Silver-catalyzed radical fluorination of alkylboronates in aqueous solution

Deboronative functionalization of alkylboron species via a radical-transfer strategy

With advances in organoboron chemistry, boron-centered functional groups have become increasingly attractive. In particular, alkylboron species are highly versatile reagents for organic synthesis, but the direct generation of alkyl radicals from commonly used, bench-stable boron species has not been thoroughly investigated. Herein, we describe a method for activating C-B bonds by nitrogen- or oxygen-radical transfer that is applicable to alkylboronic acids and esters and can be used for both Michael addition reactions and Minisci reactions to generate alkyl or arylated products.

Synthesis and Lewis Acid Properties of Neutral Silver(III) Adducts Containing the AgIII(CF3)3 Moiety

The acetonitrile AgIII complex [AgIII(CF3)3(NCCH3)] (2) has been reported independently by Eujen and Naumann in the last century, albeit with intriguing NMR discrepancy. In their reports, 2 was claimed to be obtained starting from either [AgIII(CF3)3Cl]- (3⋅Cl) or [AgIII(CF3)4]- (1) via halide abstraction using AgNO3 or acidic treatment, resp. These two synthetic routes are herein reinvestigated. The feasibility of Naumann's method is demonstrated, thus providing 2 yet accompanied by its s-triazinyl derivative [AgIII(CF3)3(C6H9N3)] (2'). The formation of 2' is unprecedented and was thereby investigated. Both 2 and 2' were isolated in pure fashion and fully characterized. In turn, halide extraction from 3⋅Cl leads to the AgIII-ONO2 anion 5 instead of 2, as evidenced by NMR spectroscopy, EA and Sc-XRD.

Silver-Catalyzed Carbofluorination of Olefins and α-Fluoroolefins with Carbamoyl Radicals

The reactivity of carbamoyl radicals, generated in situ from sodium oxamate salts, has been investigated in the context of radical carbofluorination reactions of olefins and α-fluoroolefins, respectively. Both transformations are catalyzed by silver salts and required the presence of potassium persulfate (K2S2O8) and SelectfluorTM as a radicophilic fluorine source. The reported methods provide a direct access to β-fluoroamides and β,β-difluoroamides.

Photogeneration of α-Bimetalloid Radicals via Selective Activation of Multifunctional C1 Units

Light-driven strategies that enable the chemoselective activation of a specific bond in multifunctional systems are comparatively underexplored in comparison to transition-metal-based technologies, yet desirable when considering the controlled exploration of chemical space. With the current drive to discover next-generation therapeutics, reaction design that enables the strategic incorporation of an sp3 carbon center, containing multiple synthetic handles for the subsequent exploration of chemical space would be highly enabling. Here, we describe the photoactivation of ambiphilic C1 units to generate α-bimetalloid radicals using only a Lewis base and light source to directly activate the C-I bond. Interception of these transient radicals with various SOMOphiles enables the rapid synthesis of organic scaffolds containing synthetic handles (B, Si, and Ge) for subsequent orthogonal activation. In-depth theoretical and mechanistic studies reveal the prominent role of 2,6-lutidine in forming a photoactive charge transfer complex and in stabilizing in situ generated iodine radicals, as well as the influential role of the boron p-orbital in the activation/weakening of the C-I bond. This simple and efficient methodology enabled expedient access to functionalized 3D frameworks that can be further derivatized using available technologies for C-B and C-Si bond activation.

Synthesis of substituted benzylboronates by light promoted homologation of boronic acids with N-sulfonylhydrazones

The synthesis of benzylboronates by photochemical homologation of boronic acids with N-tosylhydrazones under basic conditions is described. The reaction involves the photolysis of the N-tosylhydrazone salt to give a diazoalkane followed by the geminal carboborylation of the diazoalkane. Under the mild reaction conditions, the protodeboronation of the unstable benzylboronic acid is circumvented and the pinacolboronates can be isolated after reaction of the benzylboronic acid with pinacol. The metholodogy has been applied to the reactions of alkylboronic acids with N-tosylhydrazones of aromatic aldehydes and ketones, and to the reactions of arylboronic acids with N-tosylhydrazones of aliphatic ketones. Moreover, the employment of the DBU/DIPEA bases combination allows for homogeneous reactions which have been adapted to photochemical continuous flow conditions. Additionally, the synthetic versatility of boronates enables their further transformation via Csp3-C or Csp3-X bond forming reactions converting this methodology into a novel method for the geminal difunctionalization of carbonyls via N-tosylhydrazones.

