Photocatalytic Generation of Germyl Radicals from Digermanes Enabling the Hydro/Deuteriogermylation of Alkenes

We have developed a visible-light-induced method to photolyze digermanes through single-electron oxidation using a photocatalyst, in contrast to direct excitation, to generate germyl radicals and achieve the hydro/deuteriogermylation of alkenes. This protocol allows the previously elusive incorporation of the small trimethylgermyl group and deuterium, metabolically stable bioisosteres of tert-butyl and hydrogen, respectively, making this approach attractive in not only organic synthesis but also medicinal chemistry.

C-H and C-F bond activation of fluorinated propenes at Rh: enabling cross-coupling reactions with outer-sphere C-C coupling

The reaction of [Rh{(E)-CF[double bond, length as m-dash]CHCF3}(PEt3)3] with Zn(CH3)2 results in the methylation of the alkenyl ligand to give [Rh{(E/Z)-C(CH3)[double bond, length as m-dash]CHCF3}(PEt3)3]. Variable temperature NMR studies allowed the identification of a heterobinuclear rhodium-zinc complex as an intermediate, for which the structure [Rh(CH3)(ZnCH3){(Z)-C(CH3)[double bond, length as m-dash]CHCF3}(PEt3)2] is proposed. Based on these stoichiometric reactions, unique Negishi-type catalytic cross-coupling reactions of fluorinated propenes by consecutive C-H and C-F bond activation steps at room temperature were developed. The C-H bond activation steps provide a fluorinated ligand at Rh and deliver the fluorinated product, whereas the C-F bond activation and C-C coupling occur via outer-sphere nucleophilic attack at the fluorinated alkenyl ligand.

Modular access to alkylgermanes via reductive germylative alkylation of activated olefins under nickel catalysis

Carbon-introducing difunctionalization of C-C double bonds enabled by transition-metal catalysis is one of most straightforward and efficient strategies to construct C-C and C-X bonds concurrently from readily available feedstocks towards structurally diverse molecules in one step; however, analogous difunctionalization for introducing germanium group and other functionalities remains elusive. Herein, we describe a nickel-catalyzed germylative alkylation of activated olefins with easily accessible primary, secondary and tertiary alkyl bromides and chlorogermanes as the electrophiles to form C-Ge and C-Calkyl bonds simultaneously. This method provides a modular and facile approach for the synthesis of a broad range of alkylgermanes with good functional group compatibility, and can be further applied to the late-stage modification of natural products and pharmaceuticals, as well as ligation of drug fragments. More importantly, this platform enables the expedient synthesis of germanium substituted ospemifene-Ge-OH, which shows improved properties compared to ospemifene in the treatment of breast cancer cells, demonstrating high potential of our protocol in drug development.

Direct C-H Dehydrogenative Germylation of Terminal Alkynes with Hydrogermanes

A direct C(sp)-H germylation of terminal alkynes with triethyl germanium hydride is reported. The method is operationally simple and makes use of B(C₆F₅)₃ catalysis in combination with 2,6-lutidine as an organic base. Exclusive selectivity for dehydrogenative germylation of the alkyne over the competing hydrogermylation is observed.

Competing C-H and C-F bond activation reactions of a fluorinated olefin at Rh: a fluorido vinylidene complex as an intermediate in an unprecedented dehydrofluorination step

