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-.

Nickel-Mediated Trifluoromethylation of Phenol Derivatives by Aryl C-O Bond Activation.

The increasing pharmaceutical importance of trifluoromethylarenes has stimulated the development of more efficient trifluoromethylation reactions. Tremendous efforts have focused on copper- and palladium-mediated/catalyzed trifluoromethylation of aryl halides. In contrast, no general method exists for the conversion of widely available inert electrophiles, such as phenol derivatives, into the corresponding trifluoromethylated arenes. Reported herein is a practical nickel-mediated trifluoromethylation of phenol derivatives with readily available trimethyl(trifluoromethyl)silane (TMSCF3 ). The strategy relies on PMe3 -promoted oxidative addition and transmetalation, and CCl3 CN-induced reductive elimination. The broad utility of this transformation has been demonstrated through the direct incorporation of trifluoromethyl into aromatic and heteroaromatic systems, including biorelevant compounds.

Comparative profiling of well-defined copper reagents and precursors for the trifluoromethylation of aryl iodides.

A number of copper reagents were compared for their effectiveness in trifluoromethylating 4-iodobiphenyl, 4-iodotoluene, and 2-iodotoluene. Yields over time were plotted in order to refine our understanding of each reagent performance, identify any bottlenecks, and provide more insight into the rates of the reactions. Interestingly, differences in reactivity were observed when a well-defined [LCuCF3] complex was employed directly or generated in situ from precursors by published reports. Relative reactivities were also found to highly dependent on the nature of the iodoarenes.

Direct Difluoromethylation of Aryl Halides via Base Metal Catalysis at Room Temperature.

A stable and isolable difluoromethyl zinc reagent has been prepared through the reaction of ICF2H with diethyl zinc and DMPU. This new zinc reagent is a free-flowing solid and can be used in combination with a nickel catalyst to difluoromethylate aryl iodides, bromides, and triflates at room temperature. Such mild conditions for the catalytic difluoromethylation of these substrates are unprecedented.

Triphenylphosphine-Mediated Deoxygenative Reduction of CF3 SO2 Na and Its Application for Trifluoromethylthiolation of Aryl Iodides.

We report herein a practical method for taming Langlois' reagent CF3 SO2 Na to generate CuSCF3 by a triphenylphospine-mediated deoxygenative reduction process. This chemistry highlights a novel utilization of the inherent CF3 S skeleton of Langlois' reagent as a CF3 S feedstock under mild conditions. The CuSCF3 intermediate generated by this protocol can react with a wide array of supporting ligands to furnish several air-stable [LCu(SCF3 )] complexes as valuable trifluoromethylthiolating agents. In addition, the CuSCF3 intermediate can be directly employed for the trifluoromethylthiolation of (hetero)aryl iodides with operational simplicity and atomic efficiency.

Unsymmetrical N-Aryl-1-(pyridin-2-yl)methanimine Ligands in Organonickel(II) Complexes: More Than a Blend of 2,2'-Bipyridine and N,N-Diaryl-α-diimines?

The new organonickel complexes [(R-PyMA)Ni(Mes)X] [R-PyMA = N-aryl-1-(pyridin-2-yl)methanimine; aryl = phenyl, 2,6-Me2-, 3,5-Me2-, 2,4,6-Me3-, 2,6-iPr2-, 3,5-(OMe)2-, 2-NO2-4-Me-, 4-NO2-, 2-CF3-, and 2-CF3-6-F-phenyl; Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or I] were obtained as approximate 1/1 cis and trans isomeric mixtures or pure cis isomers depending on the PyMA ligand and X. The [(R-PyMA)Ni(Mes)X] complexes with X = Br or Cl were directly synthesized from the precursors trans-[(PPh3)2Ni(Mes)X], while [(PyMA)Ni(Mes)X] derivatives with X = F or I were obtained from [(PyMA)Ni(Mes)Br] through X exchange reactions. Although density functional theory (DFT) calculations show a preference for the sterically favored cis isomers, both isomers could be observed in many cases; in three cases, even single crystals for X-ray diffraction could be obtained for the trans isomers. Possible intermediates for the isomerization were investigated by DFT calculations. All complexes were studied by multiple spectroscopic means, electrochemistry, and spectroelectrochemistry (for the reduction processes). The long-wavelength metal-to-ligand charge-transfer (MLCT) absorptions vary markedly with the R substituent of the ligand and the cathodic electrochemical potentials to a far smaller degree. Both are almost invariable upon variation of X. All of this is in line with Ni-based and π*-based lowest unoccupied molecular orbitals (LUMOs). In line with the unsymmetric character of the NPy^Nmethanimine ligand, electrochemistry and MLCT transitions seem to not correspond to the same type of π* LUMO, making these PyMA ligands more interesting than the symmetric heteroaromatic polypyridine ligands such as 2,2'-bipyridine (bpy; NPy^NPy) and N,N-diaryl-substituted aliphatic α-diimines (Nmethanimine^Nmethanimine) such as the diaza-1,3-butadienes (DAB). First attempts to use these complexes in Negishi-type cross-coupling reactions were successful.

