T. Brent Gunnoe

Ru(II) Catalysts Supported by Hydridotris(pyrazolyl)borate for the Hydroarylation of Olefins: Reaction Scope, Mechanistic Studies, and Guides for the Development of Improved Catalysts

Carbon-carbon bond formation is the central method by which synthetic chemists add complexity, which often represents value, to molecules. Uniting a carbon chain with an aromatic substrate to yield an alkyl arene product is thus a molecular means of creating value-added materials. A traditional method for generating alkyl arenes is Friedel-Crafts catalysis, in which an alkyl halide or olefin is activated to react with an aromatic substrate. Unfortunately, despite the development of new generations of solid-state catalysts, the reaction often requires relatively harsh conditions and frequently gives poor to moderate selectivity. Conversely, a halide can first be incorporated into the aromatic ring, and the aryl halide can subsequently be joined by a variety of catalytic coupling techniques. But generating the aryl halide itself can be problematic, and such methods typically are not atom-economical. The addition of aromatic C-H bonds across the C-C double bonds of olefins (olefin hydroarylation) is therefore an attractive alternative in the preparation of alkyl arenes. Despite the dominance and practical advantages of heterogeneous catalysts in industrial synthesis, homogeneous systems can offer an enhanced ability to fine-tune catalyst activity. As such, well-defined homogeneous catalysts for the hydroarylation of olefins provide a potentially promising avenue to address issues of selectivity, including the production of monoalkylated arene products and the control of linear-to-branched ratios for synthesis of long-chain alkyl arenes, and provide access to more ambient reaction conditions. However, examples of homogeneous catalysts that are active for the conversion of unactivated aromatic and olefin substrates to alkyl arene products that function via metal-mediated C-H activation pathways are limited. In this Account, we present results from research aimed at the development of Ru(II) catalysts supported by the hydridotris(pyrazolyl)borate (Tp) ligand for the addition of aromatic C-H bonds across olefins. On the basis of detailed mechanistic studies with TpRu(L)(NCMe)R catalysts, in which the neutral ancillary ligand L is varied, we have arrived at guidelines for the development of improved catalysts that are based on the octahedral-d6 motif.

Catalytic Oxy‐Functionalization of Methane and Other Hydrocarbons: Fundamental Advancements and New Strategies

The controlled conversion of methane to methanol requires C-H bond cleavage and C-O bond formation. A catalytic cycle incorporating 1,2-CH-addition and net oxygen insertion with late transition metals has been proposed for this conversion. This Minireview discusses the current state of the art for each step of the proposed catalytic cycle.

Anti-Markovnikov hydroamination and hydrothiolation of electron-deficient vinylarenes catalyzed by well-defined monomeric copper(I) amido and thiolate complexes

Monomeric Cu(I) amido and thiolate complexes that are supported by the N-heterocyclic carbene ligand 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) catalyze the hydroamination and hydrothiolation of electron-deficient vinylarenes with reactivity patterns that are consistent with an intermolecular nucleophilic addition of the amido/thiolate ligand of (IPr)Cu(XR) (X = NH or S; R = Ph, CH2Ph) to free vinylarene.

Activation of carbon–hydrogen bonds and dihydrogen by 1,2-CH-addition across metal–heteroatom bonds

The controlled conversion of hydrocarbons to functionalized products requires selective C-H bond cleavage. This perspective provides an overview of 1,2-CH-addition of hydrocarbons across d(0) transition metal imido complexes and compares and contrasts these to examples of analogous reactions that involve later transition metal amide, hydroxide and alkoxide complexes with d(6) and d(8) metals.

A rhodium catalyst for single-step styrene production from benzene and ethylene

A more direct way to synthesize styrene Foam cups, foam pellets, plastic cutlery: All are made of polystyrene, which in turn is made of styrene. The massive manufacturing scale of this commodity chemical places a premium on the efficiency of its synthesis. The current industrial route requires three steps to make styrene from benzene and ethylene. Vaughan et al. present a rhodium catalyst that achieves the coupling in a single step by using a recyclable copper salt as an oxidant. Although the catalyst is slow for industrial application, it demonstrates the viability of a more direct process. Science, this issue p. 421 A catalytic oxidation points the way toward a more efficient method for making the key ingredient in Styrofoam. Rising global demand for fossil resources has prompted a renewed interest in catalyst technologies that increase the efficiency of conversion of hydrocarbons from petroleum and natural gas to higher-value materials. Styrene is currently produced from benzene and ethylene through the intermediacy of ethylbenzene, which must be dehydrogenated in a separate step. The direct oxidative conversion of benzene and ethylene to styrene could provide a more efficient route, but achieving high selectivity and yield for this reaction has been challenging. Here, we report that the Rh catalyst (FlDAB)Rh(TFA)(η2–C2H4) [FlDAB is N,N′-bis(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene; TFA is trifluoroacetate] converts benzene, ethylene, and Cu(II) acetate to styrene, Cu(I) acetate, and acetic acid with 100% selectivity and yields ≥95%. Turnover numbers >800 have been demonstrated, with catalyst stability up to 96 hours.

