Pi allyl complexes

A palladium-based method has been developed for the alkylation of the phenolic oxygen of tyrosine residues. Fig. 5f (61). In this reaction, allylic carbonates, esters, and carbamates are activated by palladium(O) complexes in aqueous solution to form electrophilic pi-allyl complexes. These species react at pH 8-10 with the phenolate anions of tyrosine residues, which results in the formation of an aryl ether and the regeneration of the Pd(0) catalyst. The reaction requires P(m-C6H4S03 )3 as a water-soluble phosphine ligand. Activated pi-allyl complexes that do not react with tyrosine residues undergo P-hydride elimination under the basic conditions to yield diene by-products. A particularly attractive feature of this method is its ability to use substrates with charged groups in the allylic positions. This ability allows hydrophobic substrates, such as lipids, to be solubilized to facilitate protein modification. 

Compared to the externally adsorbed complex, encaging has a profound effect on the t/c ratio of the olefins formed. As suggested by the authors (36) this is firm evidence for catalysis via an anti pi-allyl complex rather than via carbanion chemistry, favoring in absence of any constraints the cis-isomer. The abundant formation of t-but-2-ene in for the encaged complex, points to the presence of steric constraints on the transition state and suggests that the less bulky syn pi-allyl intermediate is favored (36) (equation XI). 

The first application of phase transfer catalysis in metal carbonyl chemistry was reported by Alper in 1977(23). It was found that metal carbonyl anions could be readily generated by this technique and used to prepare pi-allyl, cluster, and ortho-metalated complexes(24). 

Bis-pi-allyl Pd and Pt complexes have been found to catalyze the addition of allyl tributyltin to aldehydes [26]. These catalysts are formed in situ from Pd- and PtCl2-phosphine complexes and the allylstannanes (Fig. 7, step 1). The allylation step is depicted as a metallocene reaction of the aldehyde and an his-allylmetal complex (Fig. 7, step 3). The catalyst is regenerated by attack of the allylic stannane on the alcoholate-palladium complex formed in step 3. Representative additions of allyl and methallyl tributyltin to aldehydes with the pi-allyl platinum catalyst are summarized in Table 14. 

Figure 7. Allylation of aldehydes by CH2=CHCH2SnBu3 catalyzed by a bis-pi-allyl palladium complex.

While pz -allyl complexes are out of the scope of this review many complexes exist in which pz-o-co-ordination is present. In such cases the pz -bond resembles a normal metal-olefin pi-bond. Examples are the tetramer [PtClallyl]4 (D.XVIII) or the dimer [PtCl-acac]2 shown in D.XIX, 2°). 

The structure and dynamics of Rh(77 -C3H5)3 were studied by DFT analysis. The chemistry of Ir(77 -allyl)3 and Rh( j -allyl)3 was compared. While addition of phosphorus ligands such as P(OPh)3 to Rh( 7 -allyl)3 gave monovalent Rh( j -allyl)L, the iridium analog provided stable mixed sigma/pi-tris(allyl) complexes, as evidenced by the structural characterization of mono-, bi-, and tridentate ligand complexes. The addition of CO also provided substantial differences, as shown in Scheme 56. ... 

This is but one example mention should be made of the pioneering work metal complexes of Yermakov and Ballard and their coworkers. covering a multitude of pi-allyl... 

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

A recently described method for insertion of a carbon monoxide molecule into the monoepoxide of a conjugated diene gives /3-lactones in high yield. This is achieved by reaction of iron pentacarbonyl with the starting vinyloxirane to give the 7r-allyl iron complex (66), which on oxidation with cerium(IV) ammonium nitrate gives the /3-lactone. In some cases, y-Iactone products can also be obtained from this reaction (8lJCS(Pi)270). 

The detailed mechanism by which these additions and eliminations occur is of interest. The rate expression in all cases is consistent with the first step of the exchange being the formation of a dimeric complex according to Equation 10 (ol = vinyl or allylic ester or chloride). Pi com-... 

The ready hydrogenation and isomerization of methyl oleate and palmitoleate with Fe(CO)s confirm the results of Ogata and Misono (18) with monounsaturated aliphatic compounds. In the isomerization of monoolefins Manuel (15) suggested the occurrence of equilibria involving either 7r-olefin HFe(CO)3 and a-alkyl Fe(CO)3 complexes, or TT-olefin Fe(CO)3 and 7r-allyl HFe(CO)3 complexes. The formation of olefin-iron tetracarbonyl complexes has been reported (19). The reaction of butadiene and Fe2(CO)9 has been observed to lead to the formation of butadiene-Fe(CO)4 and butadiene-[Fe(CO)4]2 complexes in which one or both double bonds are pi-bonded to the iron (16). A mechanism involving both monoene-Fe(CO)4 (I) and allyl-HFe(CO)3 complexes (II) is postulated for the isomerization of methyl oleate (Scheme II) and for its homogeneous hydrogenation. 

Lone pair Z Base and Adjacent CH Complex Metal Hydride MH4- Organo- metallic R-M Allylic Z-C=C Simple Pi Bonds C=C CsC Aromatics 0... 

On this basis, then, it appears that the stereoselectivity of olefin isomerization can be adequately explained within the framework of classical carbonium ion theory without invoking such dubious species as the pi-complex or cyclic carbonium ion. The attractiveness of this mechanism is further enhanced by the fact that it is similar to the allyl carbanion mechanism proposed for the base catalyzed isomerization (145). Perhaps it would not be too great an extrapolation to presume that a similar mechanism, involving allyl radicals, may obtain over reduced metal catalysts. 

More complex transfers of sites have been demonstrated by saturation transfer studies used in the elucidation of sigma-pi rearrangements of allyl... 

---