Reductive Elimination Reactions Research Articles

Thermal and Photoreductive Elimination from the Tellurium Center of π-Conjugated Tellurophenes

This study introduces small molecule tellurophenes that can undergo photoreductive elimination. A tellurophene compound with strong light absorption properties and extended π-conjugation, 2,5-bis[5-(N,N'-dihexylisoindigo)]tellurophene (1), has been synthesized. Halogen oxidative addition to the tellurium center from various halogen sources gives the dibromo- (1Br2) and dichloro- (1Cl2) adducts, leading to a red-shift in the optical absorption properties. In the presence of excess opposing halogen, 1Br2 and 1Cl2 can interconvert, with equilibrium favoring the dichlorotellurophene adduct. Reductive elimination reactions were studied using optical absorption spectroscopy, NMR spectroscopy, thermogravimetric analysis, and matrix-assisted laser desorption/ionization (MALDI) analysis. Thermal reductive elimination from 1Br2 and 1Cl2 occurs in the solid-state to restore 1. Photoreductive elimination occurs under irradiation with green (505 nm) light in solution in the presence of a halogen trap with some decomposition. This is the first example of photoreductive elimination from a mononuclear tellurophene complex.

Our Journey to the Chemistry of Metallabenzynes

This personal account summarizes our work on the chemistry of transition-metal-containing metallabenzynes, organometallic compounds derived from formal replacement of a C atom in benzyne by an isolobal transition-metal fragment. Metallabenzynes with osmium and rhenium have been synthesized and well characterized. They have aromatic character on the basis of the criteria of reactivity, geometry, aromatic stabilization energy, and magnetic properties. They can undergo typical reactions of aromatic systems (e.g., electrophilic substitution reactions) and organometallic complexes (e.g., reductive elimination reactions to form carbene complexes).

Theoretical studies on the reductive elimination reaction mechanism from neutral palladium(IV) sulfinate complexes

Detailed reaction mechanisms of reductive elimination from neutral palladium(IV) sulfinate complex have been investigated with the aid of density functional theory calculations. The calculation results reveal that the neutral palladium(IV) sulfinate complexes have four possible reductive elimination pathways via the C–S, C–C, C–Cl, and desulfitative C–C bond formation to give different products, and the formation of the C–S bond‐containing product is kinetically more favorable over the formation of other products. Present calculations provide new insights into the organopalladium(IV) chemistry and C–S bond activation. Copyright © 2013 John Wiley & Sons, Ltd.

Regioselective C–H Activation Preceded by Csp2–Csp3 Reductive Elimination from Cyclometalated Platinum(IV) Complexes

Reductive elimination reactions of the cyclometalated platinum(IV) compounds [PtMe2Cl{C6H4CH═NCH2(4-ClC6H4)}L] and [PtMe2Br{C6H4CH═NCH2(C6H5)}L] (L = SMe2, PPh3) to form Csp3–Csp2 bonds, followed by either exclusive Csp2–H bond activation (L = SMe2) or competition between Csp2–H and Csp3–H bond activation (L = PPh3), are reported. Isomerization to give endo products instead of the expected exo complex was observed for the ligand C6H4CH═NCH2(2-BrC6H5), and formation of an endo six-membered platinacycle occurs for the ligand 2,4,6-Me3C6H2CH═NCH2(2-BrC6H4).

Supra-molecular synthons based on gold…π(arene) interactions

Colloquially described as modern day “Gold Rush”, use of gold in catalysis is a research area attracting intensive and increasing attention. This is evidenced, for example, by nearly exponential increase in publications appearing in the past decade in the field of homogeneous catalysis with gold [1]. Molecular gold or in the form of nanoparticles/thin films prove to be efficient catalysts offering benefits over other transition metals, properties that relate, in no small part, to electrophilic nature of gold which, in turn, is correlated with reduction in energy of the 6s atomic orbital, owing to relativistic effects [2]. Such is the propensity for molecular gold to associate with alkynes, in particular, the term alkynophilicity is often used to describe this property. Other advantages of gold often relate to gold not being likely to be involved in oxidative addition or reductive elimination reactions unlike square planar palladium(II), for example. Not surprisingly, there are many recent and thorough reviews of gold in the catalysis literature e.g., [3–7]. Given that gold is a carbophilic Lewis acid, the question then arises: does this property influence the type of supramolecular association formed by gold in its crystal structures? Gold holds a special fascination for crystal engineers owing to its propensity to form Au...Au or aurophilic interactions [8, 9]. Not only do Au...Au interactions lead to fascinating supra-molecular architectures, the fact that the energy of stabilisation afforded to their crystal structures can match that provided by conventional hydrogen bonding [10] led to competition studies between these synthons [11–14]. By contrast, considerably less attention has been directed to ascertaining formation of Au...π(arene) interactions [15]; analogous interactions are well established in the supramolecular chemistry of the main group elements [16–18]. Herein, an overview of Au...π(arene) interactions, formed both intraand inter-molecularly, operating in the crystal structures of molecular gold compounds is given, and their presence in macro-molecular structures is also discussed, along with future prospects for this type of interaction.

