Atom Transfer Chemistry Research Articles

Mechanochemical reduction of alkyl and aryl halides using mesoporous zinc oxide.

In this study, we propose a mechanochemical approach that combines mesoporous ZnO (m-ZnO) as a mechanoredox catalyst and silane-mediated atom transfer chemistry to achieve efficient hydrodehalogenation of organic halides. The reaction can be conducted under mild conditions without the use of a large amount of organic solvent. Substrates ranging from activated alkyl halides to unactivated aryl halides were converted to the corresponding debrominated hydrogenation products in moderate to excellent isolated yields (50-95%). In addition, m-ZnO can be recycled and reused without appreciable loss of catalytic activity.

Electrocatalytic Atom Transfer Radical Addition with Turbocharged Organocopper(II) Complexes.

The utility and scope of Cu-catalyzed halogen atom transfer chemistry have been exploited in the fields of atom transfer radical polymerization and atom transfer radical addition, where the metal plays a key role in radical formation and minimizing unwanted side reactions. We have shown that electrochemistry can be employed to modulate the reactivity of the Cu catalyst between its active (CuI) and dormant (CuII) states in a variety of ligand systems. In this work, a macrocyclic pyridinophane ligand (L1) was utilized, which can break the C-Br bond of BrCH2CN to release •CH2CN radicals when in complex with CuI. Moreover, the [CuI(L1)]+ complex can capture the •CH2CN radical to form a new species [CuII(L1)(CH2CN)]+ in situ that, on reduction, exhibits halogen atom transfer reactivity 3 orders of magnitude greater than its parent complex [CuI(L1)]+. This unprecedented rate acceleration has been identified by electrochemistry, successfully reproduced by simulation, and exploited in a Cu-catalyzed bulk electrosynthesis where [CuII(L1)(CH2CN)]+ participates as a radical donor in the atom transfer radical addition of BrCH2CN to a selection of styrenes. The formation of these turbocharged catalysts in situ during electrosynthesis offers a new approach to the Cu-catalyzed organic reaction methodology.

On the Mechanism of Halogen Atom Transfer from C−X Bonds to α‐Aminoalkyl Radicals: A Computational Study

AbstractGeneration of carbon centered radicals from organic halides represents a powerful tool in modern organic chemistry, especially in the context of photoredox catalysis. However, activation of carbon–halogen bonds is usually promoted by toxic and hazardous tin reagents. Alternatively, α‐aminoalkyl radicals have emerged as a cheap and efficient halogen atom transfer (XAT) reagents, although the activation mechanism is still underexplored with respect to hydrogen atom transfer (HAT) chemistry. Herein, we report a computational systematic evaluation of four different α‐aminoalkyl radicals on the Halogen Atom Transfer (XAT) mechanism. We have evaluated up to 32 reactions, including two different types of substrates (Ph−X and Cy−X). This systematic study aims to provide a big picture on the key effects in this reactivity including the hybridization of carbon, the nature of the halogen, and the electronics/sterics of the α‐aminoalkyl radical.

Carbonyl-Directed Aliphatic Fluorination: A Special Type of Hydrogen Atom Transfer Beats Out Norrish II.

Recently, our group reported that enone and ketone functional groups, upon photoexcitation, can direct site-selective sp3 C-H fluorination in terpenoid derivatives. How this transformation actually occurred remained mysterious, as a significant number of mechanistic possibilities came to mind. Herein, we report a comprehensive study describing the reaction mechanism through kinetic studies, isotope-labeling experiments, 19F NMR, electrochemical studies, synthetic probes, and computational experiments. To our surprise, the mechanism suggests intermolecular hydrogen atom transfer (HAT) chemistry is at play, rather than classical Norrish hydrogen atom abstraction as initially conceived. What is more, we discovered a unique role for photopromoters such as benzil and related compounds that necessitates their chemical transformation through fluorination in order to be effective. Our findings provide documentation of an unusual form of directed HAT and are of crucial importance for defining the necessary parameters for the development of future methods.

Ligand Taxonomy for Bioinorganic Modeling of Dioxygen-Activating Non-Heme Iron Enzymes.

Novel functions emerge from novel structures. To develop efficient catalytic systems for challenging chemical transformations, chemists often seek inspirations from enzymatic catalysis. A large number of iron complexes supported by nitrogen-rich multidentate ligands have thus been developed to mimic oxo-transfer reactivity of dioxygen-activating metalloenzymes. Such efforts have significantly advanced our understanding of the reaction mechanisms by trapping key intermediates and elucidating their geometric and electronic properties. Critical to the success of this biomimetic approach is the design and synthesis of elaborate ligand systems to balance the thermodynamic stability, structural adaptability, and chemical reactivity. In this Concept article, representative design strategies for biomimetic atom-transfer chemistry are discussed from the perspectives of "ligand builders". Emphasis is placed on how the primary coordination sphere is constructed, and how it can be elaborated further by rational design for desired functions.

