Co(III)-catalyzed regioselective benzannulation of substituted pyridones with 1,6-diynes via dual C-H bond activation

A Co(III)-catalyzed site-selective C5 and C6 benzannulation of substituted pyridones with 1,6-diynes via dual C-H bond activation has been reported. The scope of the benzannulation reaction was examined with various substituted 2-pyridyl pyridones and 1,6-diynes. The combination of cuprous acetate and silver carbonate plays a crucial role in the success of the reaction. A plausible reaction mechanism was proposed and supported by deuterium labelling studies and radical trapping experiments.

Palladium-catalyzed site-selective functionalization of unactivated alkenes with vinylcyclopropanes aided by weakly coordinating native amides

Herein, we have demonstrated a palladium-catalyzed regioselective allylation of unactivated alkenes with vinylcyclopropanes assisted by weak-coordinating native amides. The reaction exhibits wide substrate scope and excellent β-selectivity. Substrate diversification was performed to demonstrate the synthetic utility of the reaction. Mechanistic investigations were carried out to provide an insight into the reaction mechanism.

Palladium-catalyzed (Z)-selective allylation of phosphine oxides with vinylethylene carbonates to construct phosphorus allyl alcohols

Allylphosphine oxide compounds are important building blocks with broad applications in organic synthesis and pharmaceutical science. Herein, we report an unprecedented palladium-catalyzed allylation of phosphine oxides with vinylethylene carbonates, producing various phosphorus allyl alcohols in excellent yields with high Z-selectivity. In addition, gram-scale synthesis and further functional group transformations demonstrate the practical utility of this synthetic method.

Late-Stage Diversification of Phosphines by C-H Activation: A Robust Strategy for Ligand Design and Preparation

ConspectusThe advent of the twenty-first century marked a golden era in the realm of synthetic chemistry, exemplified by groundbreaking advancements in the field of C-H activation, which is a concept that quickly transitioned from mere academic fascination to an essential element within the synthetic chemist's toolkit. This methodological breakthrough has given rise to a wealth of opportunities spanning a wide range of chemical disciplines. It has facilitated the late-stage diversification of elaborate organic frameworks, encompassing the spectrum from simple methane to complex polymers, thus refining the lead optimization process and easing the production of diverse molecular analogues. Among these strides forward, the development of phosphorus(III)-directed C-H activation stands out as an increasingly significant and inventive approach for the design and synthesis of ligands, substantially redefining the contours of synthetic methodology.Phosphines, renowned for their roles as ligands and organocatalysts, have become fundamentally important in modern organic chemistry. Their efficiency as ligands is significantly affected by coordination with transition metals, which is essential for their involvement in catalytic processes, influencing both the catalytic activity and the selectivity. Historically, the fabrication of phosphines predominantly relied on synthesis employing complex, multistep procedures. Addressing this limitation, our research has delved into ligand design and synthesis through innovative catalytic P(III)-directed C-H activation strategies. In this Account, we have explored a spectrum of procedures, including direct arylation using metal catalysis, and ventured further into domains such as C-H alkylation, alkenylation, aminocarbonylation, alkynylation, borylation, and silylation. These advances have enriched the field by providing efficient methods for the late-stage diversification of biaryl-type monophosphines as well as enabled the C-H activation of triphenylphosphine and its derivatives. Moreover, we have successfully constructed libraries of diverse axially chiral binaphthyl phosphine ligands, showcasing their potency in asymmetric catalysis. Through this Account, we aim to illuminate the exciting possibilities presented by P(III)-directed C-H activation in propelling the boundaries of organic synthesis. By highlighting our pioneering work, we hope to inspire further developments in this promising field of chemistry.

Atroposelective hydroarylation of biaryl phosphines directed by phosphorus centres

Prized for their ability to generate chemical complexity rapidly, catalytic carbon-hydrogen (C-H) activation and functionalization reactions have enabled a paradigm shift in the standard logic of synthetic chemistry. Directing group strategies have been used extensively in C-H activation reactions to control regio- and enantioselectivity with transition metal catalysts. However, current methods rely heavily on coordination with nitrogen and/or oxygen atoms in molecules and have therefore been found to exhibit limited generality in asymmetric syntheses. Here, we report enantioselective C-H activation with unsaturated hydrocarbons directed by phosphorus centres to rapidly construct libraries of axially chiral phosphines through dynamic kinetic resolution. High reactivity and enantioselectivity are derived from modular assembly of an iridium catalyst with an endogenous phosphorus atom and an exogenous chiral phosphorus ligand, as confirmed by detailed experimental and computational studies. This reaction mode significantly expands the pool of enantiomerically enriched functional phosphines, some of which have shown excellent efficiency for asymmetric catalysis.

