Palladium tr allyl-complex

Palladium-catalyzed oxidation of 1,4-dienes has also been reported. Thus, Brown and Davidson obtained the l,3-diacetate 25 from oxidation of l,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 Tt-allyl intermediate, which subsequently undergoes nucleophilic attack by acetate. This principle, i.e. rearrangement of a (cr,7r-allyl)- to a (Tt-allyl)palladium complex, has been applied in nonoxidative palladium-catalyzed reactions of 1,4-dienes by Larock and coworkers. Akermark and coworkers have demonstrated the stereochemistry of this process by the transformation of l,4-cyclohexadiene to the (Tr-allyl)palladium complex 26 by treatment... 

Interception of the Tr-allyl palladium complex by soft nucleophiles, particularly malonates, has been described above. Alkenes, alkynes and carbon monoxide can also insert into the Tr-allyl palladium complex, generating a u-alkyl palladium species. When an internal alkene is involved, a useful cycbzation reaction takes place (sometimes called a palladium-ene reaction).Addition of palladium(O) to the allylic acetate 225 gave the cyclic product 226 (1.225). The reaction proceeds via the -ir-allyl palladium complex (formed with inversion of configuration), followed by insertion of the alkene cis- to the palladium and p-hydride elimination. In some cases it is possible to trap the a-alkyl palladium species with, for example, carbon monoxide. 

The mechanisms of formation of Tr-allyl-palladium complexes, both from tetrachloropalladate(n) and allyl alcohol in acidic aqueous solution and from palladium dichloride and alkenes in, for example, dimethyl-formamide, have been discussed. Allylpalladium halide dimers react... 

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). 

Hard carbon nucleophiles of organometallic compounds react with ir-allyl-palladium complexes. A steroidal side-chain is introduced regio- and stereo-selectively by the reaction of the steroidal Tr-allylpalladium complex 319 with the alkenylzirconium compound 320[283]. 

Several tr-allyl palladium chloride complexes have been prepared from steroidal olefins. Reaction of cholest-4-ene (65), for example, with bis(benzonitrile)palladium dichloride produced dimeric complexes of structures (66) and (67) (76). 

Stereochemistry A Pd(0) complex displaces allylic leaving groups (with inversion of configuration) to generate cationic Tr-allyl palladium species A. This complex is electronically deficient and undergoes attack of a suitable soft nucleophile (with inversion) to give a product with overall retention (Scheme 5.8). 

Many alkenes will react with PdCl2, forming a itt-allyl complex and one mole of HCl. A base is normally added to react with the HCl produced. Solvents that have proven useful include acetic acid, chloroform, methanol, and DMF. Where isomeric itt-allyl palladium complexes can be formed, one often obtains a mixture of products. This is particularly true for reactions run in acetic acid. The addition of CUCI2 causes an increase in regioselectivity, resulting in abstraction of the allylic C-H bond that leads to the more-substituted Tr-aUyl complex (equation 53). 

Another example of tr-allyl ligand transfer has been described recently by Heck 159). Treatment of tr-allyl derivatives of palladium chloride with NaCo(CO)4 in ether gives tr-allyl cobalt complexes. 

In 1968, van Helden and co-workers studied the corresponding carbopalladation reaction of 1,2-propadiene with the Tr-allylpaUadium species 5, affording alkenyl)allyl)palladium complex 6 (Scheme 3).f4H6]sinu]ar results were observed by Hughes and PowellJ s] rate-determining step for these insertion reactions is the actual insertion step, not the coordination of the 1,2-diene onto PdJ ... 

An attempt has been made to predict the sites of nucleophilic attack on [M(CO)3(fl--hydrocarbon)] complexes using the perturbation theory of reactivity. For the model allyl substrate [Co(CO)3( j -C3H5)] the site preference CO > M > C3H5 was predicted for reaction with hard nucleophiles in polar solvents. On the other hand, with soft nucleophiles initial attack at the ir-allyl ligand was favored. Mechanistic studies have suggested only a small energy difference between attack by alkoxide ions on the allyl ligand and the metal in related ( Tr-allyl) palladium(II) complexes. ... 

An electron-rich metal can deprotonate the dicarbonyl derivative, affording the hydridopalladium intermediate 23, which can undergo a Tr-allyl 24 formation through diene insertion (which can be assimilated to a hydridopalladation of olefin) (Scheme 7). The attack of the enolate to the -jr-allyl species occurs with good enantioselectivity in the presence of the chiral ligand. The final product 21 is released and the palladium(O) complex 22 is regenerated. 

Another difference between the two mechanisms is that the former involves 1,2 and the latter 1,3 shifts. The isomerization of 1-butene by rhodium(I) is an example of a reaction that takes place by the metal hydride mechanism,69 while an example of the Tr-allyl complex mechanism is found in the Fe3(CO)i2-cataIyzed isomerization of 3-ethyI-l-pentene. A palladium acetate or palladium complex catalyst was used to convert alkynones RCOGsCCH2CH2R to 2,4-alkadien-l-ones RCOCH=CHCH=CHCHR. 71... 

