“Inverse-Electron-Demand” Ligand Substitution:  Experimental and Computational Insights into Olefin Exchange at Palladium(0)

• Published in: Journal of the American Chemical Society 2004-11, Vol.126 (45), p.14832-14842
• Main Authors: Popp, Brian V, Thorman, Joseph L, Morales, Christine M, Landis, Clark R, Stahl, Shannon S
• Format: Article
• Language: English
• Subjects: Alkenes - chemistry; Alkynes - chemistry; Benzene Derivatives - chemistry; Chemistry; Exact sciences and technology; Inorganic chemistry and origins of life; Kinetics; Kinetics and mechanism of reactions; Ligands; Magnetic Resonance Spectroscopy; Models, Chemical; Models, Molecular; Organometallic Compounds - chemistry; Palladium - chemistry; Phenanthrolines - chemistry; Styrenes - chemistry; Thermodynamics
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0459734

The mechanism of olefin substitution at palladium(0) has been studied, and the results provide unique insights into the fundamental reactivity of electron-rich late transition metals. A systematic series of bathocuproine−palladium(0) complexes bearing trans-β-nitrostyrene ligands (nsX = XC6H4CHCHNO2; X = OCH3, CH3, H, Br, CF3), (bc)Pd0nsX (3 X), was prepared and characterized, and olefin-substitution reactions of these complexes were found to proceed by an associative mechanism. In cross-reactions between (bc)Pd(nsCH 3 ) and nsX (X = OCH3, H, Br, CF3), more-electron-deficient olefins react more rapidly (relative rate:  nsCF 3 > nsBr > nsH > nsOCH 3 ). Density functional theory calculations of model alkene-substitution reactions at a diimine−palladium(0) center reveal that the palladium center reacts as a nucleophile via attack of a metal-based lone pair on the empty π* orbital of the incoming olefin. This orbital picture contrasts that of traditional ligand-substitution reactions, in which the incoming ligand donates electron density into an acceptor orbital on the metal. On the basis of these results, olefin substitution at palladium(0) is classified as an “inverse-electron-demand” ligand-substitution reaction.