Hydrometallic processing

The insolubility of 1 has the major inconvenience that it prevents studying in detail the mechanistic factors which would operate during this hydrometalation process, as in general only the organic substrate and final zirconocene product are observed by NM R spectroscopy. 

One of the oldest ruthenium-catalyzed C=C bond coupling reactions deals with the selective dimerization of functionalized alkenes, especially the dimerization of acrylates [ 1,2]. It usually involves either an initial hydrometallation process, oxidative coupling, or vinyl C-H bond activation (Scheme 1). 

Thermodynamic analysis of the overall hydrometallation process is straightforward using the cycle of Scheme 3 we get the following expression for equation (4) AbP = D(M—H) - D(M—R) - X, where X = 36-38 kcal mol . Further estimating A5 t -27 to -30 eu, it may be concluded that equation (4) is thermodynamically favored as long as the metal-hydride bond is no more than 27-30 kcal mol" stronger than the metal-alkyl bond. 

A quite different case is presented by the bent metallocene systems, [Cp2MH(alkene)] where M = Nb or Ta. Here the metal-alkene complex is relatively quite stable, so much so that the kinetics of formation of a stable alkyl metal complex cannot be easily studied. Nonetheless it is possible to get a handle on the hydrometallation kinetics by means of dynamic NMR methods (Table 1). The increased rate for propene versus ethylene results from both steric destabilization of the ground state and electronic stabilization of the transition state for the former. The first is typical of alkene complexes the second implies that some partial positive charge develops at the -carbon during the hydrometallation process, also seen in the trend for substituted styrenes. 

These dimers are less reactive or unreactive in catalytic reactions of interest, and thus some substitution on the cyclopentadienyl ligands is necessary to prevent dimer formation through steric hindrance to association. Pentamethylcy-clopentadienyl (Cp ) ligands are useful for this, and lead to catalytic systems that are highly reactive and yet exquisitely selective in the insertion of mono-substituted alkenes. One such precatalyst system is the yttrium complex Cp 2YMe THF. Although Lewis bases normally depress catalytic activity because they compete for empty coordination sites on the catalyst, in this precatalyst the single THF of solvation appears to catalyze the hydrometallation process [Eq. (5)] [15] in the same manner that Lewis bases catalyze the hydroboration of olefins with 9-BBN [16] ... 

Methods for the hydrometallic processing of metallic wastes have been developed, for... 

Kammel R. NATO Conf. Ser.1984 6-10, Hydrometall. Process Fundam., 617-628. 

C. L. Karr, S. K. Sharma, W. Hatcher, and T. R. Harper, in Modeling and Simulation of the Control Hydrometallic Processes, Proceedings of an International Symposium, V. G. Papan-... 

Of the larger clusters, the 1,2,3-triphenylphosphirene derivatives of the iridium carbonyl clusters [HIr4(CO)9L( u-PPh2)] (L = CO, PPhs) resulting from substitution, insertion and hydrometallation processes are discussed by Hitchcock et... 

Hydrometallation and carbometallation of alkynylsilanes proceeds regio-and stereospecifically, the metal becoming attached to the silicon-bearing carbon atom in what is normally a co-addition process (hydrostannylation, however, shows the opposite regioselectivity). Electrophilic cleavage, with retention, of the carbon-metal bond then leads to vinylsilanes of various types. 

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. 

Similar intramolecular hydroarylations of alkynes and alkenes, which obviate the need for a halide or triflate group on the aryl ring, are now well established. Sames group screened over 60 potential catalysts and over 200 reaction conditions, and found that Ru(m) complexes and a silver salt were optimal. This process appears to tolerate steric hindrance and halogen substrates on the arene (Equations (175)—(177)). The reaction is thought to involve alkene-Ru coordination and an electrophilic pathway rather than a formal C-H activation of the arene followed by alkene hydrometallation, and advocates the necessary cautious approach to labeling this reaction as a C-H functionalization... 

In principle, carbometallation of an alkene (RCH=CH2) with a coordinatively unsaturated organotransition metal compound (R1 M I. ) can produce a monomeric carbometallation product 1 (Scheme 6). This reaction may not, however, stop at this stage. It can be accompanied by other processes of which (i) hydrogen-transfer hydrometallation to produce a potentially thermodynamically more favorable mixture of a 1,1-disubstituted alkene and a hydrometallation product 2 and (ii) polymerization to produce polyalkenes 3 are representative. The extents to which these side-reactions occur are functions of relative rates of various competing processes. For example, accumulation of the monomeric carbometallation product 1 can be favored in cases where the starting R1 MTL is more reactive toward alkenes than 1. The organometal/alkene ratio is also an important parameter, since neither of the two side-reactions can proceed after all of the starting alkene has reacted. 

Detailed mechanistic study on these intramolecular hydrosilylation of allylic O-silyl ethers 59 and allylic A -silylamincs 63 using deuterium labeling techniques shows that 5-endo cyclization giving 60 or 64 proceeds via a Chalk-Harrod type hydrometalation catalytic cycle, while 4-exo cyclization process yielding 61 or 65 includes a Seitz-Wrighton type silylmetalation mechanism89. 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

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