Cp*Co(III)-Catalyzed Regioselective [4 + 2]-Annulation of N-Chlorobenzamides with Vinyl Acetate/Vinyl Ketones

An efficient and straightforward strategy for the synthesis of isoquinolones through [4 + 2]-annulation of N-chlorobenzamides with vinyl acetate in the presence of CoCp*(III) catalyst in a regioselective manner is described. Furthermore, the annulation reaction was diversified by using vinyl ketones. By utilizing this strategy, biologically valuable isoquinolone derivatives were prepared in good yields. Subsequently, isoquinolone derivatives were further transformed into 1-chloroisoquinolines in the presence of POCl₃. Furthermore, mechanistic investigations such as deuterium labeling study and competition experiment were performed to support the proposed reaction mechanism.

Cobalt(III)-Catalyzed Regioselective [4 + 2]-Annulation of N-Chlorobenzamides with Substituted Alkenes

A Co(III)-catalyzed redox-neutral [4 + 2] annulation of N-chlorobenzamides/acrylamides with substituted alkenes at ambient temperature is demonstrated. Using this protocol, pharmaceutically important 3,4-dihydroisoquinolinone derivatives were synthesized in good yields. Intriguingly, the synthetically useful functional group of allylic coupling partners such as sulfonyl, carbonate, acetate, phosphate, amide, nitrile, and silane were retained in the final cyclized product. The present annulation reaction was compatible with various substituted benzamides and allylic coupling partners. To support the proposed reaction mechanism, competition experiments, deuterium labeling studies, and kinetic isotope effect studies were performed.

Sterically Controlled Late-Stage Functionalization of Bulky Phosphines

The fine-tuning of metal-phosphine-catalyzed reactions relies largely on accessing ever more precisely tuned phosphine ligands by de-novo synthesis. Late-stage C-H functionalization and diversification of commercial phosphines offers rapid access to entire libraries of derivatives based on privileged scaffolds. But existing routes, relying on phosphorus-directed transformations, only yield functionalization of Csp 2 -H bonds in a specific position relative to phosphorus. In contrast to phosphorus-directed strategies, herein we disclose an orthogonal functionalization strategy capable of introducing a range of substituents into previously inaccessible positions on arylphosphines. The strongly coordinating phosphine group acts solely as a bystander in the sterically controlled borylation of bulky phosphines, and the resulting borylated phosphines serve as the supporting ligands for palladium during diversification through phosphine self-assisted Suzuki-Miyaura reactions.

Upgrading Carbazolyl-Derived Phosphine Ligands Using RhI-Catalyzed PIII-Directed C-H Bond Alkylation for Catalytic CO2-Fixation Reactions

We report an Rh(I)-catalyzed C-H bond alkylation of PhenCarPhos [N-(2-(diphenylphosphaneyl)phenyl)carbazole] and some congener phosphine ligands with alkenes. The C-H bond functionalization occurred exclusively at the C1 position of the carbazolyl unit because the trivalent phosphine acts as a directing group. This protocol provides straightforward access to a large library of C1-alkyl substituted PhenCarPhos, which outperformed common commercial or unfunctionalized phosphines and their precursors in the Pd-catalyzed carbon dioxide-fixation reactions with propargylic amines.

Rhodium-catalyzed selective direct arylation of phosphines with aryl bromides

The widespread use of phosphine ligand libraries is frequently hampered by the challenges associated with their modular preparation. Here, we report a protocol that appends arenes to arylphosphines to access a series of biaryl monophosphines via rhodium-catalyzed P(III)-directed ortho C-H activation, enabling unprecedented one-fold, two-fold, and three-fold direct arylation. Our experimental and theoretical findings reveal a mechanism involving oxidative addition of aryl bromides to the Rh catalyst, further ortho C-H metalation via a four-membered cyclometalated ring. Given the ready availability of substrates, our approach opens the door to developing more general methods for the construction of phosphine ligands.

