Formal Giese addition of C(sp3)-H nucleophiles enabled by visible light mediated Ni catalysis of triplet enone diradicals

Hantzsch Ester as Efficient and Economical NAD(P)H Mimic for In Vitro Bioredox Reactions

Biocatalysis has emerged as a valuable and reliable tool for industrial and academic societies, particularly in fields related to bioredox reactions. The cost of cofactors, especially those needed to be replenished at stoichiometric amounts or more, is the chief economic concern for bioredox reactions. In this study, a readily accessible, inexpensive, and bench-stable Hantzsch ester is verified as the viable and efficient NAD(P)H mimic by four enzymatic redox transformations, including two non-heme diiron N-oxygenases and two flavin-dependent reductases. This finding provides the potential to significantly reduce the costs of NAD(P)H-relying bioredox reactions.

2-Nitro-cyclopropyl-1-carbonyl Compounds from Unsaturated Carbonyl Compounds and Nitromethane via Enolonium Species

2-Nitrocyclopropanes bearing ketones, amides, esters, and carboxylic acids in the 1 position may be accessed as single diastereoisomers in one operation from the corresponding unsaturated carbonyl compounds. The source of the nitro-methylene component is nitromethane. The reaction proceeds at room temperature under mild conditions. The products may be converted into, e.g., cyclopropyl-amino acids in a single step. Both nitrocyclopropanes and amino-cyclopropanes are unique moieties found in biologically active compounds and natural products.

Photoredox-Catalyzed Giese Reactions: Decarboxylative Additions to Cyclic Vinylogous Amides and Esters

An effective strategy has been developed for the photoredox-catalyzed decarboxylative addition of cyclic amino acids to both vinylogous amides and esters leading to uniquely substituted heterocycles. The additions take place exclusively trans to the substituent present on the dihydropyridone ring affording stereochemical control about the new carbon-carbon bond. These reactions are operationally simplistic and afford the desired products in good to excellent isolated yields.

Visible-light mediated cross-coupling of aryl halides with sodium sulfinates via carbonyl-photoredox/nickel dual catalysis

Photoinduced nickel-catalyzed cross-coupling of arylsulfinates (ArSO2−) with (hetero)aryl halides (Ar’-X) via visible light photoexcitation of 2-chloro-thioxanthen-9-one (Cl-TXO) has been achieved in moderate to excellent yields. This photocoupling exhibited a broad...

Electrophilic Thiocyanato Reagent Assisted Oxa-Michael/Thiocyanation of α,β-Unsaturated Ketones

A route for thiocyanation-functionalization of the electron-deficient C═C double bond was developed. Regioselective thiocyanation-etherification of α,β-unsaturated ketones was achieved. The desired products were obtained in moderate to high yields under mild conditions. It was suggested that the nucleophile was activated by the electrophilic thiocyanato reagent, and difunctionalization was achieved through a 1,4-addition/thiocyanation pathway.

Visible-Light Mediated Metal-Free 6π-Photocyclization of N-Acrylamides: Thioxanthone Triplet Energy Transfer Enables the Synthesis of 3,4-Dihydroquinolin-2-ones

An efficient thioxanthone-catalyzed triplet energy transfer process for the synthesis of 3,4-dihydroquinolin-2-ones via a 6π-photocyclization is reported. Featuring a rare example of a metal-free formal C(sp²)-H/C(sp³)-H arylation mediated by visible-light, this work hopes to inspire further interest in these small molecules as sustainable alternatives to existing transition-metal photocatalysts in related processes.

Nickel-Catalyzed Etherification of Phenols and Aryl Halides through Visible-Light-Induced Energy Transfer

Notwithstanding some progress in nickel-catalyzed etherification of alkanols and arylhalides, the ability of such a Ni-catalyzed transformation employing phenols to diaryl ethers is unsuccessful due to phenolates with much lower reduction potentials, which suppress the oxidation of nickel(II) intermediates into requisite Ni(III) species. We herein report visible-light-initiated, nickel-catalyzed O-arylation of phenols with arylhalides using t-BuNH(i-Pr) as the base and thioxanthen-9-one as the photosensitizer under visible light. This photocoupling exhibits a broad substrate scope.

