Insertion reactions into metal-hydrogen bonds

Another very interesting reaction involving insertion of dicyclopentadienyl-stannylene into metal-hydrogen bond with displacement of its ligands has been described by J. G. Noltes et al.16S). The resulting product was identified by X-ray structural analysis. 

Another general method is based on oxygen insertion into metal-hydrogen bonds (50,72,79-81) by any of several known mechanisms. Hydrogen abstraction by superoxo complexes followed by oxygenation of the reduced metal, as in the catalytic reaction of Eqs. (3)-(4) (50,72), works well but is limited by the low availability of water-soluble transition metal hydrides and slow hydrogen transfer (equivalent of reaction (3)) for sterically crowded complexes. 

In general, the insertion reaction of carbon dioxide into metal hydrogen bonds is formally much akin to the analogous process involving olefins (Scheme 1). This analogy is particularly appropriate since the binding of... 

Hydroperoxo complexes are prepared75 by protonation of peroxo complexes, by insertion of dioxygen into metal-hydrogen bonds, by hydrogen abstraction by metal dioxygen complexes, by reduction of superoxo complexes or by reaction of the metal ion with hydrogen peroxide. Well-defined stable species have been characterized for Cu,76 Ir, Pt, and other metals, for example, by syntheses of the type ... 

While insertion of carbon monoxide into metal-hydrogen bonds is an elusive reaction, the analogous insertion with isocyanides and Pt-H compounds occurs readily. For example, [tmns-Pt(CNR)(H)L2]Cl undergoes a retroionization reaction and is converted to the corresponding formamidoyl complexes ... 

The reaction of transition metal hydrides and metal alkyls with CO2 frequently results in the formation of metal formates and carboxylates via an insertion of CO2 into a metal hydride or metal carbon bond. Step 2 of Scheme 1 (15-19). In some instances, the mechanism for this reaction has been investigated in detail. It has been found that the reaction can proceed by either a dissociative mechanism to produce a coordinatively unsaturated metal hydride as an intermediate, or it can occur by an associative mechanism (20-25). Thus, the metal hydride shown in Scheme 1 may or may not be required to be coordinatively unsaturated. Organometallic and metal phosphine complexes are again the two classes of complexes most commonly involved in CO2 insertions into metal hydrogen bonds (15-19). 

Isocyanide insertion reactions into heteroatom—hydrogen, carbon-halogen, carbon—hydrogen bonds, and metal carbenes 13CSR5257. Last advances in synthesis of added value compounds and materials by laccase-mediated biocatalysis 12COC2508. 

Cycloaddition reactions, especially [2+2] cycloadditions, are prominent in sulfene chemistry and even insertion of sulfene into metal-hydrogen bonds is observed. ... 

Metalorganic substrates derived from niobium and tantalum also undergo insertion reactions into metal-carbon and metal-hydrogen bonds. Also, stepwise insertion into Nb(OR)5 is observed . Similarly, chromium trialkoxides undergo insertion reactions with aryl isocyanates ". ... 

Like carbene insertions into carbon-hydrogen bonds, metal nitrene insertions occur in both intermolecular and intramolecular reactions.For intermole-cular reactions, a manganese(III) meio-tetrakis(pentafluorophenyl)porphyrm complex gives high product yields and turnovers up to 2600 amidations could be effected directly with amides using PhI(OAc)2 (Eq. 51). The most exciting development in intramolecular C—H reactions thus far has been the oxidative cychzation of sulfamate esters (e.g., Eq. 52), as well as carbamates (to oxazolidin-2-ones), ° and one can expect further developments that are of synthetic... 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

Nickel,40 41 like almost all metal catalysts (e.g., Ti and Zr) used for alkene dimerization, effects the reaction by a three-step mechanism.12 Initiation yields an organometallic intermediate via insertion of the alkene into the metal-hydrogen bond followed by propagation via insertion into the metal-carbon bond [Eq. (13.8)]. Intermediate 11 either reacts further by repeated insertion [Eq. (13.9)] or undergoes chain transfer to yield the product and regenerate the metal hydride catalyst through p-hydrogen transfer [Eq. (13.10)] ... 

