Design, synthesis, and carbon-heteroatom coupling reactions of organometallic nickel(IV) complexes

A Transmetalation Reaction Enables the Synthesis of [18F]5-Fluorouracil from [18F]Fluoride for Human PET Imaging

Translation of new ¹⁸F-fluorination reactions to produce radiotracers for human positron emission tomography (PET) imaging is rare because the chemistry must have useful scope and the process for ¹⁸F-labeled tracer production must be robust and simple to execute. The application of transition metal mediators has enabled impactful ¹⁸F-fluorination methods, but to date none of these reactions have been applied to produce a human-injectable PET tracer. In this article we present chemistry and process innovations that culminate in the first production from [¹⁸F]fluoride of human doses of [¹⁸F]5-fluorouracil, a PET tracer for cancer imaging in humans. The first preparation of nickel σ-aryl complexes by transmetalation from arylboronic acids or esters was developed and enabled the synthesis of the [¹⁸F]5-fluorouracil precursor. Routine production of >10 mCi doses of [¹⁸F]5-fluorouracil was accomplished with a new instrument for azeotrope-free [¹⁸F]fluoride concentration in a process that leverages the tolerance of water in nickel-mediated ¹⁸F-fluorination.

Oxidatively-induced aromatic cyanation mediated by Ni(iii)

A Ni(ii) complex with two t-butylisocyanide ligands supported by a N3C(-) type tetradentate ligand was synthesized and characterized. Quantitative generation of the aromatic cyanation product, (tBu)N3C-CN, is observed by reacting this Ni(ii) complex with 1 equiv. of an oxidant. Reactivity studies suggest that this oxidatively-induced cyanation involves a heterolytic cleavage of the N-(t)Bu bond and is mediated by Ni(iii).

Ligand effects on the properties of Ni(iii) complexes: aerobically-induced aromatic cyanation at room temperature

Ligand effect studies on (^RN3C)Ni^IIIL₂ complexes reveal aerobically-induced aromatic cyanation with tBuNC at room temperature for the (^NpN3C)Ni system.

Design of a single protein that spans the entire 2-V range of physiological redox potentials

The reduction potential (E°') is a critical parameter in determining the efficiency of most biological and chemical reactions. Biology employs three classes of metalloproteins to cover the majority of the 2-V range of physiological E°'s. An ultimate test of our understanding of E°' is to find out the minimal number of proteins and their variants that can cover this entire range and the structural features responsible for the extreme E°'. We report herein the design of the protein azurin to cover a range from +970 mV to -954 mV vs. standard hydrogen electrode (SHE) by mutating only five residues and using two metal ions. Spectroscopic methods have revealed geometric parameters important for the high E°'. The knowledge gained and the resulting water-soluble redox agents with predictable E°'s, in the same scaffold with the same surface properties, will find wide applications in chemical, biochemical, biophysical, and biotechnological fields.

Gas-Phase and Computational Study of Identical Nickel- and Palladium-Mediated Organic Transformations Where Mechanisms Proceeding via M(II) or M(IV) Oxidation States Are Determined by Ancillary Ligands

Gas-phase studies utilizing ion-molecule reactions, supported by computational chemistry, demonstrate that the reaction of the enolate complexes [(CH2CO2-C,O)M(CH3)](-) (M = Ni (5a), Pd (5b)) with allyl acetate proceed via oxidative addition to give M(IV) species [(CH2CO2-C,O)M(CH3)(η(1)-CH2-CH═CH2)(O2CCH3-O,O')](-) (6) that reductively eliminate 1-butene, to form [(CH2CO2-C,O)M(O2CCH3-O,O')](-) (4). The mechanism contrasts with the M(II)-mediated pathway for the analogous reaction of [(phen)M(CH3)](+) (1a,b) (phen = 1,10-phenanthroline). The different pathways demonstrate the marked effect of electron-rich metal centers in enabling higher oxidation state pathways. Due to the presence of two alkyl groups, the metal-occupied d orbitals (particularly dz(2)) in 5 are considerably destabilized, resulting in more facile oxidative addition; the electron transfer from dz(2) to the C═C π* orbital is the key interaction leading to oxidative addition of allyl acetate to M(II). Upon collision-induced dissociation, 4 undergoes decarboxylation to form 5. These results provide support for the current exploration of roles for Ni(IV) and Pd(IV) in organic synthesis.

