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The sterically crowded triarylphosphine bearing two phenothiazine moieties as redox sites was synthesized. The cyclic voltammogram of the phosphine exhibits three-step reversible oxidation, and the...

Various kinds of sterically protected low-coordinate phosphorus compounds with Mes* (2,4,6-tri-tert-butylphenyl) group such as diphosphenes, 1-phosphaethenes, 1-phosphaallenes, 1,3-diphosphaallenes...

Utilizing 2,4,6-tri-tert-butylphenyl as a sterically demanding group (abbreviated as Mes*), we have been successful in isolation, characterization, physical and chemical property investigation of low-coordinate organophosphorus compounds such as those carrying P=P and P=C bonds. Diphosphene, 1-phosphaethene, 1,3-diphosphaallene, 1-phosphaallene, and 3,4-diphosphinidenecyclobutenes are discussed in terms of stereochemistry including E/Z isomerization around P=P or P=C bond and R/S resolution of rotational isomers around crowded P–C bonds with chiral HPLC column. (π-Allyl)palladium complexes ligated with 3,4-diphosphinidenecyclobutene are useful in such catalytic organic reactions as the Tsuji-Trost reaction by which from a chiral s-alcohol, the corresponding chiral amine is obtained with retention of chirality suggesting double inversion process.

The character of bis(2,4,6-tri-tert-butylphenyl)diphosphene is described experimentally and theoretically. The diphosphene is stabilized by steric protection and the structure can be characterized by spectroscopic as well as crystallographic analyses. Theoretical calculation on the diphosphene strongly suggests that the P=P bond is an isolated double bond and that the P–C bonds are single covalent bond. The reactivity has been investigated including photolysis, oxidation, sulfurization, selenation, transition-metal complex formation, and carbene addition. Plausible mechanistic scheme for the reaction of the diphosphene with 3,4,5,6-tetrachlorocyclohexa-3,5-diene-1,2-dione (or tetrachloro-o-benzoquinone) to a pentavalent spiro product is discussed based on the product analysis.

The character of bis(2,4,6-tri-t-butylphenyl)diphosphene is described experimentally and theoretically. The diphosphene is sterically protected and the structure can be characterized by spectroscop...

Diphosphenes with various kinds of sterically protecting groups are described and discussed with regard to 31P NMR spectroscopic data, X-ray results, preparative methods, and some reactions including end-on coordination complex formations. New results of theoretical calculations on the particular diphosphene (E)-Mes*P=PMes* are also described.

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.

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-dime...

Two diphosphines bearing an ethynylene and a 1,2-phenylene linkers between the sterically crowded triarylphosphine moieties have been synthesized by employing Sonogashira and Suzuki-Miyaura couplings as key reactions, respectively. Intramolecular interactions between the two phosphine moieties were studied by UV–Vis spectroscopy and cyclic voltammetry. The UV–Vis spectrum of the ethynylene bridged diphosphine exhibits visible absorption in longer wavelength than that of the 1,2-phenylene bridged derivative reflecting lower LUMO spreading over the diphenylacetylene moiety. On the other hand, the cyclic voltammogram of the 1,2-phenylene bridged diphosphine shows larger difference of the first and the second oxidation potentials reflecting larger positive charge repulsion upon oxidation. The trend observed in the UV–Vis absorptions is similar to that of the structurally related diamines, whereas the voltammograms show the opposite trend, or dependence on the distance between the redox centers, reflecting the limited π-conjugation of the triarylphosphines.

Novel 2,6-diaryl-4-hydroxybenzenethiol, 2,6-bis(2,4,6-triisopropylphenyl)-4-hydroxybenzenethiol, was synthesized from the corresponding 2,6-diaryl-4-bromoiodobenzene as a potential precursor of the corresponding thiobenzoquinone. The X-ray crystallography showed that the two aryl groups are nearly orthogonal to the central benzene ring and shield the mercapto group, but there still remains open space and the oxidation of the 4-hydroxybenzenethiol by MnO2 led to the formation of the corresponding disulfide. The oxidation of the 4-hydroxybenzenethiol as well as the corresponding disulfide was studied, and the spectroscopic studies suggested the formation of the labile thiobenzoquinone.

