Control of Olefin Hydroarylation Catalysis via a Sterically and Electronically Flexible Platinum(II) Catalyst Scaffold

Highly Anti-Markovnikov Selective Oxidative Arene Alkenylation Using Ir(I) Catalyst Precursors and Cu(II) Carboxylates

The Ir(I) complex [Ir(μ-Cl)(coe)2]2 (coe = cis-cyclooctene) is a catalyst precursor for benzene alkenylation using Cu(II) carboxylate salts. Using [Ir(μ-Cl)(coe)2]2, propenylbenzenes are formed from the reaction of benzene, propylene, and CuX2 (X = acetate, pivalate, or 2-ethylhexanoate). The Ir-catalyzed reactions selectively produce anti-Markovnikov products, trans-β-methylstyrene, cis-β-methylstyrene, and allylbenzene, along with minor amounts of the Markovnikov product, α-methylstyrene. The selectivity for the anti-Markovnikov products changed as the reaction progressed. For example, in a reaction that uses 240 equiv of Cu(OHex)2 (related to Ir), the selectivity for the anti-Markovnikov products increases from 18:1 at 3 h to 42:1 at 42 h with 30 psig of propylene at 150 °C. Studies of product stability have revealed that the increase in the selectivity for anti-Markovnikov products is not the result of an isomerization process or the selective decomposition of specific products. Rather, the change in selectivity correlates with the ratio of Cu(II) to Cu(I) in the solution, which decreases as the reaction progresses. We propose that the identity of the active catalyst changes as Cu(I) is accumulated, resulting in the formation of an active catalyst that is more selective for anti-Markovnikov products. Using a 4:1 Cu(I)/Cu(II) ratio at the start of the reaction, a 65(3):1 anti-Markovnikov/Markovnikov ratio is observed.

Pd(II) and Rh(I) Catalytic Precursors for Arene Alkenylation: Comparative Evaluation of Reactivity and Mechanism Based on Experimental and Computational Studies

We combine experimental and computational investigations to compare and understand catalytic arene alkenylation using the Pd(II) and Rh(I) precursors Pd(OAc)₂ and [(η²-C₂H₄)₂Rh(μ-OAc)]₂ with arene, olefin, and Cu(II) carboxylate at elevated temperatures (>120 °C). Under specific conditions, previous computational and experimental efforts have identified heterotrimetallic cyclic PdCu₂(η²-C₂H₄)₃(μ-OPiv)₆ and [(η²-C₂H₄)₂Rh(μ-OPiv)₂]₂(μ-Cu) (OPiv = pivalate) species as likely active catalysts for these processes. Further studies of catalyst speciation suggest a complicated equilibrium between Cu(II)-containing complexes containing one Rh or Pd atom with complexes containing two Rh or Pd atoms. At 120 °C, Rh catalysis produces styrene >20-fold more rapidly than Pd. Also, at 120 °C, Rh is ∼98% selective for styrene formation, while Pd is ∼82% selective. Our studies indicate that Pd catalysis has a higher predilection toward olefin functionalization to form undesired vinyl ester, while Rh catalysis is more selective for arene/olefin coupling. However, at elevated temperatures, Pd converts vinyl ester and arene to vinyl arene, which is proposed to occur through low-valent Pd(0) clusters that are formed in situ. Regardless of arene functionality, the regioselectivity for alkenylation of mono-substituted arenes with the Rh catalyst gives an approximate 2:1 meta/para ratio with minimal ortho C-H activation. In contrast, Pd selectivity is significantly influenced by arene electronics, with electron-rich arenes giving an approximate 1:2:2 ortho/meta/para ratio, while the electron-deficient (α,α,α)-trifluorotoluene gives a 3:1 meta/para ratio with minimal ortho functionalization. Kinetic intermolecular arene ethenylation competition experiments find that Rh reacts most rapidly with benzene, and the rate of mono-substituted arene alkenylation does not correlate with arene electronics. In contrast, with Pd catalysis, electron-rich arenes react more rapidly than benzene, while electron-deficient arenes react less rapidly than benzene. These experimental findings, in combination with computational results, are consistent with the arene C-H activation step for Pd catalysis involving significant η¹-arenium character due to Pd-mediated electrophilic aromatic substitution character. In contrast, the mechanism for Rh catalysis is not sensitive to arene-substituent electronics, which we propose indicates less electrophilic aromatic substitution character for the Rh-mediated arene C-H activation.

