Alkylmetal bonds

These values are not absolute velocity constants since transfer to alkyl-aluminum occurs during the reaction. However, the transfer reactions will be proportional to the concentration of active centers. Hence, at similar conversions the fractions of active alkylmetal bonds are likely to be approximately the same, and the constants will be proportional to the true velocity constants. Determining relative propagation velocity constants is complicated by the participation of a termination reaction. At low temperatures the polymer is insoluble and the catalyst is embedded in a semi-solid mass, resulting in very slow rates of polymerization. At temperatures of 41°-60° C. reasonably good first-order reactions with respect to monomer are found, but at higher temperatures there is a rapid fall-off in reaction rate with time (Figure 4). The velocity constants in Table III were calculated from the linear portion of the rate-time curve, and no account was taken of termination reactions. 

The homolytic cleavage of alkylmetal bonds, particularly those of Main Group metals, is known from Paneth s classic experiments to occur at high temperatures. The reverse of Equation 4, however, does not usually represent the energeti-... 

In these alkylmetals of the main group elements, ionization occurs from the highest occupied molecular orbital (HOMO) which has a-bonding character, i.e., they are a-donors. Consequently... 

Recently, another type of catalytic cycle for the hydrosilylation has been reported, which does not involve the oxidative addition of a hydrosilane to a low-valent metal. Instead, it involves bond metathesis step to release the hydrosilylation product from the catalyst (Scheme 2). In the cycle C, alkylmetal intermediate generated by hydrometallation of alkene undergoes the metathesis with hydrosilane to give the hydrosilylation product and to regenerate the metal hydride. This catalytic cycle is proposed for the reaction catalyzed by lanthanide or a group 3 metal.20 In the hydrosilylation with a trialkylsilane and a cationic palladium complex, the catalytic cycle involves silylmetallation of an alkene and metathesis between the resulting /3-silylalkyl intermediate and hydrosilane (cycle D).21... 

Addition of an alkene to a compound containing a metal-H bond usually results in insertion, and it does in this case, too, to give the stabler 1° alkylmetal. Addition of CuBr to this complex might result in transmetallation, to give a C2-Cu bond. Addition of the copper compound to the unsaturated imide gives conjugate addition, perhaps by coordination of the C3=C4 tz bond and insertion into the C2-Cu bond. Workup gives the observed product. 

The catalytic asymmetric preparation of a-chiral amines, by addition of organo-metallic reagents to C=N bonds, is one of the most important reactions in homogeneous catalysis [26]. However, the catalytic asymmetric addition of simple alkylmetals has been achieved only in recent years. 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

Once complexed to palladium(II), the alkene is generally activated towards nucleophilic attack, with nucleophiles ranging from chloride to phenyllithium undergoing reaction. The reaction is, however, quite sensitive to conditions and displacement of the alkene by the nucleophile (path a) or oxidative destruction of the nucleophile can become an important competing reaction. Nucleophilic attack occurs predominately to exclusively at the more-substituted position of the alkene (the position best able to stabilize positive charge) and from the face opposite the metal (trans attack, path b) to produce a new carbon-nucleophile bond and a new carbon-metal bond. This newly formed a-alkylmetal complex (2) is... 

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. 

Equation (1) depicts an early example of an intermolecular addition of an alkane C-H bond to a low valent transition metal complex [12], Mechanistic investigations provided strong evidence that these reactions occur via concerted oxidative addition wherein the metal activates the C-H bond directly by formation of the dative bond, followed by formation of an alkylmetal hydride as the product (Boxl). Considering the overall low reactivity of alkanes, transition metals were able to make the C-H bonds more reactive or activate them via a new process. Many in the modern organometallic community equated C-H bond activation with the concerted oxidative addition mechanism [10b,c]. 

Most coordination catalysts have been reported to be formed in binary or ternary component systems consisting of an alkylmetal compound and a protic compound. Catalysts formed in such systems contain associated multinuclear species with a metal (Mt)-heteroatom (X) active bond ( >Mt X Mt—X > or — Mt—X—Mt—X— Mt = Al, Zn, Cd and X = 0, S, N most frequently) or non-associated mononuclear species with an Mt X active bond (Mt = Al, Zn and X = C1, O, S most frequently). Metal alkyls, such as triethylaluminium, diethylzinc and diethylcadmium, without pretreatment with protic compounds, have also been reported as coordination polymerisation catalysts. In such a case, the metal heteroatom bond active in the propagation step is formed by the reaction of the metal-carbon bond with the coordinating monomer. Some coordination catalysts, such as those with metal alkoxide or phenoxide moieties, can be prepared in other ways, without using metal alkyls. There are also catalysts consisting of a metal alkoxide or related compound and a Lewis acid [1]. 