The intramolecular C–H insertion of the Mes*-substituted phosphanylidenecarbene [Mes*P=C:] (Mes* = 2,4,6-t-Bu3C6H2) and physicochemical properties of the cyclized product, 6,8-di-tert-butyl-3,4-dihydro-4,4-dimethyl-1-phosphanaphthalene were studied based on ab initio calculations. Whereas the alternative Fritsch–Buttenberg–Wiechell-type rearrangement requires almost no activation energy, the intramolecular cyclization needs an activation energy of 12.3 kcal/mol at the MP2(full)/6-31G(d) condition. DFT calculations supported that the optimized structure of the cyclization product of Mes*P=C: suggests remarkable conjugation effects between the nearly coplanar P=C skeleton and the aryl moiety.

Sterically crowded triarylphosphines bearing anthraquinones were synthesized by Suzuki–Miyaura coupling of arylboronic acids derived from (bromoaryl)phosphines with haloanthraquinones. The anthraquinone bearing the two triarylphosphine moieties at the 2,6-positions shows the smaller difference between the oxidation potential of the triarylphosphine moieties and the reduction potential of the anthraquinone moiety, and the more red-shifted visible absorption responsible for the reddish brown color as compared with the 1,5-derivative. A sterically crowded triarylphosphine–naphthoquinone oligomer composed of alternately aligned three triarylphosphine and four naphthoquinone moieties were also synthesized by repeated Suzuki–Miyaura coupling using arylboronic acids derived from (bromoaryl)phosphines and chloronaphthoquinone derivatives. The 31 P NMR spectrum of the oligomer consists of several peaks in a narrow range reflecting distribution of the diastereomers arising from the helicity of the propellers composed of the three aromatic rings on the phosphorus atom. The oligomer exhibits a purple color resulting from the intramolecular charge transfer from the triarylphosphine moiety to the neighboring naphthoquinone moiety, and the wavelength and the intensity of the charge transfer band have a demonstrable correlation with the redox potentials and the number of the neighboring triarylphosphine–naphthoquinone pairs, respectively.

A reaction of the radical cation of tris(2,4,6-triisopropylphenyl)phosphine with oxygen gave a phosphonium salt resulting from the introduction of the oxygen atom to the phosphorus and the formation of a five membered ring by participation of the ortho isopropyl group. On the other hand, oxidation of the phosphine with mCPBA afforded the corresponding phosphine oxide. X-ray crystallography showed that the C P C bond angles and the P C bond lengths of the phosphine oxide are similar to those of the corresponding phosphine. The corresponding arsine underwent analogous reactions to afford the arsonium salt and the arsine oxide.

Utilizing phosphorus version of the Fritsch-Buttenberg-Wiechell (FBW) reaction, arylphosphaalkynes are prepared from 2,2-dihalo-1-aryl-1-phosphaalkenes, and theoretical calculations were carried out to obtain some basic insight into intramolecular rearrangement in the phosphorus system of interest. [Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements for the following free supplemental resource: Density functional theory (DFT) and ab initio MP2 calculations.] [Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfer, and Silicon for the following free supplemental files: Additional tables, figures, and references.]

Synthesis, redox properties, and electronic spectra of the sterically crowded triarylphosphines conjugated with π-electron systems, especially electron acceptors such as carbonyl group, are briefly reviewed. The sterically crowded triarylphosphines conjugated with various π-electron systems were synthesized from the common synthetic intermediate, (bromoaryl)phosphine, by conventional manner. The sterically crowded triarylphosphines conjugated with the electron acceptors exhibit visible absorption and fluorescence with large Stokes shift. Large solvent effect, redshift in polar solvent, and good correlation with the difference of the oxidation potential of the phosphine moiety and the reduction potential of the acceptor moiety suggest polar excited state resulting from HOMO-LUMO transition.

Review: [31P NMR spectroscopic data, X-ray results, preparative methods, and complex formations, theoretical calculations on (E)-Mes*P=PMes*; 85 refs.

1,3-Di(tert-butyl)-2,4-bis[2,4,6-tri(tert-butyl)phenyl]-1,3-diphosphacyclobutane-2,4-diyl was formed from [2,4,6-tri(tert-butyl)phenyl]phosphaacetylene and t-BuLi. In addition, the X-ray diffraction analysis was carried out, together with theoretical calculations of the structure and NMR data.

