Oxidative Addition-reductive Elimination Research Articles

How Racemic Secondary Alkyl Electrophiles Proceed to Enantioselective Products in Negishi Cross-Coupling Reactions

Two mechanisms, namely, the Ni(0)–Ni(II) and Ni(I)–Ni(III) mechanisms, for nickel-bis(oxazolinyl)pyridine complex catalyzed Negishi cross-coupling reaction were investigated with density functional calculations. The Ni(I)–Ni(III) mechanism, containing sequential steps of transmetalation–oxidative addition–reductive elimination, is more favorable than the Ni(0)–Ni(II) mechanism, based on the energetic span model. The enantioselectivity of the coupled product from a racemic secondary alkyl electrophile was calculated by the relative reaction rate (rS/rR) of the reductive elimination step that forms the coupled product in the S-enantiomer over that leading to the R-enantiomer. The rS/rR can be calculated from the relative free energy of the transition states for these two reductive elimination pathways in the Ni(I)–Ni(III) mechanism. The calculated enantioselectivity for the model reaction is consistent with the experimental report. The influence of the asymmetric steric hindrance of the catalyst ligand on the reductive elimination step is also discussed.

ChemInform Abstract: Ni(0)-Catalyzed Reactions of Aryl and Vinyl Halides with Alkenes and Alkynes.

Ni(P(OPh)3)4 and Ni(P(OEt)3)4 catalyze tile reaction of aryl and vinyl halides with olefins (l-leck reaction) and alkynes. © 1997 Elsevier Science Ltd. The formation of C-C bonds by the reaction of aryl halides with olefins (Heck reaction) in presence of palladium catalysts is a well studied reaction) We have been investigating the use of other metal catalysts for this reaction based on the known mechanism of the Pd catalyzed vinylation reaction. The key step in the reaction mechanism is the oxidative addition of a low valent coordinatively unsaturated metal complex to the aryl halide followed by olefin coordination, migratory insertion and reductive elimination. We have reported the use of Co(1), Rh (I) (Wilkinson's Catalyst) and Ir(1) (Vaska's complex) successfully based on this rationale, 2 We decided to investigate other new catalysts for the Heck reaction based on the reaction mechanism viz. a metal complex capable of oxidative addition reductive elimination and undergoing ligand dissociation and association. NiCI2(PPh3)2 is known to catalyze the reaction of aryl and aliphatic halides with olefins in the presence of Zn dust to give mixtures of addition and substitution products) But it is not a very useful catalyst for the Heck reaction because it gives different types of products with ethylacrylate (addition product) and styrene (substitution product). The highly active and air sensitive complex, Ni(PPh3)4, generated in situ by the action of Et3AI on Ni(acac)2, has been used for the intramolecular reaction of aryl halides with olefins to form indoles: Ni(P(OPh)3)4 and Ni(P(OEt)3)4 are stable metal complexes of Ni(0) and are well known to undergo ligand dissociation, oxidative addition and reductive elimination reactions and have been used to catalyze the reaction of allyl acetates with nucleophiles, s We report here, the first use of these stable Ni(0) metal complexes, for the activation of aryl halides for reaction with olefins (Heck reaction) and alkynes.

On the Electronic Impact of Abnormal C4‐Bonding in N‐Heterocyclic Carbene Complexes

Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for the abnormal carbene complexes, specifically upon reaction with Lewis acids and in oxidative addition-reductive elimination sequences. These reactivities as well as analytical investigations using X-ray diffraction and X-ray photoelectron spectroscopy indicate that the C4 bonding mode increases the electron density at the metal center substantially, classifying such C4-bound carbene ligands amongst the most basic neutral donors known thus far. A direct application of this enhanced electron density at the metal center is demonstrated by the catalytic H(2) activation with abnormal carbene complexes under mild conditions, leading to a catalytic process for the hydrogenation of olefins.

Palladium-Catalyzed Chloroimination of Imidoyl Chlorides to a Triple Bond: An Intramolecular Reaction Leading to 4-Chloroquinolines

In this paper, a new type of effective chloroimination was reported. This reaction afforded 4-chloro-2-perfluoroalkyl quinolines from fluorinated imidoyl chlorides in high yields. This is the first achievement of oxidative addition-reductive elimination type C-Cl bond activation by chloropalladation.

Carbon−Hydrogen Bond Activation: Two, Three, or More Mechanisms?

