Phosphorus-containing Heterocycles Research Articles

Dithieno[3,2-b; 2',3'-f]phosphepinium-Based Near-Infrared Fluorophores: px-π* Conjugation Inherent to Seven-Membered Phosphacycles.

Positively charged phosphorus-containing heterocycles are characteristic core skeletons for functional molecules. While various phosphonium-containing five- or six-membered-ring compounds have been reported, the seven-membered-ring phosphepinium have not been fully studied yet. In this study, dithieno[3,2-b; 2',3'-f]phosphepinium ions containing electron-donating aminophenyl groups were synthesized. An X-ray crystallographic analysis of the resulting donor-acceptor-donor dyes revealed a bent conformation of the central seven-membered ring. These compounds exhibit fluorescence in the near-infrared region with a bathochromic shift of ca. 70 nm compared to a phosphepine oxide congener and a large Stokes shift. High fluorescence quantum yields were obtained even in polar solvents due to the suppression of the nonradiative decay process. A theoretical study revealed that the phosphepinium skeleton is highly electron-accepting owing to the orbital interaction between a px orbital of the phosphonium moiety and a π* orbital of the 1,3,5-hexatriene moiety. Due to the lower-lying px orbital in the phosphonium moiety compared to that of the phosphine oxide and the bent conformation of the seven-membered ring, the phosphepinium ring permits effective px-π* conjugation. A large structural relaxation with a contribution of a quinoidal resonance structure is suggested in the excited state, which should be responsible for the bright emission with a large Stokes shift.

Rh(III)-Catalyzed Regioselective Alkyne Insertion in the Context of C-H Activation and Lactonization Reactions: Synthesis of Fused 1,2-Oxaphospholenes.

Herein, we report an expedient synthesis of fused phosphorus-containing heterocycles via Rh-catalyzed cascade C-H activation, alkyne insertion, and lactonization reactions. The substrate scope and the group tolerance are good, as various substituted benzoic acids, N-methoxybenzamides, and 2-phenylindoles could go through the reactions smoothly to afford the corresponding products in moderate to high yields. Additionally, excellent regioselectivities are shown in the alkyne insertion with the assistance of an electron-withdrawing phosphonate group.

Pnictogen-Rich Heterocycles Derived from a Phosphadiazonium Cation.

Heterocycles that pair main group elements and nitrogen are extremely important within the π-conjugated heterocycles research community. Compared to the vast number of boron-nitrogen heterocycles, those that include phosphorus are less common. Furthermore, the use of phosphorus-nitrogen triple bonds of any type to prepare such compounds is unprecedented. Here, we pair pyridyl hydrazonide ligands with phosphadiazonium cations and demonstrate that the chelated Mes*NP group is directly implicated in the photophysical and redox properties observed for the resulting heterocycles. In doing so, we introduce a novel building block for the production of phosphorus-containing heterocycles that could find use in small molecule activation and catalysis or as the functional component of emerging organic electronics.

(3+2)-Cyclization Reactions of Unsaturated Phosphonites with Aldehydes and Thioketones.

By exploiting the unique reactivity of ethynyl-phosphonites we obtain novel P(V)-containing five-membered heterocycles via (3+2)-cyclization reactions with aldehydes or cycloaliphatic thioketones in satisfactory to excellent yields. Whereas reactions with thioketones to yield 1,3-thiaphospholes-3-oxides occur smoothly at room temperature with equimolar amounts of the starting materials in absence of any catalyst, the analogous conversions with aldehydes to generate 3-oxides of 1,3-oxaphospholes require addition of triethylamine as a base. We postulate a step-wise (3+2)-cyclization mechanism for the formation of the 1,3-thiaphosphole ring based on DFT quantum chemical calculations. With this study, we introduce new cyclization reactions originating from unsaturated phosphonites as central synthetic building blocks to yield previously inaccessible stable phosphorus-containing heterocycles with unexplored potential for the molecular sciences.

