Fe(III)-Triggered Radical Arylation of Arene Moieties from Cyclopropanols to Construct Dibenzocyclohepta/octanones: Synthesis of N-Acetylcolchinol-O-methyl ether

Tricyclic 6-7-6 and 6-8-6 carbon ring systems are present in numerous biologically active natural molecules. However, simple and efficient synthetic approaches to these scaffolds remain challenging. Herein, we report a versatile strategy for constructing these ring systems via Fe(NO3)3-triggered radical arylation of arenes starting from cyclopropanols. This synthetic utility has been demonstrated in the synthesis of the natural product N-acetylcolchinol-O-methyl ether.

Radical N2-Retention Cyclizations of Aryl Diazoniums: Access to 7/8/9-Membered Heterocycles

We report herein a modular approach to synthesizing diverse functionalized 7/8/9-membered poly-N-containing heterocycles via oxidative radical N2-retention cyclizations of allylic aryl diazonium salts using CF3SO2Na as a CF3 radical source. A range of trifluoromethylated benzotriazepines, benzotriazocines, and benzotriazonines were obtained in moderate to good yields. This transition-metal-free protocol demonstrates atom economy, safe conditions, broad functional group tolerance, and availability of readily accessible reagents.

Synthesis of Vicinal anti-Amino Alcohols from N-tert-Butanesulfinyl Aldimines and Cyclopropanols

The stereoselective synthesis of vicinal amino alcohols derivatives from 1-substituted cyclopropanols and chiral N-tert-butanesulfinyl imines is described. Cyclopropanols are easily prepared from carboxylic esters upon reaction with ethylmagnesium bromide in the presence of titanium tetraisopropoxide and undergo carbon-carbon bond cleavage by means of diethylzinc to produce, upon base deprotonation, enolized zinc homoenolates, which react with chiral sulfinyl imines in a highly regio- and stereoselective manner.

Regiocontrol via Electronics: Insights into a Ru-Catalyzed, Cu-Mediated Site-Selective Alkylation of Isoquinolones via a C-C Bond Activation of Cyclopropanols

A site-selective C(3)/C(4)-alkylation of N-pyridylisoquinolones is achieved by employing C-C bond activation of cyclopropanols under Ru(II)-catalyzed/Cu(II)-mediated conditions. The regioisomeric ratios of the products follow directly from the electronic nature of the cyclopropanols and isoquinolones used, with electron-withdrawing groups yielding predominantly the C(3)-alkylated products, whereas the electron-donating groups primarily generate the C(4)-alkylated isomers. Density functional theory calculations and detailed mechanistic investigations suggest the simultaneous existence of the singlet and triplet pathways for the C(3)- and C(4)-product formation. Further transformations of the products evolve the utility of the methodology thereby yielding scaffolds of synthetic relevance.

Arylation of Cyclopropanol with Pyrrole: Asymmetric Synthesis of Indolizidine 167B, Indolizidine 209D, and Monomorine I

A Fe(NO₃)₃-mediated ring-opening arylation of cyclopropanol with the electron-rich pyrrole has been developed, which might proceed through oxidative radical ring opening of cyclopropanol followed by cyclization to the pyrrole motif and then aromatization. This method enables direct arylation of cyclopropanol without prefunctionalization and thus allows rapid access to a diverse array of chiral 5,6,7,8-tetrahydroindolizines from easily available chiral amino acid esters. The synthetic utility has been demonstrated by the asymmetric synthesis of alklaoids (-)-indolizidine 167B, (+)-indolizidine 209D, (+)-monomorine I, and a natural product analogue.

Copper-Promoted Aerobic Oxidative [3+2] Cycloaddition Reactions of N,N-Disubstituted Hydrazines with Alkynoates: Access to Substituted Pyrazoles

A copper-promoted aerobic oxidative [3+2] cycloaddition reaction for the synthesis of various substituted pyrazoles from N,N-disubstituted hydrazines with alkynoates in the presence of bases is developed. This work involves a direct C(sp³)-H functionalization and the formation of new C-C/C-N bonds. In this strategy, inexpensive and easily available Cu₂O serves as the promoter and air acts as the green oxidant. The reaction exhibits the advantages of high atom and step economy, high regioselectivity, and easy operation.

