Alkenes regioselective addition

The regioselective addition of nitryl iodide to alkenes, followed by base-induced elimination, gives nitroalkenes. Nitryl iodide is generally prepared by the reaction of AgN02 and iodine. 

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... 

One of the earliest uses of palladium(II) salts to activate alkenes towards additions with oxygen nucleophiles is the industrially important Wacker process, wherein ethylene is oxidized to acetaldehyde using a palladium(II) chloride catalyst system in aqueous solution under an oxygen atmosphere with cop-per(II) chloride as a co-oxidant.1,2 The key step in this process is nucleophilic addition of water to the palladium(II)-complexed ethylene. As expected from the regioselectivity of palladium(II)-assisted addition of nucleophiles to alkenes, simple terminal alkenes are efficiently converted to methyl ketones rather than aldehydes under Wacker conditions. 

Azasulfenylation of alkenesf s The adducts of dimethyl(methylthio)-sulfonium tetrafluoroborate (1) with alkenes react slowly but smoothly with various nitrogen nucleophiles to give products of overall rrarw-addition to the alkene. Regioselectivity depends on the substitution pattern of the alkene and on the nucleophilicity of the attacking reagent, and is subject to some control. The mechanism of this azasulfenylation is not certain it may involve an episulfonium ion. 

Preparatively, it is important that mineral acids, carboxylic acids, and terf-carbenium ions can be added to alkenes via carbenium ion intermediates. Because of their relatively low stability, primary carbenium ions form more slowly in the course of such reactions than the more stable secondary carbenium ions, and these form more slowly than the even more stable tertiary carbenium ions (Hammond postulate ). Therefore, mineral and carboxylic acids add to unsymmetrical alkenes regioselectively to give Markovnikov products (see Section 3.3.3 for an explanation of this term). In addition, these electrophiles add most rapidly to those alkenes from which tertiary carbenium ion intermediates can be derived. 

Xfi-Dienes. The reagent undergoes a light-catalyzed regioselective addition to al-kenes, particularly mono-, 1,1-di-, and trisubstituted alkenes. The adducts undergo de-hydrobromination to a,p-unsaturated bromomethyl sulfones. On treatment with potassium... 

The radical addition of perfluoroalkyl halides to teraiinal alkynes is performed under conditions similar to those used for alkenes. Regioselective attack of the radical at the less-hindered carbon is observed, e.g. formation of 1. The monoaddition product so formed is often a mixture of E- and Z-vinylic halides (see Table 5). 

In regioselective addition to a chiral alkene, four diastereomers may be formed by cis or trans addition to both diastereotopic faces of the double bond. 

One of the methods for the synthesis of alkyl azides is based on the regioselective addition of mercury(II) azide, generated from mercury(II) acetate and sodium azide, to alkenes in water/te-trahydrofuran (1 1) to afford /i-azidoalkylineiciiry compounds which are then reductively demercurated to alkyl azides5-7. Completely diastereoselective cis addition occurred with norbornene (exo addition)5 and cyclopropenes6. 

The regioselective amination of 2,3-dihydrofuran with morpholine catalyzed by palladium thiocyanate diphosphine complexes (45 turnovers)102, and the regioselective addition of aniline to acrylonitrile and analogous electron-poor alkenes catalyzed by dialkylpalladium(dmpe) complexes (hundreds of turnovers)103 have recently been reported, but the scope of the reactions is limited to the specific type of alkene. 

Metal complex catalyzed hydrosilations usually give terminal silylation, even with internal alkenes. The additions occur in a cis fashion and the stereochemistry at silicon is retained. However, great variations in regioselectivity have been observed for the same reagents with different catalysts. No stereochemical generalizations can be made. The... 

Steric effects are used to explain the regioselective addition of radicals to alkenes. The radical preferentially attacks the less substituted (hindered) end of the double bond to give a less strained transition state with lower AH0 in an anti-Markovnikov-type reaction. If we introduce substituents on to the double bond, the rate of addition is lowered because of greater steric interaction. Thus, for example, the methyl radical will add three times more quickly to ethene (CH2CH2) than to the disubstituted alkene ( )-2-butene. 

The regioselective addition of HX to alkenes produces the more substituted alkyl halide, which is known as the Markovnikov (Markovnikoff) product. Markovnikov s rule states that on addition of HX to an alkene, H attaches to the carbon with fewest alkyl groups and X attaches to the carbon with most alkyl groups . This can be explained by the formation of the most stable intermediate carbocation. 

The live-ring closure of prostereogenic radicals constructs two new stereogenic centers with good diastereoselectivity, as demonstrated by the trimethyltin hydride mediated cyclization of an ester-substituted diene4. The reaction proceeds via regioselective addition of the stannyl radical to the terminal end of the less substituted alkene and subsequent cyclization. 

The diphenylmethyl oxazolidinone proves to be effective in promoting selective additions of radicals to unsymmetrical fumarates 21 [25]. Thus, addition of isopropyl radical to 16 proceeds regioselectively, addition to the carboethoxyl side of the alkene being favored in the absence of Lewis acid 22/23 = 11 1. Diaster-eoselectivity for formation of 17 in the absence of Lewis acid was poor, about 1.6 1. Several lanthanide and prelanthanide Lewis acids used stoichiometrically improve... 

A study of several substituted alkenes in methanol developed some generalizations pertaining to the capture of bromonium ions by methanol For both E- and Z-disubstituted alkenes, the addition of both methanol and Br was completely anti stereospecific. The reactions were also completely regioselective, in accordance with Markovnikov s rule, for disubstituted alkenes, but not for mono substituted alkenes. The lack of high regioselectivity of the addition to monosubstituted alkenes can be interpreted as competitive addition of solvent at both the mono- and unsubstituted carbons of the bromonium ion. This competition reflects conflicting steric and electronic effects. Steric factors promote addition of the nucleophile at the unsubstituted position, whereas electronic factors have the opposite effect. 

Thus alkynes, like alkenes, undergo electrophilic addition reactions. We will see that the same electrophilic reagents that add to alkenes also add to alkynes and that— again like alkenes—electrophilic addition to a terminal alkyne is regioselective When an electrophile adds to a terminal alkyne, it adds to the sp carbon that is bonded to the hydrogen. The addition reactions of alkynes, however, have a feature that alkenes do not have Because the product of the addition of an electrophilic reagent to an alkyne is an alkene, a second electrophilic addition reaction can occur. 

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