On the Origin of Rh-Catalyzed Selective Ring-Opening Amidation of Substituted Cyclopropanols to Access β2-Amino Ketones

Copper-catalyzed synthesis of primary amides through reductive N-O cleavage of dioxazolones

A new method for the synthesis of primary amides is developed, in which dioxazolones are treated with a copper catalyst under mild reaction conditions. A broad scope of dioxazolones is exhibited as well as dioxazolones containing biologically active structural motifs. These robust and mild reaction conditions allow the transformation of dioxazolones to primary amides, in which sensitive functional groups such as hydroxyl, aldehyde, trialkylsilyl, and unsaturated carbon units are tolerated with excellent chemoselectivity.

Three-Component Synthesis of Multiple Functionalized Allenes via Copper/Photoredox Dual Catalyzed 1,4-Alkylcyanation of 1,3-Enynes

Efficient access to multiple functionalized allenes via a three component 1,4-alkylcyanation of enynes with cyclic alcohol derivatives in the presence of trimethylsilyl cyanide (TMSCN) under copper/photoredox dual catalysis has been developed. Both easily transformable aldehyde and cyano groups were introduced to tetra-substituted allenes through light-induced C-C bond cleavage of cyclic butanol and pentanol derivatives. The reactions proceeded smoothly under mild conditions with broad functional groups tolerance.

Et2 Zn-Mediated Gem-Dicarboxylation of Cyclopropanols with CO2

An unprecedented Et2 Zn-mediated gem-dicarboxylation of C─C/C─H single bond of cyclopropanols with CO2 is disclosed, which provides a straightforward and efficient methodology for the synthesis of a variety of structurally diverse and useful malonic acids in moderate to excellent yields. The protocol features mild reaction conditions, excellent functional group compatibility, broad substrate scope, and facile derivatization of the products. DFT calculations confirm that the transition-metal-free transformation proceeds through a novel ring-opening/α-functionalization/ring-closing/ring-opening/β-functionalization (ROFCOF) process, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) plays dual important roles in the transformation.

Amidative β-Scission of Alcohols Enabled by Dual Catalysis of Photoredox Proton-Coupled Electron Transfer and Inner-Sphere Ni-Nitrenoid Transfer

The photoredox/Ni dual catalysis is an appealing strategy to enable unconventional C-heteroatom bond formation. While significant advances have been achieved using this system, intermolecular C(sp3)-N bond formation has been relatively underdeveloped due to the difficulty in C(sp3)-N reductive elimination. Herein, we present a new mechanistic approach that utilizes dioxazolones as the Ni(II)-nitrenoid precursor to capture carbon-centered radicals by merging proton-coupled electron transfer (PCET) with nickel catalysis, thus forming synthetically versatile N-alkyl amides using alcohols. Based on mechanistic investigations, the involvement of (κ2-N,O)Ni(II)-nitrenoid species was proposed to capture photoredox PCET-induced alkyl radicals, thereby playing a pivotal role to enable the C(sp3)-N bond formation.

Biorelevant Weakly Coordinating Directing Group Assisted C-H Alkenylation with Cyclopropanols via Sequential C-H/C-C Activation

A weakly coordinating biorelevant intrinsic directing group (DG) assisted site-selective C-H alkenylation via sequential C-H/C-C bond activation has been accomplished under Ru(II)-catalysis using readily accessible cyclopropyl alcohol as an alkenyl surrogate. Utilization of an intrinsic DG, exclusive regioselectivity, functional group diversity, late-stage natural product and drug mutations are the important practical features.

Transition metal-free [3 + 3] annulation of cyclopropanols with β-enamine esters to assemble nicotinate derivatives

A metal-free and efficient approach for the synthesis of structurally important nicotinates through 4-HO-TEMPO-mediated [3 + 3] annulation of cyclopropanols with β-enamine esters is presented. This protocol features high atom efficiency, green waste, simple operation and broad substrate scope. Moreover, the experiments of gram-scale synthesis and recovery of oxidants make this strategy more sustainable and practical.

Rh2(II)-Catalyzed Selective C(sp3)-H Bond Electrophilic Amination of Aryl Azide-Tethered 1,3-Dicarbonyl Compounds

Herein, we report the accomplishment of Rh2(II)-catalyzed intramolecular amination of aryl azide-tethered 1,3-dicarbonyls to access privileged heterocyclic scaffolds with exclusive diastereoselectivity under simple reaction conditions. This method also allows an unconventional direct α-amination at electron-deficient C(sp3)-H bonds of aryl azide-tethered 1,3-diketones to afford fused 2-azatricyclo[4.4.0.02,8]decanones and 2,2-disubstituted indolines, which are present in several biologically active alkaloids. Kinetic isotope experiments revealed that the nucleophilic addition of enol π-bonds on the transient electrophilic rhodium-nitrenoid intermediate enables C-N bond formation.

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.

Rhodium(III)-Catalyzed Intramolecular Annulation and Aromatization for the Synthesis of Pyrrolo[1,2-a]quinolines

A rhodium(III)-catalyzed protocol for the synthesis of pyrrolo[1,2-a]quinolines through intramolecular annulation of o-alkynyl amino aromatic ketones and subsequent aromatization is reported. This transformation builds the pyrrole and quinoline moieties of the pyrrolo[1,2-a]quinoline in one pot and achieves a flexible introduction of different substituent groups at 4- and 5-positions on products that were difficult to prepare by other means. The reaction proceeds smoothly on a gram scale, and the products are amenable to downstream synthetic manipulations.