Synthesis of Polyfunctionalised 2-Piperidinones Catalysed by Fe(acac)3

Herein is reported the synthesis of polyfunctionalised piperidines and 2-piperidinones, through hydrogen atom transfer (HAT) chemistry catalysed by Fe(acac)3. The nature and substitution around the Michael acceptor, as well as the allylic amine, allowed access to all positions of the piperidine ring. The chemistry tolerates a range of different functionalities, allowing the investigation of new and diverse scaffolds.

Non-Heme Iron Catalysts for Olefin Epoxidation: Conformationally Rigid Aryl-Aryl Junction To Support Amine/Imine Multidentate Ligands.

Atom-transfer chemistry represents an important class of reactions catalyzed by metalloenzymes. As a functional mimic of non-heme iron enzymes that deliver oxygen atoms to olefins, we have designed monoiron complexes supported by new N-donor chelates. These ligands take advantage of heme-like conformational rigidity of the π-conjugated molecular backbone, and synthetic flexibility of tethering non-heme donor groups for additional steric and electronic control. Iron complexes generated in situ can be used to carry out catalytic epoxidation of a wide range of olefin substrates by using mCPBA as a terminal oxidant. The fate of initial iron-peracid adduct and the involvement of iron-oxo species in this process were investigated further by mechanistic probes and isotope exchange studies. Our findings suggest that anilidopyridyl-derived [N,N]-bidentate motif could serve as a versatile structural platform to build non-heme ligands for catalytic oxidation chemistry.

Electron and Oxygen Atom Transfer Chemistry of Co(II) in a Proton Responsive, Redox Active Ligand Environment.

The bis-pyrazolato pyridine complex LCo(PEt3)2 serves as a masked form of three-coordinate CoII and shows diverse reactivity in its reaction with several potential outer sphere oxidants and oxygen atom transfer reagents. N-Methylmorpholine N-oxide (NMO) oxidizes coordinated PEt3 from LCo(PEt3)2, but the final cobalt product is still divalent cobalt, in LCo(NMO)2. The thermodynamics of a variety of oxygen atom transfer reagents, including NMO, are calculated by density functional theory, to rank their oxidizing power. Oxidation of LCo(PEt3)2 with AgOTf in the presence of LiCl as a trapping nucleophile forms the unusual aggregate [LCo(PEt3)2Cl(LiOTf)2]2 held together by Li+ binding to very nucleophilic chloride on Co(III) and triflate binding to those Li+. In contrast, Cp2Fe+ effects oxidation to trivalent cobalt, to form (HL)Co(PEt3)2Cl+; proton and the chloride originate from solvent in a rare example of CH2Cl2 dehydrochlorination. An unexpected noncomplementary redox reaction is reported involving attack by 2e reductant PEt3 nucleophile on carbon of the 1e oxidant radical Cp2Fe+, forming a P-C bond and H+; this reaction competes in the reaction of LCo(PEt3)2 with Cp2Fe+.

Lewis Acid Assisted Nitrate Reduction with Biomimetic Molybdenum Oxotransferase Complex.

The reduction of nitrate (NO3-) to nitrite (NO2-) is of significant biological and environmental importance. While MoIV(O) and MoVI(O)2 complexes that mimic the active site structure of nitrate reducing enzymes are prevalent, few of these model complexes can reduce nitrate to nitrite through oxygen atom transfer (OAT) chemistry. We present a novel strategy to induce nitrate reduction chemistry of a previously known catalyst MoIV(O)(SN)2 (2), where SN = bis(4- tert-butylphenyl)-2-pyridylmethanethiolate, that is otherwise incapable of achieving OAT with nitrate. Addition of nitrate with the Lewis acid Sc(OTf)3 (OTf = trifluoromethanesulfonate) to 2 results in an immediate and clean conversion of 2 to MoVI(O)2(SN)2 (1). The Lewis acid additive further reacts with the OAT product, nitrite, to form N2O and O2. This work highlights the ability of Sc3+ additives to expand the reactivity scope of an existing MoIV(O) complex together with which Sc3+ can convert nitrate to stable gaseous molecules.

