Nickel-Catalyzed Negishi Arylations of Propargylic Bromides: A Mechanistic Investigation

Although nickel-catalyzed stereoconvergent couplings of racemic alkyl electrophiles are emerging as a powerful tool in organic chemistry, to date there have been no systematic mechanistic studies of such processes. Herein, we examine the pathway for enantioselective Negishi arylations of secondary propargylic bromides, and we provide evidence for an unanticipated radical chain pathway wherein oxidative addition of the C–Br bond occurs through a bimetallic mechanism. In particular, we have crystallographically characterized a diamagnetic arylnickel(II) complex, [(i-Pr-pybox)NiIIPh]BAr^F_4, and furnished support for [(i-Pr-pybox)NiIIPh]^+ being the predominant nickel-containing species formed under the catalyzed conditions as well as a key player in the cross-coupling mechanism. On the other hand, our observations do not require a role for an organonickel(I) intermediate (e.g., (i-Pr-pybox)NiIPh), which has previously been suggested to be an intermediate in nickel-catalyzed cross-couplings, oxidatively adding alkyl electrophiles through a monometallic pathway

Anodic deposition of a robust iridium-based water-oxidation catalyst from organometallic precursors

Artificial photosynthesis, modeled on natural light-driven oxidation of water in Photosystem II, holds promise as a sustainable source of reducing equivalents for producing fuels. Few robust water-oxidation catalysts capable of mediating this difficult four-electron, four-proton reaction have yet been described. We report a new method for generating an amorphous electrodeposited material, principally consisting of iridium and oxygen, which is a robust and long-lived catalyst for water oxidation, when driven electrochemically. The catalyst material is generated by a simple anodic deposition from Cp*Ir aqua or hydroxo complexes in aqueous solution. This work suggests that organometallic precursors may be useful in electrodeposition of inorganic heterogeneous catalysts

SBOL Visual: A Graphical Language for Genetic Designs

Synthetic Biology Open Language (SBOL) Visual is a graphical standard for genetic engineering. It consists of symbols representing DNA subsequences, including regulatory elements and DNA assembly features. These symbols can be used to draw illustrations for communication and instruction, and as image assets for computer-aided design. SBOL Visual is a community standard, freely available for personal, academic, and commercial use (Creative Commons CC0 license). We provide prototypical symbol images that have been used in scientific publications and software tools. We encourage users to use and modify them freely, and to join the SBOL Visual community: http://www.sbolstandard.org/visual

Rhodium and iridium NNO-Scorpionate complexes: synthesis, structure, and reactivity with aluminum alkyls

Synthesis of rhodium and iridium NNO (NNO = bis(3,5-dimethyl-pyrazol-1-yl)acetate) complexes is reported. Complexes have been fully characterized by 1H and 13C{1H} NMR spectroscopy, mass spectrometry and X-ray diffraction. Addition of K(NNO) to [M(cod)Cl]2 results in formation of a new, fluxional species, M(cod)(NNO), which crystallizes as a bimetallic M(I)-M(I)dimer bridged by two NNO ligands. In contrast, addition of the protic starting material NNO-H to [Ir(cod)2][BArF24] (BArF24 = tetrakis(3,5-trifluoromethylphenyl)borate) resulted in the Ir(III) hydride species[Ir(cod)(NNO)(H)]BArF24. Finally, the reactivity of the newly synthesized complexes with alkyl aluminum reagents, AlR3 (R = Et, iBu) is explored

Evidence for Reversible Cyclometalation in Alkane Dehydrogenation and C–O Bond Cleavage at Iridium Bis(phosphine) Complexes

Methyl <i>tert</i>-butyl ether is found to undergo C–O bond cleavage in the formation of an iridium­(III) methallyl at a cationic bis­(phosphine) iridium complex. An exploration of this transformation has revealed a cyclometalated complex which was previously postulated to form reversibly in the first example of alkane dehydrogenation by a homogeneous transition-metal complex. The competence of this cyclometalated species in alkane dehydrogenation is demonstrated in a stoichiometric example, giving an isolable olefin dihydride. Detection and assignment of this elusive species confirms a previous hypothesis that reversible intramolecular phosphine cyclometalation can precede intermolecular alkane dehydrogenation

