Versatile Cp*Co(III)(LX) Catalyst System for Selective Intramolecular C–H Amidation Reactions

Asymmetric Catalytic Aziridination to Synthesize Spiro-aziridine Oxindoles

Asymmetric catalytic aza-Michael-initiated ring closure of methyleneindolinones with N-tosyloxycarbamates has been established. The reaction using a chiral nickel complex catalyst enabled the formation of a series of spiro-aziridine oxindoles in good yields (up to 99 %) with high stereoselectivity (up to 97 % ee, >19 : 1 dr) under mild reaction conditions. Ring-opening of spiro-aziridine oxindole leads to formation of glycinate-bearing oxindoles with retention of configuration.

Catalytic amounts of sodium-sulfonate-naphthol enable mechanically robust, ultra-transparent, super-fire-resistant and easily recyclable polycarbonate

Polycarbonate is an advanced engineering plastic widely used in aerospace, high-speed rail and 5G communications. However, it remains a huge challenge to synthesize polycarbonate materials using a strategy that simultaneously integrates green-preparation, service-stage advanced-performance and end-of-life easy-recyclability. Herein, we propose an ultrahigh-efficiency and green halogen/phosphorus-free strategy to prepare a mechanically robust, highly transparent, super-fire-resistant and chemically easily recyclable polycarbonate plastic. By chemical copolymerization of only catalytic amounts of sodium sulfonate-naphthol (0.3-0.5 mol%, namely 3400-5600 ppm), the corresponding polycarbonates exhibit >85 MPa tensile strength, >67 kJ m-2 notched impact strength, >90% transparency, >36% ultra-high limiting oxygen index and 1.6 mm thin-wall UL-94 V-0 rating during the service-stage. Especially, at the end-of-life, these polycarbonates can be easily depolymerized back to the raw monomer bisphenol A and high-value 2-oxazolidinone under mild conditions (50 °C for 4 h), achieving ultra-high atom-economic chemical recycling. Starting from the source of a chemical structure, this work opens up a new perspective for constructing life cycle-managed plastic materials with advanced high-performance and full-recyclability, contributing to the global circular economy through sustainable material design.

C-H amination of enolizable and nonenolizable ketones

We present a method for the amination of enolizable and non-enolizable ketones in the alpha (or beta) position to the carbonyl group. This approach is based on the conversion of the corresponding cyanohydrins to carbonazidates, precursors for thermal intramolecular nitrene insertion reactions into the adjacent C-H bond. Hydrolysis of the resulting carbamates under basic conditions with simultaneous regeneration of the carbonyl group yields amino ketones.

Attaining the Utmost Stability and Energy of Carbonyl Azides by the Synergistic Improvement of Conjugation and H-bonding

Carbonyl azides are important precursors to isocyanates and are used as energetic compounds. However, the further development of these compounds is limited by their inherently poor stability. In this study, we present a new family of carbonyl azides, 5-nitro-1H-1,2,4-triazol-3-yl-carbamoyl-azide (NTCA), which was synthesized through in situ oxidation cleavage of amino-tetrazole. Compared with its precursor (nitrocarbamoyl azide, HNCA), X-ray data and quantum calculations indicate that NTCA has much stronger conjugation (dihedral angle decreased from 13.39° to 1.35°) and more H-bonds (increase from 2 to 7 pairs). As a result, NTCA exhibits the highest thermal stability (decomposition temperature of 212 °C) and highest density (1.820 g cm-3) among all known carbonyl azides. In addition, a series of Curtius rearrangements were performed to generate substituted ionic derivatives, which also exhibit high stability and energy. This study provides an effective strategy for synthesizing carbonyl azides with high stability and energy, paving the way for future practical applications.

Strategies for Accessing cis-1-Amino-2-Indanol

cis-1-amino-2-indanol is an important building block in many areas of chemistry. Indeed, this molecule is currently used as skeleton in many ligands (BOX, PyBOX…), catalysts and chiral auxiliaries. Moreover, it has been incorporated in numerous bioactive structures. The major issues during its synthesis are the control of cis-selectivity, for which various strategies have been devised, and the enantioselectivity of the reaction. This review highlights the various methodologies implemented over the last few decades to access cis-1-amino-2-indanol in racemic and enantioselective manners. In addition, the various substitution patterns on the aromatic ring and their preparations are listed.

