Bond Metathesis Research Articles

Transition Metal Catalyzed Carbon—Carbon Bond Activation

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Organometallic Alkane CH Activation

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Evidence for unimolecular CO2elimination in C–N bond metathesis reactions of basic carbamatozinc complexes Zn4O(O2CAm)6(Am = N-diethylamino, N-piperidyl, N-pyrrolidyl)

The tetranuclear basic zinc carbamates Zn4O(O2CAm)6(1, Am =N-diethylamino; 2, Am =N-piperidyl; 3, Am =N-pyrrolidyl) were shown by transient FTIR spectroscopy to undergo C-N bond metathesis reactions that result in exchange of the carboxyl group with bulk carbon dioxide and exchange of the amino group with bulk secondary amine (transamination). The net rate for CO2 exchange was measured by monitoring the uptake of 13CO2 and the concomitant release of 12CO2. This revealed a CO2-dependent and CO2-independent component to the CO2 exchange process. The CO2-dependent process was interpreted in terms of preassociation of the incoming CO2 with the complex prior to the N-CO2 bond cleavage process while the CO2-independent process was interpreted in terms of a unimolecular elimination of CO2 from the complex. The transamination reaction gave rates that are independent of the concentration of the incoming amine. Furthermore, the transamination rate for each complex was within a factor of four of the CO2-independent CO2 exchange rate for that complex. These data were interpreted in terms of a common rate-limiting process for CO2 exchange and transamination that involves unimolecular elimination of CO2. Rate-limiting unimolecular dissociation of intact carbamato ligands was eliminated as a possible pathway by showing that ligand exchange dynamics is rapid relative to the overall rate of C-N bond metathesis. The ability of these metal carbamate complexes to undergo facile C-N bond metathesis reactions of this type has implications in heterocumulene metathesis and CO2 fixation chemistry.

Hydrogen for Fluorine Exchange in C6F6and C6F5H by Monomeric [1,3,4-(Me3C)3C5H2]2CeH: Experimental and Computational Studies

The net reaction of monomeric Cp'(2)CeH [Cp' = 1,3,4-(Me(3)C)(3)(C(5)H(2))] in C(6)D(6) with C(6)F(6) is Cp'(2)CeF, H(2), and tetrafluorobenzyne. The pentafluorophenylmetallocene, Cp'(2)Ce(C(6)F(5)), is formed as an intermediate that decomposes slowly to Cp'(2)CeF and C(6)F(4) (tetrafluorobenzyne), and the latter is trapped by the solvent C(6)D(6) as a [2+4] cycloadduct. In C(6)F(5)H, the final products are also Cp'(2)CeF and H(2), which are formed from the intermediates Cp'(2)Ce(C(6)F(5)) and Cp'(2)Ce(2,3,5,6-C(6)F(4)H) and from an unidentified metallocene of cerium and the [2+4] cycloadducts of tetra- and trifluorobenzyne with C(6)D(6). The hydride, fluoride, and pentafluorophenylmetallocenes are isolated and characterized by X-ray crystallography. DFT(B3PW91) calculations have been used to explore the pathways leading to the observed products of the exergonic reactions. A key step is a H/F exchange reaction which transforms C(6)F(6) and the cerium hydride into C(6)F(5)H and Cp'(2)CeF. This reaction starts by an eta(1)-F-C(6)F(5) interaction, which serves as a hook. The reaction proceeds via a sigma bond metathesis where the fluorine ortho to the hook migrates toward H with a relatively low activation energy. All products observed experimentally are accommodated by pathways that involve C-F and C-H bond cleavages.

Transition metal-catalyzed carbon–carbon bond activation

This tutorial review deals with recent developments in the activation of C-C bonds in organic molecules that have been catalyzed by transition metal complexes. Many chemists have devised a variety of strategies for C-C bond activation and significant progress has been made in this field over the past few decades. However, there remain only a few examples of the catalytic activation of C-C bonds, in spite of the potential use in organic synthesis, and most of the previously published reviews have dwelt mainly on the stoichiometric reactions. Consequently, this review will focus mainly on the catalytic reaction of C-C bond cleavage by homogeneous transition metal catalysts. The contents include cleavage of C-C bonds in strained and unstrained molecules, and cleavage of multiple C-C bonds such as C[triple bond]C triple bonds in alkynes. Multiple bond metathesis and heterogeneous systems are beyond the scope of this review, though they are also fascinating areas of C-C bond activation. In this review, the strategies and tactics for C-C bond activation will be explained.

Asymmetric Cyclization of ω‐Formyl‐1,3‐dienes Catalyzed by a Zerovalent Nickel Complex in the Presence of Silanes.