Recent Advances in Metal-Catalyzed Heterocyclic C-P Bond Formation.

The phosphorus-containing heterocycles are an important class of compounds in organic chemistry. Because of their potential application in many fields, especially, the synthetic pesticides, medicine and catalyst, the phosphorus-containing heterocycles have attracted continuous attention from organic synthesis scientists. The development of efficient and low-cost catalytic systems is of great interest for the construction of heterocycles C-P bond. Usually, the phosphorus-containing heterocycles is prepared via direct carbon-hydrogen (C-H) bond activation or pre-functionalized of heterocycles with phosphorus-hydrogen (P-H) bond of phosphorus compounds reaction by metal-catalyzed. This review summarizes recent progress in the heterocycles C-P bond formation reactions by metal-catalyzed, which mainly focus on the discussion of the reaction mechanism. It aims to provide efficient methods for the future synthesis and application in this field.

Reactions of α-Chloro-α-phosphonopropionic Acid Trichloride with Nucleophilic Reagents

Alcoholysis of α-chloro-α-phosphonopropionic acid trichloride with allyl and propargyl alcohols was studied. It was found that the reaction proceeded selectively at the carbonyl carbon atom. The reaction of α-chloro-α-phosphonopropionic acid trichloride with triethyl phosphite yielded α-methyl-β-diethylphosphatovinylphosphonic acid dichloride. A method was proposed for the synthesis of previously unknown phosphonobarbiturates and of a number of other phosphorus-containing heterocycles via the reaction of α-chloro-α-phosphonopropionic acid trichloride with urea or thiourea in the presence of pyridine. In all the cases, complex salts of pyridine with α-chloro-α-phosphonopropionic acid were isolated from the aqueous phase and characterized. The reaction of the trichloride with dimethyl sulfoxide involved its sulfur and oxygen atoms.

Calculation of 15N and 31P NMR Chemical Shifts of Azoles, Phospholes, and Phosphazoles: A Gateway to Higher Accuracy at Less Computational Cost.

A number of computational schemes for the calculation of 15N and 31P NMR chemical shifts and shielding constants in a series of azoles, phospholes, and phosphazoles was examined. A very good correlation between calculated at the CCSD(T) level and experimental 15N and 31P NMR chemical shifts was observed. It was found that basically solvent, vibrational, and relativistic corrections are of the same order of magnitude and alternate in sign, being, on average, of about 2-3 ppm in absolute value but, being much larger (up to 14 ppm) in the case of solvent molecules explicitly introduced into computational space. At the DFT level, the performance of nine exchange-correlation functionals including six conventional gradient functionals and three hybrid functionals was studied. The most accurate results were reached with the OLYP and Keal-Tozer's family of functionals, KT1, KT2, and KT3, while the most popular B3LYP and PBE0 functionals showed the most unreliable results. On the basis of these data, we highly recommend OLYP and KT2 functionals for the computation of 15N and 31P NMR chemical shifts at the DFT level in the diverse series of nitrogen- and phosphorus-containing heterocycles. Benchmark calculations of 15N and 31P NMR chemical shifts in a series of larger nitrogen- and phosphorus-containing heterocycles were performed at the DFT level in comparison with experiment and revealed the OLYP functional in combination with the aug-pcS-3/aug-pcS-2 locally dense basis set scheme as the most effective computational scheme.

Recent progress in the synthesis of phosphorus-containing indole derivatives.

Phosphorus-containing indole derivatives represent a special class of phosphorus-containing nitrogen heterocycles, which not only widely exist in pharmaceutical agents, but also have widespread applications in materials science and organic synthesis as ligands and intermediates. The synthesis of such compounds is of current interest, with two major synthetic strategies being established. This review summarizes the recent progress in this field, including the advantages and limitations of each subdivisional tactic, briefly discusses the reaction mechanisms and challenges, and outlines the synthetic opportunities still open.

Synthesis of anionic phosphorus-containing heterocycles by intramolecular cyclizations involving N-functionalized phosphinecarboxamides.

