Addition reactions of alkenes and alkynes

The importance of such reactions to synthetic organic chemistry is paramount. It is our intention in this and the following chapter to show the great diversity, utility, and specificity of addition reactions of alkenes and alkynes. 

Such structure reactions have existed in organic chemistry since long. Wilson gives an example of classification of electrophilic addition reactions of alkenes and alkynes. Patterns in organometalhc chemistry with applications in organic synthesis have been discussed by Schwartz and Labinger. 

Various addition reactions of alkenes and alkynes have been termed insertion processes. As the term insertion per se is a nonchemical term, it might be conveniently substituted with hydrometallation (H—M), carbometallation (C—M), heteroatom-metallation or heterometallation (X—M), and metallometallation (M—M), depending on the (7-bond that is added to tt compounds. These addition reactions involving Pd are represented by the general equations using alkenes as representative tt compounds as shown in Scheme 7. The alkenes in Scheme 7 may be replaced with alkynes and other 7T compounds. 

A major factor in determining the magnitude of relative rates is the solvent used. Thus, high alkene alkyne reactivity ratios are found in organic solvents of low dielectric constant (acetic acid), but the rates of reaction are comparable when water is the solvent. This is true not only for the hydration but also for bromination. Polar solvents, water in particular, are evidently capable of minimizing the energy difference between the two transition states. Whether this is accomplished by very strong solvation or by some other mechanism is not entirely clear. Addition reactions of alkenes and alkynes with trifluoroacetic acid also take place at comparable rates. ... 

Until recently, iron-catalyzed hydrogenation reactions of alkenes and alkynes required high pressure of hydrogen (250-300 atm) and high temperature (around 200°C) [21-23], which were unacceptable for industrial processes [24, 25]. In addition, these reactions showed low or no chemoselectivity presumably due to the harsh reaction conditions. Therefore, modifications of the iron catalysts were desired. 

Organometallic complexes of the /-elements have been reported that will perform both intra-and intermolecular hydroamination reactions of alkenes and alkynes, although these lie outside of the scope of this review.149-155 Early transition metal catalysts are not very common, although a number of organometallic systems exist.156-158 In these and other cases, the intermediacy of a metal imido complex LnM=NR was proposed.159,160 Such a species has recently been isolated (53) and used as a direct catalyst precursor for N-H addition to alkynes and allenes (Scheme 35).161,162... 

One of the most general and useful reactions of alkenes and alkynes for synthetic purposes is the addition of electrophilic reagents. This chapter is restricted to reactions which proceed through polar intermediates or transition states. Several other classes of addition reactions are also of importance, and these are discussed elsewhere. Nucleophilic additions to electrophilic alkenes were covered in Chapter 1, and cycloadditions involving concerted mechanisms will be encountered in Chapter 6. Free-radical addition reactions are considered in Chapter 10. 

Reactions of alkenes and alkynes that generate a carbon-metal bond by nucleophilic addition to a metal ir-complex and subsequently undergo carbon monoxide insertion to yield a carbonyl product are... 

Research on intermolecular hydroacylation has also attracted considerable attention. The transition-metal-catalyzed addition of a formyl C-H bond to C-C multiple bonds gives the corresponding unsymmetrically substituted ketones. For the intermolecular hydroacylation of C-C multiple bonds, ruthenium complexes, as well as rhodium complexes, are effective [76-84]. In this section, intermolecular hydroacylation reactions of alkenes and alkynes using ruthenium catalysts are described. 

Several heteroatom nucleophiles, for example, amines, alcohols, thiols, carboxylates, and dialkylphosphines, undergo Michael addition reactions with alkene- and alkyne-substituted carbene complexes. Reaction of alkyne-substituted chromium carbenes with urea affords products derived from Michael... 

In the remainder of Chapter 12, the oxidation of alkenes, alkynes, and alcohols— three functional groups already introduced in this text—is presented (Figure 12.8). Addition reactions to alkenes and alkynes that increase the number of C—O bonds are described in Sections 12.8-12.11. Oxidation of alcohols to carbonyl compounds appears in Sections 12.12-12.14. 

The principal reactions of alkenes and alkynes are addition reactions rather than substitution reactions. For example, contrast the reactions of ethane and ethylene with CI2. 

Carbon-carbon double bonds are reaction sites and so represent functional groups. Most addition reactions involving alkenes and alkynes proceed rapidly at room temperature. By contrast, many substitution reactions of the alkanes require catalysts and high temperatures. 

Addition is the characteristic reaction of alkenes and alkynes. Since the carbons of a double or triple bond do not have the maximum number of attached atoms, they can add additional groups or atoms. Double bonds undergo addition once and triple bonds can undergo addition twice. The reactivity of alkenes and alkynes is due to the presence of pi-bonds. Unlike sigma bonds, pi-bonds are directed away from the carbons the electrons are loosely held, very accessible, and quite attractive electron-deficient species (electrophiles) seeking an electron source. 

Additions. Hydrocarboxylation of alkenes" and alkynes with HCOOH under CO is accomplished by Pd catalysis. A similar reaction undergone by a propar-gylic carbonate after an S 2 process leads to an itaconic diester segment.The intermediate is converted into a cyclopentenone when the propargyl carbonate... 

Radical Addition to Alkenes and Alkynes. Samarium(II) iodide has proven effective for initiation of various radical addition reactions to alkenes and alkynes. Typically, tin reagents are used in the initiation of these radical cyclization reactions however, the Sml2 protocol often provides significant advantages over these more traditional routes. 

The fluorination of alkenes 41 and 43 and alkynes 45 with difluoroiodotoluene in the presence of iodine affords vt c-fluoroiodoalkanes 42 and 44 and fluoroiodoalkenes 46 in moderate to good yields (Scheme 3.16) [40]. This reaction proceeds in a Markovnikov fashion and with prevalent an//-stereoselectivity via initial addition of the electrophilic iodine species followed by nucleophilic attack of fluorine anion. The analogous reaction of alkenes and alkynes with difluoroiodotoluene in the presence of diphenyl diselenides affords the respective products of phenylselenofluorination in good yields [41],... 

The presence of carbon-carbon double or triple bonds in hydrocarbons markedly increases their chemical reactivity. The most characteristic reactions of alkenes and alkynes are addition reactions, in which a reactant is added to the two atoms that form the multiple bond. A simple example is the addition of a halogen to ethylene ... 

Despite its molecular formula, benzene for the most part does not behave as if it were unsaturated. For instance, it does not decolorize bromine solutions the way alk-enes and alkynes do (Sec. 3.7a), nor is it easily oxidized by potassium permanganate (Sec. 3.17a). It does not undergo the typical addition reactions of alkenes or alkynes. Instead, benzene reacts mainly by substitution. For example, when treated with bromine (Br2) in the presence of ferric bromide as a catalyst, benzene gives bromobenzene and hydrogen bromide as products. 

The TT-electrons in both alkenes and alkynes provide a Lewis-base site for interaction with electrophilic reagents, which are Lewis acids, so one of the typical reactions of alkenes and alkynes is electrophilic addition as illustrated in Equations 11.2 and 11.3. Such reactions of alkynes can be stopped after the addition of one equivalent of a reagent, but the use of excess reagent leads to the formation of saturated products in which a second equivalent has been added. Some aspects of the chemistry of alk)mes will be explored in the experiments in this chapter. 

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