Dihydroguaiazulenide Complexes and Catalysts of Group 8-12 Transition Metals: Ligands from Renewable Feedstock Replace, even Outmatch Petrochemical Based Cyclopentadienyl Chemistry

We present an in-depth study of the sterically demanding Cp-synthon (8-H-GuaH)Li isolated from natural product guaiazulene (Gua) as a ligand transfer reagent towards late transition metal complex precursors. The synthesis and full characterization of selected, essentially unexplored homo- and heteroleptic 8-H-guaiazulenide complexes of iron, ruthenium, cobalt, rhodium, platinum, copper and zinc are discussed in detail. In order to demonstrate their potential in catalytic applications, [(GuaH)PtMe3 ] was selected. The latter proved an even higher catalytic activity in light induced olefin hydrosilylation at catalyst loads as low as 5 ppm than classical [CpPtMe3 ] in a typical test reaction of silicone elastomer fabrication. Our results demonstrate that traditional petrochemical based Cp metal chemistry and catalysis can be replaced, sometimes even outmatched by superior catalysts based on cheap building blocks from renewable feedstock.

Annulative π-Extension by Cp*Co(III)-Catalyzed Ketone-Directed peri-Annulation: An Approach to Access Fused Arenes

A masked-bay-region selective first-row transition-metal Cp*Co(III)-catalyzed annulative π-extension of arene-derived ketones is achieved to afford K-region-functionalized benzo[e]pyrenes, benzotetraphenes, and pyrenes. Comprehensive density functional theory studies buttress the mechanistic pathway comprising key steps like peri-C-H activation, alkyne 1,2-migratory insertion, and nucleophilic attack toward ketone, this attack being the rate-determining step. In addition, π-conjugated 1,1'-bipyrenes, potential photocatalyst pyrene-quinones, and putative n-type semiconductor cyano group-containing dibenzo[de,qr]tetracenes are also accessed.

Cobalt(III)-Catalyzed Directed C-7 Selective C-H Alkynylation of Indolines with Bromoalkynes

A cobalt(III)-catalyzed directed C-7 alkynylation of indolines with easily accessible bromoalkynes has been developed. The reaction has a broad substrate scope with excellent yields and represents a powerful route to the synthesis of 7-alkynyl-substituted indolines. In addition, the reaction can be extended to the coupling of N-aryl 7-azaindoles, highlighting the synthetic practicability of the strategy.

Chemical insights into the synthetic chemistry of five-membered saturated heterocycles-a transition metal-catalyzed approach

Drug design and delivery is primarily based on the hunt for new potent drug candidates and novel synthetic techniques. Recently, saturated heterocycles have gained enormous attention in medicinal chemistry as evidenced by the medicinal drugs listed in the FDA Orange Book. Therefore, the demand for novel saturated heterocyclic syntheses has increased tremendously. Transition metal (TM)-catalyzed reactions have remained the prime priority in heterocyclic syntheses for the last three decades. Nowadays, TM catalysis is well adorned by combining it with other techniques such as bio- and/or enzyme-catalyzed reactions, organocatalysis, or using two different metals in a single catalysis. This review highlights the recent developments of the transition metal-catalyzed synthesis of five-membered saturated heterocycles.

Comprehensive Theoretical Study of Cp*IrIII-Catalyzed Intermolecular Enantioselective Allylic C-H Amidation: Reaction Mechanism, Electronic Processes, and Regioselectivity

Density functional theory was used to elucidate the reaction mechanism of Cp*Ir^III-catalyzed intermolecular regioselective C(sp³)-H amidation of alkenes with methyl dioxazolones. All substrates, intermediates, and transition states were fully optimized at the ωB97XD/6-31G(d,p) level (LANL2DZ(f) for Ir). The computational results revealed that this amidation occurred through the Ir^III/Ir^V catalytic cycle, involving four important elementary steps: C-H bond activation, oxidative addition of methyl dioxazolone, reductive elimination, and proto-demetalation, and the first was the rate-determining step. The C-H bond activation showed good α- and branch-regioselectivity, decided by the distortion energy of 2-pentene and the interaction energy of the transition state, respectively. The oxidative addition of dioxazolone occurred in one elementary step with CO₂ disassociation. The reductive elimination showed good branch-regioselectivity determined by the distorted energy of the allyl group. In the proto-demetalation, hydrogen directly transferred from the oxygen atom to the nitrogen atom. Moreover, to clarify the effect of the substituted groups, selected 12 substrates were also discussed in this text.

The recent advances in cobalt-catalyzed C(sp3)-H functionalization reactions

Over the past decades, reactions involving C-H functionalization have become a hot theme in organic transformations because they have a lot of potential for the streamlined synthesis of complex molecules. C(sp³)-H bonds are present in most organic species. Since organic molecules have massive significance in various aspects of life, the exploitation and functionalization of C(sp³)-H bonds hold enormous importance. In recent years, the first-row transition metal-catalyzed direct and selective functionalization of C-H bonds has emerged as a simple and environmentally friendly synthetic method due to its low cost, unique reactivity profiles and easy availability. Therefore, research advancements are being made to conceive catalytic systems that foster direct C(sp³)-H functionalization under benign reaction conditions. Cobalt-based catalysts offer mild and convenient reaction conditions at a reasonable expense compared to conventional 2^nd and 3^rd-row transition metal catalysts. Consequently, the probing of Co-based catalysts for C(sp³)-H functionalization is one of the hot topics from the outlook of an organic chemist. This review primarily focuses on the literature from 2018 to 2022 and sheds light on the substrate scope, selectivity, benefits and limitations of cobalt catalysts for organic transformations.

