Choosing between Ti(II) and Ti(III): selective reduction of titanocene dichloride by elemental lanthanides

The reduction of titanocene dichloride Cp2TiCl2 with lanthanide metals has led to the discovery of a surprising lanthanide effect: while with most lanthanides, a divalent [Cp2Ti] equivalent was obtained, the use of samarium or ytterbium only led to the reduction to trivalent [Cp2TiCl]-type complexes, including the structurally characterized heterobimetallic complex [Cp2Ti(μ-Cl)2SmCl2(THF)3]. These results were corroborated by reactivity studies (alkyne coupling and radical reactions), EPR spectroscopy and electrospray mass spectrometry, providing new insights into the reduction chemistry of lanthanide metals.

Europium is Different: Solvent and Ligand Effects on Oxidation State Outcomes and C-F Activation in Reactions Between Europium Metal and Pentafluorophenylsilver

Unique outcomes have emerged from the redox transmetallation/ protolysis (RTP) reactions of europium metal with [Ag(C₆ F₅ )(py)] (py=pyridine) and pyrazoles (RR'pzH). In pyridine, a solvent not normally used for RTP reactions, the products were mainly Eu^II complexes, [Eu(RR'pz)₂ (py)₄ ] (RR'pz=3,5-diphenylpyrazolate (Ph₂ pz) 1; 3-(2-thienyl)-5-trifluoromethylpyrazolate (ttfpz) 2; 3-methyl-5-phenylpyrazolate (PhMepz) 3). However, use of 3,5-di-tert-butylpyrazole (tBu₂ pzH) gave trivalent [Eu(tBu₂ pz)₃ (py)₂ ] 4, whereas the bulkier N,N'-bis(2,6-difluorophenyl)formamidine (DFFormH) gave divalent [Eu(DFForm)₂ (py)₃ ] 5. In tetrahydrofuran (thf), the usual solvent for RTP reactions, C-F activation was observed for the first time with [Ag(C₆ F₅ )(py)] in such reactions. Thus trivalent [{Eu₂ (Ph₂ pz)₄ (py)₄ (thf)₂ (μ-F)₂ }{Eu₂ (Ph₂ pz)₄ (py)₂ (thf)₄ (μ-F)₂ }] (6), [Eu₂ (ttfpz)₄ (py)₂ (dme)₂ (μ-F)₂ ] (7), [Eu₂ (tBu₂ pz)₄ (dme)₂ (μ-F)₂ ] (8) were obtained from the appropriate pyrazoles, the last two after crystallization from 1,2-dimethoxyethane (dme). Surprisingly 3,5-dimethylpyrazole (Me₂ pzH) gave the divalent cage [Eu₆ (Me₂ pz)₁₀ (thf)₆ (μ-F)₂ ] (9). This has a compact ovoid core held together by bridging fluoride, thf, and pyrazolate ligands, the last including the rare μ₄ -1η⁵ (N₂ C₃ ): 2η² (N,N'): 3κ(N): 4κ(N') pyrazolate binding mode. With the bulky N,N'-bis(2,6-diisopropylphenyl)formamidine (DippFormH), which often favours C-F activation in RTP reactions, neither oxidation to Eu^III nor C-F activation was observed and [Eu(DippForm)₂ (thf)₂ ] (10) was isolated. By contrast, Eu reacted with Bi(C₆ F₅ )₃ and Ph₂ pzH or tBu₂ pzH in thf without C-F activation, to give [Eu(Ph₂ pz)₂ (thf)₄ ] (11) and [Eu(tBu₂ pz)₃ (thf)₂ ] (12) respectively, the oxidation state outcomes corresponding to that for use of [Ag(C₆ F₅ )(py)] in pyridine.

Chromium-Catalyzed Allylic Defluorinative Ketyl Olefin Coupling

Herein we report a chromium-catalyzed allylic defluorinative ketyl olefin coupling between aldehydes and α-trifluoromethyl alkenes, which provides novel and efficient access to diverse gem-difluorohomoallylic alcohols. Remarkably, the high chemoselectivity of this reaction enables the conversion of the formyl moiety in the presence of various easily reducible functionalities including ketone, organohalides, aziridine, sulfone, alkyne, and unactivated alkene. The utility of this method is demonstrated by various simple derivatizations of the attached hydroxyl group of the coupling products. The preliminary mechanistic investigations suggest a reaction pathway with a rate-limiting C-C forming step followed by facile β-fluoro elimination.