Iron-catalyzed dual decarboxylative coupling of α-amino acids and dioxazolones under visible-light to access amide derivatives

An iron-catalyzed decarboxylative C-N coupling of α-amino acids with dioxazolones is described herein to synthesize amide derivatives under visible-light. The desired products can be given in good to excellent yields under simple, mild, and oxidant-free conditions. This protocol provides a practical route for the transformation of α-amino acids to the corresponding amides. Computational studies were carried out to shed light on the mechanism of this reaction.

Electrophilic Hydrazination of Cyclopropanols Using Azodicarboxylates via Copper(II) Catalysis: An Umpolung Strategy to Access β-Hydrazino Ketone Motifs

The scope of an umpolung approach to expand synthetic access to bifunctional γ-keto hydrazine intermediates via electrophilic amination of β-homoenolates derived from cyclopropanol precursors that took advantage of azodicarboxylates or azodicarboxamides as electron-deficient nitrogen sources was examined. This new synthetic procedure avails commercially available or readily accessible starting materials along with a ligand-free Cu(II) salt as an inexpensive catalyst. Using this operationally simple reaction, which proceeds under mild conditions (open-flask and ambient temperature) and is suitable for multigram scale, preparative applications were established with a range of aryl- and alkyl-substituted cyclopropanols and azodicarboxylate/azodicarboxamide substrates (26 examples, 74-95% yields). Further, the obtained products have been shown to provide convenient synthetic access to γ-hydroxy hydrazide, γ-amino hydrazide, and heterocyclic derivatives.

Weak Chelation-Assisted C4-Selective Alkylation of Indoles with Cyclopropanols via Sequential C-H/C-C Bond Activation

A Rh-catalyzed weak chelation-guided C4-alkylation of indoles has been accomplished using cyclopropanols as an alkylating agent via the cascade C-H and C-C bond activation. The substrate scope, functional group tolerance, and late-stage mutation of drug molecules are the important practical features.

Rh(III)-catalyzed Intra- and Intermolecular 3,4-Difunctionalization of 1,3-Dienes via Rh(III)-π-allyl Amidation with 1,4,2-Dioxazolones

We herein report a modular strategy, which enables Rh(III)-catalyzed diastereoselective 3,4-amino oxygenation and diamination of 1,3-dienes using different O- and N-nucleophiles in combination with readily available 3-substituted 1,4,2-dioxazolones (78 examples, 37-91% yield). Previous attempts to functionalize the internal double bond rested on the use of plain alcoholic solvents as nucleophilic coupling partners thus dramatically limiting the scope of this transformation. We have now identified hexafluoroisopropanol as a non-nucleophilic solvent which allows the use of diverse nucleophiles and greatly expands the scope, including an unprecedented amino hydroxylation to selectively install valuable, unprotected β-amino alcohols across 1,3-dienes. Moreover, various elaborate alcohols prove to be compatible providing unique access to complex organic molecules. Finally, this method is employed in a series of intramolecular reactions to deliver valuable nitrogen heterocycles as well as γ- and δ-lactones.

Reaction of Dioxazolones with Boronic Acids: Copper-Mediated Synthesis of N-Aryl Amides via N-Acyl Nitrenes

Dioxazolones, as direct amide sources, have been used with boronic acids in the presence of copper(I) chloride to access N-aryl amides at room temperature. The versatility of the developed reaction is proven by ample scope having a wide range of functional group tolerance. The reaction optimization conditions revealed that a fluorine additive demonstrated improved reactivity toward the intended transformation. The addition of triphenylphosphine resulted in N-acyl iminophosphorane, suggesting the involvement of an N-acyl nitrene intermediate.

Photoinduced Transition-Metal-Free Chan-Evans-Lam-Type Coupling: Dual Photoexcitation Mode with Halide Anion Effect

Herein, we report a photoinduced transition-metal-free C(aryl)-N bond formation between 2,4,6-tri(aryl)boroxines or arylboronic acids as an aryl source and 1,4,2-dioxazol-5-ones (dioxazolones) as an amide coupling partner. Chloride anion, either generated in situ by photodissociation of chlorinated solvent molecules or added separately as an additive, was found to play a critical cooperative role, thereby giving convenient access to a wide range of synthetically versatile N-arylamides under mild photo conditions. The synthetic virtue of this transition-metal-free Chan-Evans-Lam-type coupling was demonstrated by large-scale reactions, synthesis of 15N-labeled arylamides, and applicability toward biologically relevant compounds. On the basis of mechanistic investigations, two distinctive photoexcitations are proposed to function in the current process, in which the first excitation involving chloro-boron adduct facilitates the transition-metal-free activation of dioxazolones by single electron transfer (SET), and the second one enables the otherwise-inoperative 1,2-aryl migration of the thus-formed N-chloroamido-borate adduct.

Nickel-Catalyzed Carbonylation of Cyclopropanol with Benzyl Bromide for Multisubstituted Cyclopentenone Synthesis

Herein, we reported a Ni-catalyzed carbonylation of cyclopropanol with benzyl bromide to afford multisubstituted cyclopentenone under 1 atm of CO. The reaction proceeds through cascade carbonylation of benzyl bromides, followed by generation of nickel homoenolate from cyclopropanols via β-C elimination to afford 1,4-diketones, which undergoes intramolecular Aldol condensation to furnish highly substituted cyclopentenone derivatives in moderate to good yields. The reaction exhibits high functional group tolerance with broad substrate scope.