Rhodium(III)-Catalyzed Directed C-H Dienylation of Anilides with Allenes Leads to Highly Conjugated Systems

DFT Study on the Mechanisms and Selectivities in Rh (III)-Catalyzed [5 + 1] Annulation of 2-Alkenylanilides and 2-Alkylphenols with Allenyl Acetates

The mechanisms and regio-, chemo-, and stereoselectivity were theoretically investigated in the Rh(III)-catalyzed [5 + 1] annulation of 2-alkenylanilides and 2-alkylphenols with allenyl acetates. Two different reactants, 2-alkenylanilides and 2-alkylphenols, were selected as model systems in the density functional theory calculations. The obtained theoretical results show that both these reactants exhibit similar steps, namely, (1) N-H/O-H deprotonation and C-H activation, (2) allenyl acetate migratory insertion, (3) β-oxygen elimination, (4) intramolecular nucleophilic addition of the nitrogen/oxygen-rhodium bond resulting in [5 + 1]-annulation, and (5) protonation with the formation of the desired product and regeneration of the Rh(III) catalyst. The theoretical evidence suggests that the selectivity is determined at the step of allenyl acetate's migratory insertion. Moreover, the regioselectivity is driven by electronic effects, while the interaction energies (C-H···π and C-H···O interactions) play a more imperative role in controlling the stereoselectivity. The obtained theoretical results not only well rationalize the experimental observations but also provide important mechanistic insights for related types of [5 + 1]-annulation reactions.

Pyrazolidinone-Aided Ru(II)-Catalyzed Regioselective C-H Annulation with Allenes

Regioselective annulation of allenes via C-H activation represents an elegant synthetic approach toward the construction of valuable scaffolds. Considering the importance of allenes, herein we developed an unprecedented Ru(II)-catalyzed highly regioselective redox-neutral C-H activation/(4 + 1)-annulation of 1-arylpyrazolidinones employing allenyl acetates to access pyrazolo[1,2-a]indazol-1-one derivatives. Additionally, allenyl cyclic carbonates, which were never tested in C-H activation, were utilized to construct a similar class of heterocycles having a pendent alcohol functionality. Notably, double C-H functionalization was achieved by a simple modification of reaction conditions. The synthetic significance of this methodology is underscored by late-stage modification of natural products, broad substrate scope, gram-scale synthesis, and postfunctionalizations.

Iso-Pentadienyl Carbonate as a Five Carbon Synthon in Manganese(I)-Catalyzed Selective Linear 1,3-Dienylation

Selective linear 1,3-dienylations are essential transformations, and numerous synthetic efforts have been documented. However, a general method enabling access to electron-rich, -poor, and biologically relevant dienyl molecules is in high demand. Hence, we report a straightforward method of manganese(I)-catalyzed C-H dienylation of arenes by using iso-pentadienyl carbonate as a five carbon synthon. This is a highly unprecedented report for selective linear 1,3-dienylation using manganese C-H activation catalysis. Our method facilitates the synthesis of varieties of dienes, including those suitable for normal or inverse electron demand Diels-Alder reactions, dienyl glycoconjugates, and unnatural amino acids. Extensive mechanistic studies, including isolation of C-H activated organo-manganese complex and isotopic analyses, have supported the proposed mechanism of this dienylation. The synthetic applicability of this method eased to deliver a 6/6/5-fused tricyclic nagilactone scaffold.

Merging Rh-Catalyzed C-H Functionalization and Cascade Cyclization to Enable Propargylic Alcohols as Three-Carbon Synthons

Reported herein is a reactivity of propargyl alcohols as "Three-Carbon Synthons" in a Rh(III)-catalyzed C-H functionalization of acetanilides, leading to the synthesis of core structures of isocryptolepine, γ-carbolines, dihydrochromeno[2,3-b]indoles, and diindolylmethanes (DIM) derivatives. The transformation involves a rhodium(III)-catalyzed C-H functionalization and heteroannulation to yield indoles followed by a cascade cyclization with both external and internal nucleophiles to afford diverse products. The role of the hydroxy group, the key function of the silver additive, the origin of the reverse regioselectivity and the rate-determining step, are rationalized in conformity with the combination of experimental, noncovalent interaction analysis and DFT studies. This protocol is endowed with several salient features, including one-pot multistep cascade approach, exclusive regioselectivity, good functional group tolerance and synthesis of variety of molecular frameworks.