New Insights in the Formation of Five- Versus Seven-Membered Platinacycles: A Kinetico-Mechanistic Study

The reaction of Pt(IV) organometallic cyclometalated complexes of type [PtXAr(2)(Ar'CHNCH(2)CH(2)NMe(2))] to produce 5- and 7-membered Pt(II) metalacycles by a formal C-C reductive elimination/C-H oxidative addition/Ar-H reductive elimination sequence has been studied from a preparative and kinetico-mechanistic perspective. The detection and characterization of key intermediates has also been achieved via the careful selection of reaction conditions, including time, extracted from the kinetic studies. From the data collected, it is clear that a fine-tuning of the reactivity is possible with respect to the formation of the alternative 5- and 7-membered cyclometalated complexes (i.e., [PtX(Ar-Ar'CH═NCH(2)CH(2)NMe(2))] and [PtX(Ar-Ar'CH═NCH(2)CH(2)NMe(2))]). In all cases a common reductive elimination reaction occurs to form a non cyclometalated intermediate compound of type [PtX(Ar)(Ar-Ar'CH═NCH(2)CH(2)NMe(2))], which leads to the selective formation of the above-mentioned complexes by the actuation of an unexpected equilibrium between its cis-(X,NMe(2)) and trans-(X,NMe(2)) isomers. While the cis-(X,NMe(2)) isomer produces the 7-membered metallacycle, the trans-(X,NMe(2)) form leads exclusively to the 5-membered analogue. The isomerization process is dominated by the steric hindrance existing between the Ar-Pt and Ar-Ar'CHN-Pt moieties in the cis-(X,NMe(2)) isomer that forces a Z conformation of the imine, thus leading exclusively to the 7-membered ring formation only for the less hindered X = Cl systems.

Reductive Elimination from Cyclometalated Platinum(IV) Complexes To Form Csp2–Csp3 Bonds and Subsequent Competition between Csp2–H and Csp3–H Bond Activation

Reductive elimination reactions of the cyclometalated platinum(IV) compounds [PtMe2Br{C6H4CH═NCH2(4-ClC6H4)}L] (L = SMe2, PPh3) to form Csp3–Csp2 bonds, followed by either exclusive Csp2–H bond act...

Reactions of Late Transition Metal Complexes with Molecular Oxygen

Limited natural resources, high energy consumption, economic considerations, and environmental concerns demand that we develop new technologies for the sustainable production of chemicals and fuels. New methods that combine the selective activation of C-H bonds of hydrocarbons with oxidation by a green oxidant such as molecular oxygen would represent huge advances toward this goal. The spectacular selectivity of transition metals in cleaving C-H bonds offers the potential for the direct use of hydrocarbons in the production of value-added organics such as alcohols. However, the use of oxygen, which is abundant, environmentally benign, and inexpensive (particularly from air), has proven challenging, and more expensive and less green oxidants are often employed in transition-metal-catalyzed reactions. Advances in the use of oxygen as an oxidant in transition-metal-catalyzed transformations of hydrocarbons will require a better understanding of how oxygen reacts with transition metal alkyl and hydride complexes. For alkane oxidations, researchers will need to comprehend and predict how metals that have shown particularly high activity and selectivity in C-H bond activation (e.g. Pt, Pd, Rh, Ir) will react with oxygen. In this Account, we present our studies of reactions of late metal alkyls and hydrides with molecular oxygen, emphasizing the mechanistic insights that have emerged from this work. Our studies have unraveled some of the general mechanistic features of how molecular oxygen inserts into late metal hydride and alkyl bonds along with a nascent understanding of the scope and limitations of these reactions. We present examples of the formation of metal hydroperoxide species M-OOH by insertion of dioxygen into Pt(IV)-H and Pd(II)-H bonds and show evidence that these reactions proceed by radical chain and hydrogen abstraction pathways, respectively. Comparisons with recent reports of insertion of oxygen into other Pd(II)-H complexes, and also into Ir(III)-H and Rh(III)-H complexes, point to potentially general mechanisms for this type of reaction. Additionally, we observed oxygen-promoted C-H and H-H reductive elimination reactions from five-coordinate Ir(III) alkyl hydride and dihydride complexes, respectively. Further, when Pd(II)Me(2) and Pt(II)Me(2) complexes were exposed to oxygen, insertion processes generated M-OOMe complexes. Mechanistic studies for these reactions are consistent with radical chain homolytic substitution pathways involving five-coordinate M(III) intermediates. Due to the remarkable ability of Pt(II) and Pd(II) to activate the C-H bonds of hydrocarbons (RH) and form M-R species, this reactivity is especially exciting for the development of partial alkane-oxidation processes that utilize molecular oxygen. Our understanding of how late transition metal alkyls and hydrides react with molecular oxygen is growing rapidly and will soon approach our knowledge of how other small molecules such as olefins and carbon monoxide react with these species. Just as advances in understanding olefin and CO insertion reactions have shaped important industrial processes, key insight into oxygen insertion should lead to significant gains in sustainable commercial selective oxidation catalysis.

