Electrophiles, addition with

Electrophilic substitution begins like electrophilic addition, with an attack on a region of high electron density to form a positively charged intermediate ... 

This is ordinary electrophilic addition, with rate-determining protonation as the first... 

Typical electrophiles that attack olefins like Br+ (Eq. 64)105) and (OH)+ (Eq. 65) 105) do lead to electrophilic addition with ring enlargement101 a). Most interesting is the ability for electrophiles that normally do not attack olefins to also react. Thus, an oxycarbonium ion generated from an acetal smoothly alkylates the double bond of this composite functional group in an overall highly stereocontrolled 1,1,2-trialkylation of a simple ketone as illustrated in Eq. 66 106). The chemo- and... 

Ground-state alkenes generally undergo electrophilic addition with alcohols in the presence of a Bronsted acid catalyst, yielding the Markovnikov product ... 

A 3-acyl-4,5-unsubstituted-2(3//)-oxazolone 157 smoothly undergoes electrophilic addition with Br2 (or NBS) and PhSeCl in methanol to give frani-5-bromo-4-methoxy- and frani-4-methoxy-5-phenylselenenyl-2-oxazolidinones 158, respectively, with full regio- and trans-selectivity (Fig. 5.39). Both substituents thus... 

For example, attack at the Markovnikov position of Me3N—CH=CH2 would give an ion with positive charges on adjacent atoms. The compound CF3CH=CH2 has been reported to give electrophilic addition with acids in an anti-Markovnikov direction, but it has been shown88 that, when treated with acids, this compound does not give simple electrophilic addition at all the apparently anti-Markovnikov products are formed by other pathways. 

This is ordinary electrophilic addition, with rate-determining protonation as the first step.164 Certain other alkynes have also been hydrated to ketones with strong acids in the absence of mercuric salts.165 Simple alkynes can also be converted to ketones by heating with formic acid, without a catalyst.166... 

The stoichiometric equivalents of halogen fluorides, i.e. chlorine monofluoride, bromine monofluoride and iodine monofluoride, have found a wide application in addition reactions to double bonds. The equivalents are obtained by reacting A -haloamides or free halogens in combination with hydrogen fluoride or its salts as the source of fluoride ions. The reactions proceed under mild conditions at — 80 to 20 "C in anhydrous hydrofluoric acid or diethyl ether, tetrahydro-furan, dichloromethane or chloroform mainly by electrophilic addition with Markovnikov-type regioselectivity (anti addition).26-28... 

Electrophilic additions with activated alkynes also occur readily with ri2-pyrrole complexes.12bl3b In methanol solution, the pyrrole (20) and 1-methylpyrrole (21) complexes undergo conjugate addition cleanly at C-3 with 3-butyn-2-one to give the p-enone-substituted pyrrole complexes 56 and 58 in high yield (Figure 13). In contrast to what is observed... 

This demonstrates the enormous increase in the rates of electrophilic additions with increasing ring strain. 

Compoimds containing functional groups which can be generally represented as X=Y=Z may react by electrophilic addition with suitable species A-B, where A is the electrophilic and B the nucleophilic centre. A general representation is shown in equation (88). This general... 

A more complicated electrophilic addition with questions of stability and selectivity added. 

Most osmium complexes of phenols [26,44], anilines [24,45], and anisoles [23, 46,47] undergo electrophilic addition with a high regiochemical preference for para addition. While electrophilic additions to phenol complexes are typically carried out in the presence of an amine base catalyst, the other two classes generally require a mild Lewis or Bronsted acid to promote the reaction. The primary advantage of the less activated arenes is that the 4H-arenium species resulting from electrophilic addition are more reactive toward nucleophilic addition reactions (see below). 

Alkynes undergo addition reactions, especially electrophilic addition, with many of the same compounds that add to alkenes. 

The reaction of l-phenylprop5me (56) with 0.1 M HCl in acetic acid solution produced five products, as shown in equation 9.66. The major product (68) was the result of s)m addition of HCl, with attachment of the proton to C2 of the reactant. There was also a stereoisomer (69) with the same regiochemistry, indicating some anti addition of HCl. Both of these products are consistent with the intermediacy of a vinyl cation-chloride ion pair. Trace amounts of the regioisomers 70 and 71 apparently were formed by electrophilic addition with initial protonation of Cl of the alkyne. Propiophenone (72) was also formed in the reaction. Apparently solvent reacts with the vinyl cation intermediate to produce vinyl acetates, which then react with trace water to hydrolyze to unstable enols that tautomerize to the ketone. 

The mechanistic details of each step have been left unspecified. The hydrocarbon activation is believed to proceed through a c complex. This may undergo oxidative addition to a Pt(IV) alkyl hydride which undergoes reductive elimination of H" to give the Pt(II)—R species. Alternatively, the process has been viewed as a concerted electrophilic addition with transfer... 

Ether cleavage forms only a substitution product because any alkene that would be formed in an elimination reaction would undergo electrophilic addition with HBr or HI to form the same alkyl halide that is obtained from the substitution reaction. 

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