Hydrometallation reactions

Some hydrometalation reactions have been shown to be catalyzed by zirconocene. For instance, CpiZrCf-catalyzed hydroaluminations of alkenes [238] and alkynes [239] with BU3AI have been observed (Scheme 8-34). With alkyl-substituted internal alkynes the process is complicated by double bond migration, and with terminal alkynes double hydrometalation is observed. The reaction with "PrjAl and Cp2ZrCl2 gives simultaneously hydrometalation and C-H activation. Cp2ZrCl2/ BuIi-cat-alyzed hydrosilation of acyclic alkenes [64, 240] was also reported to involve hydrogen transfer via hydrozirconation. 

The first step of the reduction by cobalt(II) chloride and NaBH4 involves the production of cobalt hydride species which is capable of exchanging hydrogen ligands with the medium. The second step is a hydrometallation reaction followed by a reductive cleavage of the carbon-cobalt bond. The hydrocobaltation seems to be reversible, as indicated by deuterium label incorporation93. 

The addition of metal-hydrogen bonds across carbon-carbon multiple bonds, called hydrometallations, are very important, versatile transformations in organic synthesis. First, they allow the synthesis of new organometallic compounds. The products thus formed may be further transformed into other valuable compounds. The two most important reactions, hydroboration and hydrosilylation, will be treated here in detail, whereas other hydrometallation reactions (hydroalanation, hydro-zirconation) will be discussed only briefly. Hydrostannation, a very important transformation of substituted unsaturated compounds, has no significance in the chemistry of hydrocarbons possessing nonactivated multiple bonds. 

Hydrometallation reactions of carbon-carbon MULTIPLE BONDS Hydroalumination Diisobutylaluminum hydride, 189 Hydroboration General methods Dibromoborane-Dimethyl sulfide,... 

In this chapter, recent advances in asymmetric hydrosilylations promoted by chiral transition-metal catalysts will be reviewed, which attained spectacular increase in enantioselectivity in the 1990s [1], After our previous review in the original Catalytic Asymmetric Synthesis, which covered literature through the end of 1992 [2], various chiral Pn, Nn, and P-N type ligands have been developed extensively with great successes. In addition to common rhodium and palladium catalysts, other new chiral transition-metal catalysts, including Ti and Ru complexes, have emerged. This chapter also discusses catalytic hydrometallation reactions other than hydrosily-lation such as hydroboration and hydroalumination. 

Despite mechanistic complications, however, it appears very likely that most, if not all, of the facile and synthetically attractive carbometallation reactions involve, at a critical moment, concerted addition of carbon-metal bonds where the synergistic HOMO-LUMO interactions shown in Scheme 4.3, akin to those for the concerted hydrometallation reactions, provide a plausible common mechanism. This mechanism requires the ready availability of a metal empty orbital. It also requires that addition of carbon-metal bonds be strictly syn, as has generally been observed. Perhaps more important in the present discussion is that concerted syn carbometallation must proceed via a transition state in which a carbon-metal bond and a carbon-carbon bond become coplanar. Under such constraints, one can readily see how chirally discriminated carbon-metal bonds can select either re or si face of alkenes. In principle, the mechanistic and stereochemical considerations presented above are essentially the same as for related concerted syn hydrometalla-tion. In reality, however, carbometallation is generally less facile than the corresponding hydrometallation, which may be largely attributable to more demanding steric and... 

Carbo- and hydrometallation reactions are frequently catalyzed by group 4 metals (Ti, Zr) and are thus highly associated with the question of how the Ziegler-Natta polymerization or Tebbe processes should be controlled or... 

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... 

All of the proposed mechanisms for the reduction of alkynes with metal hydride-transition metal halide combinations involve an initial hydrometallation of the ir-system by the transition metal hydride, formed by the reaction of the original metal hydride with the transition metal halide, to form the vi-nylmetallic intermediate (99 equation 38). For the reduction of alkenes, similar alkylmetallic intermediates are implied to be formed. In the case of the reduction of alkenes with NaBH4 in the presence of Co" in alcohol solution, the hydrometallation reaction appears to be reversible as evidenced by the incorporation of an excess of deuterium when NaBD4 was used in the reduction. ... 

