Activation of SF6 at Platinum Complexes: Formation of SF3 Derivatives and Their Application in Deoxyfluorination Reactions

Hydrogen Bonding in Platinum Indolylphosphine Polyfluorido and Fluorido Complexes

The reaction of the Pt complexes cis-[Pt(CH3 )(Ar){Ph2 P(Ind)}2 ] (Ind=2-(3-methyl)indolyl, Ar=4-tBuC6 H4 (1 a), 4-CH3 C6 H4 (1 b), Ph (1 c), 4-FC6 H4 (1 d), 4-ClC6 H4 (1 e), 4-CF3 C6 H4 (1 f)) with HF afforded the polyfluorido complexes trans-[Pt(F(HF)2 )(Ar){Ph2 P(Ind)}2 ] 2 a-f, which can be converted into the fluoride derivatives trans-[Pt(F)(Ar){Ph2 P(Ind)}2 ] (3 a-f) by treatment with CsF. The compounds 2 a-f and 3 a-f were characterised thoroughly by multinuclear NMR spectroscopy. The data reveal hydrogen bonding of the fluorido ligand with HF molecules and the indolylphosphine ligand. Polyfluorido complexes 2 a-f show larger |1 J(F,Pt)|, but lower 1 J(H,F) coupling constants when compared to the fluorido complexes 3 a-f. Decreasing 1 J(P,Pt) coupling constants in 2 a-f and 3 a-f suggest a cis influence of the aryl ligands in the following order: 4-tBuC6 H4 (a) ≈4-CH3 C6 H4 (b) Collapse

Key Words

• fluorido complexes
• hydrogen bonding
• indolylphosphine
• platinum
• polyfluorido complexes

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MESH Headings Collapse

Grants

• 387284271 - SFB 1349 Deutsche Forschungsgemeinschaft
• Deutsche Forschungsgemeinschaft

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Affiliation(s)

Stefan Sander Department of ChemistryHumboldt-Universität zu BerlinBrook-Taylor-Str. 212489BerlinGermany
Elizabeth J. Cosgrove Department of ChemistryHumboldt-Universität zu BerlinBrook-Taylor-Str. 212489BerlinGermany
Robert Müller Institut für ChemieTechnische Universität Berlin, Theoretische Chemie/Quantenchemie, Sekr.C7Straße des 17. Juni 13510623BerlinGermany
Martin Kaupp Institut für ChemieTechnische Universität Berlin, Theoretische Chemie/Quantenchemie, Sekr.C7Straße des 17. Juni 13510623BerlinGermany
Thomas Braun Department of ChemistryHumboldt-Universität zu BerlinBrook-Taylor-Str. 212489BerlinGermany

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Oxidative Fluorination of Selenium and Tellurium Compounds using a Thermally Stable Phosphonium SF5 - Salt Accessible from SF6

Fluorinated group 16 moieties are attractive building blocks in synthetic chemistry but only few synthetic methods are available to prepare them. Herein, we report a new oxidative fluorination reagent capable of stabilizing reactive fluorinated anions. It consists of an SF5 - anion and a chemically inert phosphonium cation and is exceptionally thermally stable. Accordingly, it was used to generate the SeF5 - and TeF5 - anions from the elemental chalcogens and to prepare the unknown tetrafluoro(phenyl)-λ5 -selenate PhSeF4 - and -tellurate PhTeF4 - from the corresponding diphenyl dichalcogenides. In addition, we show that further derivatization of [PhTeF4 ]- by oxidation to trans-PhTeF4 O- and subsequent alkylation gives access to a new class of trans-(alkoxy)(phenyl)tetrafluoro-λ6 -tellanes (trans-PhTeF4 OR), thus providing an approach to introduce the functional group into organic molecules.

Platinum-Catalyzed Hydrofluorination of Alkynes: Hydrogen Bonding to Indolylphosphine Ligands to Provide Fluoride Reactivity

The reaction of the Pt complexes cis-[Pt(CH3 )2 {R2 P(Ind)}2 ] (Ind=2-(3-methyl)indolyl, R=Ph (1 a), 4-FC6 H4 (1 b), 4-CF3 C6 H4 (1 c)) with HF afforded the fluorido complexes trans-[Pt(F(HF)2 )(CH3 ){R2 P(Ind)}2 ] 2 a-c, which can be converted into trans-[Pt(F)(CH3 ){R2 P(Ind)}2 ] (3 a-c) by treatment with CsF. Addition of 3-hexyne to 2 a-c gave alkyne complexes trans-[Pt(C,C-η2 -C2 H5 C≡CC2 H5 )(CH3 ){R2 P(Ind)}2 {F(HF)2 }] (4 a-c) at which a fluoride is stabilised as polyfluoride in the coordination sphere by hydrogen bonding to the indolyl-substituted phosphine ligands. Subsequent heating of a solution of 4 a in the presence of PVPHF led to fluoroalkene formation. Selective catalytic hydrofluorination of alkynes to yield (Z)-fluoroalkenes were developed. The ability of hydrogen bonding to polyfluoride favours the fluorination step as demonstrated by studies with complexes bearing no indolyl groups at the phosphine ligands.

