Aromatization-Driven Deconstructive Diversification of Spiro-Dihydroquinazolinones Derived from Unstrained Ketones

A novel and robust deconstructive functionalization reaction of spiro-dihydroquinazolinones with sulfenylating reagents in the presence of base has been realized under visible light irradiation. This reaction enabled the direct ring-opening of unstrained cyclic ring systems, producing skeletally diverse functionalized quinazolinones with moderate to good yields. A range variety of sulfenylating reagents including diaryl disulfide, thiosulfonate, dithiosulfonate and 1-[(trifluoromethyl)thio]-2,5-pyrrolidinedione were compatible for this transformation. In addition, diaryl diselenide and selenosulfonate could also couple with spiro-dihydroquinazolinones to form C-Se Bonds. Mechanistic studies revealed that the reaction proceeds via a radical-radical coupling pathway.

Access to quaternary-carbon-containing β-alkyl amides via persulfate-promoted domino alkylation/smiles rearrangement of alkenes

In this study, we present an efficient approach for synthesizing all-carbon quaternary-centered β-alkyl amides. This method entails a persulfate-promoted cascade alkylative annulation/arylation of N-(arylsulfonyl)acrylamide with 4-alkyl-1,4-dihydropyridines (DHP). The reaction mechanism comprises four consecutive steps: (1) in situ generation of alkyl radical intermediates, (2) radical addition to the alkene moiety, (3) 1,4-aryl migration, and (4) finally desulfonylation.

Benzothiazolines Acting as Carbanion and Radical Transfer Reagents in Carbon-Carbon Bond Construction

Traditionally employed as hydrogenation reagents, benzothiazolines have emerged as versatile carbanion and radical transfer reagents, playing a vital role in the construction of various carbon-carbon bonds. The cutting-edge progress in photochemistry and radical chemistry have prompted the study of visible light-driven radical reactions, bringing benzothiazolines into a vibrant focus. Their chemical processes have been uncovered to encompass a variety of activation mechanisms, with five distinct modes having been identified. This work reviews the innovative applications of benzothiazolines as donors of alkyl or acyl groups, achieving hydroalkylation or hydroacylation and alkyl or acyl substitution. By examining their diverse activation mechanisms, this review highlights the potential of benzothiazolines serving as alkyl and acyl groups for further research and development. Moreover, this review will offer exemplary applications and inspiration to synthetic chemists, contributing to the ongoing evolution of benzothiazolines utility in organic synthesis.

Photocatalytic C-C bond thio(seleno)esterification of 1,2-diketone-derived pro-aromatic intermediates

We report an organophotocatalyst-enabled oxidant-free C-S/C-Se bond coupling of (un)symmetrical 1,2-diketones via pro-aromatic dihydroquinazolinones/benzothiazolines, employing readily accessible disulfides/diselenides. In this scalable and redox-neutral method, various dialkyl, di(hetero)aryl, and alkyl-aryl 1,2-diketones are expediently converted to S-aryl (S-alkyl) alkyl/(hetero)aryl thioesters and Se-alkyl aryl selenoesters with broad functional group compatibility in high efficiency.

A Radical Precursor Based on the Aromatization of p-Quinol Esters Enabled by Pyridine-Boryl Radical

A class of prearomatic carboxylic acid p-quinol ester radical precursors has been developed successfully, which could undergo homolytic cleavage of the para C-O bond of p-quinol esters via pyridine-boryl radical-induced aromatization in the presence of pyridines and diboron reagents, affording the corresponding alkyl radical via decarboxylation from the carboxyl radical in situ. In addition, the prearomatic radical precursors were further applied in radical substitution with phenylsulfonyl compounds and radical self-coulpings. This method not only provides a new approach to the generation of a radical intermediate but also expands the application of boron radicals.