Csp2-H Amination Reactions Mediated by Metastable Pseudo-Oh Masked Aryl-CoIII-nitrene Species

Cobalt-catalyzed C–H amination via M-nitrenoid species is spiking the interest of the research community. Understanding this process at a molecular level is a challenging task, and here we report a well-defined macrocyclic system featuring a pseudo-O_h aryl-Co^III species that reacts with aliphatic azides to effect intramolecular C_sp2–N bond formation. Strikingly, a putative aryl-Co=NR nitrenoid intermediate species is formed and is rapidly trapped by a carboxylate ligand to form a carboxylate masked-nitrene, which functions as a shortcut to stabilize and guide the reaction to productive intramolecular C_sp2–N bond formation. On one hand, several intermediate species featuring the C_sp2–N bond formed have been isolated and structurally characterized, and the essential role of the carboxylate ligand has been proven. Complementarily, a thorough density functional theory study of the C_sp2–N bond formation mechanism explains at the molecular level the key role of the carboxylate-masked nitrene species, which is essential to tame the metastability of the putative aryl-Co^III=NR nitrene species to effectively yield the C_sp2–N products. The solid molecular mechanistic scheme determined for the C_sp2–N bond forming reaction is fully supported by both experimental and computation complementary studies.

A well-defined pseudo-O_h aryl-Co^III species reacts with aliphatic azides to effect intramolecular C_sp2−N bond formation via a carboxylate masked-Co^III-nitrene, which serves as a shortcut to guide the reaction to productive C_sp2−N bond formation.

Alcohol-Incorporating Diels-Alder Dimerization of In Situ Formed ortho-Quinamine via Co-Nitrenoid Insertion

We disclose herein a Cp*Co(III)(LX)-catalyzed dearomative Diels-Alder dimerization of 2,6-disubstituted phenyl azidoformates. Upon the postulated cobalt-nitrenoid insertion into the neighboring ortho-carbon, the key intermediate of ortho-quinamine was generated for the subsequent dimeric cycloaddition process. A series of experimental and computational studies suggested that the quinolinol ligand of the cobalt catalyst plays a crucial role in the alcoholic solvent incorporation into the o-quinamine moiety, thereby enabling the Diels-Alder dimerization to furnish the bridged tricyclic bisamidation products.

Visible-light-induced phosgenation of amines by chloroform oxygenation using chlorine dioxide

We report the visible-light-induced in situ preparation of COCl₂ through the oxygenation of chloroform in the presence of chlorine dioxide, which leads to the safe constructions of carbamoyl chlorides with good-to-high yields and wide substrate scopes. In addition, this method can also be applied to the synthesis of various carbonates.

Iridium-Catalyzed Amidation of In Situ Prepared Silyl Ketene Acetals to Access α-Amino Esters

Disclosed herein is a convenient Ir-catalyzed amidation of esters to access α-amido esters. Initially prepared silyl ketene acetals are directly employed, without separate purification, for subsequent amidation with an oxycarbonylnitrenoid precursor using the Cp*(LX)Ir(III) catalyst. The α-amidation was facile for both α-aryl and α-alkyl esters. Density functional theory studies revealed that the generation of a putative Ir-nitrenoid is facilitated by the chelation of the countercation additive during the N-O bond cleavage of the nitrene precursor.

Cp*Co(III)-catalyzed thiocarbamate directed C−H aminocarbonyl-ation, amination, and cascade annulation of pyrroles

Cobalt(III)-catalyzed thiocarbamate directed aminocarbonylation and amination of C−H bond are described to access diverse amides. Biologically relevant pyrrolo[1,2-c]imidazoles were readily accessed via one-pot intramolecular cyclization at thiocarbamoyl directing group. Notably,...

Rh(III)-Catalysed Cascade C-H Imidization/Cyclization of N-Methoxybenzamides with Isoxazolones for the Assembly of Dihydroquinazolin-4(1H)-one Derivatives

By virtue of isoxazolones as viable imidizating reagents, an efficient Rh(III)-catalysed redox-neutral C-H imidization/cyclization cascade has been developed for the specific assembly of dihydroquinazolin-4(1H)-ones with the equipment of a quaternary...

