Cascade Approach to Highly Functionalized Biaryls by a Nucleophilic Aromatic Substitution with Arylhydroxylamines

Regioselective synthesis of 2-aminophenols from N-arylhydroxylamines

A novel strategy for the synthesis of 2-aminophenols in the absence of metals and oxidants was described. The 2-aminophenols were efficiently obtained through a cascade [3,3]-sigmatropic rearrangement and in situ hydrolysis process by using readily available N-arylhydroxylamines and the in situ-generated methyl chlorosulfonate from commercially available sulfuryl chloride and methanol under mild conditions. This method could be scaled up and has a wide substrate scope with great functional group tolerance and high regioselectivity. The 2-aminophenol products could be readily converted into structurally diverse functional molecules in good yields under various conditions.

Modular Assembly of 2-Aminoaniline Derivatives by Merging Hydroxylamine-Passerini and Hetero-Cope Rearrangement

A metal-free three-component protocol that combines a hydroxylamine-Passerini reaction and hetero-Cope rearrangement was realized, which enables the modular assembly of a wide range of structurally new and interesting 2-aminoanilines bearing an α-hydroxyamide substructure.

Synthesis of ortho-Phosphated (Hetero)Arylamines through Cascade Atherton-Todd Reaction/[3,3]-Rearrangement from Arylhydroxylamines and Dialkyl Phosphites

A practical and facile strategy for the synthesis of ortho-phosphated (hetero)arylamines from readily available arylhydroxylamines and dialkyl phosphites via cascade Atherton-Todd reaction/[3,3]-rearrangement was developed. This method is amenable to various arylhydroxylamines such as phenylhydroxylamines, naphthylhydroxylamines, and pyridylhydroxylamines, has mild reaction conditions, is oxidant-free, and has good functional-group compatibility and excellent regioselectivity.

Copper-catalyzed construction of (Z)-benzo[cd]indoles: stereoselective intramolecular trans-addition and SN-Ar reaction

Substituted benzo[cd]indoles are one of the most attractive frameworks because of their wide range of biological and optical activities. Herein, a copper-catalyzed one-step synthesis of biologically important polysubstituted benzo[cd]indoles starting from 8-alkynyl-1-naphthylamine derivatives is reported. In this protocol, many substituents tolerated the reaction conditions and produced (Z)-benzo[cd]indoles in good yields. Preliminary mechanistic studies indicated that the reaction proceeds via a stereoselective intramolecular trans-addition and S_N-Ar reaction with high selectivity and high yields. The synthesized polysubstituted (Z)-benzo[cd]indoles possess sulfonamide building blocks, which make them candidates for bioactive molecules.

Transition-Metal-Free Cascade Approach for the Synthesis of Functionalized Biaryls by SNAr of Arylhydroxylamines with Arylsulfonium Salts

We report a transition-metal-free protocol for the synthesis of functionalized biaryls through nucleophilic aromatic substitution (S_NAr) of arylhydroxylamines to arylsulfonium salts. With this protocol, structurally diverse functionalized biaryls were obtained smoothly in moderate to good yields. Merits of this transformation include mild reaction conditions, broad substrate scope, great functional group tolerance, feasibility of a one-pot procedure, and ease of handing and scale-up.

Diverse and chemoselective sigmatropic shift rearrangements of multisubstituted N,O-diarylhydroxylamines

N,O-Diarylhydroxylamines generally favor the [3,3] sigmatropic shift rearrangement. Possible N/O[1,3] sigmatropic shift rearrangements of multisubstituted N,O-diarylhydroxylamines were investigated experimentally with rationally designed substrates, which were generally in situ prepared from suitable nitroaryl halides and N-arylhydroxylamines via aromatic nucleophilic substitution. The results indicate that both N- and O-(2,4,6-trimethylphenyl)hydroxylamines still favor the [3,3] sigmatropic shift followed by tautomerization rather than N[1,3] and O[1,3] sigmatropic shifts and the rearranged products of N-(2,4,6-trimethylphenyl)hydroxylamines further undergo an intramolecular nucleophilic addition to afford dibenzo[b,d]furan-4a(9bH)-amine derivatives, while N-(4-mono- and 3,5-disubstituted phenyl)-O-(2,4,6-trinitrophenyl)hydroxylamines favorably first undergo the O[1,3] sigmatropic shift followed by tandem Smiles rearrangement and amide/ester exchange reactions, generating 2-arylaminoaryl benzoate derivatives. N-Phenyl-O-(2,4,6-trinitrophenyl)hydroxylamines undergo tandem double O[1,3] sigmatropic shift rearrangement to produce formal O[1,5] shift products. However, O-(2,6-dinitrophenyl)-N-(4-substituted phenyl)hydroxylamines undergo tandem O[1,3] and double [3,3] sigmatropic shift rearrangements to give formal 3,5-shift products. The proposed mechanism is rationalized by density functional theory (DFT) calculations. The current investigation provides not only a comprehensive understanding of the chemoselective sigmatropic shift rearrangements of N,O-diarylhydroxylamines, but also some novel synthetic strategies for dibenzo[b,d]furanamines, diarylamines, diaryl ethers, 2'-amino-[1,1'-biphenyl]-2(1H)-one, and 2'-amino-[1,1'-biaryl]-4-ol derivatives.