Cross-Coupling Reactions Enabled by Well-Defined Ag(III) Compounds: Main Focus on Aromatic Fluorination and Trifluoromethylation

AgIII compounds are considered strong oxidizers of difficult handling. Accordingly, the involvement of Ag catalysts in cross-coupling via 2e- redox sequences is frequently discarded. Nevertheless, organosilver(III) compounds have been authenticated using tetradentate macrocycles or perfluorinated groups as supporting ligands, and since 2014, first examples of cross-coupling enabled by AgI /AgIII redox cycles saw light. This review collects the most relevant contributions to this field, with main focus on aromatic fluorination/perfluoroalkylation and the identification of AgIII key intermediates. Pertinent comparison between the activity of AgIII RF compounds in aryl-F and aryl-CF3 couplings vs. the one shown by its CuIII RF and AuIII RF congeners is herein disclosed, thus providing a more profound picture on the scope of these transformations and the pathways commonly associated to C-RF bond formations enabled by coinage metals.

PhI(OAc)2-mediated aminoacyloxylation of β,γ-unsaturated hydrazones using Togni reagent II as an acyloxyl precursor

A metal-free protocol for the direct construction of C(sp2)-N and C-O bonds via a PhI(OAc)2-mediated dehydrogenative aminoacyloxylation of β,γ-unsaturated hydrazones with Togni reagent II is reported. Initiated by the carboxyl-containing species generated in situ from Togni reagent II, this method offers a new solution for regioselective functionalization at a remote site on β,γ-unsaturated hydrazones, thus providing a straightforward method for the synthesis of acyloxyl-substituted pyridazines. This reaction features a broad substrate scope and mild conditions.

Divergent Synthesis of β-Fluoroamides via Silver-Catalyzed Oxidative Deconstruction of Cyclopropanone Hemiaminals

An expedient approach for the synthesis of challenging β-fluoroamides from readily accessible cyclopropanone equivalents is reported. Following the addition of pyrazole used here as a transient leaving group, silver-catalyzed regiospecific ring-opening fluorination of the resulting hemiaminal leads to a β-fluorinated N-acylpyrazole intermediate reactive to substitution with amines, ultimately affording β-fluoroamides. The process could also be extended to the synthesis of β-fluoroesters and γ-fluoroalcohols via the addition of alcohols or hydrides as terminal nucleophiles, respectively.

Multicomponent Reaction: Pd/Cu-Catalyzed Coupling and Boration of Acyl Chlorides and Alkynes to β-Boryl Ketones

An unprecedented and challenging multicomponent reaction has been developed that allows for the direct transformation of acyl chlorides with alkynes into the corresponding saturated β-boryl ketones via Pd/Cu-catalyzed coupling and boration with ethyl acetate as the hydrogen sources. Various β-boryl ketones were synthesized in good to excellent yields with broad functional group tolerance. In addition, the introduction of boron groups into the products provides substantial opportunities for further conversions.

Amide-Ligand-Promoted Silver-Catalyzed C-H Fluorination via Radical/Polar Crossover

We describe an efficient method for benzylic C-H fluorination via sequential hydrogen-atom transfer (HAT) and oxidative radical-polar crossover utilizing the Ag(I)/Selectfluor system. Amide ligands, such as benzamide and sulfonamide, substantially facilitate the processes leading to a carbocation intermediate, which subsequently reacts with nucleophilic fluorinating reagent to form a C-F bond. This protocol is applicable to the fluorination of all 1°, 2°, and 3° C-H bonds as well as to late-stage C-H fluorination of bioactive molecules.

