Revisiting the 7‐Phospanorbornene Family: New P‐Alkyl Derivatives

A series of 1‐alkyl‐3‐methyl‐2,5‐dihydro‐1H‐phosphole oxides were converted to the corresponding phosphole oxides that, by the Diels–Alder reaction with N‐maleimide derivatives or with another unit of phosphole oxide, yielded trapped phosphole oxides or phosphole oxide dimers, respectively, as new 7‐phosphanorbornene 7‐oxides. The stereostructures of three derivatives were evaluated by single crystal X‐ray analysis. The regio‐ and stereospecific dimerization was studied by B3LYP/6‐31G(d,p) quantum chemical calculations, whose results were in accord with syntheses. Novel mechanistic features were explored. The geometrical data obtained by single crystal X‐ray analysis validated the results of quantum chemical calculations, as the deviation was less than 3%.     

Derives α-carbonyles et “aromaticite” des phospholes trivalents

A two-step synthesis of 2-acetyl and 2-carbethoxy-phospholes involves the metalation of monomeric 3,4-dimethyl phosphole sulfides by t-butyl-lithium followed by reaction with ethyl acetate and ethyl carbonate. The phosphole sulfides thus obtained are reduced by PBu₃ or P(CH₂CH₂CN)₃ to the phospholes 20–24. In the 2-acetyl-phosphole series, variable electron transfer from the dienic system and from the phosphorus atom to the carbonyl group is monitored by ¹H and ³¹P NMR. The main decomposition path of 2-carboxy phosphole 25 in the mass spectrometer involves a PC intracyclic bond cleavage with loss of CO, in sharp contrast with pyrrole and thiophene derivatives 26 and 27. These facts militate against strong 2p_π–3p_π electronic delocalisation in the phosphole nucleus.     

Electron Impact Stabilities of Phospholes and Related Compounds and their Relative “Aromaticity”

Abstract

Electron impact mass spectral fragmentation of a number of phosphole, phosphole oxide, phosphole selenide, and phosphole sulfide derivatives has been used as a probe for part of an on-going examination of the inherent stability, or “aromaticity” of these heterocycles. Ion current measurements and large fragment fragmentation patterns obtained from these compounds as well as from carbocyclic, and other heterocyclic analogs will be presented as one approach to determining the relative stabilities of these systems. The stability patterns obtained appear to be consistent with the relative stabilities established by other methods.     

Phospholes with Reduced Pyramidal Character from Steric Crowding. 1. Synthesis and NMR Characterization of 1-(2,4-Di-tert-butyl-6-methylphenyl)-3-methylphosphole

The sterically crowded 1-(2,4-di-tert-butyl-6-methylphenyl)-3-methylphosphole was synthesized by dehydrohalogenation of the corresponding 3,4-dibromophospholane, in order to probe the possibility that the steric congestion would cause some flattening of the phosphorus pyramid and an increase in electron delocalization. The phosphole was a recrystallizable solid with (31)P NMR delta 1.8. Semiempirical calculations indicated that the pyramidal shape was retained but was noticeably flatter than in 1-phenylphosphole. In the low energy conformation, the phosphole and phenyl ring planes are approximately orthogonal, with the 2-tert-butyl group in the less crowded position that is syn to the lone pair on phosphorus. The 6-methyl group is positioned under the phosphole ring. This conformational prediction was amply confirmed by several chemical shift and coupling effects in the (13)C NMR spectrum. The (1)H NMR spectrum displayed an unusually large four-bond coupling (6 Hz) of (31)P to the m-phenyl proton syn to the lone pair (and none to the anti-meta proton), consistent with the orthogonal conformation. The oxide of the phosphole showed more stability than that of less crowded phospholes and gave a (31)P NMR signal that was detectable over a several hour period at room temperature. The oxide proceeded to give the usual Diels-Alder dimer and also formed a cycloadduct with N-phenylmaleimide. The phosphoryl group of the latter was reduced with trichlorosilane to give the phosphine. This new 7-phosphanorbornene derivative gave the most downfield (31)P NMR shift (delta 153.3) of any member of this family, all of which are characterized by remarkable deshielding in the syn isomer.     