Deacylative Homolysis of Ketone C(sp3)-C(sp2) Bonds: Streamlining Natural Product Transformations

The homolytic cleavage of C-C bonds adjacent to specific functional groups has lately emerged as a versatile approach for molecular diversification. Despite the ubiquity and synthetic utility of ketones, radical fragmentation of their α-C-C bonds has proven to be a formidable challenge. Here, we present a broadly applicable deacylative strategy designed to homolytically cleave aliphatic ketones of various complexities, including transformations of cycloalkanones into carboxylic acids tethered to C-centered free radicals that can be engaged in diverse radical-based processes. The method involves ketone activation through treatment with hydrogen peroxide, yielding gem-dihydroperoxides. Subsequent single-electron-transfer reduction mediated by a low-valent metal complex generates alkyl radicals that can be captured selectively with a radicophile of choice, including through catalytic cross-coupling. The logic of our deacylative functionalization is exemplified by the total synthesis of 14 natural products, one analogue, and two drugs starting from readily available natural products, showcasing its transformative power in complex settings. This approach obviates the need for complex reagents and allows the controlled conversion of ketones to reconstructed products, making the process highly applicable across a spectrum of domains.

Ketone Skeletal Modification via a Metallaphotoredox-Catalyzed Deacylation and Acylation Strategy

Herein, we describe a dual catalytic strategy that employs dihydroquinazolinones, derived from ketone analogs, as versatile intermediates for acylation via α C-C cleavage with 2-pyridyl esters, facilitating the efficient synthesis of a variety of ketones. The reaction accommodates a wide range of ketones and carboxylic acids, showing tolerance to various functional groups. The versatility of this synthetic technique is further highlighted through its application in the late-stage modification of pharmaceuticals and biologically active natural products.

Copper-Catalyzed Aromatization-Driven Ring-Opening Amination and Oxygenation of Spiro Dihydroquinazolinones

Mild and inexpensive copper-catalyzed aromatization-driven ring-opening amination and oxygenation of spiro dihydroquinazolinones are presented, respectively. These protocols provide facile and atom-economical access to the aminated and the carbonyl-containing quinazolin-4(3H)-ones in good yields with good functional group compatibility, which are difficult to obtain by conventional methods. Remarkably, a telescoped procedure involving the condensation and the ring-opening/functionalization for simple cycloalkanone was found to be accessible. Mechanistic studies suggest a radical pathway for this transformation.

Direct Excitation Strategy for Deacylative Couplings of Ketones

The homolysis of chemical bonds represents one of the most fundamental reactivities of excited molecules. Historically, it has been exploited to generate radicals under ultraviolet (UV) light irradiation. However, unlike most contemporary radical-generating mechanisms, the direct excitation to homolyze chemical bonds and produce aliphatic carbon-centered radicals under visible light remains rare, especially in metallaphotoredox cross couplings. Herein, we present our design of the dihydropyrimidoquinolinone (DHPQ) reagents derived from ketones, which can undergo formal deacylation and homolytic C-C bond cleavage to release alkyl radicals without external photocatalysts. Spectroscopic and computational analysis reveal unique optical and structural features of DHPQs, rationalizing their faster kinetics in alkyl radical generation than a structurally similar but visible-light transparent radical precursor. Such a capability allows DHPQ to facilitate a wide range of Ni-metallaphotoredox cross couplings with aryl, alkynyl and acyl halides. Other catalytic and non-catalyzed alkylative transformations of DHPQs are also feasible with various radical acceptors. We believe this work would be of broad interest, aiding the synthetic planning with simplified operation and expanding the synthetic reach of photocatalyst-free approaches in cutting-edge research.

Organo-photoredox catalyzed gem-difluoroallylation of ketone-derived dihydroquinazolinones via C(sp3)-C bond and C(sp3)-F bond cleavage

An organo-photoredox catalyzed gem-difluoroallylation of both acyclic and cyclic ketone derivatives with α-trifluoromethyl alkenes has been demonstrated, thus giving access to a diverse set of gem-difluoroalkenes in moderate to high yields. Pro-aromatic dihydroquinazolinones can be either pre-formed or in situ generated for ketone activation. This reaction is characterized by readily available starting materials, mild reaction conditions, and broad substrate scope. The feasibility of this reaction has been highlighted by the late-stage modification of several natural products and drug-like molecules as well as the in vitro antifungal activity.