Palladium-catalyzed C(sp3)-H nitrooxylation of masked alcohols

A palladium-catalyzed β-C(sp³)-H nitrooxylation of aliphatic alcohols with AgNO₂ is reported. An 8-formylquinoline-derived oxime is installed as an exo-type directing group for sp³ C-H activation and selectfluor acts as the oxidant. The reaction tolerates a variety of functional groups and shows good selectivity for β-C-H nitrooxylation of alcohols.

Photocatalytic Oxo-Amination of Aryl Cyclopropanes through an Unusual SN2-Like Ring-Opening Pathway: Won >99% ee

One-pot oxo-amination of unactivated cyclopropanes with safe, green dioxygen as an oxidant and low-cost amines as nitrogen sources has generated interest since this can directly result in uniform β-located difunctional units. Formation of the three-electron cation radical followed by the nucleophilic attack of amines to open the strained ring of cyclopropanes catalyzed by classic noble-complex photocatalysts was a promising strategy. However, this ring-opening pathway could not maintain the entire second-order nucleophilic substitution (S_N2) conversion, which generally led to unsatisfactory enantioselectivity (enantiomeric excess (ee) value ∼60%). Here, we demonstrate that for such a one-step oxo-amination of cyclopropanes with benign dioxygen and pyrazoles, a highly uniform inversion of configuration could be first accomplished through a TiO₂ photocatalyst. This strategy features low-cost, semiheterogeneous photocatalysis and environmentally friendly reaction conditions, without using any sacrificial reagent or additive. Importantly, our protocol not only provides a relatively broad substrate scope tolerant to a certain range of substituted cyclopropanes and pyrazoles, resulting in various β-amino ketone products (∼50 examples) with excellent conversions and yields, but also retains excellent enantioselectivity (ee value ∼99%). A concerted S_N2 ring opening raised from an oxetane cation intermediate rather than a conventional three-electron cation radical prior to attaching to dioxygen was proposed.

Nickel-Catalyzed 1,2-Arylboration of Unactivated Alkenes to Access Boryl-Functionalized Aliphatic Amines

We report herein a three-component 1,2-arylboration of alkenyl amines bearing a cleavable picolinamide directing group. With aryl halides as electrophiles and B₂Pin₂ as nucleophiles, a wide range of alkenes could be converted into valuable boryl-functionalized aliphatic amines. The reaction proceeds with high levels of chemo- and regiocontrol and exhibits high functional group tolerance. In addition, the pinacol boronic ester group could undergo various transformations, indicating that the protocol could potentially provide a platform for versatile regioselective difunctionalization of alkenyl amines.

Hydroxy-directed fluorination of remote unactivated C(sp3)-H bonds: a new age of diastereoselective radical fluorination

We report a photochemically induced, hydroxy-directed fluorination that addresses the prevailing challenge of high diastereoselectivity in this burgeoning field. Numerous simple and complex motifs showcase a spectrum of regio- and stereochemical outcomes based on the configuration of the hydroxy group. Notable examples include a long-sought switch in the selectivity of the refractory sclareolide core, an override of benzylic fluorination, and a rare case of 3,3'-difluorination. Furthermore, calculations illuminate a low barrier transition state for fluorination, supporting our notion that alcohols are engaged in coordinated reagent direction. A hydrogen bonding interaction between the innate hydroxy directing group and fluorine is also highlighted for several substrates with 19F-1H HOESY experiments, calculations, and more.

Recent Advances in the Construction of Fluorinated Organoboron Compounds

Fluorinated organoboron compounds are important synthetic building blocks that combine the unique characteristics of a fluorinated motif with the versatile synthetic applications of organoboron moiety. This review article guides the research on fluorinated organoboron compounds mainly from four aspects in recent years: selective monodefluoroborylation of polyfluoroarenes and polyfluoroalkenes, selective borylation of fluorinated substrates, selective fluorination of organoboron compounds, and borofluorination of alkynes/olefins. In addition, this review will provide a necessary guidance and inspiration for the research on the valuable synthetic building block fluorinated organoboron compounds.

