Metal-alkyl complexes Electrophilic abstraction

Additions of electrophiles to metal-alkyl complexes can also induce abstraction of a hydride from the -position. Reactions of electrophiles at the 3 Carbons of alkyl, vinyl, and alkynyl groups have all been reported. Abstraction of a hydride at the -position by... 

Electrophilic transition metal complexes can react with organic ylides to yield alkylidene complexes. A possible mechanism would be the initial formation of alkyl complexes, which are converted into the final carbene complexes by electrophilic a-abstraction (Figure 3.18). This process is particularly important for the generation of acceptor-substituted carbene complexes (Section 4.1). 

However, with substrates prone to form carbocations, complete hydride abstraction from the alkane, followed by electrophilic attack of the carbocation on the metal-bound, newly formed alkyl ligand might be a more realistic picture of this process (Figure 3.38). The regioselectivity of C-H insertion reactions of electrophilic transition metal carbene complexes also supports the idea of a carbocation-like transition state or intermediate. 

Equations 3.64-3.66 illustrate routes to allyl complexes from dienes, diene complexes, and olefins. Allyl complexes have been prepared by the insertion of a conjugated diene into a metal hydride, alkyl, or acyl linkage, as illustrated for the cobalt complexes in Equation 3.64. ° Alternatively, allyl complexes have been prepared by nucleophilic or electrophilic attack on a coordinated diene. Equation 3.65 shows the formation of allyl complexes by the addition of carbanions to a cationic diene complex, and Equation 3.66 shows the formation of a cationic diene complex by the protonation of a neutral 1,3-diene complex. Allyl complexes have also been formed by the abstraction of an allylic proton from a metal-olefin complex, either by a base or by the metal itself. This reaction has been proposed as a step in the isomerization of olefins (Equation 3.67) and in the allylic oxidation of olefins (Equation 3.68). - ... 

Electrophilic reagents react not only at the a-carbon of alkyl groups, but at the a-carbon of carbene and carbyne complexes. In these cases, the electrophile can either form a stable adduct by coordination to the nucleophilic carbon, or it can abstract a labile group. As noted in the section of Chapter 13 on carbene complexes, nucleophilic early metal alkylidene complexes, such as Cp HCH, coordinate Lewis acids at the carbene carbon. CpjTiCHj coordinates Me AlCl to form Tebbe s reagent. 

For instance, if the metal is lost by Sn2 attack on coordinated carbon, this constitutes R loss, and alkyl migration to an electrophilic centre such as coordinated CO may resemble R loss. R- loss may take place by simple homolysis, or by alkyl group transfer. Moreover, as Yamamoto has pointed out an electroneutral metal-carbon bond lengthening may be a prelude to more complex processes such as 0-elimination, or may lead to internal hydrogen abstraction rather than to actual free ligand release. 

CH Activation is sometimes used rather too loosely to cover a wide variety of situations in which CH bonds are broken. As Sames has most recently pointed out, the term was first adopted to make a distinction between organic reactions in which CH bonds are broken by classical mechanistic pathways, and the class of reactions involving transition metals that avoid these pathways and their consequences in terms of reaction selectivity. For example, radicals such as RO- and -OH readily abstract an H atom from alkanes, RH, to give the alkyl radical R. Also in this class, are some of the metal catalyzed oxidations, such as the Gif reaction and Fenton chemistry see Oxidation Catalysis by Transition Metal Complexes). Since this reaction tends to occur at the weakest CH bond, the most highly substituted R tends to be formed, for example, iPr-and not nPn from propane. Likewise, electrophilic reagents such as superacids see Superacid), readily abstract a H ion from an alkane. The selectivity is even more strongly in favor of the more substituted carbonium ion product such as iPr+ and not nPr+ from propane. The result is that in any subsequent fimctionalization, the branched product is obtained, for example, iPrX and not nPrX (Scheme 1). 

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