Synthesis and photobiological evaluation of Ru(II) complexes with expanded chelate polypyridyl ligands

Photoreactive Ru(II) complexes capable of ejecting ligands have been used extensively for photocaging applications and for the creation of "photocisplatin" reagents. The incorporation of distortion into the structure of the coordination complex lowers the energy of dissociative excited states, increasing the yield of the photosubstitution reaction. While steric clash between ligands induced by adding substituents at the coordinating face of the ligand has been extensively utilized, a lesser known, more subtle approach is to distort the coordination sphere by altering the chelate ring size. Here a systematic study was performed to alter metal-ligand bond lengths, angles, and to cause intraligand distortion by introducing a "linker" atom or group between two pyridine rings. The synthesis, photochemistry, and photobiology of five Ru(II) complexes containing CH₂, NH, O, and S-linked dipyridine ligands was investigated. All systems where stable in the dark, and three of the five were photochemically active in buffer. While a clear periodic trend was not observed, this study lays the foundation for the creation of photoactive systems utilizing an alternative type of distortion to facilitate photosubstitution reactions.

Electron-Deficient Ru(II) Complexes as Catalyst Precursors for Ethylene Hydrophenylation

Ruthenium(II) complexes with the general formula TpRu(L)(NCMe)Ph (Tp = hydrido(trispyrazolyl)borate, L = CO, PMe3, P(OCH2)3CEt, P(pyr)3, P(OCH2)2(O)CCH3) have previously been shown to catalyze arene alkylation via Ru-mediated arene C–H activation including the conversion of benzene and ethylene to ethylbenzene. Previous studies have suggested that the catalytic performance of these TpRu(II) catalysts increases with reduced electron-density at the Ru center. Herein, three new structurally related Ru(II) complexes are synthesized, characterized, and studied for possible catalytic benzene ethylation. TpRu(NO)Ph2 exhibited low stability due to the facile elimination of biphenyl. The Ru(II) complex (TpBr3)Ru(NCMe)(P(OCH2)3CEt)Ph (TpBr3 = hydridotris(3,4,5-tribromopyrazol-1-yl)borate) showed no catalytic activity for the conversion of benzene and ethylene to ethylbenzene, likely due to the steric bulk introduced by the bromine substituents. (Ttz)Ru(NCMe)(P(OCH2)3CEt)Ph (Ttz = hydridotris(1,2,4-triazol-1-yl)borate) catalyzed approximately 150 turnover numbers (TONs) of ethylbenzene at 120 °C in the presence of Lewis acid additives. Here, we compare the activity and features of catalysis using (Ttz)Ru(NCMe)(P(OCH2)3CEt)Ph to previously reported catalysis based on TpRu(L)(NCMe)Ph catalyst precursors.

Advances in Group 10 Transition-Metal-Catalyzed Arene Alkylation and Alkenylation

On a large scale, the dominant method to produce alkyl arenes has been arene alkylation from arenes and olefins using acid-based catalysis. The addition of arene C-H bonds across olefin C═C bonds catalyzed by transition-metal complexes through C-H activation and olefin insertion into metal-aryl bonds provides an alternative approach with potential advantages. This Perspective presents recent developments of olefin hydroarylation and oxidative olefin hydroarylation catalyzed by molecular complexes based on group 10 transition metals (Ni, Pd, Pt). Emphasis is placed on comparisons between Pt catalysts and other group 10 metal catalysts as well as Ru, Ir, and Rh catalysts.

Reversible PtII-CH3 deuteration without methane loss: metal-ligand cooperation vs. ligand-assisted PtII-protonation

Di(2-pyridyl)ketone dimethylplatinum(ii), (dpk)Pt^II(CH₃)₂, reacts with CD₃OD at 25 °C to undergo complete deuteration of Pt-CH₃ fragments in ∼5 h without loss of methane to form (dpk)Pt^II(CD₃)₂ in virtually quantitative yield. The deuteration can be reversed by dissolution in CH₃OH or CD₃OH. Kinetic analysis and isotope effects, together with support from density functional theory calculations indicate a metal-ligand cooperative mechanism wherein DPK enables Pt-CH₃ deuteration by allowing non-rate-limiting protonation of Pt^II by CD₃OD. In contrast, other model di(2-pyridyl) ligands enable rate-limiting protonation of Pt^II, resulting in non-rate-limiting C-H(D) reductive coupling. Owing to its electron-poor nature, following complete deuteration, DPK can be dissociated from the Pt^II-centre, furnishing [(CD₃)₂Pt^II(μ-SMe₂)]₂ as the perdeutero analogue of [(CH₃)₂Pt^II(μ-SMe₂)]₂, a commonly used Pt^II-precursor.

