Ruthenium-Catalyzed Cycloisomerization of 2-Alkynylanilides: Synthesis of 3-Substituted Indoles by 1,2-Carbon Migration

Synthesis of Non-canonical Tryptophan Variants via Rh-catalyzed C-H Functionalization of Anilines

Tryptophan and its non-canonical variants play critical roles in pharmaceutical molecules and various enzymes. Facile access to this privileged class of amino acids from readily available building blocks remains a long-standing challenge. Here, we report a regioselective synthesis of non-canonical tryptophans bearing C4-C7 substituents via Rh-catalyzed annulation between structurally diverse tert-butyloxycarbonyl (Boc)-protected anilines and alkynyl chlorides readily prepared from amino acid building blocks. This transformation harnesses Boc-directed C-H metalation and demetalation to afford a wide range of C2-unsubstituted indole products in a redox-neutral fashion. This umpolung approach compared to the classic Larock indole synthesis offers a novel mechanism for heteroarene annulation and will be useful for the synthesis of natural products and drug molecules containing non-canonical tryptophan residues in a highly regioselective manner.

Experimental and Computational Studies of Steric Factors in the Isomerization of Internal Alkynes within the Coordination Environment of Half-Sandwich Metal Complexes

The isomerization of internal alkynes Ar1C≡CAr2 within the coordination environment of low-valent half-sandwich [Ru(dppe)Cp]+ complexes via a 1,2-migration process affords vinylidene species [Ru{=C=C(Ar1)Ar2}(dppe)Cp]+. The rearrangement reactions of symmetrically and asymmetrically substituted substrates featuring different electron-donating and -withdrawing groups and of varying steric bulk were modelled using density functional theory (DFT), and the conclusions supported by experimental observations. Examination of the reaction pathway and associated activation barriers reveal a high solvent dependency for the generation of the key intermediate species [Ru(dppe)Cp]+ from [RuCl(dppe)Cp] by halide dissociation in the presence of Na+ salts of weakly coordinating anions, with the lattice enthalpy of the NaCl by-product playing a critical role in the overall thermochemical balance of the reaction. The activation barriers associated with the reaction of [Ru(dppe)Cp]+ with Ar1C≡CAr2, and the relative energies of the alkyne complexes [Ru(η2-Ar1C≡CAr2)(dppe)Cp]+, are sensitive to the electron density of the alkyne and conformational changes associated with the 'bend-back' of the substrate. The latter differs by up to 66.1 kJ/mol, which in turn impacts the barrier height of the subsequent 1,2-migration step involved in the rearrangement process and ultimately the overall thermochemical nature of the complete reaction. The relative importance of these factors is evinced by the successful rearrangement of the very sterically congested 1(9-anthryl)-2(9-phenanthryl) acetylene into the fully characterized diaryl vinylidene complex, which was isolated in 89 % yield.

Controlling Chemoselectivity in Ruthenium(II)-Induced Cyclization of Aniline-Functionalized Alkynes

The cyclization of heteroatom-functionalized alkynes induced by d6-transition-metal centers has traditionally been associated with the vinylidene pathway. However, recent evidence suggests that d6-transition-metal centers can also activate alkynes through non-vinylidene pathways. In this study, we conducted a comprehensive experimental and theoretical investigation into the reactions between the Ru(II) complex [Ru([9]aneS3)(bpy)(OH2)]2+ and 2-alkynylanilines. Our study revealed that the selectivity between the vinylidene and non-vinylidene pathways can be tuned by reaction temperature, substrate, and solvent polarity. This strategic control allows for the preferential formation of either C2- or C3-metalated indole zwitterion complexes. Additionally, we identified a rare decyclization mechanism that enables the conversion of C2-metalated indoles to C3-metalated indoles, underscoring the significance of product stability in these pathways. Overall, this work demonstrates practical approaches to control the preference between vinylidene and non-vinylidene pathways, which is crucial for the design of new catalysts and metalated heterocyclic complexes.

Gold(I)-Catalyzed Synthesis of 2,2'-Biindoles via One-Pot Double Cycloisomerization Strategy

The first systematic, concise and target-directed gold(I)-catalyzed synthesis of a family of 2,2'-biindoles containing different substitution patterns is described. The developed protocol involves the synthesis of 1,3-diyne-anilines followed by a one-pot gold(I)-catalyzed double cycloisomerization, giving rise to an efficient, broad and general protocol to get different 2,2'-biindoles under mild reaction conditions. Due to the methodological restriction of present methods for accessing this class of compounds, herein we present our synthetic proposal which allowed the preparation of several examples of 2,2'-biindoles. Their functionalization-guided us to the discovery that the chemical stability, is substitution structure-dependent.

