Phospholes – Development and Recent Advances

Trifunctional Silyl Groups as Anchoring Units in the Preparation of Luminescent Phosphole-Silica Hybrids

A synthetic strategy to β-silylphospholes with three methoxy, ethoxy, chloro, hydrido, or phenyl substituents at silicon has been developed, starting from trimethoxy, triethoxy, or triphenyl silyl substituted phenyl phosphanides and 1,4-diphenyl-1,3-butadiyne. These trifunctional silylphospholes were attached to the surface of uniform spheric silica particles (15 μm) and, for comparison, to a polyhedral silsesquioxane (POSS)-trisilanol as a molecular model to explore their luminescent properties in comparison with the free phospholes. Density functional theory calculations were performed to investigate any electronic perturbation of the phosphole system by the trifunctional silyl anchoring unit. For the immobilized phospholes, cross-polarization magic-angle-spinning NMR measurements (¹³C, ²⁹Si, and ³¹P) were carried out to explore the bonding situation to the silica surface. Thermogravimetric analysis and X-ray photoelectron spectroscopy measurements were performed to approximate the amount of phospholes covering the silica surface. Identity and purity of all novel phospholes have been established with standard techniques (multinuclear NMR, mass spectrometry, and elemental analysis) and X-ray diffraction for the POSS derivative.

Nickel Complexes in C‒P Bond Formation

This review is a comprehensive account of reactions with the participation of nickel complexes that result in the formation of carbon-phosphorus (C‒P) bonds. The catalytic and non-catalytic reactions with the participation of nickel complexes as the catalysts and the reagents are described. The various classes of starting compounds and the products formed are discussed individually. The several putative mechanisms of the nickel catalysed reactions are also included, thereby providing insights into both the synthetic and the mechanistic aspects of this phosphorus chemistry.

Recent Progress Concerning the N-Arylation of Indoles

This review summarizes the current state-of-the-art procedures in terms of the preparation of N-arylindoles. After a short introduction, the transition-metal-free procedures available for the N-arylation of indoles are briefly discussed. Then, the nickel-catalyzed and palladium-catalyzed N-arylation of indoles are both discussed. In the next section, copper-catalyzed procedures for the N-arylation of indoles are described. The final section focuses on recent findings in the field of biologically active N-arylindoles.

Heavier element-containing aromatics of [4n+2]-electron systems

While the implication of the aromaticity concept has been dramatically expanded to date since its emergence in 1865, the classical [4n+2]/4n-electron counting protocol still plays an essential role in evaluating the aromatic nature of compounds. Over the last few decades, a variety of heavier heterocycles featuring the formal [4n+2] π-electron arrangements have been developed, which allows for assessing their aromatic nature. In this review, we present recent developments of the [4n+2]-electron systems of heavier heterocycles involving group 13-15 elements. The synthesis, spectroscopic data, structural parameters, computational data, and reactivity are introduced.

Phosphetes via Transition Metal Free Ring Closure - Taking the Proper Turn at a Thermodynamic Crossing

A transition metal free route to phosphetes featuring an exocyclic alkene unit is presented. In this approach phosphanides are added to a variety of diynes generating phosphaallylic intermediates which depending on the reaction conditions transform either to phosphetes or the corresponding phospholes. Investigation of the reaction mechanism by combined quantum chemical and experimental means identifies phosphole formation as thermodynamically controlled reaction path, whereas kinetic control furnishes the corresponding phosphetes. Structural and luminescence properties of the rare class of phosphetes are explored, as well as for selected key intermediates.

Synthesis, Structural Characterization, and Optical Properties of Benzene-Fused Tetracyclic and Pentacyclic Stiboles

The expectation that antimony (Sb) compounds should display phosphorescence emissions based on the “heavy element effect” prompted our interest in the introduction of antimony to a biaryl as the bridging atom in a fused heterole system. Herein, the synthesis, molecular structures, and optical properties of novel benzene-fused heteroacenes containing antimony or arsenic atoms are described. The stiboles and arsole were prepared by the condensation of dibromo(phenyl)stibane or dichloro(phenyl)arsine with dilithium intermediates derived from the corresponding dibromo compounds. Nuclear magnetic resonance (NMR) spectroscopy and X-ray crystal analysis revealed that the linear pentacyclic stibole was highly symmetric in both the solution and crystal states. In contrast, the curved pentacyclic stibole adopted a helical structure in solution, and surprisingly, only M helical molecules were crystallized from the racemate. All synthesized compounds produced very weak or no emissions at room temperature or in the solid state. In contrast, the linear penta- and tetracyclic stiboles exhibited clear phosphorescence emissions in the CHCl₃ frozen matrix at 77 K under aerobic conditions.

Theoretical Determination of pKas of P(1)-H Phospholes and their Comparison with N(1)-H Azoles

pKas of P(1)-H deprotonation of all phosphole group compounds, phosphole (C4H4PH), 1,2 and 1,3-diphospholes (C3H3PPH), 1,2,3, and 1,2,4-triphospholes (C2H2P2PH), tetraphosphole (CHP3PH), and pentaphosphole (P4PH) are determined by DFT calculations. We have compared these to the pKas of the analogue azole group compounds and the factors that cause the differences of these values are discussed in terms of the stabilities of both the phosphole anions and azole anions.

