Phosphazene Research Articles

Phosphorus-Containing Superbases: Recent Progress in the Chemistry of Electron-Abundant Phosphines and Phosphazenes.

The renaissance of Brønsted superbases is primarily based on their pronounced capacity for a large variety of chemical transformations under mild reaction conditions. Four major set screws are available for the selective tuning of the basicity: the nature of the basic center (N, P, …), the degree of electron donation by substituents to the central atom, the possibility of charge delocalization, and the energy gain by hydrogen bonding. Within the past decades, a plethora of neutral electron‐rich phosphine and phosphazene bases have appeared in the literature. Their outstanding properties and advantages over inorganic or charged bases have now made them indispensable as auxiliary bases in deprotonation processes. Herein, an update of the chemistry of basic phosphines and phosphazenes is given. In addition, due to widespread interest, their use in catalysis or as ligands in coordination chemistry is highlighted.

Cooligomerization of γ-butyrolactone with (meth)acrylates catalyzed by N-heterocyclic carbene: Low possibility of hybrid copolymerization

Copolymerization between lactones and vinyl monomers has significant implications for the synthesis of new polymeric materials; however, a highly reactive cyclic monomer, e.g., ε-caprolactone (CL), leads to a copolymeric product but with a complex structure remaining to be elucidated. Herein, we present cooligomerizations of γ-butyrolactone (γBL), a low-polymerizable cyclic monomer, with (meth)acrylates catalyzed by an N-heterocyclic carbene (NHC) under conditions in which no reaction occurs in the absence of either comonomer. Several experimental studies indicate that the product is not a hybrid cooligomer with an ester linkage in the main chain but a graft cooligomer. The reaction mechanism involves the generation of the γBL enolate by the NHC to initiate oligomerizations of γBL and (meth)acrylates followed by the transesterification of oligo (γBL) with an ester unit of (meth)acrylates. These results prompted us to investigate the previously reported structure of copolymer of CL and methyl methacrylate obtained by a phosphazene base catalyst. The hydrolysis experiment revealed that the copolymeric product turned out not to be a hybrid random copolymer.

Noncovalent Protection for Direct Synthesis of α-Amino-ω-hydroxyl Poly(ethylene oxide)

The synthesis of poly(ethylene oxide) (PEO) with amino end group, a key functionality for PEGylation, is a long-standing challenge. Multistep routes based on postmodification or covalent protection have been adopted to circumvent ethoxylation of the amino group by ethylene oxide (EO). Here, we report a noncovalent protection strategy for one-step synthesis of PEO amine. An amino (di)alcohol is mixed with a small amount of mild phosphazene base and excess triethylborane (Et3B) before addition of EO. The complexation of the amino group with Et3B guarantees that polymerization of EO occurs selectively from the hydroxyl group through the bicomponent metal-free catalysis. Simply by precipitation in diethyl ether, the protective Et3B as well as the catalyst can be removed to afford α-amino-ω-hydroxyl PEO with controlled molar mass, low dispersity, and complete end functionality. The effect of initiator structure and retention of Et3B on the storage (oxidative) stability of PEO amine is also revealed.

Organocatalyzed regio-regular polymerization of α-aryl trimethylene carbonate

Polymerization of α-substituted trimethylene carbonate (TMC) can give access to novel aliphatic polycarbonates. To combine the tunability provided by an aromatic substituent and the controlled polymerization conditions available for the synthesis of aliphatic polycarbonates, we have developed a regio-regular ring opening polymerization (ROP) of α-aryl trimethylene carbonate (α-ArTMC). Regio-regular ROP of α-methyl trimethylene carbonate (α-MeTMC) has been reported previously. Recently, it has been shown that phosphazene bases can catalyze the regio-regular ROP of α-methyl-trimethylene carbonate (α-MeTMC). When ROP of α-PhTMC was carried out under these conditions, we obtained a regio-random polymer (Xreg = 0.03). Using 1,5,7-triazabicyclo-[4.4.0]-dec-5-ene (TBD), as the catalyst, we were able to polymerize α-ArTMCs with improved regio-regularity. A large dependence of the regio-regularity on para substituent of the aryl ring was observed. By varying the substituent pattern in the aromatic ring, regio-regularities ranging from Xreg = 0.51 to Xreg = 0.89 were obtained. Changing the α-substituent in TMC from a methyl group to a phenyl group resulted in a substantial increase in glass transition temperature (Tg) of the corresponding polymer. The Tg of the polymers was shown to increase with an increase in regio-regularity to a certain extent. Overall a range of Tgs from 39 to 66 °C was observed upon changing regio-regularity and substituent pattern.