Recent advances in electrochemically enabled construction of indoles from non-indole-based substrates

Indole motifs are important heterocycles found in natural products, pharmaceuticals, agricultural chemicals, and materials. Although there are well-established classical name reactions for indole synthesis, these transformations often require harsh reaction conditions, have a limited substrate scope, and exhibit poor regioselectivity. As a result, organic synthesis chemists have been exploring efficient and practical methods, leading to numerous strategies for synthesizing a variety of functionalized indoles. In recent years, electrochemistry has emerged as an environmentally friendly and sustainable synthetic tool, with widespread applications in organic synthesis. This technology allows for elegant synthetic routes to be developed for the construction of indoles under external oxidant-free conditions. This feature article specifically focuses on recent advancements in indole synthesis from non-indole-based substrates, as well as the mechanisms underlying these transformations.

Migration of Condensed Aromatic Hydrocarbons During Alkyne-Vinylidene Rearrangements

Diarylacetylenes ArC≡CAr featuring condensed aromatic hydrocarbon fragments (Ar) such as naphthalene, anthracene, phenanthrene and pyrene were converted into vinylidene ligands by 1,2-migration reactions within the coordination sphere of half-sandwich complexes [MII(dppe)Cp]+ (MII = RuII, FeII). Comparison of the extent of conversion of the alkyne substrates to the vinylidene complexes [Ru{=C=CAr2}(dppe)Cp]+ with those obtained from acetylenes functionalized by smaller groups (H, CH3, Ph) show that the molecular volume (VM) of the migrating group and relief of steric congestion plays a role during the rearrangement process. Conversely, the H-atoms from the larger condensed ring aryl groups that are in close proximity to the migrating sites also have a significant influence on the efficacy and extent of the reaction by restricting access of the alkyne to the metal center, resulting in a less effective migration reaction. This combination of competing steric factors (acceleration due to relief of steric congestion and restricted access of the alkyne moiety to the reaction site) is exemplified by the facile migration of 1-pyryl entities and the low yields of vinylidene products formed from 1,2-bis(9-anthryl)acetylene.

Vinylidene rearrangements of internal borylalkynes via 1,2-boryl migration

Vinylidene rearrangement of alkynes is a well-established and powerful method for alkyne transformations, while use of borylalkynes has remained largely unexplored. This paper describes vinylidene rearrangements of internal borylalkynes using a cationic ruthenium complex. This rearrangement is applicable to alkynes with both tri-(B(pin), B(dan)) and tetracoordinate (B(mida)) boryl groups, and the reaction rate is dramatically affected by the Lewis acidity of the boryl group. Mechanistic study revealed that the rearrangement proceeds via 1,2-boryl migration regardless of the coordination number of the boron center. The migration mode was elucidated by theoretical calculations to indicate that the migration of the tricoordinate boryl groups is an electrophilic process in contrast to the previous vinylidene rearrangements of internal alkynes with two carbon substituents.

Synthesis of Branch-Type 3-Allylindoles from N-Alkyl-N-cinnamyl-2-ethynylaniline Derivatives Using π-Allylpalladium Chloride Complex as a Catalyst

The reaction of N-alkyl-N-cinnamyl-2-ethynylaniline derivatives 1 via annulation and aza-Claisen-type rearrangement easily afforded corresponding branch-type 3-allylindoles 2 with high regioselectivities in good yields using π-allylpalladium chloride complex as a catalyst.

Iridium-catalyzed regioselective B(3,6)-dialkenylation or B(4)-alkenylation of o-carboranes via B-H activation and 1,2-carbon migration of alkynes

An efficient Ir-catalyzed cage boron alkenylation of 1-(2'-picolyl)-o-carboranes with diarylacetylenes has been developed, leading to a wide variety of B-H geminal addition products via 1,2-carbon migration of alkynes. The steric effect of cage carbon substituents has a great impact on the regioselectivity of such alkenylation reactions.

A Platinum(II)-Based Molecular Cage with Aggregation-Induced Emission for Enzymatic Photocyclization of Alkynylaniline

Enzymes facilitate chemical conversions through the collective activity of aggregated components, but the marriage of aggregation-induced emission (AIE) with molecular containers to emulate enzymatic conversion remains challenging. Herein, we report a new approach to construct a Pt^II -based octahedral cage with AIE characteristics for the photocyclization of alkynylaniline by restricting the rotation of the pendant phenyl rings peripheral to the Pt^II corner. With the presence of water, the C-H⋅⋅⋅π interactions involving the triphenylphosphine fragments resulted in aggregation of the molecular cages into spherical particles and significantly enhanced the Pt^II -based luminescence. The kinetically inert Pt-NP chelator, with highly differentiated redox potentials in the ground and excited states, and the efficient coordination activation of the platinum corner facilitated excellent catalysis of the photocyclization of alkynylaniline. The enzymatic kinetics and the advantages of binding and activating substrates in an aqueous medium provide a new avenue to develop mimics for efficient photosynthesis.