Classic and newly proposed mechanisms for metal-mediated hydrogen atom transfer (HAT) were analyzed with density functional theory (DFT) and Bader's atoms in molecules (AIM) analysis. The two classic mechanisms for HAT are σ-bond metathesis (σBM) and oxidative addition/reductive elimination (OA/RE); the σBM mechanism was modeled by methane addition to [Cp2Sc(CH3)], and the transition state (TS) for HAT is characterized by two Sc−C and C−H bond critical points (BCP) and one ring critical point (RCP) in the center of the four-centered geometry of the TS (1). The OA/RE mechanism, modeled by methane addition to [Cp*Ir(PMe3)(CH3)]+, proceeds through an IrV intermediate (2) that is characterized by two Ir−C, BCPs, one Ir−H BCP, and no C−H interactions. Newly proposed mechanisms of intermediate character, such as metal-assisted σBM (MAσBM), oxidative hydrogen migration (OHM), and oxidatively added transition state (OATS)/σ-complex assisted metathesis (σ-CAM), were analyzed with AIM analysis, and the bonding patterns were compared to σBM and OA/RE. This analysis revealed a spectrum of mechanisms with a variety of changing bonding patterns between the two classic extremes.

遷移金属化合物の特性を活用した多成分連結反応の新展開

Recently, transition-metal promoted multi-component reactions have attracted much attention as powerful tools for onepot construction of complex molecules with high efficiency. For example, a number of couplings of three multiple-bonds via the formation of metallacycles using early transition metals have been reported. Late transition metal-catalyzed fourcomponent reactions involving catalytic cycles with combination of oxidative addition-reductive elimination have been investigated. Metathesis using the Grubbs catalysis is also embedded to the multi-component reactions.

Synthesis and study of platinum silylene complexes of the type (R3P)2Pt=SiMes2 (Mes = 2,4,6-trimethylphenyl)

The synthesis and characterization of neutral platinum silylene complexes (R3P)2Pt=SiMes2 (R = i-Pr (1) or cyclohexyl (2), Mes = 2,4,6-trimethylphenyl) is reported. The dimesitylsilylene ligand in 2 is displaced by a number of ligands including phosphines, alkenes, alkynes, and O2. Complex 2 reacts with ROH substrates (R = H, Me, Et) to give (Cy3P)2Pt and Mes2Si(OR)(H) and with H2 to give trans-(Cy3P)2Pt(H)SiHMes2 (3). Reaction of H2SiMes2 with (Cy3P)2Pt gave cis-(Cy3P)2Pt(H)SiHMes2 (4). EXSY NMR experiments of 4 reveal that exchange of silicon and platinum hydrides occurs via reductive elimination – oxidative addition and not via a silylene intermediate.Key words: silylene, EXSY, platinum, hydride.

Inter- and Intramolecular C−H Activation by a Cationic Iridium(III) Center via Oxidative-Addition Reductive-Elimination and σ-Bond Metathesis Pathways

Inter- and Intramolecular C−H Activation by a Cationic Iridium(III) Center via Oxidative-Addition Reductive-Elimination and σ-Bond Metathesis Pathways

C–H and Si–C bond activation on diruthenium bridging methylene complexes: evidence for reversible oxidative addition of Si–C(sp3) bonds

Interconversion between silyl-µ-methylene complexes [(C5H5)2Ru2(µ-CH2)(H)(SiR3)(CO)2] and [(C5H5)2Ru2(µ-CH2)(SiR3)2(CO)2] and µ-silylmethylene complexes [(C5H5)2Ru2(µ-CHSiR3)(µ-CO)(CO)(L)](L = CO, PPh3) follows reversible oxidative addition-reductive elimination processes of C–H and Si–C(sp3) bonds.

Synthese von Phosphaalkenclustern durch reduktive Phosphinidenfragmentierung zweizähniger Halogenphosphane RXP— CH2 — PXR mit Fe2(CO)9 / Synthesis of Phosphaalkene Clusters by Reductive Phosphinidene Fragmentation of Bidentate Halogenphosphines RXP—CH2—PXR with Fe2(CO)9

Reaction of halogen functional methylenebisphosphines 1 XRP–CH2–PRX (X = Cl, Br) with excess Fe2(CO)9 affords μ3-phosphinidene clusters (5) containing phosphaalkene ligands R–P=CH2. The fragmentation of the P–C–P skeletons in 1 proceeds stepwise, phosphide complexes 2 and novel Fe3 clusters 3 with RP–CH2–PR bridging units being formed as intermediates of the oxidative addition reductive elimination process. The X-ray structural analysis for 5b (R = NEt2) reveals an Fe3(CO)9PR butterfly core to which the phosphaalkene ligand R–P=CH2 (P=C bond length 1.766(4) A) is coordinated in a μ3,η2-manner. In case of methylenebisphosphines 1 with bulky substituents (R = 2,4,6-iPr3C6H2) the phosphinidene fragmentation of the P–C–P skeleton is accompanied by the formation of 2,3-dihydrobenzo[b]-phosphole ligands.

