Allylic and Propargylic Electrophiles

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. 

Allylic electrophiles react readily with Pd(0) complexes to yield rf-allyl Pd(II) complexes, which retain electrophilic character and react with nucleophiles to yield the product of allylic substitution and Pd(0). Thus, catalytic amounts of Pd(0) provide an additional mechanism (in addition to SnI, Sn2, and Sn2 ) by which an allylic substitution can proceed. Because the metal generally attacks the allylic electrophile 

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In general, allylic and propargyllic electrophiles are much more reactive than alkenyl and alkynyl electrophiles. Thus, a wide variety of electrophiles containing halogens, e.g. I, Br, and Cl, and oxygen groups, e.g. sulfonates, phosphates, carboxylates, carbonates, alkyl and aryl ethers, and even silyl... 

Scheme 4.24 Cross-coupling of dialkylzincs with allyl and propargyl electrophiles [105-107].

Characteristic feateres of Pd-catalyzed reactions of allylic and propargylic electrophiles with soft carbon nucleophiles are smmnarized with typical examples in the following. 

Whereas allyl, benzyl and propargyl electrophiles are among the most reactive towards Pd, Ni and other transition metals, ordinary alkyl halides and related alkyl electrophiles that are not /3, -unsaturated are among the least reactive carbon electrophiles with respect to oxidative addition to Pd or Ni. Most of the alkyl derivatives are also associated... 

The metabolism of oleftnic or acetylenic substances to electrophilic metabolites is greatly reduced when the oleftnic or acetylenic moieties are not terminal, or are terminal but contain alkyl substituents on the allylic and propargylic positions. 1-Heptene, for example, is bioactivated to electrophilic metabolites whereas 3-hexene, 2-methyl-l-hepteneand3,3-dimethyl-l-hexenearenot [36,37]. Similarly, 1-decyneismetabolizedto electrophilic metabolites whereas 3- and 5-decynes are essentially not [36, 37]. 

Allylic and propargylic heteroatom-substituted carbanions can yield rearranged or unrearranged products on treatment with an electrophile. The regio- and stereoselectivity of these reactions depends on the precise structure of the carbanion, on the metal and solvent chosen [199], and on the structure of the electrophile [150, 200-203], and can be difficult to predict. 

The presence of an alkenyl, phenyl, or alkynyl group one carbon away from a halogen, oxygen, or another electronegative atom, such as sulfur, makes allylic, benzyllic, and propargyllic electrophiles very reactive in oxidative addition... 

As expected according to the HSAB principle, hard electrophiles such as silyl halides and triflates react at the enolate oxygen atom to form allenyl ketene acetals, while soft electrophiles such as carbonyl compounds attack at C2. Only allylic and propargylic halides react regioselectively at G4 of the allenyl enolate to give substituted conjugated dienes. 

Coupling reactions of alkenyl- and arylmetals with allyl, benzyl, and propargyl electrophiles... 

As discussed in Section 1.2.4, the Pd- or Ni-catalyzed cross-coupling involving allyl, benzyl, and propargyl electrophiles takes place with a wide variety of leaving groups. Indeed, virtually all types of allylic alcohol derivatives, including phosphates, carbonates, carboxylates, ethers, and silyl ethers, can be employed. 

Dlenes and 1,5-Enynes via Pd-Catalyzed Cross-Couplings with Allyl, Benzyl, Propargyl Electrophiles... 

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