Rapid Access to Fluorinated Anilides via DAST-Mediated Deoxyfluorination of Arylhydroxylamines

A new strategy for the synthesis of fluorinated anilides in the absence of metals and oxidants has been developed. This deoxyfluorination of N-arylhydroxylamines with diethylaminosulfur trifluoride (DAST) proceeded smoothly under mild conditions, and the ortho- or para-fluorinated aromatic amine products were prepared in moderate to good yields. Structurally diverse fluorinated anilides, including heterocyclic and pharmaceutically relevant molecules, can be efficiently constructed by this protocol.

Cross-Coupling through Ag(I)/Ag(III) Redox Manifold

In ample variety of transformations, the presence of silver as an additive or co-catalyst is believed to be innocuous for the efficiency of the operating metal catalyst. Even though Ag additives are required often as coupling partners, oxidants or halide scavengers, its role as a catalytically competent species is widely neglected in cross-coupling reactions. Most likely, this is due to the erroneously assumed incapacity of Ag to undergo 2e^- redox steps. Definite proof is herein provided for the required elementary steps to accomplish the oxidative trifluoromethylation of arenes through Ag^I /Ag^III redox catalysis (i. e. CEL coupling), namely: i) easy Ag^I /Ag^III 2e^- oxidation mediated by air; ii) bpy/phen ligation to Ag^III ; iii) boron-to-Ag^III aryl transfer; and iv) ulterior reductive elimination of benzotrifluorides from an [aryl-Ag^III -CF₃ ] fragment. More precisely, an ultimate entry and full characterization of organosilver(III) compounds [K]⁺ [Ag^III (CF₃ )₄ ]^- (K-1), [(bpy)Ag^III (CF₃ )₃ ] (2) and [(phen)Ag^III (CF₃ )₃ ] (3), is described. The utility of 3 in cross-coupling has been showcased unambiguously, and a large variety of arylboron compounds was trifluoromethylated via [Ag^III (aryl)(CF₃ )₃ ]^- intermediates. This work breaks with old stereotypes and misconceptions regarding the inability of Ag to undergo cross-coupling by itself.

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

This review discusses recent advances in the chemistry of saturated boronic acids, boronates, and trifluoroborates. Applications of the title compounds in the design of boron-containing drugs are surveyed, with special emphasis on α-amino boronic derivatives. A general overview of saturated boronic compounds as modern tools to construct C(sp³ )-C and C(sp³ )-heteroatom bonds is given, including recent developments in the Suzuki-Miyaura and Chan-Lam cross-couplings, single-electron-transfer processes including metallo- and organocatalytic photoredox reactions, and transformations of boron "ate" complexes. Finally, an attempt to summarize the current state of the art in the synthesis of saturated boronic acids, boronates, and trifluoroborates is made, with a brief mention of the "classical" methods (transmetallation of organolithium/magnesium reagents with boron species, anti-Markovnikov hydroboration of alkenes, and the modification of alkenyl boron compounds) and a special focus on recent methodologies (boronation of alkyl (pseudo)halides, derivatives of carboxylic acids, alcohols, and primary amines, boronative C-H activation, novel approaches to alkene hydroboration, and 1,2-metallate-type rearrangements).

Regioselective Monoborylation of Spirocyclobutenes

We present a strategy for the synthesis of spirocyclic cyclobutanes with modulable exit vectors based on the regioselective monoborylation of spirocyclobutenes. Using an inexpensive copper salt and a commercially available bidentate phosphine, a broad variety of borylated spirocycles have been prepared with complete regiocontrol. The boryl moiety provides a synthetic handled for further functionalization, allowing access to a wide array of spirocyclic building blocks from a common intermediate.