Ru(II)- or Rh(III)-Catalyzed Annulation of Aromatic/Vinylic Acids with Alkylidenecyclopropanes via C-H Activation

An efficient and new route for the synthesis of (E)-4-benzylideneisochroman-1-ones through tandem cascade annulation of benzoic acids with alkylidenecyclopropanes using Ru(II) as a catalyst is demonstrated. It is important to note that the reaction delivers selectively E-stereoselective 4-benzylideneisochroman-1-one derivatives in moderate to good yields, which has completely diverse selectivity as compared with previous methods. Further, the annulation was explored with less-reactive β C-H activation of vinylic acids with alkylidenecyclopropanes, leading to the highly useful α-pyrone derivatives in the presence of an Rh(III) catalyst.

Cobalt(III)-Catalyzed Regio- and Chemoselective [4 + 2]-Annulation of N-Chlorobenzamides/Acrylamides with 1,3-Dienes at Room Temperature

Co(III)-catalyzed regio- and chemoselective redox-neutral C-H annulation of arylamides/acrylamides with 1,3-dienes is described. The present annulation reaction was well suited with a less-reactive 1,3-butadiene. By employing this protocol, pharmaceutically important 3,4-dihydroisoquinolinones were synthesized in good yields. Furthermore, the prepared 3,4-dihydroisoquinolinones were converted into highly important oxirane derivatives in good yields. A plausible mechanistic cycle is proposed and supported by a competition experiment and kinetic isotopic effect (KIE) studies.

Cobalt(III)-Catalyzed Chemo- and Regioselective [4 + 2]-Annulation of Aromatic Sulfoxonium Ylides with 1,3-Diynes

Air-stable, highly abundant, and cost-effective Co(III)-catalyzed redox-neutral [4 + 2]-annulation of aromatic sulfoxonium ylides with 1,3-diynes providing useful substituted 1-naphthol derivatives in a regioselective manner is described. Further, the prepared 1-naphthols having internal alkyne were converted into useful polycarbocyclic molecules and spiro-dienone derivatives in good-to-excellent yields. A possible reaction mechanism involving ortho C-H activation as a key step was proposed and supported by deuterium labeling and kinetic isotope labeling studies.

The cobalt(ii)-catalyzed acyloxylation of picolinamides with bifunctional silver carboxylate via C–H bond activation

The cobalt(ii)-catalyzed C–H bond acyloxylation of picolinamides with bifunctional silver carboxylate has been developed. The mild and practical esterification provides an atom-economic route to access to polysubstituted naphthalene compounds.

Ruthenium–NHC complex-catalyzed P(iii)-directed C–H borylation of arylphosphines

Bidentate NHC-based ruthenium catalyst for P(III)-directed ortho C–H borylation of arylphosphines.

BBr3 -Mediated P(III)-Directed C-H Borylation of Phosphines

Transition-metal-catalyzed C-H borylation has been widely used in the preparation of organoboron compounds. Here, we developed a general protocol on metal-free P(III)-directed C-H borylation of phosphines mediated by BBr₃ , resulting in the formation of products bearing both phosphorus and boron. The development of the metal-free strategy to mimic previous metallic processes has shown low cost, superior practicality, and environmental friendliness. Density functional theory (DFT) calculations demonstrate the preferred pathway for this metal-free directed C-H borylation process.

Microwave-Assisted Ruthenium- and Rhodium-Catalyzed Couplings of α-Amino Acid Ester-Derived Phosphinamides with Alkynes

Two different types of new phosphinamide α-amino ester derivatives have been prepared in moderate to high yields via ruthenium(II) and rhodium(III)-catalyzed ortho-C-H functionalization under microwave irradiation. Specifically, the ortho-alkenylated phosphinamides were produced through coupling of phosphinamides containing an α-substituted or α,α-disubstituted α-amino ester with internal alkynes under ruthenium catalysis. In contrast, Ru and the more effective Rh-catalyzed coupling of the α-unsubstituted glycine ester phosphinamide with alkynes resulted in formation of oxidative annulation products, phosphaisoquinolin-1-ones. The developed methods feature the use of easily accessible starting materials, short reaction time, exclusive E-stereoselectivity (for ortho-alkenylation) and good functional group tolerance. The alkenylation reaction was readily scaled up to gram scale. Furthermore, the obtained alkenylated phosphinamide could be transformed into P-containing dipeptides through hydrolysis of the ester group in the catalysis product and subsequent condensation with an α-amino ester.

