Among the more prominent features that elevate their status as a unique and versatile class of compounds, include variable oxidation states, multivalency, asymmetry and metal-binding properties. Their presence in bioactive natural products, endogenous biomolecules, small molecule therapeutic agents and pro-drugs substantiates their role in modern synthetic chemistry and chemical biology.
Moreover, their central use as ligands and effectors in asymmetric catalysis, as well as key functional groups for the development of new synthetic methods, has taken the field to new heights. This Thematic Series highlights and details some of the novel methods that are advancing the field of organophosphorus chemistry. The Thematic Series covers topics that range from new synthetic methods and phosphorus-based ligands in asymmetric catalysis to bisphosphonates as promising enzyme inhibitors.
More specifically, the Thematic Series spans new methods in C—P bond formation, chiral phosphines in nucleophilic organocatalysis, chiral N -phosphinyl auxiliaries, cyclic phosphonamide reagents in the total synthesis of natural products, phosphinate-containing heterocycles, new routes to phosphinoyl-indoles and phosphinoyl-isocoumarins and new chemistries of H-phosphonates. The Thematic Series also details work on new metathesis-based reactions of vinyl phosphonates and phosphate tethers, novel phosphorus-based ligands in asymmetric catalysis, novel rasta resin—triphenylphosphine oxides and their use as recyclable heterogeneous reagents, the Atherton—Todd reaction, cyclic phosphonium ionic liquids with distinct properties, photo-removable phosphate protecting groups, new methods of C—H functionalization using phosphoryl-related directing groups, the exciting chemistry of substituted phosphanylidenecarbenes and phosphoryl azides, and bisphosphonate ethers as promising inhibitors of geranylgeranyl diphosphate synthase GGDPS.
The articles and reviews capture the emerging potential of organophosphorus compounds and exciting opportunities in the field, and hopefully, will inspire and motivate investigators in the field to investigate new chemistry in this area. Taken collectively, organophosphorus chemistry embodies a broad, vibrant and continual growing scientific area with this Thematic Series highlighting recent advances in the field.
We are grateful and indebted to the authors for their hard work and exciting contributions to this Thematic Series and look forward to continued contributions in this area. National Center for Biotechnology Information , U. Beilstein J Org Chem. Published online Sep 4. First developed in the s and s, proliferation of such compounds through the second half of the twentieth century led to the development of the Chemical Weapons Convention and the establishment of the Organization for the Prohibition of Chemical Weapons OPCW in the early s [ 3 , 4 ].
However, events in the s in Iraq [ 5 ], s in Tokyo [ 6 ] and more recently in Syria [ 7 ] indicate the threat still posed by these chemicals.
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Whilst there are established detection and decontamination technologies readily available, recent efforts in the application of supramolecular chemistry to CWA hazard mitigation have gained interest and shown significant promise [ 8 ]. Examples include the catalytic breakdown of OP materials by metallo-organic cages [ 9 ], the development of hydrogen bond-donating organocatalysts [ 10 ] and the use of supramolecular gels as CWA-responsive materials [ 11 ]. All of the G- and V-series agents contain aliphatic side-chains, potentially suitable for inclusion into the hydrophobic cavities of appropriate cavitands through the hydrophobic effect.
Water-soluble sulfonato-calixarenes SCX; Figure 1 are increasingly well-known host systems that can form strong inclusion complexes driven by the hydrophobic effect [ 15 ]. Formation of SCX inclusion complexes with small organic molecules is well-known and examples of guests include small, neutral organic molecules e. Recently, Worek and co-workers have reported on the development of SCXs modified with hydroxamic acid, which is known to detoxify OP nerve agents [ 19 ].
The functionalized calixarenes show a clear enhancement in the half-life of the detoxification reactions of V-series agents and GD compared to the reactive components in the absence of the calixarene host. The development of new responsive materials, such as sensors and catalysts, is generally conducted with simulants, also commonly referred to as analogues or surrogates [ 20 ], prior to the studies using actual CWAs.
Due to the inherent toxicity and the control of the actual CWAs, a range of simulant materials have been proposed that possesses similar properties, such as chemical structure and physico-chemical behavior, but with considerably lower toxicity [ 21 ]. In some cases, agent—simulant correlation is understood sufficiently to suggest simulant selection [ 22 ], but this may not be the case with regard to supramolecular complex formation.
Molecular mechanics MMFF calculations followed by geometry refinement by semiempirical methods PM6 on GD, simulants, calixarenes and their complexes were used to further investigate complex formation and to suggest how GD and the simulants interacted with the calixarenes. Soman GD was produced at Dstl and its purity was confirmed by in-house methods. All NMR experiments were recorded at 9. A total of 16 scans were acquired. A total of scans were acquired.
Solvent environments were D 2 O or buffered D 2 O. Where buffer was used, this was a 0. To each vial was added an aliquot of a solution of the p -sulfonatocalix[ n ]arene 0. Calixarenes, GD and simulants were constructed using the Build option.
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A full conformational search of calixarene hosts, in the acid form, using molecular mechanics MMFF was followed by geometry refinement by semi-empirical methods PM6. Acidic protons were removed from both calixarenes, the sulfonyl groups given partial double bond character and sodium counter ions added to generate their sodium sulfonate forms. GD and simulant complex geometries were determined as for the acid forms. Simulations were attempted using explicitly solvated systems 4 to 18 water molecules and a continuum model, but these failed to refine successfully.
Typically, perturbations in the guest molecule proton environments were greater in magnitude than those experienced by the calixarene host protons.