We report that the 2-phosphaethynolate anion (PCO−) reacts with propargylamines in the presence of a proton source to afford novel N-derivatized phosphinecarboxamides bearing alkyne functionalities. Deprotonation of these species gives rise to novel five- and six-membered anionic heterocycles resulting from intramolecular nucleophilic attack of the resulting phosphide at the alkyne functionality (via 5-exo-dig or 6-endo-dig cyclizations, respectively). The nature of the substituents on the phosphinecarboxamide can be used to influence the outcome of these reactions. This strategy represents a unique approach to phosphorus-containing heterocylic systems that are closely related to known organic molecules with interesting bio-active properties.

Reactions of 5-substituted 1,3,4-thiadiazole-2-thiones with chloroacetylenephosphonates

Reactions of 1-chloroacetylene-2-phosphonates with thiadiazole-2-thiones in anhydrous acetonitrile occur regio- and chemoselectively to form novel fused phosphorus-containing heterocycles, 2-substituted 5-(dialkoxyphosphoryl)thiazolo[2,3-b][1,3,4]thiadiazol-4-ylium chlorides. The formation of these fused cyclic compounds is associated only with the thione form of the starting thiadiazoles. Reactions of the salts of thiadiazole-2-thiols with 1-chloroacetylene-2-phosphonates result in the formation of compounds of linear structure.

Structural and theoretical insights into the AIE attributes of phosphindole oxide: the balance between rigidity and flexibility.

Multiple intramolecular motions consume the excited-state energy of luminogenic molecules upon photoexcitation and lower the emission efficiency. The low frequency rotational motion of aromatic rings can be facilely restricted by steric constraint in the condensed phase, but the high frequency bond stretching motion can hardly be suppressed by aggregation. In this work, three phosphorus-containing heterocycles, 1,2,3,4,5-pentaphenylphosphole-1-oxide (PPPO), 1,2,3-triphenylphosphindole-1-oxide (TPPIO), and 1,2,3-triphenylphosphindole (TPPI), were synthesized and characterized. Their optical properties, crystal-packing manners, electronic features, and fluorescence dynamics were systematically investigated, and theoretical calculations were performed to decipher structure-property relationships. The results reveal that these luminogens are weak emitters in solutions but show strong emission in aggregates, exhibiting obvious aggregation-induced emission (AIE) features. The aggregation-insensitive stretching motion, which is dominant in PPPO, is lowered in TPPIO, enabling TPPIO to fluoresce much more efficiently than PPPO in aggregates. The stretching motion is even more lowered in TPPI, but its relatively planar conformation suffers emission quenching due to strong π-π stacking interactions in aggregates. Therefore, a twisted molecular conformation consisting of a rigid stator and a rotatable periphery is demonstrated to be a rational design for more efficient AIE luminogens.

Synthesis of phosphiranes

Phosphiranes, three-membered phosphorus-containing heterocycles, have been prepared via reaction of tertiary phosphines dilithium derivatives with carbon tetrachloride. The elaborated method can be applied for preparation of some phosphiranes which are not easily accessible.

Phosphorus-containing copolyesters: The effect of ionic group and its analogous phosphorus heterocycles on their flame-retardant and anti-dripping performances