Diversity-Oriented Synthesis of Aliphatic Fluorides via Reductive C(sp3 )-C(sp3 ) Cross-Coupling Fluoroalkylation

Monofluorinated alkyl compounds are of great importance in pharmaceuticals, agrochemicals and materials. Herein, we describe a direct nickel-catalyzed monofluoromethylation of unactivated alkyl halides using a low-cost industrial raw material, bromofluoromethane, by demonstrating a general and efficient reductive cross-coupling of two alkyl halides. Results with 1-bromo-1-fluoroalkane also demonstrate the viability of monofluoroalkylation, which further established the first example of reductive C(sp³ )-C(sp³ ) cross-coupling fluoroalkylation. These transformations demonstrate high efficiency, mild conditions, and excellent functional-group compatibility, especially for a range of pharmaceuticals and biologically active compounds. Mechanistic studies support a radical pathway. Kinetic studies reveal that the reaction is first-order dependent on catalyst and alkyl bromide whereas the generation of monofluoroalkyl radical is not involved in the rate-determining step. This strategy provides a general and efficient method for the synthesis of aliphatic fluorides.

Iodine(III)-Mediated Fluorination/Semipinacol Rearrangement Cascade of 2-Alkylidenecyclobutanol Derivatives: Access to β-Monofluorinated Cyclopropanecarbaldehydes

A hypervalent iodine(III)-mediated ring-contractive fluorination reaction of 2-alkylidenecyclobutanol derivatives is presented. The protocol allows the facile synthesis of β-monofluorinated cyclopropanecarbaldehydes via a fluorination/semipinacol rearrangement cascade using nucleophilic Py·HF as the fluorine source. For challenging electron-rich arene substrates, the installation of a protecting group on the free alcohol is pivotal for maintaining the reaction efficiency. The synthetic utility was demonstrated by the scalability of this reaction and further transformations of the products.

Construction of Complex Cyclobutane Building Blocks by Photosensitized [2 + 2] Cycloaddition of Vinyl Boronate Esters

Cyclobutyl moieties in drug molecules are rare, and in general, they are minimally substituted and stereochemically simple. Methods to assemble structurally complex cyclobutane building blocks suitable for rapid diversification are thus highly desirable. We report herein a photosensitized [2 + 2] cycloaddition with vinyl boronate esters affording straightforward access to complex, densely functionalized cyclobutane scaffolds. Mechanistic studies suggest an activation mode involving energy transfer to the styrenyl alkene rather than the vinyl boronate ester.

Borylation of Unactivated C(sp3)-H Bonds with Bromide as a Traceless Directing Group

A palladium-catalyzed alkyl C-H borylation with bromide as a traceless directing group is described, providing a convenient approach to access alkyl boronates bearing a β-all-carbon quaternary stereocenter. The protocol features a broad substrate scope, excellent site selectivity, and good functional group tolerance.

Rhodium-catalysed selective C-C bond activation and borylation of cyclopropanes

Transition metal (TM)-catalysed directed hydroboration of aliphatic internal olefins which facilitates the construction of complex alkylboronates is an essential synthetic methodology. Here, an efficient method for the borylation of cyclopropanes involving TM-catalysed directed C-C activation has been developed. Upon exposure to neutral Rh(i)-catalyst systems, N-Piv-substituted cyclopropylamines (CPAs) undergo proximal-selective hydroboration with HBpin to provide valuable γ-amino boronates in one step which are otherwise difficult to synthesize by known methods. The enantioenriched substrates can deliver chiral products without erosion of the enantioselectivities. Versatile synthetic utility of the obtained γ-amino boronates is also demonstrated. Experimental and computational mechanistic studies showed the preferred pathway and the origin of this selectivity. This study will enable the further use of CPAs as valuable building blocks for the tunable generation of C-heteroatom or C-C bonds through selective C-C bond activation.

