Fluorine Atom Transfer Research Articles

Photocatalytic Multisite Functionalization of Unactivated Terminal Alkenes by Merging Polar Cycloaddition and Radical Ring-Opening Process.

Although highly appealing for rapid access of molecular complexity, multi-functionalization of alkenes that allows incorporation of more than two functional groups remains a prominent challenge. Herein, we report a novel strategy that merges dipolar cycloaddition with photoredox promoted radical ring-opening remote C(sp3)-H functionalization, thus enabling a smooth 1,2,5-trifunctionalization of unactivated alkenes. A highly regioselective [3+2] cycloaddition anchors a reaction trigger onto alkene substrates. The subsequent halogen atom transfer (XAT) selectively initiates ring-opening process, which is followed by a series of 1,5-hydrogen atom transfer (1,5-HAT) and intermolecular fluorine atom transfer (FAT) events. With this method, site-selective introduction of three different functional groups is accomplished and a broad spectrum of valuable β-hydroxyl-ϵ-fluoro-nitrile products are synthesized from readily available terminal alkenes.

Photocatalytic Multisite Functionalization of Unactivated Terminal Alkenes by Merging Polar Cycloaddition and Radical Ring‐Opening Process

AbstractAlthough highly appealing for rapid access of molecular complexity, multi‐functionalization of alkenes that allows incorporation of more than two functional groups remains a prominent challenge. Herein, we report a novel strategy that merges dipolar cycloaddition with photoredox promoted radical ring‐opening remote C(sp3)−H functionalization, thus enabling a smooth 1,2,5‐trifunctionalization of unactivated alkenes. A highly regioselective [3+2] cycloaddition anchors a reaction trigger onto alkene substrates. The subsequent halogen atom transfer (XAT) selectively initiates ring‐opening process, which is followed by a series of 1,5‐hydrogen atom transfer (1,5‐HAT) and intermolecular fluorine atom transfer (FAT) events. With this method, site‐selective introduction of three different functional groups is accomplished and a broad spectrum of valuable β‐hydroxyl‐ϵ‐fluoro‐nitrile products are synthesized from readily available terminal alkenes.

Halogen Atom Transfer-Induced Homolysis of C-F Bonds by the Excited-State Boryl Radical.

A novel reactivity toward C-F bond functionalization has been developed, which could be designated as fluorine atom transfer (FAT). A photoexcited state of an N-heterocyclic carbene-ligated boryl radical exhibits a transcendent reactivity, capable of activating chemically inert carbon-fluorine bonds through homolysis. Combined experimental and computational studies suggest that the ligated boryl radical species directly abstracts a fluorine atom from the organofluoride substrates to provide valuable carbon-centered radicals.

Photocatalytic, modular difunctionalization of alkenes enabled by ligand-to-metal charge transfer and radical ligand transfer.

Ligand-to-metal charge transfer (LMCT) is a mechanistic strategy that provides a powerful tool to access diverse open-shell species using earth abundant elements and has seen tremendous growth in recent years. However, among many reaction manifolds driven by LMCT reactivity, a general and catalytic protocol for modular difunctionalization of alkenes remains unknown. Leveraging the synergistic cooperation of iron-catalyzed ligand-to-metal charge transfer and radical ligand transfer (RLT), here we report a photocatalytic, modular difunctionalization of alkenes using inexpensive iron salts catalytically to function as both radical initiator and terminator. Additionally, strategic use of a fluorine atom transfer reagent allows for general fluorochlorination of alkenes, providing the first example of interhalogen compound formation using earth abundant element photocatalysis. Broad scope, mild conditions and versatility in converting orthogonal nucleophiles (TMSN3 and NaCl) directly into corresponding open-shell radical species are demonstrated in this study, providing a robust means towards accessing vicinal diazides and homo-/hetero-dihalides motifs catalytically. These functionalities are important precursors/intermediates in medicinal and material chemistry. Preliminary mechanistic studies support the radical nature of these transformations, disclosing the tandem LMCT/RLT as a powerful reaction manifold in catalytic olefin difunctionalization.

