Ir-allyl complexes of palladium

B3 OLEFIN AND ir-ALLYLIC.COMPLEXES OF PALLADIUM, PLATINUM AND IRIDIUM, M.S. Lupin, S.D. Robinson and B.L. Shaw... 

The use of ir-allyl complexes of palladium as intermediates in nucleophilic displacement reactions enables both the stereochemistry and regiochemistry of the reaction to be controlled. 

Palladium forms the largest series of ir-allylic complexes of any metal, and many of these complexes are readily prepared. The majority are chloro-bridged complexes of the type [PdCl(all)]2, where all = an allylic radical. 

Allyl complexes of palladium and iridium are conveniently generated by oxidative addition between Pd(0) and Ir(I) catalysts and allylic acetates and carbonates. Chiral allyl complexes... 

The synthesis of Tj -ir-allyl complexes from palladium(II) may be achieved by transmetalation with an allylic Grignard such as 4 or directly from alkenes such as 5. 

It is apparent from mechanistic considerations that an active species in the palladium-catalyzed dimerization of butadiene is a zero-valent palladium complex, which forms bis-ir-allylic complex 20. 

Diphenylacetylene and methylphenylacetylene react with palladium halides to give a very interesting series of compounds some of which are almost certainly ir-allylicpalladium complexes. Thus diphenylacetylene (ChHjo) reacts with palladium(II) chloride in ethanol to give hexaphenyl-benzene and a complex of the composition [PdCl(Ci4Hi0)2(OC2H6)] 148, 149) which we formulate as the ir-allylic complex (XLVI) (see Section... 

This C—H substitution process results in a Markovnikov orientation, with the H that is allylic to the more substituted end of the alkene preferentially abstracted. The stereochemistry of the resulting ir-allyl complex does not represent the stereochemistry of the starting alkene, as the complexes are capable of isomerization under the conditions in which they are formed. Typically, a thermodynamic mixture is obtained, with the syn form of the complex predominating over the anti form (equation 1). The syn form is more stable due to unfavorable steric interactions that the anti form encounters with the coordination sphere of the palladium. 

A similar palladium-catalyzed cyclization procedure has recently been developed which involves enol ethers capable of (3-H elimination.373 Significant evidence has been accumulated suggesting that an oxa-ir-allyl complex is not an intermediate in these reactions, but that it is better characterized as an enolate addition to a Pdn-alkene complex.376 377 Synthetic applications of this reaction have also appeared.376-379... 

In considering the diastereoselectivity associated with reactions of 7r-allylpalladium complexes, the stereochemical nature of two processes must be evaluated. The first is the formation of the ir-allylpalla-dium complex, and the second is the allylic functionalization of a nucleophile reacting with the allyl complex. As previously described (Section 3.3.2.1.1) the generation of ir-allyl complexes from alkenes by their reaction with stoichiometric amounts of palladium proceeds so as to leave the substituents on the allyl termini preferentially in the syn rather than the anti position and the palladium situated on the steri-cally less congested face of the allyl ligand, regardless of the stereochemistry of the starting alkene (12). 

The nature of the ligands on the palladium in ir-allyl complexes can influence the regioselectivity exhibited by soft carbon nucleophiles. ir-Allylpalladium complexes generated from methylenecycloalkanes provide an example of the effect of ligands on regiochemistry. The complexes derived from methylene-cyclopentane and methylenecycloheptane both exhibit exclusive exocyclic addition by the anion of methyl(methylsulfonyl) acetate with triphenylphosphine ligands on the Pd (equation 223). In contrast, the complex derived from methylenecyclohexane yields a 62 38 ratio of exocyclic endocyclic addition (equation 226). 

Alkene complexes of Pdd2 in the presence of base give palladium ir-allyl complexes ... 

Palladium(II) salts are known to isomerize double bonds along a hydrocarbon chain as well as to cause cis-trans isomerizations (50, 82, 179). These isomerizations are believed to occur either by Pd(II)-hydride or via ir-allyl complexes. Recently, Pd(II)-catalyzed cis-trans isomerizations that apparently do not proceed by either mechanism have been reported. For instance, cis- and fram-l,2-didi,uteroethylenes are isomerized without deuterium scrambling—a result that is inconsistent with hydride mechanisms [136). Likewise vinyl chlorides and esters are isomerized, and again it could be shown that Pd(II)-hydrides were not involved 117, 123, 124). The isomerization of vinyl ethers has also been reported recently 257). Plausible mechanisms for this isomerization include the following ... 

The solvent may determine the stereochemistry in the stoichiometric formation of 7r-allyl-palladium complexes from allyl halides. Strong donor solvents such as acetonitrile and dimethylsulfoxide lead to the expected m-ir-allyl complexes cis-16, whereas benzene, dichloro-methane or tetrahydrofuran give the. vva-addition product Irans-1639. Using soft carbanions in tetrahydrofuran. both complexes are converted to the corresponding alkylated products 17 with clean inversion of configuration. 

