Nickel(II)/Lewis acid catalysed olefin hydroamination and hydroarylation under mild conditions

Aniline derivatives are important nitrogen-containing compounds with wide applications in chemicals, pharmaceuticals and agrochemicals. In the work described herein, nickel(II)/Lewis acid (LA) catalysed olefin hydroamination with anilines was explored for use in aniline derivative syntheses. The Ni(II)/LA catalysis proceeded smoothly under mild conditions, whereas using Ni(OAc)2 alone, the catalyst was inactive. Remarkably, the Markovnikov addition type products were obtained when substituted styrenes were used as the olefin source, while the anti-Markovnikov addition type products were obtained when the electron-deficient olefins such as acrylonitrile and acrylates were used. The mechanistic studies revealed that hydroamination of the styrene derivates proceeded via the amino-Ni(II)/LA attacking the carbocation intermediate which was generated by the protonation of the olefin, whereas for acrylonitrile and acrylates, it proceeded by a direct amino-Ni(II)/LA attack on the olefin by nucleophilic addition. In addition, the hydroarylation product was generated by the Hofmann-Martius rearrangement of the hydroamination product.

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.

Lewis acid improved dioxygen activation by a non-heme iron(II) complex towards tryptophan 2,3-dioxygenase activity for olefin oxygenation

Dioxygen activation and catalysis around ambient temperature is a long-standing challenge in chemistry. Inspired by the significant roles of the hydrogen bond network in dioxygen activation and catalysis by redox enzymes, this work presents a Lewis acid improved dioxygen activation by an Fe^II(BPMEN)(OTf)₂ complex towards tryptophan 2,3-dioxygenase (TDO) activity for 3-methylindole and common olefinic CC  bond oxygenation and cleavage (enzymatic Brønsted acid vs. chemical Lewis acid). It was found that the presence of a Lewis acid such as Sc³⁺ could substantially improve olefinic CC  bond oxygenation and cleavage activity through Fe^II(BPMEN)(OTf)₂ catalyzed dioxygen activation. Notably, a more negative ρ value in the Hammett plot of para-substituted styrene oxygenations was observed in the presence of a stronger Lewis acid, disclosing the enhanced electrophilic oxygenation capability of the putative iron(III) superoxo species through its electrostatic interaction with a stronger Lewis acid. Thereof, this work has demonstrated a new strategy in catalyst design for dioxygen activation and catalysis for olefin oxygenation, a significant process in the chemical industry.

Palladium(II)/Lewis Acid-Catalyzed Olefination of Arylacetamides with Dioxygen

The present work introduces Pd(II)/LA-catalyzed (LA: Lewis acid) olefination of arylacetamides with dioxygen as the oxidant source. This protocol tolerates with different functional groups on the substrates, and the catalytic efficiency is highly Lewis acidity-dependent on added LA, that is, a stronger LA provided a better promotional effect. The ¹H NMR studies of the semireaction between the arylacetamide and the Pd(II)/Sc(III) catalyst in HOAc-d₄ disclosed the formation of a palladacycle intermediate, and the C-H activation step was reversible, which led to the formation of the deuterated arylacetamide substrate and the palladacycle intermediate. Further semireaction between the palladacycle intermediate and the olefin disclosed that it was a clean and much faster reaction than the C-H activation step, thus revealing multiple mechanistic information for Pd(II)-catalyzed C-H activation.

Pd(II)/Lewis acid catalyzed regioselective olefination of indole with dioxygen

Transition metal ion catalyzed indole olefination through C-H activation is a convenient protocol to synthesize versatile bioactive vinylindole compounds; however, in most cases, stoichiometric amounts of oxidants were necessary to accomplish the catalytic cycle. The present study describes a Pd(II)/LA (LA: Lewis acid) catalyzed indole olefination with dioxygen as the sole oxidant. The olefination reaction with electron-rich olefins proceeded smoothly through the pyrrolyl N-carboxamide group directed remote C-H activation at the C3 position of the indole with the Pd(II)/LA catalyst, whereas Pd(II) alone was a very sluggish catalyst under identical conditions. For the electron-deficient olefins, the directing N-carboxamide group was not essential for olefination with this Pd(II)/LA catalyst, demonstrating a different olefination pathway from that of electron-rich olefins. Remarkably, ¹H NMR kinetics disclosed that olefination proceeded much faster with electron-rich olefins than with electron-deficient ones.

