Alkene addition reactions stereospecific

When two substituents add to the same side of a double bond, the addition is called syn addition. When two substituents add to opposite sides of a double bond, the addition is called anti addition. Both syn and anti additions occur in alkene addition reactions that form a carbocation intermediate. Equal amounts of the four stereoisomers are obtained so the reaction is not stereoselective and, because the four stereoisomers formed by the cis alkene are identical to the four stereoisomers formed by the trans alkene, the reaction is also not stereospecific. 

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

If dichlorocarbene is generated in the presence of an alkene, addition to the double bond occurs and a dichlorocyclopropane is formed. As the reaction of dichlorocarbene with ds-2-pentene demonstrates, the addition is stereospecific, meaning that only a single stereoisomer is formed as product. Starting from a cis alkene, for instance, only cis-disubstituted cyclopropane is produced starting from a trans alkene, only trans-disubstituted cyclopropane is produced. 

In investigating the mechanism of addition to a double bond, perhaps the most useful type of information is the stereochemistry of the reaction. The two carbons of the double bond and the four atoms immediately attached to them are all in a plane (p. 8) there are thus three possibilities. Both Y and W may enter from the same side of the plane, in which case the addition is stereospecific and syn they may enter from opposite sides for stereospecific anti addition or the reaction may be nonstereospecific. In order to determine which of these possibilities is occurring in a given reaction, the following type of experiment is often done YW is added to the cis and trans isomers of an alkene of the form ABC=CBA. We may use the cis alkene as an example. If the addition is syn, the product will be the erythro dl pair, because each carbon has a 50% chance of being attacked by Y ... 

In marked contrast, we used the cyclopropanation reaction of (phosphino)(silyl)-carbene (la) with methyl acrylate to demonstrate the carbene nature of our compound. More recently, we studied in detail the stereochemistry of this type of reaction.The (phosphino)(silyl)carbene (la) reacted efficiently with 3,3,4,4,5, 5,6,6,6-nonafluorohex-l-ene and (Z)- and ( )-2-deuteriostyrene giving the corresponding cyclopropanes in good yields. The stereochemical outcome was such that all monosubstituted alkenes gave exclusively the syn isomer (with respect to the phosphino group), and the addition of disubstituted alkenes was totally stereospecific (Scheme 8.16). 

Oxymercuration-reduction of alkenes preparation of alcohols Addition of water to alkenes by oxymercuration-reduction produces alcohols via Markovnikov addition. This addition is similar to the acid-catalysed addition of water. Oxymercuration is regiospecific and auft -stereospecific. In the addition reaction, Hg(OAc) bonds to the less substituted carbon, and the OH to the more substituted carbon of the double bond. For example, propene reacts with mercuric acetate in the presence of an aqueous THF to give a hydroxy-mercurial compound, followed by reduction with sodium borohydride (NaBH4) to yield 2-propanol. 

Electrophilic addition reactions of tetravalent tellurium compounds have been reviewed.64 2-Naph-thyltellurium trichloride (ArTeCb) adds to alkenes in an anti stereospecific manner (equation 11), whereas tellurium tetrachloride gives mixtures of 2 1 adducts with both syn and anti addition.72 A one-pot alkene inversion procedure has been developed, based upon TeCU addition to alkenes followed by treatment of the (3-chloroalkyltellurium trichloride adduct with aqueous Na2S (Scheme 37).73 Tellurium compounds such as tellurinyl acetates, ArTe(0)0Ac, prepared in situ through reaction of tellurinic acid anhydrides with acetic acid, can be employed in oxytelluration and aminotelluration procedures (Schemes 38 and 39).74 In the oxytelluration reaction intermediate triacetates of the type RCH2Te(OAc)2Ph are reduced with hydrazine to the corresponding tellurides. 