ChemInform Abstract: The Generation and Reactions of Quinone Methides

AbstractReview: 172 refs.

Uranium and Thorium Hydride Complexes as Multielectron Reductants: A Combined Neutron Diffraction and Quantum Chemical Study

The unusual uranium reaction system in which uranium(4+) and uranium(3+) hydrides interconvert by formal bimetallic reductive elimination and oxidative addition reactions, [(C(5)Me(5))(2)UH(2)](2) (1) ⇌ [(C(5)Me(5))(2)UH](2) (2) + H(2), was studied by employing multiconfigurational quantum chemical and density functional theory methods. 1 can act as a formal four-electron reductant, releasing H(2) gas as the byproduct of four H(2)/H(-) redox couples. The calculated structures for both reactants and products are in good agreement with the X-ray diffraction data on 2 and 1 and the neutron diffraction data on 1 obtained under H(2) pressure as part of this study. The interconversion of the uranium(4+) and uranium(3+) hydride species was calculated to be near thermoneutral (~-2 kcal/mol). Comparison with the unknown thorium analogue, [(C(5)Me(5))(2)ThH](2), shows that the thorium(4+) to thorium(3+) hydride interconversion reaction is endothermic by 26 kcal/mol.

Reductive elimination pathway for homocysteine to methionine conversion in cobalamin-dependent methionine synthase

Density functional theory has been applied to investigate the methyl transfer from methylcobalamin (MeCbl) cofactor to homocysteine (Hcy) as catalyzed by methionine synthase (MetH). Specifically, the S(N)2 and the reductive elimination pathways have been probed as the possible mechanistic pathways for the methyl transfer reaction. The calculations indicate that the activation barrier for the reductive elimination reaction (24.4 kcal mol(-1)) is almost four times higher than that for the S(N)2 reaction (7.3 kcal mol(-1)). This high energy demand of the reductive elimination pathway is rooted in the structural distortion of the corrin ring that is induced en route to the formation of the triangular transition state. Furthermore, the reductive elimination reaction demands the syn accommodation of the methyl group and the substrate over the upper face of the corrin ring, which also accounts for the high energy demand of the reaction. Consequently, the reductive elimination pathway for MetH-catalyzed methyl transfer from MeCbl to Hcy cannot be considered as one of the possible mechanistic routes.

Theoretical studies on the Mo-catalyzed asymmetric intramolecular Pauson-Khand-type [2 + 2 + 1] cycloadditions of 3-allyloxy-1-propynylphosphonates

Density functional theory (DFT) was used to investigate the Mo-catalyzed intramolecular Pauson-Khand reaction of 3-allyloxy-1-propynylphosphonates. All intermediates and transition states were optimized completely at the B3LYP/6-31 G(d,p) level [LANL2DZ(f) for Mo]. In the Mo-catalyzed intramolecular Pauson-Khand reaction, the C–C oxidative cyclization reaction was the chirality-determining step, and the reductive elimination reaction was the rate-determining step. The carbonyl insertion reaction into the Mo–C(sp(3)) bondwas easier than into the Mo–C=C bond. And the dominant product predicted theoretically was of (S)-chirality, which agreed with experimental data. This reaction was solventd ependent, and toluene was the best among the three solvents toluene, CH3CN, and THF.

Phosphine–alkene ligand-mediated alkyl–alkyl and alkyl–halide elimination processes from palladium(ii)

N-Diphenylphosphino-7-aza-benzobicyclo[2.2.1]hept-2-ene (2) behaves as a chelating phosphine-alkene ligand for Pd(0) and Pd(II), promoting direct alkyl-alkyl and indirect alkyl-halide reductive elimination reactions due to the stabilisation of the resulting bis(phosphine-alkene)Pd(0) complex.

