Electrophilic substitution with allylic rearrangement

Molecular orbitals demonstrate the smooth transition from the allyl silane, which has a k bond and a C-Si O bond, to the allylic product with a new K bond and a new o bond to the electrophile. The intermediate cation is mainly stabilized by O donation from the C-Si bond into the vacant p orbital but it has other a-donating groups (C—H, C-C, and C-E) that also help. The overall process is electrophilic substitution with allylic rearrangement. Both the site of attachment of the electrophile and the position of the new double bond are dictated by the silicon. 

This reaction, for which the termprototmpic rearrangement is sometimes used, is an example of electrophilic substitution with accompanying allylic rearrangement. The mechanism involves abstraction by the base to give a resonance-stabilized carbanion, which then combines with a proton at the position that will give the more... 

This section deals, in general, with electrophilic substitution in allylic systems. For more complicated molecular rearrangements which may accompany electrophilic substitution, there is no simple method of description other than by a full statement of the mechanism. The recommended nomenclature is given in square brackets, [ ]. 

There thus exists a preference for anti (or antara) hydroxylation in these cyclohexenylstannanes, where electrophilic substitutions are known to proceed faitiifully with allylic rearrangement. A more likely padiway is shown in Scheme 4, which is supported by results with optically active allylsilanes, whidi require anti attack by MCPBA on the silane conformation maximizing C—Si tr nr interaction. 

Substitution reactions with allylic rearrangement are known in nucleophilic 5 as well as in electrophilic Se reactions (Scheme 1). ... 

Should an allylic anion be formed as a result of an initial ionisation, attack of an electrophile on the anion could take place either at the (original) a carbon atom or at the (original) y carbon atom, the latter leading to substitution with rearrangement. An illustration for the case of a crotyl compound follows, viz. 

Allylic electrophiles can react with nucleophiles either with or without allylic rearrangement [213], The outcome of such reactions will depend on whether or not an allylic carbocation is formed as intermediate, and on the steric requirement and hardness of the two electrophilic centers and the nucleophile. Bimolecular substitutions at allylic electrophiles which occur with rearrangement are called Sn2 reactions. 

There are two main synthetic applications where the reaction of an allyl system with electrophiles is accompanied by an allylic rearrangement. One consists of the use of heteroatom-substituted allylic anions as homoenolate anion equivalents and the other represents a synthetically valuable alternative to the aldol reaction by addition of allyl metal compounds to aldehydes. 

In most cases, treatment of allylic halides containing one ASG with a nucleophile does not result in formation of electrophilic cyclopropanes (MIRC product) instead, other reaction pathways are followed, e.g. addition, substitution, rearrangement and elimination reactions.However, with certain alkenes or nucleophiles or under the appropriate conditions a conjugate addition-nucleophilic substitution pathway is favored, resulting in cyclopropanes substituted with one ASG. Representative examples are compiled in Tables 20 and 21 where organometallic compounds or active methylene compounds are used as the nucleophilic species in combination with allyl bromides containing an ester or a sulfone as ASG. 

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