Chromium carbides and cyclopropenylidenes

Carbon tetrabromide can be reduced with CrBr2 in THF to form a dinuclear carbido complex, [CrBr2(thf)2)][CrBr2(thf)3](μ-C), along with formation of [CrBr3(thf)3]. An X-ray diffraction (XRD) study of the pyridine adduct displayed a dinuclear structure bridged by a carbido ligand between 5- and 6-coordinate chromium centers. The carbido complex reacted with two equivalents of aldehydes to form α,β-unsaturated ketones. Treatment of the carbido complex with alkenes resulted in a formal double-cyclopropanation of alkenes by the carbido moiety to afford spiropentanes. Isotope labeling studies using a 13C-enriched carbido complex, [CrBr2(thf)2)][CrBr2(thf)3](μ-13C), identified that the quaternary carbon in the spiropentane framework was delivered by carbide transfer from the carbido complex. Terminal and internal alkynes also reacted with the carbido complex to form cyclopropenylidene complexes. A solid-state structure of the diethylcyclopropenylidene complex, prepared from 3-hexyne, showed a mononuclear cyclopropenylidene chromium(iii) structure.

Photoredox-Catalyzed Dehydrogenative Csp3-Csp2 Cross-Coupling of Alkylarenes to Aldehydes in Flow

Executing photoredox reactions in flow offers solutions to frequently encountered issues regarding reproducibility, reaction time, and scale-up. Here, we report the transfer of a photoredox-catalyzed benzylic coupling of alkylarenes to aldehydes to a flow chemistry setting leading to improvements in terms of higher concentration, shorter residence times, better yields, ease of catalyst preparation, and enhanced substrate scope. Its applicability has been demonstrated by a multi-gram-scale reaction using high-power light-emitting diodes (LEDs), late-stage functionalization of selected active pharmaceutical ingredients (APIs), and also a photocatalyst recycling method.

Single Cu(I)-Photosensitizer Enabling Combination of Energy-Transfer and Photoredox Catalysis for the Synthesis of Benzo[b]fluorenols from 1,6-Enynes

An efficient, mild, and atom-economical synthesis of benzo[b]fluorenols from 1,6-enynes has been developed under photocatalytic conditions. A single P/N heteroleptic Cu(I)-photosensitizer might exhibit both energy-transfer and photoredox catalytic activities in the formation of benzo[b]fluorenols.

Organophotocatalytic dearomatization of indoles, pyrroles and benzo(thio)furans via a Giese-type transformation

Accessing fascinating organic and biological significant indolines via dearomatization of indoles represents one of the most efficient approaches. However, it has been difficult for the dearomatization of the electron deficient indoles. Here we report the studies leading to developing a photoredox mediated Giese-type transformation strategy for the dearomatization of the indoles. The reaction has been implemented for chemoselectively breaking indolyl C=C bonds embedded in the aromatic system. The synthetic power of this strategy has been demonstrated by using structurally diverse indoles bearing common electron-withdrawing groups including (thio)ester, amide, ketone, nitrile and even aromatics at either C₂ or C₃ positions and ubiquitous carboxylic acids as radical coupling partner with high trans-stereoselectivity (>20:1 dr). This manifold can also be applied to other aromatic heterocycles including pyrroles, benzofurans and benzothiophenes. Furthermore, enantioselective dearomatization of indoles has been achieved by a chiral camphorsultam auxiliary with high diastereoselectivity.

Carbon dioxide based methodologies for the synthesis of fine chemicals

Rapid environmental changes triggered by the increase in the concentration of heat-absorbing gases such as CO2 in the atmosphere have become a major cause of concern. One of the ways to counter this growing threat will be to efficiently convert atmospheric CO2 into value-added products via the development of efficient transition-metal-catalyzed processes. Conversion of CO2 into bulk products such as CH3OH and methane as well as its incorporation into commercial polyurethane synthesis has been achieved and reviewed extensively. However, the efficient transformation of CO2 into fine chemicals and value-added chemicals has many fold advantages. Recent years have seen a rapid rise in the number of metal-mediated protocols to achieve this goal of converting CO2 into fine chemicals. These are essential developments given the requirement of several commodities and fine chemicals in various industrial processes and the utilization of atmospheric CO2 will help provide a sustainable solution to the current environmental problems. Accordingly, we present here a comprehensive compilation of catalytic processes, involving CO2 as the C1 source for reacting with substrates such as alkanes, alkenes, alkynes, amines, acid chlorides, alcohols, allyl boronates, alkenyl triflates, and many others to provide easy access to a wide variety of useful molecules. Such a technology would certainly prove to be beneficial in solving the problems associated with the environmental accumulation of CO2.

Visible-light-promoted thiocyanation of sp2 C–H bonds over heterogeneous graphitic carbon nitrides

Mesoporous graphitic carbon nitride (mpg-C₃N₄) is developed as a practical heterogeneous photocatalyst for C–S bond formation.

