Experimental and computational insights into the mechanism of FLP mediated selective C-F bond activation

C(sp3)-F Bond Activation by Lewis Base-Boryl Radicals via Concerted Electron-Fluoride Transfer

Selective C-F bond activation through a radical pathway in the presence of multiple C-H bonds remains a formidable challenge, owing to the extraordinarily strong bond strength of the C-F bond. By the aid of density functional theory calculations, we disclose an innovative concerted electron-fluoride transfer mechanism, harnessing the unique reactivity of Lewis base-boryl radicals to selectively activate the resilient C-F bonds in fluoroalkanes. This enables the direct abstraction of a fluorine atom and subsequent generation of an alkyl radical, thus expanding the boundaries of halogen atom transfer reactions.

Defluorinative thio-functionalization: direct synthesis of methyl-dithioesters from trifluoromethylarenes

A new functional group transformation allowing the synthesis of methyl-dithioesters from readily available trifluoromethyl arenes via defluorinative functionalization has been developed. This microwave-assisted method is operationally simple, rapid, and eliminates the need for pre-functionalization while accommodating a broad range of functional groups. In addition, it does not rely on highly odorous thiol sources, and utilizes the commercially available reagent BF3SMe2 complex as a multifunctional Lewis acid/sulfur source/defluorination and demethylation agent. Finally, this approach is suitable for late-stage functionalizations, as shown by the transformation of pharmaceuticals leflunomide, flufenamic acid and celecoxib into novel methyl-dithioester derivatives.

Evidence for a kinetic FLP reaction pathway in the activation of benzyl chlorides by alkali metal-phosphine pairs

Kinetic frustrated Lewis pairs (FLP) allow facile cleavage of a number of E-H bonds (E = H, Si, C, B) where both the Lewis base and Lewis acid are involved in the bond activation transition state. More recently, kinetic FLP systems have been extended to the cleavage of C-X (X = F, Cl, Br) bonds. We report on the role of sodium tetrakis(pentafluorophenyl)borate in the benzylation of triarylphosphines, where the sodium cation and phosphine support a kinetic FLP type transition state.

Photoreduction of Trifluoromethyl Group: Lithium Ion Assisted Fluoride-Coupled Electron Transfer from EDA Complex

Photoinduced single-electron reduction is an efficient method for the mono-selective activation of the C-F bond on a trifluoromethyl group to construct a difluoroalkyl group. We have developed an electron-donor-acceptor (EDA) complex mediated single-electron transfer (EDA-SET) of α,α,α-trifluoromethyl arenes in the presence of lithium salt to give α,α-difluoroalkylarenes. The C-F bond reduction was realized by lithium iodide and triethylamine, two common feedstock reagents. Mechanistic studies revealed the generation of a α,α-difluoromethyl radical by single-electron reduction and defluorination, followed by the radical addition to alkenes. Lithium salt interacted with the fluorine atom to promote the photoinduced reduction mediated by the EDA complex. Computational studies indicated that the lithium-assisted defluorination and the single-electron reduction occurred concertedly. We call this phenomenon fluoride-coupled electron transfer (FCET). FCET is a novel approach to C-F bond activation for the synthesis of organofluorine compounds.

A review of frustrated Lewis pair enabled monoselective C-F bond activation

Frustrated Lewis pair (FLP) bond activation chemistry has greatly developed over the last two decades since the seminal report of metal-free reversible hydrogen activation. Recently, FLP systems have been utilized to allow monoselective C-F bond activation (at equivalent sites) in polyfluoroalkanes. The problem of 'over-defluorination' in the functionalization of polyfluoroalkanes (where multiple fluoro-positions are uncontrollably functionalized) has been a long-standing chemical problem in fluorocarbon chemistry for over 80 years. FLP mediated monoselective C-F bond activation is complementary to other solutions developed to address 'over-defluorination' and offers several advantages and unique opportunities. This perspective highlights some of these advantages and opportunities and places the development of FLP mediated C-F bond activation into the context of the wider effort to overcome 'over-defluorination'.

Photoinduced copper-catalyzed C-N coupling with trifluoromethylated arenes

Selective defluorinative functionalization of trifluoromethyl group (-CF3) is an attractive synthetic route to the pharmaceutically privileged fluorine-containing moiety. Herein, we report a strategy based on photoexcited copper catalysis to activate the C-F bond of di- or trifluoromethylated arenes for divergent radical C-N coupling with carbazoles and aromatic amines. The use of different ligands can tune the reaction products diversity. A range of substituted, structurally diverse α,α-difluoromethylamines can be obtained from trifluoromethylated arenes via defluorinative C-N coupling with carbazoles, while an interesting double defluorinative C-N coupling is ready for difluoromethylated arenes. Based on this success, a carbazole-centered PNP ligand is designed to be an optimal ligand, enabling a copper-catalyzed C-N coupling for the construction of imidoyl fluorides from aromatic amines through double C-F bond functionalization. Interestingly, a 1,2-difluoroalkylamination strategy of styrenes is also developed, delivering γ,γ-difluoroalkylamines, a bioisostere to β-aminoketones, in synthetically useful yields. The DFT studies reveal an inner-sphere electron transfer mechanism for Cu-catalyzed selective activation of C(sp3)-F bonds.

Stereoselective Synthesis of Fluoroalkanes via FLP Mediated Monoselective C─F Activation of Geminal Difluoroalkanes

A method of desymmetrization of geminal difluoroalkanes using frustrated Lewis pair (FLP) mediated monoselective C-F activation where a chiral sulfide is the Lewis base component is reported. The stereoselective reaction provides generally high yields of diastereomeric sulfonium salts with dr of up to 95:5. The distribution of diastereomers is found to be thermodynamically controlled via facile sulfide exchange. The use of enantiopure chiral sulfides allows for high stereospecificity in nucleophilic substitution reactions and the formation of stereoenriched products.

Insights into the Mechanism of Aluminum-Catalyzed Halodefluorination

Aluminum has been reported to catalyze halodefluorination reactions, where aliphatic fluorine is substituted with a heavier halogen. Although it is known that stoichiometric aluminum halide can perform this reaction, the role of catalytic aluminum halide and organyl alane reagents is not well understood. We investigate the mechanism of the halodefluorination reaction using catalytic aluminum halide and stoichiometric trimethylsilyl halide. We explore the use of B(C₆F₅)₃ as a catalyst to benchmark pathways where aluminum acts either as a Lewis acid catalyst in cooperation with trimethylsilyl halide or as an independent halodefluorination reagent which is subsequently regenerated by trimethylsilyl halide. Computational and experimental results indicate that aluminum acts as an independent halodefluorination reagent and that reactivity trends observed between different halide reagents can be attributed to relative barriers in halide delivery to the organic fragment, which is the rate-limiting step in both the aluminum halide- and B(C₆F₅)₃-catalyzed pathways.