ALLYL PALLADIUM COMPLEXES

Esters of allylic alcohols with resin-bound carboxylic acids can be converted into palladium allyl complexes by treatment with palladium(O). These allyl complexes react with carbon... 

Arguably the best, or at least the most versatile, allylic oxidation method is based on Pd [114]. Since the intermediates are palladium-allyl complexes rather than free radicals the number of by-products is limited compared to the preceding examples (Fig. 4.41). Furthermore, a large number of nucleophiles (amines, alcohols, stabilized carbanions, carboxylates or halides) may attack the palladium-allyl complex, giving a wide variety of products. 

Steric protection can be provided by adamantyl wingtip groups that are introduced by the reaction of the l,r-diaminobiphenyl to the Schiff base (with 2-adamantone) followed by reduction with LiAlH [79,80] (see Figure 5.24). Ring closure with triethyl orthoformate to the corresponding azolium salt proceeds as usual and subsequent reaction with [Pd(allyl)Cl]j in the presence of a suitable auxiliary base (potassium tcrt-butylate) yields the respective palladium allyl complex. The paUadium(II) allyl complex can then be reacted with hydrochloric acid to form the [Pd(NHC)Cy dimer. Subsequent reaction with a silver(I) salt predictably breaks up the dimer under chloride abstraction and anion transfer from silver to palladium. 

The reactions of CO with palladium allyl complexes have also been investigated. Ris-i -allylpalladium and hu- -crotylpalladium yield the corresponding carboxylic acids after acidification. A lactone, which has been observed in the analogous nickel reaction, could not be discovered [56]. 

Rhodium and palladium allyl complexes form from the reaction of a Tl(III)allyl compound with a metal halide ... 

Tetrahydro-4-hydroxy-2-vinylfuran (2) can be prepared from 4-acetoxy-5-hexen-l,2-diol (I) with high 1,3-asymmetric induction via nucleophilic displacement on the palladium allyl complex, followed by elimination of a hydride. Thus, treatment of 1 with (dba),Pd2-chloroform and triphenylphosphane at 20 °C for 3 hours gives the tetrahydrofuran in 95 % yield and a 91 9 dia stereomeric ratio, On the basis of mechanistic considerations, the major isomer is assigned trans 1. 

Friesen, R. W. Product subclass 2 palladium-allyl complexes. Science of Synthesis 2002, 1,113-264. 

The INS spectra of the a-allyl compound allyl iodide, CH2=CHCH2l, and the r -allyl compound di-p-chloro-bis(r] -allylpalladium), [ Pd(Ti -C3H5) 2(p-Cl)2] [94] and the INS below 800 cm of nickel and palladium allyl complexes [97] have been reported. The structure of the palladium complex is shown in Fig. 7.24 [98]. The INS spectra are shown in Fig. 7.25. Peak positions and assignments are in Table 7.15. 

Here is an example from the McMurry flexibilene synthesis quoted in chapter 1. An alkyne 127 with a protected aldehyde group reacts with Cp2ZrHCl to give a vinyl zirconium complex 128 which is coupled to a palladium-allyl complex in the next step. The double bond so produced is present with the same E configuration in the final product. It is marked 129 with an arrow in the diagram.28... 

Dicarboration of conjugated dienes can occur in either a 1,2- or a 1,4-addition mode. Several examples of stepwise stereoselective c -l,4-dicarborations of open-chain and cyclic dienes, via palladium-allyl complexes, which use stoichiometric amounts of palladium salts and soft, delocalized carbanions have been reported65 67. Catalytic dicarboration of 1,3-butadiene, stereoselectively leading to ( )-alkenes with up to 70% yield, is achieved using aryl iodides and delocalized carbanions of the malonate type in the presence of palladium(II) chloride and l,2-bis(diphenylphosphino)ethane68. A Tt-allyl-palladium intermediate is thought to determine the E geometry of the product. 

O. Winkelmann, C. Nather, U. Liining, Concave reagents, 56 synthesis, structure and catalytic activity of a himacrocyhc NHC palladium allyl complex, J. Organomet. Chem., 2008, 693, 2784-2788. 

