Nickel Perfluoroalkyl Complexes Supported by Simple Acetate Coligands.

The interaction of tetramethylammonium acetate with [(MeCN)2Ni(CF3)2], [(MeCN)2Ni(C2F5)2], and [NMe4][(MeCN)Ni(CF3)3] was explored by 19F NMR spectroscopy. We show that depending on the nature of the nickel complex, one or two acetate ligands can add to the metal center and replace the nickel-bound MeCN ligands, depending on the acetate concentration. The number of acetates that could bind to nickel, and whether the resulting complex exists as a monomer or dimer, was determined to be dependent on the nature of the fluoroalkyl ligand. Moreover, we observe that oxidation of the nickel center of [(MeCN)2Ni(CF3)2] in the presence of two equivalents of acetate leads cleanly to the octahedral, paramagnetic (EPR spectroscopy), and anionic Ni(III) complex [NMe4][Ni(CF3)2(OAc)2].

Diversifying chemical space of DNA-encoded libraries: synthesis of 2-oxa-1-azaspiro(bicyclo[3.2.0])heptanes on-DNA via visible light-mediated energy transfer catalysis.

Azaspiro[3.3]heptanes are valuable synthetic targets for drug discovery programs. The challenges associated with the preparation and diversification of this moiety as compared to other small, saturated rings have led to limited applications of compounds containing this spirocycle. In this regard, important advances in the field of synthetic photochemistry have exploited the biradical nature of the triplet excited state of 2-isoxazoline-3-carboxylates, engaging these species in intermolecular coupling reactions under visible light irradiation. As a continuation of our program preparing F(sp3)-rich, structurally complex molecules for DNA-encoded library technology (DELT) applications via photocatalysis, we disclose herein the incorporation of unique and densely functionalized 2-oxa-1-azabicyclo[3.2.0]heptanes via [2+2] cycloaddition energy transfer sensitization, providing access to an unexplored library of azaspiro compounds, many of which include additional synthetic handles important for further functionalization of the DNA-conjugated products and for library production.

Alkyl (Het)Arylsulfones from SuFEx Reagents via Photochemical S-F Bond Activation.

A visible light-induced S-F bond activation of sulfur fluoride exchange (SuFEx) reagents to generate alkyl sulfones is described. The method is free of transition metals and relies on the use of commercially available commodity chemicals. In addition, this method demonstrates the first photoredox functionalization of SuFEx reagents. The reaction has a diverse substrate scope, with applications in various aryl sulfonyl fluorides, both activated and unactivated alkenes.

Visible-Light-Induced C-F Bond Activation for the Difluoroalkylation of Indoles.

An aryl disulfide mediated C-F bond activation of the trifluoromethyl group to generate valuable gem-difluoroalkylindoles is described. This method relies on readily available commodity reagents under mild reaction conditions and represents the first transition-metal-free redox-neutral C-F bond activation strategy. The reaction employs various substituted indoles and α-fluoro-substituted esters. Further, this mode of C-F activation was also amenable to the activation of trifluoromethylated arenes for the preparation of bis-benzylic gem-difluoromethylenes between indole and arene substructures, providing access to a unique chemical space.

Synthesis, structure, and electrochemical properties of [LNi(Rf)(C4F8)]- and [LNi(Rf)3]- complexes.

The new anionic nickelate complexes [(MeCN)Ni(C4F8)(CF3)]-, [(MeCN)Ni(C4F8)(C2F5)]-, [(IMes)Ni(C4F8)(CF3)]-, [(IMes)Ni(CF3)3]- (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), and [(F-NHC)Ni(Rf)3]- (F-NHC = 1,3-bis(2,4-F2Ph), 2,4,6-F3Ph- or 3,4,5-F3Ph)imidazol-2-ylidene; (Rf = CF3 or C2F5) were synthesized and structurally characterized. The electrochemical properties of all new compounds were revealed by cyclic voltammetry studies and compared to the known CF3 analogue [(MeCN)Ni(CF3)3]-. The IMes-coordinated complexes exhibited initial oxidation events that were well-separated from a second oxidation process in the cyclic voltammograms. The complexes containing F-substituted NHC ligands [(F-NHC)Ni(CF3)3]- are structurally quite similar to the IMes derivative and reveal also two separated oxidation waves in their cyclic voltammograms. The absolute potentials as well as the separation between the two waves vary with the substitution pattern, suggesting that the NHC ligand environment (NHC = N-heterocyclic carbene) is an interesting platform for the development of new redox-triggered reactions that release trifluoromethyl and perfluoroalkyl radicals upon oxidation.

Solvated Nickel Complexes as Stoichiometric and Catalytic Perfluoroalkylation Agents*.

The acetonitrile-solvated [(MeCN)Ni(C2 F5 )3 ]- was prepared in order to compare and contrast its reactivity with the known [(MeCN)Ni(CF3 )3 ]- towards organic electrophiles. Both [(MeCN)Ni(CF3 )3 ]- and [(MeCN)Ni(C2 F5 )3 ]- successfully react with aryl iodonium and diazonium salts as well as alkynyl iodonium salts to give fluoroalkylated organic products. Electrochemical analysis of [(MeCN)NiII (C2 F5 )3 ]- suggests that, upon electro-oxidation to [(MeCN)n NiIII (C2 F5 )3 ], reductive homolysis of a perfluoroethyl radical occurs, with the concomitant formation of [(MeCN)2 NiII (C2 F5 )2 ]. Catalytic C-H trifluoromethylations of electron-rich arenes were successfully achieved using either [(MeCN)Ni(CF3 )3 ]- or the related [Ni(CF3 )4 ]2- . Stoichiometric reactions of the solvated nickel complexes reveal that "ligandless" nickel is exceptionally capable of serving as reservoir of CF3 groups under catalytically relevant conditions.

[(MeCN)Ni(CF3)3]- and [Ni(CF3)4]2-: Foundations toward the Development of Trifluoromethylations at Unsupported Nickel.

Nickel anions [(MeCN)Ni(CF3)3]- and [Ni(CF3)4]2- were prepared by the formal addition of 3 and 4 equiv, respectively, of AgCF3 to [(dme)NiBr2] in the presence of the [PPh4]+ counterion. Detailed insights into the electronic properties of these new compounds were obtained through the use of density functional theory (DFT) calculations, spectroscopy-oriented configuration interaction (SORCI) calculations, X-ray absorption spectroscopy, and cyclic voltammetry. The data collectively show that trifluoromethyl complexes of nickel, even in the most common oxidation state of nickel(II), are highly covalent systems whereby a hole is distributed on the trifluoromethyl ligands, surprisingly rendering the metal to a physically more reduced state. In the cases of [(MeCN)Ni(CF3)3]- and [Ni(CF3)4]2-, these complexes are better physically described as d9 metal complexes. [(MeCN)Ni(CF3)3]- is electrophilic and reacts with other nucleophiles such as phenoxide to yield the unsupported [(PhO)Ni(CF3)3]2- salt, revealing the broader potential of [(MeCN)Ni(CF3)3]- in the development of "ligandless" trifluoromethylations at nickel. Proof-in-principle experiments show that the reaction of [(MeCN)Ni(CF3)3]- with an aryl iodonium salt yields trifluoromethylated arene, presumably via a high-valent, unsupported, and formal organonickel(IV) intermediate. Evidence of the feasibility of such intermediates is provided with the structurally characterized [PPh4]2[Ni(CF3)4(SO4)], which was derived through the two-electron oxidation of [Ni(CF3)4]2-.