Kinetic and thermodynamic control of C(sp2)-H activation enables site-selective borylation

Cobalt-Catalyzed Cross-Coupling of Glycosyl Sulfones with Zinc reagents toward the Stereoselective Synthesis of C(sp/sp2)-Glycosides

A cobalt-catalyzed desulfonylative cross-coupling of glycosyl sulfones with organozinc reagents toward the stereoselective synthesis of C-glycosides is reported. The new C-glycoside synthesis proceeds under mild reaction conditions and exhibits tolerance to a range of functional groups. Diverse alkynylated, arylated, and alkenylated products are formed with high efficiency and excellent diastereoselectivity. Mechanistic studies indicate a radical pathway.

The Role of Boron Reagents in Determining the Site-Selectivity of Pyridine(dicarbene) Cobalt-Catalyzed C-H Borylation of Fluorinated Arenes

The origin of the meta- and ortho-to-fluorine site-selectivity in the C(sp2)-H borylation of fluorinated arenes with B2Pin2 and HBPin promoted by pyridine(dicarbene)cobalt catalysts has been investigated. In situ generation of the cobalt(I)-boryl complex and treatment with three representative fluoroarenes established meta-selective C(sp2)-H oxidative addition to form predominantly the meta isomers of the corresponding cobalt(I)-aryl complexes. Attempts to observe or isolate the four-coordinate cobalt(I)-boryl complex yielded the cobalt-hydride dimer, [(iPrACNC)CoH]2, borohydride (iPrACNC)CoH2BPin, or diboryl hydride, (iPrACNC)CoH(BPin)2 depending on the amounts of B2Pin2 and HBPin present. The phosphite derivatives (iPrACNC)CoH(P(OiPr)3) and (iPrACNC)CoBPin(P(OiPr)3) were prepared and crystallographically characterized. In the catalytic borylation of 1,3-difluorobenzene, ortho-to-fluorine cobalt(I)-aryl and borohydride complexes were identified as resting states despite meta-to-fluorine borylation being the major product of catalysis. Deuterium kinetic isotope effects support irreversible but not turnover-limiting C(sp2)-H oxidative addition. Stoichiometric borylation of isolated cobalt(I)-aryl intermediates with B2Pin2 established that the meta-cobalt(I)-aryl was more reactive than the ortho-isomer and accounts for the observed cobalt(I)-aryl resting states. All cobalt(I)-aryl compounds reacted more quickly with HBPin. While ortho-cobalt(I)-aryl compounds yielded arylboronate products with high site-selectivity, meta-cobalt-aryl counterparts yielded a mixture of arylboronate isomers and free arene. Deuterium labeling experiments with DBPin confirmed that HBPin mediates reversible C(sp2)-H oxidative addition. Thus, the overall site-selectivity arises from two reinforcing effects: (i) kinetically meta-selective oxidative addition and (ii) faster reaction of the meta-cobalt-aryl isomer with B2Pin2. As B2Pin2 is converted to HBPin, C(sp2)-H reductive elimination competes against borylation of the meta-cobalt-aryl isomer, resulting in increased ortho-selective borylation.

Crystal structure and Hirshfeld-surface analysis of the pesticide etoxazole

Etoxazole (C21H23F2NO2), systematic name 4-(4-tert-butyl-2-eth-oxy-phen-yl)-2-(2,6-di-fluoro-phen-yl)-4,5-di-hydro-1,3-oxazole, is a fluorinated insecticide and acaricide that inhibits chitin biosynthesis, disrupting insect development by preventing proper exoskeleton formation. Widely used in agriculture since 1998, it is readily absorbed by plant tissues and translocates within leaves. Metabolic studies have identified several oxidative degradation products, while toxicol-ogical assessments have examined potential effects, including oxidative stress. This study presents a detailed crystallographic and Hirshfeld surface analysis of etoxazole. The mol-ecule consists of a central di-hydro-oxazole ring flanked by 2,6-di-fluoro-phenyl and 4-tert-butyl-2-eth-oxy-phenyl groups, each twisted relative to the oxazole core. The di-hydro-oxazole ring is nearly planar, with the substituted phenyl rings forming dihedral angles of 44.20 (4)° and 47.87 (4)° with the mean plane of the di-hydro-oxazole. The eth-oxy group exhibits a dihedral angle of 15.04 (11)° to the tert-butyl-phenyl ring, while the tert-butyl group itself shows minor torsional disorder [major:minor occupancies are 0.760 (6):0.240 (6)]. The mol-ecular packing is dominated by van der Waals-type inter-actions, though weak C-H⋯F and C-H⋯O inter-actions lead to pleated layers parallel to the ab plane, which further stack along the c-axis direction. A Hirshfeld surface analysis confirms the prevalence of van der Waals inter-actions in crystal stabilization.