Chemoselective Ru-Catalyzed Oxidative Lactamization vs Hydroamination of Alkynylamines: Insights from Experimental and Density Functional Theory Studies

The Ru-catalyzed intramolecular oxidative amidation (lactamization) of aromatic alkynylamines with 4-picoline N-oxide as an external oxidant has been developed. This chemoselective process is very efficient to achieve medium-sized ε- and ζ-lactams (seven- and eight-membered rings) but not for the formation of common δ-lactams (six-membered rings). DFT studies unveiled the capital role of the chain length between the amine and the alkyne functionalities: the longer the connector, the more favored the lactamization process vs hydroamination.

Macrolactonization of methyl 15-hydroxypentadecanoate to cyclopentadecanolide using KF-La/γ-Al2O3 catalyst

It has been a challenge to synthesize macrolide musk in excellent yields with high purity. KF-La/γ-Al₂O₃ catalyst was prepared from a highly basic mesoporous framework using a mild method. The prepared KF-La/γ-Al₂O₃ catalyst was employed for the synthesis of cyclopentadecanolide from methyl 15-hydroxypentadecanoate. The morphology and structure of prepared catalysts were characterized using XRD, TG-DTG, SEM, EDX, TEM, BET and CO₂-TPD. The results revealed that the K₃AlF₆ and LaOF are produced on the surface of KF-La/γ-Al₂O₃, and LaO can promote the dispersion of KF on the surface of Al₂O₃. Catalysts pore size main distribution ranges between 10 and 30 nm, the maximum CO₂ desorption temperature is 715°C when the La loading is 25%. Because F^- ion has a higher electronegativity than O^2- ion, the KF-promoted metal oxide (Al₂O₃ or/and La₂O₃) contained more strong basic sites, compared with that of the corresponding metal oxide. The yield of cyclopentadecanolide obtained at 0.5 g KF-25La/γ-Al₂O₃ catalyst and a reaction temperature of 190°C for 7 h were 58.50%, and the content after reactive distillation is 98.8%. The KF-La/γ-Al₂O₃ catalyst has a larger pore size and basic strength, which is more conducive to the macrolactonization of long-chain hydroxy ester.

Recent Advances in Organic Synthesis of 3-Amino- or 4-Aminocoumarins

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Coumarin is a privileged scaffold that contains the unique 2H-chromen-2-one motif, and its derivatives are widely distributed in nature, especially in plants. In recent years, due to their diverse pharmacological activities and remarkable photochemical properties, they have attracted significant attention from scientists, which has also prompted the research on the synthesis approaches and the availability of substrates for these compounds. This article is a brief description of the methods for the synthesis of various coumarin derivatives via two- or multi-component reactions involving 3-amino or 4-aminocoumarin reported during 2015-2021. This review may help expand the development of various analogues with coumarin as the basic unit.

Synthesis of C7-Functionalized Indoles through an Aromaticity Destruction-Reconstruction Process

A process for the synthesis of C7-functionalized indoles using para-substituted 2-alkynylanilines as starting materials was reported. The process involves a dearomatization, an 1,2-addition by organic lithium or Grignard reagents, an aromatization-driven allylic rearrangement, and a cyclization. A variety of groups including alkyl, aryl, alkenyl, or alkynyl groups were selectively installed at the C7 site of indoles leading to the formation of 2,5,7-trisubstituted indoles.

TfOH-promoted synthesis of indoles and benzofurans involving cyclative transposition of vinyl ketone

A metal-free approach to construct indole ring from vinylogous amides derived from o-alkynylanilines involving cyclization, retro-aza-Michael and amine trapping cascade is reported here. This atom-economical transformation has been extended to...

Cobalt-catalyzed C(sp3)-H bond functionalization to access indole derivatives

Herein, we develop an efficient method of cobalt-catalyzed C(sp3)-H bond functionalization to synthesize indole derivatives. The highlight of this protocol is accomplished by the sequential C-H activation. This “cobalt/ organic...

