Electrophilic Attack on Open-Chain Double Bonds with Diastereotopic Faces

3 Electrophilic Attack on Open-Chain Double Bonds with Diastereotopic Faces 

and the protonation of the enolate 5.136 in the same sense to give mainly the diastereoisomeric ketone 5.138. 

A reaction in the alternative sense 5.133 is the cycloaddition of a nitrile oxide to a terminal alkene, which gives mainly the diastereoisomer 5.141 by way of the transition structure 5.139. Nitrile oxide cycloadditions are among those dipolar cycloadditions which are electrophilic in nature. The substituent A is a hydrogen atom, and the medium-sized group is only a methyl group, so it fits the criteria that make this pathway plausible. 

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. 

Open-chain SE2 reactions are special cases of electrophilic attack on a C=C double bond in the sense 5.132, with attack specifically at C-3, and followed by the loss of an electrofugal group from the stereogenic centre. The most studied of these are the electrophilic substitution reactions of stereodefined allylsilanes, 

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