Alkylmetals reactivity

Cationic cyclopentadienyliron dicarbonyl (Fp) alkene complexes are generally reactive towards a wide variety of nucleophiles, including nitrogen nucleophiles, but they too generate stable chemical step (usually oxidation). This makes catalysis impossible and severely limits application of this methodology to organic synthesis (equation 21 ).36 However, in contrast to palladium, iron is relatively inexpensive and stoichiometric procedures... 

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]. 

All of these carbenes are reactive intermediates that must be generated from the appropriate precursors in the presence of the alkene (or arene) which is to be cyclopropanated. The following methods of carbene-transfer reactions to C-C double bonds will be discussed path a. from a-halo-a-metal (or alkylmetal) compounds by a-elimination path b. from iodine or sulfur ylides by thermal, photochemical or transition metal catalyzed decomposition ... 

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. 

With these interesting types of reactivity of selenides and functionalized selenides, it is important to show that these compounds can be rapidly prepared from readily available starting materials. At least two types of methods are available for such purposes and involve the attack of a selenolate or of an a-seleno alkylmetal , on an electrophilic carbon atom. This last reaction is particularly interesting since a new carbon-carbon double bond is formed in the process. 

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. 

With the notable exceptions of aUyl-allyl coupling (Sect, in.2.10) and enolate allylation (Sect. V.2.1.4), Pd-catalyzed allylation has been achieved mostly with alkenyl-metals (alkenyl-allyl couphng) and arylmetals (aryl-aUyl couphng). The use of alkylmetals and alkynyhnetals has been rare perhaps because their allylation can be performed satisfactorily with appropriate Grignard reagents, whose reactivity can be modified favorably with either stoichiometric or catalytic amounts of Cu compounds, as needed. 

Li. Despite some early successful results with alkyllithiums, they have scarcely been used in Pd-catalyzed alkylation. This must, in part, be due to their inability to tolerate many conventional polar functional groups. As discussed earlier, however, the high intrinsic reactivity of alkyllithiums appears to be responsible for some of the difficulties observed with alkyllithiums. In any event, it may be stated that the current scope of Pd-catalyzed alkylation with alkyllithiums is rather limited. In cases where alkylmetals containing other metals are prepared via alkyllithiums, however, it would be desirable and advisable to examine the reaction of alkyllithiums themselves to justify the use of other metals. Some of the favorable results observed with alkyllithiums are summarized in Scheme 2. 

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