Manganese/NaOPh co-catalyzed C2-selective C–H conjugate addition of indoles to α,β-unsaturated carbonyls

Manganese(I)-Catalyzed Direct Addition of C(sp3)-H Bonds to Aryl Isocyanates

The addition of C-H bonds to isocyanates catalyzed by transition metals is a highly auspicious methodology for providing synthetically and biologically important amides. However, the substrates are limited to C(sp2)-H bonds. In this work, an efficient manganese(I)-catalyzed direct addition reaction of C(sp3)-H bonds of 8-methylquinolines to aryl isocyanates has been developed, leading to the synthesis of various α-quinolinyl amide compounds in moderate to high yields. The reaction has a broad range of substrates and a good functional group tolerance. A possible mechanism is proposed based on the experimental results.

Manganese(I)-catalyzed nucleophilic addition of C(sp3)-H bonds to aldehydes

The C-H bond activation catalyzed by a manganese(I) complex has achieved significant development but is limited to C(sp2)-H bonds. In this work, an efficient manganese(I)-catalyzed direct nucleophilic addition reaction of C(sp3)-H bonds to aromatic aldehydes has been developed. This is the first example of manganese(I)-catalyzed C(sp3)-H bond transformation. A manganacycle complex was isolated and proved to be the key active intermediate in the catalytic cycle.

Efficient and chemoselective imine synthesis catalyzed by a well-defined PN3-manganese(II) pincer system

The highly efficient reductive amination of aldehydes with ammonia (NH3) and hydrogen (H2) to form secondary imines is described, as well as the dehydrogenative homocoupling of benzyl amines. Using an air-stable, well-defined PN3-manganese(II) pincer complex as a catalyst precursor, various aldehydes are easily converted directly into secondary imines using NH3 as a nitrogen source under H2 in a one-pot reaction. Importantly, the same catalyst facilitates the dehydrogenative homocoupling of various benzylamines, exclusively forming imine products. These reactions are conducted under very mild conditions, without the addition of any additives, yielding excellent selectivities and high yields of secondary imines in a green manner by minimizing wastes.

Origin of Ligand and Acid Effects on the Pd-Catalyzed Regiodivergent Coupling Reaction of Indazoles and Isoprene: A DFT Study

Comprehensive computational studies were carried out to explore the mechanisms and origin of regioselectivity in the Pd-catalyzed regiodivergent coupling reaction of indazoles and isoprene. Three different insertion models were investigated for regioselectivity 1,2- or 4,3-insertion with respect to the electrophilic sites on isoprene under two different ligands (L1 and L2) and acids ((PhO)2PO2H, (nBuO)2PO2H) via PdII-H species, allyl-π-PdII-O, and indazoles-acid-assisted PdII-π-allyl. The calculated results show that the PdII-π-allyl coordinated mechanism is the most preferred one. The noncovalent interactions between the less bulky ligand L1, substrates, and (PhO)2PO2- are found to be key factors for 1,2-insertion. The 4,3-insertion selectivity is primarily controlled by the steric repulsion of the t-Bu group of bulky ligand L2 and substrate, as well as the geometry distortion. Therefore, the regioselectivity difference of the 1,2- and 4,3-insertion on electrophilic sites is controlled by the synergistic effect of ligands and acids instead of the size of the ligand. Similarly, nucleophilic site selectivity at N1 or N2 of indazoles is governed by cooperative acid BF3 and solvent iPrOH rather than BF3 alone. Overall, our findings might open a new avenue for designing more efficient regioselective 1,2- or 4,3-addition or N1-/N2-selective nucleophilic reactions.

Manganese-catalyzed hydroarylation of multiple bonds

Transition metal-catalyzed C-H activation has become a promising strategy in organic synthesis due to its improved atom-, step- and resource economy. Considering the Earth's abundance, economic benefits, and low toxicity, 3d metal catalysts for C-H activation have received a significant focus. In particular, organometallic manganese-catalyzed C-H activation has proven to be versatile and suitable for a wide range of transformations such as C-H addition to π-components, arylation, alkylation, alkynylation, amination, and many more. Among them, manganese-catalyzed C-H addition to C-C and C-heteroatom multiple bonds exhibited unique and promising reactivity to construct a wide range of complex organic molecules. In this review, we highlight the developments in the field of manganese-catalyzed hydroarylation of multiple bonds via C-H activation with a range of applications until August 2022.