Organocatalytic Ring‐Opening Alternating Copolymerization of Epoxides and Cyclic Anhydrides by a Simple Organobase/Urea Binary Catalyst

AbstractAlthough several organocatalysts have been reported for the efficient ring‐opening alternating copolymerization (ROAC) of epoxides and cyclic anhydrides, the low catalytic activity still remains as a problem. Thus, there is still a motivation to develop new simple and high active organocatalysts. In this contribution, an organic cyclic trimeric phosphazene base (CTPB) is used as catalyst in combination with different ureas for the ROAC of phthalic anhydride with a variety of epoxides. The CTPB/urea binary catalysts exhibit high catalytic activities, producing perfectly alternating polyesters without formation of ether linkages. Polyesters with diverse structures, well‐defined chain‐ends, as well as low dispersities are successfully obtained.

Synthesis of functional and architectural polyethers via the anionic ring-opening polymerization of epoxide monomers using a phosphazene base catalyst

This focused review provides an overview of our recent work and related research regarding the precise anionic ring-opening polymerization (AROP) of substituted epoxides, including alkylene oxides, glycidyl ethers, and glycidyl amines, using t-Bu-P4 as the phosphazene base catalyst to produce functional polyethers, such as homopolymers, block copolymers (BCPs), and topologically unique polymers. First, the fundamental aspects and applicable monomer scope of t-Bu-P4-catalyzed AROP are discussed. Subsequently, the applications of well-defined polyethers prepared via t-Bu-P4-catalyzed AROP to develop functional materials, such as thermoresponsive polymers and Li+ conducting polymers, are discussed. Finally, the utility of t-Bu-P4-catalyzed AROP in the precise synthesis of star-shaped, cyclic, and multicyclic polymers is presented. Overall, we intend to illustrate the exploitable utility of the present polymerization system for fundamental and advanced polymer research. This focused review provides an overview of our recent work and related research regarding the precise anionic ring-opening polymerization (AROP) of substituted epoxides, including alkylene oxides, glycidyl ethers, and glycidyl amines, using t-Bu-P4 as the phosphazene base catalyst to produce functional polyethers, such as homopolymers, block copolymers (BCPs), and topologically unique polymers. First, the fundamental aspects and applicable monomer scope of the t-Bu-P4-catalyzed AROP are discussed. Subsequently, the applications of the AROP system for synthesizing functional materials and architectural polymers are presented.

Evidence of the Lewis-Amphoteric Character of Tris(pentafluoroethyl)silanide, [Si(C2 F5 )3

According to a first view on the geometrical and electronic structure of the tris(pentafluoroethyl)silanide, this anion appears as a Lewis base. Quantum chemical calculations on perfluoroalkylated silanides show significantly lower HOMO and LUMO energy levels in comparison to their non‐fluorinated counterparts, which implies reduced Lewis basicity and increased Lewis acidity of the [Si(C2F5)3]− ion. With these findings and a HOMO–LUMO gap of 4.80 eV similar to N‐heterocyclic silylenes (NHSis), perfluoroalkyl silanides are predestined to exhibit Lewis‐amphoteric character similar to silylenes. Deprotonation of Si(C2F5)3H with sterically demanding phosphazene bases afforded thermally stable phosphazenium salts of the [Si(C2F5)3]− anion, which add to benzaldehyde, benzophenone, CS2, and CO2 in various manners. This behavior also mirrors the reactivity of silylenes towards ketones as well as heterocumulenes and is rationalized by Lewis amphotericity being inherent in these silanides.