Recent advances in photoredox and nickel dual-catalyzed cascade reactions: pushing the boundaries of complexity

Cascade reactions that produce multiple chemical bonds in one synthetic operation are important in the efficient construction of complex molecules. In addition, photoredox and nickel dual catalysis opens a new and powerful avenue for transition-metal-catalyzed cross-coupling reactions. By combining these two concepts, photoredox and nickel dual-catalyzed cascade reactions have been recently established, and they provide an efficient and mild method for accessing a series of valuable organic compounds.

Dehydrogenative Coupling of Benzylic and Aldehydic C-H Bonds

A photoinduced dehydrogenative coupling reaction between benzylic and aldehydic C-H bonds is reported. When a solution of an alkylbenzene and an aldehyde in ethyl acetate is irradiated with visible light in the presence of iridium and nickel catalysts, a coupled α-aryl ketone is formed with evolution of dihydrogen. An analogous C-C bond forming reaction occurs between a C-H bond next to the nitrogen of an N-methylamide and an aldehydic C-H bond to produce an α-amino ketone. These reactions provide a straightforward pathway from readily available materials leading to valued structural motifs of pharmacological relevance.

A metal-free visible-light-promoted phosphorylation/cyclization reaction in water towards 3-phosphorylated benzothiophenes

A metal-free visible-light-induced phosphorylation/cyclization reaction was developed in water at room temperature for the synthesis of 3-phosphorylated benzothiophenes.

Dual Benzophenone/Copper-Photocatalyzed Giese-Type Alkylation of C(sp3 )-H Bonds

Photocatalyzed Giese-type alkylations of C(sp³ )-H bonds are very attractive reactions in the context of atom-economy in C-C bond formation. The main limitation of such reactions is that when using highly polymerizable olefin acceptors, such as unsubstituted acrylates, acrylonitrile, or methyl vinyl ketone, radical polymerization often becomes the dominant or exclusive reaction pathway. Herein, we report that the polymerization of such olefins is strongly limited or suppressed when combining the photocatalytic activity of benzophenone (BP) with a catalytic amount of Cu(OAc)₂ . Under mild and operationally simple conditions, the Giese adducts resulting from the C(sp³ )-H functionalization of amines, alcohols, ethers, and cycloalkanes could be synthesized. Preliminary mechanistic studies have revealed that the reaction does not proceed through a radical chain, but through a dual BP/Cu photocatalytic process, in which both Cu^II and low-valent Cu^I/0 species, generated in situ by reduction by the BP ketyl radical, may react with α-keto or α-cyano intermediate radicals, thus preventing polymerization.

Mn-Enabled Radical-Based Alkyl-Alkyl Cross-Coupling Reaction from 4-Alkyl-1,4-dihydropyridines

Highly efficient alkylation of β-chloro ketones and their derivatives was achieved by means of domino dehydrochlorination/Mn-enabled radical-based alkyl-alkyl cross-coupling reaction. In situ-generated α,β-unsaturated ketones and their analogues were identified as the reaction intermediates. Known bioactive compounds, such as melperone and azaperone, could be easily prepared from β-chloropropiophenone in two steps.

Benzylarylation of N-Allyl Anilines: Synthesis of Benzylated Indolines

An unprecedented benzylic C-H functionalization of methyl arenes across unactivated alkenes is presented. In the presence of MnCl₂·4H₂O and di- tert-butyl peroxide, N-allyl anilines underwent benzylation/cyclization cascade to give benzylated indolines, which are a previously unmet synthetic goal. This protocol features simple operation, broad substrate scope, and great exo selectivity.

Hydrogen Atom Transfer Reactions via Photoredox Catalyzed Chlorine Atom Generation

The selective functionalization of chemically inert C-H bonds remains to be fully realized in achieving organic transformations that are redox-neutral, waste-limiting, and atom-economical. The catalytic generation of chlorine atoms from chloride ions is one of the most challenging redox processes, where the requirement of harsh and oxidizing reaction conditions renders it seldom utilized in synthetic applications. We report the mild, controlled, and catalytic generation of chlorine atoms as a new opportunity for access to a wide variety of hydrogen atom transfer (HAT) reactions owing to the high stability of HCl. The discovery of the photoredox mediated generation of chlorine atoms with Ir-based polypyridyl complex, [Ir(dF(CF₃ )ppy)₂ (dtbbpy)]Cl, under blue LED irradiation is reported.