Evidence for a High-Valent Iron-Fluoride That Mediates Oxidative C(sp3)-H Fluorination.

[FeII(NCCH3)(NTB)](OTf)2 (NTB = tris(2-benzimidazoylmethyl)amine, OTf = trifluoromethanesulfonate) was reacted with difluoro(phenyl)-λ3-iodane (PhIF2) in the presence of a variety of saturated hydrocarbons, resulting in the oxidative fluorination of the hydrocarbons in moderate-to-good yields. Kinetic and product analysis point towards a hydrogen atom transfer oxidation prior to fluorine radical rebound to form the fluorinated product. The combined evidence supports the formation of a formally FeIV(F)2 oxidant that performs hydrogen atom transfer followed by the formation of a dimeric μ-F-(FeIII)2 product that is a plausible fluorine atom transfer rebound reagent. This approach mimics the heme paradigm for hydrocarbon hydroxylation, opening up avenues for oxidative hydrocarbon halogenation.

Skeletal Transformation Triggered by C-F Bond Activation after Photochemical Rearrangement of Fluorinated [7]Helicenes.

Tri- and tetra-fluorinated [7]helicenes are photolabile and undergo a double fluorine atom transfer. Herein, we show that the transferred product further undergoes a skeletal transformation on silica gel. The transformation begins with activation of the allylic C-F bond on the silanol surface. Then, the resulting carbocation readily undergoes a regioselective nucleophilic aromatic substitution with water, depending on the position of the fluorine substituents. Hexafluoro-2-propanol also activated the allylic C-F bond and acted as a nucleophile. These findings support the generation of a highly reactive cationic electrophilic intermediate in the successive transformations involving fluorine atoms.

Synthesis of Terminally Fluorinated [7]Helicenes and Their Application to Photochemical Domino Reactions.

The intramolecular Diels-Alder reactions of helicenes deform their π-conjugated screw-shaped skeletons. In particular, terminally tetrafluorinated [7]helicene (F4 -[7]helicene) undergoes a photoinduced Diels-Alder reaction followed by a photoinduced double fluorine atom transfer. Herein, we thoroughly investigated this photochemical domino process by decreasing the level of fluorine substitution. F3 -[7]Helicenes bearing two fluorine atoms at the dienophile terminal underwent photoinduced Diels-Alder reactions, but the whole domino process became slow. F2 -[7]Helicene, which is difluorinated only at the dienophile terminal, was also photolabile. As a result, two fluorine atoms were sufficient for the photochemical domino reaction to occur. X-ray crystallographic analysis revealed that F2 -[7]helicene was less compressed than F4 -[7]helicene, indicating that terminal polyfluorination enhanced the intramolecular arene-fluoroarene stacking interactions and thus promoted the transformations.

Silver-catalyzed synthesis of β-fluorovinylphosphonates by phosphonofluorination of aromatic alkynes.

A silver-catalyzed three-component reaction involving alkynes, Selectfluor®, and diethyl phosphite was employed for the one-pot formation of C(sp2)–F and C(sp2)–P bonds to provide an efficient access to β-fluorovinylphosphonates in a highly regio- and stereoselective manner under mild reaction conditions. This reaction is operationally simple and offers an excellent functional group tolerance as well as a broad substrate scope that includes both terminal and internal alkynes. The reaction proceeded through the oxidative generation of a P-centered radical and subsequent fluorine atom transfer.

A quantum chemical study of the effect of substituents in governing the strength of the S–F bonds of sulfenyl-type fluorides toward homolytic dissociation and fluorine atom transfer

The gas-phase homolytic S–F bond dissociation energies (BDEs) of 21 sulfenyl-type fluorides (RSF) have been obtained using the W1w thermochemical protocol. The BDEs (at 298 K) for the species in this set range from 316.2 (HCCSF) to 368.1 (H2CCHSF) kJ mol–1. We additionally report fluorine-transfer energies (FTEs), corresponding to the energetics of fluorine transfer from RSF to H2S. At 298 K, the FTEs range from –10.7 (H2AlSF) to 90.7 (MeHNSF) kJ mol–1. We have also assessed the performance of a wide range of density functional theory (DFT) and double-hybrid DFT methods (in conjunction with the A'VQZ basis set) for the calculation of these quantities. For the calculation of S–F BDEs, the M06-2X procedure offers the best performance, with a mean absolute deviation (MAD) of 1.6 kJ mol–1, whilst for the FTEs, B2K-PLYP and DSD-PBEP86 offer the best performance with MADs of 0.5 kJ mol–1.

