Migratory insertions metal-hydride bonds

Many of these catalysts are derived from metal complexes which, initially, do not contain metal hydride bonds, but can give rise to intermediate MH2 (al-kene) species. These species, after migratory insertion of the hydride to the coordinated alkene and subsequent hydrogenolysis of the metal alkyl species, yield the saturated alkane. At first glance there are two possibilities to reach MH2 (alkene) intermediates which are related to the order of entry of the two reaction partners in the coordination sphere of the metal (Scheme 1.2). 

One pervasive mechanistic feature of many of the hydrogenations described in other chapters of this handbook concerns the bonding of the unsaturated substrate to a metal center. As illustrated in generalized form in Eq. (1) for the hydrogenation of a ketone, a key step in the traditional mechanism of hydrogenation is migratory insertion of the bound substrate into a metal hydride bond (M-H). 

Chalk and Harrod provided the first mechanistic explanation for the transition metal catalyzed hydrosilation as early as in 1965. Their mechanism was derived from studies with Speier s catalyst and provided a general scheme, which could be used also for other transition metals. The catalytic cycle consists of an initial oxidative addition (see Oxidative Addition) of the Si-H bond, followed by coordination of the unsaturated molecule, a subsequent migratory insertion (see Insertion) into the metal-hydride bond and eventually a reductive elimination (see Reductive Elimination) (Scheme 3 lower cycle). The scheme provides an explanation for the observed Z-geometry in the hydrosilation of alkynes, which is a consequence of the syn-addition mechanism. The observation of silated alkenes as by-products in the hydrosilation of alkenes along with the lack of well-established stoichiometric examples of reductive elimination of aUcylsilanes from alkyl silyl metal complexes... 

More recent studies have shown that a number of other mechanisms are operative in the hydrosilation process for different metals. Mechanistic proposals for early metals, lanthanides and actinides have been elaborated on. These involve a Chalk-Harrod like initial migratory insertion into a metal-hydride bond, followed by a a-bond metathesis step (Scheme 4). An alternative mechanism, however, was proposed for Group 4 metallocene catalysis, which involves a coordinated olefin, which undergoes a-bond metathesis with the hydrosilane. ... 

Migratory insertion of CO into a transition metal-hydride bond is not common. Nevertheless, its importance in catalytic reactions such as the Fisher-Tropsch synthesis has attracted much attention T The marked difference in behavior between metal alkyls and metal hydrides with respect to the insertion of CO is largely a consequence of thermodynamic factors ... 

The versatility of CO as a synthon also stems from its ability to undergo insertion reactions into a variety of metal-heteroatom bonds. The migratory insertion of CO into transition metal-hydride bonds, while thermodynamically unfavorable, generates metal-formyl complexes M-C(0)H (Equation (19)), a few examples of which have been isolated independently. This reaction is assumed to be a key step in both the homogeneous and heterogeneous catalytic hydrogenation (i.e., reduction) of CO, including the Fischer-Tropsch synthesis of hydrocarbons and... 

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 insertions of alk5mes into metal-carbon o-bonds are less common than either the insertions of olefins into metal-carbon bonds or the insertions of alkynes into metal-hydride bonds. Nevertheless, several examples of this reaction have been studied, and many examples are part of catalytic processes. Most of the insertions of alkynes into metal-carbon bonds occur by concerted migratory insertion pathivays and provide products from cis addition of the metal and hydrocarbyl group across the carbon-carbon multiple bond, as predicted on theoretical groimds by Thom and Hoffmann. In some cases, the products from trans addition are observed, but these kinetic products are thought to result from isomerization of the vinyl group in reaction intermediates formed by cis addition. 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

The mechanism for the reaction of another metal hydride, Tp BuMeQ0 H (Tp/ B,U,M< = t-butylmethyltris(pyrazolylborate), with 02 to generate the hydroperoxide in non-aqueous solvents appears to involve migratory insertion of 02 into the metal H bond (85). 

Selectivity and Stereochemistry. An important property of transition-metal complexes is that they coordinate groups in a specific manner permitting high regio-and stereoselectivity in the catalytic reaction. The migratory insertion step is a highly stereospecific transformation. The four-center transition state 16 illustrated for the Wilkinson catalyst requires a coplanar arrangement of metal, hydride, and alkene n bond ... 

The key intermediate in the reduction of metal ions by carbon monoxide and water is the hydroxycarbonyl (18). Initially (18) was proposed to form by a migratory insertion of CO into a M—OH bond, but more recent studies have favored a direct attack of water or hydroxide on a coordinated carbonyl (4,62). This latter view is in accord with the expected reactivity of coordinated CO toward nucleophiles. Intermediate (18) may then decarboxylate to give C02 and either a reduced metal ion or a metal hydride, as in (29) and (30), respectively. 

The reaction is catalysed by many transition-metal complexes, and a mechanism for the hydrosilylation of an alkene under transition-metal catalysis is depicted in Figure Si5.7. Initial coordination of the alkene to the metal is followed by cis addition of the silicon-hydrogen bond. A hydride migratory insertion and elimination of the product silane complete the cycle. 

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