Schwartz s reasoning for optimizing these thermodynamic considerations led to the development of hydrozirconation. Hydride complexes of the late transition metals do not in general exhibit the hydrometallation reaction, probably because the alkene complexes are too stable. This may be understood from the Dewar-Chatt-Duncanson model for alkene bonding, wherein back donation of metal d-elec-trons to the alkene Tr -orbital is a major contributor. For metal centers with d -electron configurations, there should be substantial stabilization of (3) with respect to (2). Such metals are only found towards the left end of the Periodic Table, particularly Groups III A to VA. 

Investigation of hydrometallation reactions of a-olefins with alkali-metal tetra-hydrogallates have shown that reaction is very slight in ether solvents. In the presence of GaEts, however, the sodium and potassium salts react readily when the solvent is a hydrocarbon, producing tetra-alkylgallates." ... 

This hydrometalation reaction is regiospecific, forming only the primary alkyltantalum. These insertions proceed under mild conditions (e.g., 75°C and 100 kPa CO for 7-8 h) on the endo-hydridoalkene complexes. However, while the exo-hydridoalkene complexes do react with CO to form Ta alkyls, alkyltantalums are not formed from the exo-hydridoalkene complex and an isonitrile. 

Alkylcobalts form in stoichiometric reactions of pentacyanocobalt(III) hydride and alkenes. This reaction occurs both for halogenated alkenes such as tetrafluoroethylene and for alkenes that contain other electron-withdrawing groups such as carbonyls, nitriles and arenes as substituents (see Table 6) . The addition is regiospedfic, forming the more substituted alkylcobalt. Prior coordination of alkene to cobalt to form an alkene(hydrido)cobalt complex, an intermediate in hydrometalation reactions, is not important. This reaction is a radical process however, by NMR, additions of [HCo(CN)5 ] " to diastereomeric alkenes such as fumaric and maleic add salts lead to a cr-alkylcobalt by stereospecific cis addition of Co and H to the double bond . The overall reduction is not stereospecific. (r-Alkylcobalt bond formation proceeds by either a concerted addition or a rapid collapse of a radical cage. 

As far as the role of the transition metal catalysts in the hydromagnesiation is concerned, other hydrometallation reactions such as hydroa-lumination 1108] and the recently reported hydro-zincation 1109) are catalyzed by the same class of titanium complexes, so all of these reactions can be grouped together. In fact, these reactions show quite similar applicabilities to particular unsaturated compounds and similar rcgioselcctivi-ties with unsymmetrical substrates (eqs. 3.64 and 3.65). 

The photochemical hydrometallation reaction of electron deficient olefins (firmaronitrile, maleic anhydride, etc.) by CP2MH2 (M = Mo, W) was shown to occur via electron transfer, followed by proton transfer from [Cp2MH2] to the reduced olefin, and final radical recombination (scheme 20) [16]. 

The reactivity of Qo comparable to that of electron deficient conjugated olefins is nicely reflected by reactions with transition metal complexes. A variety of single crystal structures and spectroscopic studies show that the complexation of transition metals to the fullerene core proceeds in a dihapto manner or as hydrometalation reactions rather than in rf- or ] -binding mode. This was elegantly demonstrated by the reaction of Cgg with ruthenium complexes (Scheme 8) [144]. A variety of iridium complexes ( ] -Cgo)Ir(CO)Cl(PR R R )2 were synthesized by allowing Cgg to react with different Vaska-type complexes Ir(CO)Cl(PR R R )2 [145]. ] -Complex formation was also observed upon reaction of Cgo with other Ir [146] as well as Rh [147] complexes. Hydro-metallation was obtained with Cp2Zr(H)Cl [140]. 

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.)... 

Over the past few decades, Pd complexes have been shown to be effective catalysts for hydrometallation reactions involving both main group and transition metals. Not surprisingly, most of the investigations have dealt with hydrometallation reactions of group 14 metals including Si, Ge, and Sn, which are discussed in Sects. B and C. 

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