Electrophilic carbon, addition

Ester (9) can easily be made from acid (H)- You might consider two approaches to this a one-carbon electrophile addition via chloromethylation (Table T 2.2) and oxidation or FGl (Table 2,3) back to p-chlorotoluene (12). The latter is easier on a large scale. The p-chlorotoluene (12) can be made either by direct chlorination of toluene or by the diazotisation route (p T 12) again from toluene. 

An alkyne is less reactive than an alkene. A vinyl cation is less able to accommodate a positive charge, as the hyperconjugation is less effective in stabilizing the positive charge on a vinyl cation than on an alkyl cation. The vinyl cation is more stable with positive charge on the more substituted carbon. Electrophilic addition reactions allow the conversion of alkenes and alkynes into a variety of other functional groups. 

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

Section 11 16 Addition reactions to alkenylbenzenes occur at the double bond of the alkenyl substituent and the regioselectivity of electrophilic addition is governed by carbocation formation at the benzylic carbon See Table 11 2... 

Electrophilic addition (Section 11 16) An aryl group stabilizes a benzylic carbocation and con trols the regioselectivity of addition to a double bond involving the benzylic carbon Markovni kov s rule is obeyed... 

The diminished rr electron density m the double bond makes a p unsaturated aide hydes and ketones less reactive than alkenes toward electrophilic addition Electrophilic reagents—bromine and peroxy acids for example—react more slowly with the carbon-carbon double bond of a p unsaturated carbonyl compounds than with simple alkenes... 

The electron-rich carbon—carbon double bond reacts with reagents that are deficient in electrons, eg, with electrophilic reagents in electrophilic addition (6,7), free radicals in free-radical addition (8,9), and under acidic conditions with another butylene (cation) in dimerization. 

Electrophilic additions to allenes represent an interesting reaction type which is related to additions to both alkenes and alkynes. An allene could, for example, conceivably be protonated at either a terminal s[p- carbon or the central sp carbon. 

The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they act as electron-releasing groups promoting addition and also control the regiochemistry. A silyl or starmyl substituent strongly stabilizes carboca-tion character at the /3-catbon atom and thus directs the electrophile to the a-carbon. The reaction is then completed by the limination step, in which the carbon-sihcon or carbon-tin bond is broken. 

The effects of a- vs. P-fluonnation imply that fluoroolefms normally react regioselectively with electrophiles to minimize the number of fluorines p to the electron-deficient carbon in the transition state. Diverse types of electrophilic additions follow this rule (equations 6-8, for example), although there aie exceptions, especially for ionic addiPons of halomethanes to fluoroolefins [124]... 

A hypsochromic shift of 20-50 cm is observed in the double-bond stretching region, when the enamines are converted to the corresponding iminium salts by the electrophilic addition of a proton at the /3-carbon atom. The shift is accompanied by an enhancement in the intensity of the band. Leonard and co-workers (68,71-74) have used this absorption shift as a diagnostic tool for the determination of the position of the double bond... 

Markovnikov s rule is used to predict the regiochemistry of HX (electrophilic) addition reactions. The rule states that HX adds to an unsymmetrical alkene mainly in the direction that bonds H to the less substituted alkene carbon and X to the more substituted alkene carbon. 

In addition to electrophilic attack on the pyrrole ring in indole, there is the possibility for additions to the fused benzene ring. First examine the highest-occupied molecular orbital (HOMO) of indole. Which atoms contribute the most What should be the favored position for electrophilic attack Next, compare the energies of the various protonated forms of indole (C protonated only). These serve as models for adducts formed upon electrophilic addition. Which carbon on the pyrrole ring (C2 or C3) is favored for protonation Is this the same as the preference in pyrrole itself (see Chapter 15, Problem 2)1 If not, try to explain why not. Which of the carbons on the benzene ring is most susceptible to protonation Rationalize your result based on what you know about the reactivity of substituted benzenes toward electrophiles. Are any of the benzene carbons as reactive as the most reactive pyrrole carbon Explain. 

Thus, dearomatization of the furan heterocycle in the 4,5-ti complexes causes electrophilic addition mainly at the uncoordinated carbon C3. The other possibility... 

In reaction with an alkene, initially a three-membered ring Lewis acid/Lewis base-complex 5 is formed, where the carbon-carbon double bond donates r-electron density into the empty p-orbital of the boron center. This step resembles the formation of a bromonium ion in the electrophilic addition of bromine to an alkene ... 

Various side-reactions may complicate the course of the Nef reaction. Because of the delocalized negative charge, the nitronate anion 2 can react at various positions with an electrophile addition of a proton at the a-carbon reconstitutes the starting nitro alkane. 1. The nitrite anion can act as leaving group, thus leading to elimination products. 

Oxidative addition—Reaction of the carbon electrophile with palladium-(0) complex 5 to give a palladium-(II) complex 6. 

Carbon-carbon double bonds are present in most organic and biological molecules, so a good understanding of their behavior is needed. In this chapter, we ll look at some consequences of alkene stereoisomerism and then focus on the broadest and most general class of alkene reactions, the electrophilic addition reaction. 

Before beginning a detailed discussion of alkene reactions, let s review briefly some conclusions from the previous chapter. We said in Section 5.5 that alkenes behave as nucleophiles (Lewis bases) in polar reactions. The carbon-carbon double bond is electron-rich and can donate a pair of electrons to an electrophile (Lewis acid), for example, reaction of 2-methylpropene with HBr yields 2-bromo-2-methylpropane. A careful study of this and similar reactions by Christopher Ingold and others in the 1930s led to the generally accepted mechanism shown in Figure 6.7 for electrophilic addition reactions. 

Aikene chemistry is dominated by electrophilic addition reactions. When HX reacts with an unsymmetrically substituted aikene, Markovnikov s rule predicts that the H will add to the carbon having fewer alky) substituents and the X group will add to the carbon having more alkyl substituents. Electrophilic additions to alkenes take place through carbocation intermediates formed by reaction of the nucleophilic aikene tt bond with electrophilic H+. Carbocation stability follows the order... 

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

The following carbocation is an intermediate in the electrophilic addition reaction of HCl with two different alkenes. Identify both, and tell which C-H bonds in the carbocation are aligned for hyperconjugation with the vacant p orbital on the positively charged carbon. 

One of the most striking differences between conjugated dienes and typical alkenes is in their electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 6.7). Markovnikov regiochemistry is found because the more stable carbo-cation is formed as an intermediate. Thus, addition of HC1 to 2-methylpropene yields 2-chloro-2-methylpropane rather than l-chloro-2-methylpropane, and addition of 2 mol equiv of HC1 to the nonconjugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

Conjugated dienes also undergo electrophilic addition reactions readily, but mixtures of products are invariably obtained. Addition of HBr to 1,3-butadiene, for instance, yields a mixture of two products (not counting cis-trans isomers). 3-Bromo-l-butene is the typical Markovnikov product of 1,2-addition to a double bond, but l-bromo-2-butene appears unusual. The double bond in this product has moved to a position between carbons 2 and 3, and HBr has added to carbons 1 and 4, a result described as 1,4-addition. 

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