Aromatization-driven deconstructive functionalization of spiro dihydroquinazolinones via dual photoredox/nickel catalysis

Aromatization-driven deconstruction and functionalization of spiro dihydroquinazolinones via dual photoredox/nickel catalysis is developed. The aromatization effect was introduced to synergistically drive unstrained cyclic C-C bond cleavage, with the aim of overcoming the ring-size limitation of nitrogen-centered radical induced deconstruction of carbocycles. Herein, we demonstrate the synergistic photoredox/nickel catalyzed deconstructive cross-coupling of spiro dihydroquinazolinones with organic halides. Remarkably, structurally diverse organic halides including aryl, alkenyl, alkynyl, and alkyl bromides were compatible for the coupling. In addition, this protocol is also characterized by its mild and redox-neutral conditions, excellent functional group compatibility, high atom economy, and easy scalability. A telescoped procedure involving condensation and ring-opening/coupling was found to be accessible. This work provides a complementary strategy to the existing radical-mediated C-C bond cleavage of unstrained carbocycles.

Construction of C-S and C-Se Bonds from Unstrained Ketone Precursors under Photoredox Catalysis

A mild photoredox catalyzed construction of sulfides, disulfides, selenides, sulfoxides and sulfones from unstrained ketone precursors is introduced. Combination of this deacylative process with SN 2 or coupling reactions provides novel and convenient modular strategies toward unsymmetrical or symmetric disulfides. Reactivity studies favor a bromine radical that initiates a HAT (Hydrogen Atom Transfer) from the aminal intermediate resulting in expulsion of a C-centered radical that is intercepted to make C-S and C-Se bonds. Gram scale reactions, broad substrate scope and tolerance towards various functional groups render this method appealing for future applications in the synthesis of organosulfur and selenium complexes.

Pro-aromatic Dihydroquinazolinones - From Multigram Synthesis to Reagents for Gram-scale Metallaphotoredox Reactions

Dihydroquinazolinone (DHQZ) has recently been harnessed as a ketone-derived pro-aromatic reagent extensively employed in (metalla)photoredox reactions as versatile group transfer agents. In this work, we outline a column chromatography-free protocol for the multigram-scale synthesis of pro-aromatic DHQZs as well as its use in a gram-scale nickel/photoredox dual-catalyzed cross-coupling in single-batch, photoflow, and simultaneous multiple smaller batches. While the single-batch approach leveraged moderate yields, a simple plug-flow photoreactor also exhibited amenable productivity (up to 45 % yield) despite the use of a heterogeneous base. Meanwhile, performing the metallaphotoredox-catalyzed reaction in multiple smaller batches in an improvised photoreactor facilitated high yields of up to 59 % and good reproducibility, implying a convenient alternative in the absence of photoflow setups.

Methyl Ketones as Alkyl Halide Surrogates: A Deacylative Halogenation Approach for Strategic Functional Group Conversions

Alkyl halides are versatile precursors to access diverse functional groups (FGs). Due to their lability, the development of surrogates for alkyl halides is strategically important for complex molecule synthesis. Given the stability and ease of derivatization inherent in common alkyl ketones, here we report a deacylative halogenation approach to convert various methyl ketones to the corresponding alkyl chlorides, bromides, and iodides. The reaction is driven by forming an aromatic byproduct, i.e., 1,2,4-triazole, in which N'-methylpicolinohydrazonamide (MPHA) is employed to form a prearomatic intermediate and halogen atom-transfer (XAT) reagents are used to quench the alkyl radical intermediate. The reaction is efficient in yielding primary and secondary alkyl halides from a wide range of methyl ketones with broad FG tolerance. It also works for complex natural-product-derived and fluoro-containing substrates. In addition, one-pot conversions of methyl ketones to various other FGs and annulations with alkenes and alkynes through deacylative halogenation are realized. Moreover, an unusual iterative homologation of alkyl iodides is also demonstrated. Finally, mechanistic studies reveal an intriguing double XAT process for the deacylative iodination reaction, which could have implications beyond this work.

