Addition That Generates Carbocation Intermediates

This type of reaction begins when a ir bond of an alkene donates an electron pair to an acid (H+)—an acid-base reaction where the alkene is a weak base. The Jt bond is broken as the new Br—H bond is formed, and the remaining carbon of the former double bond becomes a carbocation. The reaction of cyclohexene with acid to form secondary cation 294 illustrates this process. The cationic center then reacts with the nucleophilic gegenion (Br from HBr) to produce bromocyclohexane. The latter portion of this sequence is analogous to the second step (coupling) of an Sjsfl reaction. The initial reaction usually involves formation of a solvent separated carbocation intermediate, but this depends on the solvent. A tight ion pair intermediate can react in the substitution step to give the same product. The net result of this cationic reaction is addition of H and Br across the jt bond. 

Markovnikov addition, - of simple alkenes. In other words, addition of HCl, HBr, or HI to substituted alkenes will lead to an alkyl halide where the nucleophile is attached to the more substituted carbon of the Jt-bond. The basis of this regioselectivity is, of course, formation of the more stable carbocation, 295, which is attacked by bromide ion to give 1-bromo-l-methylcyclohexane (296). This is the normal course of all reactions with acid and an alkene if the intermediate is a cation. In section 2.7.B.ii, the reaction of 100 with HCl is an example of this type of reaction. 

Chapter 2. Acids, Bases. Functional Group Exchanges 

The disconnection for carbocation addition with strong acids HX is  

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It is clear that atoms other than hydrogen can be electron deficient and function as electron pair acceptors. Can a carbon atom function as a Lewis acid The answer is yes, if the definition is modified somewhat. Various reactions generate carbocation intermediates (see 55 and 58) and a Lewis base can certainly donate electrons to that positive carbon. A species that donates electrons to carbon is called a nucleophile (see Section 6.7), so an electron donor that reacts with 55 or with 58 is a nucleophile. In addition to carbocations, which are charged species, the carbon atom in a polarized bond is electron deficient, and a nucleophile could donate electrons to the 6+ carbon. This is the basis of many organic reactions to be discussed, particularly in Chapter 11. The fundamental concept of a species donating electrons to a carbon is introduced in this section, with the goal of relating this chemical reactivity to the Lewis acid-Lewis base definitions used in previous sections. 

Competition studies reported by Kuwajima, " which also complement the results of Nakai," illustrate the limitations of the 3-effect as a tool for predicting the outcome of vinylsilane-terminated cyclizations (Scheme 4). Acylium ion initiated cyclizations of (7a) and (7b) gave the expected cyclopentenones (8a) and (8b). However, compound (7c), upon treatment with titanium tetrachloride, gave exclusively the cyclopentenone proiduct (8c) arising fr the chemoselective addition on the 1,1-disubstituted alkene followed by protodesilylation of the vinylsilane. The reversal observed in the mode of addition may be a reflection of the relative stabilities of the carbocation intermediates. The internal competition experiments of Kuwajima indicate that secondary 3-silyl cations are generated in preference to secondary carbocations (compare Schemes 3 and 4), while tertiary carbocations appear to be more stable than secondary 3-silyl cari ations, as judged by the formation of compound (te). 

Electrophilic addition of HCl to a conjugated diene involves the formation of allylic carbocation intermediates. Thus, the first step is to protonate the two ends of the diene and draw the resonance forms of the two allylic carbocations that result. Then allow each resonance form to react with C , generating a maximum of four possible products. 

Benzene s aromaticity causes it to undergo electrophilic aromatic substitution reactions. The electrophilic addition reactions characteristic of alkenes and dienes would lead to much less stable nonaromatic addition products. The most common electrophilic aromatic substitution reactions are halogenation, nitration, sulfonation, and Friedel-Crafts acylation and alkylation. Once the electrophile is generated, all electrophilic aromatic substitution reactions take place by the same two-step mechanism (1) The aromatic compound reacts with an electrophile, forming a carbocation intermediate and (2) a base pulls off a proton from the carbon that... 

Notably, 2-substitution on the indole moiety led to enhaneed selectivity, indicative of beneficial steric effects [e.g. when is H, only 11% enantiomeric excess is observed). Additionally, as solvent polarity decreased, increases in both rate and stereoselectivity were observed. Screening other organocatalysts and other proline derivatives identified L-proline as the optimal catalyst this was suggestive of the important role of proline s carbojylate moiety in generating the iminium-like indole carbocation. It was postulated that proline s carbo>ylate moiety could interact via electrostatic interactions with the carbocation intermediate, which was thought to explain solvent effects. ... 

Alkenes readily react with chlorine to give chloronium ions, which prefer to exist in the open form, that is, with a carbocation intermediate initial addition of chlorine may occur at the top or bottom face, which gives enantiomeric intermediate carbocations. Either of these intermediates may be intercepted by water to give a chlorohydrin product, with an ont/ -outcome in which water attacks at the opposite side to chlorine, and as a racemic pair. Deprotonation of the oxonium intermediate generates the chlorohydrin product. 

We may seem to have contradicted ourselves because Equation 10-1 shows a carbocation to be formed in bromine addition, but Equation 10-5 suggests a bromonium ion. Actually, the formulation of intermediates in alkene addition reactions as open ions or as cyclic ions is a controversial matter, even after many years of study. Unfortunately, it is not possible to determine the structure of the intermediate ions by any direct physical method because, under the conditions of the reaction, the ions are so reactive that they form products more rapidly than they can be observed. However, it is possible to generate stable bromonium ions, as well as the corresponding chloronium and iodonium ions. The technique is to use low temperatures in the absence of any strong nucleophiles and to start with a 1,2-dihaloalkane and antimony penta-fluoride in liquid sulfur dioxide ... 

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