Metal-Free SF6 Activation: A New SF5 -Based Reagent Enables Deoxyfluorination and Pentafluorosulfanylation Reactions

The activation of SF₆ , a potent greenhouse gas, under metal-free and visible light conditions is reported. Herein, mechanistic investigations including EPR spectroscopy, NMR studies and cyclic voltammetry allowed the rational design of a new fluorinating reagent which was synthesized from the 2-electron activation of SF₆ with commercially available TDAE. This new SF₅ -based reagent was efficiently employed for the deoxyfluorination of CO₂ and the fluorinative desulfurization of CS₂ allowing the formation of useful fluorinated amines. Moreover, for the first time we demonstrated that our SF₅ -based reagent could afford the mild generation of Cl-SF₅ gas. This finding was exploited for the chloro-pentafluorosulfanylation of alkynes and alkenes.

Reactivity of Xantphos-Type Rhodium Complexes Towards SF4 : SF3 Versus SF2 Complex Generation

S-F-bond activation of sulfur tetrafluoride at [Rh(Cl)(tBu xanPOP)] (1; tBu xanPOP=9,9-dimethyl-4,5-bis-(di-tert-butylphosphino)-xanthene) led to the formation of the cationic complex [Rh(F)(Cl)(SF2 )(tBu xanPOP)][SF5 ] (2 a) together with trans-[Rh(Cl)(F)2 (tBu xanPOP)] (3) and cis-[Rh(Cl)2 (F)(tBu xanPOP)] (4) which both could also be obtained by the reaction of SF5 Cl with 1. In contrast to that, the conversion of SF4 at the methyl complex [Rh(Me)(tBu xanPOP)] (5) gave the isolable and room-temperature stable cationic λ4 -trifluorosulfanyl complex [Rh(Me)(SF3 )(tBu xanPOP)][SF5 ] (6). Treatment of 6 with the Lewis acids BF3 or AsF5 produced the dicationic difluorosulfanyl complex [Rh(Me)(SF2 )(tBu xanPOP)][BF4 ]2 (8 a) or [Rh(Me)(SF2 )(tBu xanPOP)][AsF6 ]2 (8 b), respectively. Refluorination of 8 a was possible with the use of dimethylamine giving [Rh(Me)(SF3 )(tBu xanPOP)][BF4 ] (9). A reaction of 6 with trichloroisocyanuric acid (TClCA) gave the fluorido complex [Rh(F)(Cl)(SF2 )(tBu xanPOP)][Cl] (2 b) together with chloromethane and SF5 Cl.

Fluorination Reactions at a Platinum Carbene Complex: Reaction Routes to SF3 , S(=O)F and Fluorido Complexes

The electron-rich Pt complex [Pt(IMes)2 ] (IMes: [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolinylidine]) can be used as precursor for the syntheses of a variety of fluorido ligand containing compounds. The sulfur fluoride SF4 undergoes a rapid oxidative addition at Pt0 to yield trans-[Pt(F)(SF3 )(IMes)2 ]. A photolytic reaction of SF6 at [Pt(IMes)2 ] in the presence of IMes gave the fluorido complexes trans-[Pt(F)2 (IMes)2 ] and trans-[Pt(F)(SF3 )(IMes)2 ] along with trans-[Pt(F)(SOF)(IMes)2 ] and trans-[Pt(F)(IMes')(IMes)] (IMes': cyclometalated IMes ligand), the latter being products produced by reaction with adventitious water. trans-[Pt(F)(SOF)(IMes)2 ] and trans-[Pt(F)2 (IMes)2 ] were synthesized independently by treatment of [Pt(IMes)2 ] with SOF2 or XeF2 . A reaction of [Pt(IMes)2 ] with a HF source gave trans-[Pt(H)(F)(IMes)2 ], and an intermediate bifluorido complex trans-[Pt(H)(FHF)(IMes)2 ] was identified. Compound trans-[Pt(H)(F)(IMes)2 ] converts in the presence of CsF into trans-[Pt(F)(IMes')(IMes)].

