Christopher Uyeda

Electrocatalytic Hydrogen Evolution in Acidic Water with Molecular Cobalt Tetraazamacrocycles

A series of water-soluble molecular cobalt complexes of tetraazamacrocyclic ligands are reported for the electrocatalytic production of H(2) from pH 2.2 aqueous solutions. The comparative data reported for this family of complexes shed light on their relative efficiencies for hydrogen evolution in water. Rotating disk electrode voltammetry data are presented for each of the complexes discussed, as are data concerning their respective pH-dependent electrocatalytic activity. In particular, two diimine-dioxime complexes were identified as exhibiting catalytic onset at comparatively low overpotentials relative to other reported homogeneous cobalt and nickel electrocatalysts in aqueous solution. These complexes are stable at pH 2.2 and produce hydrogen with high Faradaic efficiency in bulk electrolysis experiments over time intervals ranging from 2 to 24 h.

A New Family of Nucleophiles for Photoinduced, Copper-Catalyzed Cross-Couplings via Single-Electron Transfer: Reactions of Thiols with Aryl Halides Under Mild Conditions (O °C)

Building on the known photophysical properties of well-defined copper-carbazolide complexes, we have recently described photoinduced, copper-catalyzed N-arylations and N-alkylations of carbazoles. Until now, there have been no examples of the use of other families of heteroatom nucleophiles in such photoinduced processes. Herein, we report a versatile photoinduced, copper-catalyzed method for coupling aryl thiols with aryl halides, wherein a single set of reaction conditions, using inexpensive CuI as a precatalyst without the need for an added ligand, is effective for a wide range of coupling partners. As far as we are aware, copper-catalyzed C-S cross-couplings at 0 °C have not previously been achieved, which renders our observation of efficient reaction of an unactivated aryl iodide at -40 °C especially striking. Mechanistic investigations are consistent with these photoinduced C-S cross-couplings following a SET/radical pathway for C-X bond cleavage (via a Cu(I)-thiolate), which contrasts with nonphotoinduced, copper-catalyzed processes wherein a concerted mechanism is believed to occur.

Transition-state charge stabilization through multiple non-covalent interactions in the guanidinium-catalyzed enantioselective Claisen rearrangement.

The mechanism by which chiral arylpyrrole-substituted guanidinium ions promote the Claisen rearrangement of O-allyl α-ketoesters and induce enantioselectivity was investigated by experimental and computational methods. In addition to stabilization of the developing negative charge on the oxallyl fragment of the rearrangement transition state by hydrogen-bond donation, evidence was obtained for a secondary attractive interaction between the π-system of a catalyst aromatic substituent and the cationic allyl fragment. Across a series of substituted arylpyrrole derivatives, enantioselectivity was observed to vary predictably according to this proposal. This mechanistic analysis led to the development of a new p-dimethylaminophenyl-substituted catalyst, which afforded improvements in enantioselectivity relative to the parent phenyl catalyst for a representative set of substrates.

Catalytic Enantioselective Claisen Rearrangements of O-Allyl β- Ketoesters **

Claisen rearrangement; organocatalysis; asymmetric catalysis; Density-functional calc; H bondingThe selective construction of contiguous quaternary stereogenic centers, a motif found inmany complex natural products, represents a significant synthetic challenge.[1] Among thelimited number of approaches for the formation of bonds between such sterically-congestedcarbon atoms, intramolecular processes such as polyene cyclizations,[2a,b] intramolecularcycloadditions,[2c] and sigmatropic rearrangements[2d,e] have been particularly effective.For addressing vicinal quaternary carbons, these transformations have only been applied in adiastereocontrolled manner using substrates containing pre-existing stereogenic centers,either as part of cleavable auxiliaries or structural features of the target molecule. Thedevelopment of catalytic asymmetric methods for the direct and selective formation of suchstereochemical arrays represents a highly desirable and challenging goal.Since its discovery in 1912,[3] the [3,3]-sigmatropic rearrangement of allyl vinyl ethers (theClaisen rearrangement) has emerged as a proven strategy for the formation of carbon–carbon bonds between vicinal stereogenic centers.[4] Diastereoselectivity is generallypredictable and high in these processes because of the concerted nature of the C–O bond-breaking and C–C bond-forming events as well as the large energetic preference for chair-like over boat-like transition states. Furthermore, important examples of enantioselectivemethods for Claisen rearrangements involving Lewis acid catalysis[5] have been identifiedrecently for select substrates with chelating functional groups.We reported recently that the achiral guanidinium ion

Reversible Substrate Activation and Catalysis at an Intact Metal–Metal Bond Using a Redox-Active Supporting Ligand

