Xavi Ribas

Copper-catalyzed aerobic oxidative functionalization of an arene C-H bond: evidence for an aryl-copper(III) intermediate.

Recent studies have highlighted the ability of Cu(II) to catalyze the aerobic oxidative functionalization of C-H bonds; however, very little is known about the mechanisms of these reactions. Here, we describe the Cu(II)-catalyzed C-H methoxylation and amidation of a macrocylic arene substrate with O(2) as the stoichiometric oxidant. Kinetic and in situ spectroscopic studies demonstrate the involvement of three different oxidation states of Cu in the catalytic mechanism, including an aryl-Cu(III) intermediate. These observations establish a novel mechanistic pathway that has implications for numerous other Cu-catalyzed aerobic oxidation reactions.

Observation of Fe(V)=O using variable-temperature mass spectrometry and its enzyme-like C–H and C=C oxidation reactions

Oxo-transfer chemistry mediated by iron underpins many biological processes and today is emerging as synthetically very important for the catalytic oxidation of C-H and C=C moieties that are hard to activate conventionally. Despite the vast amount of research in this area, experimental characterization of the reactive species under catalytic conditions is very limited, although a Fe(V)=O moiety was postulated. Here we show, using variable-temperature mass spectrometry, the generation of a Fe(V)=O species within a synthetic non-haem complex at -40 °C and its reaction with an olefin. Also, with isotopic labelling we were able both to follow oxygen-atom transfer from H(2)O(2)/H(2)O through Fe(V)=O to the products and to probe the reactivity as a function of temperature. This study pioneers the implementation of variable-temperature mass spectrometry to investigate reactive intermediates.

Direct observation of CuI/CuIII redox steps relevant to Ullmann-type coupling reactions

A series of aryl–copper(III)-halide complexes have been synthesized and characterized by NMR and UV-visible spectroscopy, cyclic voltammetry and X-ray crystallography. These complexes closely resemble elusive intermediates often invoked in catalytic reactions, such as Ullmann–Goldberg cross-coupling reactions, and their preparation has enabled direct observation and preliminary characterization of aryl halide reductive elimination from CuIII and oxidative addition to CuI centers. In situ spectroscopic studies (1H NMR, UV-visible) of a Cu-catalyzed C–N coupling reaction provides definitive evidence for the involvement of an aryl-copper(III)-halide intermediate in the catalytic mechanism. These results provide the first direct observation of the CuI/CuIII redox steps relevant to Ullmann-type coupling reactions.

Nucleophilic Aryl Fluorination and Aryl Halide Exchange Mediated by a CuI/CuIII Catalytic Cycle

Copper-catalyzed halide exchange reactions under very mild reaction conditions are described for the first time using a family of model aryl halide substrates. All combinations of halide exchange (I, Br, Cl, F) are observed using catalytic amounts of Cu(I). Strikingly, quantitative fluorination of aryl-X substrates is also achieved catalytically at room temperature, using common F(-) sources, via the intermediacy of aryl-Cu(III)-X species. Experimental and computational data support a redox Cu(I)/Cu(III) catalytic cycle involving aryl-X oxidative addition at the Cu(I) center, followed by halide exchange and reductive elimination steps. Additionally, defluorination of the aryl-F model system can be also achieved with Cu(I) at room temperature operating under a Cu(I)/Cu(III) redox pair.

The role of organometallic copper(III) complexes in homogeneous catalysis

Organometallic CuIII species have been invoked in many copper-catalyzed reactions, but until recently, experimental evidences were lacking. The advent of new spectroscopic techniques and the characterization of simple and stable catalyst models have been key to gain clear mechanistic insights into these important reactions. In this perspective, we will discuss these discoveries and the corresponding mechanistic proposals that derive from important reactions such as nucleophilic organocuprate chemistry for C–C bond formation, C–heteroatom cross-coupling reactions (Ullmann condensation reactions and halide-exchange processes) and direct C–H bond functionalizations catalyzed by copper.

Asymmetric epoxidation with H2O2 by manipulating the electronic properties of non-heme iron catalysts.

A non-heme iron complex that catalyzes highly enantioselective epoxidation of olefins with H2O2 is described. Improvement of enantiomeric excesses is attained by the use of catalytic amounts of carboxylic acid additives. Electronic effects imposed by the ligand on the iron center are shown to synergistically cooperate with catalytic amounts of carboxylic acids in promoting efficient O-O cleavage and creating highly chemo- and enantioselective epoxidizing species which provide a broad range of epoxides in synthetically valuable yields and short reaction times.