Formal [4+1] Annulation of Azoalkenes with CF3-Imidoyl Sulfoxonium Ylides and Dual Double Bond Isomerization Cascade: Synthesis of Trifluoromethyl-Containing Pyrazole Derivatives

A straightforward strategy for the metal-free construction of trifluoromethyl-containing pyrazole derivatives has been achieved from readily available α-halo hydrazones and CF₃-imidoyl sulfoxonium ylides. The cascade transformation proceeds through the formal [4+1] cycloaddition followed by an unexpected dual double bond isomerization. The protocol features mild conditions, easy operation, excellent substrate compatibility, and good regioselectivity. The synthetic utility is demonstrated by scale-up reaction and further elaboration of the obtained pyrazole products.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Rhodium(III)-catalyzed oxidative annulation of isoquinolones with allyl alcohols: synthesis of isoindolo[2,1-b]isoquinolin-5(7H)-ones

An efficient rhodium(III)-catalyzed direct C-H oxidative annulation of isoquinolones with allyl alcohols as C1 synthons has been successfully developed. This protocol enables the straightforward synthesis of structurally diverse isoindolo[2,1-b]isoquinolin-5(7H)-ones with high atom economy, tolerates a broad spectrum of functionalities, and is applicable to one-pot operation from readily available N-methoxybenzamides.

Visible-Light-Induced C4-Selective Functionalization of Pyridinium Salts with Cyclopropanols

The site-selective C-H functionalization of heteroarenes is of considerable importance for streamlining the rapid modification of bioactive molecules. Herein, we report a general strategy for visible-light-induced β-carbonyl alkylation at the C4 position of pyridines with high site selectivity using various cyclopropanols and N-amidopyridinium salts. In this process, hydrogen-atom transfer between the generated sulfonamidyl radicals and O-H bonds of cyclopropanols generates β-carbonyl radicals, providing efficient access to synthetically valuable β-pyridylated (aryl)ketones, aldehydes, and esters with broad functional-group tolerance. In addition, the mild method serves as an effective tool for the site-selective late-stage functionalization of complex and medicinally relevant molecules.

Iron catalyzed tandem ring opening/1,6-conjugate addition of cyclopropanols with p-quinone methides: new access to γ,γ-diaryl ketones

An iron(III) catalyzed tandem ring opening/1,6-conjugate addition of cyclopropanols to p-quinone methides leading to γ,γ-diaryl ketones has been described. This catalytic protocol provides a novel and efficient method to access γ,γ-diaryl ketone derivatives in good to excellent yields with high functional group tolerance. Importantly, γ,γ-diaryl ketone can be further functionalized to give a versatile set of useful products.

Rhodium(III)-Catalyzed Oxidative Cyclization of Oxazolines with Cyclopropanols: Synthesis of Isoindolinones

The synthesis of C3-substituted isoindolin-1-ones from oxazolines and cyclopropanols has been achieved with oxazoline as a bifunctional nucleophilic directing group. The reaction proceeds by the cleavage of three chemical bonds and allows the formation of three new chemical bonds, a C-N bond, a C-C bond, and a C-O bond, in a single step.

New insight into the electrochemical reduction of different aryldiazonium salts in aqueous solutions

Electrochemical reduction of different aryldiazonium salts in aqueous solution was studied in this work and it is shown that the aryldiazonium salts are converted to the corresponding aryl radical and aryl anion. The results of this research indicate that the reduction of aryldiazonium salts takes place in two single-electron steps. Our data show that when the substituted group on the phenyl ring is H, Cl, OH, NO₂, OCH₃ or SO₃⁻, the corresponding diazonium salt shows poor adsorption characteristics, but when the substituted group is methyl, the corresponding diazonium salt shows strong adsorption characteristics. In the latter case, the voltammogram exhibits three cathodic peaks. In addition, the effect of various substitutions on the aryldiazonium reduction was studied by Hammett's method. The data are show that with increasing electron withdrawing capacity of the substituent, the reduction of corresponding diazonium salt becomes easier.

Electrochemical reduction of different aryldiazonium salts in aqueous solution was studied. It is shown that the aryldiazonium salts are converted to the corresponding aryl radical and aryl anion.

C-H Bond Arylation of Pyrazoles at the β-Position: General Conditions and Computational Elucidation for a High Regioselectivity

Direct arylation of most five-membered ring heterocycles are generally easily accessible and strongly favored at the α-position using classical palladium-catalysis. Conversely, regioselective functionalization of such heterocycles at the concurrent β-position remains currently very challenging. Herein, we report general conditions for regioselective direct arylation at the β-position of pyrazoles, while C-H α-position is free. By using aryl bromides as the aryl source and a judicious choice of solvent, the arylation reaction of variously N-substituted pyrazoles simply proceeds via β-C-H bond functionalization. The β-regioselectivity is promoted by a ligand-free palladium catalyst and a simple base without oxidant or further additive, and tolerates a variety of substituents on the bromoarene. DFT calculations revealed that a protic solvent such as 2-ethoxyethan-1-ol significantly enhances the acidity of the proton at β-position of the pyrazoles and thus favors this direct β-C-H bond arylation. This selective pyrazoles β-C-H bond arylation was successfully applied for the straightforward building of π-extended poly(hetero)aromatic structures via further Pd-catalyzed combined α-C-H intermolecular and intramolecular C-H bond arylation in an overall highly atom-economical process.