13C NMR spectra of phosphole–P(IV) dimers; ABX and AA′X 13C?31P coupling in some derived structures

The easily obtained dimers of phosphole oxides, sulfides and methiodides give ¹³C NMR spectra where carbons within three (sometimes four) bonds of each ³¹P nucleus are doublets of doublets and thus constitute X in an AMX spectrum. Most of the spectra have been completely interpreted with the aid of spectral measurements at two magnetic fields. Saturation of the double bonds in dimers of methylphosphole–P(IV) derivatives causes the ³¹P nuclei to have very similar chemical shifts, with Δν not adequately different from ³J(PP) to give first-order coupling. When both ³¹P nuclei couple with a given ¹³C, a second-order (ABX) ³¹C NMR spectrum is obtained. The presence of the effect is revealed by running the ¹³C NMR spectra at high magnetic field; J(AX)+J(BX) is constant at all fields, but spacing between the lines of the multiplet varies. The spectrum of the oxide, with Δν/J=1.44 for the ³¹P spectrum at 36.43 MHz, approaches first order character at 75 MHz; the methiodide spectrum (Δν/J=4.55) is second order at 15 MHz but clearly first order at 50 MHz, and the sulfide (Δν/J=5.6) is nearly first order at 15 MHz. [2 + 2]-Photochemical intramolecular cyclization of the dimer oxides provides cage-like structures where the ³¹P nuclei are chemically equivalent, but magnetically non-equivalent, making the ¹³C signals have the characteristics of X in an AA′X coupling pattern.     

Competitive [4 + 2] cycloadditions in equimolar mixtures of 1‐arylphosphole oxides

Four different [4 + 2] cycloadditions were observed to take place in the equimolar mixtures of two different arylphosphole oxides 2a + 2b or 2b + 2c . In addition to the phosphole oxide dimers 3a‐a and 3b‐b , or 3b‐b and 3c‐c , the crossed cycloadducts 3a‐b and 3b‐a , or 3b‐c and 3c‐b were also formed in considerable portions. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:633–635, 2001     

Modular synthesis of benzo[b]phosphole derivatives via BuLi-mediated cyclization of (o-alkynylphenyl)phosphine

Treatment of an (o-alkynylphenyl)phosphine with a stoichiometric amount of BuLi effects a cyclization reaction to produce a 3-lithiobenzo[b]phosphole, which affords a variety of 3-substituted benzophospholes upon reaction with electrophiles. An example is given for the synthesis of a bis-benzo[b]phosphole, which can be further converted to the corresponding benzo[b]phosphole oxide possessing high electron affinity.     

Synthesis and Structure–Property Relationships of 2,2′‐Bis(benzo[b]phosphole) and 2,2′‐Benzo[b]phosphole–Benzo[b]heterole Hybrid π Systems

The first comprehensive study of the synthesis and structure–property relationships of 2,2′‐bis(benzo[b]phosphole)s and 2,2′‐benzo[b]phosphole–benzo[b]heterole hybrid π systems is reported. 2‐Bromobenzo[b]phosphole P‐oxide underwent copper‐assisted homocoupling (Ullmann coupling) and palladium‐catalyzed cross‐coupling (Stille coupling) to give new classes of benzo[b]phosphole derivatives. The benzo[b]phosphole–benzo[b]thiophene and ‐indole derivatives were further converted to P,X‐bridged terphenylenes (X=S, N) by a palladium‐catalyzed oxidative cycloaddition reaction with 4‐octyne through the C_β? H activation. X‐ray analyses of three compounds showed that the benzo[b]phosphole‐benzo[b]heterole derivatives have coplanar π planes as a result of the effective conjugation through inter‐ring C? C bonds. The π–π* transition energies and redox potentials of the cis and trans isomers of bis(benzo[b]phosphole) P‐oxide are very close to each other, suggesting that their optical and electrochemical properties are little affected by the relative stereochemistry at the two phosphorus atoms. The optical properties of the benzo[b]phosphole–benzo[b]heterole hybrids are highly dependent on the benzo[b]heterole subunits. Steady‐state UV/Vis absorption/fluorescence spectroscopy, fluorescence lifetime measurements, and theoretical calculations of the non‐fused and acetylene‐fused benzo[b]phosphole–benzo[b]heterole π systems revealed that their emissive excited states consist of two different conformers in rapid equilibrium.     