Enhancing the catalytic properties of well-defined electrophilic platinum complexes

Platinum complexes have been often considered as the least reactive of the group 10 triad metals. Slow kinetics are behind this lack of reactivity but, still, some industrially relevant catalytic process are dominated by platinum compounds and sometimes different selectivities can be found in comparison to Ni or Pd. Nevertheless, during the last years, it has been reported that the catalytic behaviour of well-defined platinum derivatives can be improved through a judicious choice of their electronic and steric properties, leading to highly electrophilic or low-electron count platinum systems. In this feature article, we highlight some catalytic processes in which well-defined electrophilic platinum complexes or coordinatively unsaturated systems play an important role in their catalytic activity.

Advances in Rhodium-Catalyzed Oxidative Arene Alkenylation

ConspectusAlkyl and alkenyl arenes are of substantial value in both large-scale and fine chemical processes. Billions of pounds of alkyl and alkenyl arenes are produced annually. Historically, the dominant method for synthesis of alkyl arenes is acid-catalyzed arene alkylation, and alkenyl arenes are often synthesized in a subsequent dehydrogenation step. But these methods have limitations that result from the catalytic mechanism including (1) common polyalkylation, which requires an energy intensive transalkylation process, (2) quantitative selectivity for Markovnikov products for arene alkylation using α-olefins, (3) for substituted arenes, regioselectivity that is dictated by the electronic character of the arene substituents, (4) inability to form alkenyl arenes in a single process, and (5) commonly observed slow reactivity with electron-deficient arenes. Transition-metal-catalyzed aryl-carbon coupling reactions can produce alkyl or alkenyl arenes from aryl halides. However, these reactions often generate halogenated waste and typically require a stoichiometric amount of metal-containing transmetalation reagent. Transition-metal-catalyzed arene alkylation or alkenylation that involves arene C-H activation and olefin insertion into metal-aryl bonds provides a potential alternative method to prepare alkyl or alkenylation arenes. Such reactions can circumvent carbocationic intermediates and, as a result, can overcome some of the limitations mentioned above. In particular, controlling the regioselectivity of the insertion of α-olefins into metal-aryl bonds provides a strategy to selectively synthesize anti-Markovnikov products. But, previously reported catalysts often show limited longevity and low selectivity for anti-Markovnikov products.In this Account, we present recent developments in single-step arene alkenylation using Rh catalyst precursors. The reactions are successful for unactivated hydrocarbons and exhibit unique selectivity. The catalytic production of alkenyl arenes operates via Rh-mediated aromatic C-H activation, which likely occurs by a concerted metalation-deprotonation mechanism, olefin insertion into a Rh-aryl bond, β-hydride elimination from the resulting Rh-hydrocarbon product, and net dissociation of alkenyl arene with formation of a Rh hydride. Reaction of the Rh hydride with Cu(II) oxidant completes the catalytic cycle. Although Rh nanoparticles can be formed under some conditions, mechanistic studies have revealed that soluble Rh species are likely responsible for the catalysis. These Rh catalyst precursors achieve high turnovers with >10,000 catalytic turnovers observed in some cases. Under anaerobic conditions, Cu(II) carboxylates are used as the oxidant. In some cases, aerobic recycling of Cu(II) oxidant has been demonstrated. Hence, the Rh arene alkenylation catalysis bears some similarities to Pd-catalyzed olefin oxidation (i.e., the Wacker-Hoechst process). The Rh-catalyzed arene alkenylation is compatible with some electron-deficient arenes, and they are selective for anti-Markovnikov products when using substituted olefins. Finally, when using monosubstituted arenes, consistent with a metal-mediated C-H activation process, Rh-catalyzed alkenylation of substituted arenes shows selectivity for meta- and para-alkenylation products.