Many studies of the addition of nucleophiles to palladium-allyl complexes have been conducted. Hayashi has shown that the additions of stabilized anions, such as malonate anions or amine nucleophiles, to chiral, non-racemic allyl complexes occur with inversion of configuration.Addition of excess phosphine and either diethyl malonate or dimethylamine to a chiral, non-racemic allyl complex results in nucleophilic attack with nearly complete inversion. The reaction with sodium dimethylmalonate is shown at the right of Equation 11.40. In contrast, nonstabilized carbanions such as allyl or phenyl magnesium chloride react with the same Ti -allylpalladium complex with retention of configuration as shown at the left of Equation 11.40. The stereochemistry from reaction of the Grignard reagents likely results from nucleophilic attack at the metal, followed by reductive elimination. 

The reaction of [Cp2Ta(=CH2)CH3] with the palladium allyl complex [CpPd(T]3-C3H5)] afforded 23 [Cp2Ta(ll-CH2)2PdCpl with loss of one equivalent of propene. 

A series of l-(pyridin-2-yl)benzimidazolium salts were synthesized by Chianese et al. and used as precursors to rigid, five-membered chelated biden-tate NHC-pyridine ligands [57]. The ligands were constructed with either very small (H, Me) or very bulky (2,6-diisopropylphenyl, 2,4,6-triisopropylphenyl) groups adjacent to the nitrogen and carbon donor atoms. Cationic palladium-allyl complexes were synthesized under mild conditions and used as catalysts for the allylic substitution reaction between allylic carbonates and either sodium dieth)f-2-methylmalonate or Af-methylbenzylamine. 

Jarvo et al. have reported the use of isolated NHCP palladium allyl complexes of the type [(R-2)Pd(i -C3H5)]X (X = C1, OAc, BF ) as catalysts for nucleophihc allylation reactions. They investigated a series of electron-rich monodentate and bidentate ligands (bisphosphines, functionahzed NHCs, monodentate NHCs) for their ability to promote nucleophilic attack of the corresponding palladium allyl complexes on electrophiles in, for example, the conjugate addition of allylboronic esters to a variety of ,p-unsaturated Af-acylpyrroles [15a] as well as the allylation of aldehydes by allylstannanes (Scheme 10.4) [15b]. 

Scheme 10.4 Allylation of a,p-unsaturated W-acylpyrroles and aldehydes with allylboronic esters and allylstannanes catalyzed by an NHCP palladium allyl complex.

Recently, we have developed an alternative synthesis that enables the formation of bulky and electron-rich Af-phosphinomethyl-substituted NHC metal complexes in just three steps (Scheme 10.9) [51]. Further, this procedure does not rely on the isolation of air-sensitive trialkylphosphines. The coupling of borane-protected di-tert-butylphosphine and the chloromethylated imidazohum salts R-40 (R = tBu, Mes) with w-butyl lithium generated the air-stable protected imidazolium salts R-41. In case of an Af-mesityl substitution, this compound could be converted into the palladium allyl complex Mes-42 in a high-yielding one-pot procedure. This feature could encourage the development of new trialkylphosphine-NHC hybrid ligands. 

Scheme 10.9 Efficient three-step synthesis of NEICP palladium allyl complexes. 10.4.2...

Also, pincer ligands have been prepared from l,3-bis(diisopropyl or /-butyl)(phosphinomethyl)ruthenocene. Notably, the iridium and rhodium complexes of these have been discussed. The iridium hydride complex [Ir(H)(Cl)[ 2,5-(Bu PCH2)2C5H2 Ru(Cp)]] exists in the form of a mixture of endo- and o-isomers, the exo being the more thermodynamically favorable.Trimetallic ruthenocene/ferrocene ligands 23 have been prepared in three steps from l,l -diacetylruthenocene, and further derivatized to the pentametallic bis-palladium allyl complex 24. 

Figure 24 Palladium allyl complexes used for the copolymerization of norbornene or the functional derivatives N-C02Et, N-CH20C(0)Me, N-CH2OH, and N-COOMe with ethene.

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