A new route to the phosphindole system

A new and possibly general route to the hitherto relatively inaccessible simple substituted phosphindoles, utilizing the extreme readiness of simple phosphole sulfides to undergo Diels-Alder dimerization, is described. The synthesis reported here is that of 3,6-dimethyl-l-phenylphosphin-dole sulfide which is prepared in good yield in two simple steps from the readily available 3-methyl-l-phenylphosphole.     

Triplet states of 3,3′-alkylsubstituted thiacarbocyanine dimers and dimeric complexes with cucurbit[8]uril in water

Alkylsubstituted thiacarbocyanines (3,3′-diethylthiacarbocyanine, D1, and 3,3′-disulfopropylthiacarbocyanine, D2), existing in water as monomers and dimers, manifest the ability to transition to the triplet state. The spectrum of triplet-triplet (T–T) absorption of the D2 dimers is shifted in the range higher than 590 nm by 20 nm to the red in comparison with the T–T spectrum of monomers. The D1 dimers in the presence of cucurbit[8]uril form a dimeric complex with two bands in the differential absorption spectrum. The band at 550 nm belongs to the triplet-triplet absorption of the dimeric complexes, and the band in the range of 620–700 nm is the result of charge transfer in the triplet state. The rate constants of deactivation for these triplet states coincide.     

On the mechanisms of oxidation of organic sulfides by H2O2 in aqueous solutions

The mechanism of oxidation of organic sulfides in aqueous solutions by hydrogen peroxide was investigated via ab initio calculations. Specifically, two reactions, hydrogen transfer of hydrogen peroxide to form water oxide and the oxidation of dimethyl sulfide (DMS) by hydrogen peroxide to form dimethyl sulfoxide, were studied as models of these processes in general. Solvent effects are included both via including explicitly water molecules and via the polarizable continuum model. The former was found to have a much more significant effect than the latter. When explicit water molecules are included, a mechanism different from those proposed in the literature was found. Specific interactions including hydrogen bonding with 2-3 water molecules can provide enough stabilization for the charge separation of the activation complex. The energy barrier of the oxidation of DMS by hydrogen peroxide was estimated to be 12.7 kcal/mol, within the experimental range of the oxidation of analogous compounds (10-20 kcal/mol). The major reaction coordinates of the reaction are the breaking of the O-O bond of H2O2 and the formation of the S-O bond, the transfer of hydrogen to the distal oxygen of hydrogen peroxide occurring after the system has passed the transition state. Reaction barriers of the hydrogen transfer of H2O2 are an average of 10 kcal/mol or higher than the reaction barriers of the oxidation of DMS. Therefore, a two-step oxidation mechanism in which, first, the transfer of a hydrogen atom occurs to form water oxide and, second, the transfer of oxygen to the substrate occurs is unlikely to be correct. Our proposed oxidation mechanism does not suggest a pH dependence of oxidation rate within a moderate range around neutral pH (i.e., under conditions in which hydronium and hydroxide ions do not participate directly in the reaction), and it agrees with experimental observations over moderate pH values. Also, without including a protonated solvent molecule, it has activation energies that correspond to measured activation energies.     