Air-tolerant 2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl singlet biradicals can be prepared by utilizing the unique reactivity of a kinetically stabilized P≡C triple bond compound. In this procedure, we studied the spectroscopic properties of a fundamental unsymmetrical P-heterocyclic biradical containing both PEt and PMe moieties, and the effects of the PCH2OMe group in relation to the stability of the P-heterocyclic biradical skeleton. The experimentally observed nuclear magnetic resonance and photo-absorption parameters of 1-ethyl-3-methyl-2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl were discussed based on our previous findings and density functional theory calculations, suggesting particular structural characteristics of the P-heterocyclic biradical skeleton and aromatic substituent effects on the sp2-C atoms in the 4-membered ring. Introduction of the methoxymethyl group in the P2C2 biradical moiety gave more stabilized 1,3-diphosphacyclobutane-2,4-diyl derivatives. In comparison with considerably unstable biradicals bearing propargyl substituents, relatively higher lowest unoccupied molecular orbital energies suggest reluctant oxidation of the P-heterocyclic skeleton. Copyright © 2011 John Wiley & Sons, Ltd.

X-ray structure of a stable chlorophosphine and a secondary phosphine both bearing one or more sterically bulky 2,4,6-tri-t-butylphenyl (Mes*) group(s) are discussed in terms of molecular distortion. Steric encumbrance of the bulky aromatic ring induces a boat-type structure of the six-membered aromatic ring, and the degree of distortion depends on a substituent in the ipso position.

The sterically crowded triarylphosphines bearing formyl and benzoyl groups were synthesized and characterized by X-ray crystallography. The benzoyl derivative was converted to the p -quinomethane conjugated with the triarylphosphine. The McMurry coupling of the formyl derivative afforded the diarylethene bearing the two sterically-crowded-triarylphosphine moieties. The cyclic voltammograms of these compounds show reversible redox waves corresponding to the oxidation to the radical cations of the triarylphosphines and irreversible or quasi-reversible waves corresponding to the reduction of the acceptor moieties. The electronic and the fluorescence spectra of these π-conjugated systems, especially push-pull substituted derivatives, exhibit bathochromic shift typical of the extended π-conjugated systems especially in the polar solvent, and the large Stokes shift typical of the crowded triarylphosphines is enhanced by conjugation with the acceptor moiety.

Sterically crowded triarylphosphines and tetraaryldiphosphane bearing phenothiazinyl groups as redox sites were synthesized by using 2-bromo-1,3-dialkyl-5-phenothiazinylbenzenes as key synthetic intermediates. Cyclic voltammograms of these compounds show the first redox wave corresponding to the oxidation of the phosphorus redox center and the following waves corresponding to the oxidation of the phenothiazinyl groups. Introduction of more sterically crowded triarylphosphine moiety leads to more reversible oxidation of the phosphine as well as the phenothiazine moieties. A crowded tetraaryldiphosphane bearing four phenothiazinyl groups are oxidized in two steps. Four phenothiazine moieties are oxidized nearly at once after oxidation of the diphosphane moiety. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:506–513, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20714

We have succeeded in catenating two sterically encumbered 1,3-di-t-butyl-2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl units with a spacer 1,2-(CH2)2C6H4 to obtain bis(biradicals) as considerably stable compounds. We have discussed physicochemical properties of the dimer, together with DFT calculations of model compounds. Spectroscopic data, redox properties, and X-ray structures of the oligo(biradicals) derivatives including other spacers like 1,3-(CH2)2C6H4, 1,4-(CH2)2C6H4, and 1,3,5-(CH2)3C6H3, reveal that the P-heterocyclic biradical moieties interact through nonconjugative pathways. These properties of oligo(biradicals) will facilitate to design novel molecular systems for electronics. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 21:404–411, 2010; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20625

A cyclic phosphaallyl anion generated from a phosphorus-carbon triple bond (phosphaalkyne) was allowed to react with dimethyl- and di-p-tolyl disulfide to afford the corresponding 1,3-diphosphacyclobutane-2,4-diyls bearing the sulfanyl substituent on phosphorus. The sulfanyl-bearing P-heterocyclic biradicals showed high stability permitting easy handling under ambient conditions. UV-vis spectra and electrochemical properties of the compounds indicated that the sulfanyl groups behave as electron-withdrawing groups to affect electronic properties of the cyclic biradical moiety. X-ray analysis of the p-tolylsulfanyl derivative indicated that the sulfanyl group causes no significant modification in the metric parameters around the central 4-membred biradical skeleton.

Sterically crowded triarylphosphines bearing naphthoquinone moieties were synthesized by the Suzuki−Miyaura coupling of the corresponding (phosphinoaryl)boronic acid derivatives with 2,3-dichloro-1...