Well before the wide use of organoselenium compounds in chemistry, it was discovered that electrophilic selenium compounds of the type RSeX add stereospecifically to alkenes.45 Since that time this reaction has been an important tool in the portfolio of organic chemists and has been used even for the construction of complex molecules. Comprehensive reviews on this chemistry have appeared46-49 and in recent times the synthesis of chiral selenium electrophiles and their application in asymmetric synthesis has emerged. As shown in Scheme 1, the addition reactions of selenium electrophiles to alkenes are stereospecific anti additions. They involve the initial formation of seleniranium ion intermediates 1 which are immediately opened in the presence of nucleophiles. External nucleophiles lead to the formation of addition products 2. The addition to unsymmetrically substituted alkenes follows the thermodynamically favored Markovnikov orientation. The seleniranium ion intermediates of alkenes with internal nucleophiles such as 3 will be attacked intramolecularly to yield cyclic products 4 and 5 via either an endo or an exo pathway. Depending on the reaction conditions, the formation of the seleniranium ions can be reversible. 

The complexes will effect oxyamination reaction with alkenes in a stereospecific reaction (Scheme 8).290 After reductive cleavage of the intermediate alkanolaminato complex (I) (see below) vicinal amino alcohols (II) are formed. The reaction is unusual in that the new C—N bond is always formed at the least substituted terminal alkenic carbon atom, and there is a clear preference for the imido complex to use its NR group for coordination to the osmium despite the steric restraints of R.299,300 However, the least sterically hindered part of the alkene moiety is attached to. the nitrogen atom.290,291,300 The yields of amino alcohols in the reaction can be improved by addition of tertiary alkyl bulkhead amines (see below). 

Moreno and coworkers published a study on the triplet carbene-ethene addition reaction. This process should involve two steps, namely the formation of a triplet trimethylene 1,3-diradical as an intermediate followed by intersystem crossing and formation of 1. The intermediate may live long enough to permit rotation at the CC bonds. In this way, the stereochemistry of the alkene will be lost and a non-stereospecific addition takes place. Moreno and coworkers calculated for the first step of the reaction C 2C i) H2C = CH2 a barrier of 11 kcalmol" and a reaction energy of -26 kcalmoF at the MP2/3-21G level. (It has to be mentioned in this connection that the 3-21G basis is far too small to lead to reliable energies in correlation calculations and therefore results are just of a qualitative nature ) Activation energies varied from 5 to 17 kcalmoF if CH2 was replaced by the triplet state of CH(CN), CH(BeH) and CHLi ... 

One of the most important addition reactions which involves nitrogen nucleophiles is the azidoselenenylation of alkenes because both the azido and the phenylseleno group can give rise to several useful transformations. These compounds were first prepared by Krief from the reaction of alkenes with PhSeBr followed by sodium azide in DMF [54]. Direct azidoselenenylation of alkenes can be effected with PhSeCl and sodium azide in DMSO [55]. Under these conditions the reaction is stereospecific but it is not regiospecific. Similarly, the reaction of exocyclic alkenes with N-(phenylseleno)phthalimide and azido-trimethylsilane in methylene chloride (Scheme 11) gives rise to a 1 1 mixture of the regioisomers 67 and 68 [56]. 

The best experimental conditions to introduce a phenylseleno and an azido group to the alkene double bond are those which employ diphenyl diselenide, sodium azide and iodobenzene diacetate in methylene chloride. Under these conditions, however, the addition reaction occurs through the radical mechanism illustrated in Scheme 12 [581. The addition therefore occurs with an anti Markovnikov orientation and it is not stereospecific. The reaction is initiated by the oxidation of the azido anion to the azido radical, which adds to the alkene to afford a carbon radical. This is trapped by the PhSeSePh to afford the final product and a PhSe radical, which dimerizes to give the diselenide. 

In contrast with other electrophilic additions, the peracid epoxidation is syn-stereospecific. With sterically strongly hindered alkenes the reaction takes place on the less sterically hindered side. In other cases, the stereochemistry of the reaction is affected by polar effects or the geometry of the transition state. Important conclusions regarding the mechanism of the reaction can be drawn from the steric pathways in the synthesis of the oxiranes. This has been dealt with comprehensively by Berti, who reviewed the topic up to 1971, with special emphasis on the peracid oxidation. A noteworthy account of the topic of peracid epoxidation is given in a review by Rebek. ... 

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