Anti-Markovnikov hydroallylation reaction of alkenes via scandium-catalyzed allylic C‒H activation

Compared with rare-earth (RE)…heteroatom interaction, RE…π interaction, frequently used in facilitating regio- and stereoselectivity of olefin polymerizations, is seldomly used to trigger catalytic C - H functionalization. Here, we describe a direct anti-Markovnikov hydroallylation reaction of styrene derivatives with 1-aryl-2-alkyl alkenes and α-alkenes by use of RE…π interaction. This protocol provides a straightforward and atom-efficient route for the synthesis of valuable chain elongated internal alkenes (65 examples, up to 99% yield, > 19:1 E/Z ratio). The reaction proceeds via an allylic Csp3‒H activation pathway initiated by site-selective deprotonation with the assistance of cationic imidazolin-2-iminato scandium alkyl species followed by alkene insertion into the resulting scandium-allyl bond. A catalytic amount of Lewis base additives, such as amine and tetrahydrofuran (THF) show significant effects on the reactivity and E/Z selectivity. The reaction mechanism is elucidated by experimental studies and theoretical calculations.

Controlled monodefluorination and alkylation of C(sp3)-F bonds by lanthanide photocatalysts: importance of metal-ligand cooperativity

The controlled functionalization of a single fluorine in a CF3 group is difficult and rare. Photochemical C-F bond functionalization of the sp3-C-H bond in trifluorotoluene, PhCF3, is achieved using catalysts made from earth-abundant lanthanides, (CpMe4)2Ln(2-O-3,5- t Bu2-C6H2)(1-C{N(CH)2N(iPr)}) (Ln = La, Ce, Nd and Sm, CpMe4 = C5Me4H). The Ce complex is the most effective at mediating hydrodefluorination and defluoroalkylative coupling of PhCF3 with alkenes; addition of magnesium dialkyls enables catalytic C-F bond cleavage and C-C bond formation by all the complexes. Mechanistic experiments confirm the essential role of the Lewis acidic metal and support an inner-sphere mechanism of C-F activation. Computational studies agree that coordination of the C-F substrate is essential for C-F bond cleavage. The unexpected catalytic activity for all members is made possible by the light-absorbing ability of the redox non-innocent ligands. The results described herein underscore the importance of metal-ligand cooperativity, specifically the synergy between the metal and ligand in both light absorption and redox reactivity, in organometallic photocatalysis.

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.

Recent Advances in Transition-metal-catalyzed C-C Bond Formation via C(sp2 )-F Bond Cleavage

The activation of a carbon-fluorine bond is one of the most challenging topics in modern synthetic organic chemistry due to their low reactivity compared to other carbon-halogen bonds. In this review, we present the recent developments since 2015 on cross-coupling reactions that form C-C bonds via cleavage of C(sp² )-F bonds. Not only the conventional activation of C(sp² )-F bonds, but also decarbonylative or carbonyl-retentive cleavage of C(acyl)-F bonds will be introduced. This paper mainly describes new protocols for the formation of C(sp² )-C(sp³ ), C(sp² )-C(sp² ), and C(sp² )-C(sp) bonds via transition-metal-catalyzed cleavage of C(sp² )-F bonds.

Recent advance in the C–F bond functionalization of trifluoromethyl-containing compounds

The C–F bond is the strongest single bond in organic compounds.

Lewis acid capping of a uranium(v) nitride via a uranium(iii) azide molecular square

Addition of B(C₆F₅)₃ to a tetrameric uranium(iii) azide-bridged molecular square induced N₂ loss and formation of a uranium(v) borane-capped nitride.

Reactions of a cerium(iii) amide with heteroallenes: insertion, silyl-migration and de-insertion

Reactions of Ce[N(SiMe3)Ph(F)]3 (-Ph(F) = pentafluorophenyl) toward small molecules of the type E1[double bond, length as m-dash]C[double bond, length as m-dash]E2 (E1, E2 = O, S, NR), including carbon disulfide, carbodiimide, carbon dioxide, isocyanate and isothiocyanate are reported, resulting in distinct products, including cerium(iii) dithiocarbamate, cerium(iii) guanidinate, isocyanates and unsymmetric carbodiimides. These reactions were rationalized as three consecutive stages of the same reaction pathway: insertion, silyl-migration and de-insertion.

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.

Tetrametallic Thorium Compounds with Th4E4 (E = S, Se) Cubane Cores

Tetrametallic thorium compounds with a Th₄E₄ core (E = S, Se) having a distorted cubane structure can be prepared by ligand-based reductions of elemental E with thorium chalcogenolates, prepared by in situ oxidation of Th metal with a 3:1 mixture of PhEEPh and F₅C₆EEC₆F₅. Four compounds, (py)₈Th₄S₄(μ₂-SPh)₄(SC₆F₅)₄, (py)₈Th₄S₄(μ₂-SPh)₄(SeC₆F₅)₄, (py)₈Th₄Se₄(μ₂-SePh)₄(SeC₆F₅)₄, and (py)₈Th₄Se₄(μ₂-SePh)₄(SC₆F₅)₄, were isolated and characterized by NMR spectroscopy and X-ray diffraction. These compounds clearly demonstrate the chemical impact of ring fluorination, with the less-nucleophilic EC₆F₅ ligands occupying the terminal binding sites and the EPh ligands bridging two metal centers. For this series of compounds, crystal packing and intermolecular π···π and H-bonding interactions result in a consistent motif and crystallization in a body-centered tetragonal unit cell. Solution-state ⁷⁷Se NMR spectroscopy reveals that the solid-state structures are maintained in pyridine.

Cerium Photosensitizers: Structure-Function Relationships and Applications in Photocatalytic Aryl Coupling Reactions

Two complete mixed-ligand series of luminescent Ce(III) complexes with the general formulas [(Me3Si)2NC(N(i)Pr)2]xCe(III)[N(SiMe3)2]3-x (x = 0, 1-N; x = 1, 2-N, x = 2, 3-N; x = 3, 4) and [(Me3Si)2NC(N(i)Pr)2]xCe(III)(OAr)3-x (x = 0, 1-OAr; x = 1, 2-OAr, x = 2, 3-OAr; x = 3, 4) were developed, featuring photoluminescence quantum yields up to 0.81(2) and lifetimes to 117(1) ns. Although the 4f → 5d absorptive transitions for these complexes were all found at ca. 420 nm, their emission bands exhibited large Stokes shifts with maxima occurring at 553 nm for 1-N, 518 nm for 2-N, 508 nm for 3-N, and 459 nm for 4, featuring yellow, lime-green, green, and blue light, respectively. Combined time-dependent density functional theory (TD-DFT) calculations and spectroscopic studies suggested that the long-lived (2)D excited states of these complexes corresponded to singly occupied 5dz(2) orbitals. The observed difference in the Stokes shifts was attributed to the relaxation of excited states through vibrational processes facilitated by the ligands. The photochemistry of the sterically congested complex 4 was demonstrated by C-C bond forming reaction between 4-fluoroiodobenzene and benzene through an outer sphere electron transfer pathway, which expands the capabilities of cerium photosensitizers beyond our previous results that demonstrated inner sphere halogen atom abstraction reactivity by 1-N.