Alkyls electrophilic abstraction

A further general route to heteroatom-substituted carbene complexes is based on the a-abstraction of nucleophiles from alkyl complexes (electrophilic abstraction Figure 2.13). 

Electrophilic abstraction from an alkyl complex (equation 4) is illustrated in the reactions of equations (8) and (9) equation (9) is driven by the high Si-F bond strength. 

Alkoxy radicals are electrophiles and they preferentially attack C-H bonds with high HOMO energies, for instance, the a-C-H bond of ethers and amines or the alkyl C-H bond of esters. In contrast, nucleophilic alkyl radicals abstract a hydrogen atom from the acyl group of esters, because this C-H bond has a lower LUMO energy. 

Electrophilic abstraction of a portion of a a-bound hydrocarbyl or a coordinated 7t ligand may occur. The trityl cation, Ph3C+, is a commonly-used electrophile for this task. Equation 8.73 provides an example of abstraction at the (3 position of an alkyl ligand to provide an rf-alkene complex,102 a useful route for the synthesis of these compounds. 

Abstract For many years, unactivated alkyl electrophiles that contain p hydrogens were generally regarded as unsuitable partners for palladium-catalyzed cross-coupling reactions. Recently, however, a series of studies have established that palladium complexes can in fact couple a range of alkyl electrophiles with a variety of organometallic reagents. 

As we saw in Section 7.3, some reactions that 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 CF3C=CCF3, may be able to react in the same way. 

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). 

Cooper33a has described abstraction reactions from a metal alkyl by an electrophilic reagent that goes by an SET route. Instead of the normal abstraction from an ethyl group, which occurs in the usual electrophilic abstraction, he finds a preference for a abstraction from a methyl group. Since... 

Electrophilic abstraction from an alkyl complex (Eq. 11.3) is illustrated by Eq. 11.8. 

With the exception of the nuclear amination of 4-methylthiazole by sodium amide (341, 346) the main reactions of nucleophiles with thiazole and its simple alkyl or aryl derivatives involve the abstraction of a ring or substituent proton by a strongly basic nucleophile followed by the addition of an electrophile to the intermediate. Nucleophilic substitution of halogens is discussed in Chapter V. 

Inductive and resonance stabilization of carbanions derived by proton abstraction from alkyl substituents a to the ring nitrogen in pyrazines and quinoxalines confers a degree of stability on these species comparable with that observed with enolate anions. The resultant carbanions undergo typical condensation reactions with a variety of electrophilic reagents such as aldehydes, ketones, nitriles, diazonium salts, etc., which makes them of considerable preparative importance. 

Thus we think of the chemical ionization of paraffins as involving a randomly located electrophilic attack of the reactant ion on the paraffin molecule, which is then followed by an essentially localized reaction. The reactions can involve either the C-H electrons or the C-C electrons. In the former case an H- ion is abstracted (Reactions 6 and 7, for example), and in the latter a kind of alkyl ion displacement (Reactions 8 and 9) occurs. However, the H abstraction reaction produces an ion oi m/e = MW — 1 regardless of the carbon atom from which the abstraction occurs, but the alkyl ion displacement reaction will give fragment alkyl ions of different m /e values. Thus the much larger intensity of the MW — 1 alkyl ion is explained. From the relative intensities of the MW — 1 ion (about 32%) and the sum of the intensities of the smaller fragment ions (about 68%), we must conclude that the attacking ion effects C-C bond fission about twice as often as C-H fission. 

In proceeding to a consideration of the chemical ionization mass spectra of more highly branched paraffins, it will be most convenient to consider separately the several different classes of alkyl ions found in the spectra—i.e., MW — 1+, MW — 15+, MW — 29 +, etc. We can see from Table II that a considerable amount of variation in the relative intensity of the MW — 1 ions (always the highest mass ion for which an intensity is given in the table) occurs. However, we shall show that the observed MW — 1 intensities can be approximately accounted for in terms of the concept of localized electrophilic attack by the reactant ion. First, however, we must consider the energetics of two processes which may be important in generating the spectra of branched paraffins. One of these is the abstraction of a primary hydrogen by the reactant ion. As a typical example we may write... 

In enolates formed by proton abstraction from a,(3-unsaturated ketones, there are three potential sites for attack by electrophiles the oxygen, the a-carbon, and the y-carbon. The kinetically preferred site for both protonation and alkylation is the a-carbon.65... 

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). 

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. 

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