2,2-Dimethyl- and 2-benzyl-2-methyl-1-(2,4,6-tri- tert -butylphenyl)-1-phosphaethenes were employed as ligand of mononuclear chlorogold(I) complexes, which catalyzed cycloisomerization of 1,6-enyne affording vinylcyclopentene exclusively in the absence of silver co-catalyst. The reaction mechanisms are discussed based on DFT calculations. In addition to the cycloisomerization, the phosphaalkene–chlorogold(I) complexes catalyzed cyclization of pent-4-ynoic acids providing γ-methylene-γ-lactones under basic conditions.

t-Butylcyclopentadienyllithium was allowed to react with (2,4,6-tri-t-butylphenyl)phosphonous dichloride to afford the corresponding chloro-5-cyclopentadienylchlorophosphine as a mixture of diastereomers, the structure of one of which was determined by X-ray crystallography. The mixture of 5-cyclopentadienylchlorophosphine diastereomers was treated with a base to furnish novel 2-t-butyl-6-(2,4,6-tri-t-butylphenyl)-6-phosphafulvenes, and each geometrical isomer of them was fully characterized by X-ray crystallography. The E/Z ratio of the 6-phosphafulvene obtained was considerably dependent on the diastereomeric ratio of the chlorophosphine precursor.

Two and three stable 1-sec-butyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl units were catenated to construct multi-biradical derivatives by utilizing 1,3-di-, 1,4-di-, and 1,3,5-trimethylenebenzenes as bridging groups, respectively. UV/Vis spectroscopic and cyclovoltammetric (CV) properties of the multi-biradicals indicate a non-conjugative interaction between the concatenated biradical units.

Tris(2,6-diisopropyl-4-methoxyphenyl)phosphine (1) was synthesized to investigate substituent effect of methoxy groups on the properties of crowded triarylphosphines. Methoxy groups slightly lower ...

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.

A sterically crowded triarylphosphine bearing an ester group, (4-ethoxycarbonyl-2,6-diisopropylphenyl)bis(2,4,6-triisopropylphenyl)phosphine, was synthesized by the lithiation of the corresponding (bromoaryl)phosphine followed by the reaction with excess diethyl carbonate. The ester was obtained as lemon yellow crystals and exhibits the blueshifted UV-Vis absorption and fluorescence as compared with the orange-colored corresponding aldehyde and ketone. The effect of the substitution of an ester group on the photophysical and redox properties of the sterically crowded triarylphosphine is discussed, and the properties of the sterically crowded triarylphosphines bearing an aldehyde, a ketone, and an ester groups are briefly reviewed.

Due to a facile head-to-tail [3+2] dimerization, even a sterically demanding group such as the Mes∗ (2,4,6-tri-tert-butylphenyl) group around the P C C moiety did not allow us to isolate 3-(4-cyanophenyl)-1-(2,4,6-tri-tert-butylphenyl)-1-phosphaallene from the elimination reaction of 2-bromo-3-(4-cyanophenyl)-1-(2,4,6-tri-tert-butylphenyl)-1-phosphaprop-1-ene with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), and the corresponding 1,4-diphosphafulvene containing cyano groups was obtained and characterized. Theoretical studies on the [3+2] dimerization of phosphaallene characterize possible intermediates affording 1,4-diphosphafulvenes and also suggest the cyano group effect to facilitate the saturation of the P C double bonds. On the other hand, 1,2-bis(4-cyanophenyl)-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene was obtained from 2-bromo-3-(4-cyanophenyl)-3-trimethylsiloxy-1-(2,4,6-tri-tert-butylphenyl)-1-phosphaprop-1-ene together with the 3-(4-cyanophenyl)-1-phosphaallene.

Crowded triarylphosphines carrying a nitroxide radical were synthesized to investigate intramolecular interaction between the crowded triarylphosphine redox centers and a neutral radical. A crowded...