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Reactions of Ruthenium Benzylidene Complexes with Cyclic and Acyclic Imines: Oligomerization of 1-Pyrroline and Metathesis via Tautomerism

At room temperature, NMR spectroscopy indicates that the ruthenium benzylidene complex (Cl)2(PCy3)2RuCHPh reacts with 1-pyrroline to yield (Cl)2(PCy3)(1-pyrroline)RuCHPh. Heating a solution of (Cl)2(PCy3)2RuCHPh with excess 1-pyrroline to 90 °C results in ring-opening oligomerization of the cyclic imine. The combination of ruthenium carbene complexes (Cl)2(PCy3)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh (H2IMes = 1,3-dimesityl-4,5-dihydroimidazolylidene) with acyclic imines of the type (R)NCH(R‘) results in metathesis reactions when the imine possesses a C−H bond α to the imine carbon. Imines that lack C−H bonds α to the imine carbon do not react with (Cl)2(PCy3)2RuCHPh. The primary products from the reactions of (Cl)2(PCy3)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh with acyclic imines are olefins and new Fischer carbene complexes of the type (Cl)2(L)(L‘)RuCH{N(H)R} (L = L‘ = PCy3; L = PCy3, L‘ = H2IMes). The ruthenium complex (Cl)2(PCy3)2RuCH{N(H)Pr} has been isolated from the reaction of (Cl)2(PCy3)2RuCHPh with (Pr)NCH(i-Pr) and has been fully characterized. A possible pathway for the reactions of (Cl)2(PCy3)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh with acyclic imines that involves imine to enamine tautomerism followed by CC bond metathesis reactions is discussed. The failure of the ruthenium benzylidene complexes (Cl)2(PCy3)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh to react with the CN bonds of acyclic imines in combination with observed reactivity between Ru(Cl)2(PPh3)3 and 1-pyrroline indicates that the oligomerization of 1-pyrroline with (Cl)2(PCy3)2RuCHPh likely proceeds via a Lewis acid catalyzed mechanism.

Titanocene and zirconocene σ-alkynyl complexes in CC single bond coupling and cleavage reactions

Group 4 metallocene mono- and bis-σ-alkynyl complexes of the type L 2M(σ-CCR) and L 2M(σ-CCR) 2 with M=titanium and zirconium in the oxidation states +3 and +4 and L=Cp (η 5-cyclopentadienyl) and Cp * (η 5-pentamethylcyclopentadienyl) are important compounds for stoichiometric and catalytic CC single bond coupling and cleavage reactions. Detailed investigations show five-membered metallacyclocumulenes L 2M(η 4-1,2,3,4-RC 4R) as the key intermediates in both reactions of a CC single bond cleavage of different 1,4-substituted 1,3-butadiynes RCCCCR to alkynyl groups and the opposite reaction of CC single bond formation starting from alkynyl groups under the formation of 1,4-substituted 1,3-butadiynes. Depending on different metals M and ligands L, coupling or cleavage is favoured. Combination of both reactions offered the first CC single bond metathesis in homogeneous solution, which is photocatalyzed and titanocene-mediated. It proceeds via titanocene–mono-alkynyl complexes, which are interesting species also for other stoichiometric and catalytic CC coupling reactions. Some similarities regarding the σ-to-π conversion exist between the coupling of the alkynyl groups at titano- and zirconocenes to complexed 1,3-butadiynes on one side and the coupling of phenyl groups at chromium to complexed diphenyl on the other side.

Catalytic C=N bond metathesis of carbodiimides by group 4 and 5 imido complexes supported by guanidinate ligands.

A family of guanidinate-supported imido metal complexes are novel, effective catalysts for C=N metathesis of alkyl and aryl carbodiimides and evidence suggests that this reaction proceeds via a sequential addition/elimination pathway.

Asymmetric cyclization of omega-formyl-1,3-dienes catalyzed by a zerovalent nickel complex in the presence of silanes.

A nickel(0)-catalyzed asymmetric cyclization of omega-formyl-1,3-diene in the presence of silanes in which five- or six-membered carbocycles or pyrrolidine derivatives were afforded up to 86% ee by the use of (2R,5R)-2,5-dimethyl-1-phenylphospholane as a monodentate chiral ligand was investigated. The reaction course of this asymmetric cyclization can be explained for by two possible mechanisms: one in which the cyclization proceeds via a pi-allylnickel intermediate to produce a cyclized product having an internal olefin in the side chain, and one in which the cyclization proceeds via a sigma bond metathesis of silane and the nickel-oxygen bond of oxanickelacycle to produce a cyclized compound having a terminal olefin and/or an internal olefin in the side chain. It was speculated that both of these mechanisms operate in this asymmetric cyclization depending upon the nature of silanes and the reaction conditions.

