Biaryl complexes, chelating

The chiral center most frequently encountered is the asymmetric carbon atom, a tetrahedral C atom, bonded to four different substituents. Chiral centers of this type are known for many other elements (4). However, chiral centers are also found in other polyhedra, e.g., the metal atoms in octahedral compounds containing three bidendate chelate ligands. Chirality axes, present in the atrop isomers of ortho-substituted biaryls, occur in coordination chemistry in appropriately substituted aryl, pyridyl, and carbene metal complexes. Well known examples of planar chirality in organometallic chemistry are ferrocenes, cymantrenes, and benchrotrenes containing two different substituents in 1,2- or 1,3-positions relative to each other (5-5). 

The presence of chelating groups in those complexes is necessary to stabilize the intermediate aryl-palladium complex for isolation but it does not seem necessary to cause palladation. The chelating group does, however, tremendously accelerate the palladation. Aromatic compounds reactive to electrophilic substitution apparently undergo palladation with palladium acetate in acetic acid solution fairly readily at 100 °C or above. Of course, the arylpalladium acetates presumably formed, are not stable under these conditions, and they decompose very rapidly into biaryls and palladium metal 34,35,36) ag do aryl palladium salts prepared by the exchange route 24>. If the direct palladation is carried out in the presence of suitable olefins, arylation can be achieved, so far, however, only in poor yields, arid with concurrent loss of stereospecificity and formation of isomers and other side products 37.38). 

Upon treatment with phosphine or chloride, some of the C,N-chelated diarylgold(III) chloride complexes undergo reductive elimination of the aryl ligands to give biaryls. The high-yield reaction proceeds at room temperature and can be used for the synthesis of... 

Scheme 2.25 Multiple chelation modes in rhodium complexes with biaryl tetraphosphines. 2.1.5...

Phosphorus also functions as a chelating group. The carbonyl group of a phosphine-tethered benzophenone/nickel complex was extruded upon heating to give a biaryl (Scheme 1.18) [28]. The extrusion/insertion of CO was reversible, indicating that the aryl-aryl bond also adds to nickel(O) facilely. The related oxidative addition reaction to iridiumfl) and rhodiumfl) was also reported [29]. 

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