Formal Transition-Metal-Catalyzed Phosphole C-H Activation for the Synthesis of Pentasubstituted Phospholes

Unprecedented formal transition-metal-catalyzed phosphole C-H functionalization is described in this paper. Pentasubstituted phospholes were prepared via the copper-catalyzed reaction of 1,3,4-trisubstituted phosphole with aryl iodides or bromides under distinct conditions. The developed methodology is able to accommodate a wide variety of substituents, including aryl, heteroaryl, and alkenyl.

Organophosphorus chemistry based on elemental phosphorus: advances and horizons

The results of studies on the application of elemental phosphorus for the synthesis of important organophosphorus compounds are surveyed and summarized. Currently, this trend represents a synthetically, environmentally and technologically attractive alternative to classical organophosphorus chemistry based on toxic and corrosive phosphorus chlorides. Direct phosphination and phosphinylation of organic compounds with elemental phosphorus (discussed in the first part of the review) basically extend the range of available phosphines, phosphine chalcogenides and phosphinic acids and provides further development of their synthetic potential (discussed in the second part of the review). It is shown that the breakthrough in this area is largely due to the discovery of reactions of elemental phosphorus (white and red) with various electrophiles in superbasic suspensions and emulsions derived from alkali metal hydroxides and to the development of electrochemical, electrocatalytic and catalytic activation of white phosphorus.

The bibliography includes 299 references.

Asymmetric 1,3-dipolar cycloaddition reaction of chiral 1-alkyl-1,2-diphospholes with diphenyldiazomethane

The 1,3-dipolar cycloaddition of chiral 1-alkyl-1,2-diphosphacyclopenta-2,4-dienes ((1-(−)-menthyl)oxymethyl-1,2-diphosphole and 1-(+)-neomenthyl-1,2-diphosphole) with diphenyldiazomethane leads to novel P-chiral bicyclic phosphiranes having six chiral centers. The degree of diastereoselectivity depends on the substituent at phosphorus, and dramatically increases in the case of (+)-neomenthyl group (de up to 71%). DFT calculations indicate that the cycloaddition is thermodynamically controlled.

The 1,3-dipolar cycloaddition of chiral 1-alkyl-1,2-diphosphacyclopenta-2,4-dienes with diphenyldiazomethane leads to novel P-chiral bicyclic phosphiranes having six chiral centers.

P-Chiral 1,7-diphosphanorbornenes: from asymmetric phospha-Diels-Alder reactions towards applications in asymmetric catalysis

A straightforward synthesis of P-chiral polycyclic phosphines by an asymmetric Diels-Alder reaction of 1-alkyl-1,2-diphospholes and (5R)-(l-menthyloxy)-2(5H)-furanone (MOxF) is presented. The [4 + 2] cycloaddition reaction of 1,2-diphospholes 1-3 with MOxF (4) proceeded with high diastereoselectivity (de up to 90%) resulting in the corresponding enantiopure anti-endo-1,7-diphosphanorbornenes 5a-7a. The absolute configuration of 5-7 was proved by a variety of 1D/2D NMR correlation methods. The use of the anti-endo-1,7-diphosphanorbornene 5a in the Pd-catalyzed asymmetric allylic alkylation of cinnamyl acetate 8 with cyclic β-ketoesters 9a,b provided up to 52% ee.

Application of the Extended HOMED (Harmonic Oscillator Model of Aromaticity) Index to Simple and Tautomeric Five-Membered Heteroaromatic Cycles with C, N, O, P, and S Atoms

The geometry-based HOMA (Harmonic Oscillator Model of Aromaticity) descriptor, based on the reference compounds of different delocalizations of n- and π-electrons, can be applied to molecules possessing analogous bonds, e.g., only CC, only CN, only CO, etc. For compounds with different heteroatoms and a different number of CC, CX, XX, and XY bonds, its application leads to some discrepancies. For this reason, the structural descriptor was modified and the HOMED (Harmonic Oscillator Model of Electron Delocalization) index defined. In 2010, the HOMED index was parameterized for compounds with C, N and O atoms. For parametrization, the reference molecules of similar delocalizations of n- and π-electrons were employed. In this paper, the HOMED index was extended to compounds containing the CP, CS, NN, NP, PP, NO, NS, PO, and PS bonds. For geometrical optimization of all reference molecules and of all investigated heterocompounds, the same quantum–chemical method {B3LYP/6-311+G(d,p)} was used to eliminate errors of the HOMED estimation. For some tautomeric systems, the Gn methods were also employed to confirm tautomeric preferences. The extended HOMED index was applied to five-membered heterocycles, simple furan and thiophene, and their N and P derivatives as well as for tautomeric pyrrole and phosphole and their N and P derivatives. The effects of additional heteroatom(s) in the ring on the HOMED values for furan are parallel to those for thiophene. For pyrroles, aromaticity dictates the tautomeric preferences. An additional N atom in the ring only slightly affects the HOMED values for the favored and well delocalized NH tautomers. Significant changes take place for their rare CH forms. When intramolecular proton-transfer is considered for phosphole and its P derivatives, the PH tautomers seem to be favored only for 1,2,3-triphosphole/1,2,5-triphosphole and for 1,2,3,5-tetraphosphole. For other phospholes, the CH forms have smaller Gibbs energies than the PH isomers. For phosphazoles, the labile proton in the favored form is linked to the N atom. The PH forms have smaller HOMED indices than the NH tautomers but higher than the CH ones.

