Synthesis, Electrochemistry, and Imido Transfer Reactions of (TTP)Ti(eta(2)-PhN=NPh).

Treatment of (TTP)Ti(eta(2)-RC&tbd1;CR) (R = Et or Ph) with PhN=NPh results in formation of the azobenzene adduct (TTP)Ti(eta(2)-PhN=NPh) (1) in good isolated yield. Complex 1 reacts with (TTP)Ti(eta(2)-RC&tbd1;CR) at elevated temperatures to cleanly afford 2 equiv of the phenylimido compound, (TTP)Ti=NPh (2). The azobenzene complex, 1, is also formed in low yields by the reaction of the (TTP)Ti=NPh (2) with excess 1,2-diphenylhydrazine. The electrochemistry of the azobenzene adduct (1) and the phenylimido complex (2) is investigated by cyclic voltammetry experiments.

"Inverse-electron-demand" ligand substitution: experimental and computational insights into olefin exchange at palladium(0).

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-beta-nitrostyrene ligands (ns(X) = X-C(6)H(4)CH=CHNO(2); X = OCH(3), CH(3), H, Br, CF(3)), (bc)Pd(0)ns(X) (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(ns(CH)()3) and ns(X) (X = OCH(3), H, Br, CF(3)), more-electron-deficient olefins react more rapidly (relative rate: ns(CF)()3 > ns(Br) > ns(H) > ns(OCH)()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 pi 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.

"Inverse-electron-demand" ligand substitution in palladium(0)-olefin complexes.

Ligand substitution reactions are ubiquitous in transition-metal chemistry and catalysis. Investigation of ligand substitution reactions for a series of electron-rich palladium(0)-olefin complexes, (bathocuproine)Pd(nitrostyrene) reveals an unprecedented mechanism in which the metal serves as the nucleophilic partner in an "associative" substitution pathway.