A manganese(i)tricarbonyl-catalyst for near room temperature alkene and alkyne hydroarylation

Developing more efficient catalytic processes using abundant and low toxicity transition metals is key to enable their mainstream use in synthetic chemistry. We have rationally designed a new Mn(i)-catalyst for hydroarylation reactions that displays much improved catalytic activity over the commonly used MnBr(CO)5. Our catalyst, MnBr(CO)3(MeCN)2, avoids the formation of the off-cycle manganacycle-(CO)4 species responsible for low catalyst activity, allowing near room temperature hydroarylation of alkenes and alkynes with broad functional group tolerance including late stage functionalisation and diversification of bioactive molecules.

Homogeneous Manganese-Catalyzed Hydrofunctionalizations of Alkenes and Alkynes: Catalytic and Mechanistic Tendencies

In recent years, many manganese-based homogeneous catalytic precursors have been developed as powerful alternatives in organic synthesis. Among these, the hydrofunctionalizations of unsaturated C-C bonds correspond to outstanding ways to afford compounds with more versatile functional groups, which are commonly used as building blocks in the production of fine chemicals and feedstock for the industrial field. Herein, we present an account of the Mn-catalyzed homogeneous hydrofunctionalizations of alkenes and alkynes with the main objective of finding catalytic and mechanistic tendencies that could serve as a platform for the works to come.

Selective Labeling of Peptides with o-Carboranes via Manganese(I)-Catalyzed C-H Activation

A robust method for the selective labeling of peptides via manganese(I) catalysis was devised to achieve the C-2 alkenylation of tryptophan containing peptides with 1-ethynyl-o-carboranes. The manganese-catalyzed C-H activation was accomplished with high catalytic efficiency, and featured low toxicity, high functional group tolerance and excellent E-stereoselectivity. This approach unravels a promising tool for the assembly of o-carborane with structurally complex peptides of relevance to applications in boron neutron capture therapy.

TfOH-catalyzed regioselective N2-alkylation of indazoles with diazo compounds

Selective alkylation of indazoles is still a highly challenging topic in chemistry and the synthesis of important molecules. Herein, a novel highly selective N²-alkylation of indazoles with diazo compounds is described in the presence of TfOH. Unlike the traditional metal- and base-catalysed version, this protocol highlights the regioselectivity of alkylation of indazoles and a metal-free catalysis system, affording N²-alkylated products in good to excellent yields with high regioselectivity (N²/N¹ up to 100/0) and excellent functional group tolerance. Furthermore, a gram scale synthesis was conducted successfully to give rise to the corresponding products. Mechanistic studies through control experiments provide plausible mechanistic proposals.

2-Alkylation of 3-Alkyindoles With Unactivated Alkenes

An acid-catalyzed 2-alkylation of indole molecules is developed. Only catalytic amount of the commercially available, inexpensive and traceless HI is used as the sole reaction promoter. 2,3-Disubstituted indole molecules bearing congested tertiary carbon centers are afforded as the final products in moderate to excellent yields.

Constructing a multienzyme cascade redox-neutral system for the synthesis of halogenated indoles

Inspired by biocatalytic retrosynthesis, a multienzyme cascade system containing alcohol dehydrogenase, flavin-dependent halogenase and flavin reductase was developed for the synthesis of several halogenated indoles starting from aminoalcohol. This redox-neutral...

Manganese-Catalyzed [4 + 2] Annulation of N-H Amidines with Vinylene Carbonate via C-H Activation

Manganese-catalyzed C-H bond functionalization of aryl amidines for the synthesis of 1-aminoisoquinolines in the presence of vinylene carbonate has been developed. The reaction features a broad substrate scope and proceeds under mild reaction conditions with only the carbonate anion as the byproduct.