Selective CH Functionalization of Methane, Ethane, and Propane by a Perfluoroarene Iodine(III) Complex

Direct partial oxidation of methane, ethane, and propane to their respective trifluoroacetate esters is achieved by a homogeneous hypervalent iodine(III) complex in non-superacidic (trifluoroacetic acid) solvent. The reaction is highly selective for ester formation (>99%). In the case of ethane, greater than 0.5 M EtTFA can be achieved. Preliminary kinetic analysis and density functional calculations support a nonradical electrophilic CH activation and iodine alkyl functionalization mechanism.

Flavin-Catalyzed Insertion of Oxygen into Rhenium–Methyl Bonds

Flavins and related molecules catalyze organic Baeyer-Villiger reactions. Combined experimental and DFT studies indicate that these molecules also catalyze the insertion of oxygen into metal-carbon bonds through a Baeyer-Villiger-like transition state.

Dihapto binding of aromatic molecules by π-basic transition metal complexes: development of alternatives to the {Os(NH3)5}2+ fragment

Abstract Dihapto-coordination of aromatic ligands by electron-rich transition metals can effectively dearomatize the bound aromatic molecule. The pentaammineosmium(II) system forms thermally stable η 2 -complexes with a variety of arenes and aromatic heterocycles, and has been used for a variety of organic transformations on the bound aromatic fragments. The systematic variation of isoelectronic rhenium(I) systems has provided the necessary electronic and steric characteristics needed for a less expensive, chiral alternative to the {Os(NH 3 ) 5 } 2+ system. The {TpRe(CO)(PMe 3 )} system has been shown to form stable dihapto complexes with furan, thiophene and naphthalene. Accordingly, the {TpRe(CO)(PMe 3 )} fragment and analogous {TpRe(CO)(L)} fragments represent the first class of asymmetric surrogates to the pentaammineosmium(II) system.

Variable Pathways for Oxygen Atom Insertion into Metal–Carbon Bonds: The Case of Cp*W(O)2(CH2SiMe3)

Cp*W(O)(2)(CH(2)SiMe(3)) (1) (Cp* = η(5)-pentamethylcyclopentadienyl) reacts with oxygen atom donors (e.g., H(2)O(2), PhIO, IO(4)(-)) in THF/water to produce TMSCH(2)OH (TMS = trimethylsilyl). For the reaction of 1 with IO(4)(-), the proposed pathway for alcohol formation involves coordination of IO(4)(-) to 1 followed by concerted migration of the -CH(2)TMS ligand to the coordinated oxygen of IO(4)(-) with concomitant dissociation of IO(3)(-) to produce Cp*W(O)(2)(OCH(2)SiMe(3)) (3), which undergoes protonolysis to yield free alcohol. In contrast to the reaction with IO(4)(-), the reaction of 1 with H(2)O(2) results in the formation of the η(2)-peroxo complex Cp*W(O)(η(2)-O(2))(CH(2)SiMe(3)) (2). In the presence of acid (HCl) or base (NaOH), complex 2 produces TMSCH(2)OH. The conversion of 2 to TMSCH(2)OH catalyzed by Brønsted acid is proposed to occur through protonation of the η(2)-peroxo ligand, which facilitates the transfer of the -CH(2)TMS ligand to a coordinated oxygen of the η(2)-hydroperoxo ligand. In contrast, the hydroxide promoted conversion of 2 to TMSCH(2)OH is proposed to involve hydroxide coordination, followed by proton transfer from the hydroxide ligand to the peroxide ligand to yield a κ(1)-hydroperoxide intermediate. The migration of the -CH(2)TMS ligand to the coordinated oxygen of the κ(1)-hydroperoxo produces an alkoxide complex, which undergoes protonolysis to yield free alcohol.

Selective Monooxidation of Light Alkanes Using Chloride and Iodate

We describe an efficient system for the direct partial oxidation of methane, ethane, and propane using iodate salts with catalytic amounts of chloride in protic solvents. In HTFA (TFA = trifluoroacetate), >20% methane conversion with >85% selectivity for MeTFA have been achieved. The addition of substoichiometric amounts of chloride is essential, and for methane the conversion increases from <1% in the absence of chloride to >20%. The reaction also proceeds in aqueous HTFA as well as acetic acid to afford methyl acetate. (13)C labeling experiments showed that less than 2% of methane is overoxidized to (13)CO2 at 15% conversion of (13)CH4. The system is selective for higher alkanes: 30% ethane conversion with 98% selectivity for EtTFA and 19% propane conversion that is selective for mixtures of the mono- and difunctionalized TFA esters. Studies of methane conversion using a series of iodine-based reagents [I2, ICl, ICl3, I(TFA)3, I2O4, I2O5, (IO2)2S2O7, (IO)2SO4] indicated that the chloride enhancement is not limited to iodate.