Cobalt-Nitrenoid Insertion-Mediated Amidative Carbon Rearrangement via Alkyl-Walking on Arenes

We herein disclose the Cp*Co(III)(LX)-catalyzed amidative alkyl migration using 2,6-disubstituted phenyl azidoformates. Upon the cobalt-nitrenoid insertion toward the substituted ortho carbon, an arenium cationic species bearing a quaternary carbon is generated, and a subsequent alkyl migration process is suggested to occur through an unforeseen alkyl-walking mechanism. A quinolinol ligand of the cobalt catalyst system is proposed to facilitate the final product-releasing rearomatization process by serving as an internal base. This new mechanistic mode enabled both [1,2]- and [1,4]-alkyl rearrangements to allow the structural variation of N-heterocyclic compounds.

Homogeneous catalytic C(sp3)-H functionalization of gaseous alkanes

The conversion of light alkanes into bulk chemicals is becoming an important challenge as it effectively avoids the use of prefunctionalized alkylating reagents. The implementation of such processes is, however, hampered by their gaseous nature and low solubility, as well as the low reactivity of the C-H bonds. Efforts have been made to enable both polar and radical processes to activate these inert compounds. In addition, these methodologies also benefit significantly from the development of a suitable reactor technology that intensifies gas-liquid mass transfer. In this review, we critically highlight these developments, both from a conceptual and a practical point of view. The recent expansion of these mechanistically-different methods have enabled the use of various gaseous alkanes for the development of different bond-forming reactions, including C-C, C-B, C-N, C-Si and C-S bonds.

Cobalt-Catalyzed Intermolecular C-H Amidation of Unactivated Alkanes

Alkanes are an abundant and inexpensive source of hydrocarbons; thus, development of new methods to convert the hydrocarbon feedstocks to value-added chemicals is of high interest. However, it is challenging to achieve such transformation in a direct and selective manner mainly due to the intrinsic inertness of their C-H bonds. We herein report a tailored Cp*Co(III)(LX)-catalyzed efficient and site-selective intermolecular amidation of unactivated hydrocarbons including light alkanes. Electronic modulation of the cobalt complexes led to the enhanced amidation efficiency, and these effects were theoretically rationalized by the FMO analysis of presupposed cobalt nitrenoid species. Under the current cobalt protocol, a secondary C-H bond selectivity was observed in various nonactivated alkanes to reverse the intrinsic tertiary preference, which is attributed to the steric demands of the cobalt system that imposes difficulties in accessing tertiary C-H bonds. Experimental and computational studies suggested that the putative triplet Co nitrenoids are transferred to the C-H bonds of alkanes via a radical-like hydrogen abstraction pathway.

Nitrocarbamoyl Azide O2NN(H)C(O)N3: A Stable but Highly Energetic Member of the Carbonyl Azide Family

The diazotization of nitrosemicarbazide (1) resulted in the formation and isolation of nitrocarbamoyl azide (2), which was thoroughly characterized by spectroscopic and structural methods. This compound shows surprising stability but also high reactivity and sensitivity, with a melting point of 72 °C and a detonative decomposition point at 83 °C. In addition, five selected salts were synthesized by careful deprotonation. The decomposition mechanism of 2 in solution was investigated and could be clarified by performing experiments using methanol and hydrazine as trapping reagents. The energetic and physicochemical properties of all these compounds were investigated and classified.

Electrostatic repulsion-controlled regioselectivity in nitrene-mediated allylic C–H amidations

The difference of repulsive electrostatic interactions between nitrene N and allyl carbon atoms is the dominant factor affecting the regioselectivity in metal nitrenoid-catalyzed allylic C–H amidations.

Iron-Catalyzed Regiodivergent Hydrostannation of Alkynes: Intermediacy of Fe(IV)-H versus Fe(II)-Vinylidene

We report an iron system, Cp*Fe(1,2-R₂PC₆H₄X), which controls the Markovnikov and anti-Markovnikov hydrostannation of alkynes by tuning the ionic metal-heteroatom bonds (Fe-X) reactivity. The sequential addition of nBu₃SnH to the iron-amido catalyst (1, X = HN^-, R = Ph) affords a distannyl Fe(IV)-H species responsible for syn-addition of the Sn-H bond across the C≡C bond to produce branched α-vinylstannanes. Activation of the C(sp)-H bond of alkynes by an iron-aryloxide catalyst (2, X = O^-, R = Cy) affords an iron(II) vinylidene intermediate, allowing for gem-addition of the Sn-H to the terminal-carbon producing β-vinylstannanes. These catalytic reactions exhibit excellent regioselectivity and broad functional group compatibility and enable the large-scale synthesis of diverse vinylstannanes. Many new reactions have been established based on such a synthetic Fe-X platform to demonstrate that the initial step of the catalysis is conveniently controlled by the activation of either the tin hydride or the alkyne substrate.

Iron-catalysed 1,2-aryl migration of tertiary azides

1,2-Carbon to nitrogen aryl migration of α,α-diaryl tertiary azides was realized by using FeCl₂ and N-heterocyclic carbene SIPr·HCl as a catalyst.