Doping And Band-Gap Engineering Of An Intrazeolite Tungsten(Vi) Oxide Supralatiice

ABSTRACTNew results are presented concerning the topotactic self-assembly, n-type doping and band-gap engineering of an intrazeolite tungsten(VI) oxide supralattice n(WO3)-Na56Y, where O < n ≤ 32, built-up of single size and shape (WO3)2 dimers. In particular it has been found that the oxygen content of these dimers can be quantitatively adjusted by means of a thermal vacuum induced reversible reductive-elimination oxidative-addition of dioxygen. This provides access to new n(WO3−x)−Na56Y materials (0 < x ≤ 1.0) in which the oxygen content, structural properties and electronic architecture of the dimers are changed. In this way one can precisely control the oxidation state, degree of n-doping and band-filling of a tungsten(VI) oxide supralattice through an approach which can be considered akin to, but distinct in detail to, that found in the Magneli crystallographic shear phases of non-stoichiometric bulk WO3−x. Another discovery concerns the ability to alter local electrostatic fields experienced by the tungsten(VI) oxide moieties housed in the 13Å supercages of 16(WO3)−M56Y, by varying the ionic potential of the constituent supercage M+ cations across the alkali metal series. This method provides the first opportunity to fine-tune the band-gap of a tungsten(VI) oxide supralattice. A miniband electronic description is advanced as a qualitative first attempt to understand the origin of the above effects. The implications of these discoveries are that cluster size, composition and intrinsic electrostatic field effects can be used to “chemically manipulate” (engineer) the doping and band architecture of intrazeolite supralattices of possible interest in quantum electronics and nonlinear optics.

Dichloro bis(?5) molybdenum catalysed reduction of organic halides by sodium borohydride

Dichloro bis(η5) molybdenum catalyses the reduction of organic halides by sodium borohydride smoothly and selectively. The reactions with representative organic monohalides and polyhalides are investigated. The ease of reduction depends on the structure of the halide and the solvent used. alkoxycarbonyl and cyano groups are unattacked in the reaction. An oxidative addition-reductive elimination mechanism is tentatively suggested.

Platinum(III) in the REOA mechanism of tetrachloroplatinate(2-) and hexachloroplatinate(2-): a pulse-radiolysis study

In aqueous solutions PtCl{sub 4}{sup 2{minus}} is oxidized by OH to PtCl{sub 4}OH{sup 2{minus}} and PtCl{sub 6}{sup 2{minus}} reduced by e{sub aq}{sup {minus}} or H to PtCl{sub 6}{sup 3{minus}}. These Pt(III) species both react subsequently with H{sub 2}O to produce PtCl{sub 4}(OH){sub 2}{sup 3{minus}} and H{sup +}. All these processes are fast, whereas decay of PtCl{sub 4}(OH){sub 2}{sup 3{minus}} by disproportionation is relatively slow. Kinetics of reactions and properties of intermediate species are described. The early stage of the reductive elimination/oxidative addition (REOA) mechanism of Pt(II) and Pt(IV) is proposed. 31 refs., 3 figs.

Studies of oxidative addition-reductive elimination reactions, and the crystal structure of the palladium(IV) complex Me 2( p-BrC 6H 4CH 2)Pd(phen)Br

Selectivity in reductive elimination of ethane and RMe has been observed for benzyl and phenacyl complexes Me 2RPd(L 2)Br (L 2 = bipy, phen), with product ratios dependent upon R and L 2, and cationic intermediates detected by 1H NMR spectroscopy for oxidative addition of CD 3I and phenacyl bromides to Me 2Pd(L 2). The crystal structure of fac-Me 2( p-BrC 6H 4CH 2)Pd(phen)Br has been determined.

