Jr-Allylic palladium complex

However, some binuclear -jr-allyl palladium complexes have been obtained by facile decarboxylations from palladium chloride and some... 

Palladium-catalyzed oxidation of 1,4-dienes has also been reported. Thus, Brown and Davidson28 obtained the 1,3-diacetate 25 from oxidation of 1,4-cyclohexadiene by ben-zoquinone in acetic acid with palladium acetate as the catalyst (Scheme 3). Presumably the reaction proceeds via acetoxypalladation-isomerization to give a rr-allyl intermediate, which subsequently undergoes nucleophilic attack by acetate. This principle, i.e. rearrangement of a (allyl)palladium complex, has been applied in nonoxidative palladium-catalyzed reactions of 1,4-dienes by Larock and coworkers29. Akermark and coworkers have demonstrated the stereochemistry of this process by the transformation of 1,4-cyclohexadiene to the ( r-allyl)palladium complex 26 by treatment... 

Allylation of nucleophiles. jr-Allyl palladium complexes (a) of a,/S-unsaturated epoxides react with active methylene compounds to give 1,4-adducts [allylic alcohols, equation (I)].10... 

Abstract Palladium-catalyzed cascade reactions have gained steadily increasing importance over the last decade. The important factor in these reactions is the catalytic generation of jr-allyl palladium intermediates which further undergo a variety of reactions. jr-Allyl palladium complexes can be easily formed by the treatment of allylic substrates with Pd(0). A jr-allyl palladium complexes on treatment with allylic metal species produce bis jr-allyl palladium complex. In this review, the palladium catalyzed cascade reactions involving jr-allyl palladium chemistry is described. The first part deals with catalytic reactions involving jr-allyl palladium complexes as an intermediate, while the second part features catalytic reactions involving bis jr-allyl palladium complex as an intermediate. 

Keywords Cascade reaction Palladium jr-Allyl palladium complex ... 

Bis jr-allyl palladium complex Synthetic strategies Heterocycles... 

It is very well known that jr-allyl palladium complex 1, which is a key intermediate for the Tsuji-Trost type allylation, has an electrophilic character and reacts with nucleophiles to afford the corresponding allylation products. We discovered that bis 7r-allyl palladium complex 2 is nucleophilic and reacts with electophiles such as aldehydes [27] and imines [28-32] (Scheme 2, Structure 2). We have also shown that bis 7r-allyl palladium complex 2 can act as an amphiphilic catalytic allylating agent it reacts with both nucleophilic and electrophilic carbons at once to produce double allylation products [33]. These complexes incorporate two allyl moieties that can bind with different hapticity to palladium (Scheme 3). The different complexes may interconvert by ligand coordination. The complexes 2a, 2b and 2c are called as r]3,r]3-bisallypalladium complex (also called bis-jr-allylpalladium complex), r)l,r)3-bis(allyl)palladium complex, -bis(allyl)palladium complex, respectively. Bis zr-allyl palladium complex 2 can easily be generated by reaction of mono-allylpalladium complex 1 and allylmetal species 3 (Scheme 4) [33-36]. Because of the unique catalytic activities of the bis zr-allyl palladium complex 2, a number of interesting cascade reactions appeared in the literature. The subject of the present chapter is to review some recent synthetic and mechanistic aspects of the interesting palladium catalyzed cascade reactions which in-... 

We reported that the palladium-catalyzed reaction of arynes with bis-jr-allyl palladium complex afforded 1,2-diallylated derivatives of benzene in good yields (Scheme 32) [77]. The reaction of 104 with allyltributylstan-nane 3a and allyl chloride 11 in acetonitrile in the presence of 2.5 mol % Pd2(dba)3.CHCl3/dppf catalyst at 40 °C for 12 h afforded 1,2-diallyl benzene 105 in 76% yield. The generation of benzyne 106 takes place presumably first from 104 under the conditions of the palladium catalysis, which reacts with the complex 2 in a manner similar to the diallylation of activated olefins (refer Scheme 29). 

The 1,4-addition o two nucleophiles to 1,3-dienes is an oxidation reaction and involves nucleophilic attacks on (jr-diene)- and (jr-allyl)palladium complexes. The principle and mechanism of this reaction are given in Eq.(32) and Scheme 8-6, and are exemplified with p-benzoquinone as the oxidant. 