Decarboxylative Addition of Propiolic Acids with Indoles to Synthesize Bis(indolyl)methane Derivatives with a Pd(II)/LA Catalyst

Exploring new protocols for efficient organic synthesis is crucial for pharmaceutical developments. The present work introduces a Pd(II)/LA-catalyzed (LA: Lewis acid) decarboxylative addition reaction for the synthesis of bis(indolyl)methane derivatives. The presence of Lewis acid such as Sc(OTf)₃ triggered Pd(II)-catalyzed decarboxylative addition of propiolic acids with indoles to offer the bis(indolyl)methane derivatives in moderate to good yields, whereas neither Pd(II) nor Lewis acid alone was active for this synthesis. The catalytic efficiency of Pd(OAc)₂ was highly dependent on the Lewis acidity of the added Lewis acid, that is, a stronger Lewis acid provided a higher yield of the bis(indolyl)methane derivatives. Meanwhile, this Pd(II)/LA-catalyzed decarboxylative addition reaction showed good tolerance toward versatile electron-rich or -deficient substituents on the indole skeleton and on the benzyl ring of propiolic acids. The studies on the in situ ¹H NMR kinetics of this Pd(II)/Sc(III) catalysis disclosed the formation of a transient vinyl-Pd(II)/Sc(III) intermediate generated by the pyrrole addition to the alkynyl-Pd(II)/Sc(III) species after decarboxylation, which was scarcely observed before.

An approach to unsymmetrical 3,3′-diindolylmethanes through Pd-catalyzed cascade Heck cyclization of allenamides and o-ethynylanilines

A highly efficient synthesis of unsymmetrical 3,3′-diindolylmethanes has been developed by palladium-catalyzed cascade Heck/cyclization reaction.

Regiocontrol in the oxidative Heck reaction of indole by ligand-enabled switch of the regioselectivity-determining step

Efficient control of regioselectivity is a key concern in transition-metal-catalyzed direct C-H functionalization reactions. Various strategies for regiocontrol have been established by tuning the selectivity of the C-H activation step as a common mode. Herein, we present our study on an alternative mode of regiocontrol, in which the selectivity of the C-H activation step is no longer a key concern. We found that, in a reaction where the C-H activation step exhibits a different regio-preference from the subsequent functionalization step, a ligand-enabled switch of the regioselectivity-determining step could provide efficient regiocontrol. This mode has been exemplified by the Pd(ii)-catalyzed aerobic oxidative Heck reaction of indoles, in which a ligand-controlled C3-/C2-selectivity was achieved for the first time by the development of sulfoxide-2-hydroxypyridine (SOHP) ligands.

Palladium(II)/Lewis Acid-Catalyzed Oxidative Olefination/Annulation of N-Methoxybenzamides: Identifying the Active Intermediates through NMR Characterizations

Although Pd(II)-catalyzed C-H activation in arenes has been widely successful in organic synthesis with many palladacycle compounds isolated as the intermediates in ligand-directed C-H activation, direct identification of the reaction intermediates such as the π-complex prior to the C-H activation is still not successful because of their instability. In the present study, we introduce a Pd(II)/LA (LA: Lewis acid)-catalyzed oxidative olefination/annulation reaction between N-methoxybenzamides and acrylates with oxygen as the oxidant source, in which two intermediates, including an unsymmetrical η⁶-complex and a palladacycle species without the proton releasing to the environment, were identified through NMR characterizations. The in situ formation of the heterobimetallic Pd(II)/LA species such as Pd(II)/Sc(III) may have enhanced the electrophilic properties of the Pd²⁺ cation, thus improving the stability of the π-complex, herein, an unsymmetrical η⁶-complex, and improving its catalytic efficiency. The observed insensitive electronic effect preferred the concerted metalation-deprotonation (CMD) mechanism for this C-H activation, and the detected palladacycle intermediate without the proton releasing to the environment offered an experimental clue to support the proposed CMD mechanism.

Density Functional Theory Study on the Mechanism of Iridium-Catalyzed Benzylamine ortho C-H Alkenylation with Ethyl Acrylate

Iridium-catalyzed oxidative o-alkenylation of benzylamines with acrylates was enabled by the directing group pentafluorobenzoyl (PFB). Density functional theory calculations were performed to explore the detailed reaction mechanism. The calculated results reveal that N-deprotonation prior to C-H activation is favored over direct C-H activation. Moreover, C-H activation is reversible and not the rate-determining step, which has been supported by the experimental observation. The regio- and stereoselectivity of ethyl acrylate insertion are controlled by the steric effect and the carbon atom with a larger orbital coefficient of the π* antibonding orbital in the nucleophilic attack, respectively. The migratory insertion of ethyl acrylate is computationally found to be rate-determining for the whole catalytic cycle. Finally, the seven-membered ring intermediate IM11 undergoes a sequential N-protonation and β-H elimination with the assistance of AcOH, rather than β-H elimination and reductive elimination proposed experimentally, to afford the o-alkenylated product. IM11 is unable to directly cyclize through C-N reductive elimination because both sp³-hybridized N and C atoms are unfavorable for N-C reductive elimination. The origin of the directing group PFB preventing the product and intermediates undergoing aza-Michael addition has been explained.

Electrochemical Synthesis of Bisindolylmethanes from Indoles and Ethers

An electrochemical bisindolylation of ethers was developed. Carried out under ambient conditions and in the absence of any chemical oxidants, this reaction exhibits a broad substrate scope and good functional group compatibility.