Deoxygenative Functionalizations of Aldehydes, Ketones and Carboxylic Acids

Conversion of carbonyl compounds, including aldehydes, ketones and carboxylic acids, into functionalized alkanes via deoxygenation would be highly desirable from a sustainability perspective and very enabling in chemical synthesis. This review covers the recent methodology development in carbonyl and carboxyl deoxygenative functionalizations, highlighting some typical and significant contributions in this field. These advances will be categorized based on types of bond formation, and in each part, selected examples will be discussed from their generalized mechanistic perspectives. Four summarized reactivity modes of aldehydes and ketones during the deoxygenation, namely, bis-electrophile, carbenoid, bis-nucleophile and alkyl radical, are presented, while the carboxylic acids are deoxygenated mainly via activated carbonyl or acetal intermediates.

Chemoselective Hydro(Chloro)pentafluorosulfanylation of Diazo Compounds with Pentafluorosulfanyl Chloride

Pentafluorosulfanyl chloride (SF₅ Cl) is the most prevalent reagent for the incorporation of SF₅ group into organic compounds. However, the preparation of SF₅ Cl often relies on hazardous reagents and specialized apparatus. Herein, we described a safe and practical synthesis of a bench-stable and easy-to-handle solution of SF₅ Cl in n-hexane under gas-reagent-free conditions. The synthetic application of SF₅ Cl was demonstrated through the unprecedented reaction with diazo compounds. The chemoselective hydro- and chloropentafluorosulfanylations of α-diazo carbonyl compounds were developed in the presence of K₃ PO₄ or copper catalyst, respectively. These reactions provide a direct and efficient access to various α-pentafluorosulfanyl carbonyl compounds of high value for potential applications.

Non-coordinated and Hydrogen Bonded Phenolate Anions as One-Electron Reducing Agents

In this work, the syntheses of non-coordinated electron-rich phenolate anions via deprotonation of the corresponding alcohols with an extremely powerful perethyl tetraphosphazene base (Schwesinger base) are reported. The application of uncharged phosphazenes renders the selective preparation of anionic phenol-phenolate and phenolate hydrates possible, which allows for the investigation of hydrogen bonding in these species. Hydrogen bonding brings about decreased redox potentials relative to the corresponding non-coordinated phenolate anions. The latter show redox potentials of up to -0.72(1) V vs. SCE, which is comparable to that of zinc metal, thus qualifying their application as organic zinc mimics. We utilized phenolates as reducing agents for the generation of radical anions in addition to the corresponding phenoxyl radicals. A tetracyanoethylene radical anion salt was synthesized and fully characterized as a representative example. We also present the activation of sulfur hexafluoride (SF6 ) with phenolates in a SET reaction, in which the nature of the respective phenolate determines whether simple fluorides or pentafluorosulfanide ([SF5 ]- ) salts are formed.

Reactivity of Binary and Ternary Sulfur Halides towards Transition-Metal Compounds

Binary sulfur fluorides exhibit an interesting reactivity towards transition metal complexes. They open up routes for the generation of sulfur‐containing building blocks. Often ligands with particular properties can be constructed. This includes their ability to transfer sulfur atoms or polysulfide units as well as fluorination reactions. This Minireview provides an insight into the reactivity of the binary and ternary sulfur halides S₂Cl₂, SCl₂, SF₄, SF₆ and SF₅Cl towards transition‐metal compounds.

Photoredox Catalytic α-Alkoxypentafluorosulfanylation of α-Methyl- and α-Phenylstyrene Using SF6

SF6 was applied as pentafluorosulfanylation reagent to prepare ethers with a vicinal SF5 substituent through a one-step method involving photoredox catalysis. This method shows a broad substrate scope with respect to applicable alcohols for the conversion of α-methyl and α-phenyl styrenes. The products bear a new structural motif with two functional groups installed in one step. The alkoxy group allows elimination and azidation as further transformations into valuable pentafluorosulfanylated compounds. These results confirm that non-toxic SF6 is a useful SF5 transfer reagent if properly activated by photoredox catalysis, and toxic reagents are completely avoided. In combination with light as an energy source, a high level of sustainability is achieved. Through this method, the proposed potential of the SF5 substituent in medicinal chemistry, agrochemistry, and materials chemistry may be exploited in the future.

The Versatile Behavior of Platinum Alkyne Complexes towards XeF2 : Formation of Fluorovinyl and Fluorido Complexes

Reactions of platinum(0) tolane complexes, bearing a chelating ligand with P and N donor atoms, with the electrophilic fluorinating agent XeF₂ give facile access to platinum(II) β-fluorovinyl fluorido complexes. A series of new platinum(II) β-fluorovinyl complexes have been synthesized and were structurally characterized. Further oxidation with XeF₂ led to ortho-metalated platinum(IV) fluorido compounds. Additional reactions of platinum(0) tolane complexes, bearing a chelating P,P donor ligand, with XeF₂ led to a variety of fluorido and fluorovinyl complexes.