Regio- and stereoselective synthesis of thiazoline derivatives via the thioketene-induced ring expansion of aziridines

Metal-free thioketene-induced ring expansion of aziridines gave 4-alkylthiazolines stereospecifically from 2-alkylaziridines through an intramolecular substitution at the less substituted ring carbon and 5-arylthiazolines stereoselectively from 2-arylaziridines via tandem ring cleavage and formation through intimate ion-pair intermediates after nucleophilic addition of aziridines to thioketenes generated from 4-substituted 1,2,3-thiadiazoles in the presence of a base.

Sulfonium-aided coupling of aromatic rings via sigmatropic rearrangement

Biaryl synthesis continues to occupy a central role in chemical synthesis. From blockbuster drug molecules to organic electronics, biaryls present numerous possibilities and new applications continue to emerge. Transition-metal-catalyzed coupling reactions represent the gold standard for biaryl synthesis and the mechanistic steps, such as reductive elimination, are well established. Developing routes that exploit alternative mechanistic scenarios could give unprecedented biaryl structures and expand the portfolio of biaryl applications. We have developed metal-free C-H/C-H couplings of aryl sulfoxides with phenols to afford 2-hydroxy-2'-sulfanylbiaryls. This cascade strategy consists of an interrupted Pummerer reaction and [3,3] sigmatropic rearrangement. Our method enables the synthesis of intriguing aromatic molecules, including oligoarenes, enantioenriched dihetero[8]helicenes, and polyfluorobiaryls. From our successes in aryl sulfoxide/phenol couplings and a deeper understanding of sigmatropic rearrangements for biaryl synthesis, we have established related methods, such as aryl sulfoxide/aniline and aryl iodane/phenol couplings. Overall, our fundamental interests in underexplored reaction mechanisms have led to various methods for accessing important biaryl architectures.

A four-step cascade reaction involving O[1,3] sigmatropic shift and Smiles rearrangements as key steps

4-Nitroaryl halides and N-acyl-N-arylhydroxyamines undergo cascade aromatic nucleophilic substitution-O[1,3] sigmatropic shift-Smiles rearrangement-amide and ester exchange reaction, affording 2-((4-nitroaryl)amino)aryl carboxylates.

Expeditious and Efficient ortho-Selective Trifluoromethane-sulfonylation of Arylhydroxylamines

A metal- and oxidant-free, practical and efficient method for the synthesis of highly versatile and synthetically useful ortho-trifluoromethanesulfonylated anilines from arylhydroxylamines and trifluoromethanesulfinic chloride was developed. This rapid transformation proceeded smoothly with good yields and excellent ortho-selectivity in the absence of any metals or ligands. Mechanistically, the reaction comprised a noncanonical O-trifluoromethanesulfinylation of the arylhydroxylamine, and the subsequent [2,3]-sigmatropic rearrangement to afford ortho-trifluoromethanesulfonylated aniline derivatives. The practical application of this reaction was demonstrated by further conversion into a series of functional molecules under different reaction conditions.

Rapid Access to Fluorinated Anilides via DAST-Mediated Deoxyfluorination of Arylhydroxylamines

A new strategy for the synthesis of fluorinated anilides in the absence of metals and oxidants has been developed. This deoxyfluorination of N-arylhydroxylamines with diethylaminosulfur trifluoride (DAST) proceeded smoothly under mild conditions, and the ortho- or para-fluorinated aromatic amine products were prepared in moderate to good yields. Structurally diverse fluorinated anilides, including heterocyclic and pharmaceutically relevant molecules, can be efficiently constructed by this protocol.

Mechanistic Investigation of a Synthetic Route to Biaryls by the Sigmatropic Rearrangement of Arylsulfonium Species

A comprehensive mechanistic investigation was conducted on the coupling reaction of aryl sulfoxides with phenols by using trifluoroacetic anhydride to yield biaryls. NMR experiments revealed that our previously proposed mechanism, which consists of a cascade of an interrupted Pummerer reaction and a rate-determining [3,3] sigmatropic rearrangement, is reasonable. The electronic effects of the substrates were also evaluated to elucidate the nature of the rearrangement step. Based on experimental observations and theoretical calculations, we conclude that the rearrangement is highly asynchronous and stepwise rather than concerted when electron-rich phenols are employed for the reaction.