Structure and Composition of the Anionic Chloride Complexes of Copper(II) as the Precursors of Catalysts for C–Cl Bond Metathesis

The chloride complexes of copper(II) (catalysts or catalyst precursors for various reactions of halogenated hydrocarbons) were characterized using electron, EPR, and EXAFS spectroscopy. It was found that chlorocuprates occur as mononuclear ([CuCl4]2–), binuclear ([Cu2Cl6]2–), and, probably, polynuclear species in chlorobenzene solutions. The Cu–Cl bond length in [CuCl4]2– is 2.25 ± 0.2 A, which is close to the same values for crystalline tetrachlorocuprates. It was assumed that the chloride complexes of copper with counterions occur as globules in chlorobenzene.

DFT study of H--H activation by Cp(2) LnH d(0) complexes.

The energy required to activate the H--H bond in the entire series of Cp(2)LnH complexes has been calculated by DFT (B3PW91) methods. The activation energies have been calculated to vary from 0.5 to 8.0 kcal x mol(-1), indicating an overall facile reaction. The electronegativity of the lanthanide in its most stable oxidation state is suggested to be a leading factor for interpreting the trends in activation energy. The geometry of the transition state is best viewed as an almost linear H(3)(-) ligand with short H--H distances and strong M--H interaction, through the wingtip H centers, with Ln. The exchange reaction is thus established to be a sigma bond metathesis reaction.

Immobilized Complexes of Copper(II) Chloride with Triethylenetetramine as Catalysts for the Reaction of C–Cl Bond Metathesis

The structure and activity of C–Cl bond metathesis catalysts containing CuCl2and triethylenetetramine (TETA) supported on silica were studied. The catalysts were found to contain ∼40% Cu(II) as paramagnetic complexes of the composition Cu(TETA)2Cl2and diamagnetic binuclear or polynuclear copper complexes, which are undetectable by EPR spectroscopy. The catalytic activity of the test systems in the radical reaction of C–Cl bond metathesis in the CCl4–C10H22system was maximum at a Cu(II) concentration of ∼0.5 wt % on the support. A kinetic scheme was proposed, which suggests that only mononuclear copper complexes participate at the stage of radical generation, whereas termination can also occur on binuclear or polynuclear complexes.

Catalytic Activity of Polymetalorganosiloxanes Supported on Silica in the Metathesis of a C–Cl Bond

The catalytic activity of structurally different Cu- and Ni-containing polymetalorganosiloxanes supported on silica was studied using the reaction of C–Cl bond metathesis in the carbon tetrachloride–saturated hydrocarbon (n-decane) system as an example. Catalysts with low metal contents were found to exhibit the highest activity; an increase in the metal content resulted in a decrease in both conversion and specific activity. At the initial portions of kinetic curves, the reaction was described by first-order rate equations with respect to n-decane; the reaction was of fraction order with respect to CCl4and catalyst. The activation energy of the process in the n-decane–carbon tetrachloride system on a copper-containing network catalyst was 23 ± 2 kcal/mol. The mechanism of catalysis on immobilized catalysts from the above class is discussed.

Quantum-chemical investigation of Cu-containing catalysts for CCl bond metathesis

Ab initio calculations modelling catalysts of CCl bond metathesis were carried out. The complicated catalytic system consisted of three different groups of components — amides, copper complexes in different oxidation states and inorganic support. The results of calculations, in good agreement with experimental data, explain the way of fixing the active catalyst on silica surface, describe geometrical parameters and redox properties of copper complexes with donor ligands, verify some key stages of the proposed mechanism of metathesis.

Homogeneous Conversion of Methane to Methanol. 2. Catalytic Activation of Methane by cis- and trans-Platin: A Density Functional Study of the Shilov Type Reaction

