Bis 7r-allyl palladium complex

Novel palladium-catalyzed transformations of allylic alcohol and its derivatives are developed by Yamamoto and his co-workers. Bis(7r-allyl)palladium complexes are considered to be the key intermediates for the allylation of benzyl-idenemalonitrile and benzyl chloride (Scheme 29) (for examples see Refs 176,176a-176d). Asymmetric version of these reactions is being awaited. 

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-... 

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 Ti-allyl palladium intermediates which further undergo a variety of reactions. TT-Allyl palladium complexes can be easily formed by the treatment of allylic substrates with Pd(0). A 7r-allyl palladium complexes on treatment with allylic metal species produce bis Ti-allyl palladium complex. In this review, the palladium catalyzed cascade reactions involving 7r-allyl palladium chemistry is described. The first part deals with catalytic reactions involving 7r-allyl palladium complexes as an intermediate, while the second part features catalytic reactions involving bis 7r-allyl palladium complex as an intermediate. 

Next, the effect of ligand of palladium complex on the reaction of butadiene was studied. Only vinylcyclohexene (4) was obtained by the reaction of butadiene at 90°C using bis(7r-allyl)palladium (13) and tri-chlorophosphine (1 1). 

In conjunction with this, Jeong reported the cycloadditions of bis(allyl) and bis(homoallyl) acetals of alkynals leading to bicyclic lactols. Smaltz extended its utility to the synthesis of carbocyclic nucleoside by coupling with nucleophilic substitution of a 7r-allylic palladium complex (Equation (46)). ... 

In a sense the tr-allyl compounds of the transition metals can be regarded as the simplest of the sandwich molecules. Bis(jr-allyl)nickel, the best known of such complexes, has been shown by x-ray crystallography (104,105) to have a staggered arrangement of tr-allyl moieties and hence a C2h molecular conformation. The electronic structure of the ground state of bis(jr-allyl)nickel has been investigated by both semiempirical (47) and ab initio (274,275) methods, and a semiempirical computation has been performed on bis(7r-allyl)palladium (47). 

The same study describes the reaction of bis(7r-allyl)palladium and -platinum complexes with metallic mercury 161). 

For this reaction, a mechanism involving hydrogen migration via an intermediate complex with an Fe—H bond has been proposed 19). The hydrolysis of bis(7r-allyl)palladium chloride has also been shown to proceed readily at room temperature 54, 55), viz. 

Recent studies have shown that allylpalladium complexes can also react as nucleophiles. Bis(7r-allyl)palladium, for instance, was found to be a key intermediate in the Pd-catalyzed allylation of aldehydes and imines by allylstannane. This complex has... 

When phosphane-free palladium catalysts, such as bis(dibenzylideneacetone)palladium, bis(jy4-cycloocta-l,5-diene)palladium, tris(norbornene)palladium or a catalyst generated in situ from bis(acetylacetonato)palladium and ethoxydiethylaluminum, are used with 3,3-dimethylcyclopropene, dimer 17 is obtained as the major product (76% when R1 = R2 = Me), along with three tetrameric products in a combined yield of 12.3%. Complexes such as bis(7r-allyl)pal-ladium and bis(acetonitrile)dichloropalladium also act as catalysts but mainly lead to higher oligomers.42 Other 3,3-dialkylcyclopropenes react in the same manner.36 ... 

Formation of 1 1 adducts from substituted dienes 49 can be understood by the following mechanism, hi this case, generation of bis-7r-allylie palladium is not possible for steric reasons. Instead, insertion of a diene to H-Pd bond of H-Pd-Nu occurs to afford the jr-allylpalladium complex 50, and nucleophilic attack at the allylic terminus gives 51. 

A proposed catalytic cycle of the present allylation with allyl(tributyl)stannanes (Schemes 25-27) is outlined in Scheme 28. The formation of bis-7r-aUylpalladium(II) (1) from PdCl2(PPh3)2 and aUyl(tributyl)stannane, which involves formation of -Tr-allylpal-ladium chloride dimer as an intermediate, is confirmed by an NMR spectroscopic method. Coordination of aldimine to 1 (path a) may cause the change of bis-7r-allyl structure of 1 to 7r-aUyl-a--allyl structure and the o--allyl moiety of the complex attacks the aldimine ligand to form 7r-aUyl(homoallylamino)palladium(ll), which undergoes transmetallation with... 

The Properties of -n-Allyl Transition Metal Complexes a. Stability. The thermal stability of w-allylic complexes varies from the very stable complexes of palladium, which are often stable above 200° C, to the very unstable and incompletely characterized tris(7r-allyl) complexes of chromium and iron. Most 7r-allylic complexes are oxidized by atmospheric oxygen, especially in solution. An extreme example is bis(7r-allyl)nickel which spontaneously ignites in air. However, many 7r-allylic complexes only show decomposition in air after several hours exposure. 

Benzaldimines have been reported to give high yields of condensation products with allylic stannanes, in the presence of palladium catalysts, under neutral conditions. Studies support transmetalation of the stannane to yield a bis-7r-allylpalladium complex, which binds imine 190 for allyl transfer. In the case of the catalyst 191, the chiral allyl ligand is not transferable, but determines facial selectivity in the optically emiched amine 192 (80% ee) (Scheme 5.2.42). 

Another convenient method to synthesize palladium(O) phosphine complexes involves the reduction of CpPd( 7r-all i) [70], The starting 7c-allyl complex can be readily prepared and stored as a relatively stable crystalline solid. In the presence of tertiary phosphine ligands in solution, this complex undergoes reductive elimination to give palladium(O) phosphine complexes. The reaction proceeds cleanly under mild conditions without formation of inorganic by-products, leading to a facile synthesis of kinetically unstable species. This method was used to prepare coordinatively unsaturated bis(tertiary phosphine)pa)ladium(O) complexes (eq (51)) [71]. 

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