[4 + 2] Cycloaddition of trifluoromethyl ketimines with 2-alkenyl azaarenes through selective C–F bond cleavage of CF3

A transition-metal-free [4 + 2] cycloaddition of trifluoromethyl ketimines with 2-alkenyl azaarenes through selective C–F bond cleavage of CF₃ has been developed to afford cis-tetrahydroxypyridine products in moderate yields under mild conditions.

Picking One out of Three: Selective Single C-F Activation in Trifluoromethyl Groups

The introduction of a trifluoromethyl (CF₃ ) group into an organic molecule can modify its chemical behavior and lead to changes in its physicochemical and pharmacological properties. The CF₃ group is often chosen for its chemical inertness and stability, which are related to the strong C-F bonds. In recent years, the potential of gaining straightforward access to difluorinated compounds through selective single C-F activation in CF₃ groups has been unveiled. This review describes the latest methodologies for the synthesis of difluoromethylated arenes, difluoroalkenes and difluorinated aldol products of potential synthetic and/or biological interest, employing low-valent metals, transition metal catalysis, FLP and Lewis acid mediated transformations as well as photoredox catalysis.

Application of Elemental Lanthanides in the Selective C-F Activation of Trifluoromethylated Benzofulvenes Providing Access to Various Difluoroalkenes

The selective activation of one carbon-fluorine bond in polyfluorinated aromatic molecules or in trifluoromethyl-containing substrates offers the possibility of accessing unique fluorine-containing molecules, which are difficult to obtain by other synthetic pathways. Among various metals, which can undergo C-F activation, lanthanides (Ln) are good candidates as they form strong Ln-F bonds. Lanthanide metals are strong reducing agents with a redox potential Ln³⁺/Ln of approximately -2.3 V, which is comparable to the value of the Mg²⁺/Mg redox couple. In addition, lanthanide metals display a promising functional group tolerance and their reactivity can vary along the lanthanide series, making them suitable reagents for fine-tuning reaction conditions in organic and organometallic transformations. However, due to their oxophilicity, lanthanides react readily with oxygen and water and therefore require special conditions for storage, handling, preparation, and activation. These factors have limited a more widespread use in organic synthesis. We herein present how dysprosium metal - and by analogy all lanthanide metals - can be freshly prepared under anhydrous conditions using glovebox and Schlenk techniques. The freshly filed metal, in combination with aluminum chloride, initiates the selective C-F activation in trifluoromethylated benzofulvenes. The resulting reaction intermediates react with nitroalkenes to obtain a new family of difluoroalkenes.

Silylated cyclopentadienes as competent silicon Lewis acid catalysts

The synthesis and characterization of silicon Lewis acid complexes that incorporate highly electron-deficient cyclopentadienes is reported. Several pentacarboxycyclopentadienyl and monocarboxytetracyanocyclopentadienyl complexes were prepared. A comparison of their reactivities for catalysis of the allylation of an electron-deficient benzaldehyde was established. The use of a monocarboxytetracyano silylium donor was shown to be effective for the allylation or arylation of a variety of electrophiles via an anion abstraction pathway.

(2-Fluoroallyl)boronates: new reagents for diastereoselective 2-fluoroallylboration of aldehydes

New reagents for anti-selective 2-fluoroallylboration of aromatic and aliphatic aldehydes—(2-fluoroallyl)pinacolboranes, were prepared by Pd-catalyzed borylation of 2-fluoroallyl chlorides.

Two tetranuclear 3d–4f heterometal complexes Mn2Ln2 (Ln = Dy, Gd): synthesis, structure, magnetism, and electrocatalytic reactivity for water oxidation

The two Mn₂Ln₂ complexes can catalyze water oxidation, which may arise from the cooperative catalytic effect of the Mn/Ln ions.

Activation of C–F bonds α to C–C multiple bonds

A closer look is given to the successful approaches to the C(sp³)–F activation of benzylic, allylic, propargylic and allenylic fluorides.