Construction of Bulky Ligand Libraries by Ru(II)-Catalyzed P(III)-Assisted ortho-C-H Secondary Alkylation

Modification of commercially available biaryl monophosphine ligands via ruthenium^(II)-catalyzed P^(III)-directed-catalyzed ortho C-H secondary alkylation is described. The use of highly ring-strained norbornene as a secondary alkylating reagent is the key to this transformation. A series of highly bulky ligands with a norbornyl group were obtained in excellent yields. The modified ligands with secondary alkyl group outperformed common substituted phosphines in the Suzuki-Miyaura cross-coupling reaction at a ppm mole level of Pd catalyst.

Ruthenium Catalyzed Intramolecular C-X (X=C, N, O, S) Bond Formation via C-H Functionalization: An Overview

Ruthenium catalyzed C-H activation is well known for its high tolerance towards the functional group and broad applicability in organic synthesis and molecular sciences, with significant applications in pharmaceutical industries, material sciences, and polymer industry. In the last few decades, enormous progress has been observed with ruthenium-catalyzed C-H activation chemistry. Notably, the vast majority of the C-H functionalization known in the literature are intermolecular, although the intramolecular variant provides fascinating new structural facet starting from the simple molecular scaffolds. Intramolecular C-H functionalization is atom economical and step efficient, results in less formation of undesired products which is easy to purify. This has created a lot of interest in organic chemistry in developing new synthetic strategies for such functionalization. The focus of this review is to present the relatively unexplored intramolecular functionalization of C-H bonds into C-X (X=C, N, O, S) bonds utilizing versatile ruthenium catalysts, their scope, and brief mechanistic discussion.

The Synthesis of Biarylmonophosphonates via Palladium-Catalyzed Phosphonation, Iridium-Catalyzed C-H Borylation, Palladium-Catalyzed Suzuki–Miyaura Cross-Coupling

Abstract

The iridium-catalyzed C-H borylation of diethyl phenylphosphonate results in nonselective mono and bisborylation to afford a near statistical mixture of 3-, 3,5- and 4-boryl substituted aryl phosphonates whereas 3-substituted aryl phosphonates undergo highly regioselective C-H borylation to afford the corresponding meta-phosphonate substituted arylboronic esters as the sole product; the resulting boronic esters were used as nucleophilic reagents in a subsequent palladium-catalyzed Suzuki–Miyaura cross-coupling to generate a range of biarylmonophosphonates. Gratifyingly, the Suzuki–Miyaura cross-coupling can be conducted without purifying the boronic ester which greatly simplifies the synthetic procedure.

Graphical Abstract

Cobalt(iii)-catalyzed redox-neutral [4+2]-annulation of N-chlorobenzamides/acrylamides with alkylidenecyclopropanes at room temperature

An efficient synthesis of substituted 3,4-dihydroisoquinolinones through [4+2]-annulation of N-chlorobenzamides/acrylamides having a monodentate directing group with alkylidenecyclopropanes in the presence of a less expensive, highly abundant and air stable Co(iii) catalyst via a C-H activation is demonstrated. In this reaction, the N-Cl bond of N-chlorobenzamide serves as an internal oxidant and thus an external metal oxidant is avoided. The 3,4-dihydroisoquinolinone derivatives are converted successfully into the highly useful imidoyl chloride derivatives. The deuterium labeling and kinetic isolabelling studies reveal that the C-H activation is a rate-determining step in this cyclization reaction.