ChemInform Abstract: Mechanisms for Acceleration of Halide Anation Reactions of Platinum(IV) Complexes. REOA versus Ligand Assistance and Platinum(II) Catalysis Without Central Ion Exchange

Chloride anation of trans-Pt(CN)4ClOH2− has been studied with and without Pt(CN)42− present at 25.0°C by use of stopped-flow and conventional spectrophotometry and a 1.00 M perchlorate medium. The rate law in the absence of Pt(CN)42− is compatible with a chloride assistance via an intermediate of the type Cl-Cl-Pt(CN)4···OH22−, in which the reactivity of the aqua ligand is enhanced due to a partial reduction of the platinum. This mechanism of halide assistance is in principle the same as the modified reductive elimination oxidative addition (REOA) mechanism proposed by Poe, in which the intermediate is not split into free halogen, platinum(II) and water, and in which electron transfer not necessarily involves complete reduction to platinum(II). To avoid confusion with complete reductive eliminations, reactions without split of the intermediates are here termed halide-assisted reactions. The pH-dependence indicates acid catalysis via a protonated intermediate ClClPt(CN)4···OH3−.Reaction between PtCl5OH2− and chloride is accelerated by Pt(CN)42− and gives PtCl62− as the reaction product. The rate law is derived at 35.0°C and for a 1.50 M perchlorate acid medium. The reaction takes place without central ion exchange. Alternative mechanisms with two consecutive central ion exchanges can be excluded. The role of Pt(CN)42− in this reaction is very similar to that of the assisting halide in the halide assisted anations. [p ]Reaction between trans-Pt(CN)4ClOH2− and PtCl42− gives Pt(CN)42− and PtCl5OH2− as products. The formation of an aqua complex as the primary reaction product and the rate independent of [Cl−] shows that formation of a bridged intermediate of the type Pt(II)Cl4ClPt(IV)(CN)4OH23− is formed in the initial reaction step, not five-coordinated PtCl53−.

Intramolecular, reversible C–H oxidative addition–reductive elimination at rhodium. Reaction of the intermediate with different types of C–H bonds and dihydrogen

The ortho-metallated complex [{(Ph2PCH2CH2)2N(CH2CH2PPhC6H4)}RhH](SO3CF3) thermally or chemically undergoes reductive elimination of the substituted phenyl group; the N(CH2CH2PPh2)3Rh+ fragment thus formed is stabilized by CO, halides, C2H4, or engaged in oxidative additions of H2 and C–H bonds from arenes, alkynes, and aldehydes.

Mechanisms for acceleration of halide anation reactions of platinum(IV) complexes. REOA versus ligand assistance and platinum(II) catalysis Without central ion exchange

Chloride anation of trans-Pt(CN) 4ClOH 2 − has been studied with and without Pt(CN) 4 2− present at 25.0°C by use of stopped-flow and conventional spectrophotometry and a 1.00 M perchlorate medium. The rate law in the absence of Pt(CN) 4 2− is Rate=( p 1 + p 2 [H +] ) [Cl −] 2 [complex]/(1 + q [Cl −]) with p 1=(3.0 ± 0.1) × 10 −5 M −2s −1, p 2=(3.6 ± 0.1) × 10 −5 M −3 s −1 and q=(0.62 ± 0.02) M −1. It is compatible with a chloride assistance via an intermediate of the type Cl-Cl-Pt(CN) 4···OH 2 2−, in which the reactivity of the aqua ligand is enhanced due to a partial reduction of the platinum. This mechanism of halide assistance is in principle the same as the modified reductive elimination oxidative addition (REOA) mechanism proposed by Poë, in which the intermediate is not split into free halogen, platinum(II) and water, and in which electron transfer not necessarily involves complete reduction to platinum(II). To avoid confusion with complete reductive eliminations, reactions without split of the intermediates are here termed halide-assisted reactions. The pH-dependence indicates acid catalysis via a protonated intermediate ClClPt(CN) 4···OH 3 −. The Pt(CN) 4 2−accelerated path has the rate law Rate= ▪ [Cl-] [Pt(CN) 4 2−] [complex] where k=(39.9±0.5) M −2 s −1 and K a=(4.0±0.2)10 −2 M is the protolysis constant of trans-Pt(CN) 4ClOH 2−. Reaction between PtCl 5OH 2 − and chloride is accelerated by Pt(CN) 4 2− and gives PtCl 6 2− as the reaction product. The rate law is Rate= k [Cl −] [Pt(CN) 4 2−] [PtCl 5OH 2 −] with k=(5.6 ± 0.2)10 −3 M −2 s −1 at 35.0°C and for a 1.50 M perchlorate acid medium. The reaction takes place without central ion exchange. Alternative mechanisms with two consecutive central ion exchanges can be excluded. The role of Pt(CN) 4 2− in this reaction is very similar to that of the assisting halide in the halide assisted anations. [p ]Reaction between trans-Pt(CN) 4ClOH 2 − and PtCl 4 2− gives Pt(CN) 4 2− and PtCl 5OH 2 − as products and has the rate law Rate= k[PtCl 4 2−] [ trans-Pt(CN) 4ClOH 2 −] with k=(3.32 ± 0.02) M −1 s −1 at 25 °C for a 1.00 M perchloric acid medium. The formation of an aqua complex as the primary reaction product and the rate independent of [Cl −] shows that formation of a bridged intermediate of the type Pt(II)Cl 4ClPt(IV)(CN) 4OH 2 3− is formed in the initial reaction step, not five-coordinated PtCl 5 3−.

