Electrophilic Abstraction of Alkyl Groups

Oxidative addition by the Sn2 or by the ionic mechanisms involves electrophilic attack at the metal (Eq. 8.33 and Sections 6.2 and 6.4)  

In some cases the second step does not take place, and the counterion never binds to the metal. This makes the reaction an electrophilic addition, rather than an oxidative addition to the metal, although the latter term is sometimes seen in the literature to describe this type of reaction. An example is the reaction of the highly nucleophilic Co(I) anion, cobaloxime, with an alkyl triflate, a reaction known to go with inversion. Protonation of metal complexes to give metal hydrides is also very common (Eqs. 3.30-3.31). 

Electrophilic metal ions, notably, Hg + can cleave transition metal alkyl bonds relatively easily. Two main pathways have been identified, one of which is attack at the a carbon of the alkyl, which can lead to inversion of configuration 

As a Oe ligand, HgCl2, or more likely, HgCr, can bind to an 18e metal exactly in the same way as can a proton. It is not yet clear whether the electrophilic attack takes place at the M—C bond or at the metal. The first pathway can give RHgCl directly (Eq. 8.37), the second gives an alkylmetal mercuric chloride, which can reductively eliminate to give the same product (Eq. 8.38). In the absence of an isolable intermediate it is very difficult to tell the two paths apart. This is an important process as we will see in Chapter 16, electrophilic attack by Hg(II) on the methyl derivative of coenzyme B12 is the route by which mercuric ion from various sources is converted into the toxic methylmercury cation in natural waters. 

Other electrophiles are known to abstract transition metal alkyls, as shown below  

Electrophilic metal ions, notably can cleave transition metal alkyl bonds relatively easily. Two main pathways have been identified, one of which is attack at the a carbon of the alkyl, which can lead to inversion of configuration at (bat carbon (Eq. 8.3S). In the other, attack occurs at the metal or at the M—C bond and retention of configuration is found (Eq. 8.36). The difference has been ascribed to the greater basicity of the metal in the iron case.  

Retention of configuration is not always observed in the electrophilic abstraction of vinyl groups because the electrophile sometimes gives an initial reversible addition to the carbon (Eq. 8.43). Free rotation about a C-C single bond in the carbene intermediate then leads to loss of stereochemistry. 

As we saw in Section 7.3, some reactions diat lead to overall insertion into an M—R bond go by the electrophilic abstraction of an alkyl as the first step. SO2 insertion is the best known, but it is thought that SO3, (CN)2C=C(CN)2, and CF3CSCCF3 may be able to react in the same way. 

An alternative pathway for the reaction of a metal alkyl with n electrophile is the abstraction of a substituent at the or cartxin to form a carbene. 

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An analogous ambiguity holds for nucleophilic reactions. We have already seen one facet of this problem in the oxidative addition of alkyl halides to metals (Section 6.3), which can go either by an electrophilic addition to the metal, the Sn2 process, or by SET and the intermediacy of radicals. The two processes can often give the same products. Other related cases we have s n are the promotion of migratory insertion and nucleophilic abstraction by SET oxidation of the metal (Sec. 7.1), and electrophilic abstraction of alkyl groups by halogen (Section 8.5). 

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. 

Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-551]. This approach is one of the most appropriate for the preparation of vinylidene complexes [128]. Figure 3.27 shows illustrative examples of such reactions. 

Alkyl substituents in aromatic azoloazines are reactive towards electrophilic reagents in basic media. Basic reagents readily abstract protons from such alkyl groups yielding resonance stabilized carbanions. Thus, treatment of the methyl derivatives (243) with aldehydes gives the alkenes (245) (Scheme 21) <84H(22)174i). Ready formation of the resonance stabilized anions (244) is behind the activity of the methyl group. 

Enamines are among the most powerful neutral nucleophiles and react spontaneously with alkyl halides. Silyl enol ethers are less reactive and so require a more potent electrophile to initiate reaction. Carbocations will do, and they can be generated in situ by abstraction of a halide or other leaving group from a saturated carbon centre by a Lewis acid. 

In dithioacetals the proton geminal to the sulfur atoms can be abstracted at low temperature with bases such as Bu"Li. Lithium ion complexing bases such as DABCO, HMPA and TMEDA enhance the process. The resulting anion is a masked acyl carbanion, which enables an assortment of synthetic sequences to be realized via reaction with electrophiles. Thus, a dithioacetal derived from an aldehyde can be further functionalized at the aldehyde carbon with an alkyl halide, followed by thioacetal cleavage to produce a ketone. Dithiane carbanions allow the assemblage of polyfunctional systems in ways complementary to traditional synthetic routes. For instance, the p-hydtoxy ketone systems, conventionally obtained by an aldol process, can now be constructed from different sets of carbon groups. ... 

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