Selective synthesis of pyridyl pyridones and oxydipyridines by transition-metal-free hydroxylation and arylation of 2-fluoropyridine derivatives

An efficient protocol for the construction of various pyridyl pyridone and oxydipyridine derivatives through a hydroxylation and arylation tandem reaction of 2-fluoropyridines is reported. Under simple transition-metal-free conditions, the reaction provided a series of products in good to excellent yields, and their structures were confirmed by crystal diffraction analysis. Furthermore, the controlling effect of 6-position substituents on the highly selective synthesis of pyridone and oxydipyridine was studied.

Theoretical Probe to the Mechanism of Pt-Catalyzed C-H Acylation Reaction: Possible Pathways for the Acylation Reaction of a Platinacycle

Density functional theory (DFT) and nudged elastic band (NEB) theory have been used to study the possible pathways for the acylation of cycloplatinated complex A derived from 2-phenoxypyridine, which is conceived as the key step in the platinum-catalyzed acylation of 2-aryloxypyridines. Geometry optimization indicates that the previously proposed intermediate, an arenium ion species as a result of analogous aromatic substitution, is not an energy minimum, but rather cationic Pt-arene η²-complex E is obtained as a stable intermediate. NEB simulations suggest that the minimum energy pathway for the acylation reaction has energy barrier of 33.6 kcal/mol and consists of the following steps: (1) Nucleophilic substitution at acetyl chloride by the platinum of the reactant A forms five-coordinate Pt(IV) acylplatinum complex B with an energy barrier of 21.7 kcal/mol. (2) B undergoes 1,2-acyl migration from the platinum to the cyclometalated carbon through a three-membered platinacycle transition state to give Pt-arene η²-complex E with an energy barrier of 14.0 kcal/mol. (3) E undergoes ligand exchange with chloride to form neutral Pt-arene η²-complex F. (4) F undergoes ligand substitution with acetonitrile to give the product and the energy barrier is small (10.6 kcal/mol). The rate-determining step is the 1,2-acyl migration step. It is interesting to note that intermediate F was not included in the proposed mechanism but was identified by the NEB simulations. Five-coordinate Pt(IV) acylplatinum complex B undergoes barrierless ligand coordination with chloride to form neutral formal oxidative addition acylplatinum complex D; however, D is less stable than reactant A by 2.9 kcal/mol, which also implies that the isolation of an oxidative addition product Pt(IV) complex may be very challenging. The direct reductive elimination of D to form product P has a higher energy barrier (36.6 kcal/mol).

Structural and chemical properties of half-sandwich rhodium complexes supported by the bis(2-pyridyl)methane ligand

[Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) supported by bidentate chelating ligands are a useful class of compounds for studies of redox chemistry and catalysis. Here, we show that the bis(2-pyridyl)methane ligand, also known as dipyridylmethane or dpma, can support [Cp*Rh] complexes in the formally +iii and +ii rhodium oxidation states. Specifically, two new rhodium complexes ([Cp*Rh(dpma)(L)]ⁿ⁺, L = Cl^-, CH₃CN) have been isolated and structurally characterized, and the properties of the complexes have been compared with those of [Cp*Rh] complexes bearing the related dimethyldipyridylmethane (Me₂dpma) ligand. Complex [Cp*Rh(dpma)(NCCH₃)]²⁺ displays a quasireversible rhodium(iii/ii) reduction by cyclic voltammetry; related electron paramagnetic resonance (EPR) spectroscopic studies confirm access to the unusual rhodium(ii) oxidation state. Further reduction to the formally rhodium(i) oxidation state, however, is followed by deprotonation of dpma, as observed in electrochemical studies and chemical reduction experiments. This reactivity can be understood to occur as a consequence of the presence of doubly benzylic protons in the dpma ligand, since use of the analogous Me₂dpma enables reduction to rhodium(i) without involvement of ligand deprotonation. These findings highlight the important role of the ligand backbone substitution pattern in influencing the stability of highly-reduced complexes, a key class of metal species for study of electron and proton management in catalysis.

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

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 ((Fl)DAB)Rh(TFA)(η(2)-C2H4) [(Fl)DAB 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.

Moilanen J, Roemmele TL, Boeré RT, Konu J, Tuononen HM. Group 13 complexes of dipyridylmethane, a forgotten ligand in coordination chemistry

A comprehensive study of the coordination properties of dipyridylmethane (dpma) was performed. The preparation of metal complexes of the type [(dpma)₂MCl₂]⁺, [(dpma)MCl₂]⁺, (M = Al, Ga), (dpma)InCl₃ and (dpma)(BCl₃)₂ is described along with their characterization.