Conformation calix arenes

The calix[ ]arene ( indicates the number of aromatic units in the metacyclophane) building block provides a three-dimensional skeleton that is needed to fix different donor ligands in the optimal spatial positions for binding one ion selectively [94,95]. Calix[n]arenes are easily accessible from the base-eatalyzed condensation of p-tert-butylphenol and formaldehyde [96]. Gutsche proposed to define the two faces of a calix[4]arene molecule as the lower (phenolic groups) and the upper rim. The calix[4]arene with free hydroxyl groups is conformationally flexible and the... 

Recently van Leeuwen reported the fiigt crystal structure of the diphosphite dicarbonyl rhodium catalyst HRh(CO)2(P P) [258]. Bomer et al. developed a new class of phosphonites which show promising results for the isomerization and subsequent hydroformylation of internal olefins [259]. The number of phosphite ligands based on supramolecular backbones such as calix[4]arenes is growing [260]. They are attractive because of their well defined structure combined with the ability to adopt several discrete conformations. Calix[4]arene diphosphites and calix[6]arene phosphites were first developed by BASF [261]. In... 

The upper lim cone conformed calix[4]arene-zinc porphyrin (ZP) conjugate 180 derivatized at ZP with a p5u-omellitimide (PI) electron acceptor at the distal position on calixarene with a carotenoid unit was designed and S5mthesized for stud5ung the electric field effect of photogenerated ion pair on an adjacent chromophore. The photoinduced charge separation within a ZP-PI electron donor-acceptor pair having an 8.4 A center-center distance in a linear orientation has been found to... 

Calixarenes, shown schematically in Fig. 24, are prepared by the condensation of p-tert-butylphenol with paraformaldehyde. In the cone conformation, calix[4]arene has the shape of a chalice or vase. Figure 25 exhibits this geometry, and the inclusion of a toluene molecule is apparent. Closer observation of Fig. 25 shows that an Na+ ion is also held in the base of the chalice. Thus, this cal-ixarene can include both neutral molecules and ions at the same time. The similarity to the cyclodextrins is clear, but the calixarenes are not as rigid. 

Calix[ ]arenes and calix[ ]pyrroles can dramatically change their conformation. Vase-shaped calix[ ]arenes have two rotation modes of the phenolic units the ojygen-through-the-annulus rotation and the para-substituent-through-the-annulus rotation (Figure 4.2). ... 

When this concept is applied to the stereochemistry of pillar[n]arene, rotation of the units triggers the planar-chiral inversion from pS to pR or from pR to pS. Pillar[5]arenes are cyclic pentamers, therefore they have eight possible conformers pS, pS, pS, pS, pS), (pR, pR, pR, pR, pR), (pR, pS, pS, pS, pS), pS, pR, pR, pR, pR), pR, pR, pS, pS, pS), pS, pS, pR, pR, pR), R, pR, pS, pR, pS) and pS, pS, pR, pS, pR), namely la/b, 2a/b, 3a/b and 4a/b, respectively. In many cases, these conformers interconvert at 25 °C. Thus, inhibition of the rotation of the units contributes to the separation of these planar-chiral pillar[5]arene enantiomers. In calix[ ]arenes, modification of alkyl substituents can slow the rotational motion of the phenolic units, and can immobilize the conformation of calix[n]arenes. Thus, we first introduced various lengths of allq l chains, such as ethyl, propyl, butyl, pentyl, hetg l and dodecyl chains to both rims of the pillar[5]arene. ... 

Among the solid state structures of the larger calix[ ]arenes, Raston [96] and coworkers reported the first inclusion complex of p-tert-butylcalix[9]arene C[9] with o-carborane (Fig. 7.30). In this structure, the calix[9]arene macrocycle adopts a conformation composed by three 3/4-cone clefts and a pleated-loop portion. Perrin et al. [97] showed that p-tert-butylcalix[10]arene C[10] adopts in the solid state a pinched cone conformation stabilized by a circular H-bond between OH groups and very similar to that found for calix[6]arene macrocycle [98]. 

Fig. 24.2 Different cationic calixarene derivatives upper rim (trimethyl-ammonium)methyl calix [4]arene 1 in cone conformation, 13-alternate calix[4]arene 2, flexible calix[n]arenes 3 and 4 upper rimpara-guanidinium calix[ ]arenes 5-8 [13, 14]...

Possible conformations of the calix[4]arene systems. (Figure adapted from Fischer S, P D J Groothenuis, roenen, PV P van Hoorn, F C j M van Geggel, D N Reinhoudt and M Karplus 1995. Pathways for mational Interconversion of Calix[4]arenes. Journal of the American Chemical Society 117 1611-1620.)... 

Fig. 5.32 Interconversion between various conformations of calix[4]arenes. X = H, R = H (top) X = OH, R = H (bottom). Energies in kcal/mol.

Thus, it has been shown that calix[4]aryl esters exhibit remarkably high selectivity toward Na [11-14]. This is attributable to the inner size of the ionophoric cavity composed of four 0CH2C=0 groups, which is comparable to the ion size of Na, and to the cone conformation that is firmly constructed on the rigid ca-lix[4]arene platform (Scheme 2). 