Alkynyl/chloride exchange between trans-platinum(II) and -palladium(II) chlorides and alkynylstannanes. Crystal structure of trans-[bis(1-propynyl)-bis(triethylphosphine)platinum(II)

The reaction between alkynylstannanes ( 3, 4) and trans-[(Et 3P) 2Pt(Cl) 2] ( 1) gives trans-[(Et 3P) 2Pt(Cl)CCR] ( 5) or trans-[(Et 3P) 2Pt(CCR 1) 2 ( 6) in high yield (R 1  H, Me, Ph). Complexes 5 react with alkynylstannanes to give the unsymmetrical bis(alkynyl) complexes 7, trans-[(Et 3P) 2Pt(CCR 2)CCR 1] (R 1  Me, Ph; R 2  H). Although the palladium(II) dichloride 2, trans-[(Bu 3P) 2PdCl 2], reacts much faster with alkynylstannanes than 1, the compounds trans-[(Bu 3P) 2Pd(CCR 1) 2] ( 9) are always obtained as mixtures with trans-[(Bu 3P) 2Pt(Cl)CCR 1] ( 8), except for R 1  H. Compounds 8 can be obtained pure R 1  Me or t-Bu if the reaction between 2 and 3 is carefully controlled. The dependence of the product distribution under experimental conditions in the case of the palladium(II) acetylides indicates that oxidative addition-reductive elimination reactions are involved. The crystal structure of trans-[(Et 3P) 2Pt(CCMe) 2] ( 6b) has been determined. All the products have been characterized by 13C, 31P and 195Pt NMR spectroscopy.

Hydrodesulfurization of thiophene using rhodium(III) zeolites: 13X and ZSM-5

The hydrodesulfurization of thiophene using rhodium(III)-13X and -ZSM-5 zeolites has been studied in a pulsed microreader as a function of temperature. Hydrodesulfurization catalysts were prepared by sulfidation with thiophene and with H 2S H 2 . The C 4 products included butene and butane. Neither tetrahydrothiophene nor butadiene was identified in the product gas stream. Thiophene conversion over Rh(III)-ZSM-5 sulfided at 400 °C with a 10 vol% H 2S 90 vol % H 2 mixture was at least as effective as a commercial cobalt-molybdate catalyst under similar reaction conditions. The effectiveness of the Rh(III)-ZSM-5 catalyst sulfided with the H 2S H 2 mixture at 250 °C and with thiophene at 100 °C was less than the cobalt-molybdate catalyst. The conversion percentage to C 4 hydrocarbons was greater on the Rh(III)-13X than on CoMo Al 2O 3 . Analysis by X-ray photoelectron spectroscopy (XPS) of the Rh-zeolite catalysts before and after use indicated that the active species is Rh(I). A mechanism for reaction on the Rh-zeolite is proposed where oxidative addition-reductive elimination reactions of Rh(I) Rh(III) are involved in the hydrodesulfurization process.

Oxidative addition - reductive elimination sequences in the photochemistry of some bis(phosphine)platinum complexes

The photolysis of [L 2Pt(C 2H 4)] (L = PPh 3, P( p-C 6H 4CH 3) 3 complexes in halocarbon solvents (CH 2Cl 2, CH 2Br 2) gives C 2H 4 and the coordinatively unsaturated species [L 2Pt]. Photolysis of platinum metallacycles [L 2Pt(CH 2) 4] (L = PPh 3, P(n-Bu) 3) generates alkanes, alkenes and [L 2Pt]. The [L 2Pt] centers are very reactive, and under prolonged photolysis undergo oxidative addition of CH 2Cl 2 forming the trans-[L 2Pt(CH 2Cl)Cl] complexes. Under appropriately controlled conditions the trans complexes isomerize to cis species before bimolecular C 2H 4 elimination occurs and [L 2PtCl 2] is formed as the final product. The oxidative addition-reductive elimination mechanism is discussed on the basis of spin-trapping experiments, quantum yield values, and the sensitivity to radical inhibitors and to solvents.