Rhodium(III)-catalyzed oxidative alkylation of N-aryl-7-azaindoles with cyclopropanols

An efficient Rh(iii)-catalyzed C-H oxidative alkylation of N-aryl-7-azaindoles with cyclopropanols by merging tandem C-H and C-C cleavage was developed. This transformation features mild reaction conditions, high regioselectivity, and excellent functional group compatibility. The resulting β-aryl ketone derivatives can be readily transformed into 7-azaindole-containing π-extended polycyclic heteroarenes.

Nickel-Catalyzed Arylation/Alkenylation of tert-Cyclobutanols with Aryl/Alkenyl Triflates via a CC Bond Cleavage

Herein, we first present a nickel-catalyzed arylation and alkenylation of tert-cyclobutanols with aryl/alkenyl triflates via a C-C bond cleavage. An array of γ-substituted ketones was obtained in moderate-to-good yields, thus featuring earth-abundant nickel catalysis, broad substrate scope, and simple reaction conditions. Preliminary mechanistic experiments indicated that β-carbon elimination pathways might be involved in the catalytic cycle.

Direct sulfonamidation of (hetero)aromatic C-H bonds with sulfonyl azides: a novel and efficient route to N-(hetero)aryl sulfonamides

N-Aryl sulfonamides belong to a highly important class of organosulfur compounds which are found in a number of FDA-approved drugs such as dofetilide, dronedarone, ibutilide, sotalol, sulfadiazine, sulfamethizole, vemurafenib, and many more. There is therefore continuing interest in the development of novel and convenient protocols for the preparation of these pharmaceutically important compounds. Recently, direct sulfonamidation of (hetero)aromatic C–H bonds with easily available sulfonyl azides has emerged as an attractive and powerful strategy to access N-(hetero)aryl sulfonamides where non-toxic nitrogen gas forms as the sole by-product. This review highlights recent advances and developments (2012–2020) in this fast growing research area with emphasis on the mechanistic features of the reactions.

N-Aryl sulfonamides belong to a highly important class of organosulfur compounds which are found in a number of FDA-approved drugs such as dofetilide, dronedarone, ibutilide, sotalol, sulfadiazine, sulfamethizole, vemurafenib, and many more.

Copper-Catalyzed Ring-Opening Defluorinative Alkylation of Siloxydifluorocyclopropanes: Synthesis of γ-Fluoro-δ-Ketoesters and γ,δ-Diketonitriles

In view of the importance of both fluorinated synthons and homoenolate equivalents, synthetic application of difluorocyclopropanols is desired but remains challenging due to their thermodynamic instability. Herein, we use siloxydifluorocyclopropanes as difluorocyclopropanol precursors to carry out new Cu-catalyzed ring-opening defluorinative alkylation. With α-bromo carboxylic esters as coupling partners, the reaction affords γ-fluoro-δ-ketoesters via a Cu^I/Cu^II catalytic cycle. Interestingly, by the use of α-bromoamides, the ring-opening defluorinative alkylation is followed by an additional intramolecular C-N oxidative coupling to deliver a lactam intermediate, which further undergoes defluorination, hydrolysis, ring opening, and dehydration cascade via a Cu^I/Cu^II/Cu^III catalytic pathway, leading to γ,δ-diketonitriles as the final products.

General Synthesis of Tri-Carbo-Substituted N2-Aryl-1,2,3-triazoles via Cu-Catalyzed Annulation of Azirines with Aryldiazonium Salts

The general synthesis of fully substituted N²-aryl-1,2,3-triazoles is hitherto challenging compared with that of the N¹-aryl counterparts. Herein, we describe a Cu-catalyzed annulation reaction of azirines and aryldiazonium salts. This regiospecific method allows access to a broad spectrum of tri-carbo N²-aryl-1,2,3-triazoles substituted with diverse aryl and alkyl moieties. Its utility is highlighted by the synthesis of several triazole precursors applicable in drug discovery, as well as novel chiral binaphthyl ligands bearing triazole moieties.

Is Cu(iii) a necessary intermediate in Cu-mediated coupling reactions? A mechanistic point of view

The different pathways have been summarized to disclose the key intermediate in copper-mediated coupling reactions.