Guest complexes phenols

The addition of the second methyl group on the phenol ring led to the observation of the consecutive inclusion process with a decrease in the dynamics for complex formation (Table 8, cf. 29 with 28 (R = CH3)). This result supports the previous suggestion190 that small guests can slip into the CD cavity and in one process form the stable host-guest complex. 

Vogtle has developed this approach further and employed a series of anionic templates to prepare rotaxanes (instead of the neutral template in the above reaction) [65-67]. In this approach a phenolate, thiophenolate or sulfonamide anion is non-covalently bound to the tetralactam macrocycle (46) forming a host-guest complex via hydrogen bonding (see Scheme 21). 

The second example (95a), synthesized by Dalcanale and co-worker181 is asymmetric because of one substituted quinoxaline residue (similar to calix[4]arenes containing one m-substituted phenolic unit). Since the menthyl residue attached to the quinoxaline via an ester function is chiral itself a mixture of the two diastereom-ers was obtained which could be separated by chromatography on silica gel. The subsequent reduction of the ester function in 95a led to enantio-pure (+) and (-) alcohols 95b. The compound 95a was used in host-guest complexation studies182 with benzene, fluorobenzene, 2-fluorotoluene, and isobutane in the gas phase by means of Desorption Chemical Ionization Mass Spectrometry however, studies of chiral recognition have not yet been reported. 

Coumaric acid 28 presents a more ambitious problem because difTerential protection of two competing anionic sites is to be observed In the absence of 24 any possible esterification and etherification product is formed but adding a slight excess of host 24 switches the reaction paths to the exclusive production of the ester. The absolute reaction rate drops considerably indicating that host-guest complexation effects both reaction centers of the substrate. Nevertheless the differences in their reactivities are stressed by virtue of this dynamic protection preferentially at the phenolate thus enabling the enhancement of reaction selectivity. 

Figure 4A.42 Cyclodextrin, (a) The helical structure of the starch yields mainly cyclodextrins with six to eight glucose units. fhttp //www.chemiedidaktik.uni-wuppertal.de/disido cy/cyen/info/01 manufacture cy.htmj (b) Chemical structure of the three main types of cyclodextrins, Stanisfaw Skowron, shape of single monomers. From work of Bartosz Marcin Kojak, for Wikimedia commons, GNU Free Documentation License, Version 1.2) c) Host-Guest complex of 3-cyclodextrin and phenolphthalein hydrogen bonds between the p-cyclodextrin molecule and the phenolphthalein dianion. fhttp //www.chemiedidaktik.uni-wuppertal.de/disido cy/cyde/exp/06 cy phenol.htm (aj 7ndfc,)ProfessorDr. Simone Krees, Westfdlische Wiihelms-University Munster, with kind permission)...

Cyclodextrins have had valuable industrial uses for a considerable time, particularly as agents to bind or release volatile molecules. Accurate predictions concerning the selectivity and stability of cyclodextrin-guest complexes are therefore of considerable interest both academically and practically." MD was used to simulate cyclodextrin hydrates" as a test of the applicability of the GROMOS program package to systems beyond proteins and nucleic acids. Other early MD simulations focused on interactions with guests such as enantiomers of methyl-2-chloropropionate. Comparisons between calculated thermodynamic properties for complexes formed by O -cyclodextrin with para-substituted phenols and the results of MM simulations led to improvements in force fields that described the interactions. MM2 simulations were used to support NMR data for the -cyclodextrin inclusion complex with benzoic acid. " The well-known catalytic effect of cyclodextrins has been modeled. For example, the relative rate increase of hydrolysis of S over R phenyl ester stereoisomers in the presence of -cyclodextrin... 

Furthermore, m-cresol and m-halogenated phenols have been polymerized in aqueous buffer solution only in the presence of 2,6-di-0-methyl-[)-cyclodextrin [90]. In the absence of cyclodextrin, only insoluble materials in very low yields were obtained under aqueous conditions from these phenols [81]. The structure of the host-guest complexes between cyclodextrin and the phenols was characterized by 2D NMR [48,90] and the association constants of the cyclodextrin complexes were determined by the Benesi-Hildebrand method. Firrthermore, 3-fluorophenol, 3-chlorophenol, and 3-bromophenol were successfully polymerized in various aqueous organic solvents such as methanol, acetone, or isopropanol [115]. 

Nishioka and Fujita 78> have determined the Kd values for a- and P-cyclodextrin complexes with m- and p-substituted phenols at pH 7.0. Taking into account the directionality in inclusion of a guest molecule, they assumed three and two probable orientational isomers for the cyclodextrin complexes with m- and p-substituted phenols respectively (Fig. 6). Then the observed Kd values were divided into two or three terms corresponding to the dissociation of the orientational isomers involved (Eqs. 16, 17) ... 

It is considerably easier to induce an expansion of the periodicity along the c axis, since along this direction the layered structure is stabilized mainly by weaker dispersion forces. Indeed, the structural variation of the guest species caused a significant increase of the c axis from 14.845 A (for the phenol complex) to 15.818 A (for the p-cresol... 

---