Palladium-catalyzed aerobic oxidation reactions have been the focus of industrial application and extensive research efforts for nearly 60 years. A significant transition occurred in this field approximately 20 years ago, with the introduction of catalysts supported by ancillary ligands. The ligands play crucial roles in the reactions, including promotion of direct oxidation of palladium(0) by O2, bypassing the typical requirement for Cu salts or related redox cocatalysts to facilitate oxidation of the reduced Pd catalyst; facilitation of key bond-breaking and bond-forming steps during substrate oxidation; and modulation of chemo-, regio-, or stereoselectivity of a reaction. The use of ligands has contributed to significant expansion of the scope of accessible aerobic oxidation reactions. Increased understanding of the role of ancillary ligands should promote the development of new synthetic transformations, enable improved control over the reaction selectivity, and improve catalyst activity and stability...

Ligand-Promoted Palladium-Catalyzed Aerobic Oxidation Reactions

Homogeneous catalysis with transition metal complexes containing sulfur ligands

Steric Influence on the Reactivity of the (o-Carboranedithiolato)iridium(III) Complex [Ir(η5-C5Me5)(η2-S2C2B10H10)]: New Types of Addition Reactions Involving Cyclometalation or Isomerization†

The syntheses and unique chemical properties of metalladithiolene rings (especially in [CpM(S2C2R1R2)]-type complexes) found by our research group are reviewed. The constructions of metalladithiolene rings by one-pot reaction among [CpM(CO)n], alkyne, and elemental sulfur and by the reaction between [CpM(CO)n] and sulfur-containing heterocyclic compounds are described. Several ionic and radical substitution reactions are presented. The substitutions by carbon-centered and sulfur-centered radicals occur in the metalladithiolene rings. Benzoyloxy-substitution by dibenzoyl peroxide (BPO) and succinimido-substitution by N-halosuccinimides (NXS) proceed via ionic mechanisms induced by the heterolysis of BPO and NXS in the interaction with the lone pair electrons at the sulfur atoms. These substitutions are due to the aromatic character of the metalladithiolene rings. The unique addition reactions due to the unsaturated character of the metalladithiolene rings are also described. Diazo compounds and azides reac...

Synthesis and Chemical Properties of Metalladithiolene and Metalladithiazole Rings. Unique Reactions Due to Coexistence of Aromaticity and Unsaturation

The dinuclear Cp*Co dithiolene complex [Cp*Co(btt)CoCp*] ( 1) is prepared in high yield from the reaction of the bis(dibutyltin) complex of 1,2,4,5-benzenetetrathiolate (btt) with 2 equiv of [Cp*Co(CO)I 2]. Mononuclear complexes are also obtained from 1,2,4,5-tetrakis(isopropylthio)benzene ( 2) and sodium in pyridine, from benzo[1,2- d;4,5- d ']bis(1,3-dithiolane-2,6-dione) ( 3) and ( t )BuOK in tetrahydrofuran, or from benzo[ d]-1,3-dithiolane-2-one ( 7) and ( t )BuOK to afford respectively 4a, 4b, and [Cp*Co(bdt)] ( 6), while [Cp*Co(dmit)] ( 8) is obtained by literature methods. The X-ray structures of the dinuclear complex 1 and the mononuclear complexes 4a and 6 were determined. They are all characterized by Cp* . . . btt face-to-face intermolecular interactions, leading to a recurrent 4-fold symmetry motif. The cyclic voltammograms of the [Cp*Co(dithiolene)] complexes performed in CH 2Cl 2 show reversible Co (III) to Co (II) reduction but irreversible oxidation waves. The large potential difference between the two reduction waves of the bimetallic complex 1 (269 mV) indicates a stable mixed-valence Co (III)-Co (II) state for the reduced [Cp*Co(btt)CoCp*] (-) anion. Upon trimethyl phosphite addition, the mono-P(OMe) 3 adduct [ 1.P(OMe) 3] exhibits a red shift of the low-energy absorption band to the IR region (856 nm, = 13 000 M (-1).cm (-1)), while [ 8.P(OMe) 3] exhibits a 150 nm blue shift. The stability constants of these P(OMe) 3 adducts were determined from UV-vis spectroscopic titration experiments, with, for example, log( K/mol (-1).dm (3)) values of 3.1 and 0.52 for the mono- and bis-adduct of 1, respectively. The electrochemical investigation of 1 and 8 in excess phosphite shows a strong current enhancement upon oxidation, attributable to the catalytic generation of the radical cation P(OMe) 3 (*+).

Dinuclear Cp* cobalt complexes of the 1,2,4,5-benzenetetrathiolate bischelating ligand.

Formation, structure, and reactivities of 1:1 adducts between quadricyclane and (η5-cyclopentadienyl)(1,2-diphenyl- or -dimethoxycarbonyl-1,2-ethenedithiolato)rhodium(III)

Mikako Yokoyama

Papers

Formation, structure, and reactivities of 1:1 adducts between quadricyclane and (η5-cyclopentadienyl)(1,2-diphenyl- or -dimethoxycarbonyl-1,2-ethenedithiolato)rhodium(III)

Isomerization of quadricyclane to norbornadiene catalysed by (η5-cyclopentadienyl)(substituted 1,2-benzenedithiolato)cobalt(III) complexes