Double Insertion of Coordinated Phosphanylalkyne Ligands into a Pt−C6F5 Bond

Enhanced reactivity is shown by uncoordinated C≡C bonds in the proximity of a metal in phosphanylacetylene complexes. cis-[Pt(C₆F₅)₂(thf)₂] reacts with [M(C₆F₅)₂(PPh₂C≡CPh)₂] (M=Pt, Pd) to form binuclear complexes containing the novel 2,3-bis(diphenylphosphanyl)-1,3-butadien-1-yl bridging ligand. Substitution of the solvent ligands with, for example, PPh₂H (see picture) provides species that could be characterized by X-ray crystallography.     

Positioning a Carbon–Fluorine Bond over the π Cloud of an Aromatic Ring: A Different Type of Arene Activation

It is known that the fluoro group has only a small effect on the rates of electrophilic aromatic substitutions. Imagine instead a carbon–fluorine (C?F) bond positioned tightly over the π cloud of an aryl ring—such an orthogonal, noncovalent arrangement could instead stabilize a positively charged arene intermediate or transition state, giving rise to novel electrophilic aromatic substitution chemistry. Herein, we report the synthesis and study of molecule 1 , containing a rigid C?F???Ar interaction that plays a prominent role in both its reaction chemistry and spectroscopy. For example, we established that the C?F???Ar interaction can bring about a >1500 fold increase in the relative rate of an aromatic nitration reaction, affording functionalization on the activated ring exclusively. Overall, these results establish fluoro as a through‐space directing/activating group that complements the traditional role of fluorine as a slightly deactivating aryl substituent in nitrations.     

Gold/Palladium Alloy for Carbon–Halogen Bond Activation: An Unprecedented Halide Dependence

New catalytic activity of gold/palladium alloy nanoclusters (NCs) for carbon–halogen bond activation is demonstrated. In the case of an aryl chloride, the inclusion of gold in a bimetallic catalyst is indispensable to achieve the coupling reactions. Gold has the unique effect of stabilizing palladium, such that Pd²⁺ leached from clusters by means of spillover of chloride during oxidative addition. The thus‐formed spillover intermediate further reacts heterogeneously in both Ullmann and Suzuki‐type coupling reactions through a new type of mechanism. In the case of an aryl bromide, Ullmann coupling occurs through the spillover of bromide, similar to that of aryl chloride. However, a significant fraction of palladium also leached, which diminished the Ullmann coupling activity of the aryl bromide and, as a result, the order of reactivity was ArCl>ArBr. With regard to the activation of the C?Br bond towards a Suzuki‐type reaction, the inclusion of a higher gold content in gold/palladium clusters stabilized palladium to prevent the leaching of Pd²⁺ from the clusters by means of spillover of bromide. The spillover intermediate reacts heterogeneously with PhB(OH)₂, palladium‐rich gold/palladium, or pure palladium clusters; the oxidative addition of ArBr favors the extraction of palladium from the clusters, yielding Pd²⁺ intermediates. The extracted intermediates react homogenously (Pd^2+/Pd⁰ catalysis) with PhB(OH)₂, which results in the higher selectivity of the cross‐coupling product. An initial step to observe such unprecedented halide dependency, together with the dynamic behavior of palladium on the surface of gold is the oxidative addition of Ar?X. A detailed insight into the first oxidative addition process was also examined by quantum chemical calculations.     

Altering Oxygen Binding by Redox-Inactive Metal Substitution to Control Catalytic Activity: Oxygen Reduction on Manganese Oxide Nanoparticles as a Model System**

Establishing generic catalyst design principles by identifying structural features of materials that influence their performance will advance the rational engineering of new catalytic materials. In this study, by investigating metal-substituted manganese oxide (spinel) nanoparticles, Mn₃O₄:M (M=Sr, Ca, Mg, Zn, Cu), we rationalize the dependence of the activity of Mn₃O₄:M for the electrocatalytic oxygen reduction reaction (ORR) on the enthalpy of formation of the binary MO oxide, Δ_fH°(MO), and the Lewis acidity of the M²⁺ substituent. Incorporation of elements M with low Δ_fH°(MO) enhances the oxygen binding strength in Mn₃O₄:M, which affects its activity in ORR due to the established correlation between ORR activity and the binding energy of *O/*OH/*OOH species. Our work provides a perspective on the design of new compositions for oxygen electrocatalysis relying on the rational substitution/doping by redox-inactive elements.     