The hydrofluoroolefin Z-1,3,3,3-tetrafluoropropene has been activated via an initial C-F bond activation and subsequent C-H bond activation using [Rh(H)(PEt3)3] (1) or via C-H bond activation at [Rh(CH3)(PEt3)3] (8). In both cases the formation of [Rh{(E)-CF[double bond, length as m-dash]CHCF3}(PEt3)3] (3) was observed. Importantly, the C-F activation product [Rh{(E)-CH[double bond, length as m-dash]CHCF3}(PEt3)3] (2) reacts in the presence of Z-1,3,3,3-tetrafluoropropene into 3. The latter converted into [Rh(C[triple bond, length as m-dash]CCF3)(PEt3)3] (6) by an unprecedented dehydrofluorination reaction, presumably via a vinylidene complex as intermediate. When the carbonyl complex [Rh(C[triple bond, length as m-dash]CCF3)(CO)(PEt3)3] (12) was treated with an excess of NEt3·3HF or HBF4 at low temperature, the formation of the phosphonioalkenyl compounds [Rh{(Z)-C(PEt3)[double bond, length as m-dash]CHCF3}(CO)(PEt3)2]X (X = F(HF) x , BF4) (13) was observed. The formation of 13 can be explained by an attack of PEt3 at the electrophilic α-carbon atom of an intermediate vinylidene complex. The employment of PiPr3 derivatives as model compounds allowed for the isolation of the unique fluorido vinylidene complex trans-[Rh(F)([double bond, length as m-dash]C[double bond, length as m-dash]CHCF3)(PiPr3)2] (16), which in the presence of PEt3 transforms into [Rh(C[triple bond, length as m-dash]CCF3)(PEt3)3] (6).

Hydrogermylation of Alkenes via Organophotoredox-Initiated HAT Catalysis

This Letter discloses the straightforward hydrogermylation of olefins under visible-light organophotoredox-initiated HAT catalysis conditions to yield primary and secondary alkyl germanes at room temperature. The protocol is operationally simple, metal-free, and tolerant of various functional groups. The synthesized alkyl germanes proved to be highly robust toward acidic, basic, or oxidizing conditions and chemical transformations of C_sp²-GeEt₃ or C_sp²-BPin functionalities in their presence.

Versatile Reaction Pathways of 1,1,3,3,3-Pentafluoropropene at Rh(I) Complexes [Rh(E)(PEt3 )3 ] (E=H, GePh3 , Si(OEt)3 , F, Cl): C-F versus C-H Bond Activation Steps

The reaction of the rhodium(I) complexes [Rh(E)(PEt₃)₃] (E=GePh₃ (1), H (6), F (7)) with 1,1,3,3,3‐pentafluoropropene afforded the defluorinative germylation products Z/E‐2‐(triphenylgermyl)‐1,3,3,3‐tetrafluoropropene and the fluorido complex [Rh(F)(CF₃CHCF₂)(PEt₃)₂] (2) together with the fluorophosphorane E‐(CF₃)CH=CF(PFEt₃). For [Rh(Si(OEt)₃)(PEt₃)₃] (4) the coordination of the fluoroolefin was found to give [Rh{Si(OEt)₃}(CF₃CHCF₂)(PEt₃)₂] (5). Two equivalents of complex 2 reacted further by C−F bond oxidative addition to yield [Rh(CF=CHCF₃)(PEt₃)₂(μ‐F)₃Rh(CF₃CHCF₂)(PEt₃)] (9). The role of the fluorido ligand on the reactivity of complex 2 was assessed by comparison with the analogous chlorido complex. The use of complexes 1, 4 and 6 as catalysts for the derivatization of 1,1,3,3,3‐pentafluoropropene provided products, which were generated by hydrodefluorination, hydrometallation and germylation reactions.

Rhodium(I) Complexes as Useful Tools for the Activation of Fluoroolefins

In this account we describe studies on the reactivity of rhodium(I) complexes of the type [Rh(E)(PEt3)3], where E represents hydrido, fluorido, germyl, boryl or silyl ligands, towards fluorinated olefins. The results are compared with those reported by other research groups on fluoroolefins, as well as with the chemistry of compounds [Rh(E)(PEt3)3] towards fluoroaromatics in terms of selectivity and mechanisms.