Practical and Modular Construction of C(sp3)-Rich Alkyl Boron Compounds

Alkyl boronic acids and esters play an important role in the synthesis of C(sp³)-rich medicines, agrochemicals, and material chemistry. This work describes a new type of transition-metal-free mediated transformation to enable the construction of C(sp³)-rich and sterically hindered alkyl boron reagents in a practical and modular manner. The broad generality and functional group tolerance of this method is extensively examined through a variety of substrates, including synthesis and late-stage functionalization of scaffolds relevant to medicinal chemistry. The strategic significance of this approach, with alkyl boronic acids as linchpins, is demonstrated through various downstream functionalizations of the alkyl boron compounds. This two-step concurrent cross-coupling approach, resembling formal and flexible alkyl-alkyl couplings, provides a general entry to synthetically challenging high Fsp³-containing drug-like scaffolds.

Internal Alkyne-Directed Fluorination of Unactivated C(sp3)-H Bonds

A silver-mediated internal alkyne-guided fluorination of unactivated C(sp³)-H bonds is described. The reaction provides a facile access to γ-fluorinated fluoroalkylated (Z)-alkenes from readily available alkynes in promising yields with excellent regioselectivity, stereoselectivity, and site selectivity.

Iodine Catalysis for C(sp3 )-H Fluorination with a Nucleophilic Fluorine Source

Iodine catalysis was developed for aliphatic fluorination through light-promoted homolytic C-H bond cleavage. The intermediary formation of amidyl radicals enables selective C-H functionalization via carbon-centered radicals. For the subsequent C-F bond formation, previous methods have typically been limited by a requirement for electrophilic fluorine reagents. We here demonstrate that the intermediary instalment of a carbon-iodine bond sets the stage for an umpolung, thereby establishing an unprecedented nucleophilic fluorination pathway.

Silver-Catalyzed Trifluoromethoxylation of Alkyl Trifluoroborates

A silver-catalyzed trifluoromethoxylation of alkyl trifluoroborates with trifluoromethyl arylsulfonate as the trifluoromethoxylation reagent has been reported for the first time. This reaction is performed under mild reaction conditions and has wide functional group compatibility. In addition, the mechanism of this site-specific trifluoromethoxylation is proposed as a radical pathway.

Gold-Catalyzed [3+2] Carbocycloaddition Reaction of Pinacol Alkenylboronates: Stereospecific Synthesis of Boryl-Functionalized Cyclopentene Derivatives

Gold-catalysis has enabled new synthetic opportunities in the chemistry of vinyldiazo compounds. Herein, we report the gold-catalyzed reaction of stabilized vinyldiazo compounds with pinacol alkenylboronates to provide boryl-functionalized cyclopentene derivatives through a formal [3+2] carbocycloaddition reaction, a very unusual pathway in alkenylboronate chemistry. This reaction proceeds with high regio- and stereoselectivity. The synthetic usefulness of the resulting borylated cyclopentene derivatives toward the synthesis of densely functionalized cyclopentanoids is also demonstrated.

Selective C-H trifluoromethoxylation of (hetero)arenes as limiting reagent

Methods for direct C-H trifluoromethoxylation of arenes and heteroarenes are rare, despite the importance of trifluoromethoxylated compounds for pharmaceuticals, agrochemicals, and material sciences. Especially selective C-H trifluoromethoxylation of pyridines remains a formidable challenge. Here we show a general late-stage C-H trifluoromethoxylation of arenes and heteroarenes as limiting reagent with trifluoromethoxide anion. The reaction is mediated by silver salts under mild reaction conditions, exhibiting broad substrate scope and wide functional-group compatibility. In addition, ortho-position selective C-H trifluoromethoxylation of pyridines is observed. The method is not only applicable to the gram-scale synthesis of trifluoromethoxylated products but also allows efficient late-stage C-H trifluoromethoxylation of marketed small-molecule drugs, common pharmacophores and natural products.

Selective C-H trifluoromethoxylation of pyridines remains a formidable synthetic challenge. Here, the authors report a silver-mediated late-stage C-H trifluoromethoxylation of arenes and heteroarenes as limiting reagents with trifluoromethoxide anion.