Manganese-Catalyzed Aerobic Oxytrifluoromethylation of Styrene Derivatives Using CF3SO2Na as the Trifluoromethyl Source.

A mild and practical protocol for manganese-catalyzed aerobic oxytrifluoromethylation of olefinic bonds of styrene derivatives using CF3SO2Na (Langlois' reagent) as the CF3 source is described. A distinguishing feature of this method is the generation of trifluoromethyl radicals from CF3SO2Na using the simple manganese salt/O2 system. The reaction proceeds under ambient conditions, free of added peroxide initiators, and provides moderate to good selectivities for alcohol versus ketone product.

Unliganded HIV-1 gp120 core structures assume the CD4-bound conformation with regulation by quaternary interactions and variable loops.

The HIV-1 envelope (Env) spike (gp120(3)/gp41(3)) undergoes considerable structural rearrangements to mediate virus entry into cells and to evade the host immune response. Engagement of CD4, the primary human receptor, fixes a particular conformation and primes Env for entry. The CD4-bound state, however, is prone to spontaneous inactivation and susceptible to antibody neutralization. How does unliganded HIV-1 maintain CD4-binding capacity and regulate transitions to the CD4-bound state? To define this mechanistically, we determined crystal structures of unliganded core gp120 from HIV-1 clades B, C, and E. Notably, all of these unliganded HIV-1 structures resembled the CD4-bound state. Conformational fixation with ligand selection and thermodynamic analysis of full-length and core gp120 interactions revealed that the tendency of HIV-1 gp120 to adopt the CD4-bound conformation was restrained by the V1/V2- and V3-variable loops. In parallel, we determined the structure of core gp120 in complex with the small molecule, NBD-556, which specifically recognizes the CD4-bound conformation of gp120. Neutralization by NBD-556 indicated that Env spikes on primary isolates rarely assume the CD4-bound conformation spontaneously, although they could do so when quaternary restraints were loosened. Together, the results suggest that the CD4-bound conformation represents a "ground state" for the gp120 core, with variable loop and quaternary interactions restraining unliganded gp120 from "snapping" into this conformation. A mechanism of control involving deformations in unliganded structure from a functionally critical state (e.g., the CD4-bound state) provides advantages in terms of HIV-1 Env structural diversity and resistance to antibodies and inhibitors, while maintaining elements essential for entry.

Synthesis and reactivity of new aminophenolate complexes of nickel.

New well-defined, paramagnetic nickel complexes have been prepared and characterized by X-ray crystallography. The complexes were found to be active for the cross-coupling of alkyl electrophiles (especially ethyl 2-bromobutyrate) with alkyl Grignard reagents. The ligand architecture in these new complexes could potentially be rendered chiral, opening up future possibilities for performing asymmetric cross-coupling reactions.

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].

A five-coordinate nickel(II) fluoroalkyl complex as a precursor to a spectroscopically detectable Ni(III) species.