Tetra­aqua­(7‐hydroxy‐5‐oxidoflavone‐6‐sulfonato‐κ2O4,O5)nickel(II) dimethyl­formamide solvate monohydrate

In the title compound, [Ni(C₁₅H₈O₇S)(H₂O)₄]·C₃H₇NO·H₂O, the Ni^II cation is chelated by a 7‐hydroxy‐5‐oxidoflavone‐6‐sulfonate ligand through one oxide and one carbonyl O atom, and the sixfold coordination is completed by four aqua ligands. Individual mol­ecules are linked into hydrogen‐bonded dimers by way of five pairs of O—H⋯O hydrogen bonds. These dimers, in turn, determine a three‐dimensional supra­molecular arrangement through a variety of inter­dimeric inter­actions, such as O—H⋯O, C—H⋯O and π–π stacking.     

Synthesis and Structure–Property Relationships of Phosphole‐Based π Systems and Their Applications in Organic Solar Cells

Phosphole is a chemically tunable heterole, and its π‐conjugated derivatives are potential candidates for optoelectronic materials. This account describes recent developments in the synthesis and structure–property relationships of π‐conjugated phosphole derivatives made by my research group. Thiophene–phosphole–styrene, phosphole–acetylene–arene, oligophosphole, polyphosphole, areno[c]phosphole, and phosphole–heterole π systems are synthesized using titanacycle‐mediated metathesis and palladium‐catalyzed cross‐coupling reactions. The structural, optical, and electrochemical properties of selected compounds are discussed. Initial results on some applications of thiophene–phosphole copolymers, acenaphtho[c]phospholes, and amine–terthiophene–phosphole donor–π–acceptor dyes in organic solar cells are described. These results give valuable information and guidelines for designing new phosphorus‐containing organic materials for molecular electronics.     

Solvent-stabilized molecular capsules

Pyrrogallolarenes 2 were prepared by acid-catalyzed condensation of pyrrogallol with aldehydes. Compound 2a crystallizes from a methanol solution of quinuclidine hydrochloride to give a dimeric molecular capsule surrounding one disordered quinuclidinium cation. The molecules of 2a are connected by direct hydrogen bonds and by bridging methanol and water molecules. The chloride anion is positioned outside the capsule and is hydrogen bonded to the hydroxy groups of 2a. The shortest distance between the cation and anion was found to be 6.7 A. Crystallization of 2b from aqueous acetonitrile resulted in a dimeric capsule linked by a polar belt of 16 hydrogen bonding water molecules. Four acetonitrile molecules occupy the cavity of this dimeric capsule and assume two binding sites that differ in hydrogen bonding and electronic environment. Compounds 2 also form hydrogen-bonded dimeric molecular capsules in alcohols and aqueous acetonitrile solutions. These assemblies readily encapsulate tetramethylammonium, tetramethylphosphonium, quinuclidinium, and tropylium cations to give complexes stable on the NMR time scale at 233 K.     

水电解制氢非贵金属催化剂的研究进展

氢能作为零碳排放能源是被公认的最清洁能源之一,如何有效可持续地产氢是未来人类步入氢能经济首先要解决的问题。电解水技术基于电化学分解水的原理,利用可再生电能或太阳能驱动水分解为氢气和氧气,被认为是最有前途和可持续性的产氢途径。然而,无论是光解水还是电解水,均需要高活性、高稳定性的非贵金属氢析出和氧析出催化剂以使水电解反应经济节能。本文介绍了我们研究所近三年在水电解方面的研究进展,其中着重介绍了：（ⅰ）氢析出催化剂,包括利用低温磷化过渡金属（氢）氧化物的方法制备过渡金属磷化物,同时过渡金属硫化物、硒化物以及碳化物等均被成功合成并被应用为有效的阴极析氢催化剂;（ⅱ）氧析出催化剂,主要包括金属磷化物、硫化物、氧化物/氢氧化物等;（ⅲ）双功能催化剂,主要包括过渡金属磷化物、硒化物、硫化物等。最后,总结展望了发展水电解非贵金属催化剂所面临的挑战与未来发展方向。     

Selective hydrogen peroxide oxidation of sulfides to sulfoxides or sulfones with MWW-type titanosilicate zeolite catalyst under organic solvent-free conditions