Copper‐Catalyzed Skeletal Rearrangements of O‐Propargylic Oximes via Cleavage of a Carbon–Oxygen Bond

In this account, we describe recent developments in copper‐catalyzed skeletal 2,3‐rearrangement reactions of O‐propargylic oximes to form four‐membered cyclic nitrones, pyridine N‐oxides, and amidodienes via N‐allenylnitrone intermediates. The sequence of events leading up to our encounter with O‐propargylic oximes is also presented.     

Diastereoselective Synthesis of Highly Substituted Five-Membered-Ring Oxygen Heterocycles by Zirconocene-Mediated C–C Coupling Reactions

As homoenolate equivalents , zirconocene-1-aza-1,3-diene complexes were used for the first time in stereoselective synthesis. By insertion of an unsymmetrical ketone in the Zr–C σ bond, sterically demanding trisubstituted dihydro- and tetrahydrofuran derivatives were prepared in good overall yields and with high diastereoselectivities (see below).     

Carbon–Nitrogen Bond Cleavage by a Thorium‐NHC‐bpy Complex

Actinide complexes demonstrate unparalleled reactivity towards small molecules. However, utilizing these powerful transformations in a predictable and deliberate manner remains challenging. Therefore, developing actinide systems that not only perform noteworthy chemistry but also demonstrate controllable reactivity is a key goal. We describe a bis(NHC)borate thorium‐bpy complex ( 1 ) that is capable of reductively cleaving the R?NC bond in a series of organic isocyanides. In contrast to most actinide‐mediated bond activations, the dealkylation event mediated by 1 is remarkably general and yields very well‐defined products that assist in mechanistic elucidation. Synthesis of the rearranged but‐3‐enyl product from the reaction of 1 and cyclopropylmethyl isocyanide supports the notion of a radical‐based mechanism.     

Structural Basis for Copper–Oxygen Mediated C−H Bond Activation by the Formylglycine‐Generating Enzyme

The formylglycine‐generating enzyme (FGE) is a unique copper protein that catalyzes oxygen‐dependent C−H activation. We describe 1.66 Å‐ and 1.28 Å‐resolution crystal structures of FGE from Thermomonospora curvata in complex with either Ag^I or Cd^II providing definitive evidence for a high‐affinity metal‐binding site in this enzyme. The structures reveal a bis‐cysteine linear coordination of the monovalent metal, and tetrahedral coordination of the bivalent metal. Similar coordination changes may occur in the active enzyme as a result of Cu^I/II redox cycling. Complexation of copper atoms by two cysteine residues is common among copper‐trafficking proteins, but is unprecedented for redox‐active copper enzymes or synthetic copper catalysts.     

Selective Carbon–Carbon Bond Cleavage for the Stereoselective Synthesis of Acyclic Systems

Most of the efforts of organic chemists have been directed to the development of creative strategies to build carbon–carbon and carbon–heteroatom bonds in a predictable and efficient manner. In this Review, we show an alternative approach where challenging molecular skeletons could be prepared through selective cleavage of carbon–carbon bonds. We demonstrate that it has the potential to be a general principle in organic synthesis for the regio‐, diastereo‐, and even enantioselective preparation of adducts despite the fact that C? C single bonds are among the least reactive functional groups. The development of such strategies may have an impact on synthesis design and can ultimately lead to new selective and efficient processes for the utilization of simple hydrocarbons.     

