Indole-ynones as Privileged Substrates for Radical Dearomatizing Spirocyclization Cascades

Photoredox-Catalyzed Strain-Release-Driven Synthesis of Functionalized Spirocyclobutyl Oxindoles

Spirocyclobutyl oxindoles have garnered substantial attention in drug discovery and pharmaceuticals owing to their wide range of biological activities. Strain-release in small-ring compounds is a powerful strategy to enable efficient access to complex molecules. In this study, we successfully realized a photoredox-catalyzed strain-release radical spirocyclization approach to attain functionalized spirocyclobutyl oxindoles. A diverse array of radicals, such as sulfonyl, phosphonyl, and trifluoromethyl, were added efficiently to the strained C-C σ-bond of bicyclobutanes (BCBs) to afford a library of spirocyclobutyl oxindoles. Furthermore, the obtained products could be transformed into valuable building blocks. The observed reactivity and selectivity have been rationalized based on density functional theory calculations.

Shining light on tryptamine-derived isocyanides: access to constrained spirocylic scaffolds

Dearomatization of indoles through a charge transfer complex constitutes a powerful tool for synthesizing three-dimensional constrained structures. However, the implementation of this strategy for the dearomatization of tryptamine-derived isocyanides to generate spirocyclic scaffolds remains underdeveloped. In this work, we have demonstrated the ability of tryptamine-derived isocyanides to form aggregates at higher concentration, enabling a single electron transfer step to generate carbon-based-radical intermediates. Optical, HRMS and computational studies have elucidated key aspects associated with the photophysical properties of tryptamine-derived isocyanides. The developed protocol is operationally simple, robust and demonstrates a novel approach to generate conformationally constrained spirocyclic scaffolds, compounds with high demand in various fields, including drug discovery.

Diastereoselective synthesis of functionalized spiroindolines via intramolecular ipso-iodocyclization/nucleophile addition cascade reactions of indole-tethered ynones

Herein, we describe a highly diastereoselective approach for synthesizing polyfunctionalized spiroindolines from indolyl-ynones involving an ipso-iodocyclization/nucleophile addition cascade. The developed strategy allows the formation of a spirocyclic core and the installation of two functional groups in a single operation. Also this strategy is accompanied by the generation of two C-C and one C-I bonds and two contiguous stereocenters.

Seleno/Thio-functionalized ipso-Annulation of N-Propiolyl-2-arylbenzimidazole to Construct Azaspiro[5,5]undecatrienones

Till date, the ipso-cyclization of propiolamides is limited to provide azaspiro[4,5]decatrienones. Herein, we present the first example of ipso-carbocyclization, leading to azaspiro[5,5]-undecatrienones from N-propiolyl-2-arylbenzimidazoles, involving both the radical-based and electrophilic reactions. This report establishes an access to a wide range of chalcogenated (SCN/SCF3/SePh) benzimidazo-fused azaspiro[5,5]undecatrienones in good yields.

Ag(I)-catalyzed dearomatizing spirocyclization/nucleophile addition cascade reactions of indole-tethered ynones

Ag(I)-catalyzed highly diastereoselective construction of divergent spiroindolines is disclosed herein. The approach proceeds via dearomatizing spirocyclization of indole-tethered ynones followed by C-nucleophile or hydride trapping. The established strategy is accompanied by the generation of two new C-C bonds and two contiguous stereocenters. This strategy features a broad range of (hetero)arenes as C-nucleophiles and excellent diastereoselectivity.

Electrosynthesis of Spiro-indolenines via Dearomative Arylation of Indoles in Batch and Continuous Flow

An electrosynthesis of spiro-indolenines in batch and continuous flow was achieved through dearomative arylation of indoles with good functional group compatibility. User-friendly undivided cells were used under catalyst- and oxidant-free conditions. Moreover, the use of a flow electrolysis cell gave high daily productivity and excellent scale-up potential under less supporting electrolyte and higher substrate concentration conditions.

Visible-Light-Promoted Direct C3-H Cyanomethylation of 2H-Indazoles

The efficient visible-light-promoted cyanomethylation of 2H-indazoles in the presence of Ir(ppy)₃ as the photocatalyst and bromoacetonitrile as the cyanomethyl radical source was achieved under mild conditions, providing a series of C3-cyanomethylated derivatives in good yields.

Lewis Base-Boryl Radicals Enabled Borylation Reactions and Selective Activation of Carbon-Heteroatom Bonds