2-(2,4,6-Tri-tert-butylphenyl)-1-phosphaethyne was allowed to react with trimethylsilyldiazomethane derivatives to afford the corresponding 1,2,4-diazaphospholes via [2+3] cycloaddition followed by H/SiMe3 migration, in spite of the steric encumbrance of the substrates. The aromatic character of 1,2,4-diazaphosphole is indicated by the NMR spectroscopic data and the X-ray structure analysis as well as by theoretical calculation to confirm the planar delocalized ring skeleton. The crystal structure of the obtained 1,2,4-diazaphospholes showed remarkable hydrogen bonding character in relation to molecular aggregation due to the presence of bulky aryl groups. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

A kinetically stabilized phosphaalkyne bearing a bulky Mes ∗ (2,4,6-tri- t -butylphenyl) group is useful compound to prepare an enormous number of highly stable 1,3-diphosphacyclobutane-2,4-diyls through reactions with a lithium reagent and an electrophile. By utilizing this synthetic protocol, we prepared several non-symmetrical 1,3-diphosphacyclobutane-2,4-diyls in which the substituents on the phosphorus are different. Furthermore, we succeeded in preparation and characterization of novel air-tolerant symmetrical 2,4-bis(2,4,6-tri- t -butylphenyl)-1,3-diphosphacyclobutane-2,4-diyls bearing the identical alkyl substituents on the phosphorus atoms. Structures and properties of the 1,3-diphosphacyclobutane-2,4-diyls indicate characters as singlet ground-state carbon centered biradicals. In addition to those biradicals, we succeeded in preparation and isolation of a novel P-heterocyclic air-stable neutral radical as well as a P-heterocyclic cation radical.

Reactions of a sterically protected 1,2-di(2-thienyl)-3,4-bis[(2,4,6-tri-t-butylphenyl)phosphinidene]cyclobutene were investigated. The diphosphinidenecyclobutene reacted with elemental sulfur or transition metal reagents to form a thiaphosphirane derivative or the corresponding transition metal complexes, respectively. Reactions of the di(2-thienyl)diphosphinidenecyclobutene with butyllithium followed by treatment with electrophiles afforded functionalized di(2-thienyl)diphosphinidenecyclobutene derivatives.

Two Mes∗P C (Mes∗ = 2,4,6-tri-t-butylphenyl) parts were successfully combined with one or two disiloxane chain(s) by the reaction of 2-silyl-1-phosphaethenyllithium with dichlorotetramethyldisiloxane, and the structures of bis(1-bromo-2-phosphaethenyl)disiloxane and 3,7-diphosphinidene-2,2,4,4,6,6,8,8-octamethyl-1,5-dioxa-2,4,6,8-tetrasilacyclooctane were characterized by spectroscopic analyses and X-ray crystallography.

The acetylenes possessing one and two 1-phospholyl groups were synthesized by reaction of the alkynyl Grignard reagents with the 1-chlorophosphole and converted to the corresponding phosphole sulfides. Reaction of the 1-phenylethynylphosphole sulfide with CpCo(CO)2 resulted in η4-complexation on the phosphole moiety. The structures of the di(1-phospholyl)acetylene disulfide and the [η4-(1-phenylethynylphosphole sulfide)]cobalt(I) complex were characterized by X-ray crystallography. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:344–349, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20230

(E)-3,3-Diphenyl-1-(2,4,6-tri-tert-butylphenyl)-2-(trimethylsilyl)-1,3-diphosphapropene was prepared from (2-(2,4,6-tri-tert-butylphenyl)-1-(trimethylsilyl)-2-phosphaethenyl)lithium and chlorodiphenylphosphine, and the properties involving conformational aspects, coordination to metals, and sulfurization were investigated. The structure of 2-(trimethylsilyl)-1,3-diphosphapropene shows C 1 symmetry with the phosphino lone pair almost perpendicular to the P=C π-system, whereas the 2-chloro- and 2-methyl-1,3-diphosphapropene derivatives show nearly C s symmetry with the phosphino lone pair almost coplanar with the P=C plane. A very small 2 J pp coupling constant for the 2-(trimethylsilyl)-l,3-diphosphapropene was observed by 31 P NMR spectroscopy, indicating that the conformation of the C 1 symmetry was kept in solution. On coordination of the phosphorus atoms to the carbonyltungsten(O) moiety, the inherently predominant C 1 form of the 2-(trimethylsilyl)-1,3-diphosphapropene changed to C s symmetry. On the other hand, sulfurization of the 2-(trimethylsilyl)-l,3-diphosphapropene afforded the corresponding 3-thioxo-2-(trimethylsilyl)-1,3-diphosphapropene, indicating C 1 symmetry with the P=S bond almost perpendicular to the P=C moiety. The 2-(trimethylsilyl)-1,3-diphosphapropene acts as a P2 chelate ligand of a dichloropalladium(II) complex, which showed moderate catalytic activity in the Sonogashira cross-coupling reaction. The structure of the dichloropalladium(II) complex bearing the ligated 2-(trimethylsilyl)-1,3-diphosphapropene was analyzed by X-ray crystallography, indicating structure effects on the properties.

