Phosphorus cavitand

The flexibility and the different conformations adopted by the re-sorc[4]arene can be rigidified in the cone shape configuration by bridging the phenol functions with different substituents [38]. We will report here on the tetra-bridged phosphorus cavitands (phosphocavitand), whose general structure is presented in Scheme 2 [39]. 

The general synthetic route for the preparation of bridged phosphorus cavitand is outlined in Scheme 5. From the resorc[4]arene, bearing various substituents at the lower rim, the cyclization step, which leads to the formation of the four fused eight-membered rings, was performed with three-co-ordinated and four-coordinated phosphorus reagent to give respectively tet-ra-phosphite, tetra-phosphonite, and tetra-phosphate or tetra-phosphonate derivatives. 

The name cavitand refers to the rigidified bridged resorc[4]arene. lUPAC name for the tetra-briged phosphorus cavitand 5,9,13,17-tetra-R-l,21,23,25-tetrakis-R -2,20 3,19-... 

This geometry is undoubtedly imposed by the skeletal stabilizing unit. Other eight membered phosphorus(III)-nitrogen rings, (MePNMe)i, (9) and (PrNO G NP) (11) are crown shaped containing all chemically equivalent phosphorus atoms. In XI, because the phosphorus atoms are of two sets and one set is in a highly protected environment, the possibility of selective, "cavitand," coordination at phosphorus sites exists. 

The cavitands are essentially synthesized from their resorc[4]arene precursors which are readily obtained by resorcinol condensation with aldehydes. The main feature comes from the different configurations that are expected for this tetrameric species and the relative thermodynamical stability of each isomer, which has been widely investigated by several authors. In addition, the conformational mobility of the resorc[4]arene molecules will depend on substitution at the upper and lower rims [28, 36, 40, 41]. The first attempt to synthesize a phosphorus bridged cavitand was to treat resorc[4]arene la (1, R=CH3) with phenylphosphonic dichloride or phenylphosphonothioic dichloride. Only inseparable isomer mixtures were obtained and isolation of the desired cavitands was not possible [42]. The first isolated phosphorylated resorcinol-based cavitand was described in 1992 by Markovsky et al., who prepared compound D from la and four equivalents of o-phenylenechlorophos-phate in the presence of triethylamine [43, 44]. For this compound, a tautomeric temperature and solvent dependent equilibrium exists between the spirophosphorane structure and the cyclic phosphate form (Scheme 4). 

The main feature for cation recognition by tetra-bridged phosphorylated cavitands arises from the cooperative effect of the four phosphorus groups and the aromatic molecular cavity. In the phosphorus(IV) cavitands guest binding will be achieved through O (P=0) or S (P=S) coordination with different affinity for hard or soft metal ions. On the other hand, transition metal rim complexes described above can act as host for metal cation. 

The high affinity of ammonium cations for tetra-bridged phosphorus(IV) P=0 cavitands was used to complex bis-ammonium guests. The highly insoluble NjAT-dimethyl-bipyridinium dication [paraquat " ] was readily dissolved in chloroform solution in presence of 12b in a 2 1 host-guest ratio. 

An alternative to deepening the cavity in the manner of cavitand 4 comes in the form of the stereoselective bridging of resorcinarenes. The first successes in this regard were carried out with a number of phosphorus derivatives, e.g., Stereoselective bridging with... 

However, the application of calixarenes and other cavitands such as the phosphorus-bridged ones or P-cyclodextrin in low-temperature gas sensors, particularly quartz crystal microbalance (QCM)- and... 

In 1992 Maikovsky and his group were the first to publish work concerning bridging with phosphorus when they introduced spirophosphorane rnoieties to t e resorcinarene framework (Scheme 9.5) [196]. Cavitand 58 is itself in equilibrium with the unbridged form 59 an equilibrium controlled by both solvent and temperature. There was no discussion concerning the stereochemistry of these reversibly formed bridges (only one isomer shown in 58). 

Extending this line of thought, Puddephatt s group reported in 1993 cavitands bridged with P(III) moieties. They demonstrated the formation of several tetra-gold (I) and tetra-copper(I) complexes and showed that they were capable of the size-selective binding of halide ions [197]. Molecular mechanics and NMR data were consistent with the stereochemistry of the free ligand in which the lone pairs on the phosphorus point towards the center of the pocket (60). 

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