INFLUENCES ON THE RELATIVE RATES FOR C-N BOND-FORMING REDUCTIVE ELIMINATION AND BETA-HYDROGEN ELIMINATION OF AMIDES - A CASE-STUDY ON THE ORIGINS OF COMPETING REDUCTION IN THE PALLADIUM-CATALYZED AMINATION OF ARYL HALIDES

Typical decomposition by beta-hydrogen elimination has limited the pro ductive catalytic organometallic chemistry of late transition metal am ido complexes. However, one reaction that has been shown to involve a late metal amido complex with beta-hydrogens and elude extensive beta- hydrogen elimination is the palladium-catalyzed amination of aryl brom ides to give arylamines. The primary side products formed in these cat alytic aminations are arenes, the products of aryl halide reduction. I t would seem reasonable that both arylamine and arene products result from competitive reductive elimination of amine and beta-hydrogen elim ination from a common amido aryl intermediate. Our results do substant iate competitive beta-hydrogen elimination and reductive elimination i nvolving an amido group, but also reveal a second pathway to reduction that occurs when employing Pd(II) precursors. This second pathway for aryl halide reduction was shown principally by the observations that (1) stoichiometric reactions of aryl halide complexes or catalytic rea ctions employing [P(o-tolyl)(3)]Pd-2(0) showed less arene side product than did catalytic reactions employing Pd(II) precursors, (2) increas ing amounts of Pd(II) catalyst gave increasing amounts of arene produc t, and (3) reactions catalyzed by Pd(II) precursors showed amine:arene ratios at early reaction times that were lower than ratios after comp lete reaction. In addition to data concerning arene formation during P d(II) reduction, we report data that demonstrate how electronic and st eric factors control the relative rates for amine vs arene formation. The relative amounts of reduction product and amination product depend on the size of the phosphine and substitution pattern of the amide li gands. Systematic variation of phosphine size demonstrated that increa sing the size of this Ligand gave increasing amounts of arylamine prod uct, increasing size of the amido group gave increasing amounts of ary lamine product, while decreased nucleophilicity of the amide gave decr eased amounts of arylamine product. Further, the presence of electron withdrawing groups on the palladium-bound aryl ring accelerated the re ductive elimination reaction, relative to beta-hydrogen elimination, a nd this result is consistent with previously observed acceleration of carbon-heteroatom bond-forming reductive eliminations with isolable pa lladium complexes.