1,2-Aryl Migration Induced by Amide C-N Bond-Formation: Reaction of Alkyl Aryl Ketones with Primary Amines Towards α,α-Diaryl β,γ-Unsaturated γ-Lactams

Rearrangement reactions incorporated into cascade reactions play an important role in rapidly increasing molecular complexity from readily available starting materials. Reported here is a Cu-catalyzed cascade reaction of α-(hetero)aryl-substituted alkyl (hetero)aryl ketones with primary amines that incorporates an unusual 1,2-aryl migration induced by amide C-N bond formation to produce a class of structurally novel α,α-diaryl β,γ-unsaturated γ-lactams in generally good-to-excellent yields. This cascade reaction has a broad substrate scope with respect to primary amines, allows a wide spectrum of (hetero)aryl groups to smoothly undergo 1,2-migration, and tolerates electronically diverse α-substituents on the (hetero)aryl ring of the ketones. Mechanistically, this 1,2-aryl migration may stem from the intramolecular amide C-N bond formation which induces nucleophilic migration of the aryl group from the acyl carbon center to the electrophilic carbon center that is conjugated with the resulting iminium moiety.

Enantioselective synthesis of indole derivatives by Rh/Pd relay catalysis and their anti-inflammatory evaluation

An efficient Rh/Pd relay catalyzed intermolecular and cascade intramolecular hydroamination for the synthesis of exclusive trans 1-indolyl dihydronaphthalenols (up to 88% yield, 99% ee) is described under mild conditions. Moreover, the in silico and in vitro screening showed that the novel 1-indolyl dihydronaphthalenol products are potent lead compounds for treating inflammation disease.

Vinylidene, allenylidene, cyclic oxycarbene, and η2-alkyne complexes from reactions of (η5-C5Me5)OsCl(PPh3)2 with alkynes and alkynols

Reactions of Cp*OsCl(PPh3)2 (Cp* = pentamethylcyclopentadienyl) with alkynes and alkynols are described. Treatment of Cp*OsCl(PPh3)2 with phenylacetylene and trimethylsilylacetylene gave the vinylidene complexes Cp*OsCl(=C=CHPh)(PPh3) and Cp*OsCl(=C=CH2)(PPh3), respectively. Treatment of Cp*OsCl(PPh3)2 with the internal alkyne dimethyl acetylenedicarboxylate produced the η2-alkyne complex Cp*OsCl(η2-MeO2C≡CCO2Me)(PPh3). Treatment of Cp*OsCl(PPh3)2 with the propargylic alcohol HC≡CC(OH)Ph2 gave the osmium allenylidene complex Cp*OsCl(=C = C=CPh2)(PPh3). The outcomes of the reactions of Cp*OsCl(PPh3)2 with ω-alkynols HC≡C(CH2)nOH are dependent on the length of the -(CH2)n- linker. The reaction with 3-butyn-1-ol produced the cyclic oxycarbene complex Cp*OsCl{=C(CH2)3O}(PPh3) exclusively. The reactions with 4-pentyn-1-ol produced a mixture of the hydroxyalkyl vinylidene complex Cp*OsCl{=C=CH(CH2)3OH}(PPh3) and the cyclic oxycarbene complex Cp*OsCl{=C(CH2)4O}(PPh3) in about 10:1 molar ratio. The reaction with 5-hexyn-1-ol gave exclusively the hydroxyalkyl vinylidene complex Cp*OsCl{=C=CH(CH2)4OH}(PPh3).

Synthesis of N-indolated amino acids or peptides from 2-alkynylanilines via a dearomatization process

A process for the rapid synthesis of N-indolated amino acids or peptides from readily available 2-alkynylanilines via dearomatization was reported.

Gold(i)-catalyzed redox transformation of o-nitroalkynes with indoles for the synthesis of 2,3′-biindole derivatives

A gold(i)-catalyzed cascade reaction of o-nitroalkynes with indoles has been reported for the rapid assembly of 2-indolyl indolone N-oxides, which exhibit high anticancer potency against SCLC cells.

Iron(ii)-induced cycloisomerization of alkynes via"non-vinylidene" pathways for iron(ii)-indolizine and -indolizinone complexes

Reactions between pyridine-functionalized alkynes and an Fe(ii) precursor supported by 2,5,8-trithia[9](2,9)-1,10-phenanthrolinophane afforded the first Fe(ii)-indolizine and -indolizinone complexes. Structural analysis and theoretical calculations revealed the existence of unconventional "non-vinylidene" pathways and challenged the generality of vinylidene intermediacy in Fe(ii)-induced alkyne transformations.

