Addition anti with bridging electrophiles

This species is similar to the bromonium ion that is responsible for stereospecific anti addition in the electrophilic mechanism. Further evidence for the existence of such bridged radicals was obtained by addition of Br- to alkenes at 77 K. The ESR spectra of the resulting species were consistent with bridged structures. ... 

Alkynes react with bromine via an electrophilic addition mechanism. A bridged bromonium ion intermediate has been postulated for alkyl-substituted acetylenes, while vinyl cations are suggested for aryl-substituted examples.119 1-Phenylpropyne gives mainly the anti addition product in acetic acid, but some of the syn isomer is formed.120 The proportion of dibromide formed and stereoselectivity are enhanced when lithium bromide is added to the reaction mixture. 

The next two are examples of additions of electrophiles that form bridged cationic intermediates a cychc bromonium ion in the first case (Section 12-5) and a cyclic mercurinium ion in the second (Section 12-7). Additions therefore proceed stereospecifically anti, because the cychc ion can be attacked only from the direction opposite the location of the bridging electrophile (Figure 12-3). With Br2, identical atoms add to both alkene carbons, so regiochemistry is not a consideration. In oxymer-curation, the nucleophile is water, and it wiU add to the most substituted alkene carbon, because the latter is tertiary and possesses the greatest partial positive charge. Stereochemistry must be considered because addition of the electrophile occurs with equal probability from the top and from the bottom and stereocenters are created. So we have... 

Selenenyl chlorides add to alkenes, often via an AdE2 mechanism involving a bridged seleniranium ion intermediate (19) (equation 14). These reactions are therefore highly stereospecitic, resulting in anti addition. The regiochemistry of the process can be under either kinetic or thermodynamic control. In some cases, initial anti-Markovnikov products were observed at low temperature and Markovnikov adducts dominated after further equilibration. Analogous electrophilic additions to acetylenes and aUenes (Scheme 9) have also been reported. When selenenyl hahdes react with alkenes in the presence of other nucleophiles such... 

The basic mechanisms that are considered to be involved in electrophilic additions to alkynes are outlined below. The first involves a discrete vinyl cation. In general, this reaction will lead to a mixture of the two stereoisomeric addition products. Mechanisms B and C depict bridged intermediates formed without (B) or with (C) participation of a second electrophilic molecule. Mechanisms B and C should lead to anti addition. Mechanism D is a termolecular process that would be expected to be a stereospecific anti addition. Mechanisms A and B are of the Adg2 type, whereas C and D are classified as Ad S. Each of these mechanisms may involve a prior complex formation between the alkyne and an electrophile. 

Additions and Eliminations.—It is argued that electrophilic exo-additions to norbor-nenes and related compounds arise from an extension of the HOMO in the exodirection. The stronger hyperconjugative interaction of the methano-bridge than the ethano-bridge with the 7c-system and the rear-side interaction of the anti C—H bond at C-7 with the n-system point to the differential extension of the HOMO in... 

The electrophile in oxymercuration reactions, HgX or Hg, is a soft acid and strongly polarizing. It polarizes the tt electrons of an alkene to the extent that a three-center, two-electron bond is formed between mercury and the two carbons of the double bond. A three-center, two-electron bond implies weaker bridging in the mercurinium ion than in the three-center, four-electron bonding of the bromonium ion. Oxymercuration of simple alkenes is usually a stereospecific anti addition. This result is consistent with the involvement of a mercurinium intermediate which is opened by nucleophilic attack. 

In Summary Halogens add as electrophiles to alkenes, producing vicinal dihalides. The reaction begins with the formation of a bridged halonium ion. This intermediate is opened stereospecifically by the halide ion displaced in the initial step to give overall anti addition to the double bond. In subsequent sections, we shall see that other stereochemical outcomes are possible, depending on the electrophile. 

The reaction of mCPBA with the methano-bridged [5,6] open fulleroid (77) has been reported to result in anomalous selective electrophilic addition at the bridgehead anti-Bredt double bond to give (80) rather than the usual epoxidation product (79). The mechanistic preference for the unprecedented stepwise addition of mCPBA over the concerted epoxidation has been rationalized in terms of the notable ar-orbital misalignment of (78) by >30°, calculated at the B3LYP/6-31G(d) level. ... 

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