Alkene addition reactions stereoselective

An addition to an alkene can form up to two new chiral centers, and a reaction that occurs with only a syn or only an anti addition mechanism will give a product with predictable stereochemistry. Conversion of alkynes to alke-nes can also occur with either syn or anti stereoselectivity. When these alkyne reductions are taken in combination with alkene addition reactions, target molecules with a wide variety of stereochemical relationships can be prepared. 

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. 

The overall result of the sequence hydroboration -I- oxidation is a regioselective and-Markownikoff-addition of water to an alkene. This reaction is an important method in organic synthesis, since it can be made stereoselective and even enantioselective. 

Since alkenes are relatively impotent precursors to aziridines, especially with regard to stereoselective reactions, substantially greater advances have been made in this field by means of the addition reactions between imines and a range of car-bene equivalents. 

This means that if a reaction is carried out on a compound that has no stereoisomers, it cannot be stereospecific but at most stereoselective. The concerted reactions, including SN2 displacements, E2 elimination of alkyl halides, anti and Syn addition to alkenes are all stereoselective. In the case of chiral or geometric substrates the nature of the product depends on the unique stereoelectronic requirement of the reaction. These are examples of stereospecific reactions. 

In the case of electrophilic addition, the reactions of tricyclic dienes 1 with several electrophilic reagents have been investigated.1 7 Interestingly, some of these compounds undergo addition reactions with remarkable syn stereoselectivity. For example, the reaction of dimethyl tricy-clo[4.2.2.02,5]deca-3,9-diene-7,8-dicarboxylate with iodine azide solution, prepared in situ from an excess of sodium azide and iodine monochloride, in acetonitrile at — 5 C provided the. yyn-4-azido-3-iodo derivative 2 (Table 1) in 90% yield.1,2,4,6 The formation of the 5,>,n-4-azido-3-iodo derivative 2 is thought to be the first example of a syn addition of iodine azide to an alkene.1,2 The formation of the syn-product is best explained by the twist strain theory,8 according to which the syn transition structure A is favored over the an/7-coplanar transition structure B.1... 

Other alkyl hypohalites usually add to carbon-carbon multiple bonds in a free-radical process.155-158 Ionic additions may be promoted by oxygen, BF3, or B(OMe)3.156-160 While the BF3-catalyzed reaction of alkyl hypochlorites and hypo-bromites gives mainly halofluorides,159 haloethers are formed in good yields but nonstereoselectively under other ionic conditions.156-158 160 In contrast, tert-BuOI reacts with alkenes in the presence of a catalytic amount of BF3 to produce 2-iodoethers.161 Since the addition is stereoselective, this suggests the participation of a symmetric iodonium ion intermediate without the involvement of carbocationic intermediates. 

Addition of pyridine bases to the catalytic system caused a considerable increase in the rate and selectivity of the reaction, reaching 80% yield of styrene oxide for 100% styrene conversion (r.t., 30 min). In the presence of this pyridine-modified system, the reactivity of alkenes is in the order styrene > trisubstituted > cis-disubstituted > trans-disubstituted> monosubstituted. The epoxidation of alkenes is not stereoselective. In the absence of pyridine, cis-stilbene was converted into a 1.8 1 trans cis epoxide mixture, whereas the cis isomer prevails in the presence of excess pyridine ligands. Neither chlorohydrins nor pyridine N-oxides are involved in this catalytic system. Attempts to isolate the reactive intermediate led to the characterization of a relatively stable... 

The prototype of copper-mediated conjugate addition reactions is the transformation of acceptor-substituted alkenes and alkynes into the corresponding adducts (Scheme 1). Whereas full control of the regio- and chemoselectivity in these Michael additions has been possible for a long time,3 the emphasis of the last decade has been put on the use of new copper reagents, the broadening of the substrate scope, and the control of the stereoselectivity of the conjugate addition. 

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