The C-H activation of methane catalyzed by cis- and trans-platin in aqueous solution has been studied by density functional based computational methods. By analogy with the Shilov reaction, the initial step is the replacement of an ammonia ligand by methane, followed by the formation of a methyl complex and the elimination o a proton. The computations utilize the B3LYP hybrid functionals, effective core potentials, and double-{zeta} to polarized double-{zeta} basis sets and include solvation effects by a dielectric continuum method. In contrast with the Shilov reaction studied by Seigbahn and Crabtree (J.Am.Chem.Soc. 1996, 118, 4443), in the platins the replacement of an ammonia ligand by methane is found to be effectively rate determining, in that the energy barriers to C-H activation are comparable with those of the initial substitution reaction, viz. {approximately} 34 and 44 kcal/mol for cis- and trans-platin, respectively. Several reaction pathways for C-H activation and subsequent proton elimination were identified. For cis-platin the energy barriers associated with the oxidative addition and {sigma}-bond metathesis type mechanisms were found to be comparable, while for trans-platin oxidative addition is predicted to be strongly preferred over {sigma}-bond metathesis, which, interestingly, also proceeds through a Pt(IV) methyl hydrido complex as reaction intermediate.more » In line with accepted ideas on trans influence, the methyl and hydride ligands in the Pt(IV) complexes that arise in the oxidative addition reactions were always found to be cis to each other. On the basis of the population analyses on the Pt(IV) complexes it is suggested that the Pt-H and Pt-CH{sub 3} bonds are best described as covalent bonds and, further, that the preference of the hydride and methyl anions to be cis to each other is a consequence of such covalent bonding. In light of these findings, the energies of several methyl Pt(IV) hydrido bisulfate complexes were also recalculated, with CH{sub 3} and H places cis to each other. The revised results provide evidence for the thermodynamic feasibility of oxidative addition of methane to catalysts such as [Pt(NH{sub 3}){sub 2}(OSO{sub 3}H){sub 2}] or [Pt(NH{sub 3}){sub 2}(OSO{sub 3}H)(H{sub 2}SO{sub 4})]{sup +}.« less

Copper (II) chloride–dmf catalytic system in solution and on silica

The connections between composition and formation conditions of catalysts based on immobilised complexes of Cu(II) and dmf as precursors and their catalytic activity in C–Cl bond metathesis in system CCl 4– n-C 10H 22 were studied. High activity and selectivity could be obtained providing that a lamina of dmf containing mononuclear copper complexes forms in pores of a support. It was shown that the composition and structure of surface complexes does not depend on concentrations of copper ions or dmf in precursor solution. The loss of catalytic activity when the support cannot fix the lamina (e.g., charcoal in contrast with silica) is observed. The key stage of catalysis could be a reduction of Cu(II) to Cu(I).

Designer clusters: synthesis and characterization of Cp*2Rh2Co3(CO)8B3HCl (Cp* =η5-C5Me5)

Sigma bond metathesis of B–H in Cp*2Rh2B3H6Cl with Co–Co in Co2(CO)8 leads directly to the title compound.

Mechanistic and IR Spectroelectrochemical Studies for Alkali Metal Ion Catalyzed Multiple Bond Metathesis Reactions of Carbon Dioxide

Abstract The reaction of the tetrakis(alkyl or aryl isocyanide) complex, Ni(CNR)4 (R = 2,6-Me2C6H3, Me), with carbon dioxide in the presence of Li+ leads to the formation of Ni(CO)2(CNR)2 and the alkyl- or arylisocyanate, RN=C=O. 13C-labeling studies indicate that the carbonyl ligands of Ni(CO)2(CNR)2 are produced by multiple bond metatheses between CO2 and CNR (R = 2,6-Me2C6H3, Me) not by deoxygenation of CO2.

Mechanism of olefin polymerization by a soluble zirconium catalyst

A mechanistic study has been carried out on the homogeneous olefin polymerization/oligomerization catalyst formed from Cp 2ZrMe 2 and methylaluminoxane, (MeAlO) x , in toluene. Formal transfer of CH 3 from Zr to Al yields low concentrations of Cp 2ZrMe + solvated by [(Me 2AlO) y (MeAlO) x− y ] y . The cationic Zr species initiates ethylene oligomerization by olefin coordination followed by insertion into the Zr–CH 3 bond. Chain transfer occurs by one of two competing pathways. The predominant one involves exchange of Cp 2Zr–P + (P=growing ethylene oligomer) with Al–CH 3 to produce another Cp 2ZrMe + initiator plus an Al-bound oligomer. Terminal Al–C bonds in the latter are ultimately cleaved on hydrolytic workup to produce materials with saturated end groups. Concomitant chain transfer occurs by sigma bond metathesis of Cp 2Zr–P + with ethylene. Metathesis results in cleavage of the Zr–C bond of the growing oligomer to produce materials also having saturated end groups; and a new initiating species, Cp 2Zr-CHCH 2 +. The two chain transfer pathways afford structurally different oligomers distinguishable by carbon number and end group structure. Oligomers derived from the Cp 2ZrMe + channel are C n ( n=odd) alkanes; those derived from Cp 2Zr–CHCH 2 + are terminally mono-unsaturated C n ( n=even) alkenes. Chain transfer by beta hydride elimination is detectable but relatively insignificant under the conditions employed. Propylene and 1-hexene react similarly but beta hydride elimination is the predominant chain transfer step. The initial Zr-alkyl species produces a Cp 2ZrH + complex that is the principle chain initiator. Chain transfer is fast relative to propagation and the products are low molecular weight oligomers.