Selective oxidation of sulfides to sulfoxides and sulfones with hydrogen peroxide under organic solvent-free conditions was demonstrated by the MWW-type titanosilicate zeolite catalyst. Sulfides were oxidized smoothly to give sulfoxides with good selectivities at ambient temperature using 1.0–1.2 equiv of hydrogen peroxide with the MWW-type titanosilicate zeolite catalyst. Especially, the Ti-MWW with an interlayer-expanded structure (Ti-IEZ-MWW) catalyst showed high activity with good chemoselectivity for the oxidation of various sulfides. The catalyst is recyclable for at least five cycles, and the only byproduct is water. Sulfides were directly oxidized to give sulfones in high yields by 2.5 equiv of hydrogen peroxide with the MWW-type titanosilicate zeolite catalyst under organic solvent-free conditions.     

Environment‐Sensitive Fluorescent Probe: A Benzophosphole Oxide with an Electron‐Donating Substituent

Electron‐donating aryl groups were attached to electron‐accepting benzophosphole skeletons. Among several derivatives thus prepared, one benzophosphole oxide was particularly interesting, as it retained high fluorescence quantum yields even in polar and protic solvents. This phosphole‐based compound exhibited a drastic color change of its fluorescence spectrum as a function of the solvent polarity, while the absorption spectra remained virtually unchanged. Capitalizing on these features, this phosphole‐based compound was used to stain adipocytes, in which the polarity of subcellular compartments could then be discriminated on the basis of the color change of the fluorescence emission.     

Anodic oxidation of 1-naphthylamine in acetonitrile

The anodic oxidation of 1-naphthylamine (ArNH₂) has been studied at the platinum electrode in acetonitrile by controlled potential electrolysis, chronopotentiometry, cyclic voltammetry, cyclic voltammetry and spectrometric methods. From the electrochemical data a complex ECE type of mechanism is inferred with an overall efficiency of one electron per molecule of ArNH₂. The electrochemical steps are reversible and the chemical process seems to follow second order kinetics and is extremely fast. The cation radical produced in the first charge transfer step suffers fast decomposition to give dimers which are easier to oxidize than ArNH₂. The dimeric products were identified by comparison with authentic samples. Further complications arise due to slow reactions of the oxidized dimers. Electrochemical studies in basic media (pyridine and 2,6-lutidine) and acid media (anhydrous trifluoroacetic acid) were also performed.     

Products derived from oxidations of 3,4-dimethyl-1-phenylphosphole-1-oxide: A 3-phenyl-5,6-dimethyl-2,3-oxaphosphabicyclo[2.2.2]octene-3-oxide derivative as a precursor of phenyl metaphosphonic anhydride

Oxidation of 3,4-dimethyl-1-phenylphosphole with peracids or peroxides gives a relatively stable P-oxide, which can be used in Diels-Alder reactions to give derivatives with the 7-phosphanorbornene framework. Oxygen insertion into a C–P bond of this framework occurs smoothly with m- chloroperbenzoic acid (MCPBA) providing derivatives of the 2,3-oxaphosphabicyclo [2.2.2] octene ring system. The phosphole can be converted to this system in a one-pot synthesis by reaction with excess MCPBA in the presence of N-phenylmaleimide as dienophile. The phosphole oxide undergoes mono-epoxidation with MCPBA. Thermal or photochemical fragmentation of the 2,3-oxaphosphabicyclo [2.2.2] ocetene is a useful source of the 3-coordinate species Ph–PO₂, a meta-anhydride of phenylphosphonic acid. This species was trapped successfully with a variety of alcohols.     

Enantiomerically pure disulfides: key compounds in the kinetic resolution of chiral PIII-derivatives with stereogenic phosphorus

Enantiomerically pure disulfides were reacted with various chiral P^III-derivatives with stereogenic phosphorus such as tertiary phosphines, halogenophosphines, phosphinite and phosphole under kinetic resolution conditions to afford enantiomerically enriched phosphine oxides or sulfides with ee values up to 50%. Enantiomeric excess rose to 70% under dynamic kinetic resolution conditions in the case of tert-butylphenylchlorophosphine.