The low trans selectivity and increased propensity for 1,4-m addition is thought to arise from a direct interaction with the metal-porphyrin peroxo complex similar to that suggested for the 2,5-dimethoxyphenyl derivatives shown in Scheme 8-13. It was demonstrated that the 2,5-dimethoxyphenyl derivatives worked in the aerobic oxidation, even without p-benzoquinone being present [64], Interestingly, in this case the 1,4-cis addition product predominates. It was proposed that the (jr-allyl)palladium complex is activated as shown in Scheme 8-13. 

Palladium(II)-catalyzed 1,4-additions to conjugated dienes, in which at least one alkoxide function is added, require the presence of an alcohol function. In all cases known so far, this involves an alkoxypalladation of tlie conjugated diene to give an intermediate 4-alkoxy-l,2,3-(jr-allyl)palladium complex. Subsequent nucleophilic attack on the jr-allyl... 

Allylic carbonates produce the required alkoxide by decarboxylation of the carbonate anion that is displaced in the formation of the 7t-allyl palladium intermediate. Deprotonation creates the active nucleophile, which rapidly traps the jr-allyl palladium complex to give the allylated product and regenerates the palladium(O) catalyst. 

The counter-phase transfer catalysis via jr-allyl—palladium complexes exhibits an unusual solvent effect [16]. Both the reduction with sodium formate and the car-... 

Intramolecular cyclization of alcohols to 1,3-dienes can lead to fused tetrahydrofurans and tetrahydropyrans. In this reaction, using 5 mol% Pd(0Ac)2, the first step involves a trans oxypalladation to form the heterocyclic ring and a jr-allyl palladium complex. In the presence of 0.2 equiv. of LiCl, AcO adds trans to the r-allyl, leading to a net cA-l,4 addition of oxygen nucleophiles to the diene. When LiCl is omitted, the AcO adds to the same face of the r-allyl as the palladium, giving net tra/ 5-1,4-addition of oxygen groups to the diene (Scheme 41). 

The entire allyl palladium alkylation process discussed in Sections 12.6.A and 12.6.B can be made catalytic in palladium. The basis for this catalytic activity involves the use of an allylic substituted species (339, allylic acetate, allylic chloride, etc.) rather than an allylic hydrocarbon. A rerra/ /5-palladium(0) complex (L4Pd) reacts with the allylic substrate to form a jr-allyl palladium complex (369). Substitution with a nucleophile generates 370 with liberation of palladium species L4Pd, which is recycled to react with additional 339.223... 

D.1. Reactions with Nucleophiles. Previously, a jr-allylic palladium complex was generated by reaction of palladium reagents with allylic hydrocarbons prior to reaction with nucleophiles. In the catalytic version of this reaction, an allylic halide or an allylic acetate is used with a palladium(O) reagent. Why use a palladium complex when enolate alkylation is a well-known process (sec. 9.3.A) A typical enolate coupling reaction is the conversion of 2-methylcyclopentane-l,3-dione (373) to the enolate anion by reaction with NaOH, allowing reaction with allyl bromide. Under these conditions only 34% of 374 was obtained. When allyl acetate was used in place of allyl bromide in this reaction and tetra w(triphenylphosphino)palladium was used as a catalyst, a 94% yield of 374 was obtained.224 in this reaction, formation of the Jt-allyl palladium complex facilitated coupling with the nucleophilic enolate derived from 373, which exhibited poor reactivity in the normal enolate alkylation sequence. 

Cross-coupling Reactions. Metal-mediated C-C and C-X bond formation via various cross-coupling reactions has emerged as a powerful tool in organic synthesis. Palladium-catalyzed processes are ubiquitous and Pd(dba)2 is frequently employed as a catalyst precursor. Cross-coupling sequences involving jr-allyl palladium complexes generally proceed with overall inversion of... 

By contrast, the essential feature of the jr-allyl palladium complexes is their particularly strong electrophilic character which allows smooth attack by various nucleophiles. This mode of activation has been extensively applied for reaUzing selectively various coupling reactions (see below). 

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