Migratory Aryl Cross-Coupling

A fundamental property of cross-coupling reactions is regiospecificity, meaning that the site of bond formation is determined by the leaving group's location on the electrophile. Typically, achieving a different substitution pattern requires the synthesis of a new, corresponding starting-material isomer. As an alternative, we proposed the development of cross-coupling variants that would afford access to multiple structural isomers from the same coupling partners. Here, we first demonstrate that a bulky palladium catalyst can facilitate the efficient, reversible transposition of aryl halides by temporarily forming metal aryne species. Despite the nearly thermoneutral equilibrium governing this process, combining it with the gradual addition of a suitable nucleophile results in dynamic kinetic resolution of the isomeric intermediates and high yields of unconventional product isomers. The method accommodates a range of oxygen- and nitrogen-centered nucleophiles and tolerates numerous common functional groups. A Curtin-Hammett kinetic scheme is supported by computational and experimental data, providing a general mechanistic framework for extending this migratory cross-coupling concept.

Electrochemical DABCOylation enables challenging aromatic C-H amination

The selective amination of aromatic C-H bonds is a powerful strategy to access aryl amines, functionalities found in many pharmaceuticals and agrochemicals. Despite advances in the field, a platform for the direct, selective C-H amination of electronically diverse (hetero)arenes, particularly electron-deficient (hetero)arenes, remains an unaddressed fundamental challenge.1-10 In addition, many (hetero)arenes present difficulty in common selective pre-functionalization reactions, such as halogenation11, or metal-catalyzed borylation12 and silylation13. Here, we report a general solution to these limitations that enables selective C-H amination across a comprehensive scope of (hetero)arenes. Key to this strategy's success is the oxidative generation of highly electrophilic N-radical dications from bicyclic tertiary amines (DABCO) that reacts across a wide range of arenes with high selectivity. Notably, this platform constitutes the first anodically generated N-radical cations that engage in aromatic C-H amination over well-reported hydrogen atom transfer (HAT) with weak C-H bonds.14-16 This C-H amination reaction that allows selective functionalization of both electron-rich and deficient arenes, as well as pyridines, is a rarity in the general area of non-directed aromatic C-H functionalization.1-4 This sustainable electrochemical DABCOylation reaction provides access to many complex drug-like aryl- and pyridinylpiperazines with high functionality tolerance, chemoselectivity, and site-selectivity.17.

A Convenient Approach for the Synthesis of Multidentate N-Heterocyclic Carbene Ligand Precursors

Multidentate bis-NHCs ligands with various spacers are expected to combine the advantages of coordination chemistry and catalysis. These offer opportunities to construct various organometallic frameworks involving transition metals and main group elements. Therefore, developing a general procedure for synthesizing a variety of carbene salt precursors using a convenient technique is key in this context. The extended protocol of a solvent-free approach for synthesizing various bridged bis-imidazolium carbene salts, including tris and tetrakis-imidazolium precursors, is reported here. This method can be performed in the laboratory, leading to high yields (80-95 %) and isolated as analytically pure, multigram, bench-stable compounds.

Rapid and General Amination of Aryl Boronic Acids and Esters Using O-(Diphenylphosphinyl)hydroxylamine (DPPH)

O-(Diphenylphosphinyl)hydroxylamine (DPPH) is a general reagent for the conversion of (hetero)aryl boronic acids and esters to primary anilines. The transformation proceeds rapidly at rt and exhibits a broad substrate scope and exceptional functional-group tolerance. In terms of rate, the reaction is relatively insensitive to the electronic properties of the substrate, in contrast to similar reactions using electrophilic amination reagents such as hydroxylamine-O-sulfonic acid. Consequently, this protocol is particularly useful for accessing electron-deficient (hetero)aryl anilines, which had been challenging to prepare using prior methods.

Crystal structure and Hirshfeld-surface analysis of an etoxazole metabolite designated R13

The etoxazole metabolite R13, systematic name 4-(4-tert-butyl-2-eth-oxy-phen-yl)-2-(2,6-di-fluoro-phen-yl)oxazole (C21H21F2NO2), results from the oxidation of etoxazole, a chitin synthesis inhibitor belonging to the oxazoline class, widely used as an insecticide/acaricide since 1998. The structure of R13 features a central oxazole ring with attached 2,6-di-fluoro-phenyl and 4-t-butyl-2-eth-oxy-phenyl moieties. The overall conformation gives dihedral angles between these rings and the oxazole of 24.91 (5)° (with di-fluoro-phen-yl) and 15.30 (6)° (with t-butyl-eth-oxy-phen-yl), indicating an overall deviation from planarity. Additionally, torsion angles of the eth-oxy and t-butyl groups define the orientation of these substituents relative to their benzene ring. In the crystal packing, no significant hydrogen bonds are present, but a Hirshfeld surface analysis highlights weak inter-molecular contacts leading to π-π-stacked dimers linked by weak C-H⋯N contacts. The packing analysis confirms that most inter-molecular inter-actions involve hydrogen atoms.

