Electrophiles alkene addition reactions

Two SN1 reactions occur during the biosynthesis of geraniol, a fragrant alcohol found in roses and used in perfumery. Geraniol biosynthesis begins with dissociation of dimethylallyl diphosphate to give an allylic carbocation, which reacts with isopentenyl diphosphate (Figure IT 15). From the viewpoint of isopentenyl diphosphate, the reaction is an electrophilic alkene addition, but from tile viewpoint of dimethylallyl diphosphate, the process in an Sjjl reaction in which the carbocation intermediate reacts with a double bond as the nucleophile. 

Table 8 provides the absolute rates of addition of (CF3)3C, (CF3)2CF-, CF3CF2-, and CF3 to a group of alkenes of variable reactivity. It can be seen from the Table that both the perfluoro-iso-propyl and perfluoro-ferf-butyl radicals give evidence of much greater electrophilicity in their alkene addition reactions. For example, the latter radical reacts significantly (6.8 times) faster than CF3- with the nucleophilic a-methylstyrene (IP = 8.9 eV), while reacting somewhat (1.6 times) slower than CF3- with the more electrophilic pentafluorostyrene (IP = 9.2 eV). Comparative plots of all of the available rate data for alkene additions of CF3- and (CF3)3C- vs alkene IPs, as seen in Fig. 4, leave no doubt as to the relative electrophilicity of the two species. Moreover, the rates of addition of the very electrophilic, but non-o, planar perfluoro-ferf-butyl radical to the more nucleophilic... 

Nitro-stabilized anions also undergo additions to aldehydes, ketones, and electrophilic alkenes these reactions appear in Chapters 27 and 29. 

The addition of electrophiles to aikenes is a useful and general reaction that makes possible the synthesis of many different kinds of compounds. Although we ve studied only the addition of HX thus far, many other electrophiles also add to aikenes. In this chapter, we ll see how aikenes are prepared, we ll discuss many further examples of alkene addition reactions, and we ll review the wide variety of compounds that can bo made from aikenes. 

Do not memorize the products of alkene addition reactions. Instead, for each reaction, ask yourself, "What is the electrophile " and "What nucleophile is present in the greatest concentration "... 

In their qualitative chemistry, conjugated dienes behave very much like alkenes They readily react with electrophiles in addition reactions. Just as in the case of alkenes. this addition proceeds to give the most stable intermediate. For conjugated dienes, this normally turns out to be a re.sonance stabilized allylic cation ... 

There are two main classes of molecules (substrates) that can perform oxidative additions to metal centers non-electrophilic and electrophilic. Oxidative addition reactions with either class of substrates are favored by metal complexes that are more electron rich. Common non-electrophilic substrates are H2, Si-H bonds, P-H bonds, S-H bonds, B-H bonds, N-H bonds, S-S bonds, C-H bonds, alkenes, and alkynes. An important criterion for these non-electrophillic substrates is that they require a sterically accessible open coordination site on the metal (16e configuration or lower) onto which they need to pre-coordinate before initiating the oxidative addition to the metal center. For these substrates, both ligand atoms typically become cisoidally coordinated to the metal center after the oxidative addition as anionic (T-donors (subsequent ligand rearrangements, of course can occur). H2 is the most important and common for catalysis and a well-studied reaction is shown in Equation (5). 

We have already studied one addition reaction of alkenes—hydrogenation—in which a hydrogen atom is added at each end of a double (or triple) bond. In this chapter we shall study other alkene addition reactions that do not involve the same mechanism as hydrogenation. We can depict this type of reaction generally, using E for an electrophilic portion of a reagent and Nu for a nucleophilic portion, as follows. 

Electrophilic aromatic substitution represents the second instance in which we have encountered a C = C double bond attacking an electrophile. The first instance was in our discussion of alkene addition reactions in Section 5.3. Notice the similarities in the first step where a C = C double bond attacks an electrophilic atom (H or E ). In Step 2, however, alkene addition results in the attack of a nucleophile on the carbocation, while EAS results in abstraction of a hydrogen by base. In one reaction, the C = C double bond is destroyed, while in the other, the C = C double bond is regenerated. 

A characteristic of transition states of the carbene-to-alkene addition reactions, particularly sensitive to the philicity of a carbene, is the angle of slope of the carbene plane relative to the double-bond plane. According to calculations [44, 45] one may hold that the carbenes for which in the transition states of addition to alkenes the angle a < 45° are electrophilic. The angle a > 50° is typical of nucleophilic carbenes, while the 45° < a < 50° region relates to the ambiphilic carbenes. Ab initio [44, 52, 53] and semiempirical (MNDO) [54] calculations of pathways of addition reactions of various carbenes have verified this dependence. 

The regioselectivity of electrophilic addition to an alkyne can be explained just as the regioselectivity of alkene addition reactions was explained in Section 6.4. Of the two possible transition states for the reaction, the one with a partial positive charge on the more substituted (secondary) carbon is more stable. 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

The double bond m the alkenyl side chain undergoes addition reactions that are typical of alkenes when treated with electrophilic reagents... 

The propylene double bond consists of a (7-bond formed by two ovedapping orbitals, and a 7t-bond formed above and below the plane by the side overlap of two p orbitals. The 7t-bond is responsible for many of the reactions that ate characteristic of alkenes. It serves as a source of electrons for electrophilic reactions such as addition reactions. Simple examples are the addition of hydrogen or a halogen, eg, chlorine ... 

Toluene, an aLkylben2ene, has the chemistry typical of each example of this type of compound. However, the typical aromatic ring or alkene reactions are affected by the presence of the other group as a substituent. Except for hydrogenation and oxidation, the most important reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution on the methyl group. Addition reactions to the double bonds of the ring and disproportionation of two toluene molecules to yield one molecule of benzene and one molecule of xylene also occur. 

The initial discussion in this chapter will focus on addition reactions. The discussion is restricted to reactions that involve polar or ionic mechanisms. There are other important classes of addition reactions which are discussed elsewhere these include concerted addition reactions proceeding through nonpolar transition states (Chapter 11), radical additions (Chapter 12), photochemical additions (Chapter 13), and nucleophilic addition to electrophilic alkenes (Part B, Chi iter 1, Section 1.10). 

Reactions of alkynes with electrophiles are generally similar to those of alkenes. Because the HOMO of alkynes (acetylenes) is also of n type, it is not surprising that there IS a good deal of similarity between alkenes and alkynes in their reactivity toward electrophilic reagents. The fundamental questions about additions to alkynes include the following. How reactive are alkynes in comparison with alkenes What is the stereochemistry of additions to alkynes And what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions in addition reactions of alkenes raises the question of whether similar species can be involved with alkynes, where the ring would have to include a double bond ... 

---