Leaving groups in electrophilic substitution

Thus, not only should alkylmercuric iodides and dialkylmercurials have a more nucleophilic carbon compared with the others, but their respective -Hg-X functions should also be better leaving groups in electrophilic substitution reactions. This latter prophesy is supported in part by the results of Hughes et al. from their kinetic studies of mercury exchange reactions (20). 

Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are structurally similar—both contain a double bond and a pyrophosphate ester unit—but the chemical reactivity expressed by each is different. The principal site of reaction in dimethylallyl pyrophosphate is the carbon that bears the pyrophosphate group. Pyrophosphate is a reasonably good leaving group in nucleophilic substitution reactions, especially when, as in dimethylallyl pyrophosphate, it is located at an allylic carbon. Isopentenyl pyrophosphate, on the other hand, does not have its leaving group attached to an allylic carbon and is far- less reactive than dimethylallyl pyrophosphate toward nucleophilic reagents. The principal site of reaction in isopentenyl pyrophosphate is the carbon-carbon double bond, which, like the double bonds of simple alkenes, is reactive toward electrophiles. 

Hydrogen as the Leaving Group in Simple Substitution Reactions A. Hydrogen as the Electrophile 11-1 Hydrogen Exchange Deuterio-de-hydrogenation or deuteriation... 

As mentioned in Section II.B.5, the tert-butyldi phony I silyl group attached to the olefinic carbon is a poor electrofugal group and hence does not leave easily in electrophilic substitution reactions140. On the other hand, the epoxide 172, obtained from the corresponding vinylsilane having such a bulky silyl substituent, gives the carbonyl compound upon... 

The leaving group ability of Me Sn group in electrophilic substitutions allowed the synthesis of diaryl ketones in good yields (40-78%) through the reaction of Me SnAr (synthesized by S l reactions) with ArCOCl [48]. These reactions were regioselective, allowing the synthesis of diaiyl ketones not usually available under Friedel-Crafts reactions. 

Sn2 substitutions of allylic electrophiles, additions to oxo-carbenium ions, and many others. Suitable leaving groups in these substitution reactions include alkyl chlorides, bromides, iodides, mesylates, tosylates, and acetates. Often these processes are conducted under phase-transfer conditions, or require the use of 18-crown-6 to solubilize the salt and enhance the nucleophilicity of the acetate anion. The use of ionic solvents, such as butyl methylimidazolium tetrafluoroborate (Btnim BF4), has also proven useful. The following examples are representative (eqs 2-9). 

In the discussion of electrophilic aromatic substitution (Chapter 11) equal attention was paid to the effect of substrate structure on reactivity (activation or deactivation) and on orientation. The question of orientation was important because in a typical substitution there are four or five hydrogens that could serve as leaving groups. This type of question is much less important for aromatic nucleophilic substitution, since in most cases there is only one potential leaving group in a molecule. Therefore attention is largely focused on the reactivity of one molecule compared with another and not on the comparison of the reactivity of different positions within the same molecule. 

Non-heteroatom-substituted carbene complexes can also be generated by treatment of electrophilic transition metal complexes with ylides (e.g. diazoalkanes, phosphorus ylides, nucleophilic carbene complexes, etc. Section 3.1.3). Alkyl complexes with a leaving group in the a-position are formed as intermediates. These alkyl complexes can undergo spontaneous release of the leaving group to yield a carbene complex (Figure 3.2). 

Calb et al. have thoroughly investigated the use of allylic electrophiles containing heterocyclic leaving groups in regioselective allylic substitution (Scheme 8.7) [22]. 

The hydroarylation of olefins is relatively uncommon in photochemistry, despite a high interest in this process which allows the formation of an aryl-carbon bond via the direct activation of an aromatic, with no need for leaving groups in both components of the reaction. The process follows a photo-EOCAS (Electrophile-Olefin Combination Aromatic Substitution) mechanism [32], and is initiated by a PET reaction between an electron-rich aromatic and an electron-poor olefin, as illustrated in Scheme 3.14. 

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