Dipeptides Synthesis by Using Phosphazene Bases to Enhance the Solubility of Amino Acids

Dipeptides Synthesis by Using Phosphazene Bases to Enhance the Solubility of Amino Acids

Healing with Polymer Containing Phosphazene of Concrete Exposed to Freezing and Thawing

Healing with Polymer Containing Phosphazene of Concrete Exposed to Freezing and Thawing

Ruthenium complexes of phosphine–amide based ligands as efficient catalysts for transfer hydrogenation reactions

This work presents three mononuclear Ru(ii) complexes of tridentate phosphine-carboxamide based ligands providing a NNP coordination environment. The octahedral Ru(ii) ion shows additional coordination with co-ligands; CO, Cl and CH3OH. All three Ru(ii) complexes were thoroughly characterized including their crystal structures. These Ru(ii) complexes were utilized as catalysts for the transfer hydrogenation of assorted carbonyl compounds, including some challenging biologically relevant substrates, using isopropanol as the hydrogen source. The binding studies illustrated the coordination of the isopropoxide ion by replacing a Ru-ligated chloride ion followed by the generation of the Ru-H intermediate that was isolated and characterized and was found to be involved in the catalysis.

Recent advances in the synthesis and applications of phosphoramides.

Phosphoramide, as an important framework of many biologically active molecules, has attracted widespread attention in recent decades. It is not only widely used in pharmaceuticals because of its excellent biological activities, but it also shows good performance in organic dyes, flame retardants and extractors. Thus, it is of great significance to develop effective and convenient methods for the synthesis of phosphoramides. In this review, the recent advancements made in the synthesis routes and applications of phosphoramides are discussed. The synthetic strategies of phosphoramides can be separated into five categories: phosphorus halides as the substrate, phosphates as the substrate, phosphorus hydrogen as the substrate, azides as the substrate and other methods. The latest examples of these methods are provided and some representative mechanisms are also described.

Brønsted Base‐Catalyzed Formal Reductive [3+2] Annulation of 4,4,4‐Trifluorocrotonate and α‐Iminoketones

A formal reductive [3+2] annulation of 4,4,4-trifluorocrotonate and α-iminoketones was developed under Brønsted base catalysis. A single phosphazene base efficiently catalyzes the one-pot tandem reaction involving two mechanistically different elementary processes, namely the chemoselective reduction of an imine moiety of α-iminoketones with thiols as the reductant and the subsequent intermolecular Michael addition of an enolate of α-aminoketones concomitant with lactam formation. This operationally simple method provides β-trifluoromethyl-substituted γ-lactams with a tetrasubstituted carbon as a single diastereomer.

Copolymerize Conventional Vinyl Monomers to Degradable and Water‐Soluble Copolymers with a Fluorescence Property

AbstractIn the copolymerization of acrylamide (Am) and hydroxyethyl acrylate (HEA), the initial product is a carbon chain polymer with hydroxyl and amide side groups. However, in the present work, these two conventional vinyl monomers are combined into a degradable heterochain copolymer by using a phosphazene base, t‐BuP4, as a catalyst. NMR not only confirms that the backbone of the resulting copolymer is composed of ester–ether units and amide units but also suggests that the resulted secondary amide group retains an active hydrogen and can further react with another olefin, which would result in branching or gelation. Thermogravimetric analysis (TGA) shows a signal indicating decomposition at 205–509 °C, verifying the uniform composition of the sample. More interestingly, the synthesized copolymer can dissolve in water and exhibits aggregation‐enhanced emission (AEE). Additionally, the aqueous copolymer solution has a high selectivity and sensitivity for Ba2+, Fe2+, and Fe3+ cations. Hence, it is believed that this method for constructing degradable polymers may offer innovative insights into polymer synthesis and application.

