Nucleophilic substitution rearrangement

Another feature of systems that are subject to B-strain is their reluctance to form strained substitution products. The cationic intermediates usually escape to elimination products in preference to capture by a nucleophile. Rearrangements are also common. 2-Methyl-2-adamantyl p-nitrobenzoate gives 82% methyleneadamantane by elimination and 18% 2-methyl-2-adamantanol by substitution in aqueous acetone. Elimination accounts for 95% of the product from 2-neopentyl-2-adaman l p-nitrobenzoate. The major product (83%) from 2-r-butyl-2-adamantyl p-nitrobenzoate is the rearranged alkene 5. 

In Volume 13 reactions of aromatic compounds, excluding homolytic processes due to attack of atoms and radicals (treated in a later volume), are covered. The first chapter on electrophilic substitution (nitration, sulphonation, halogenation, hydrogen exchange, etc.) constitutes the bulk of the text, and in the other two chapters nucleophilic substitution and rearrangement reactions are considered. 

After the migration has taken place, the atom at the migration origin (A) must necessarily have an open sextet. In the third step, this atom acquires an octet. In the case of carbocations, the most common third steps are combinations with a nucleophile (rearrangement with substitution) and loss of H (rearrangement with elimination). 

Nucleophilic Substitution with Rearrangement of Propargylic Derivatives... 

Scheme 20.34 Enyne-allenes via nucleophilic substitution with rearrangement.

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. 

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. 

For another classical problem of nucleophilic aliphatic substitution and rearrangement, namely pinacolic deaminations and rearrangements, the use of cyclohexane derivatives with a tert-hvXyX group instead of open-chain alkane derivatives was very helpful. The stereochemistry of these reactions was studied first by Bernstein and Whitmore (1939). Specific labeling with used first by Collins and his coworkers (Benjamin et al., 1957), helped significantly to elucidate the complex pathway of pinacolic deamination. Even summarizing the results would take far too much space here. 

Mesyl chloride Nucleophilic substitution with rearrangement... 

With this terminology we may not only describe a fully concerted, one-step Sn2 reaction (AnDn), and a stepwise SnI reaction involving intermediate ions that diffuse apart (Dn -H An), but we may also concisely represent a stepwise reaction involving a transient ion pair (E An). The lUPAC nomenclature system can also be used to describe other substitution reactions. Among them are the SnI (substitution nucleophilic unimolecular with rearrangement) reaction, equation 1.8, which is denoted by lUPAC as an (1/Dn -I- 3/An) reaction. The numbers before the slash symbols indicate atoms involved in the dissociation and association steps. Thus, 1/Dn means that the nucleofuge dissociates from one atom (1), while the 3/An term means that the nucleophile associates at an allylic position (3). 

Similarly, the Sn2 (substitution nucleophilic bimolecular with rearrangement) reaction, equation 8.9, is described as a (3/1/AnDn) process. 

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. 

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