Nickel-catalyzed, silyl-directed, ortho-borylation of arenes via an unusual Ni(II)/Ni(IV) catalytic cycle

Nickel-catalyzed C-H bond functionalization reactions provide an impressive alternative to those with noble metal catalysts due to their unique reactivity and low cost. However, the regioselective C(sp2)-H borylation reaction of arenes accomplished by nickel catalyst remains limited. We herein disclose a silyl-directed ortho C(sp2)-H borylation of substituted arenes with a Ni(cod)2/PMe3/KHMDS catalyst system. Using readily available starting materials, this protocol provides easy access to ortho-borylated benzylic hydrosilanes bearing flexible substitution patterns. These products can serve as versatile building blocks for the synthesis of sila or sila/borine heterocycles under mild conditions. Control experiments and DFT calculations suggest that a catalytic amount of base prompts the formation of Ni(II)-Bpin-ate complex, likely related to the C(sp2)-H bond activation. This borylation reaction might follow an unusual Ni(II)/Ni(IV) catalytic cycle.

Multicomponent Cross-Dehydrogenative Coupling of Imidazo[1,2-a]pyridine: Access to Abnormal Mannich and Mannich-Type Reaction

This study showcases successfully switchable approaches to accomplish the C3-aryl methylation and C3- amino methylation of privileged nitrogen-containing pharmaceutical compounds "imidazopyridines" with distinct amines, which surmounts the long-standing requirement for a superfluous directing group. These two transformations manifest pronounced regio- and chemo-divergent behavior, successfully demonstrating unprecedented multicomponent "abnormal Mannich and Mannich-type" reactions. The remarkable environmentally benign protocol has been efficiently extended to concise the synthesis and late-stage derivatization.

C(sp)-H, S-H, and Sn-H Bond Activation with a Cobalt(I) Pincer Complex

This study focuses on the stoichiometric reactions of {2,6-(iPr2PO)2C6H3}Co(PMe3)2 with terminal alkynes, thiols, and tin hydrides as part of an effort to develop catalytic, two-electron processes with cobalt. This specific Co(I) pincer complex proves to be effective for cleaving the C(sp)-H, S-H, and Sn-H bonds to give oxidative addition products with the general formula {2,6-(iPr2PO)2C6H3}CoHX(PMe3) (X = alkynyl, thiolate, and stannyl groups) along with the free PMe3. These reactions typically reach completion when the substituents on acetylene, sulfur, and tin are electron-withdrawing groups (e.g., phenyl, pyridyl, and alkenyl groups). In contrast, alkyl-substituted acetylenes, 1-pentanethiol, and tributyltin hydride are partially converted due to the equilibria with the corresponding oxidative addition products. The Co(I) pincer complex is not a hydrothiolation catalyst but capable of catalyzing the hydrostannation of terminal alkynes with Ph3SnH to produce β-(Z)-alkenylstannanes selectively.

Engineering Frustrated Lewis Pair Active Sites in Porous Organic Scaffolds for Catalytic CO2 Hydrogenation

Frustrated Lewis pairs (FLPs), featuring reactive combinations of Lewis acids and Lewis bases, have been utilized for myriad metal-free homogeneous catalytic processes. Immobilizing the active Lewis sites to a solid support, especially to porous scaffolds, has shown great potential to ameliorate FLP catalysis by circumventing some of its inherent drawbacks, such as poor product separation and catalyst recyclability. Nevertheless, designing immobilized Lewis pair active sites (LPASs) is challenging due to the requirement of placing the donor and acceptor centers in appropriate geometric arrangements while maintaining the necessary chemical environment to perform catalysis, and clear design rules have not yet been established. In this work, we formulate simple guidelines to build highly active LPASs for direct catalytic hydrogenation of CO2 through a large-scale screening of a diverse library of 25,000 immobilized FLPs. The library is built by introducing boron-containing acidic sites in the vicinity of the existing basic nitrogen sites of the organic linkers of metal-organic frameworks collected in a "top-down" fashion from the CoRE MOF 2019 database. The chemical and geometrical appropriateness of these LPASs for CO2 hydrogenation is determined by evaluating a series of simple descriptors representing the intrinsic strength (acidity and basicity) of the components and their spatial arrangement in the active sites. Analysis of the leading candidates enables the formulation of pragmatic and experimentally relevant design principles which constitute the starting point for further exploration of FLP-based catalysts for the reduction of CO2.

Applications of Transition Metal-Catalyzed ortho-Fluorine-Directed C-H Functionalization of (Poly)fluoroarenes in Organic Synthesis

The synthesis of organic compounds efficiently via fewer steps but in higher yields is desirable as this reduces energy and reagent use, waste production, and thus environmental impact as well as cost. The reactivity of C-H bonds ortho to fluorine substituents in (poly)fluoroarenes with metal centers is enhanced relative to meta and para positions. Thus, direct C-H functionalization of (poly)fluoroarenes without prefunctionalization is becoming a significant area of research in organic chemistry. Novel and selective methodologies to functionalize (poly)fluorinated arenes by taking advantage of the reactivity of C-H bonds ortho to C-F bonds are continuously being developed. This review summarizes the reasons for the enhanced reactivity and the consequent developments in the synthesis of valuable (poly)fluoroarene-containing organic compounds.