Insertion, into metal-hydrogen bonds olefins

However, because of the difficulties involved in handling this reagent, other methods for preparing these compounds are generally preferred. Olefins and acetylenes insert into metal-hydrogen bonds to give alkyl and alkenyl complexes, respectively. Examples are... 

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... 

Olefin Insertion into Metal-Hydrogen and Metal-Alkyl Bonds 1.22.6 Databases... 

Insertion of olefins and acetylenes into metal-hydrogen bonds... 

Of equal importance to carbonyl insertion into a metal-carbon bond is olefin insertion into a metal-hydrogen bond. 

Therefore, for either antipode, the diastereomeric activated complex controlling optical yield could be either the one corresponding to the formation of the x-complex or the one corresponding to the olefin insertion into the metal-hydrogen bond. In the case of rhodium, it appears from the results of the hydroformylation of 1,2-dimethylcyclohexene and of 2-methylmethylidencyclohexane, that the second case is more probable 10). In the case of platinum, the fact that isomerization of the substrate, which is very likely to occur via metal alkyl-complex formation, proceeds at a rate similar to or even higher than the hydroformylation rate seems to indicate that the same situation can also be assumed. 

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 catalytic oligomerization of olefins in the presence of OAC and the olefin polymerization in the presence of transition metals are based on similar olefin insertions into the metal-carbon and metal-hydrogen bonds (see Section 3.2). However, in organoaluminium compounds, the structure of the active center is defined more simply and more reliably. Data on its coordination state, thermodynamic and kinetic parameters have been reported (e.g. Table 13). 

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. 

The addition of hydrogen to olefinic or acetylenic bonds is symmetry-forbidden [87, 88]. However, the participation of a catalyst subdivides the addition of H2 to an unsaturated system into a series of successive steps which do not suffer from these symmetry restrictions. These successive steps are oxidative addition of hydrogen, insertion of the coordinated unsaturated system into a metal-hydrogen bond, and reductive elimination of the hydrogenation product. Irrespective of the individual mechanism there is overwhelming evidence from D2 addition experiments that the catalytic addition of H2 to carbon-carbon double and triple bonds is a cA-addition [20]. 

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). 

Homogeneous catalysis with defined soluble transition metal complexes as catalysts has become one of the most effective means of transforming simple olefins into more valuable materials. The technically important hydroformylation of olefins to aldehydes or alcohols the Wacker process the dimerization of propylene to linear hexenes the oligomerization of ethylene to linear a-olefins are only a few examples. A feature common to all these processes is the insertion of a substrate olefin molecule, which is coordinatively bonded to the transition metal center M, into a metal-carbon or metal-hydrogen bond present at the same center ... 

It is generally assumed that olefin insertion into metal-carbon or metal-hydrogen bonds takes place by a concerted reaction path, that means through a more or less polar, cyclic transition state, with simultaneous bond breaking and bond making, e.g.-. 

Olefin insertions into transition metal-carbon and transition metal-hydrogen bonds are fundamental reactions in homogeneous catalysis. With unsymmetrically substituted olefins, a remarkable regioselectivity is frequently observed, whereby the orientation of the olefin depends on the metal, the ligands, and the olefin itself. Empirical rules of regioselectivity are given, and interpreted on the base of the electronic structure of the reaction partners. 

The thermodynamic driving force of olefin insertion into a metal-hydrogen bond is, of course, equal and with opposite sign to the thermodynamic barrier that prevents /3-hydride elimination from the metal alkyl hydride compounds, that is, the reverse of reaction (91) (R = H see Scheme 1). This decomposition mechanism will thus be more favorable for middle and late transition complexes, where metal-hydrogen bonds are much stronger than metal-alkyl bonds. 

Exposure of fluoroethene to [Ta(H)2(OSiBu 3)3] yields [TaH(Et)(OSiBu 3)3] and [TaH(F)(OSiBu 3)3]. The latter is formed by insertion of the olefin into the metal-hydrogen bond followed by / -fluoride elimination. The ethene which is produced in turn gives the ethyl compound [TaH(Et)(OSiBu 3)3]. 

Migratory insertions are one step of many different types of catalytic processes, several of which are conducted on large industrial scales and are presented in later chapters of this text. For example, the mechanism of carbonylation processes, such as hydroformylation, includes ttie insertion of CO into a metal-carbon bond. Likewise, catalytic hydrogenation occurs by insertion of an olefin into a metal-hydride bond, and olefin polymerizations and couplings of olefins with haloarenes occur by insertions of olefins into metal-carbon bonds. The reverse of these reactions, p-hydride, p-alkyl, and p-aryl eliminations, are principal pathways for the decomposition of metal-alkyl complexes. 

The insertion of alkynes into metal-hydride bonds occurs during a number of catalytic processes, including alkyne hydrogenation, hydrosilylation, silylformylation, hydroesterification and dimerization. This insertion chemistry is more complex mechanistically than the insertions of olefins into metal hydrides. In some cases, ds addition products have... 

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