Metal-hydrogen bonds, insertion reactions

Reactions 2.3.2.2 and 23.23 are examples of insertion of an alkene into a metal-hydrogen bond. Such reactions are important in all homogeneous catalytic reactions where metal hydrides and alkenes are involved. 

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. 

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)] ... 

Hydrogen cyanide can be added across olefins in the presence of Ni, Co, or Pd complexes (Scheme 56) (123). Conversion of butadiene to adiponitrile is a commercial process at DuPont Co. The reaction appears to occur via oxidative addition of hydrogen cyanide to a low-valence metal, olefin insertion to the metal-hydrogen bond, and reductive elimination of the nitrile product. The overall reaction proceeds with cis... 

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. 

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... 

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. 

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 ... 

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. 

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. 

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 many reactions that involve insertion of alkenes or alkynes into metal-carbon or metal-hydrogen bonds provide further examples of hypercoordination of carbon atoms during reactions. For example, an alkene may coordinate to the coordinatively unsaturated metal atom of a metal hydride complex prior to inserting into the metal-hydrogen bond [Eq. (1.9)] ... 

Carbon-Hydrogen Bond Insertion In the early 1960s the activation of alkanes by metal systems was realized from the related development of oxidative addition reactions " " in which low-valent metal complexes inserted into carbon-heteroatom, silicon-hydrogen, and hydrogen-hydrogen bonds. The direct oxidative addition of metals into C-H bonds was found in the cyclometallation reaction [Eq. (6.61)].The reverse process of oxidative addition is called reductive elimination, which involves the same hypercoordinate carbon species. 

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). 

The most widely studied reactions of CO are those with metal-carbon and metal-hydrogen bonds. Here it will suffice to recall the first organometallic complexes obtained from an insertion of CO into a metal-carbon bond ... 

Scheme 48 shows a mechanism for the Rh(I)-catalyzed reaction proposed by Wilkinson (11a, 107). The reaction starts with the insertion of coordinated olefin into the metal-hydrogen bond in the hydrido-... 

The mechanism of the reaction was studied for cyclopentadienyl lanthanide complexes, [26, 29, 31]. A monomeric metal hydride is proposed to be the active species. The catalytic cycle turns via a fast and irreversible insertion of the olefin into the metal hydrogen bond to form an alkyl species which reacts with the silane in the rate determining step with regeneration of the hydride (Scheme 12). 

The overall reaction from 10.25 to 10.26 is an insertion into a metal-hydrogen bond. It is, however, only an apparent insertion, as the Rh — H bond dissociates in the diazene diazenido equilibrium (10-9), as already emphasized by Sutton in 1975. Other interesting cases are the reactions of tungsten mono- and bis-hydrido complexes with diazonium salts. The monohydrido complex 10.27 yields the aryldiazene complex 10.28 (Smith and Hillhouse, 1988) in an 1,1-insertion (10-11). The bishydrido complex 10.29 (10-12), however, adds one of the two H-atoms at the... 

Metal formyl complexes have been proposed as important intermediates in the metal-catalyzed reduction of CO by H2 1,2, 3, 4). While the insertion of CO into alkyl and aryl carbon-metal bonds is well known (5), the insertion of CO into a metal-hydrogen bond to give a metal formyl complex has not been observed. (The intermediacy of metal formyl compounds in the substitution reactions of metal hydrides has been considered.) To ascertain the reasons for the failure to observe metal formyl complexes in the reactions of metal hydrides with CO, we have developed a new synthesis of metal formyl complexes and have studied their properties. 

---