Metal-oxygen bonds, insertion reaction

Metal-oxygen bond, 27 195, 196 insertion reactions, 28 136-141 strength and selectivity, oxidative dehydrogenation of alkanes, 40 26-28... 

The insertion of carbon dioxide into a transition metal-oxygen bond, e.g., a metal alkoxide, results in an organic carbonate ester, coordinated in either a monodentate or bidentate manner. Only a limited number of such reactions have been observed, and little mechanistic information is available. The reactions may proceed by interaction of C02 with ROH or RO in solution followed by metal coordination, in a manner similar to the C02 reactions with the early transition metal dialkylamides. Alternatively, direct attack of C02 on the alkoxide oxygen might occur, or a C02 adduct may form as an intermediate. 

An alternative mechanism (Scheme 7) was proposed by Mimoun, which involves olefin coordination to the metal.112-116 The insertion of the alkene into the metal-oxygen bond is thought to be the rate-limiting step of this reaction mechanism. 

The insertion into metal-oxygen bonds of, for example, a transition metal alkoxidc, results in organic carbonate esters. The reactions with hydroxy complexes yield the corresponding bicarbonate comidexes. 

Oxaosmacyclopropane (24) undergoes several metal-oxygen bond additions. It reacts with carbon diselenide by insertion into the O—Os bond. Reaction with HI affords diiodide (51) presumably via the alcohol (Scheme 1) <83JOM(244)C53>. Reaction with other electrophiles affords ring-opened Cations <82JOM(231)335>. 

The insertion of olefins into the metal-oxygen bonds of isolated alkoxo, phenoxo, or hydroxo compounds has been observed directly in a few cases. As will be noted in Chapter 11, a hydroxy or alkoxyalkyl group can be formed by insertion of an olefin into a metal-oxygen bond or by attack of hydroxide or an alkoxide on a coordinated olefin. Many studies described in Chapter 11 imply that this type of compound is formed by nucleophilic attack on a coordinatively saturated olefin complex, and this reaction has been proposed as the C-0 bond-forming step during oxidations of olefins catalyzed by palladium complexes. However, Henry provided some of the first evidence that the C-0 bond forms by insertion of olefins into metal-hydroxo and -alkoxo complexes under certain reaction conditions. ... 

The intermediate at the left in the scheme appears to be inspired by a similar ruthenium complex (yide infra) and is also consistent with previous studies in our group, showing that insertion of electrophiles occur into the metal-nitrogen rather than into the metal-oxygen bond of F (PPh3)2(n -PhNO) [106-108]. The complex on the right side of the scheme represent a conunonly supposed intermediate for the first deoxygenation reaction of nitrobenzene. However, no such a complex has ever been isolated or observed and it may also be better considered as a transition state rather than a true intermediate. 

The reaction of molecular oxygen complexes of iridium or platinum with sulphur dioxide to produce sulphato-complexes involves both insertion into a metal-oxygen bond and bond formation between the sulphur and the leaving oxygen, with subsequent rearrangement of the peroxo-sulphite intermediate thus generated ... 

The S02 molecule has unshared pairs of electrons on both the sulfur and oxygen atoms. As a result, it forms numerous complexes with transitions metals in which it is known to attach in several ways. These include bonding through the sulfur atom, through an oxygen atom, by both oxygen atoms, and various bridging schemes. In most cases, the complexes involve soft metals in low oxidation states. Another important reaction of sulfur dioxide is known as the insertion reaction, in which it is placed... 

Metal Hydrides. It is likely that the reduction of aldehydes to alcohols by cobalt hydrocarbonyl (27) is an example of a carbonyl insertion reaction with a metal hydride. It is not clear which way the hydrocarbonyl adds to the carbonyl groups —whether it forms a cobalt-carbon bond (2), or a cobalt-oxygen bond (90). 

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. 

Reactions other than Lewis acid-base associations/dissociations are frequently observed wit donor molecules, leading notably to solvolysis, oxygen or sulfur abstraction, insertion reaction and carbon-carbon coupling reactions. The tendency to form metal-element multiple bonds i remarkable in this respect the activation of dinitrogen by tantalum or niobium is unique. Th formation and chemistry of constrained reactive metallacycles open another promisin fast-developing area, on the frontier with organometallic chemistry. 

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