Hydrometallation transition metal hydrides

Addition reactions of three kinds of main group metal compounds, namely R—M X (carbometallation, when R are alkyl, alkenyl, aryl or allyl groups), H—M X (hydrometallation with metal hydrides) and R—M —M"—R (dimetallation with dimetal compounds) to alkenes and alkynes, are important synthetic routes to useful organometallic compounds. Some reactions proceed without a catalyst, but many are catalysed by transition metal complexes. 

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. 

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. ... 

This problem may be generally solved by catalyzed hydrometallation, which proceeds as shown in Scheme 1. Here the actual hydrometallating species is a transition metal hydride, but only catalytic amounts are needed. The following survey of such methods is brief more details on the two most important systems, hydroalumination and hydrosilylation, may be found in Volume 8, Chapters 3.11 and 3.12 respectively. 

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) ... 

Asymmetric hydrometallation of ketones and imines with H-M (M = Si, B, Al) catalyzed by chiral transition-metal complexes followed by hydrolysis provides an effective route to optically active alcohols and amines, respectively. Asymmetric addition of metal hydrides to olefins provides an alternative and attractive route to optically active alcohols or halides via subsequent oxidation of the resulting metal-carbon bonds (Scheme 2.1). 

The hydroacylation of olefins with aldehydes is one of the most promising transformations using a transition metal-catalyzed C-H bond activation process [1-4]. It is, furthermore, a potentially environmentally-friendly reaction because the resulting ketones are made from the whole atoms of reactants (aldehydes and olefins), i.e. it is atom-economic [5]. A key intermediate in hydroacylation is a acyl metal hydride generated from the oxidative addition of a transition metal into the C-H bond of the aldehyde. This intermediate can undergo the hydrometalation ofthe olefin followed by reductive elimination to give a ketone or the undesired decarbonyla-tion, driven by the stability of a metal carbonyl complex as outlined in Scheme 1. 

The reverse process, decarbonylation, is also fast but can be arrested by maintaining a pressure of carbon monoxide above the reaction mixture. The reverse of hydrometallation involves the elimination of a hydride from the adjacent carbon of a metal alkyl to form an alkene complex. This process is known as [3-hydride elimination or simply [3 elimination. It requires a vacant site on the metal as the number of ligands increases in the process and so is favoured by a shortage of ligands as in 16-electron complexes. The metal and the hydride must be syn to each other on the carbon chain for the elimination to be possible. The product is an alkene complex that can lose the neutral alkene simply by ligand exchange. So (3 elimination is an important final step in a number of transition-metal-catalysed processes but can be a nuisance because, say, Pd-Et complexes cannot be used as p elimination is too fast. 

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. 

For main group metals, which exhibit relatively small differences between M—H and M—C bond strengths and no strong metal-alkene interactions, the thermodynamics of hydrometallation should be even more favorable than for early transition metals. This does not appear to be the case, especially for hydroalumination (Volume 8, Chapter 3.11). The reason is almost certainly the additional stability of the metal hydride reagent conferred by aggregation organoaluminum hydrides exist as rather tight dimers. 

An important example of transition-metal (T-alkyl synthesis by hydrometalation is the synthesis of bis(cyclopentadienyl)alkylzirconium chlorides ( j -Cp)2ZrRCl from alkenes and biscyclopentadienylchlorozirconium hydride ( ACp)2ZrHCl. The facility and general applicability of this hydrometalation, coupled with the synthetic versatility of the resultant alkylzirconium reagents makes this hydrometalation an important synthetic method in organic chemistry. ... 

E. PALLADIUM-CATALYZED HYDROMETALLATION INVOLVING TRANSITION METAL COMPLEXES AS HYDRIDE SOURCES... 

RS-M" L c-i)- The other is the oxidative addition of thiols to low-valent transition metals (M"L ) to give the corresponding transition metal complexes bearing both hydride and thiolate ligands (RS-M" L c 2-H). The reaction of the former complexes (RS-M" L c-i) with carbon-carbon unsaturated compounds such as alkynes may proceed via thiometallation, in which relatively more bulky is bonded at the terminal carbon of alkynes. On the other hand, in the reaction of the latter complexes (RS-M" L c-2 H) with alkynes both hydrometallation and thiometallation processes are possible. These processes proceed via sy -addition. Alternative pathway for the addition of thiols to alkynes involves coordination of alkynes to transition metals and then nucleophilic addition of thiols (or thiolate anions) to the alkynes. These processes take place via a f -addition. By controlling these pathways, regio- and stereoselective hydrothiolation of alkynes is expected to be attained successfully. 

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