Metal chelating dienes

S.2. rj n-Alkyl Complexes by Nucleophilic Attack on Metal-Chelating Diene Complexes. 

The formation of a metal-substrate bond of intermediate stability is of key importance in a successful hydrogenation. If this bond is too strong, the subsequent migratory insertion may be hindered and hydrogenation cannot take place. Strongly chelating dienes, such as 1,5-cyclooctadiene (COD) or cyclopentadiene, form stable complexes that, in turn, are used as catalysts in hydrogenations. 

The second approach involves the addition of a chelated form of the transition metal, for example, Ci2-dien plus zinc(II) (where Ci2-dien is 4-dodecyldiethylenediamine), into the mobile phase (Fig. 2.7). In this example, the triamine chelate binds strongly to the metal ion, forming a complex cationic counterion. The metal chelate represents a conformationally semirigid structure with a local polarized charge center. As such, not only is there the typical electrostatic attraction of anions for the positively charged... 

Figure 2.7 Structure of Ci2-dien-Zn(II) metal chelate complex.

With AgC104, 1,5-hexadiene, 1,7-octadiene, and 1,9-decadiene form 3 2, 1 1, and 1 1 complexes, respectively (85a). The 1 1 complexes are considered (85a) to be coordination polymers while the 3 2 complex of 1,5-hexadiene has been shown (45a) to have chelated diene-silver ion entities connected through a bridging diene molecule to produce a distorted trigonal coordination about the metal. 

The oligomerizations of dienes catalyzed by nickel18 and cobalt19 complexes formed by electrochemical syntheses have already been described. Furthermore, it is worth mentioning here the electrosynthesis, patented by Lehmkuhl20,21, metal chelates of which have been proposed as catalysts for some polymerization reactions. The reaction is a good example of paired electrochemical syntheses, in which both the anodic and cathodic processes contribute to the overall reaction ... 

Since the investigations which used ZnCb and BF3-OEt2 as catalysts, other Lewis acids have been shown to catalyze the cyclocondensation reaction. MgBr2, for example, was used by Pearson and Danishefsky to add the power of chelation control to the reaction. When chiral a-alkoxy aldehydes are condensed with dienes under the influence of MgBr2, products derived from an ACF transition state predominate. The chelation of the metal by the a-alkoxy group of the aldehyde forces the aldehyde side chain to occupy the same side of the aldehyde as the metal. The diene, therefore, can attack the aldehyde from the least hindered exo face giving rise to the trans-KC product (Scheme 5). When an aldehyde is used that cannot form a chelate to the metal (such as benzaldehyde) syn (endo) selectivity is observed. 

Formation of Carbon-Transition and Inner Transition Metal Bond 169 5.8.2.5. from Chelating Dienes... 

Chelating Diene Complexes Reaction with Metal Complexes. 

I. Chelating Diene Complexes by Direct Reaction with Metal Complexes via Ligand Displacement. 

Investigations aimed at finding new and easily accessible ligands for asymmetric transformations are an active area of research. It is widely known that chelating dienes such as 1-5-cycIooctadiene (cod) and norbomadiene (nbd) are stable ligands for late transition metal complexes, but use of their chiral versions has not been widely explored. Hayashi et and Defieber et have independently... 

The idea that steric compression on intrinsically highly active metallacarbenes affords cis polymers is vindicated by recent work where we used Lewis-base solvents such as ethers with early transition metal chloride catalysts, based on Ta, Nb, Mo and W, to drive up cis contents (see Figure 3) [6]. The same effect was also achieved using chelating dienes as spectator ligands with noble metal salt catalysts, and significantly by carrying out polymerizations in supercritical CO2 [7]. 

Esters are processing and long-term stabilizers, sulfur containing compounds are antioxidants for PO and diene rubbers, while oxamides are metal chelators. 

Chelating dienes have recently emerged as a promising class of chiral ligands for transition-metal-catalyzed processes as a consequence of independent concurrent studies by Hayashi [148,149] and by Carreira [150-153]. The first example with a chiral rhodium-diene complex for conjugate addition reactions employed chiral C2-symmetric diene ligand 193 (Equation 36) [148, 149]. Addition of boronic acid 192 to cyclohexenone catalyzed by the putative complex formed in situ from 193 and Rh(I) afforded adduct 194 in 90% yield and 99% ee [148]. 

More synthetic interest is generated by the potentially very useful hydration of dienes. As shown on Scheme 9.6, methylethylketone (MEK) can be produced from the relatively cheap and easily available 1,3-butadiene with combined catalysis by an acid and a transition metal catalyst. Ruthenium complexes of several N-N chelating Hgands (mostly of the phenanthroline and bipyridine type) were found active for this transformation in the presence of Bronsted acids with weakly coordinating anions, typically p-toluenesulfonic acid, TsOH [18,19]. In favourable cases 90 % yield of MEK, based on butadiene, could be obtained. 

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