Biaryls, chelate formation

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). 

Chelation-assisted C-H/olefin coupling can be applied to the atroposelective alkylation of biaryl compounds. The reaction of 2-(l-naphthyl)-3-methylpyri-dine with ethylene using [RhCl(coe)2]2 where coe is cyclooctene, and PCy3 results in the formation of an ethylation product in 92% yield (Eq. 21) [20]. In place of the PCy3 ligand, the use of (R) - (1 - [ (S) - 2- diphenylphosphino ] ferro-cenyl)ethyl methyl ether [(R),(S)-PPFOMe] leads to the atropselective alkyla-... 

As a rule, biaryl compounds with four ortho substituents are configurationally stable, even under forcing conditions. If equipped with two chelating groups next to the axis, however, an atropisomerisation can occur in the presence of an appropriate transition metal such as copper, permitted by the resulting metallacycle with its lower rotational barrier. If the metal or the substrate itself is chirally modified, this opens up the possibility for the conversion of a racemic biaryl substrate into atropisomerically pure material by DKR, often already in situ, in the course of its formation. Many Ullmann couplings of chiral... 

Biaryl moieties are ubiquitous in natural products and pharmaceuticals [45]. Consequentially, efficient and direct synthetic protocols for the formation of biaryl molecules are highly desirable. The earliest report of chelate-assisted C-H arylation from 1998 describes the use of Wilkinson s catalyst [RhCl(PPh3)3] for the ortho-C-H functionahzation of 2-arylpyridines with tetraphenylstan-nane (Scheme 23.13) [46]. Monoarylated products can be obtained by using a blocking group on one of the ortho positions on the pyridine moiety of the substrate (13) that prevents arylation of the second ortho-C-H bond. In the absence of a blocking substituent in the substrate 14, diarylated products are formed. 

---