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 reactivity of -Tr-allylpalladium-phosphine complexes generated stoichiometrically or from alkenes allylically substituted with a leaving group, is essentially identical and, as a result, allyl species will be generally considered in this section without distinction as to the origin of the palladium complex. 

An extraordinary effect is noted for cydizalion of the carbonate derived from dihydrocar-vone (Table 14, entries 16 and 17). When R = H, no reaction is observed with the depicted substrate (Table 14, entry 16) or with the regioisomeric allylic carbonate22. Palladium cannot coordinate to the olefin because of steric reasons. However, cyclization occurs smoothly in the presence of an alkyne group in the molecule (Table 14, entry 17), because precoordination of palladium(O) to the alkyne enables the intramolecular formation of the initial palladium(O)-alkene complex and subsequent ionization to the tr-allyl complex. The product has been further elaborated via an ene-yne cyclization and eventually transformed into the alkaloid (-)-den-drobine. Entry 18 shows the preparation of a precursor for (-)-aspochalasin B23. Cyclization to the 11-membered ring proceeds with high diastereoselectivity which has been used to advantage during further transformations. 

Also based on PS-PEG supports, ligands 142, 143, and 144 have been prepared and palladium complexes formed from them screened for activity in asymmetric transformations on tr-allylic functionalities. 

An alternative to the direct electrophific allylic amination of an alkene is the reaction of an allylic acetate or carbonate with a transition metal (typically a palladium or rhodium complex) to give a Tr-allyl metal species that reacts with a nitrogen nucleophile to give an allylic amine (see Section 1.2.4). 

The conducted experiments [70,71] demonstrate that PVP-Pd complexes are active, selective and stable catalysts. The composition of such catalysts represents a composite system including Pd(II) and Pd(0). The role of the polymer ligands evidently consists in the stabilization of the particular valent states of palladium which are optimum for the substrate hydrogenation. One can assume that in the given catalytic system, Pd(0) promotes the activation of hydrogen, whereas complex-bound Pd(II) promotes the formation of a tr-allyl complex with unsaturated double bonds of the substrate and thus its activation. Furthermore, pyridine rings promote substrate orientation. This assumption enables polymer-metal heterogenized catalysts to be considered as models of catalytic enzyme systems. 

Carbopalladation is the reaction of a cr-bonded organopalladium complex I with an unsaturated molecule (such as an alkene 2) to yield the migratory insertion product 3 (Scheme 1). The reaction is tremendously flexible, allowing for a wide variety of structural types for both reactants 1 and 2. The precursors of palladium complexes 1 are commonly alkenyl or aryl halides or triflates (8 and 9, respectively), the reaction of which is more commonly termed the Heck reaction. Allylic systems 10, which react to provide -Tr-allylpalladium complexes, can participate in the reaction as can benzylic precursors 11. Acylpalladium complexes 12 also react and are commonly generated in the same reaction vessel by Pd-catalyzed carbonylation. Their unsaturated reaction partners include alkenes 2, alkynes 4, dienes 6, allenes, and arenes, all of which can be electron rich or poor. Carbopalladation occurs in a syn fashion allowing the installation of stereocenters (2- 3) or control of alkene geometry (4- 5). 

In contrast with palladium(II), platinum(II) shows little tendency to form TT-allylic complexes. Only one tt-allylic platinum(II) complex, tr-allyl-7T -cyclopentadienylplatinum(II) has been prepared so far (4), We have now shown that several acyclic and cyclic 1,3-dienes react with sodium chloro-platinite in alcohols to give products of composition [PtCl (diene)l these are very insoluble and do not appear to involve tt -allylic bonding to the platinum i.e, they are true olefin complexes. 

The reaction of alkenyl epoxides with organometallic species (lithium, magnesium, copper, and boron) affords allylic alcohols, following an Sn and/or Sn mechanism. These processes can accommodate only little organic functionality and exhibit low regio- and/or stereoselectivity. Under smooth conditions, C—C bond formation proceeds by nucleophilic alkylation of vinyl epoxides in the presence of catalytic amounts of zerovalent palladium. Regio- and stereoselectivity can be achieved via the formation of a Tr-allylpal-ladium complex. Trost and Molander and Tsuji and co-workers simultaneously reported the first studies in 1981. Since then, numerous papers have dealt with this subject. Essentially, after chelation and oxidative addition of the palladium onto the vinyl epoxide, the zwitterionic 7r-allylpalladium complex deprotonates the nucleophile, which can in principle attack either carbon 2 (proximal attack) or 4 (distal attack) (Scheme 1). 

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