Azetidines with All-Carbon Quaternary Centers: Merging Relay Catalysis with Strain Release Functionalization

Given the importance and beneficial characteristics of decorated azetidines in medicinal chemistry, efficient strategies for their synthesis are highly sought after. Herein, we report a facile synthesis of the elusive all-carbon quaternary-center-bearing azetidines. By adopting a well-orchestrated polar-radical relay strategy, ring strain release of bench-stable benzoylated 1-azabicyclo[1.1.0]butane (ABB) can be harnessed for nickel-catalyzed Suzuki Csp2-Csp3 cross-coupling with commercially available boronic acids in broad scope (>50 examples), excellent functional group tolerance, and gram-scale utility. Preliminary mechanistic studies provided insights into the underlying mechanism, wherein the ring opening of ABB with a catalytic quantity of bromide accounts for the conversion of ABB into a redox-active azetidine, which subsequently engages in the cross-coupling reaction through a radical pathway. The synergistic bromide and nickel catalysis could intriguingly be derived from a single nickel source (NiBr2). Application of the method to modify natural products, biologically relevant molecules, and pharmaceuticals has been successfully achieved as well as the synthesis of melanocortin-1 receptor (MC-1R) agonist and vesicular acetylcholine transporter (VAChT) inhibitor analogues through bioisosteric replacements of piperidine with azetidine moieties, highlighting the potential of the method in drug optimization studies. Aside from the synthesis of azetidines, we demonstrate the ancillary utility of our nickel catalytic system toward the restricted Suzuki cross-coupling of tertiary alkyl bromides with aryl boronic acids to construct all-carbon quaternary centers.

Photocatalytic Hydro Tri/Difluoromethylation of Alkenes with Bench Stable Tri/Difluoromethylating Reagents

Herein, we demonstrate the synthesis and characterization of bench stable tri/difluoromethylating reagents and their potential applications in redox neutral hydro tri/difluoromethylation of alkenes enabled by visible light. The new tri/difluoromethylating reagents are obtained on a gram-scale through simply cyclocondensation of commercially available anthranilamide with phenyltrifluoro or difluoromethyl ketone. Preliminary mechanistic studies indicated that a canonical photoredox catalytic cycle is being operative. DFT studies support this and further reveal that deprotonation occurs before radical cleavage. DFT studies also show that the better yield with HCF2 reagent is attributed to the favorable expulsion of the corresponding radical moiety.

Ruthenium-Catalyzed Activation of Nonpolar C-C Bonds via π-Coordination-Enabled Aromatization

Activation of C-C bonds allows editing of molecular skeletons, but methods for selective activation of nonpolar C-C bonds in the absence of a chelation effect or a driving force derived from opening of a strained ring are scarce. Herein, we report a method for ruthenium-catalyzed activation of nonpolar C-C bonds of pro-aromatic compounds by means of π-coordination-enabled aromatization. This method was effective for cleavage of C-C(alkyl) and C-C(aryl) bonds and for ring-opening of spirocyclic compounds, providing an array of benzene-ring-containing products. The isolation of a methyl ruthenium complex intermediate supports a mechanism involving ruthenium-mediated C-C bond cleavage.

Photocatalytic Alkylation/Arylative Cyclization of N-Acrylamides of N-Heteroarenes and Arylamines with Dihydroquinazolinones from Unactivated Ketones

We describe a visible-light photoredox-catalyzed alkylation/arylative cyclization of N-acrylamides─from 2-arylindoles, 2-arylbenzimidazoles, or N-substituted anilines─with ketone-derived dihydroquinazolinones, accessing indolo- and benzimidazolo[2,1-a]isoquinolines or 2-oxindoles. The consecutive incorporation of alkyl- and aryl-carbogenic motifs across a C=C bond via formal cleavage of ketone α-C-C and arene C-H bonds leads to the formation of five- and six-membered rings, with an all-carbon quaternary stereocenter. This dicarbofunctionalization elaborates aromatization-driven radical C-C functionalization of unactivated aliphatic ketones to construct diverse cyclic structures with functionality tolerance.

