Substitution with rearrangement

Nucleophilic Substitution with Rearrangement of Propargylic Derivatives... 

Scheme 20.34 Enyne-allenes via nucleophilic substitution with rearrangement.

There have been many proposals for the occurrence of a so-called SN2 mechanism that would produce a concerted substitution with rearrangement for allylic halides. One possible example is... 

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. 

BIMOLECULAR ELECTROPHILIC SUBSTITUTION WITH REARRANGEMENT Se2, [Se2 (open)]... 

The various mechanisms outlined in Section 2 (p. 16) can, in principle, all be subject to nucleophilic catalysis. Without enumerating them again, the arguments and discussions of Section 3 can readily be applied to these substitutions with rearrangement. 

Formation of (195) is another type of substitution with rearrangement reaction (route VIII of Scheme 1), in which migration of the double bond takes place. 

Although rearrangements are usually seen as annoying side reactions, a clever chemist can use a rearrangement to accomplish a synthetic goal. Problem 11-15 shows how an alcohol substitution with rearrangement might be used in a synthesis. 

In general it is very difficult to make any distinction between a concerted process and the involvement of a short-lived carbanion. For clarity a number of alkene rearrangements in the following text are written as two-step processes, but it should be emphasised that they could involve concerted mechanisms. Products arising from substitution with rearrangement are frequently encountered in reactions of cyclic fluoroalkenes and in fluoride-ion-induced rearrangements (Subsection 6, below). 

AUyl compounds are highly reactive towards nucleophiles and examples of dissociative (5 1) and associative (5 2) reactions, and nucleophilic substitution with rearrangement are well documented . Differentiation between these mechanisms when azide ion is the nucleophile is cften extremely difficult due to the possibility of rearrangement of the allylic azide resulting from substitution, and for this reason the relative nucleophilic strength of azide ion in allylic 5n reactions has not been delineated. 

Mesyl chloride Nucleophilic substitution with rearrangement... 

Nucleophilic substitution with rearrangement s. 16, 615 Cohaltic fluoride... 

Zakharkin and co-workers have studied, in detail, the synthesis and reactions of fluorovinyl-substituted carbaboranes (Vol. 1, p. 89). In Scheme 12 are shown some of the reactions described. Apparently the ability of the o-carbaboranyl nucleus to stabilize a negative charge during nucleophilic addition-elimination reactions exceeds that of either chlorine or trifluoro-methyl, attack on the double bond occurring p to the nucleus. > 4,4-Dichloro-3,3-difluoro-l-phenylcyclobutene reacts with aryl-lithium compounds, sodium thiolates NaSAr (Ar = Ph,p-MeC,H4,orn-H2N-CeH4), or pyridine or other heterocyclic bases by substitution with rearrangement. 

Nucleophilic substitution with rearrangement Halogen interchange... 

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