ConspectusThe past decades have witnessed tremendous progress on radical reactions. However, in comparison with carbon, nitrogen, oxygen, and other main group element centered radicals, the synthetic chemistry of boron centered radicals was less studied, mainly due to the high electron-deficiency and instability of such 3-center-5-electron species. In the 1980s, Roberts and co-workers found that the coordination of a Lewis base (amines or phosphines) with the boron center could form 4-center-7-electron boryl radicals (Lewis base-boryl radicals, LBRs) that are found to be more stable. However, only limited synthetic applications were developed. In 2008, Curran and co-workers achieved a breakthrough with the discovery of N-heterocyclic carbene (NHC) boryl radicals, which could enable a range of radical reduction and polymerization reactions. Despite these exciting findings, more powerful and valuable synthetic applications of LBRs would be expected, given that the structures and reactivities of LBRs could be easily modulated, which would provide ample opportunities to discover new reactions. In this Account, a summary of our key contributions in LBR-enabled radical borylation reactions and selective activation of inert carbon-heteroatom bonds will be presented.Organoboron compounds have shown versatile applications in chemical society, and their syntheses rely principally on ionic borylation reactions. The development of mechanistically different radical borylation reactions allows synthesizing products that are inaccessible by traditional methods. For this purpose, we progressively developed a series of NHC-boryl radical mediated chemo-, regio-, and stereoselective radical borylation reactions of alkenes and alkynes, by which a wide variety of structurally diverse organoboron molecules were successfully prepared. The synthetic utility of these borylated products was also demonstrated. Furthermore, we disclosed a photoredox protocol for oxidative generation of NHC-boryl radicals, which enabled useful defluoroborylation and arylboration reactions.Selective bond activation is an ideal way to convert simple starting materials to value-added products, while the cleavage of inert chemical bonds, in particular the chemoselectivity control when multiple identical bonds are present in similar chemical environments, remains a long-standing challenge. We envisaged that finely tuning the properties of LBRs might provide a new solution to address this challenge. Recently, we disclosed a 4-dimethylaminopyridine (DMAP)-boryl radical promoted sequential C-F bond functionalization of trifluoroacetic acid derivatives, in which the α-C-F bonds were selectively snipped via a spin-center shift mechanism. This strategy enables facile conversion of abundantly available trifluoroacetic acid to highly valuable mono- and difluorinated molecules. Encouraged by this finding, we further developed a boryl radical enabled three-step sequence to construct all-carbon quaternary centers from a range of trichloromethyl groups, where the three C-Cl bonds were selectively cleaved by the rational choice of suitable boryl radical precursors in each step. Furthermore, a boryl radical promoted dehydroxylative alkylation of α-hydroxy carboxylic acid derivatives was achieved, allowing for the efficient conversion of some biomass platform molecules to high value products.

Access to Diverse Seleno-spirocyclohexadienones via Ag(II)-Catalyzed Selenylative ipso-Annulation with Se and Boronic Acids

An efficient and straightforward synthesis of diversified seleno-azaspiro-2,5-cyclohexadienones from N-(4-methoxy aryl)propiolamides using elemental selenium and boronic acids has been demonstrated. The reaction proceeds through silver-catalyzed oxidative dearomatization in the presence of potassium persulfate (K₂S₂O₈) as the oxidant. Further, this approach was extended to N-(4-methoxy aryl)propiolates and biaryl ynones to access the corresponding selenylated oxospiro-2,5-cyclohexadienones and spiro[5,5]trienones, respectively. The present three-component method offers the diverse substitutions on selenium involving two C-Se and one C-C bond formations.

Visible-Light-Induced Cascade Cyclization of 3-(2-(Ethynyl)phenyl)quinazolinones to Phosphorylated Quinolino[2,1-b]quinazolinones

3-(2-(Ethynyl)phenyl)quinazolinones were designed and synthesized as a class of novel and efficient skeletons for phosphorylation/cyclization reactions. Under visible light irradiation, a series of phosphorylated quinolino[2,1-b]quinazolinones (35 examples, up to 87% yield) were first synthesized from 3-(2-(ethynyl)phenyl)quinazolinones and diarylphosphine oxides by using 4CzIPN as a photocatalyst under mild conditions. This reaction was also applicable under sunlight irradiation. Moreover, the reaction efficiency could be significantly improved under continuous-flow conditions.

PIFA-mediated selenylative spirocyclization of indolyl ynones: facile access to selenated spiro[cyclopentenone-1,3'-indoles]

A fast selenylative spirocyclization of indolyl ynones mediated by PIFA has been developed. This transformation was enabled by the reactive RSeOCOCF₃ species generated in situ from diselenides with PIFA, involving an electrophilic dearomative cascade cyclization. This protocol provides a facile and efficient method for the synthesis of selenated spiro[cyclopentenone-1,3'-indoles] and tolerates broad functional groups.

Selectivity, Speciation, and Substrate Control in the Gold-Catalyzed Coupling of Indoles and Alkynes

A convenient and mild protocol for the gold-catalyzed intermolecular coupling of substituted indoles with carbonyl-functionalized alkynes to give vinyl indoles is reported. This reaction affords 3-substituted indoles in high yield, and in contrast to the analogous reactions with simple alkynes which give bisindolemethanes, only a single indole is added to the alkyne. The protocol is robust and tolerates substitution at a range of positions of the indole and the use of ester-, amide-, and ketone-substituted alkynes. The use of 3-substituted indoles as substrates results in the introduction of the vinyl substituent at the 2-position of the ring. A combined experimental and computational mechanistic study has revealed that the gold catalyst has a greater affinity to the indole than the alkyne, despite the carbon–carbon bond formation step proceeding through an η²(π)-alkyne complex, which helps to explain the stark differences between the intra- and intermolecular variants of the reaction. This study also demonstrated that the addition of a second indole to the carbonyl-containing vinyl indole products is both kinetically and thermodynamically less favored than in the case of more simple alkynes, providing an explanation for the observed selectivity. Finally, a highly unusual gold-promoted alkyne dimerization reaction to form a substituted gold pyrylium salt has been identified and studied in detail.

Access to cyclopentenones via copper-catalyzed 5-endo trifluoromethylcarbocyclization of ynones

A copper-catalyzed 5-endo-selective trifluoromethylcarbocyclization of ynones is realized for the direct construction of trifluoromethylated cyclopentenones.

Copper catalyzed dearomatization by Michael-type addition of indolyl ynones: divergent synthesis of functionalized spiroindoles and cyclopenta[c]quinolin-3-ones

Described herein is a copper-catalyzed multifunctionalization of indolyl ynones, allowing the synthesis of dihalogen-substituted spiroindoles. This Cu catalysis is also applicable to the synthesis of cyclopenta[c]quinolin-3-ones via decarbonylation.