Bis-7r-allyl complexes

One other reaction deserves mention. From bis(cyclooctadiene)nickel and butadiene (31), and in the presence of an isocyanide (RNC, R = cyclohexyl, phenyl, tcrt-butyl) two organic oligomeric products are obtained, 1 -acylimino-11 -vinyl-3,7-cycloundecadiene and 1 -acylimino-3,7,11 -cyclo-dodecatriene. In each, one isocyanide has been incorporated. An analogous reaction with carbon monoxide had been reported earlier. The proposed mechanism of these reactions, via a bis-7r-allyl complex of nickel, is probably related to the mechanism described for allylpalladium complexes above. 

One of the most recent developments in the field of Ni-catalyzed reactions of alkyl halides with organozinc derivatives is a study of Terao et al.411 They reported the use of three additives in the couplings 1,3-butadiene, N,N-bis(penta-2,4-dienyl)benzylamine 308a, and 2,2-bis(penta-2,4-dienyl)malonic acid dimethyl ester 308b. Addition of tetraene 308b to the reaction mixtures significantly increased the product yields (Scheme 157). The remarkable effect of these additives was explained by the formation of the bis-7r-allylic complex 309 as the key intermediate (Scheme 158). 

Cyclization of butadiene catalysed by Ni(0) catalysts proceeds via 7r-allylnickel complexes. At first, the metallacyclic bis-7i-allylnickel complex 6, in which Ni is bivalent, is formed by oxidative cyclization. The bis-7r-allyl complex 6 may also be represented by cr-allyl structures 7, 8 and 9. Reductive elimination of 7, 8 and 9 produces the cyclic dimers 1, 2 and 3 by [2+2], [2+4] and [4+4] cycloadditions. Selectivity for 1, 2 and 3 is controlled by phosphine ligands. The catalyst made of a 1 1 ratio of Ni and a phosphine ligand affords the cyclic dimers 1, 2 and 3. In particular, 1 and 3 are obtained selectively by using the bulky phosphite 11. 1,2-Divinylcyclobutane (1) can be isolated only at a low temperature, because it undergoes facile Cope rearrangement to form 1,5-COD on warming. Use of tricyclohexylpho-sphine produces 4-vinylcyclohexene (2) with high selectivity. 

The crown ether-catalyzed generation of the Co( CO)4 ion in ether or hydrocarbon solvents has been described (32). Treatment of the anion with 2,3-bis(bromomethyl)naphthalene in tetrahydrofuran gives what is formulated as a bis-7r-allyl complex, while ketones were isolated using monocyclic dibromides as substrates. 

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). 

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. 

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 discovery of bis(7r-allyl)nickel led to the recognition that most of the transition metals form such complexes in which exclusively allyl groups are attached to the central metal atom (24).4 Further investigation demonstrated very clearly that in the transition allyl metal compounds both types of metal-carbon bonds, tj and it, play important roles, and that even differences in the participation of the two types of bonds are possible. By a rather peculiar... 

Furthermore, it has recently been demonstrated (14) that the pentaco-ordinate 7r-allyl complex similar to (IV) reacts with allyl bromide in methanol to form bis(allyl) and methyl 3-butenoate. This is consistent... 

In the case of cyclooctenyl bromide, a reaction proceeding exclusively through a bis(7r-allyl)nickel complex can be excluded, since reaction of bis(7r-cyclooctenyl)nickel in dimethylformamide with carbon monoxide... 

The mechanism of the homocoupling of dienes is one of the representative reactions proceeding through a n-allylruthenium intermediate. Indeed, a bis 7r-allylruthenium complex was produced by oxidative cyclization of two dienes and the coupling of the terminal carbon atoms led to a cationic (diene) (allyl)hydridoruthenium species. 

The TBAF-catalyzed allylation of aldehydes with 10 proceeds efficiently in refluxing THF.92 In contrast, a Pd-TBAF co-catalyst system enables the allylation at room temperature (Equation (20)).93 This mild allylation is due to the formation of a bis-7r-allylpalladium complex as the actual allylating agent. 

In 1991, Shiu et al.m and Sarkar et a/.319 reported the reactions of zero-valent bis(3,5-dimethylpyrazolyl)methane type complexes [Mo (CO)4(N, N)] with allyl halides to give a series of 7r-allyl complexes [MoX (CO)2(N N)(7 3-allyl)] (N N = H2CPz2, H2CPz 2, PhHCPz2, PhHCPz 2). These complexes also can be made by reaction of [MoBr(CO)2(NCMe)2 (V-allyl)] with N N.318... 

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-... 

Reaction of 1,3-butadienes and Pd salts give munerous C4 and Cg products, depending on the exact reaction conditions. Either r-allyl C4 complexes or bis-7r-allyl Cg dimers form, which can undergo ftirther reactions with nucleophiles, CO, or solvent (Scheme 26). 

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

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. 

Soluble single species catalysts are also known, such as the bis(7r-allyl nickel halides) [7]. These can be prepared separately or in situ by reacting bis-allyl nickel (which is an ologomerization catalyst for butadiene but does not give high molecular weight polymer) with an equimolar quantity of nickel halide, and thus bears some resemblance to the catalysts from titanium subhalides and alkyl titanium halides. It is of interest to note that the active species is the monomeric form of the initiator as TT-complex with butadiene (XII) [61]. 

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). 

It is known that allylation of aldehydes with allyltrimethylsilane occurs in the presence of a catalytic amount of TBAF under reflux in THF [333]. In contrast, a Pd-TBAF co-catalyst system effectively promotes the allylation even at room temperature (Scheme 10.131) [376]. A plausible reaction mechanism for this allylation involves the formation of bis-7r-allylpalladium complex 121, which can be observed in a stoichiometric reaction of Ti-allylpalladium chloride dimer, allyltrimethylsilane, and TBAF. 

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 ... 

Butadiene dimerisation is very sensitive to catalysis by zero-valent nickel complexes, which can direct the reaction towards 1,2-cycloaddition or (4-1-4) cycloaddition, through a bis-7r-allyl intermediate, with small amounts of Diels-Alder product . Larger quantities of the latter were obtained in other cases . This and other examples of activation of olefins by transition metal complexes have been associated with excitation of the coordinated 7r-system . 

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