Hexachloroiridate ion, IrClJ-, is a complex inert to substitution and is known to undergo outer-sphere electron transfer with other inorganic species (cf. Cecil and Littler, 1968). Some of its reactions have been treated in Tables 12 and 14 and shown to be of the non-bonded electron-transfer type. Its reaction with various alkylmetals has been thoroughly studied, and some results are shown in Table 16 (entries nos. 14 and 15). Except for sterically hindered tetralkyltins the Marcus theory makes incorrect predictions for these reactions, and non-bonded electron transfer does not appear to be feasible. 

In a broad class of reactions of CH bonds with low-valent transition metals, however, the metal inserts into a CH bond of the alkane to give an alkylmetal hydride in which there is often a preference for formation of the least substituted alkyl. In any subsequent fimctionalization, the linear (or least branched) product is obtained, for example, nPrX and not iPrX. Since the branched product can be obtained... 

Isomeric (s-cis- and (i-fra/w-V-conjugated diene)zirconocene and -haf-nocene complexes exhibit pronounced differences in their characteristic structural data as well as their spectroscopic features. These differences exceed by far the consequences expected to arise simply from the presence of conformational isomers of the 1,3-diene unit. While (f-rra/u-butadiene)-zirconocene (3a) shows a behavior similar to a transition metal olefin TT-complex, the (.r-cu-diene)ZrCp2 isomer 5a exhibits a pronounced alkylmetal character (23, 45). Typical features are best represented by a tr, 7T-type structure for 5 (55). However, the distinctly different bonding situation of the butadiene Tr-system/bent-metallocene linkage is not only reflected in differences in physical data between the dienemetallocene isomers 3 and 5, but also gives rise to markedly different chemical behavior. Three examples of this are discussed in this section the reactions of the 3/5 isomeric mbcture with carbon monoxide, ethylene, and organic carbonyl compounds. 

By the nature of its molecular mechanism, the carbonyl-insertion reaction represents a typical reaction mode of o alkyltransition metal complexes. Formation of the new C—C cr-bond takes place during a 1,2-alkyl-migration step, transforming an alkylmetal carbonyl moiety [cts-M(CO)R] into an acylmetal unit (M—COR) (89). In general, (s-cir-diene)-zirconocene complexes 5 appear to exhibit a substantial alkylmetal character (90). Therefore, it is not too surprising that some members of this class of compounds [in contrast to most other dienetransition metal complexes (97)] react with carbon monoxide with C—C bond formation (45). However, as demonstrated by X-ray structural data for 5 (Tables V... 

Investigations into nonsynthetic aspects of the Direct Reaction have usually focused on their possible uses to extract metals from solids. The substrate may be a pure metal, a metal alloy, or a compound between a metal and a semimetal. In these applications, the yield may or may not be important. It is not even necessary that the metal-carbon bond be sufficiently stable to isolate the alkylmetals formed even if the initially... 

Electrochemical oxidations of alkylmetal donors are irreversible in all cases studied owing to rapid homolytic cleavage of the metal-carbon bond at the cation-radical stage (Eq. 3) ... 

Metallocenes are useful electron donors as judged by their low (vertical) ionization potentials in the gas phase and oxidation potentials in solution (see Table 2). In fact, the electron-rich (19 e ) cobaltocene with an oxidation potential of E°ox = -0.9 V relative to the SCE [45] is commonly employed as a very powerful reducing agent in solution. Unlike the alkylmetals (vide supra), the HOMOs of metallocenes reside at the metal center [46] which accounts for two effects (i) Removal of an electron from the HOMO requires minimal reorganization energy which explains the facile oxidative conversion from metallocene to metallocenium. (ii) The metal-carbon bonding orbitals are little affected by the redox process, and thus the resulting metallocenium ions are very stable and can be isolated as salts. 

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