Regioselective Dichotomy in Ru(II)-Catalyzed C-H Annulation of Aryl Pyrazolidinones with 1,3-Diynes

Herein, we present a substrate-controlled regiodivergent strategy for the selective synthesis of C3 or C2-alkynylated indoles via ruthenium-catalyzed [3 + 2]-annulation of readily available pyrazolidinones and 1,3-diynes. Remarkably, C3-alkynylated indoles were obtained in good yields when 1,4-diarylbuta-1,3-diynes were employed as the coupling partners. On the other hand, dialkyl-1,3-diynes led to the selective formation of C2-alkynylated indoles. The key features of the strategy are the operationally simple conditions and external-oxidant-free, broad-scope, and substrate-switchable indole synthesis. Scale-up reactions and further transformations expanded the synthetic utility of the protocol.

Divergent Reactivity of Phosphorylated and Related Allenes: [4 + 2] Cycloaddition with 3,6-Diphenyltetrazine, Self-Addition Leading to Dimers and [Pd]-Complex Formation

Phosphorus-based naphthalenes are formed by self-dimerization-cum-cyclization of α-aryl allenylphosphonates or allenylphosphine oxides using catalytic Pd(OAc)₂in the presence of PPh₃ and Et₃N . This reaction involves [4 + 2]-cycloaddition with the (β,γ) double bond of one allene as the dienophile; the double bonds at the α-aryl-(β',γ') group and (α,β)-carbons of the second allene act as the diene part. A subsequent proton shift also takes place. Upon treating allenylphosphine oxides with Pd(OAc)₂ [stoichiometry 2:1] in the presence of PPh₃/Ag₂CO₃, a [Pd]-complex is isolated and structurally characterized. This complex can be used as a catalyst for C-C bond-forming reactions of phosphorus-based allenes with 2-iodophenol. Densely substituted 3,6-diphenylpyridazines are conveniently obtained in excellent yields by a thermally induced regioselective Inverse Electron Demand Diels-Alder (IEDDA) reaction of allenes with 3,6-diphenyltetrazine, followed by a [1,3]-H shift.

Regioselective Annulation of Allenylphosphine Oxides with Aromatic Amides under Ruthenium(II) Catalysis

Engaging allenes in transition-metal-catalyzed C-H bond activation strategy is immensely promising to access high-value scaffolds. However, such a reaction manifold remains largely elusive using cheap and sustainable ruthenium catalysis. We disclose an unprecedented ruthenium-catalyzed (4 + 2) annulation between aromatic amides and allenylphosphine oxides, furnishing NH-free isoquinolinones in high yields. This operationally simple methodology leverages weak coordination assistance, displays high selectivity, and is amenable to the late-stage functionalization of several biologically relevant motifs.

Rh(III)-Catalyzed Synthesis of Indazolo[2,3-a]quinolines: Vinylene Carbonate as C1 and C2 Building Blocks

A rhodium-catalyzed cyclization of azobenzenes and vinylene carbonate via C-H bond activation to construct indazolo[2,3-a]quinolines has been developed. This protocol offers an efficient method for synthesis of the titled products in good yields with broad functional group tolerance. In this reaction, three C-C bonds and C-N bond are formed in one pot, and vinylene carbonate (VC) acts as C1 and C2 synthons as well as "vinylene transfer" agent and acylation reagent in the construction of target-fused heterocycles. Moreover, the products exhibit favorable fluorescence properties, which indicate their potential application as fluorescent materials and biosensors.

Rh(iii)-Catalyzed [5 + 1] annulation of 2-alkenylanilides and 2-alkenylphenols with allenyl acetates

Herein, we report a mild and highly regioselective Rh(iii)-catalyzed non-oxidative [5 + 1] vinylic C-H annulation of 2-alkenylanilides with allenyl acetates, which has been elusive so far. The reaction proceeds via vinylic C-H activation, regioselective 2,3-migratory insertion, β-oxy elimination followed by nucleophilic cyclization to get direct access to 1,2-dihydroquinoline derivatives. The strategy was also successfully extended to C-H activation of 2-alkenylphenols for constructing chromene derivatives. In the overall [5 + 1] annulation, the allene serves as a one carbon unit. The acetate group on the allene is found to be crucial both for controlling the regio- and chemoselectivity of the reaction and also for facilitating β-oxy elimination. The methodology was scalable and also further extended towards late stage functionalization of various natural products.

