Sigma bond electrophiles

Sigma-bond electrophiles can be further subdivided into three types those where E is C, those where E is a heteroatom, and Brpnsted acids, in which E is H. Sigma-bond electrophiles of the first class include alkyl halides, alkyl sulfonates and other pseudohalides, oxonium ions (e.g., Me3COH2), and sul-fonium ions (e.g., Me3S+). Sigma-bond electrophiles of the first class may also... 

Common cr-bond electrophiles of the second class include Br2 and other elemental halogens, peracids (RCO3H the terminal O of the 0-0 bond is electrophilic), and RSX and RSeX (X is Br or Cl S or Se is electrophilic). Sigma-bond electrophiles of the second class do not undergo spontaneous cleavage of the E-X bond because E is usually too electronegative to suffer electron-deficiency. 

Heterolytic Splitting of H-H, Si-H, and Other sigma Bonds on Electrophilic Metal Centers Gregory J. Kubas... 

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

In the case of complexes with normal sigma bonds between both metals, electrophilic attack takes place with retention of configuration ... 

The sp2 hybridized carbanion 496 can also be viewed as an sp3 hybridized anion and can therefore look like 497 or 498. In 487, the electron pair is antiperiplanar to the two C —0 bonds of the dioxane ring, so that the carbanion orbital can be delocalized by an overlap with the antibonding orbitals of the two C —0 sigma bonds (n-o interaction). On that basis, carbanion 496 would be closer to 497 than 498, and the equatorial approach of the electrophile is thus readily understood. Banks has however given a different explanation based on the work of Klein (152, 153). 

The pi bond as a nucleophile. A strong electrophile attracts the electrons out of the pi bond to form a new sigma bond, generating a carbocation. The (red) curved arrow shows the movement of electrons, from the electron-rich pi bond to the electron-poor electrophile. 

A wide variety of electrophilic additions involve similar mechanisms. First, a strong electrophile attracts the loosely held electrons from the pi bond of an alkene. The electrophile forms a sigma bond to one of the carbons of the (former) double bond, while the other carbon becomes a carbocation. The carbocation (a strong electrophile) reacts with a nucleophile (often a weak nucleophile) to form another sigma bond. 

Like an alkene, benzene has clouds of pi electrons above and below its sigma bond framework. Although benzene s pi electrons are in a stable aromatic system, they are available to attack a strong electrophile to give a carbocation. This resonance-stabilized carbocation is called a sigma complex because the electrophile is joined to the benzene ring by a new sigma bond. 

An intermediate in electrophilic aromatic substitution or nucleophilic aromatic substitution with a sigma bond between the electrophile or nucleophile and the former aromatic ring. The sigma complex bears a delocalized positive charge in electrophilic aromatic substitution and a delocalized negative charge in nucleophilic aromatic substitution, (p. 756)... 

It was also shown, that dicarbollide ions could be subjected to similar reactions with other electrophilic agents, such as acyl chlorides,14 trialkylsilyl chlorides,15 diphenyl chloro phosphine16 etc., and yield boron sigma-bonded derivatives of dicarba-wufo-unde-caborates. 

Figure 2.12 The interaction of the HOMO of the nucleophile and the LUMO of the electrophile to produce a sigma bond. The orbital arrangement on the right of the figure gives no interaction because the orbitals have equal positive and negative overlap (recall Fig. 2.7).

The AdE2 is the conceptual reverse of the El elimination. The first step, Ag, (Association, Electrophilic) is a new electron flow path and has the following general form. The arrow starts from the pi bond, breaking it and forming a new sigma bond to the electrophile, creating a carbocation next to the carbon-electrophile bond. 

An alkene is an average electron source, and an aromatic compound is usually worse therefore to get electrophilic addition to alkenes and aromatic compounds to occur one needs a good electron sink. Often a loose association of an electrophile with the pi electron cloud (called a pi-complex) occurs before the actual sigma bond formation step. The best electrophiles, carbocations, add easily. For an overview of electrophilic additions to alkenes, see Section 4.4. 

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