Allyl systems, reactivity toward nucleophilic substitu

Allyl derivatives 11 with identical substituents at Cl and C3 are an important class of substrates for enantioselective allylic substitution (Scheme 10). Starting from either enantiomer (11 or ent-ll) the same allyl-palladium complex 12 is formed. Therefore, the first part of the catalytic cycle leading to this intermediate usually is irrelevant for the stereoselectivity of the overall reaction [31]. The two termini of the free allyl system are enantiotopic. If the catalyst is chiral, they become diasterotopic in the allyl-metal complex and, therefore, may exhibit different reactivities toward nucleophiles. Under the influence of a suitable chiral ligand attached to palladium, nucleophilic attack can be rendered regioselective leading preferentially either to product 13 or its enantiomer ent-l3. 

In addition to steric effects, there are other important substituent effects that influence both the rate and mechanism of nucleophilic substitution reactions. As we discussed on p. 302, the benzylic and allylic cations are stabilized by electron delocalization. It is therefore easy to understand why substitution reactions of the ionization type proceed more rapidly in these systems than in alkyl systems. Direct displacement reactions also take place particularly rapidly in benzylic and allylic systems for example, allyl chloride is 33 times more reactive than ethyl chloride toward iodide ion in acetone." These enhanced rates reflect stabilization of the Sjv2 TS through overlap of the /2-type orbital that develops at carbon." The tt systems of the allylic and benzylic groups provide extended conjugation. This conjugation can stabilize the TS, whether the substitution site has carbocation character and is electron poor or is electron rich as a result of a concerted Sjv2 mechanism. 

The extent of the rate enhancement due to adjacent substituents is dependent on the nature of the transition state. The most important factor is the nature of the TT-type orbital which develops at the trigonal bipyramidal carbon in the transition state. If this carbon is cationic in character, electron donation from adjacent substituents becomes stabilizing. If bond formation at the transition state is far advanced, electron withdrawal should be more stabilizing. Substituents such as carbonyl therefore have their greatest effect on reactions with strong nucleophiles. Adjacent alkoxy substituents can stabilize Sn2 transition states that are cationic in character. Since the vinyl and phenyl groups can stabilize either type of transition state, the allyl and benzyl systems show enhanced reactivity toward both strong and weak nucleophiles. ... 

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