1 Introduction

2 Reactivity Towards Fluoroolefins

2.1 Reactivity of Hexafluoropropene

2.2 Reactivity of (E)-1,2,3,3,3-Pentafluoropropene

2.3 Reactivity of 2,3,3,3-Tetrafluoropropene and (E)-1,3,3,3-Tetrafluoropropene

2.4 Reactivity of 3,3,3-Trifluoropropene

2.5 Reactivity of Pentafluorostyrene

3 Conclusion and Perspective

Activation of tetrafluoropropenes by rhodium(i) germyl and silyl complexes

The reaction of the rhodium(i) complexes [Rh(E)(PEt3)3] (E = GePh3 (1), Si(OEt)3 (5)) with HFO-1234yf (2,3,3,3-tetrafluoropropene) afforded [Rh(F)(PEt3)3] (2) and the functionalized olefins Z-CF3CH[double bond, length as m-dash]CH(E) (E = GePh3 (4a), Si(OEt)3 (7)). Conceivable reaction pathways were assessed using DFT calculations. Reactions of [Rh(E)(PEt3)3] with HFO-1234ze (E-1,3,3,3-tetrafluoropropene) yielded the rhodium fluorido complex 2 and [Rh{(E)-CH[double bond, length as m-dash]CH(CF3)}(PEt3)3] (9) via two different reaction pathways. Using complexes 1 and 5 as catalysts, functionalized building blocks were obtained.

C-H and C-F bond activation reactions of pentafluorostyrene at rhodium complexes

The rhodium(i) complexes [Rh(Bpin)(PEt₃)₃] (1), [Rh(H)(PEt₃)₃] (5) and [Rh(Me)(PEt₃)₃] (14) were employed in reactions with pentafluorostyrene affording coordination of the olefin and C-F or C-H bond activation. Control of the reaction conditions allowed for selective activation reactions at different positions at the fluorinated aromatic ring. The rhodacycle trans-[Rh(F)(CH₂CH₂(2-C₆F₄))(PEt₃)₂] (7) was identified as an intermediate for an activation at the 2-position. Reactivity studies of the latter with CO led to the generation of trans-[Rh(F)(CH₂CH₂C₆F₄)(CO)(PEt₃)₂] (10). Stoichiometric and catalytic hydroboration reactions were achieved using complexes 1 or 5 as catalysts.

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.

Mechanistic Dichotomy of Magnesium- and Zinc-Based Germanium Nucleophiles in the C(sp3 )-Ge Cross-Coupling with Alkyl Electrophiles

Robust procedures for two mechanistically distinct C(sp³ )-Ge bond formations from alkyl electrophiles and germanium nucleophiles are reported. The germanium reagents were made available as bench-stable solutions by lithium-to-magnesium and lithium-to-zinc transmetalation, respectively. The germanium Grignard reagent reacts with various primary and secondary alkyl electrophiles by an ionic nucleophilic displacement. Conversely, the coupling of the corresponding zinc reagent requires a nickel catalyst, which then engages in radical bond formations with primary, secondary, and even tertiary alkyl bromides. Both methods avoid the regioselectivity issue of alkene hydrogermylation and enable the synthesis of a wide range of functionalized alkyl-substituted germanes.

Ni-catalyzed migratory fluoro-alkenylation of unactivated alkyl bromides with gem-difluoroalkenes

We describe a nickel-catalyzed highly regio- and stereoselective migratory fluoro-alkenylation of unactivated alkyl bromides. A unique catalytic cycle merging alkyl nickel chain-walking and defluorinative coupling enables the introduction of a broad array of fluoroalkenyl moieties into carbon chains. Control experiments with other halogenated alkenes demonstrated the essential role of fluorine atoms in this reaction. Notably, the reaction proceeds under mild conditions and allows for the synthesis of a variety of valuable monofluoroalkenes.

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.

Et3GeH versus Et3SiH: controlling reaction pathways in catalytic C–F bond activations at a nanoscopic aluminum chlorofluoride

Catalytic C–F activations at Lewis-acidic amorphous aluminum chlorofluoride (ACF) with Et₃GeH and Et₃SiH for reaction pathway control are presented.

Structure and dynamic NMR behavior of rhodium complexes supported by Lewis acidic group 13 metallatranes

Z-type complexes featuring Rh → Al and Rh → Ga interactions show distorted Rh centers and fluxionality on the NMR timescale.