C−F Bond Activation by a Saturated N‐Heterocyclic Carbene: Mesoionic Compound Formation and Adduct Formation with B(C 6 F 5 ) 3

Access to NHC-Boryl Mono- and Bis-Selenide and Utility as Mild Selenium Transfer Reagent Including to the C-F Bond

Reactions of 5-SIDipp ⋅ BH3 (5-SIDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene) (1) with diphenyldiselenide provide access to 5-SIDipp-boryl mono- (5-SIDipp ⋅ BH2 SePh) (2) and bis-selenide (5-SIDipp ⋅ BH(SePh)2 ) (3). The facile cleavage of the B-Se bond makes 2 a neutral source of selenium nucleophiles in substitutions reactions with benzyl bromides, and provide access to the corresponding selenoethers. The direct transformations of one of the C(sp2 )-F bonds of C5 F5 N and C6 F5 CF3 to C-Se bonds have also been achieved by the use of 2 without employing transition-metal catalysts. While it was previously established that C6 F6 could undergo complete defluoroselenation under harsh conditions, we successfully achieved partial defluorination of C6 F6 by employing 2 as a mild selenide transfer reagent. During the formation of C-Se bonds through the cleavage of C-F bonds, the potential by-product NHC ⋅ BH2 F undergoes ring expansion of the NHC, leading to the formation of the six-membered diaazafluoroborinane (7).

Photoinduced Dual C-F Bond Activation of Hexafluorobenzene Mediated by Boron Atom

The reaction of laser-ablated boron atoms with hexafluorobenzene (C6 F6 ) was investigated in neon and argon matrices, and the products are identified by matrix isolation infrared spectroscopy and quantum-chemical calculations. The reaction is triggered by a boron atom insertion into one C-F bond of hexafluorobenzene on annealing, forming a fluoropentafluorophenyl boryl radical (A). UV-Vis light irradiation of fluoropentafluorophenyl boryl radical causes generation of a 2-difluoroboryl-tetrafluorophenyl radical (B) via a second C-F bond activation. A perfluoroborepinyl radical (C) is also observed upon deposition and under UV-Vis light irradiation. This finding reveals the new example of a dual C-F bond activation of hexafluorobenzene mediated by a nonmetal and provides a possible route for synthesis of new perfluorinated organo-boron compounds.

Enhancing Diradical Character of Chichibabin's Hydrocarbon through Fluoride Substitution

In this work, 5-SIDipp [SIDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] (1) derived Chichibabin's hydrocarbon with an octafluorobiphenylene spacer (3) has been reported. The addition of two equivalents of 5-SIDipp with decafluorobiphenyl in presence of BF3 gives the double C-F bond activated imidazolium salt with two tetrafluoroborate anions, 2. Further reduction of 2 gives the fluorine substituted 5-SIDipp based Chichibabin's hydrocarbon, 3. Quantum chemical calculations suggested a singlet state of 3 with a singlet-triplet energy gap (ΔES-T ) of 3.7 kcal mol-1 , which is substantially lower with respect to the hydrogen substituted NHC-based Chichibabin's hydrocarbons (10.7 kcal mol-1 , B3LYP). As a result, the diradical character (y) of 3 (y=0.62) is also noticeably higher than the hydrogen substituted CHs (y=0.41-0.43). The ▵ES-T was found to be higher in CASSCF (22.24 kcal mol-1 ) and CASPT2 (11.17 kcal mol-1 ) for 3 and the diradical character (d) is 44.6 %.

B(C6 F5 )3 -Catalyzed Tandem Friedel-Crafts and C-H/C-O Coupling Reactions of Dialkylanilines

Tandem Friedel-Crafts (FC) and C-H/C-O coupling reactions catalyzed by tris(pentafluorophenyl) borane (B(C₆ F₅ )₃ ) were achieved without using any other additive in the absence of solvent. This process can be used for the reactions between a series of dialkylanilines and vinyl ethers with good isolated yields of bis(4-dialkylaminophenyl) compounds. Based on combined theoretical and experimental studies, the possible reaction mechanism was proposed. B(C₆ F₅ )₃ can activate the C=C and C-O bond for FC and C-H/C-O coupling reactions respectively. The FC reaction is slow, which is followed by a fast C-H/C-O coupling.

