Nucleophilic addition to rr-allylpalladium complexes

Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. 

The catalytic version of allylation of nucleophiles via 7r-allylpaUadium intermediates was discovered in 1970 using allylic esters and aUyl phenyl ethers as substrates (Scheme Formation of 7r-allylpaUadium complexes by oxidative addition of various allylic compounds to Pd(0) and subsequent reaction of electrophilic rr-allylpalladium complexes with soft carbon nucleophiles are the basis of the catalytic allylation. After the reaction, Pd(0) is regenerated, which undergoes oxidative addition to the allylic compounds again, making the whole reaction catalytic. The efficient catalytic cycle is ascribed to the characteristic feature that Pd(0) is more stable than Pd(II). Allylation of carbon nucleophiles with allyhc compounds via TT-allylpalladium complexes is called the Tsuji-Trost reaction. The reaction has wide synthetic applications, particularly for cyclization. " ... 

The direct, Pd(II)-catalyzed addition of heteroatom and stabilized carbon nucleophiles to alkenes is generally not a successful reaction. An exception is the addition of water, which gives carbonyl compounds and has been developed into an important indnstrial process, the Wacker process. This has been reviewed extensively.By contrast, the stoichiometric addition of nucleophiles such as amines is facile. - However, if alkenes could be converted catalytically into Tr-allylpalladium complexes, the problems with nucleophilic addition to alkenes could be circumvented and amines and other heteroatom nucleophiles could be employed. A range of alkenes have been converted into rr-allyl complexes in a stoichiometric fashion,t "t but catalytic reactions have proved more difficult. However, aUyl acetates and similar compounds readily exchange the acetate group for heteroatom nucleophiles in a Pd(0)-catalyzed reaction, which proceeds via 7T-allylpalladinm(ll) intermediates (Scheme 1). Since this reaction has been developed into a very important synthetic reaction, an efficient procedure for catalytic conversion of alkenes into aUyl acetates would have great synthetic potential. 

Substitution reactions of allylic substrates with nucleophiles have been shown to be catalyzed by certain palladium complexes [2, 42], The catalytic cycle of the reactions involves Jt-allylpalladium as a key intermediate (Scheme 2-22). Oxidative addition of the allylic substrate to a palladium(o) species forms a rr-allylpal-ladium(n) complex, which undergoes attack of a nucleophile on the rr-allyl moiety to give an allylic substitution product. The substitution reactions proceed in an Sn or Sn- manner depending on catalysts, nucleophiles, and substituents on the substrates. Studies on the stereochemistry of the allylic substitution have revealed that soft carbon nucleophiles represented by sodium dimethyl malonate attack the TT-allyl carbon directly from the side opposite to the palladium (Scheme 2-23). 

By the isolation of nonracemic chiral it-allylpalladium complexes82,83, ss, it was unambiguously demonstrated that the oxidative addition step occurs with inversion of configuration. Loss of chirality is mainly due to anti attack of free palladium(O) species on the rr-allyl complex (sec Section 1.5.6.1.2.1.). When the chiral allyl complexes are subjected to nucleophilic addition with soft carbanions, inversion of configuration is observed and thus retention of configuration for the overall process results. 

Conjugated dienes coordinated to a transition metal can readily be transformed into a rr-allylmetal complex by functionalization at the 4-position.f This makes dienes useful substrates for catalytic transformations since the 7r-allyl complex formed can undergo further reaction. t A number of Pd-catalyzed reactions of conjugated dienes are known that proceed via rr-allylpalladium intermediates and lead to useful 1,4- or 1,2-funtional-ization of the diene. There are two types of reactions of this kind (i) Pd(0)-catalyzed reactions that involve initial oxidative addition and (ii) Pd(II)-catalyzed reactions that involve electrophilic activation of the diene by the metal followed by nucleophilic attack. This section deals with C— N and C—O bond formation via these two types of reactions. This topic has previously been reviewed in connection with Pd-catalyzed additions to conjugated dienes. [5] [n] -jijg present review will focus mainly on the work published since 1997 but will also briefly discuss previous work. 

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