MAPPING THE REACTION PATHWAY IN PALLADIUM-CATALYZED CROSS-COUPLING REACTIONS

The formation of the cyclooctatetraene (COT) complex Pd(dppf)(COT) (dppf = 1,1’-bis(diphenyl-phosphino)ferrocene) from Pd(dppf)X2 (X = Cl, Br, I) and dilithium cyclooctatetraenide in situ affords an entry into the cross-coupling chemistry of this system. The reaction of Pd(dppf) (COT) with an alkenyl bromide or iodide results in the formation of the appropriate (η1alkenyl)palladium halide via an oxidative addition of a η2-coordinated alkene species. The η1-2-alkenyl complexes react with norbornene to give insertion products, while Grignard reagents give the products of nucleophilic attack and carbon-carbon coupling. The full characterization of the intermediate Pd(dppf)(CH═CHC6H4OCH3)Br (9) and the norbornene insertion product Pd(dppf)(C7H10CH═CHC6H4OCH3) (15) is reported. A set of putatie intermediates for the coupling of alkenyl and benzyl moieties by palladium complexes is demonstrated spectroscopically. The cross-coupling cycle for the case of alkenyl-aryl coupling is delineated by an analysis of cross-coupling reactions employing 13C-labeled catalysts and unlabeled substrates (and vice versa). Platinum analogues of all the intermediates in this cross-coupling catalytic cycle have been observed. © 1990, American Chemical Society. All rights reserved.