In reactions which do not proceed via a /weyo-n-allyl complex, almost complete stereoselectivity in favor of the product with retenlion of configuration is observed, but regioselectivity may cause problems. The electron-withdrawing properties of the alkyl substituenl in the allylic aceiatc 22 is responsible for the exclusive formation of the distal product 2354 rather than its steric demand, since the nucleophile approaches the ir-allyl complex trans to the substituent. After further derivatization, the products 23 can be converted to polyfused ring systems by an intramolecular tandem palladium ene-Heck insertion reaction. 

Allylic acetoxylation with palladium(II) salts is well known however, no selective and catalytic conditions have been described for the transformation of an unsubstituted olefin. In the present system use 1s made of the ability of palladium acetate to give allylic functionalization (most probably via a palladium-ir-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast to copper(II) chloride (CuClj) as a reoxidant,8 our catalyst combination is completely regioselective for allcyclic alkenes with aliphatic substrates, evidently, both allylic positions become substituted. As yet, no allylic oxidation reagent is able to distinguish between the two allylic positions in linear olefins this disadvantage is overcome when the allylic acetates are to... 

This catalytic cycle is shown in Scheme 16.17 for the reactions of cyclohexadiene catalyzed by paUadium-phosphine complexes. By this mechanism, the combination of a diene and either a Ni(0) or Pd(0) complex and acid or a cationic nickel(ll) or palladium(ll) hydride generates a cationic ir-allyl complex. Similarly, the combination of a vinylarene and one of these combinations of species generates a cationic phenethylpalladium complex. These allyl and phenethyl complexes have been isolated using this procedure. - The amine then attacks the ir-allyl or ir-phenethyl complex to form tfie organic product and regenerate Ni(0) or Pd(0). This nucleophilic attack on ir-allyl and ir-benzyl complexes was discussed in detail in Chapter 11. 

The expression brought into focus is used since, prior to these studies, a number of oiganometallic complexes were described in the literature, particularly those of palladium, which were incorrectly formulated, often as olefin complexes. Only recently have these compounds been recognized as ir-allylic complexes. 

The spectrum of a ir-allyl system where H has been substituted is often v ry simple but when substitution at or occurs considerably more complex spectra are observed. Nonetheless, the spectrum of 77-crotyl-palladium chloride can be interpreted satisfactorily on the basis of an AMM X system 16, 44). In some cyclohexenyl complexes of palladium, the hydrogens are substituted by a (—CH2—group. A comparison of their spectra with those of analogous rr-allyl complexes confirms the assignments of the H hydrogens to the higher doublet in the spectra of the unsubstituted complexes 33, 61, 99). In the case of the platinum complex... 

These highly useful complexes can be generated in other ways. For instance, they can be formed by a modification of the Heck reaction. Insertion of dienes into vinyl or aryl-palladium(II) bonds generates -ir-allyl complexes. These can be trapped by external nucleophiles, but the product of p-hydride elimination will also be obtained (Scheme 9.73). After insertion of one double bond of the diene 9.258 into the carbon-palladium bond of the t -intermediate 9.262, p-hydride elimination may occur to give the expected Heck product 9.260. However, the t -intermediate 9.265 is in equilibrium with its -equivalent 9.266 and, if an appropriate nucleophile is present, this complex may be trapped to give the allylated product 9.261. Intramolecular trapping is more efficient. This has been used, for instance, to prepare dihydroindoles 9.269 (Scheme 9.74). 

C.i. (T- and rr-Bonded Complexes of Palladium ir-Allyl and Cyclopentadienyl Complexes... 

Both kinetic and labelling experiments on isomerization of enol propionates and vinyl halides indicate that the normally considered mechanisms of inter- or intra-molecular hydride transfer or of reversible ir-allyl complex formation are not applicable here. Rather, in the presence of palladium(n) compounds, the reaction appears to proceed through a 1,3-acetoxonium ion with a palladium-carbon a-bond, e.g. the ion (51). ... 

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

The proposed mechanism (Scheme 7-11) includes (a) oxidative addition of a pro-tonated alcohol to Pd(0) to provide the ir-allyl palladium complex 44, (b) nucleophilic replacement of H2O by PhSH, (c) insertion of CO into the Pd-C bond, and (d) re-... 

Similar studies on the reactions of butadiene and bis(ir-allyl)palladium were carried out by Wilke and co-workers (4). Unlike the reactions with nickel complexes, no cyclization took place, and 1,6,10-dodecatriene... 

Oxidative addition involving carbon-to-oxygen bonds is of relevance to the catalysis with palladium complexes. The most reactive carbon-oxygen bond is that between allylic fragments and carboxylates. The reaction starts with a palladium zero complex and the product is a ir-allylic palladium(II) carboxylate Figure 2.16. 

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]. 

The 7r-allylpalladium complex [PdCl(7r-C iH6)]2 on heating decomposes quantitatively into allyl chloride and palladium. The analogous ir-meth-allylpalladium complex gives a 96% yield of methallyl chloride but a little isobutene is also formed 123). 

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