Electrophilic addition nucleophilic trapping

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. 

Finally, in the case of the geometrical isomers 164a,b, only products from an exo addition to the endocyclic double bond followed by homoallylic rearrangement are observed140, both in methanol and in methylene chloride. The electrophilic attack is exo specific, while the subsequent nucleophilic trapping by methanol or chloride proceeds non-stereospecifically giving equal amounts of 165 and 166 (equation 133). 

In contrast, the a-peroxy lactones, also members of the dioxetane family, display a higher reactivity toward nucleophiles, in view of the inherent polarization of the peroxide bond by the carbonyl functionality. Consequently, the nucleophilic attack is expected to take place at the more sterically hindered but more electrophilic alkoxy-type oxygen atom of the peroxide bond. A recent detailed study of the oxidation of various di-, tri-and tetrasubstituted alkenes 6 with dimethyl a-peroxy lactone (7) revealed, however, much complexity, as illustrated in Scheme 7 for R = CH3, since cycloaddition (8), ene-reaction (9 and 10) and epoxidation (11) products were observed. In the presence of methanol, additionally the trapping products 12 and 13 were obtained, at the expense of the polyester 14. The preferred reaction mode is a sensitive function of the steric demand imposed by the attacking alkene nucleophile. 

The normal course of reaction of alkenes involves addition of Lewis acids (electrophiles) yielding an intermediate carbocation which is trapped by a weak nucleophile [114]. The most common electrophilic addition reactions are summarized in Ligure 6.1. If the olefin is unsymmetrically substituted, the question of regioselectivity arises. We begin by examining the effects on the olefin n system of three classes of substituents as... 

The general reaction sequence has been named more specifically as tandem vicinal dialkylation, nucleophilic-electrophilic carbacondensation,11 dicarbacondensation1213 or conjugate addition-enolate trapping,14 usually in reference to the fact that most of the reaction examples available create two new vicinal carbon-carbon bonds. Many noncarbon nucleophiles and electrophiles also are known, resulting... 

Electrophilic addition to double bonds gives three-membered ring intermediates with Br2, with Hg2+, and with peroxy-acids (in which case the three-membered rings are stable and are called epoxides). All three classes of three-membered rings react with nucleophiles to give 1,2-difunctionalized products with control over (1) regioselectivity and (2) stereoselectivity. Protonation of a double bond gives a cation, which also traps nucleophiles, and this reaction can be used to make alkyl halides. Some of the sorts of compounds you can make by the methods of this chapter are shown below. 

The Adg2 is the major addition pathway of alkenes and dienes. Electrophile addition to a pi bond or protonation of a pi bond, path Ag, produces the most stable cation, which is then trapped by a nucleophile, path An. (Section 4.4.2 discusses the various addition types.) If the electrophile is a proton, this reaction is the reverse of the El reaction. The reaction commonly produces a mixture of syn and anti addition. 

The trapping of a cation by a nucleophile. An, can be considered the electrophilic addition of a cation to a lone pair. 

Cations react rapidly and indiscriminately with nucleophiles. Thus if a reaction which is suspected to go via a cationic intermediate is carried out in the presence of an added nucleophile, and an adduct containing the new nucleophile is obtained, this provides evidence for a cationic intermediate which is trapped by the added nucleophile. For example, the addition of bromine to alkenes is thought to go via a cationic intermediate 4 (reaction 5.11). If chloride or nitrite ions are added to the reaction mixture, the chloride or nitrite adducts 5 and 6 are obtained, even though the chloride and nitrite ions do not react with ethene (or 1,2-dibromoethane) directly at a rate that would account for the amount of these products in the reaction mixture. This provides strong evidence for the intermediate bromoethyl cation 4, which will be trapped by the added chloride or nitrite ions (reactions 5.12 and 5.13). The structure of bromoalkyl cations is discussed further in the section on electrophilic addition. 

Electrophilic addition Addition to a double bond through attack by an electrophile followed by trapping of the resulting carbocation by a nucleophile. 

Brown notes that the formation in solvolysis of tight ion pairs rather than of free carbocations turns the rate of reactions of ions with nucleophiles to be lower than that of reaching an equilibrium between epimers. Therefore in order to trap the classical 2-norbornyl cation it is necessary not to use solvolysis, but other reactions in which more free carbocations are formed. The electrophilic addition of acids to norbomene belong to these reactions. Brown considers results obtained to be due to the capture of the classical 2-norbomyl cation before a complete equilibrium is established ... 

---