Cooperative Ligand-Promoted P(III)-Directed Ruthenium-Catalyzed Remote Meta-C-H Alkylation of Tertiary Phosphines

Herein, we disclose a ruthenium-catalyzed meta-selective C-H activation of phosphines by using intrinsic P^(III) as a directing group. 2,2,6,6-Tetramethylheptane-3,5-dione acts as the ligand and exhibits an excellent performance in boosting the meta-alkylation. The protocol allows an efficient and straightforward synthesis of meta-alkylated tertiary phosphines. Several meta-alkylated phosphines were evaluated for Pd-catalyzed Suzuki coupling and found to be superior to commercially available ortho-substituted phosphines. The practicability of this methodology is further demonstrated by the synthesis of difunctionalized phosphines.

Ruthenium (II) Catalyzed C(sp2 )-H Bond Alkenylation of 2-Arylbenzo[d]oxazole and 2-Arylbenzo[d]thiazole with Unactivated Olefins

Functionalization of the bio-relevant heterocycles 2-arylbenzo[d]oxazole and 2-arylbenzo[d]thiazole has been achieved through Ru(II)-catalyzed alkenylation with unactivated olefins leading to selective formation of the mono-alkenylated products. This approach has a broad substrate scope with respect to the coupling partners, affords high yields, and works for gram scale synthesis using a readily available Ru-based catalyst. Mechanistic studies reveal a C-H activation pathway for the dehydrogenative coupling leading to the alkenylation. However, the results of the ESI-MS-guided deuterium kinetic isotope effect studies indicate that the C-H activation stage may not be the rate-determining step of the reaction. The use of a radical scavenging agent such as TEMPO did not show any detrimental effect on the reaction outcome, eliminating the possibility of the involvement of a free-radical pathway.

Metal-catalyzed silylation of sp3C–H bonds

Metal-catalyzed activations of inert sp³C–H bonds have recently brought a revolution in the synthesis of useful molecules and molecular materials, due to the interest of the formed sp³C–SiR₃ silanes, stable organometallic species, and for further functionalizations that sp³C–H bonds cannot reach directly.

Copper-catalyzed arylation of polycyclic aromatic hydrocarbons by the P[double bond, length as m-dash]O group

The first example of a directed and regioselective arylation of polycyclic aromatic hydrocarbons (PAHs) by using a P[double bond, length as m-dash]O directing group is reported herein. The protocol uses a cheap copper catalyst, and results in a breakthrough meta-selective C-H functionalization of arylphosphine oxide compounds. Substrates with potential fluorescence properties, for example, pyrene and fluoranthene, were successfully arylated under the system, thus achieving an efficient modification of fluorescent molecules containing the P[double bond, length as m-dash]O functional group.

External oxidant-compatible phosphorus(III)-directed site-selective C-H carbonylation

The first development of an external oxidant-compatible system involving a phosphorus(III)-directed C-H functionalization has been uncovered. An efficient C-H esterification of indoles with CO and alcohols has been reported in which the high reactivity and the exclusive C7-selectivity derives from the selection of a P(III)-directing group and the utilization of benzoquinone as an external oxidant with palladium catalysis. This strategy shows many advantages, involving an easily accessible and removable directing group, the use of cheap carbonylation sources, a broad substrate scope, and excellent positional selectivity. Two cyclopalladated intermediates were confirmed by x-ray analysis, uncovering key mechanistic features of this P(III)-directed C-H metalation event.

Diastereoselective Synthesis of P-Chirogenic and Atropisomeric 2,2'-Bisphosphino-1,1'-binaphthyls Enabled by Internal Phosphine Oxide Directing Groups

Diphosphine ligands that merge both axial and P-centered chirality may exhibit superior or unique properties. Herein we report the diastereoselective introduction of P-centered chirality at the 2-position of the axially chiral 2'-(phosphine oxide)-1,1'-binaphthyl scaffold. A lithium-bromide exchange reaction of a 2-bromo-2'-(phosphine oxide)-1,1'-binaphthyl and treatment with dichlorophosphines followed by a nucleophilic organometallic reagent afforded unsymmetrical 2-phosphino-2'-(phosphine oxide)-1,1'-binaphthyls with binaphthyl axial chirality and one or two phosphorus stereocenters with a variety of P substituents. The final diastereomerically pure 2,2'-bisphosphino-1,1'-binaphthyls were obtained by reduction of the phosphine oxide directing group. Preliminary results demonstrated that a ligand with this hybrid chirality could induce higher stereoselectivity in the metal-complex-catalyzed asymmetric hydrogenation of a dialkyl ketone.