Iridium-Catalyzed Redox-Neutral C2 and C3 Dual C-H Functionalization of Indoles with Nitrones toward 7H-Indolo[2,3-c]quinolines

An iridium-catalyzed redox-neutral C-2 and C-3 dual C-H functionalization of indoles with nitrones has been developed, furnishing a range of 7H-indolo[2,3-c]quinolines with high efficiency and regioselectivity under mild reaction conditions. Notably, sequential multiple C-H bond cleavage and C-C bond formation constitute the key events of this process, in which nitrone serves as a building block and an oxidant. Distinct from the previous methods toward 7H-indolo[2,3-c]quinolines, this newly developed reaction features readily available substrates, operational simplicity, broad scope, good to high efficiency, and excellent regioselectivity.

Solvent-Switched Manganese(I)-Catalyzed Regiodivergent Distal vs Proximal C-H Alkylation of Imidazopyridine with Maleimide

A sustainable Mn(I)-catalyzed exclusive solvent-dependent functionalization of imidazopyridine with maleimide via an electrophilic metalation at the distal (in 2,2,2-trifluoroethanol (TFE)) and chelation assisted at the proximal (in tetrahydrofuran (THF)) has been developed. The strategy was successfully applied to the drug Zolimidine and a broad range of substrates, thereby reflecting the method's versatility.

Acyl silane directed Cp*Rh(III)-catalysed alkylation/annulation reactions

Studies into the Cp*Rh(iii)-catalysed hydroarylation of alkenes with aryl acyl silanes led to the discovery of a new synthetic strategy to access unique silicon derived indene frameworks. Rather than protodemetalation of the metal enolate formed following insertion of an alkene into the aryl C-H bond, intramolecular aldol condensation of the acyl silane occurred to generate a series of 2-formyl- and 2-acetyl-3-silyl indenes. This represents only the second example of rhodium-catalysed C-H functionalisation employing acyl silanes as weakly coordinating directing groups and the intramolecular aldol condensation strategy was extended to access analogous silicon derived benzofurans.

Cascade Reaction to Selectively Synthesize Multifunctional Indole Derivatives by IrIII -Catalyzed C-H Activation

An effective and condition-controlled way to synthesize with high selectivity a variety of functionalized indoles with potent biological properties has been developed. Notably, 2,4-dialkynyl indole products were obtained by direct double C-H bond alkynylation, whereas alkynyl at the C4 position could convert to carbonyl to generate 2-alkynyl-3,4-diacetyl indoles fast and effectively. Additionally, a one-pot relay catalytic reaction led to 2,5-di-alkynyl-3,4-diacetyl indoles when using a carbonyl group as the directing group and by controlling the type and quantity of additives. A possible mechanism was proposed based on many studies including deuterium-exchange experiments, the necessary conditions of product conversion, and the effect of water on the reaction.

Manganese-Catalyzed [3 + 2] Cyclization of Ketones and Isocyanates via Inert C-H Activation

Stoichiometric cyclomanganation of aromatic ketones and further reactions of the thus-formed manganacycles with isocyanates were first reported by Kaesz and Liebeskind in 1975 and 1990, respectively. The buildup of a closed manganese catalytic cycle for the reaction of ketones and isocyanates remains an unsolved problem. Herein, an unprecedented trio of Me₂Zn/AlCl₃/AgOTf is developed to build up manganese catalysis, which enables the [3 + 2] cyclization of ketones with isocyanates via inert C-H activation to access 3-alkylidene phthalimidines in a straightforward manner unachieved by other transition metal catalyses.

Manganese-Catalyzed C(sp2)–H Addition to Polar Unsaturated Bonds

Transition-metal-catalyzed nucleophilic C–H addition of hydrocarbons to polar unsaturated bonds could intrinsically avoid prefunctionalization of substrates and formation of waste byproducts, thus featuring high step- and atom-economy. As the third most abundant transition metal, manganese-catalyzed C–H addition to polar unsaturated bonds remains challenging, partially due to the difficulty in building a closed catalytic cycle of manganese. In the past few years, we have developed manganese catalysis to enable the sp2-hydrid C–H addition to polar unsaturated bonds (e.g., imines, aldehydes, nitriles), which will be discussed in this personal account.

1 Introduction

2 Mn-Catalyzed N-Directed C(sp2)–H Addition to Polar Unsaturated Bonds

3 Mn-Catalyzed O-Directed C(sp2)–H Addition to Polar Unsaturated Bonds

4 Conclusion