Electrophiles, and Lewis acids

Most reactions of alkenyl and allylic silanes require strong carbon electrophiles and Lewis acid catalysts are often involved. The most useful electrophiles from a synthetic standpoint are carbonyl compounds, iminium ions, and electrophilic alkenes. 

Most reactions of alkenylsilanes require strong carbon electrophiles, and Lewis acid catalysts are often involved. Reaction with acyl chlorides is catalyzed by aluminum chloride or stannic chloride.57... 

Tab. 6.1.3. Typical electrophiles and Lewis acids used for detachment.

O hare, D., 2011. Separating electrophilicity and Lewis acidity the synthesis, characterization, and electrochemistry of the electron deficient tns(aryl)boranes B(C6F5)3 (C6Ci5) (n = 1-3). J. Am. Chem. Soc. 133, 14727-14740. 

An electrophilic center is an electron-deficient atom that is capable of accepting a pair of electrons. Notice that this definition is very similar to the definition for a Lewis acid. In fact, the terms electrophile and Lewis acid are synonymous. 

The positive charge on carbon and the vacant p orbital combine to make carbo cations strongly electrophilic ( electron loving or electron seeking ) Electrophiles are Lewis acids (Section 117) They are electron pair acceptors and react with Lewis bases (electron pair donors) Step 3 which follows and completes the mechanism is a Lewis... 

Trifluoroacetates of silver, mercury(II), thallium(lll), lead(IV), and lodme(III) are synthetically valuable reagents that combine the properties of strong electrophiles, oxidizers, and Lewis acids Furthermore, trifluoroacetate anions are stable to oxidation, are weak nucleophiles, and usually do not cause any contamination of the reaction mixture... 

Ether cleavage can also be effected by reaction with acetic anhydride and Lewis acids such as BF3, FeCl3, and MgBr2.97 Mechanistic investigations point to acylium ions generated from the anhydride and Lewis acid as the reactive electrophile. 

Several modifications of the Simmons-Smith procedure have been developed in which an electrophile or Lewis acid is included. Inclusion of acetyl chloride accelerates the reaction and permits the use of dibromomethane.174 Titanium tetrachloride has similar effects in the reactions of unfunctionalized alkenes.175 Reactivity can be enhanced by inclusion of a small amount of trimethylsilyl chloride.176 The Simmons-Smith reaction has also been found to be sensitive to the purity of the zinc used. Electrolytically prepared zinc is much more reactive than zinc prepared by metallurgic smelting, and this has been traced to small amounts of lead in the latter material. 

The term halogen bonding (XB) [3-8] indicates any D X-Y interaction in which X is the electrophilic halogen (Lewis acid, XB donor), D is a donor of electron density (Lewis base, XB acceptor), and Y is carbon, nitrogen, halogen, etc. (Fig. 1). 

Correlation between electrophilicity-nucleophilicity and Lewis acidity-basicity ... 

C4 is nucleophilic (enol ether), and CIO is electrophilic. The Lewis acid makes CIO more electrophilic by coordinating to 013. After conjugate addition, 08 traps the C3 carbocation. Proton-Li+ exchange gives the product. 

Indeed, it is often difficult to control electrophilic attack so that monosubstitution occurs. A further problem is that pyrrole polymerizes in the presence of strong acids and Lewis acids, so that... 

It has been noted that the ortho para ratio for protodetritiation of p-[ H] toluene varies with acid medium, and although side reactions are present with both sulphuric acid and Lewis acids, they are usually absent in trifluoroacetic acid. See (a) Taylor, R. Electrophilic Aromatic Substitution. J. Wiley and Sons, New York, 1990, pp. 61-64 (b) Baker, R. Eabcm C. Taylor, R. J. Chem. Soc. 1961,4927. 

As in the alkylation reaction, the reactive intermediate can be a dissociated acylium ion or a complex of the acyl chloride and Lewis acid.43 Recent mechanistic studies have indicated that with benzene and slightly deactivated derivatives, it is the protonated acylium ion that is the kinetically dominant electrophile.44... 

In contrast to some related reviews, which use reaction class or electrophiles as organizational elements, this chapter is divided into three main sections according to catalyst class (i) Bronsted acid catalysis by phosphoric acid and phosphoramide derivatives, (ii) N—H hydrogen bond catalysis by organic base and ammonium systems, and (iii) combined acid catalysis including Bronsted-acid-assisted Bronsted acid, Lewis-acid-assisted Bronsted acid, and Lewis-acid-assisted Br0nsted acid systems (Figure 5.1). 

Acids and Lewis acids react with quinoline at the basic nitrogen atom to form quinolinium salts, and there is a question over the nature of the substrate for electrophilic attack, i.e. is it quinoline or the quinolinium cation The answer is not a simple one and appears to depend upon the reagents and reaction conditions. Thus, whereas acetyl nitrate at 20 °C gives mainly 3-nitroquinoline (Scheme 3.2), fuming nitric acid in concentrated sulfuric acid containing sulfur trioxide at 15-20 °C yields a mixture of 5-nitroquinoline (35%) and 8-nitroquinoline (43%) (Scheme 3.3). In the case of acetyl nitrate, the reaction may proceed by the 1,4-addition of the reagent to quinoline, followed by electrophilic attack upon the 1,4-dihydro derivative. 

In Friedel-Crafts acylation of aromatics with acid chlorides and Lewis acid metal halides the reactive electrophile is considered to be formed in the interaction of the reagent and the catalyst. First the highly polarized donor-acceptor complex 1 is formed, which can further give other complexes and ion pairs.24 The various... 

The addition of thiols to C—C multiple bonds may proceed via an electrophilic pathway involving ionic processes or a free radical chain pathway. The main emphasis in the literature has been on the free radical pathway, and little work exists on electrophilic processes.534-537 The normal mode of addition of the relatively weakly acidic thiols is by the electrophilic pathway in accordance with Markovnikov s rule (equation 299). However, it is established that even the smallest traces of peroxide impurities, oxygen or the presence of light will initiate the free radical mode of addition leading to anti-Markovnikov products. Fortunately, the electrophilic addition of thiols is catalyzed by protic acids, such as sulfuric acid538 and p-toluenesulfonic acid,539 and Lewis acids, such as aluminum chloride,540 boron trifluoride,536 titanium tetrachloride,540 tin(IV) chloride,536 540 zinc chloride536 and sulfur dioxide.541... 

Keteniminium salts are more electrophilic than ketenes and are thus able to react with less nucleophilic olefins. Ketoketeniminium salts can be conveniently prepared from the corresponding a-chloroenamines and Lewis acids.3 However, the method cannot be applied well to the preparation of the less stable aldoketeniminium salts. 

The reaction of allyl silanes with aldehydes and ketones activated as electrophiles by Lewis acids is a very useful method for preparing homoallylic alcohols. Since allyl silanes are only modestly nucleophilic, strong electrophiles are needed to ensure a good reactivity match. 

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