Dihydropyracyloporphyrins

A pyrrole ethyl ester with a fused dihydropyracylene unit was prepared by reacting 5-nitro-1,2-dihydropyracylene with ethyl isocyanoacetate in the presence of a phosphazene base. Cleavage of the ester moiety, followed by reaction with an acetoxymethylpyrrole and acetic acid in refluxing ethanol, afforded a tripyrrane, and subsequent ‘3 + 1’ condensation with a pyrrole dialdehyde gave a dihydropyracyloporphyrin. Cyclotetramerization of the dihydropyracylopyrrole with benzaldehyde and BF3·Et2O gave a sterically crowded tetrakis(dihydropyracylo)-porphyrin and the reduced five-membered rings proved to be an excellent NMR probe for conformational mobility. The new porphyrins and their nickel(II) complexes also showed potentially valuable highly red shifted UV–vis spectra.

The Coordination Chemistry of the N-Donor-Substituted Phosphazanes.

Phosph(III)azanes, featuring the heterocyclobutane P2 N2 ring, have now been established as building blocks in main-group coordination and supramolecular compounds. Previous studies have largely involved their use as neutral P-donor ligands or as anionic N-donor ligands, derived from deprotonation of amido-phosphazanes [RNHP(μ-NR)]2 . The use of neutral amido-phosphazanes themselves as chelating, H-bond donors in anion receptors has also been an area of recent interest because of the ease by which the proton acidity and anion binding constants can be modulated, by the incorporation of electron-withdrawing exo- and endo-cyclic groups (R) and by the coordination of transition metals to the ring P atoms. We observed recently that the effect of P,N-chelation of metal atoms to the P atoms of cis-[(2-py)NHP(μ-Nt Bu)]2 (2-py=2-pyridyl) not only pre-organises the N-H functionality for optimum H-bonding to anions but also results in a large increase in anion binding constants, well above those for traditional organic receptors like squaramides and ureas. Here, we report a broader investigation of ligand chemistry of [(2-py)NHP(μ-t NBu)]2 (and of the new quinolyl derivative [(8-Qu)NHP(μ-Nt Bu)]2 (8-Qu=8-quinolyl). The additional N-donor functionality of the heterocyclic substituents and its position has a marked effect on the anion and metal coordination chemistry of both species, leading to novel structural behaviour and reactivity compared to unfunctionalized counterparts.

Highly Efficient Amide Michael Addition and Its Use in the Preparation of Tunable Multicolor Photoluminescent Polymers.

The amide bond is one of the most pivotal functional groups in chemistry and biology. It is also the key component of proteins and widely present in synthetic materials. The majority of studies have focused on the formation of the amide group, but its postmodification has scarcely been investigated. Herein, we successfully develop the Michael additions of amide to acrylate, acrylamide, or propiolate in the presence of phosphazene base at room temperature. This amide Michael addition is much more efficient when the secondary amide instead of the primary amide is used under the same conditions. This reaction was applied to postfunctionalize poly(methyl acrylate-co-acrylamide), P(MA-co-Am), and it is shown that the amide groups of P(MA-co-Am) could be completely modified by N,N-dimethylacrylamide (DMA). Interestingly, the resulting copolymer exhibited tailorable fluorescence with emission wavelength ranging from 380 to 613 nm, which is a desired property for luminescent materials. Moreover, the emissions of the copolymer increased with increasing concentration in solution for all excitation wavelengths from 320 to 580 nm. Therefore, this work not only develops an efficient t-BuP4-catalyzed amide Michael addition but also offers a facile method for tunable multicolor photoluminescent polymers, which is expected to find a wide range of applications in many fields, such as in anticounterfeiting technology.