Analysis of 31 P-NMR spectra also indicated formation of inclusion complexes for all simulants studied as evidenced by a downfield shift perturbation in 31 P resonances upon addition of the calixarene species. In the case of DEMP and DIMP the 31 P resonances were invariant to the addition of 4-hydroxybenzene sulfonic acid, indicating both a lack of pH effects and confirming that complexation is driven by the hydrophobic cavity of the cyclic hosts. In the case of PMP, however, considerable perturbation of the P-centre was observed, indicating the formation of different solution species.
Therefore, as this could not be separated from the complexation process, PMP data is reported in buffered solutions using only 1 H-NMR for data fitting analyses. For PMP monitoring, the proton resonances of the t Bu group of the molecule indicated a binding interaction, however, the P-CH 3 environments in particular gave rise to skewed plots in D 2 O and did not furnish expected curves in buffered solutions.
It may also indicate the presence of multiple binding modes or conformations in the PMP—host complex. The calculated association constants, K 11 , are given in Table 1. The presence of buffer appears to lower the complex affinity, as has been reported by others for phosphate buffers. In the case of the larger SCX6 host, there is an indication of complex formation with DEMP but the data could not be fitted satisfactorily to allow association constants to be determined.
In the case of PMP, low affinity complexes were observed for both SCX host species in buffered solutions, and determination of complex affinity in non-buffered solutions was not feasible, as discussed earlier. Errors stated are those obtained from the data fitting process and fit plots are presented in the Supplementary Materials.
Consequently, buffered solutions phosphate, pH 7, 0. Job plots constructed by monitoring the t Bu environment indicated the formation of stoichiometry complexes of GD with both calixarene hosts Figure 6 , however, analysis of the P-CH 3 group provided little indication of complex formation. This is suggestive of a host—guest complex conformation in which the pinacolyl side-chain is included in the cavity and the P-center is closer to the solvent environment.
In the case of SCX6 Job plots were less clear, indicative of weaker interaction between the host and guest. This is expected, given that the SCX hosts are achiral and chiral recognition using calixarenes is usually achieved through either the incorporation of chiral functionality or of asymmetry [ 28 , 29 ]. Calculations were undertaken in the gas phase and solvated state with explicit water molecules introduced into the simulations. The latter calculations encountered problems upon refinement at semi-empirical level so discussion has been restricted to the gas phase results.
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We have previously shown that, at this level of theory, solution phase host—guest complexes are quite adequately represented by gas phase models [ 30 , 31 ]. While binding affinities were not reproduced well, the resulting geometries of the complexes give some insights into the potential binding of GD and simulants by the two calixarenes Figure 7 and Figure 8. It is clear that where differences in binding affinities occur upon buffering, it is the formation of sodium salts of the calixarenes which reduces the access to the macrocyclic cavity.
In the case of SCX6, this is accompanied by large conformational changes in which the sodium cations bridge adjacent sulfonate groups Figure 8. Inspection of the different complexes shows that hydrogen bonding between the interior of the macrocyclic hosts and their guests is largely restricted to their sulfonic acid forms Table 2 and Table 3.
Only PMP, of all the species tested, can act as a hydrogen bond donor which leads to synergistic interactions in the case of both SCX4 and SCX6 and could potentially lead to higher stability complexes in the unbuffered system. One alkyl substituent is buried within the calixarene cavity suggesting further stabilization through hydrophobic interactions. Under buffered conditions the sulfonate salts of the macrocycles are much less amenable to binding PMP.
While the calixarenes studied are also likely to adopt numerous conformers in solution, the models chosen here represent the most stable based upon conformational analysis. A majority were below three angstrom, consistent with literature data. While there is considerable literature on the inclusion properties of SCX4 and SCX6, many of the papers focus on complexes with a potential biological importance [ 27 , 33 , 34 , 35 , 36 ].
In agreement with the literature, this appears to be driven by the hydrophobic inclusion of aliphatic groups [ 16 , 37 ]. In contrast to reports by ourselves and others on the inclusion of OP CWAs and related compounds with cyclodextrins [ 13 , 23 ], the affinity of the complexes is in most cases fairly weak.
This may be a result of non-optimized size match between the aliphatic side-chains of the guest species and the internal hydrophobic cavity, or the relative hydrophobic nature of the cyclodextrin and SCX cavities. The computational simulations indicate that binding to all guests except PMP is less likely when the both hosts are in the sodium buffered form. Under these conditions, the sodium cations form networks with the sulfonate groups which block the approach of hosts.
Even so, fewer hydrogen bonding interactions are predicted with the calixarene sodium salts when moving from neutral to buffered solutions of SCX4 and SCX6.
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Computational modeling approaches have been used to further elucidate the mechanism of binding, and to explain differences in observed complexation behavior between agents and simulants. It appears that in buffered solutions the effects of the cations significantly reduce the abilities of guest molecules to bind to the calixarene hosts.
This, and poor host—guest complementarity, contributes to the weak binding observed for buffered SCX6 complexes. This work adds to our recent efforts to more fully understand the supramolecular behavior of CWAs and to furnish agent—simulant correlations to aid in simulant selection and extrapolation of simulant-only data. All authors contributed to the writing of this paper.
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Journal List Molecules v. Published online Jan Jayne A. Ede , 1 Peter J. Cragg , 2 and Mark R. Peter J. P Find articles by Peter J. Mark R. Author information Article notes Copyright and License information Disclaimer. Received Nov 9; Accepted Jan This material is licensed under the terms of the Open Government Licence except where otherwise stated. This article has been cited by other articles in PMC. Associated Data Supplementary Materials moleculess Keywords: supramolecular, hydrophobic effect, chemical warfare agent, nerve agent, complexation, inclusion complex, computational. Open in a separate window.
Figure 1. Figure 2.
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