Carbon–Carbon Bond Formation from Azaallyl and Imine Couplings about Metal–Metal Bonds

Typical C-C bond-forming processes feature oxidative addition, insertion, and reductive elimination reactions. An alternative strategy toward C-C bond formation involves the generation of transient radicals that can couple at or around one or more metal centers. Generation of transient azaallyl ligands that reductively couple at CH positions possessing radical character is described. Two C-C bonds form, and the redox non-innocence of the resulting pyridine-imines may be critical to formation of a third C-C bond via dinuclear di-imine oxidative coupling. Unique metal-metal bonds are a consequence of the chelation.

ChemInform Abstract: Carbon—Heteroatom Bond‐Forming Reductive Elimination from Palladium(IV) Complexes

AbstractReview: 78 refs.

Mechanism of intermolecular hydroacylation of vinylsilanes catalyzed by a rhodium(I) olefin complex: a DFT study

Density functional theory (DFT) was used to investigate the Rh(I)-catalyzed intermolecular hydroacylation of vinylsilane with benzaldehyde. All intermediates and transition states were optimized completely at the B3LYP/6-31G(d,p) level (LANL2DZ(f) for Rh). Calculations indicated that Rh(I)-catalyzed intermolecular hydroacylation is exergonic, and the total free energy released is -110 kJ mol(-1). Rh(I)-catalyzed intermolecular hydroacylation mainly involves the active catalyst CA2, rhodium-alkene-benzaldehyde complex M1, rhodium-alkene-hydrogen-acyl complex M2, rhodium-alkyl-acyl complex M3, rhodium-alkyl-carbonyl-phenyl complex M4, rhodium-acyl-phenyl complex M5, and rhodium-ketone complex M6. The reaction pathway CA2 + R2 → M1b → T1b → M2b → T2b1 → M3b1 → T4b → M4b → T5b → M5b → T6b → M6b → P2 is the most favorable among all reaction channels of Rh(I)-catalyzed intermolecular hydroacylation. The reductive elimination reaction is the rate-determining step for this pathway, and the dominant product predicted theoretically is the linear ketone, which is consistent with Brookhart's experiments. Solvation has a significant effect, and it greatly decreases the free energies of all species. The use of the ligand Cp' (Cp' = C(5)Me(4)CF(3)) decreased the free energies in general, and in this case the rate-determining step was again the reductive elimination reaction.

Unprecedented Directing Group Ability of Cyclophanes in Arene Fluorinations with Diaryliodonium Salts

For the first time it is shown that exceptionally electron-rich arene rings can be fluorinated exclusively during the reductive elimination reactions of diaryliodonium fluorides. The 5-methoxy[2.2]paracyclophan-4-yl directing group simultaneously reduces unproductive aryne chemistry and eliminates ligand exchange reactions by a combination of steric and electronic effects. Use of the cyclophane directing group permits an unprecedented degree of control in fluorination reactions of diaryliodonium salts.

ChemInform Abstract: Bystanding F+ Oxidants Enable Selective Reductive Elimination from High‐Valent Metal Centers in Catalysis

AbstractReview: ca. 100 refs.

Bystanding F+ oxidants enable selective reductive elimination from high-valent metal centers in catalysis.

Reductive elimination from partially or completely oxidized metal centers is a vital step in a myriad of carbon-carbon and carbon-heteroatom bond-forming reactions. One strategy for promoting otherwise challenging reductive elimination reactions is to oxidize the metal center using a two-electron oxidant (that is, from M((n)) to M((n+2))). However, many of the commonly used oxidants for this type of transformation contain oxygen, nitrogen, or halogen moieties that are subsequently capable of participating in reductive elimination, thus leading to a mixture of products. In this Minireview, we examine the use of bystanding F(+) oxidants for addressing this widespread problem in organometallic chemistry and describe recent applications in Pd(II) /Pd(IV) and Au(I) /Au(III) catalysis. We then briefly discuss a rare example in which one-electron oxidants have been shown to promote selective reductive elimination in palladium(II)-catalyzed C-H functionalization, which we view as a promising future direction in the field.

Mechanistic analysis of trans C–N reductive elimination from a square-planar macrocyclic aryl-copper(iii) complex

A net trans C-N reductive elimination reaction is observed from a macrocyclic aryl-Cu(III) complex, and a mechanistic study of this reaction indicates that coordinating ligands play a role in mediating this unusual transformation.