Pd-Catalyzed Heck-Type Reactions of Allenes for Stereoselective Syntheses of Substituted 1,3-Dienes

A highly stereoselective Pd-catalyzed Heck-type reaction of allenes in which the stereochemistry of both olefins is set simultaneously was developed. The ligand CyJohnPhos was crucial to achieving stereoselectivity, while minimizing isomerization of the starting material through hydropalladation. The stereodetermining factors were proposed to be A1,3 strain between the catalyst and allene substituent, which influences the σ-π-σ equilibrium of the coupled allene intermediate, as well as eclipsing interactions of R groups in the β-hydride elimination. Good functional group tolerance and stereoselectivities for formation of the Z,E isomer were demonstrated. The methodology was further expanded to include the regioselective formation of 2,4-dienoates and 2,4-dienamides with a variety of substitution patterns, albeit in reduced stereoselectivities favoring the E,E isomer. A plausible mechanism is proposed to account for the observed selectivities and substituent effects.

Rh(III)-Catalyzed and synergistic dual directing group-enabled redox-neutral [3+3] annulation of N-phenoxyacetamides with α-allenols

By virtue of α-allenols as innovative three-carbon annulation components, the Rh(III)-catalyzed redox-neutral C-H coupling of N-phenoxyacetamides with α-allenols has been realized for the assembly of 4-alkylidene chroman-2-ol frameworks via an unusual [3+3] annulation. This transformation features good functional group tolerance, specific regio-/chemoselectivity and potential synthetic utility. Mechanistic studies reveal that synergistic coordination modes between the dual directing groups (-ONHAc and -OH) and the rhodium metal center account for the observed exclusive selectivity.

Stereoselective rhodium-catalyzed 2-C-H 1,3-dienylation of indoles: dual functions of the directing group

A rhodium-catalyzed intermolecular highly stereoselective 1,3-dienylation at the 2-position of indoles with non-terminal allenyl carbonates has been developed by using 2-pyrimidinyl or pyridinyl as the directing group. The reaction tolerates many functional groups affording the products in decent yields under mild conditions. In addition to C-H bond activation, the directing group also played a vital role in the determination of Z-stereoselectivity for the C-H functionalization reaction with 4-aryl-2,3-allenyl carbonates, which is confirmed by the E-selectivity observed with 4-alkyl-2,3-allenyl carbonates. DFT calculations have been conducted to reveal that π-π stacking involving the directing 2-pyrimidinyl or pyridinyl group is the origin of the observed stereoselectivity. Various synthetic transformations have also been demonstrated.

Copper-Catalyzed Cross-Nucleophile Coupling of β-Allenyl Silanes with Tertiary C-H Bonds: A Radical Approach to Branched 1,3-Dienes

Described herein is a distinctive approach to branched 1,3-dienes through oxidative coupling of two nucleophilic substrates, β-allenyl silanes, and hydrocarbons appending latent functionality by copper catalysis. Notably, C(sp³)-H dienylation proceeded in a regiospecific manner, even in the presence of competitive C-H bonds that are capable of occurring hydrogen atom transfer process, such as those located at benzylic and other tertiary sites, or adjacent to an oxygen atom. Control experiments support the intermediacy of functionalized alkyl radicals.

Metallaphotoredox-catalyzed C–H activation: regio-selective annulation of allenes with benzamide

We have developed an efficient annulation of benzamides with allenes using cobalt and photoredox dual catalysis under an oxygen atmosphere. The transformation features an alternative strategy for the regeneration of a cobalt catalyst with the aid of Eosin Y.

Cobalt-Catalyzed C8-Dienylation of Quinoline-N-Oxides

An efficient Cp*Co^III -catalyzed C8-dienylation of quinoline-N-oxides was achieved by employing allenes bearing leaving groups at the α-position as the dienylating agents. The reaction proceeds by Co^III -catalyzed C-H activation of quinoline-N-oxides and regioselective migratory insertion of the allene followed by a β-oxy elimination, leading to overall dienylation. Site-selective C-H activation was achieved with excellent selectivity under mild reaction conditions, and 30 mol % of a NaF additive was found to be crucial for the efficient dienylation. The methodology features high stereoselectivity, mild reaction conditions, and good functional-group tolerance. C8-alkenylation of quinoline-N-oxides was achieved in the case of allenes devoid of leaving groups as coupling partners. Furthermore, gram-scale preparation and preliminary mechanistic experiments were carried out to gain insights into the reaction mechanism.

Access to 5H-benzo[a]carbazol-6-ols and benzo[6,7]cyclohepta[1,2-b]indol-6-ols via rhodium-catalyzed C–H activation/carbenoid insertion/aldol-type cyclization

The rhodium-catalyzed mono-ortho C–H activation/carbenoid insertion/aldol-type cyclization of 3-aldehyde-2-phenyl-1H-indoles with diazo compounds has been developed.