Nucleophilic substitution at an allylic carbon

Nucleophilic substitution at an allylic carbon takes place either by an SN2 mechanism or by an Sn2 mechanism. No allylic rearrangement takes place in the former case, but it does occur in the latter case. 

Nucleophilic substitution at an allylic carbon can also take place by an Sn2 mechanism, in which case no allylic rearrangement usually takes place. However, allylic rearrangements can also take place under Sn2 conditions, by the following mechanism, in which the nucleophile attacks at the y carbon rather than the usual... 

Nucleophilic Substitution at an Allylic Carbon. Allylic Rearrangements... 

Nucleophilic substitution at an allylic substrate under S 2 conditions may proceed via nucleophilic attack at the y-carbon, especially when substitution at the a-carbon sterically impedes the normal 8 2 reaction. These S 2 reactions with cyclohexenyl systems generally proceed via an anti addition of the nucleophile to the double bond, as depicted below (best overlap of participating... 

Sn2 substitution at a benzylic carbon is analogous to substitution at an allylic carbon (Section 10.4) in that -rr delocalization facilitates the flow of electrons from the nucleophile to the LUMO of the halide. In a benzylic halide, the LUMO encompasses both the CH2CI unit and the -rr system of the aromatic ring. 

Another common reaction of alkenes uses diatomic halogens such as bromine (Br2) to form 1,2-dibromides (see Chapter 10, Section 10.4.1). In this reaction, the alkene reacts as a Lewis base with the bromine atom to form a bromonium ion. When 1,3-butadiene (3) reacts with bromine, both 1,2 and 1,4 addition products are formed, just as with the HBr reaction. The products are 3,4-dibromo-l-butene (32) and a mixture otE- and Z-l,4-dibromo-2-butene (33 and 34). Initial reaction with bromine gives bromonium ion 29 however, when this reacts with bromide ion, there are two sites for reaction. If bromide attacks the less stericaUy hindered carbon atom, the product is 32, but the bromine ion may also attack the C=C unit to give products 33 + 34. Nucleophilic attack of this type is called an Sj reaction (nudeophilic substitution at an allylic carbon with displacement of the leaving group). 

Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are structurally sim liar—both contain a double bond and a pyrophosphate ester unit—but the chemical reactivity expressed by each is different The principal site of reaction m dimethylallyl pyrophosphate is the carbon that bears the pyrophosphate group Pyrophosphate is a reasonably good leaving group m nucleophilic substitution reactions especially when as in dimethylallyl pyrophosphate it is located at an allylic carbon Isopentenyl pyrophosphate on the other hand does not have its leaving group attached to an allylic carbon and is far less reactive than dimethylallyl pyrophosphate toward nucleophilic reagents The principal site of reaction m isopentenyl pyrophosphate is the carbon-carbon double bond which like the double bonds of simple alkenes is reactive toward electrophiles... 

In DMF, with carbanions (malonates or nitronates) as nucleophiles and phenylthio or phenylsulphonyl leaving groups, a chain radical substitution takes place at an allylic carbon. This SRN1 like reaction was found to be activated by visible light, the only possible absorbing species being a charge transfer complex between the anion and the substrate [218]. 

For trisubstituted olefins, the nucleophile attacks predominantly at the less substituted end of the allyl moiety, e.g. to afford a 78 22 mixture of 13 and 14 (equation 7). Both the oxidative addition of palladium(O) and the subsequent nucleophilic attack occur with inversion of configuration to give the product of net retention7. The synthesis of the sex pheromone 15 of the Monarch butterfly has been accomplished by using bis[bis(l,2-diphenylphosphinoethane)]palladium as a catalyst as outlined in equation 87. A substitution of an allyl sulfone 16 by a stabilized carbon nucleophile, such as an alkynyl or vinyl system, proceeds regioselectively in the presence of a Lewis acid (equation 9)8. The... 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

The scope of allylic electrophiles that react with amines was shown to encompass electron-neutral and electron-rich ciimamyl methyl carbonates, as well as furan-2-yl and alkyl-substituted allylic methyl carbonates. An ort/io-substituted cinnamyl carbonate was found to react with lower enantioselectivity, a trend that has been observed in later studies of reactions with other nucleophiles. The electron-poor p-nitrocinnamyl carbonate also reacted, but with reduced enantioselectivity. Allylic amination of dienyl carbonates also occur in some cases with high selectivity for formation of the product with the amino group at the y-position over the s-position of the pentadienyl unit [66]. Arylamines did not react with allylic carbonates under these conditions. However, they have been shown to react in the presence of the metalacyclic iridium-phosphoramidite catalysts that are discussed in Sect. 4. 

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