Metal insertion hydrometallation

The insertion of alkene to metal hydride (hydrometallation of alkene) affords the alkylmetal complex 34, and insertion of alkyne to an M—R (R = alkyl) bond forms the vinyl metal complex 35. The reaction can be understood as the cis carbometallation of alkenes and alkynes. 

Addition of hydride bonds of main group metals such as B—H, Mg—H, Al—H, Si—H and Sn—H to alkenes and alkynes to give 513 and 514 is called hydrometallation and is an important synthetic route to compounds of the main group metals. Further transformation of the addition product of alkenes 513 and alkynes 514 to 515,516 and 517 is possible. Addition of B—H, Mg—H, Al—H and Sn—H bonds proceeds without catalysis, but their hydrometallations are accelerated or proceed with higher stereoselectivity in the presence of transition metal catalysts. Hydrometallation with some hydrides proceeds only in the presence of transition metal catalysts. Hydrometallation starts by the oxidative addition of metal hydride to the transition metal to generate transition metal hydrides 510. Subsequent insertion of alkene or alkyne to the M—H bonds gives 511 or 512. The final step is reductive elimination. Only catalysed hydrometallations are treated in this section. 

The initially expected (75) cis-hydrometallation or olefin-insertion step with fumarate (R = C02Me) yields the threo isomer 8, which then undergoes the k2 step with retention to give racemic 1,2-dideuterosuccinate. Such retention is necessary to give the usually observed (7, p. 407) overall cis addition of H2 to olefinic bonds, but this study provided the first direct experimental proof, the difficulty being the scarcity of stable metal alkyl-hydride intermediates. The Cp2MoH2 complex also catalyzes hydrogenation of 1,3- or 1,4-dienes to monoenes (197). 

Usually, the reaction displays m-stereoselectivity giving (-E)-organomctallics. However, isomerization occurs very often leading to mixtures of (Z)- and (/. )-vinyl organometallics. The accepted general mechanism for the hydrometallation reaction involves the initial coordination of the alkyne to a vacant orbital of the metal, followed by insertion of the hydrogen-metal bond to a 7T-bond of the alkyne (Scheme 34).132... 

A catalytic cycle (going clockwise from the top) shows the various stages of alkene coordinatio hydrometallation to produce an alkyl metal species, coordination of carbon monoxide followed insertion, and finally reductive cleavage with hydrogen to produce the metal-hydride intermedia... 

The mechanisms of the two key steps are worth discussion. Hydrometallation occurs by initial n-complex formation followed by addition of the metal to one end of the alkene and hydrogen to the other. Both of these regioisomers are formed. The carbonyl insertion reaction is another migration from the metal to the carbon atom of a CO ligand. 

A general picture for the mechanism is shown in Scheme 4, which is based upon a theoretical analysis by Thom and Hoffmann. Here distinction between (2) and (2a) reflects the general assumption, supported by calculations, that the insertion step requires the M—H and C==C groups to be cis and coplanar, which need not be the case for the first-formed and/or thermodynamically most stable alkene complex (2). Thom and Hoffmann conclude that most or all metal hydrides will have some pathway that leads to hydrometallation without a large kinetic barrier, so long as none of the key intermediates along the way is too stable. The same inference was drawn for the bent metallocene systems discussed earlier (Figure 1) a kinetic barrier to insertion, found only for the cP-cases, is a consequence of the thermodynamic stabilization of alkene complex (2). ... 

The cis ligand alkene then inserts into the M—H bond (hydrometallation) of (6) to afford an alkyl metal complex (7), which interacts with another molecule of alkene to induce the coupling of the alkyl and silyl ligands and to regenerate the active catalyst (5). This last reaction is the rate-determining step of the catalytic cycle. 

Nitriles are resistant to hydrometallation by transition metal hydride complexes. The complex Cp2ZrHCl reacts with nitriles A rhenium complex bridged by adinuc-lear hydride undergoes insertion with several isonitriles and with acetonitrile. The product in the case of the isonitrile results from a 1,1-inertion (see 11.2.8) ... 

The key step in nearly all of the catalytic processes to be discussed is olefin insertion into a metal hydride [Eq. (2)] or organometallic species [Eq. (3)]. These hydrometallation and carbometallation processes also form the basis for the polymerization of alkenes. Olefin insertions generally occur with the same regiose-lectivity as hydroboration reactions [9], with the bulky metal and associated ligands residing at the least hindered site of the two carbon reactive unit. 

The (postulated) hydrometallation pathway of hydroalkoxylation (or hydration) of olefins (Scheme 2b) relies on O-H bond activation by oxidative addition of RO-H to metal centers, a process that is studied eagerly in the organometallic chemistry community [3, 24]. However, the insertion of olefins into the M-H bonds of metal... 

Various addition reactions of alkenes and alkynes have been termed insertion processes. As the term insertion per se is a nonchemical term, it might be conveniently substituted with hydrometallation (H—M), carbometallation (C—M), heteroatom-metallation or heterometallation (X—M), and metallometallation (M—M), depending on the (7-bond that is added to tt compounds. These addition reactions involving Pd are represented by the general equations using alkenes as representative tt compounds as shown in Scheme 7. The alkenes in Scheme 7 may be replaced with alkynes and other 7T compounds. 

Since most of the facile and general hydro- and carbometallation reactions involve syn-addition, the preparation of trans-a.jS-substituted alkenylmetals via 5yn-addition of alkynes would require carbometallation of terminal alkynes placing the metal in the internal position. Although such reactions exemplified by carbopalladationf are known, they are still more exceptional than normal. From the perspective of the current discussion, more commonly used are (i) some anfi-hydrometallation reactions of proximally heterofunctional internal alkynes and (ii) the hydroboration-migratory insertion tandem process of 1-haloaIkynes. Whereas the H migration produces (Z)-/3-substituted alkenylboranes (Sect. D.iii), the corresponding C migration provides trans-a,/3-substituted alkenyhnetals. (See Table 15.)... 

The insertion of alkenes into M-H bonds has been examined in Chap. 4. This reaction is very important because, it leads to the dimerization, oligomerization and polymerization of alkenes. It is broad and concerns not only transition metals, but also main-group metals (group 13 Lewis acids), lanthanides and actinides. For instance, AlEt3 is an excellent initiator of olefin polymerization. This reaction can also be considered as the hydrometallation or the hydroelementation of an olefin, and stoichiometric examples have been shown. If the element E does not have the property of a Lewis acid allowing olefin pre-coordination onto a vacant site and thus facilitating insertion, the insertion reaction is not possible without a catalyst. 

Catalytic hydrofunctionalization of the multiple bonds involves oxidative addition of H-Het to low valent metal complex and intermediate formation of H-MLn-Het complex. As a next step, either heterometallation (insertion into M-Het) or hydrometallation (insertion into M-H) may take place (Scheme 13). Involvement of both pathways was proposed in practical hydrofunctionalization reactions with various substrates and different metal complexes [20-28]. 

For producing ri -coordinated allyl metal species, two pathways are proposed as shown in Scheme 4, and in either case an acid is involved, often added as a cocatalyst or in situ generated path (a) formation of metal hydride species followed by coordination of C-C double bond and subsequent migratory insertion into M-H bond [hydrometallation], and path (b) coordination of C-C double bond followed by protonation of the coordinated alkene [41]. To the terminal carbon of the rj -allyl system, an amine attacks from external side. This type of hydroamination has different characteristics in that the formation of C-H bond precedes by the formation of C-N bond, by contrast to the reactions of other mechanisms which have the opposite bond-forming order, that is, the formation of C-N bond occurs first. 

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