Triple photochemical domino reaction of a tetrafluorostilbene terminating in double fluorine atom transfer

In domino reactions, the product formed in one step undergoes a subsequent transformation under identical reaction conditions. Owing to the spontaneous nature of these reactions, it is difficult to isolate the key intermediates, and these are thus usually presumed. Here we perform a photoinduced domino reaction consisting of three photochemical steps. First, oxidative photocyclisation of a tetrafluorostilbene derivative generates tetrafluoro[7]helicene, which readily undergoes a photoinduced intramolecular Diels–Alder reaction. The resulting product then undergoes a double fluorine atom transfer under the same photochemical conditions. As a result, the four originally adjacent fluorine atoms are separated into two pairs in the final product. One advantage of a photochemical domino process over a thermal one is that the process can be suspended and restarted. Hence, precise control of the irradiation time allows us to isolate the thermally stable intermediates and characterise them using X-ray crystallography, thus confirming the until-now putative domino process.

Amidyl Radicals by Oxidation of α-Amido-oxy Acids: Transition-Metal-Free Amidofluorination of Unactivated Alkenes.

A three-component transition-metal-free amidofluorination of unactivated alkenes and styrenes is presented. α-Amido-oxy acids are introduced as efficient and easily accessible amidyl radical precursors that are oxidized by a photoexcited organic sensitizer (Mes-Acr-Me) to the corresponding carboxyl radical. Sequential CO2 and aldehyde/ketone fragmentation leads to an N-centered radical that adds to an alkene. Commercial Selectfluor is used to trap the adduct radical through fluorine-atom transfer. The transformation features by high functional-group tolerance, broad substrate scope, and practical mild conditions. Mechanistic studies support the radical nature of the cascade.

Radical Carbofluorination of Unactivated Alkenes with Fluoride Ions.

The copper-assisted radical carbofluorination of unactivated alkenes with fluoride ions is described. With [Cu(L3)F2]H2O (L3 = 4,4'-di(methoxycarbonyl)-2,2'-bipyridine) as the fluorine source and [Ag(DMPhen)(MeCN)]BF4 (DMPhen = 2,9-dimethyl-1,10-phenanthroline) as the chloride scavenger, the reaction of unactivated alkenes with CCl4 in acetonitrile provided the corresponding carbofluorination products in satisfactory yields. The protocol exhibited a wide functional group compatibility and broad substrate scope and could be extended to the use of a variety of activated alkyl chlorides other than CCl4. A copper-catalyzed fluorotrifluoromethylation of unactivated alkenes was then successfully developed with CsF as the fluorine source and Umemoto's reagent as the trifluoromethylating agent. A mechanism involving the fluorine atom transfer from Cu(II)-F complexes to alkyl radicals is proposed.

Synthesis of α-Fluoroketones from Vinyl Azides and Mechanism Interrogation.

An efficient and mild fluorination of vinyl azides for the synthesis of α-fluoroketones is described. The mechanistic studies indicated that a single-electron transfer (SET) and a subsequent fluorine atom transfer process could be involved in the reaction.

ChemInform Abstract: Fluoro‐Ene Reaction versus [2 + 2] Cycloaddition in the Thermal C2—C6 Cyclization of Enyne‐Allenes: An Experimental and Theoretical Investigation.

A series of fluoroalkyl-substituted enyne-allenes have been synthesized by a new route with the aim of elucidating the possibility of a fluoro-ene reaction via an intermediate fulvenyl diradical generated in the thermal C2–C6 (Schmittel) cyclization reaction. As a result of the strong C–F bond, fluorine atom transfer was not observed. Instead, 1H-cyclobuta[a]indenes were formed in good yields despite the high strain energy. DFT calculations at the B3LYP level of theory indicated that although the fluoro-ene reaction is the most exothermic reaction available, cyclobutene formation is kinetically favored over the [1,5]-fluorine shift by 5–15 kcal mol–1 in various model compounds.