Of the presently known reactions, production of the formate complex predominates. Catalysis of the hydrogen reduction of C02, which apparently involves insertion into a metal-hydrogen bond, is considered later. Here we consider the insertion reaction itself. 

The only claim for the production of a metallocarboxylic acid from the insertion of C02 into a metal-hydrogen bond in the opposite sense is based on the reaction of C02 with [HCo(N2)(PPh3)3] (108, 136). The metallocarboxylic acid is said to be implicated since treatment of the product in benzene solution with Mel followed by methanolic BF3 yielded a considerable amount of methyl acetate as well as methyl formate derived from the cobalt formate complex. Metallocarboxylic acid species formed by attack of H20 or OH- on a coordinated carbonyl are considered in the section on CO oxidation. 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

Figure 2.8 Top The relationship between insertion of an alkene into a metal-hydrogen bond and the reverse /3-ehmination reaction for a rhodium complex. Bottom /3-elimination leading to the formation of a metal hydride and release of a polymer molecule with an alkene end group.

The catalytic cycle for the cobalt-based hydroformylation is shown in Fig. 5.7. Most cobalt salts under the reaction conditions of hydroformylation are converted into an equilibrium mixture of Co2(CO)8 and HCo(CO)4. The latter undergoes CO dissociation to give 5.20, a catalytically active 16-electron intermediate. Propylene coordination followed by olefin insertion into the metal-hydrogen bond in a Markovnikov or anti-Markovnikov fashion gives the branched or the linear metal alkyl complex 5.24 or 5.22, respectively. These... 

The basic mechanism of hydrogenation is shown by the catalytic cycle in Fig. 7.3. This cycle is simplified, and some reactions are not shown. Intermediate 7.9 is a 14-electron complex (see Section 2.1). Phosphine dissociation of Wilkinson s complex leads to its formation. Conversion of 7.9 to 7.10 is a simple oxidative addition of H2 to the former. Coordination by the alkene, for example, 1-butene, generates 7.11. Subsequent insertion of the alkene into the metal-hydrogen bond gives the metal alkyl species 7.12. The latter undergoes reductive elimination of butane and regenerates 7.9. 

Heterocumulenes undergo insertion reactions with numerous substrates. In general, carbodiimides react faster than isocyanates and isothiocyanates, in that order. Insertions of carbodiimides into metal-hydrogen, metal-halogen, metal-mitrogen, metal-oxygen and metal-sulfur bonds are reported. Also insertions of carbodiimides into carbon-hydrogen bonds are known. 

Reductive dimerization of formaldehyde has. however, also been proposed to explain ethylene glycol synthesis [12]. The formation of formyl species might be considered as a key step, at least under special conditions, despite the unfavourable thermodynamics. The reaction is an endothermic process at room temperature [45] and the direct insertion of CO into a metal-hydrogen bond has never been observed. 

SCF and CAS SCF calculations on mono and bimetallic transition metal hydride complexes are reported. The importance of including the non dynamical correlation elTects for the study of the cis-trans isomerism in dihydrido complexes and for the study of the CO insertion reaction into the metal hydride bond is stressed. The metal to metal hydrogen transfer in a class of bimetallic d — d hydride complexes is analyzed and the feasibility of the transfer discussed as a function of the coordination pattern around the two metal centers. 

Olefin isomerization has been widely studied, mainly because it is a convenient tool for unravelling basic mechanisms involved in the interaction of olefins with metal atoms (10). The reaction is catalyzed by cobalt hydrocarbonyl, iron pentacarbonyl, rhodium chloride, palladium chloride, the platinum-tin complex, and by several phosphine complexes a review of this field has recently been published (12). Two types of mechanism have been visualized for this reaction. The first involves the preformation of a metal-hydrogen bond into which the olefin (probably already coordinated) inserts itself with the formation of a (j-bonded alkyl radical. On abstraction of a hydrogen atom from a diflFerent carbon atom, an isomerized olefin results. 

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