Masaaki Yoshifuji

Poly(Biradicals): Oligomers of 1,3-Diphosphacyclobutane-2,4-diyl Units†

occupies a special place as a product of low-coordinated organophosphorus chemistry. This biradical has a range of potential applications owing to its considerable stability, as described by Niecke et al. and our group. Some of our air-stable 1,3-diphosphacyclobutane-2,4-diyls of type I are quite electron-rich and are easily oxidized to the corresponding P-heterocyclic cation radicals (II). The 1,3-diphosphacyclobutane-2,4-diyl moiety may thereby serve as a module for redox-functionalized molecular systems. In addition to the biradical derivatives, we succeeded in preparing a neutral monoradical III by use of a synthetic intermediate for I (1). Covalent assembly of redox-active biradical units derived from I is an attractive strategy to develop new materials for molecular electronics and spintronics applications. As an example of double catenation of stable biradical units, Bertrand and co-workers succeeded in binding two 1,3dibora-2,4-diphosphoniocyclobutane-1,3-diyl moieties (IV) with the mor p-phenylene unit to build p-conjugative communication between the P2B2 biradical centers. [9] We are interested in distinct “multi-biradical” systems in which each biradical unit experiences a static interaction through a nonconjugative covalent spacer. Such nonconjugative catenation will lead to stable polyradical systems without strong or shared interaction between individual biradical components. Therefore, on the basis of our synthetic method for 1,3diphosphacyclobutane-2,4-diyls, we demonstrate multi-biradical derivatives in which the P-heterocyclic biradical moieties are catenated with dior trimethylenephenyl groups. According to our previous report (Scheme 1), anion 1 was prepared from phosphaalkyne Mes*C P

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 Structure of Sterically Crowded Triarylphosphine Bearing 4-Bromo-2,6-Bis(4-tert-Butylphenyl)Phenyl Group

Abstract A novel sterically crowded triarylphosphine bearing a 2,6-diaryl-4-bromophenyl group, [4-bromo-2,6-bis(4-tert-butylphenyl)phenyl]bis(2,4,6-triisopropylphenyl)phosphine (1), was synthesized. The X-ray crystallography revealed that the bond angles and lengths around the phosphorus atom (110.9°, 1.846 Å) are comparable to those of tris(2,4,6-triisopropylphenyl)phosphine and its derivatives. Triarylphosphine 1 is reversibly oxidized to the radical cation at the potential close to that of the typical sterically crowded (bromoaryl)phosphine, (4-bromo-2,6-diisopropylphenyl)bis(2,4,6-triisopropylphenyl)phosphine, and exhibits the UV absorption at long wavelength (λmax 332 nm) characteristic of the sterically crowded triarylphosphines. Thus, triarylphosphine 1 can be regarded to be a promising candidate for the key synthetic intermediate or building block to the functional molecules bearing sterically crowded triarylphosphine moieties. GRAPHICAL ABSTRACT

Studies on Stable 1,3-Diphosphacyclobutane-2,4-diyls

Various kinds of stable 1,3-diphosphacyclobutane-2,4-diyls, sterically protected by bulky Mes* (2,4,6-tri-t-butylphenyl) groups, were prepared from Mes*-phosphaethyne. The structures and physical and chemical characters were studied. They are discussed in terms of ground-state singlet biradicals and have been confirmed by spectroscopic analyses, X-ray crystallography and theoretical calculations. EPR studies were also carried out to validate the paramagnetic species related to the 1,3-diphosphacyclobutane-2,4-diyls. The reactions of the biradical species are described including oxidation and sulfurization.

Design, Synthesis, and Application of Phosphaalkenes to Unique Palladium and Gold Catalysts

A 2-silyl-1,3-diphosphapropene was utilized as an asymmetrical P2 ligand of stable palladium(II) catalysts. Several phosphaalkene ligands were utilized for synthesis of novel gold(I) complexes which exhibited unique catalytic activity in 1,6-enyne cycloisomerization, indicating effects of P = C double bond, Au–Au contact, and sulfanyl group.

NMR Studies of 1-tert-Butyl-3-methyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl

Abstract Intensive 1H-, 13C-, and 31P-NMR studies of 1-tert-butyl-3-methyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl were perfomed to reveal several unusual coupling constants.

Studies on Intramolecular Cyclization of 2,4,6-Tri-Tert-Butylphenyl–P=C:In Memory of Professor Dr. Friedrich Bickelhaupt, Scheikundig Laboratorium, Vrije Universiteit, Amsterdam

GRAPHICAL ABSTRACT Abstract The intramolecular C–H insertion of the Mes*-substituted phosphanylidenecarbene [Mes*P=C:] (Mes* = 2,4,6-t-Bu3C6H2) and characters of the cyclization product, 6,8-di-tert-butyl-3,4-dihydro-4,4-dimethyl-1-phosphanaphthalene, were studied based on ab initio and density functional theory calculations.

Synthesis, Structure, and Properties of [4-Methyl-2,6-Bis(2-Methylphenyl)Phenyl]Bis(2,4,6-Triisopropylphenyl)Phosphine

GRAPHICAL ABSTRACT Abstract A novel sterically crowded triarylphosphine, [4-methyl-2,6-bis(2-methylphenyl)phenyl]bis(2,4,6-triisopropylphenyl)phosphine (1), was synthesized by the reaction of 4-methyl-2,6-bis(2-methylphenyl)phenylcopper(I) with chlorobis(2,4,6-triisopropylphenyl)phosphine. Triarylphosphine 1 exhibits a low inversion barrier, a reversible oxidation at low potential, and a UV absorption in long wavelength typical of sterically crowded triarylphosphines. The mCPBA oxidation of 1 gave the corresponding phosphine oxide of more crowded and rigid structure.

Diverse Reactions of Sterically Protected 1,3-Diphosphacyclobutane-2,4-diyls with Hydride

Abstract Stable 1,3-diphosphacyclobutane-2,4-diyls carrying a substituent such as benzyl moiety react with hydride to cause reductive P–C bond cleavage leading to the cyclic phosphaallyl anion, which is the synthetic intermediate of biradicals.

Modeling the Direct Activation of Dihydrogen by a P2C2-Cyclic Biradical: Formation of a Cyclic Bis(P-H λ5-phosphorane)

Abstract 1-tert-Butyl-3-methyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl reacts with hydride followed by proton quench to give a 2,4-diphospha-1,3-cyclobutadiene with diylidic structure.