Switching on elusive organometallic mechanisms with photoredox catalysis

Transition metal-catalyzed cross-coupling reactions have become one of the most utilized carbon–carbon and carbon–heteroatom bond-forming reactions in chemical synthesis. More recently, nickel catalysis has been shown to participate in a wide variety of C–C bond forming reactions, most notably Negishi, Suzuki–Miyaura, Stille, Kumada, and Hiyama couplings^1,2. Despite the tremendous advances in C–C fragment couplings, the ability to forge C–O bonds in a general fashion via nickel catalysis has been largely unsuccessful. The challenge for nickel-mediated alcohol couplings has been the mechanistic requirement for the critical C–O bond forming step (formally known as the reductive elimination step) to occur via a Ni(III) alkoxide intermediate. In this manuscript, we demonstrate that visible light-excited photoredox catalysts can modulate the preferred oxidation states of nickel alkoxides in an operative catalytic cycle, thereby providing transient access to Ni(III) species that readily participate in reductive elimination. Using this synergistic merger of photoredox and nickel catalysis, we have developed a highly efficient and general carbon–oxygen coupling reaction using abundant alcohols and aryl bromides. More significantly, we have developed a general strategy to “switch on” important yet elusive organometallic mechanisms via oxidation state modulations using only weak light and single-electron transfer (SET) catalysts.

Highly Regioselective Indoline Synthesis under Nickel/Photoredox Dual Catalysis

Nickel/photoredox catalysis is used to synthesize indolines in one step from iodoacetanilides and alkenes. Very high regioselectivity for 3-substituted indoline products is obtained for both aliphatic and styrenyl olefins. Mechanistic investigations indicate that oxidation to Ni(III) is necessary to perform the difficult C-N bond-forming reductive elimination, producing a Ni(I) complex, which in turn is reduced to Ni(0). This process serves to further demonstrate the utility of photoredox catalysts as controlled single electron transfer agents in multioxidation state nickel catalysis.

Halide-Dependent Mechanisms of Reductive Elimination from Gold(III)

Two unique organometallic halide series (Ph3P)Au(4-Me-C6H4)(CF3)(X) and (Cy3P)Au(4-F-C6H4)(CF3)(X) (X = I, Br, Cl, F) have been synthesized. The PPh3-supported complexes can undergo both C(aryl)-X and C(aryl)-CF3 reductive elimination. Mechanistic studies of thermolysis at 122 °C reveal a dramatic reactivity and kinetic selectivity dependence on halide ligand. For X = I or F, zero-order kinetic behavior is observed, while for X = Cl or Br, kinetic studies implicate product catalysis. The selectivity for C(aryl)-CF3 bond formation increases in the order X = I < Br < Cl < F, with exclusively C(aryl)-I bond formation when X = I, and exclusively C(aryl)-CF3 bond formation when X = F. Thermodynamic measurements show that Au(III)-X bond dissociation energies increase in the order X = I < Br < Cl, and that ground state Au(III)-X bond strength ultimately dictates selectivities for C(aryl)-X and C(aryl)-CF3 reductive elimination.

Aromatic Methoxylation and Hydroxylation by Organometallic High-Valent Nickel Complexes

Herein we report the synthesis and reactivity of several organometallic Ni(III) complexes stabilized by a modified tetradentate pyridinophane ligand containing one phenyl group. A room temperature stable dicationic Ni(III)-disolvento complex was also isolated, and the presence of two available cis coordination sites in this complex offers an opportunity to probe the C-heteroatom bond formation reactivity of high-valent Ni centers. Interestingly, the Ni(III)-dihydroxide and Ni(III)-dimethoxide species can be synthesized, and they undergo aryl methoxylation and hydroxylation that is favored by addition of oxidant, which also limits the β-hydride elimination side reaction. Overall, these results provide strong evidence for the involvement of high-valent organometallic Ni species, possibly both Ni(III) and Ni(IV) species, in oxidatively induced C-heteroatom bond formation reactions.

Oxidation of Ni(II) to Ni(IV) with Aryl Electrophiles Enables Ni-Mediated Aryl-CF3 Coupling

This communication describes the synthesis and reactivity of Ni(IV)(aryl)(CF3)2 complexes supported by trispyrazolylborate and 4,4'-di-tert-butylbipyridine ligands. We demonstrate that isolable Ni(IV) complexes can be accessed under mild conditions via the oxidation of Ni(II) precursors with S-(trifluoromethyl)dibenzothiophenium triflate as well as with diaryliodonium and aryl diazonium reagents. The Ni(IV) intermediates undergo high yielding aryl-CF3 bond-forming reductive elimination. These studies support the potential viability of Ni(IV) intermediates in nickel-catalyzed coupling reactions involving diaryliodonium and aryldiazonium electrophiles.

Organonickel(IV) chemistry: a new catalyst?

With scorpionate ligands finding their way into organonickel chemistry, the state of the art of present-day nickel(IV) chemistry is highlighted. Will rapid CX coupling reactions emerge as a domain of higher-oxidation-state nickel chemistry?

Nickel-catalyzed direct thiolation of unactivated C(sp3)–H bonds with disulfides

The first nickel-catalyzed thiolation of unactivated C(sp³)–H bonds with disulfides is described.