Conventional and unconventional alkyne activations by Ru and Os for unprecedented dimetalated quinolizinium complexes

Two types of unexpected quinolizinium complexes were obtained from the reactions between pyridine-functionalized propargylic alcohol HC[triple bond, length as m-dash]CC(OH)(Ph)(CH₂(2-py)) (L1) and cis-[M(L^L)₂Cl₂] (M = Ru, Os; L^L = 1,1-bis(diphenylphosphino)methane (dppm), 2,2'-bipyridine (bpy)). Their molecular structures revealed that L1 can be activated by Ru and Os via the conventional "vinylidene-involving" or unconventional "non-vinylidene-involving" pathways.

Ruthenium-Induced Cyclization of Heteroatom-Functionalized Alkynes: Progress, Challenges and Perspectives

Metal-induced cyclization of functionalized alkynes represents one of the most general approaches to prepare organic heterocycles. Although Ru^II centers are well-established to promote alkyne to vinylidene rearrangements and many Ru^II -mediated alkyne cyclizations have been rationalized to be the results of post-vinylidene transformations, recent discoveries indicate that Ru^II centers can serve as electrophiles and induce alkyne cyclizations without vinylidene intermediacy. In this Minireview, an overview of the Ru^II -induced cyclization of heteroatom-functionalized alkynes in the last decade is provided, with an emphasis on the discoveries and validations of the unconventional "non-vinylidene-involving" pathways.

A competitive and highly selective 7-, 6- and 5-annulation with 1,3-migration through C-H and N-H - alkyne coupling

We demonstrated a highly competitive and selective C-C and N-C cross-coupled 7-, 6- and 5-annulation utilizing 2-ethynylanilides to afford functionalized 1H-benzo[b]azepin-2(5H)-ones, 2-quinolinones, and 3-acylindoles in high yield. ZnCl2 was found to be the smart catalyst for 7- and 5-annulation with 1,3-migration through C-H and N-H functionalization, respectively, whereas molecular iodine performed the C-H functionalized 6-annulation with a nonconventional 1,3 H-shift. The mechanism was investigated by intermediate trapping, control, and labeling experiments.

Rh-Catalyzed nitrene alkyne metathesis/formal C–N bond insertion cascade: synthesis of 3-iminoindolines

A Rh-catalyzed nitrene/alkyne metathesis (NAM) cascade reaction terminated by a formal C–N bond insertion has been developed, which provides facile access to the tricyclic 3-iminoindolines in good yields with broad substrate scope.

Recent advances in the synthesis of indoles from alkynes and nitrogen sources

This review highlights ten years of success in the synthesis of indoles using alkynes and nitrogen sources as substrates.

Copper-catalyzed cascade cyclization reaction of 3-aminocyclobutenones with electron-deficient internal alkynes: synthesis of fully substituted indoles

A novel copper-catalyzed cascade cyclization reaction of 3-aminocyclobutenones with electron-deficient internal alkynes has been developed. This reaction provides a new method for the synthesis of fully substituted indoles by formation of four new bonds and two rings in a single step.

Synthesis of lactone-fused pyrroles by ruthenium-catalyzed 1,2-carbon migration-cycloisomerization

A ruthenium-catalyzed cycloisomerization of 3-amino-4-alkynyl-2H-chromen-2-ones via 1,2-carbon migration was developed. Various 1-arylchromeno[3,4-b]pyrrol-4(3H)-ones were synthesized in good to excellent yields. The reaction was applied to the formal total synthesis of marine natural products Ningalin B and Lamellarin H. The efficient synthesis of γ-butyrolactone-fused pyrrole derivatives was also achieved.

Isolation of a C3-metalated indolizine complex and a phosphonium ring-fused bicyclic metallafuran from the osmium-induced transformation of pyridine-tethered alkynes

The first C3-metalated indolizine complex was prepared from the reaction between cis-[Os(dppm)2Cl2] (dppm = 1,1-bis(diphenylphosphino)methane) and propargylic pyridine, HC[triple bond, length as m-dash]CC(OH)(2-py)2. A phosphonium ring-fused bicyclic osmafuran complex was also prepared from the reaction between cis-[Os(dppm)2Cl2] and pyridyl ynone, HC[triple bond, length as m-dash]C(C[double bond, length as m-dash]O)(2-py). The formation of these two products revealed the intermediacy of metal-vinylidene species regarding [Os(dppm)2Cl]+-mediated alkyne transformations. Comparison of the d6 transition-metal precursors employed to activate HC[triple bond, length as m-dash]CC(OH)(2-py)2 suggests that precursors with higher π-basicity favor the vinylidene-involving pathway.