Fluoro‐Ene Reaction Versus [2+2] Cycloaddition in the Thermal C2–C6 Cyclization of Enyne‐Allenes: An Experimental and Theoretical Investigation

AbstractA series of fluoroalkyl‐substituted enyne‐allenes have been synthesized by a new route with the aim of elucidating the possibility of a fluoro‐ene reaction via an intermediate fulvenyl diradical generated in the thermal C2–C6 (Schmittel) cyclization reaction. As a result of the strong C–F bond, fluorine atom transfer was not observed. Instead, 1H‐cyclobuta[a]indenes were formed in good yields despite the high strain energy. DFT calculations at the B3LYP level of theory indicated that although the fluoro‐ene reaction is the most exothermic reaction available, cyclobutene formation is kinetically favored over the [1,5]‐fluorine shift by 5–15 kcal mol–1 in various model compounds.

A Convenient Photocatalytic Fluorination of Unactivated CH Bonds

Fluorination reactions are essential to modern medicinal chemistry, thus providing a means to block site-selective metabolic degradation of drugs and access radiotracers for positron emission tomography imaging. Despite current sophistication in fluorination reagents and processes, the fluorination of unactivated CH bonds remains a significant challenge. Reported herein is a convenient and economic process for direct fluorination of unactivated CH bonds that exploits the hydrogen abstracting ability of a decatungstate photocatalyst in combination with the mild fluorine atom transfer reagent N-fluorobenzenesulfonimide. This operationally straightforward reaction provides direct access to a wide range of fluorinated organic molecules, including structurally complex natural products, acyl fluorides, and fluorinated amino acid derivatives.

A Convenient Photocatalytic Fluorination of Unactivated CH Bonds

AbstractFluorination reactions are essential to modern medicinal chemistry, thus providing a means to block site‐selective metabolic degradation of drugs and access radiotracers for positron emission tomography imaging. Despite current sophistication in fluorination reagents and processes, the fluorination of unactivated CH bonds remains a significant challenge. Reported herein is a convenient and economic process for direct fluorination of unactivated CH bonds that exploits the hydrogen abstracting ability of a decatungstate photocatalyst in combination with the mild fluorine atom transfer reagent N‐fluorobenzenesulfonimide. This operationally straightforward reaction provides direct access to a wide range of fluorinated organic molecules, including structurally complex natural products, acyl fluorides, and fluorinated amino acid derivatives.

ChemInform Abstract: Silver‐Catalyzed Radical Phosphonofluorination of Unactivated Alkenes.

We report herein a mild and catalytic phosphonofluorination of unactivated alkenes. With catalysis by AgNO3, the condensation of various unactivated alkenes with diethyl phosphite and Selectfluor reagent in CH2Cl2/H2O/HOAc at 40 °C led to the efficient synthesis of β-fluorinated alkylphosphonates with good stereoselectivity and wide functional group compatibility. A mechanism involving silver-catalyzed oxidative generation of phosphonyl radicals and silver-assisted fluorine atom transfer is proposed.

Formal fluorine atom transfer radical addition: silver-catalyzed carbofluorination of unactivated alkenes with ketones in aqueous solution

Using a combination of ketones and Selectfluor to mimic α-fluoroketones, formal fluorine-atom-transfer radical addition (F-ATRA) reactions were successfully developed in high efficiency.

Silver-Catalyzed Radical Phosphonofluorination of Unactivated Alkenes

We report herein a mild and catalytic phosphonofluorination of unactivated alkenes. With catalysis by AgNO3, the condensation of various unactivated alkenes with diethyl phosphite and Selectfluor reagent in CH2Cl2/H2O/HOAc at 40 °C led to the efficient synthesis of β-fluorinated alkylphosphonates with good stereoselectivity and wide functional group compatibility. A mechanism involving silver-catalyzed oxidative generation of phosphonyl radicals and silver-assisted fluorine atom transfer is proposed.