Facile C−H Bond Cleavage via a Proton-Coupled Electron Transfer Involving a C−H···CuII Interaction

The present study provides mechanistic details of a mild aromatic C-H activation effected by a copper(II) center ligated in a triazamacrocylic ligand, affording equimolar amounts of a Cu(III)-aryl species and Cu(I) species as reaction products. At low temperatures the Cu(II) complex 1 forms a three-center, three-electron C-H...Cu(II) interaction, identified by pulse electron paramagnetic resonance spectroscopy and supported by density functional theory calculations. C-H bond cleavage is coupled with copper oxidation, as a Cu(III)-aryl product 2 is formed. This reaction proceeds to completion at 273 K within minutes through either a copper disproportionation reaction or, alternatively, even faster with 1 equiv of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), quantitatively yielding 2. Kinetic studies of both reactions strongly implicate a rate-limiting proton-coupled electron transfer as the key C-H activation step, a mechanism that does not conform to the C-H activation mechanism in a Ni(II) analogue or to any previously proposed C-H activation mechanisms.

Efficient and selective peracetic Acid epoxidation catalyzed by a robust manganese catalyst.

A manganese catalyst containing a tetradentate ligand derived from triazacyclononane exhibits high catalytic activity in epoxidation reactions using peracetic acid as oxidant. The system exhibits broad substrate scope and requires small (0.1-0.15 mol %) catalyst loading. The catalyst is remarkably selective toward aliphatic cis-olefins. Mechanistic studies point toward an electrophilic oxidant delivering the oxygen atom in a concerted step.

Regioselective oxidation of nonactivated alkyl C-H groups using highly structured non-heme iron catalysts

Selective oxidation of alkyl C-H groups constitutes one of the highest challenges in organic synthesis. In this work, we show that mononuclear iron coordination complexes Λ-[Fe(CF(3)SO(3))(2)((S,S,R)-MCPP)] (Λ-1P), Δ-[Fe(CF(3)SO(3))(2)((R,R,R)-MCPP)] (Δ-1P), Λ-[Fe(CF(3)SO(3))(2)((S,S,R)-BPBPP)] (Λ-2P), and Δ-[Fe(CF(3)SO(3))(2)((R,R,R)-BPBPP)] (Δ-2P) catalyze the fast, efficient, and selective oxidation of nonactivated alkyl C-H groups employing H(2)O(2) as terminal oxidant. These complexes are based on tetradentate N-based ligands and contain iron centers embedded in highly structured coordination sites defined by two bulky 4,5-pinenopyridine donor ligands, a chiral diamine ligand backbone, and chirality at the metal (Λ or Δ). X-ray diffraction analysis shows that in Λ-1P and Λ-2P the pinene rings create cavity-like structures that isolate the iron site. The efficiency and regioselectivity in catalytic C-H oxidation reactions of these structurally rich complexes has been compared with those of Λ-[Fe(CF(3)SO(3))(2)((S,S)-MCP)] (Λ-1), Λ-[Fe(CF(3)SO(3))(2)((S,S)-BPBP)] (Λ-2), Δ-[Fe(CF(3)SO(3))(2)((R,R)-BPBP)] (Δ-2), Λ-[Fe(CH(3)CN)(2)((S,S)-BPBP)](SbF(6))(2) (Λ-2SbF(6)), and Δ-[Fe(CH(3)CN)(2)((R,R)-BPBP)](SbF(6))(2) (Δ-2SbF(6)), which lack the steric bulk introduced by the pinene rings. Cavity-containing complexes Λ-1P and Λ-2P exhibit enhanced activity in comparison with Δ-1P, Δ-2P, Λ-1, Λ-2, and Λ-2SbF(6). The regioselectivity exhibited by catalysts Λ-1P, Λ-2P, Δ-1P, and Δ-2P in the C-H oxidation of simple organic molecules can be predicted on the basis of the innate properties of the distinct C-H groups of the substrate. However, in specific complex organic molecules where oxidation of multiple C-H sites is competitive, the highly elaborate structure of the catalysts allows modulation of C-H regioselectivity between the oxidation of tertiary and secondary C-H groups and also among multiple methylene sites, providing oxidation products in synthetically valuable yields. These selectivities complement those accomplished with structurally simpler oxidants, including non-heme iron catalysts Λ-2 and Λ-2SbF(6).

Evidence for a Precursor Complex in C-H Hydrogen Atom Transfer Reactions Mediated by a Manganese(IV) Oxo Complex

Evidence for a Precursor Complex in C-H Hydrogen Atom Transfer Reactions Mediated by a Manganese(IV) Oxo Complex