DFT exploration of [3 + 2] cycloaddition reaction of 1H-phosphorinium-3-olate and 1-methylphosphorinium-3-olate with methyl methacrylate

A Molecular Electron Density Theory (MEDT) study of the regio- and stereoselectivity of the [3 + 2] cycloaddition (32CA) reaction of 1H-phosphorinium-3-olate and 1-methylphosphorinium-3-olate with methyl methacrylate was carried out using the B3LYP/6-31G(d) method. In order to test the method dependence for the most favorable reaction path leading to the 1H-substituted 6-exo cycloadduct (CA) various functionals using higher basis sets were taken into consideration in the gas phase. An analysis of the energetic parameters indicates that the reaction path leading to 6-exo CA are kinetically as well as thermodynamically favored in the gas phase, THF and ethanol. The calculated energetic parameters of the 32CA reaction of these phosphorus derivatives were compared with those of methyl acrylate and their nitrogen analogues. Investigation of the global electron density transfer at the TSs indicates that these 32CA reactions have non-polar character, while electron localisation function topological analysis of the C-C bond formation along the most favorable reaction path indicates that these 32CA reactions take place through a non-concerted two-stage one-step mechanism, via highly asynchronous TSs.

Novel enantiopure monophospholes: synthesis, spatial and electronic structure, photophysical characteristics and conjugation effects

A rational and highly efficient method to access lithium 2,3,4,5-tetraphenylphospholide directly from white phosphorus, diphenylacetylene and lithium has been developed.

Synthesis and photophysical properties of novel benzophospholo[3,2-b]indole derivatives

The parent benzophospholo[3,2-b]indole was prepared by the reaction of dichlorophenylphosphine with a dilithium intermediate, which was prepared in two steps from 2-ethynyl-N,N-dimethylaniline. Using the obtained benzophosphole-fused indole as a common starting material, simple modifications were carried out at the phosphorus center of the phosphole, synthesizing various functionalized analogs. The X-ray structure analysis of trivalent phosphole and phosphine oxide showed that the fused tetracyclic moieties are planar. The benzophosphole-fused indoles, such as phosphine oxide, phospholium salt, and borane complex, exhibited strong photoluminescence in dichloromethane (Φ = 67–75%).

π-Conjugated phospholes and their incorporation into devices: components with a great deal of potential

This review serves as a brief introduction to phospholes and discusses their unique favorable properties for application in organic electronic materials. Over the past several years, π-conjugated phospholes have been slowly making their way into devices. We report here the mode of synthesis of these π-conjugated phospholes as well as discuss the performances of the devices.

Enhancing the reactivity of 1,2-diphospholes in cycloaddition reactions

Two different approaches have been employed to enhance the reactivity of 1-alkyl-1,2-diphospholes – the introduction of electron-withdrawing groups either at the phosphorus atoms or in the para-position of the arene ring. The alkylation of sodium 1,2-diphospha-3,4,5-triphenylcyclopentadienide with alkyl halides Hal-CH₂-R (R = CN, COOEt, OMe, CH₂OEt) results in corresponding 1-alkyl-3,4,5-triphenyl-1,2-diphospholes (alkyl = CH₂CN (1a), CH₂COOEt (1b), CH₂OMe (1c), and (CH₂)₂OEt (1d)), which spontaneously undergo the intermolecular [4 + 2] cycloaddition reactions at room temperature to form the mixture of the cycloadducts, 2a–c, respectively. However the alkylation of sodium 1,2-diphospha-3,4,5-tri(p-fluorophenyl)cyclopentadienide with ethyl iodide leads to stable 1-ethyl-3,4,5-tris(p-fluorophenyl)-1,2-diphosphole (1e), which forms the [4 + 2] cycloadduct 2,3,4,4a,5,6-hexa(p-fluorophenyl)-1-ethyl-1,7,7a-triphospha-4,7-(ethylphosphinidene)indene (2e) only upon heating up to 60 °C. With further heating to 120 °C with N-phenylmaleimide, the cycloadducts 2a–c and 2e undergo the retro-Diels–Alder reaction and form only one product of the [4 + 2] cycloaddition reaction 3a–с, 3e with good yields up to 65%.

Phosphole-based ligands in catalysis

This review provides an overview of phosphole-based ligand families (monophospholes, multidentate hybrid phosphole ligands, diphosphole and 2,2′-biphosphole-based ligands) and their potential in metal- and organo-catalyzed reactions (asymmetric reactions included).