CO2 Capture by 2-(Methylamino)pyridine Ligated Aluminum Alkyl Complexes

A set of novel, easily synthesized aluminum complexes, Al[κ2-N,N-2-(methylamino)pyridine]2R (R = Et, iBu) are reported. When subjected to 1 atm of CO2 pressure, each hemilabile pyridine arm dissociates and facilitates cooperative activation of the CO2 substrate reminiscent of a Frustrated Lewis Pair. This reaction has limited precedent for Al/N based Lewis Pair systems, and this is the first system readily shown to sequester multiple equivalents of CO2 per aluminum center. The ethyl variant then reacts further, inserting a third equivalent of CO2 into the aluminum alkyl to generate an aluminum carboxylate. Examples of this type of reactivity are rare under thermal conditions. A joint experimental/computational study supports the proposed reaction mechanism

CO 2

A set of novel, easily synthesized aluminum complexes, Al[κ(2)-N,N-2-(methylamino)pyridine](2)R (R = Et, iBu) are reported. When subjected to 1 atm of CO(2) pressure, each hemilabile pyridine arm dissociates and facilitates cooperative activation of the CO(2) substrate reminiscent of a Frustrated Lewis Pair. This reaction has limited precedent for Al/N based Lewis Pair systems, and this is the first system readily shown to sequester multiple equivalents of CO(2) per aluminum center. The ethyl variant then reacts further, inserting a third equivalent of CO(2) into the aluminum alkyl to generate an aluminum carboxylate. Examples of this type of reactivity are rare under thermal conditions. A joint experimental/computational study supports the proposed reaction mechanism

Symmetrical Hydrogen Bonds in Iridium(III) Alkoxides with Relevance to Outer Sphere Hydrogen Transfer

International audienceA chelating ligand formed by deprotonation of 2-(2′-pyridyl)-2-propanol stabilizes a distorted trigonal bipyramidal geometry in a 16e- d6 5-coordinate iridium complex with the alkoxide acting as a π donor. Ambiphilic species such as AcOH bearing both nucleophilic and electrophilic functionality form adducts with the unsaturated iridium complex which contain strong intramolecular O***H***O hydrogen bonds that involve the basic alkoxide oxygen. Density functional theory (DFT) calculations on the isolated cations reproduce with high accuracy the geometrical features obtained via X-ray diffraction and corroborate the presence of very short hydrogen bonds with O***O distances of about 2.4 Å. Calculations further confirm the known trend that the hydrogen position in these bonds is sensitive to the O***O distance, with the shortest distances giving rise to symmetrical O***H***O interactions. Dihydrogen is shown to add across the Ir-O π bond in a presumed proton transfer reaction, demonstrating bifunctional behavior by the iridium alkoxide

Formation of a Delocalized Iridium Benzylidene with Azaquinone Methide Character via Alkoxycarbene Cleavage

Lewis base directed α,α-dehydrogenation of 2-(methoxymethyl)­aniline gives a cationic iridium­(III) alkoxycarbene. This alkoxycarbene is found to be subject to pyridine-promoted C–O bond cleavage in the formation of a delocalized iridium benzylidene. X-ray diffraction and DFT calculations support a structural distortion away from a classic benzylidene structure stemming from azaquinone methide character. Treatment of the iridium azaquinone methide with ethylene results in formal ethylene insertion into the vinylic C–H bond, demonstrating an unusual mechanism for net C–C bond formation via benzylic C–O cleavage

Formation of a Delocalized Iridium Benzylidene with Azaquinone Methide Character via Alkoxycarbene Cleavage

Lewis base directed α,α-dehydrogenation of 2-(methoxymethyl)­aniline gives a cationic iridium­(III) alkoxycarbene. This alkoxycarbene is found to be subject to pyridine-promoted C–O bond cleavage in the formation of a delocalized iridium benzylidene. X-ray diffraction and DFT calculations support a structural distortion away from a classic benzylidene structure stemming from azaquinone methide character. Treatment of the iridium azaquinone methide with ethylene results in formal ethylene insertion into the vinylic C–H bond, demonstrating an unusual mechanism for net C–C bond formation via benzylic C–O cleavage