2-Silylated Dihydroquinazolinone as a Photocatalytic Energy Transfer Enabled Radical Hydrosilylation Reagent

The hydrosilylation of alkenes is one of the most important methods for the synthesis of organosilicon compounds. In addition to the platinum-catalyzed hydrosilylation, silyl radical addition reactions are notable as economic reactions. An efficient and widely applicable silyl radical addition reaction was developed by using 2-silylated dihydroquinazolinone derivatives under photocatalytic conditions. Electron-deficient alkenes and styrene derivatives underwent hydrosilylation to give addition products in good to high yields. Mechanistic studies indicated that the photocatalyst functioned not as a photoredox catalyst but as an energy transfer catalyst. DFT calculations clarified that the triplet excited state of 2-silylated dihydroquinazolinone derivatives released a silyl radical through the homolytic cleavage of a carbon-silicon bond, and this was followed by the hydrogen atom transfer pathway, not the redox pathway.

An Oxidant-Free and Mild Strategy for Quinazolin-4(3H)-One Synthesis via CuAAC/Ring Cleavage Reaction

An oxidant-free and highly efficient synthesis of phenolic quinazolin-4(3H)-ones was achieved by simply stirring a mixture of 2-aminobenzamides, sulfonyl azides, and terminal alkynes. The intermediate N-sulfonylketenimine underwent two nucleophilic additions and the sulfonyl group eliminated through the power of aromatization. The natural product 2-(4-hydroxybenzyl)quinazolin-4(3H)-one can be synthesized on a large scale under mild conditions with this method.

Visible-Light-Photocatalyzed Dicarbofunctionalization of Conjugated Alkenes with Ketone-Based Dihydroquinazolinones

A visible-light-photocatalyzed 1,2-arylalkylation of N-(arylsulfonyl)acrylamides with ketone-based dihydroquinazolinones is described. The formal C-C bond cleavage of aliphatic ketones is unified with tandem radical alkylation/1,4-aryl migration/desulfonylation to forge two different types of vicinal C-C bonds and construct an all-carbon quaternary α-stereocenter, thus enhancing the carbogenic complexity and tolerating diverse functionalities. Additional to telescopic synthesis and product diversification, this method features a radical dicarbofunctionalization of conjugated N-(arylsulfonyl)acrylamides with a nucleophilic alkyl radical precursor (dihydroquinazolinone) utilizing oxygen as a green oxidant at ambient temperature.

Copper-Catalyzed C(sp3 )-Amination of Ketone-Derived Dihydroquinazolinones by Aromatization-Driven C-C Bond Scission

Herein, we describe the development of a copper-catalyzed C(sp³ )-amination of proaromatic dihydroquinazolinones derived from ketones. The reaction is enabled by the intermediacy of open-shell species arising from homolytic C-C bond-cleavage driven by aromatization. The protocol is characterized by its operational simplicity and generality, including chemical diversification of advanced intermediates.

Trifluoromethylation of Carbonyl and Unactivated Olefin Derivatives by C(sp3 )-C Bond Cleavage

Herein, we report a Cu-mediated trifluoromethylation of carbonyl-type compounds and unactivated olefins enabled by visible-light irradiation via σ C(sp³ )-C bond-functionalization. The reaction is distinguished by its modularity, mild conditions and wide scope-even in the context of late-stage functionalization-thus offering a complementary approach en route to valuable C(sp³ )-CF₃ architectures from easily accessible precursors.

Dihydroquinazolinones as adaptative C(sp3) handles in arylations and alkylations via dual catalytic C-C bond-functionalization

C–C bond forming cross-couplings are convenient technologies for the construction of functional molecules. Consequently, there is continual interest in approaches that can render traditionally inert functionality as cross-coupling partners, included in this are ketones which are widely-available commodity chemicals and easy to install synthetic handles. Herein, we describe a dual catalytic strategy that utilizes dihydroquinazolinones derived from ketone congeners as adaptative one-electron handles for forging C(sp³) architectures via α C–C cleavage with aryl and alkyl bromides. Our approach is achieved by combining the flexibility and modularity of nickel catalysis with the propensity of photoredox events for generating open-shell reaction intermediates. This method is distinguished by its wide scope and broad application profile––including chemical diversification of advanced intermediates––, providing a catalytic technique complementary to existing C(sp³) cross-coupling reactions that operates within the C–C bond-functionalization arena.

Although derived from feedstock chemicals and therefore in principle abundant, ketones are not widely used as cross-coupling partners in organic synthesis. Herein, the authors use ketone derivatives as one-electron handles for forging C(sp3) architectures via dual photo- and nickel catalysis.