Selective Catalytic Olefin Epoxidation with MnII-Exchanged MOF-5

Partial substitution of ZnII by MnII in Zn4O(terephthalate)3 (MOF-5) leads to a distorted all-oxygen ligand field supporting a single MnII site, whose structure was confirmed by Mn K-edge X-ray absorption spectroscopy. The MnII ion at the MOF-5 node engages in redox chemistry with a variety of oxidants. With tBuSO2PhIO, it produces a putative MnIV-oxo intermediate, which upon further reaction with adventitious hydrogen is trapped as a MnIII–OH species. Most intriguingly, the intermediacy of the high-spin MnIV–oxo species is likely responsible for catalytic activity of the MnII-MOF-5 precatalyst, which in the presence of tBuSO2PhIO catalyzes oxygen atom transfer reactivity to form epoxides from cyclic alkenes with >99% selectivity. These results demonstrate that MOF secondary building units serve as competent platforms for accessing terminal high-valent metal–oxo species that consequently engage in catalytic oxygen atom transfer chemistry owing to the relatively weak ligand fields provided by the SBU.

Light-Mediated Reductive Debromination of Unactivated Alkyl and Aryl Bromides

Cleavage of carbon–halogen bonds via either single-electron reduction or atom transfer is a powerful transformation in the construction of complex molecules. In particular, mild, selective hydrodehalogenations provide an excellent follow-up to the application of halogen atoms as directing groups or the utilization of atom transfer radical addition (ATRA) chemistry for the production of hydrocarbons. Here we combine the mechanistic properties of photoredox catalysis and silane-mediated atom transfer chemistry to accomplish the hydrodebromination of carbon–bromide bonds. The resulting method is performed under visible light irradiation in an open vessel and is capable of the efficient reduction of a variety of unactivated alkyl and aryl substrates.

ChemInform Abstract: Fascinating Hydrogen Atom Transfer Chemistry of Alkenes Inspired by Problems inTotal Synthesis.

AbstractReview: (15 refs.).

Fascinating hydrogen atom transfer chemistry of alkenes inspired by problems in total synthesis.

Fascinating hydrogen atom transfer chemistry of alkenes inspired by problems in total synthesis.

Oxygen Atom Transfer and Oxidative Water Incorporation in Cuboidal Mn3MOn Complexes Based on Synthetic, Isotopic Labeling, and Computational Studies

The oxygen-evolving complex (OEC) of photosystem II contains a Mn(4)CaO(n) catalytic site, in which reactivity of bridging oxidos is fundamental to OEC function. We synthesized structurally relevant cuboidal Mn(3)MO(n) complexes (M = Mn, Ca, Sc; n = 3,4) to enable mechanistic studies of reactivity and incorporation of μ(3)-oxido moieties. We found that Mn(IV)(3)CaO(4) and Mn(IV)(3)ScO(4) were unreactive toward trimethylphosphine (PMe(3)). In contrast, our Mn(III)(2)Mn(IV)(2)O(4) cubane reacts with this phosphine within minutes to generate a novel Mn(III)(4)O(3) partial cubane plus Me(3)PO. We used quantum mechanics to investigate the reaction paths for oxygen atom transfer to phosphine from Mn(III)(2)Mn(IV)(2)O(4) and Mn(IV)(3)CaO(4). We found that the most favorable reaction path leads to partial detachment of the CH(3)COO(-) ligand, which is energetically feasible only when Mn(III) is present. Experimentally, the lability of metal-bound acetates is greatest for Mn(III)(2)Mn(IV)(2)O(4). These results indicate that even with a strong oxygen atom acceptor, such as PMe(3), the oxygen atom transfer chemistry from Mn(3)MO(4) cubanes is controlled by ligand lability, with the Mn(IV)(3)CaO(4) OEC model being unreactive. The oxidative oxide incorporation into the partial cubane, Mn(III)(4)O(3), was observed experimentally upon treatment with water, base, and oxidizing equivalents. (18)O-labeling experiments provided mechanistic insight into the position of incorporation in the partial cubane structure, consistent with mechanisms involving migration of oxide moieties within the cluster but not consistent with selective incorporation at the site available in the starting species. These results support recent proposals for the mechanism of the OEC, involving oxido migration between distinct positions within the cluster.