Synthesis of non-C2 symmetrical NOBIN-type biaryls through a cascade N-arylation and [3,3]-sigmatropic rearrangement from O-arylhydroxylamines and diaryliodonium salts

We developed herein a regioselective construction of non-C₂ symmetrical NOBIN-type biaryls through a cascade N-arylation and [3,3]-sigmatropic rearrangement from O-arylhydroxylamines and diaryliodonium salts under mild conditions. The employment of copper salt could inhibit the further O-arylation of the newly formed biaryl products, otherwise, O-arylated NOBIN-type products were furnished in moderate to good isolated yields. The products of this protocol can be further converted into highly valuable functional molecules and heterocycles.

O-Cyclopropyl hydroxylamines: gram-scale synthesis and utility as precursors for N-heterocycles

O-Cyclopropyl hydroxylamines, now accessible via a novel and scalable synthetic route, have been demonstrated to be bench-stable and practical precursors for the synthesis of N-heterocycles via a di-heteroatom [3,3]-sigmatropic rearrangement. In order to study the reactivity of these compounds in depth, a robust synthesis of both ring-substituted and ring-unsubstituted O-cyclopropyl hydroxylamines has been developed. Metal-free conditions for the facile N-arylation of these precursors were also identified. It was found that the N-arylated O-cyclopropyl hydroxamates can efficiently undergo a one-pot [3,3]-sigmatropic rearrangement/cyclization/rearomatization cascade under base-mediated conditions to furnish a structurally diverse set of substituted tetrahydroquinolines.

Copper-Catalyzed Synthesis of Axially Chiral Biaryls with Diaryliodonium Salts as Arylation Reagents

NOBIN and BINAM derivatives harboring biaryl frameworks are recognized as a class of important atropisomers with versatile applications. Here, we present an efficient synthetic route to access such compounds through copper-catalyzed domino arylation of N-arylhydroxylamines or N-arylhydrazines with diaryliodonium salts and [3,3]-sigmatropic rearrangement. This reaction features mild conditions, good substrate compatibility, and excellent efficiency. The practicality of this protocol was further extended by the synthesis of biaryl amino alcohols.

Enantioselective Synthesis of 3,3'-Disubstituted 2-Amino-2'-hydroxy-1,1'-binaphthyls by Copper-Catalyzed Aerobic Oxidative Cross-Coupling

A challenging direct asymmetric catalytic aerobic oxidative cross-coupling of 2-naphthylamine and 2-naphthol, using a novel Cu^I /SPDO system, has been successfully developed for the first time. Enantioenriched 3,3'-disubstituted NOBINs were achieved and could be readily derived to divergent chiral ligands and catalysts. This reaction features high enantioselectivities (up to 96 % ee) and good yields (up to 80 %). The DFT calculations suggest that the F-H interactions between CF₃ of L17 and H-1,8 of 2-naphthol, and the π-π stacking between the two coupling partners could play vital roles in the enantiocontrol of this cross-coupling reaction.

Metal-free synthesis of biarenes via photoextrusion in di(tri)aryl phosphates

A metal-free route for the synthesis of biarenes has been developed. The approach is based on the photoextrusion of a phosphate moiety occurring upon irradiation of biaryl- and triaryl phosphates. The reaction involves an exciplex as the intermediate and it is especially suitable for the preparation of electron-rich biarenes.

Synthesis of ortho-Phenolic Sulfilimines via an Intermolecular Sulfur Atom Transfer Cascade Reaction

To expand the toolbox for the synthesis of ortho-phenolic sulfilimines, sigmatropic rearrangements were introduced to the field of sulfilimine chemistry. Herein we report a N-H sulfenylation/[2,3]-sigmatropic rearrangement cascade reaction. This mild reaction enables commercially available thiols to serve as the sulfenylation reagent and generates water as the sole byproduct. Moreover, the reaction has a wide substrate scope and can be conducted on a gram scale with excellent reaction efficiency.

Regioselective Synthesis of Benzo-Fused Tetrahydroisoquinoline-Based Biaryls through a Tandem One-Pot Halogenation of p-Benzynes from Enediynes and Suzuki-Miyaura Coupling

A regioselective halogenation of p-benzyne derived from a nonaromatic enediyne core via Bergman cyclization and Suzuki-Miyaura coupling of the resulting haloarene in one-pot is disclosed. For the one-pot protocol to work, the reaction conditions were modified compared to an earlier reported procedure ( J. Org. Chem. 2019 , 84 , 2911 - 2921 ) by reducing the amount of lithium iodide and exclusion of pivalic acid. Under these modified conditions, the products, benzo-fused tetrahydroisoquinoline-based biaryl derivatives were obtained in overall high to excellent yields (71-90%).

Ir(iii)-Catalyzed direct C–H functionalization of N-phenylacetamide with α-diazo quinones: a novel strategy for producing 2-hydroxy-2′-amino-1,2′-biaryl scaffolds

A novel strategy is reported for the construction of 2-hydroxy-2′-amino-1,1′-biaryl scaffolds via Ir(iii)-catalyzed direct C–H bond activation under mild reaction conditions.