A series of copolyesters containing ionic groups (PETIs-K) were synthesized by melt co-polycondensation using terephthalic acid, ethylene glycol, and potassium salt of bishydroxyethyl ester of 10-hydroxy-10-oxo-10H-phenoxaphosphine-2,8-dicarboxylic acid (DHPPO-K) as monomers. The copolyesters only with analogous phosphorus-containing heterocycles (PETPs) were also synthesized and used as a control. The dynamic rheological behaviors confirmed that the complex viscosity of PETIs-K increased with increasing temperature due to the existence of ionic aggregates, which restricted melt dripping, while PETPs had flow behaviors similar to PET: the complex viscosity decreased with increasing temperature. The ionic groups induced the decomposition of the polymer backbone to form a more stable residue at high temperatures, which made PETIs-K exhibit good flame retardancy even with low phosphorus contents. Cone test showed the peak heat release rate and total smoke production of PETP5 decreased to 34% and 58% of pure PET. Compared with PETP5, these values for PETI5-K were reduced by 31% and 50%, respectively. EDX and ICP-AES results demonstrated that 92% of phosphorus for PETI10-K remained in char residues; however, this value for PETP10 was only 45%. Combined with results of XPS, Raman spectroscopy and Py-GC/MS, it can be proved that the ionization of phosphorus took main effects in condensed phase via promoting carbonization to form stable graphitic char, which avoided the decrease of carbonization due to the competitive effects of phosphorus in gas phase.

ChemInform Abstract: Palladium‐Catalyzed Intramolecular Direct Arylation for Phosphorus Heterocycle Synthesis.

A palladium-catalyzed intramolecular direct arylation of bromo-substituted phosphine oxides is reported for the synthesis of phosphorus-containing heterocycles with good to excellent yields (78–98%)

Palladium-Catalyzed Intramolecular Direct Arylation for Phosphorus Heterocycle Synthesis

A palladium-catalyzed intramolecular direct arylation of bromo-substituted phosphine oxides is reported for the synthesis of phosphorus-containing heterocycles with good to excellent yields (78–98%)

New Chiral Zwitterionic Phosphorus Heterocycles: Synthesis, Structure, Properties and Application as Chiral Solvating Agents

A family of new chiral zwitterionic phosphorus-containing heterocycles (zPHC) have been derived from methylene-bridged bis(imidazolines). These structures were unambiguously determined, including single-crystal XRD analysis for two compounds. The stability, acid/base and electronic properties of these dipolar phosphorus heterocycles were subsequently investigated. zPHCs can be successfully employed as a new class of chiral solvating agents for the enantiodifferentiation of chiral carboxylic and sulfonic acids by NMR spectroscopy. The stoichiometry and binding constants for the donor-acceptor complexes formed were established by NMR titration methods.

ChemInform Abstract: Ring Transformations of Phosphorus-Containing Heterocycles. Part 6. The Formation of the 4-Methylene-1,4-dihydrophosphinine Ring System in the Reaction of 3,4-Dimethyl-3-phospholene 1-Oxides with Dichlorocarbene.

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Antimicrobial activity of novel fused nitrogen, sulfur and phosphorus containing-heterocycles and relevant phosphonates with difurylpyridazine species

Treatment of 5,6-di(2-furyl)-3-thioxo-2,3-dihydropyridazine-4-carbonitrile with phosphonate carboanions stabilised with electron withdrawing group (CO2R, CN, SR’) led, under microwave irradiation and the effect of basic catalysis, to excellent yields of a variety of condensed sulfur, nitrogen and phosphorus-containing heterocycles as major products, along with fused thieno- and thiazole ring systems bearing a phosphonate moiety. On the other hand, fused thiazaphosphinine oxide and thiopyranone phosphonate were obtained in equal yields from the reaction of 5,6-di(2-furyl)-3-thioxo-2,3-dihydropyridazine-4-carbonitrile with diethyl vinylphosphonate. Screening results of antimicrobial potency for the products were discussed in terms of structure-activity relationships (SAR), and an attempt was made to define the structural features for lead compounds.

Phosphinines as ligands in homogeneous catalysis: recent developments, concepts and perspectives

Phosphinines belong to an intriguing class of phosphorus-containing heterocycles with interesting steric, electronic and coordinating properties. This article focuses on recent developments, concepts and perspectives in the field of phosphinines and phosphinine-based ligands and their application as ligands in homogeneous catalytic reactions.

Synthesis of Chiral Organophosphorus Compounds by the Reaction of Dibenzylphosphine Oxide with α,β‐Unsaturated Ketones of the Terpene Series.

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