Electrophiles allylic leaving groups

Recently, Lambert and co-workers645,646 have reported the preparation of the trimesityl-substituted Mes3Si+ cation 249 using the so-called allyl leaving group approach [Eq. (4.162)] instead of the hydride transfer reaction applied in earlier studies. The electrophile attacks the double bond and the intermediate carbenium... 

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... 

This type of process is analogous to the nucleophilic allylic rearrangements discussed in Chapter 10 (p. 420). There are two principal pathways. The first of these is analogous to the SeI mechanism in that the leaving group is first removed, giving a resonance-stabilized allylic carbanion, and then the electrophile attacks. 

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

Calb et al. have thoroughly investigated the use of allylic electrophiles containing heterocyclic leaving groups in regioselective allylic substitution (Scheme 8.7) [22]. 

A second type of organopalladium intermediates are 7r-allyl complexes. These complexes can be obtained from Pd(II) salts and allylic acetates and other compounds with potential leaving groups in an allylic position.79 The same type of 7i-allyl complexes can be prepared from alkenes by reaction with PdCl2 or Pd(02CCF3)2.80 The reaction occurs by electrophilic attack on the n electrons followed by loss of a proton. The proton loss probably proceeds via an unstable species in which the hydrogen is bound to... 

Several new leaving groups have been discovered recently which merit special discussion. Allyl sul-fones, surprisingly, function as substrates for palladium catalysis.86 As the sulfone group had previously been proven to be able to stabilize an adjacent carbanion, this result allowed allyl sulfones now to be considered as synthons for 1,1- and 1,3-dipoles (equation 10). That is, the allyl sulfone can be used alternately as a nucleophile and electrophile, greatly extending its synthetic utility. 

A final example of this class of allyl-X compounds is a vinylcyclopropane that is geminally substituted on the three-membered ring with electron withdrawing groups. Upon addition of a Pd° complex, the allylic C—C bond ruptures, with the stabilized carbanion serving as a leaving group (equation 11).96 The complex can be independently reacted with electrophiles at the carbanion as well as with nucleophiles on the allyl moiety. 

Because of the ability of some leaving groups to stabilize an a-carbanion, the pH at which the substitution is performed can be critical. Electrophiles with such leaving groups (e.g. R-N02 [36, 37], R-S(=0)2R [38, 39], R-S(=O)R[40]) will usually undergo substitution only under neutral or acidic conditions, what limits the choice of suitable nucleophiles. Some nucleophilic displacements of nitro and sulfonyl groups, both under acidic and basic reaction conditions, are shown in Schemes 4.6 and 4.7. Allylic nitro groups can also be readily displaced by catalysis with palla-... 

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