Regioselective Reactions Based on Chelation and Participation of Heteroatoms

8 Regioselective Reactions Based on Chelation and Participation of Heteroatoms 

Unactivated aromatic and alkyl groups can be functionalized by formation of chelate Pd complexes and interesting applications have been published, although most of them are stoichiometric reactions. 

Aromatic compounds 413, bearing hetero atom-containing groups at positions suitable for forming mainly five-membered or sometimes six-membered chelating rings, undergo cyclopalladation at an ort/io-position to form a cr-arylpalladium bond as in 414 by virtue of the stabilization due to the chelation of these hetero atoms. The ort/io-palladation products 414 are stable and can be isolated [173]. After the first report on the preparation of the azobenzene and A, A-dimethylbenzy-lamine complexes 415 and 416 [174], numerous complexes have been prepared. 

The cr-arylpalladium bonds in these eomplexes are reactive and undergo insertion and substitution reactions, and the reactions offer useful methods for the regiospe-cihc functionalization of the aromatic rings. Alkenes, alkynes and CO insert under certain conditions. Some classical examples are cited in the following. 

The hrst example is insertion of styrene to Ar,AT-dimethylbenzylamine complex (416) to form the stilbene derivative 417, which takes place smoothly at room temperature in AcOH [14]. Alkynes insert smoothly. Two moles of diphenylacetylene insert into the benzyl methyl sulfide complex 418 to afford the eight-membered heterocycle 419 [175]. 

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