Alkylmetal complexes, formation

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... 

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... 

Very reactive electrophiles such as tetracyanoethylene and isocyanates undergo metal-assisted cycloaddition reactions, the key step of which is thought to involve nucleophilic attack on a metal-complexed olefin to result in the formation of an >/ -alkylmetal complex. 

The reversibility of the zwitter ion adduct formation in Eq. 8.6 also affected the rate law of the formation of amino-substituted alkylmetal complexes. Thus, kinetic studies indicated [33] that the rate of the formation of /i-aminoalkyl complex 4 in Scheme 8.20 was second-order with respect to the concentration of the amine, namely rate = [amine] [complex]. This is consistent with a reaction sequence shown in Scheme 8.20 involving a reversible formation of the zwitter ionic intermediate, followed by the rate-determining deprotonation by the second amine molecule. The observed rate constant appeared to contain contributions from both the equilibrium constant of the first step and the rate constant of the second deprotonation, so that the direct comparison of the rate of the initial nucleophilic attack at the coordinated alkene between Pd and Pt complexes was not possible. However, the higher overall reactivity (ca. 70 times) of Pd complex than Pt complex was consistent with the higher ionization potential of Pd than Pt. This difference in the ionization potential then would lead to the weaker jt basicity of Pd(II) than Pt(II) for jt back-donation to alkene jt orbital, and therefore facilitated the nucleophilic attack at the Pd-alkene complex more than that at the Pt complex. 

Protonolysis of electron-rich alkylmetals may proceed via initial electrophilic attack at metal, i.e. hydride complex formation. Different from the case of the halogenolysis, the subsequent C-H bond formation occurred via internal reductive elimination with overall retention of configuration (Eq. 8.24) [129]. 

The hydrogenation mechanisms discussed above indicate the reversible formation of intermediates. The degree of reversibility depends on the nature of the catalyst and the alkene, and on reaction conditions. Some nonreducible internal alkenes undergo slow hydrogen exchange and isomerization, indicating that reversible steps can occur in these cases. The reversible formation of alkylmetal intermediates provides a ready explanation for the isomerization of alkenes in the presence of certain metal complexes (see Section 4.3.2). 

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]. 

Fischer carbene ligands can also be attacked by nucleophiles because of a minor, if any, contribution of dTt-pTt back-bonding to the metal-carbon bond. The attack may lead to formation of alkylmetal and new carbene complexes (Scheme 8.16). If there is a C-H bond next to the carbene carbon, deprotonation sometimes occurs upon attack of base to give vinylmetal complexes (Scheme 8.16)... 

Scheme 8.74, path B is reminiscent of the electrophilic attack at oxygen of acylmetal complex shown in Eq. 8.5. Another electrophilic route to carbene complex is the reaction of alkylmetals with Ph3C" , as shown in Eq. 8.25 [136]. An electrophilic attack that is similar to Scheme 8.74, path B but appears potentially more signihcant in catalysis is that of alkynylmetal complex to generate vinyli-dene ligand. Although Scheme 8.17 described direct formation of the vinylidene complex from M+ and terminal alkyne, this complex is sometimes derived by treatment of M-C = CR with H+ via -attack [137]. 

Much information has been gained on the mechanism of C-H bond-forming reductive elimination (see Equation 8.9). In addition to creating an understanding of C-H bond formation, this information has been used to understand the mechanism of the opposite reaction, the oxidative addition of C-H bonds. Because reductive eliminations of alkanes are faster from three- and five-coordinate species than from four- and six-coordinate species, square planar and octahedral complexes often dissociate or associate a dative ligand prior to reductive elimination. However, elimination to form a C-H bond from a four- or six-coordinate complex can also be fast enough that it occurs directly from the alkylmetal-hydride complexes prior to ligand dissociation. 

Reactions of mtemal olefins are more complex than reactions of terminal olefins (Scheme 16.3). As mentioned previously, terminal nitriles are often formed from reactions of internal olefins. The formation of terminal nitriles results from insertion of the internal olefin to form a branched alkylmetal intermediate (A in Scheme 16.3) that undergoes isomerization to the terminal alkyl intermediate B prior to reductive elimination of the final linear nitrile faster than it undergoes reductive elimination to form the branched nitrile. Internal olefins react more slowly than terminal olefins, and this relative rate can be traced to the slower insertion of internal olefins into metal hydrides. Lewis acids, such as ZnClj and AlClj, promote these reactions of isolated alkenes. 

---