An electron rich Ni(I)-Ni(I) bond supported by a doubly reduced naphthyridine-diimine (NDI) ligand reacts rapidly and reversibly with Ph2SiH2 and Et2SiH2 to form stable adducts. The solid-state structures of these complexes reveal binding modes in which the silanes symmetrically span the Ni-Ni bond and exhibit highly distorted H-Si-H angles and elongated Si-H bonds. This process is facilitated by the release of electron density stored in the π-system of the NDI ligand. Based on this dinuclear mode of activation, [NDI]Ni2 complexes are shown to catalyze the high-yielding hydrosilylation of alkenes, dienes, alkynes, aldehydes, ketones, enones, and amides. In comparative studies of alkyne hydrosilylations, the [NDI]Ni2 catalyst is found to be significantly more active than its mononuclear counterparts for aryl-substituted substrates.

Enantioselective Thiourea-Catalyzed Intramolecular Cope-Type Hydroamination

Catalysis of Cope-type rearrangements of bis-homoallylic hydroxylamines is demonstrated using chiral thiourea derivatives. This formal intramolecular hydroamination reaction provides access to highly enantioenriched α-substituted pyrrolidine products and represents a complementary approach to metal-catalyzed methods.

Dinuclear Nickel Complexes in Five States of Oxidation Using a Redox-Active Ligand

Redox-active nitrogen donor ligands have exhibited broad utility in stabilizing transition metal complexes in unusual formal oxidation states and enabling multielectron redox reactions. In this report, we extend these principles to dinuclear complexes using a naphthyridine-diimine (NDI) framework. Treatment of ((i-Pr)NDI) with Ni(COD)2 (2.0 equiv) yields a Ni(I)-Ni(I) complex in which the two metal centers form a single bond and the ((i-Pr)NDI) ligand is doubly reduced. A homologous series of ((i-Pr)NDI)Ni2 complexes in five oxidation states were synthesized and structurally characterized. Across this series, the ligand ranges from a neutral state in the most oxidized member to a dianionic state in the most reduced. The interplay between metal- and ligand-centered redox activity is interrogated using a variety of experimental techniques in combination with density functional theory models.

Selective nitrite reduction at heterobimetallic CoMg complexes.

Heme-containing nitrite reductases bind and activate nitrite by a mechanism that is proposed to involve interactions with Brønsted acidic residues in the secondary coordination sphere. To model this functionality using synthetic platforms that incorporate a Lewis acidic site, heterobimetallic CoMg complexes supported by diimine-dioxime ligands are described. The neutral (μ-NO2)CoMg species 3 is synthesized from the [(μ-OAc)(Br)CoMg](+) complex 1 by a sequence of one-electron reduction and ligand substitution reactions. Data are presented for a redox series of nitrite adducts, featuring a conserved μ-(η(1)-N:η(1)-O)-NO2 motif, derived from this synthon. Conditions are identified for the proton-induced N-O bond heterolysis of bound NO2(-) in the most reduced member of this series, affording the [(NO)(Cl)CoMg(H2O)](+) complex 6. Reduction of this complex followed by protonation leads to the evolution of free N2O. On the basis of these stoichiometric reactivity studies, the competence of complex 1 as a NO2(-) reduction catalyst is evaluated using electrochemical methods. In bulk electrolysis experiments, conducted at -1.2 V vs SCE using Et3NHCl as a proton source, N2O is produced selectively without the competing formation of NH3, NH2OH, or H2.

Access to formally Ni(I) states in a heterobimetallic NiZn system

Heterobimetallic NiZn complexes featuring metal centers in distinct coordination environments have been synthesized using diimine-dioxime ligands as binucleating scaffolds. A tetramethylfuran-containing ligand derivative enables a stable one-electron-reduced S = 1/2 species to be accessed using Cp2Co as a chemical reductant. The resulting pseudo-square planar complex exhibits spectroscopic and crystallographic characteristics of a ligand-centered radical bound to a Ni(II) center. Upon coordination of a π-acidic ligand such as PPh3, however, a five-coordinate Ni(I) metalloradical is formed. The electronic structures of these reduced species provide insight into the subtle effects of ligand structure on the potential and reversibility of the NiII/I couple for complexes of redox-active tetraazamacrocycles.

Reductive Cyclopropanations Catalyzed by Dinuclear Nickel Complexes.

Dinuclear Ni complexes supported by naphthyridine-diimine (NDI) ligands catalyze the reductive cyclopropanation of alkenes with CH2 Cl2 as the methylene source. The use of mild terminal reductants (Zn or Et2 Zn) confers significant functional-group tolerance, and the catalyst accommodates structurally and electronically diverse alkenes. Mononickel catalysts bearing related N chelates afford comparatively low cyclopropane yields (≤20 %). These results constitute an entry into catalytic carbene transformations from oxidized methylene precursors.