(π-Allyl)palladium triflate coordinated with 1,2-bis(4-methoxyphenyl)-3,4-bis(2,4,6-tri- t -butylphenylphosphinidene)cyclobutene (DPCB–OMe), [Pd(η 3 -C 3 H 5 )(DPCB–OMe)]OTf, efficiently catalyzes cyclodehydration of cis –2-butene-1,4-diol with active methylene compounds such as acetylacetone and ethyl acetoacetate in toluene in the presence of pyridine. The reactions can be performed in air, giving 2-vinyl-2,3-dihydrofurans in good to high yields.

According to a protocol for the synthesis of phosphaallenes we recently established, 3-cyclopropyl-1-(2,4,6-tri-tert-butylphenyl)-1-phosphallene was obtained from (Z)-2-bromo-3-cyclopropyl-1-(2,4,6-tri-tert-butylphenyl)-1-phosphapropene. A novel bidentate ligand, 1,2-bis(cyclopropyl)-3,4-bis(2,4,6-tri-tert-butylphenyl)-3,4-diphosphinidenecyclobutene, was prepared by oxidative homocoupling of 3-cyclopropyl-1-(2,4,6-tri-tert-butylphenyl)-1-phosphaallene in the presence of butyllithium, together with the generation of hydrogen gas. The 1,2-bis(cyclopropyl)-3,4-diphosphinidenecyclobutene was allowed to react with (tht)AuCl (tht = tetrahydrothiophene) to afford the corresponding digold(I) complex of a six-membered metallacycle containing the P=C-C=P skeleton. The molecular structures of the 2-bromo-3-cyclopropyl-1-phosphapropene, 1,2-bis(cyclopropyl)-3,4-diphosphinidenecyclobutene and the digold complex were unambiguously determined by X-ray crystallography and are discussed from the point of view of the cyclopropyl conjugation.

To construct the novel redox systems possessing the trimesitylphosphine- or trimesitylarsine-type substructure as a reversible redox site, dimesityl(3-phenothiazinomesityl)phosphine, dimesityl(4-phenothiazinoduryl)phosphine, and the corresponding arsines were synthesized (mesityl = 2,4,6-trimethylphenyl, duryl = 2,3,5,6-tetramethylphenyl). The key synthetic intermediates, (3-bromomesityl)-dimesitylphosphine, (4-bromoduryl)dimesitylphosphine, and the analogous arsines were prepared by successive addition of the corresponding Grignard reagents to phosphorus or arsenic trichloride. The (bromoaryl)phosphines and -arsines were converted to the corresponding (iodoaryl)phosphines and -arsines and coupled with phenothiazine in the presence of copper to afford the phenothiazinophosphino- and phenothiazinoarsinobenzenes. The cyclic voltammograms of the phenothiazinopnictogenobenzenes thus obtained exhibit two-step redox waves corresponding to oxidation on the pnictogen as well as phenothiazine redox centers. The phenothiazino group contributes to stability of the redox systems, and the phenothiazinophosphinobenzenes display two-step nearly reversible redox waves at -78 °C. On the other hand, the cyclic voltammograms of the phenothiazinoarsinobenzenes consist of the first reversible wave followed by the second irreversible wave, suggesting decomposition at the unstable arsenic redox center. The pnictogen redox centers of the phenothiazinopnictogenobenzenes are unstable as compared with those of the corresponding trimesityl derivatives. Chemical oxidation of the phenothiazinophosphinobenzenes and phenothiazinoarsinobenzenes by tris(4-bromophenyl)aminium perchlorate, which can oxidize trimesitylphosphine and trimesitylarsine to the corresponding cation radicals, was studied by EPR. However, only the nitrogen-centered cation radical was observed probably because of the instability of the phosphorus as well as arsenic radical centers.

Kinetically stabilized 3-oxo-1,3-diphosphapropenes were prepared by oxidation of 1,3-diphosphapropenes with m-CPBA or from reaction of the corresponding phosphaethenyllithium with phosphinic chloride. The 3-oxo-1,3-diphosphapropenes were used as P,O-unsymmetrical bidentate ligands and the catalytic activity of the palladium(II) complexes in some cross-coupling reactions was investigated.