NaH should serve as an efficient template ion to yield cone conformers. Conformer distribution for the reaction of p-tert-h Wy calix[4]arene and ethyl bromoacetate is given in Table I. 

Table 1 Conformer Distribution for the Reaction of p-Zerz-Butyl Calix[4]arene and Ethyl Bromoacetate [16]...

Calixarenes are cyclophane-type molecules, in which at least four aromatic units are joined via methylene or related groups [9]. It is known that calix[4]arenes with hydroxy groups in the 2-position normally prefer a conelike conformation, however, depending on the substituents a more or less rapid dynamic interconversion with three possible partial cone conformations is possible (Fig. 1) [9, 10]. 

The preferred conformation of the unsubstituted calixarene shown in Fig. 1 (X = CH2) is 1,3-alternating, whereby two opposite rings are perpendicular to the plane formed by the four methylene groups and the other two are parallel to the same plane [11]. In the case of calix[6]arenes the conformational mobility is enhanced and 1,3,5-alternating or 1,4-alternating winged conformations have been identified as the preferred ones [9]. Very few heteratom-... 

Fig. 1. Known calixarenes 1 and possible conformations for calix[4]arenes...

In contrast, porphyrinogens 16 (Fig. 5) possess only 16 Ti-electrons and as a consequence the delocalization over the whole macrocycle is absent. The conformations of porphyrinogens are not planar any more and can approximate to the conformations known for calix[4]arenes (vide supra). Compounds 16 (Fig. 5) may, therefore, be considered as heteroatomic calixarene derivatives, of which some have additional heteroatoms in the bridging positions [15, 28-30]. 

As in the case of the tetrameric macrocycles discussed above, compounds 17 and 18 can be considered as porphyrinogen analogues or as heteroatom-bridged heteroaromatic calix[4]arene derivatives. The two different conformations observed for 17 and 18 have analogues in compounds 16 (Fig, 5) [15, 28-30],... 

Li" " and Na+ (preferentially Na" ), but not K", Mg or Ca. Molecules such as these, whose cavities can be occupied only by spherical entities, have been called spherands. Other types are calixarenes (e.g., 22). There is significant hydrogen bonding involving the phenolic OH units in [4]calixarenes, but this diminishes as the size of the cavity increases in larger ring calixarenes. There are also calix[4]-arenes, which have been shown to have conformational isomers (see p. 167) in... 

It is not only p-substituted phenols that have been connected by methylene groups an extended calixarene based on naphthalene units [57] has also been synthesized and its acidity constants have been measured (see Table 25 Poh and Lim, 1989, 1990). In contrast to the calix[4]arenes, the pKa of the expanded calixarene [57] did not differ much from the of the analogue [58]. This may have two explanations (i) the conformation of the expanded calixarene is different from phenol-based calix[4]arenes and a special stabilization of the anion does not occur, (ii) this stabilization occurs but it is also present in the analogue. A stabilization by the neighbouring... 

Calixarenes, which are macrocyclic compounds, are one of the best building blocks to design molecular hosts in supramolecular chemistry [158]. Synthesis of calix[4]arenes, which have been adamantylated, has been reported [105, 109]. In calix[4]arenes, adamantane or its ester/carboxylic acid derivatives were introduced as substituents (Fig. 29). The purpose of this synthesis was to learn how to employ the flexible chemistry of adamantane in order to construct different kinds of molecular hosts. The X-ray structure analysis of p-(l-adamantyl)thiacalix[4]arene [109] demonstrated that it contained four CHCI3 molecules, one of which was located inside the host molecule cavity, and the host molecule assumed the cone-like conformational shape (Fig. 30). 

At this stage it could be useful to make a comment on the structural features of complexes 4-17. The coordination number of the metal affects strongly the shape of the calix[4]arene fragment. In the case of five-coordinate and, eventually, four-coordinate metals, the calix[4]arene moiety displays a cone, which changes to an elliptical conformation in the case of a six-coordinate metal.4,10 Such conformation changes, which appear in the solid state, are also detectable in solution, as in the... 

Increasing the reducing agent/W ratio and using Na-naphthalenide led to the isolation of the two-electron-reduced, diamagnetic compound 19. The C2v symmetry of the h NMR spectrum and the X-ray analysis are in agreement with the cen-trosymmetric structure sketched in Scheme 2 for 19 [W = W, 2.614(1) A]. The six-coordination of the metal and the inclusion of the alkali metal cation removes the planarity ofthe 04 core and the cone conformation of the calix[4]arene. The sodium cation within the calix[4]arene cavity is r 3-bonded to two opposite arene rings. 

A different synthetic access to a 1 -metallacyclopropene, which can be a versatile organometallic synthon, is displayed in Scheme 33. The mono-alkyne derivatives of W(IV)-calix[4]arene are easily accessible through the thermal displacement of cyclohexene from 32 using the appropriate acetylenes. The reaction led to complexes 34 and 172-174. The proposed 3-metallacyclopropene has been confirmed from the spectroscopic and the X-ray data. The H NMR data reveal a cone conformation of the calixarene with a four-fold symmetry, for which the... 

The stereochemistry and conformation of di-Schiff bases containing calix[4]arene unit U(IV) complexes have been deduced on the basis of the 2D TOCSY and T-ROESY measurements.116... 

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