Synthesis of cyclomanganated complexes derived from 2,5-diphenyl-1,3,4-oxadiazole and their reactivity with respect to 1,1-diphenyldiazomethane: Evidence for a fluxional trihaptobenzylic coordination mode

The cyclometallation of 2,5-diphenyl-1,3,4-oxadiazole with benzylpentacarbonylmanganese afforded the related mono- and binuclear complexes. The thermolytic coupling of 1,1-diphenyldiazomethane with monometallated 2,5-diphenyl-1,3,4-oxadiazole afforded substantial amounts of a new trihaptobenzylic complex. In the solid state, an X-ray diffraction analysis confirmed the coordination of manganese metal to one of the phenyl groups introduced by the 1,1-diphenyldiazomethane and to one of ligand’s nitrogen centre. In solution, this trihaptobenzylic complex displays a fluxionality which is assumed to be related to the equilibration of its helical Λ and Δ enantiomers.     

Using ligand exchange reactions to control the coordination environment of Pt(II) acetylide complexes: applications to conjugated metallacyclynes

Ligand exchange of cis-bis(diphenylphosphino)ethylene (dppee) with trans-(Ph₃P)₂Pt(CCR)₂ easily generates the cis-complexes (dppee)Pt(CCR)₂ in 64-95% yield. This transformation is used to convert pyridine-containing macrocycle 7 to its cis-analogue 8 and the macrocyclic bipyridine analogue 10 to the unique macrocyclic ligand 11. X-ray structural characterization of trans-complexes 5a and 5b and cis-complexes 6a and 6b are reported, as is the structure of the strained macrocycle 8.     

Facile Carbon–Fluorine Bond Activation and Subsequent Functionalisation of 1,1‐Difluoroethylene and Tetrafluoroethylene Promoted by Adjacent Metal Centres

The bridging fluoroolefin ligands in the complexes [Ir₂(CH₃)(CO)₂(μ‐olefin)(dppm)₂][OTf] (olefin=tetrafluoroethylene, 1,1‐difluoroethylene; dppm=μ‐Ph₂PCH₂PPh₂; OTf^?=CF₃SO₃^?) are susceptible to facile fluoride ion abstraction. Both fluoroolefin complexes react with trimethylsilyltriflate (Me₃SiOTf) to give the corresponding fluorovinyl products by abstraction of a single fluoride ion. Although the trifluorovinyl ligand is bound to one metal, the monofluorovinyl group is bridging, bound to one metal through carbon and to the other metal through a dative bond from fluorine. Addition of two equivalents of Me₃SiOTf to the tetrafluoroethylene‐bridged species gives the difluorovinylidene‐bridged product [Ir₂(CH₃)(OTf)(CO)₂(μ‐OTf)(μ‐C?CF₂)(dppm)₂][OTf]. The 1,1‐difluoroethylene species is exceedingly reactive, reacting with water to give 2‐fluoropropene and [Ir₂(CO)₂(μ‐OH)(dppm)₂][OTf] and with carbon monoxide to give [Ir₂(CO)₃(μ‐κ¹:η²‐C?CCH₃)(dppm)₂][OTf] together with two equivalents of HF. The trifluorovinyl product [Ir₂(κ¹‐C₂F₃)(OTf)(CO)₂(μ‐H)(μ‐CH₂)(dppm)₂][OTf], obtained through single C? F bond activation of the tetrafluoroethylene‐bridged complex, reacts with H₂ to form trifluoroethylene, allowing the facile replacement of one fluorine in C₂F₄ with hydrogen.     

Strain‐Induced Double Carbon–Carbon Bond Activations of Cycloparaphenylenes by a Platinum Complex: Application to the Synthesis of Cyclic Diketones

The carbon–carbon (C?C) bond activation of [n]cycloparaphenylenes ([n]CPPs) by a transition‐metal complex is herein reported. The Pt⁰ complex Pt(PPh₃)₄ regioselectively cleaves two C?C σ bonds of [5] CPP and [6]CPP to give cyclic dinuclear platinum complexes in high yields. Theoretical calculations reveal that the relief of ring strain drives the reaction. The cyclic complex was further transformed into a cyclic diketone by using a CO insertion reaction.