Synthesis and structural characterization of two-coordinate low-valent 14-group metal complexes bearing bulky bis(amido)silane ligands

A series of germylene, stannylene and plumbylene complexes [η(2)(N,N)-Me(2)Si(DippN)(2)Ge:] (3a), [η(2)(N,N)-Ph(2)Si(DippN)(2)Ge:] (3b), [η(2)(N,N)-Me(2)Si(DippN)(2)Sn:] (4), [η(2)(N,N)-Me(2)Si(DippN)(2)Pb:](2) (5a), and [η(2)(N,N)-Ph(2)Si(DippN)(2)Pb:] (5b) (Dipp = 2,6-iPr(2)C(6)H(3)) bearing bulky bis(amido)silane ligands were readily prepared either by the transamination of M[N(SiMe(3))(2)](2) (M = Sn, Pb) and [Me(2)Si(DippNH)(2)] or by the metathesis reaction of bislithium bis(amido)silane [η(1)(N),η(1)(N)-R(2)Si(DippNLi)(2)] (R = Me, Ph) with the corresponding metal halides GeCl(2)(dioxane), SnCl(2), and PbCl(2), respectively. Preliminary atom-transfer chemistry involving [η(2)(N,N)-Me(2)Si(DippN)(2)Ge:] (3a) with oxygen yielded a dimeric oxo-bridged germanium complex [η(2)(N,N)-Me(2)Si(DippN)(2)Ge(μ-O)](2) (6). All complexes were characterized by (1)H, (13)C, (119)Sn NMR, IR, and elemental analysis. X-ray single crystal diffraction analysis revealed that the metal centres in 3b, 4, and 5b are sterically protected to prevent interaction between the metal centre and the nitrogen donors of adjacent molecules while complex 5a shows a dimeric feature with a strong intermolecular Pb···N interaction.

Low-Coordinate Germylene and Stannylene Heterocycles featuring Sterically Tunable Bis(amido)silyl Ligands

A series of monomeric heterocyclic metallylenes [{(i)Pr(2)Si(NR)(2)}M:] (M = Ge and Sn; R = Dipp = 2,6-(i)Pr(2)C(6)H(3) or SiPh(3)) have been prepared. Preliminary atom-transfer chemistry involving the new low-valent germylenes with the chalcogen sources Me(3)NO and S(8) yielded the corresponding dimeric oxo- and sulfido complexes (e.g., [{(i)Pr(2)Si(NDipp)(2)}Ge(μ-E)](2); E = O and S). Structural analyses of the metallylenes and their oxidized products reveal that incorporation of the umbrella-shaped triarylsilyl groups (SiPh(3)) within the NSiN chelate confers additional steric protection about the group 14 centers relative to a Dipp group. The inclusion of sterically modifiable -SiAr(3) (Ar = aryl) units as part of a bis(amido) ligand array represents a new approach in this field and holds considerable promise with regard to attaining increasingly higher degrees of steric bulk.

Manganese amido-imine bisphenol Hangman complexes

A modular ligand macrocycle is composed from two phenolic groups linked to a cyclohexane bridge through an amide bond and an imine bond. The stability of the asymmetric linkers to metathesis permits a macrocyclic platform to be assembled from the condensation of two different phenolic groups in a single-step, high yield, reaction. The primary coordination sphere may be tuned with functional groups on one phenolic group. The other phenolic group may be modified with a scaffold possessing a proton transfer group. In this way, control over the secondary coordination sphere of the macrocycle is achieved. Manganese complexes of the amido-imine linked macrocycle catalytically epoxidizes 1,2-dihydronapthalene using sodium hypochlorite as the oxidant. The amido-imine macrocycles represent a new metal active site capable of supporting high oxidation states and attendant atom transfer chemistry while at the same time permitting control over the primary and secondary sphere of the metal center.

Mo(TmMe)(O)2Cl]: An Alternative Functional Model of Sulfite Oxidase

The hydrotris(methimazolyl)borate anion (TmMe) has been used to synthesize an alternative functional model ([Mo(TmMe)(O)2Cl]) of the metalloenzyme sulfite oxidase. It has been shown that the complex undergoes oxygen atom transfer chemistry and that it performs the primary function of the enzyme, sulfite oxidation. A method using ion chromatography has been developed to definitively prove that sulfite is oxidized to sulfate. Employment of a soft tripodal ligand has allowed us to tune the redox potentials of our complex so that they are significantly closer to those reported for sulfite oxidase.

Small Molecule Activation at Uranium Coordination Complexes: Control of Reactivity via Molecular Architecture

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Small molecule activation at uranium coordination complexes: control of reactivity via molecular architecture

Electron-rich uranium coordination complexes display a pronounced reactivity toward small molecules. In this Feature article, the exciting chemistry of trivalent uranium ions coordinated to classic Werner-type ligand environments is reviewed. Three fundamentally important reactions of the [(((R)ArO)3tacn)U]-system are presented that result in alkane coordination, CO/CO2 activation, and nitrogen atom-transfer chemistry.