Electrophiles bridging

Electrophilic Attack on a C=C Double Bond by Bridging Electrophiles. Heteroatom electrophiles, like peracids, sulfenyl halides and the halogens nearly always give bridged products in the first step. The difference between these electrophiles and the proton or carbon electrophiles... 

Bridging electrophiles, as in epoxidation, are fairly well behaved in the sense 5.132, possibly because the acute approach angle does not leave room for the medium-sized group to sit inside. Bromination, however, is reversible in the first step, and the stereochemistry actually observed, although often in the sense 5.132, is partly governed by the relative ease with which each of the diastereoisomeric epibromonium ions is opened. As a consequence, the ratio of diastereoisomers is not reliably a measure of the relative rates of attack on the diastereotopic faces of the alkene. 

These allylsilanes undergo r/ntZ-reaetions with bridging electrophiles such as osmium tetroxide or m-chloroperbenzoic acid. ... 

The stereochemistry of the second step of an addition initiated by a nonbridging electrophile like a proton will be controlled by which surface of the intermediate cation 5.57 is more easily attacked by the nucleophile. The addition of hydrogen chloride to an alkene is not stereospecifically anti, because the chloride does not necessarily attack the cation either specifically anti or syn to the proton,478 in contrast to addition initiated by bridging electrophiles like bromine, or metallic electrophiles like the mercuric ion, described below. The stereochemistry will depend instead on ion pairing or on the substituents in the cation 5.57, and how they influence the conformation at the time the nucleophile attacks. 

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... 

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