Incorporation of redox-inactive cations promotes iron catalyzed aerobic C-H oxidation at mild potentials

The synthesis and characterization of the Schiff base complexes Fe(ii) (2M) and Fe(iii)Cl (3M), where M is a K+ or Ba2+ ion incorporated into the ligand, are reported. The Fe(iii/ii) redox potentials are positively shifted by 440 mV (2K) and 640 mV (2Ba) compared to Fe(salen) (salen = N,N'-bis(salicylidene)ethylenediamine), and by 70 mV (3K) and 230 mV (3Ba) compared to Fe(Cl)(salen), which is likely due to an electrostatic effect (electric field) from the cation. The catalytic activity of 3M towards the aerobic oxidation of allylic C-H bonds was explored. Prior studies on iron salen complexes modified through conventional electron-donating or withdrawing substituents found that only the most oxidizing derivatives were competent catalysts. In contrast, the 3M complexes, which are significantly less oxidizing, are both active. Mechanistic studies comparing 3M to Fe(salen) derivatives indicate that the proximal cation contributes to the overall reactivity in the rate determining step. The cationic charge also inhibits oxidative deactivation through formation of the corresponding Fe2-μ-oxo complexes, which were isolated and characterized. This study demonstrates how non-redox active Lewis acidic cations in the secondary coordination sphere can be used to modify redox catalysts in order to operate at milder potentials with a minimal impact on the reactivity, an effect that was unattainable by tuning the catalyst through traditional substituent effects on the ligand.

A ruthenium-catalyzed alkenylation–annulation approach for the synthesis of indazole derivatives via C–H bond activation

A new approach to indazole derivatives through a Ru(ii)-catalyzed C–H activation–annulation reaction, which proceeds via C–C and C–N bond forming reactions, is reported.

Transformation of Unsaturated Fatty Acids/Esters to Corresponding Keto Fatty Acids/Esters by Aerobic Oxidation with Pd(II)/Lewis Acid Catalyst

Utilization of renewable biomass to partly replace the fossil resources in industrial applications has attracted attention due to the limited fossil feedstock with the increased environmental concerns. This work introduced a modified Wacker-type oxidation for transformation of unsaturated fatty acids/esters to the corresponding keto fatty acids/esters, in which Cu²⁺ cation was replaced with common nonredox metal ions, that is, a novel Pd(II)/Lewis acid (LA) catalyst. It was found that adding nonredox metal ions can effectively promote Pd(II)-catalyzed oxidation of unsaturated fatty acids/esters to the corresponding keto fatty acids/esters, even much better than Cu²⁺, and the promotional effect is highly dependent on the Lewis acidity of added nonredox metal ions. The improved catalytic efficiency is attributed to the formation of heterobimetallic Pd(II)/LA species, and the oxidation mechanism of this Pd(II)/LA catalyst is also briefly discussed.

Computational Studies of Carboxylate-Assisted C-H Activation and Functionalization at Group 8-10 Transition Metal Centers

Computational studies on carboxylate-assisted C-H activation and functionalization at group 8-10 transition metal centers are reviewed. This Review is organized by metal and will cover work published from late 2009 until mid-2016. A brief overview of computational work prior to 2010 is also provided, and this outlines the understanding of carboxylate-assisted C-H activation in terms of the "ambiphilic metal-ligand assistance" (AMLA) and "concerted metalation deprotonation" (CMD) concepts. Computational studies are then surveyed in terms of the nature of the C-H bond being activated (C(sp²)-H or C(sp³)-H), the nature of the process involved (intramolecular with a directing group or intermolecular), and the context (stoichiometric C-H activation or within a variety of catalytic processes). This Review aims to emphasize the connection between computation and experiment and to highlight the contribution of computational chemistry to our understanding of catalytic C-H functionalization based on carboxylate-assisted C-H activation. Some opportunities where the interplay between computation and experiment may contribute further to the areas of catalytic C-H functionalization and applied computational chemistry are identified.

Pd-Catalyzed Aerobic Oxidative Biaryl Coupling: Non-Redox Cocatalysis by Cu(OTf)2 and Discovery of Fe(OTf)3 as a Highly Effective Cocatalyst

Copper salts find widespread use in Pd-catalyzed oxidation reactions, and they are typically used as oxidants or redox-active cocatalysts. Here, we probe the origin of a dramatic acceleration effect of Cu(OTf)₂ in the C-H/C-H aerobic oxidative coupling of o-xylene. NMR spectroscopic analysis of the Pd^II catalyst in the presence of Cu(OTf)₂, together with other experimental and DFT computational studies of the catalytic reaction, show that Cu(OTf)₂ activates the Pd^II catalyst for C-H activation via a non-redox pathway and has negligible impact on catalyst reoxidation. These observations led to the testing of other metal triflate salts as cocatalysts, the results of which show that Fe(OTf)₃ is even more effective than Cu(OTf)₂.