Preparation and Properties of 1,2-Diferrocenyl-Substituted 3,4-Diphosphinidenecyclobutenes and Their Complexes: The First Application of a Diphosphinidenecyclobutene Unit as a Linker of Functional Moieties

Catalytic applications of transition-metal complexes bearing diphosphinidenecyclobutenes (DPCB)

The preparation and characterization of sterically protected diphosphinidenecyclobutenes bearing two two-coordinate phosphorus atoms are described from the viewpoint of the development of a novel type of bidentate ligand for transition-metal complexes. They form complexes with group 6 metals such as chromium, molybdenum and tungsten, group 8 metals such as ruthenium, group 10 metals such as palladium and platinum, and group 11 metals such as copper and gold. Some of them can be used as catalysts for synthetic reactions such as cross-coupling of the Sonogashira type, Suzuki-Miyaura coupling, Ullmann coupling, Kosugi-Migita-Stille coupling, cyanation, polymerization of ethylene, dehydrogenative hydrosilylation of ketones, hydroamination of 1,3-butadienes, and direct alkylation or amination of allylic alcohols of Tsuji-Trost reactions.

Synthesis and Redox Properties of Crowded Triarylphosphines Possessing Ferrocenyl Groups

Crowded triarylphosphines possessing ferrocenyl groups [(4-ferrocenyl-2,6-diisopropylphenyl)(n)(2,4,6-triisopropylphenyl)(3-n)P (5a, n = 1; 5b, n = 2; 5c, n = 3)] were synthesized by the reaction of the corresponding arylcopper(I) reagents with the diarylchlorophosphines. Structures of the triarylphosphines were studied by 1H, 13C, and 31P NMR spectroscopies, and the characteristic patterns of the proton signals of the 2,6-isopropyl groups and upfielded 31P chemical shifts suggest structural similarities of the triarylphosphine moiety to the previously reported tris(2,4,6-triisopropylphenyl)phosphine (2). X-ray crystallography of 5c also revealed that the structure around the phosphorus is similar to that of 1, where the average bond angle and length around the phosphorus atom are 110.8 degrees and 1.854 A, respectively. According to the electrochemical measurements, phosphines 5a, 5b, and 5c are reversibly oxidized in two, three, and four steps, respectively, which suggests significant electronic interaction among the triarylphosphine and the ferrocene redox centers as well as weak interaction among the ferrocene redox centers. The EPR spectra obtained at cryogenic temperature after oxidation of phosphines 5a, 5b, and 5c are superpositions of those for the cation radicals of the crowded triarylphosphine and ferricinium. The solution spectra obtained at 293 K, which consist of two lines typical of the cation radical of the crowded triarylphosphines, become weaker as the number of the ferrocenyl groups increases and the cation radical of 5c does not show EPR signals. These findings suggest not only instability of the tetra(cation radical) of 5c but also the course of oxidation where the ferrocenyl groups in the periphery of the molecules are oxidized at first.