Activation of Aromatic C-F Bonds by a N-Heterocyclic Olefin (NHO)

A N-heterocyclic olefin (NHO), a terminal alkene selectively activates aromatic C-F bonds without the need of any additional catalyst. As a result, a straightforward methodology was developed for the formation of different fluoroaryl-substituted alkenes in which the central carbon-carbon double bond is in a twisted geometry.

Activation of C–F, Si–F, and S–F Bonds by N-Heterocyclic Carbenes and Their Isoelectronic Analogues

Reactions involving C–F, Si–F, and S–F bond cleavage with N-heterocyclic carbenes and isoelectronic species are reviewed. Most examples involve activation of aromatic C–F bond via an SNAr pathway and nucleophilic substitution of fluorine in electron-deficient olefins. The mechanism of the C–F bond activation depends on the reaction partners and the reaction can proceed via addition–elimination, oxidative addition (concerted or stepwise) or metathesis. The adducts formed upon substitution find applications in organic synthesis, as ligands and as stable radical precursors, but in most cases, their full potential remains unexplored.

1 Introduction

1.1 The C–F Bond

1.2 C–F Bond Activation: A Short Summary

1.3 C–F Bond Activation: A Special Case of SNAr

1.4 N-Heterocyclic Carbenes (NHCs)

1.5 The Purpose of this Article

2 C–F bond Activation in Acyl Fluorides

3 Activation of Vinylic C–F Bonds

4 Activation of Aromatic C–F Bonds

5 X–F Bond Activation (X = S or Si)

6 C–F Bond Activation by Main Group Compounds Isoelectronic with NHCs

7 Conclusions and Outlook

Saturated N-Heterocyclic Carbene Based Thiele's Hydrocarbon with a Tetrafluorophenylene Linker

The synthesis of a SIPr [1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] derived Kekulé diradicaloid with a tetrafluorophenylene spacer (3) has been described. Two synthetic routes have been reported to access 3. The cleavage of C-F bond of C₆ F₆ by SIPr in the presence of BF₃ led to double C-F activated compound with two tetrafluoro borate counter anions (2), which upon reduction by lithium metal afforded 3. Alternatively, 3 can be directly accessed in one step by reacting SIPr with C₆ F₆ in presence of Mg metal. Compounds 2 and 3 were well characterized spectroscopically and by single-crystal X-ray diffraction studies. Experimental and computational studies support the cumulenic closed-shell singlet state of 3 with a singlet-triplet energy gap (ΔE_S-T ) of 23.7 kcal mol^-1 .

The crystal structure of 1,1′-(9-ethyl-9H-carbazole-3,6-diyl)bis(3-ethyl-1H-imidazol-3-ium) bis(hexafluorophosphate(IV)), C24H27N5F12P2

C24H27N5F12P2, monoclinic, P21/n (no. 14), a = 12.395(4) Å, b = 16.630(6) Å, c = 26.899(10) Å, β = 90.395(7)°, V = 5545(4) Å3, Z = 8, Rgt(F) = 0.0650, wRref(F2) = 0.1651, T = 113(2) K.

C-H and C-F Bond Activation Reactions of Fluorinated Propenes at Rhodium: Distinctive Reactivity of the Refrigerant HFO-1234yf

The reaction of [Rh(H)(PEt₃ )₃ ] (1) with the refrigerant HFO-1234yf (2,3,3,3-tetrafluoropropene) affords an efficient route to obtain [Rh(F)(PEt₃ )₃ ] (3) by C-F bond activation. Catalytic hydrodefluorinations were achieved in the presence of the silane HSiPh₃ . In the presence of a fluorosilane, 3 provides a C-H bond activation followed by a 1,2-fluorine shift to produce [Rh{(E)-C(CF₃ )=CHF}(PEt₃ )₃ ] (4). Similar rearrangements of HFO-1234yf were observed at [Rh(E)(PEt₃ )₃ ] [E=Bpin (6), C₇ D₇ (8), Me (9)]. The ability to favor C-H bond activation using 3 and fluorosilane is also demonstrated with 3,3,3-trifluoropropene. Studies are supported by DFT calculations.