A Track-Based Molecular Synthesizer that Builds a Single-Sequence Oligomer through Iterative Carbon-Carbon Bond Formation

Summary We report an artificial molecular machine that moves along a track, iteratively joining building blocks to form an oligomer of single sequence with a continuous backbone of carbon-carbon bonds. The rotaxane features a macrocycle bearing an aldehyde-terminated chain and an axle containing different phosphonium ylides separated by rigid spacers. Each ylide is large enough to block the passage of the macrocycle, trapping the ring between the stopper at the terminus of original threading and the next ylide along the track. Once a building block is reachable, it is removed from the track through a Wittig reaction that adds it to the terminus of the growing chain. Operation on a four-barrier tetra(phosphonium salt) track produces a tetra(diphenylpropane) of single sequence linked through alkene bonds. The prototype extends the principle for molecular machines that build polymers by moving along tracks to the synthesis of sequence-encoded chains with continuous carbon backbones.

Phosphorus-nitrogen compounds. Part 48. Syntheses of the phosphazenium salts containing 2-pyridyl pendant arm: Structural characterizations, thermal analysis, antimicrobial and cytotoxic activity studies

The phosphazenium salts (protic ionic liquids, PILs/protic molten salts, PMOSs) ( 6a-6d and 7a ) of the free phosphazene bases ( 4a-4d and 5a ) were prepared by the reactions of the corresponding cyclotriphosphazenes with the bulky gentisic acid. The structures of the PMOS were evaluated using the elemental analyses, FTIR, 1 H, 13 C{ 1 H} and 31 P{ 1 H} NMR data. The molecular and crystal structures of 4a and 6c were established by X-ray crystallography. The thermal properties of the PMOS were determined using TG and DTA techniques. On the other hand, the antimicrobial activities of the free phosphazene bases ( 4a-4d and 5a-5d ) and PMOSs ( 6a-6d and 7a ) were screened against the selected bacteria and yeast strains. The antimicrobial activities of the free phosphazene bases and the PMOSs were compared. The interactions of the phosphazenes and their salts with plasmid DNA were elucidated by the agarose gel electrophoresis. The evaluations of the cytotoxic activities of these compounds were also studied against to L929 fibroblast and breast cancer cells (MDA-MB-231).

Tuning the toxixity of phosphonium based biocides using the bioisosteric CH2F moiety

Tertiary alkyl, aryl or amino phosphines PR3 (R = Me, nBu, C2H4CN, NEt2) and the bis(phosphine) POP were allowed to react with fluoroiodomethane to produce fluoromethyl phosphonium salts. New crystal structures of forming fluoromethyl phosphonium salts [R3PCH2F]I with R = Me, C2H2CN and NEt2 were obtained and gave additional information of the nearly unknown P-CH2F moiety. The toxicity of the water soluble salt [Me3PCH2F]I was investigated compared to the biocide THPS.

Rapid one-pot radiosynthesis of [carbonyl-11C]formamides from primary amines and [11C]CO2

BackgroundFormamides are common motifs of biologically-active compounds (e.g. formylated peptides) and are frequently employed as intermediates to yield a number of other functional groups. A rapid, simple and reliable route to [carbonyl-11C]formamides would enable access to this important class of compounds as in vivo PET imaging agents.ResultsA novel radiolabelling strategy for the synthesis of carbon-11 radiolabelled formamides ([11C]formamides) is presented. The reaction proceeded with the conversion of a primary amine to the corresponding [11C]isocyanate using cyclotron-produced [11C]CO2, a phosphazene base (2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine, BEMP) and phosphoryl chloride (POCl3). The [11C]isocyanate was subsequently reduced to [11C]formamide using sodium borohydride (NaBH4). [11C]Benzyl formamide was obtained with a radiochemical yield (RCY) of 80% in 15 min from end of cyclotron target bombardment and with an activity yield of 12%. This novel method was applied to the radiolabeling of aromatic and aliphatic formamides and the chemotactic amino acid [11C]formyl methionine (RCY = 48%).ConclusionsThis study demonstrates the feasibility of 11C-formylation of primary amines with the primary synthon [11C]CO2. The reactivity is proportional to the nucleophilicity of the precursor amine. This novel method can be used for the production of biomolecules containing a radiolabelled formyl group.