Mechanistic proposals for nickel-catalyzed coupling reactions often invoke five-coordinate alkyl- or aryl-bound Ni(II) and/or high-valent nickel(III) species, but because of their reactive nature, they have been difficult to study and fingerprint. In this work, we invoked the stabilizing properties of fluoroalkyl ligands to access such nickel species bearing ligands that are commonplace in organic coupling reactions. We show that five-coordinate Ni(II) complexes containing nickel-carbon bonds can readily be prepared given the appropriate precursor, and we also present evidence for the formation of Ni(III) species upon chemical and electrochemical oxidation of the five-coordinate complexes.

Lithium bromide-induced structural changes in a nickel bis-alkoxide complex.

The bis-alkoxide [(DEAMP)2Ni] (1, DEAMP = 1-(diethylamino)-2-methylpropan-2-olate) was found to react with trace amounts of lithium bromide to afford the bis-LiBr adduct 2, in which the oxygens of the DEAMP ligand coordinate to lithium to form a chiral-at-metal complex. This new complex is five-coordinate at nickel, and contains nickel and oxygen atoms which are all chiral. One diastereomer precipitates from pentane solution. The two lithium ions rigidify the new structure in the solid state by coordinating to the oxygen and bromide atoms.

Scrutinizing formally NiIV centers through the lenses of core spectroscopy, molecular orbital theory, and valence bond theory.

Nickel K- and L2,3-edge X-ray absorption spectra (XAS) are discussed for 16 complexes and complex ions with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS alone is shown to be an ambiguous metric of physical oxidation state for these Ni complexes. Meanwhile, L2,3-edge XAS reveals that the physical d-counts of the formally NiIV compounds measured lie well above the d6 count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF62- is considered using high-level molecular orbital approaches as well as advanced valence bond methods. The emergent electronic structure picture reveals that even highly electronegative F-donors are incapable of supporting a physical d6 NiIV center. The reactivity of NiIV complexes is then discussed, highlighting the dominant role of the ligands in this chemistry over that of the metal centers.

Nickel-catalyzed synthesis of aryl trifluoromethyl sulfides at room temperature.

Inexpensive nickel-bipyridine complexes were found to be active for the trifluoromethylthiolation of aryl iodides and aryl bromides at room temperature using the convenient [NMe(4)][SCF(3)] reagent.

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.

Redox trends in terpyridine nickel complexes.

A synthesis has been developed that allows the isolation of four-coordinate [(tpy)Ni-Br] (1, tpy = terpyridine) in high yield. Complex 1 has been structurally characterized, and the X-ray data reveal a square-planar geometry, unlike the known [(tpy')Ni-I] (tpy' = 4,4',4''-tri-tert-butyl-terpyridine) but similar to [(tpy)Ni-CH(3)]. In the solid-state, EPR spectroscopy indicates, however, that unlike [(tpy)Ni-CH(3)], the electronic structure of 1 is a metal-centered, not a ligand-centered radical. Density functional theory (DFT) analyses support this assignment. The preparation of 1 also facilitated the analysis of the redox potentials of a series of terpyridine nickel derivatives. It was found that the overall ligand sphere (one vs two coordinated terpyridine ligands) plays more of a role in determining the redox potentials of these derivatives than do the formal oxidation states of the nickel ions in the solution phase.

An organocatalytic asymmetric Nazarov cyclization.

An organocatalytic asymmetric Nazarov cyclization of diketoesters that proceeds by means of a dual activation mechanism has been developed. Screening of a number of catalysts led to a new thiourea that incorporates a primary amino group. The method gives rise to cyclic products with two adjacent quaternary asymmetric carbon atoms, one of which is an all-carbon stereocenter, with complete or nearly complete diastereoselectivity and in high or very high enantiomeric excess. A brief and very convenient synthesis of the acyclic diketoester starting materials through nucleophilic addition to a ketene is also described.

Rapid and selective catalytic oxidation of secondary alcohols at room temperature by using (N-heterocyclic carbene)-ni(0) systems.

The selective, anaerobic catalytic oxidation of secondary alcohols at room temperature by using an in situ (N-heterocyclic carbene)-Ni(0) system is presented. The use of non-anhydrous, non-degassed 2,4-dichlorotoluene as both the oxidant and the solvent allows for very short reaction times and very high yields. In addition, a well-defined (N-heterocyclic carbene)-Ni(0) complex was synthesized and applied to these oxidation reactions.