Guest complexes alcohols

We conclude that the neutral substrate enters 1 to form a host-guest complex, leading to the observed substrate saturation. The encapsulated substrate then undergoes encapsulation-driven protonation, presumably by deprotonation of water, followed by acid-catalyzed hydrolysis inside 1, during which two equivalents of the corresponding alcohol are released. Finally, the protonated formate ester is ejected from 1 and further hydrolyzed by base in solution. The reaction mechanism (Scheme 7.7) shows direct parallels to enzymes that obey Michaelis-Menten kinetics due to the initial pre-equilibrium followed by a first-order rate-limiting step. 

Polar O—H bonds are found in molecules such as water and alcohols. Crystal structures of several crown ether complexes have indicated that the cavity need only be partially filled by water.260-262 Several host-guest complexes of alcohols with the pyridino crown (83) have been reported.263 Longer chain and branched alcohols do not in general form crystalline adducts with this ligand. 

Actually, coordination complexes of different metal salts of DBTA with hydroxycarboxylic acid esters, hydroxycarboxylic acids and alcohols as well as host-guest complexes of DBTA with chiral phosphine oxides and racemic alcohols can be prepared and used for separation of optical isomers. In the next subchapters theoretical and practical aspects of these recent resolution processes are summarised. 

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. 

Of the new Pt catalysts reported since 1990 platinum complexes with new ligands and activators are noteworthy. Cyclodextrin complexes of platinum (as host-guest complexes) have been employed as hydrosilylation catalysts active at elevated temperature after releasing the guest compound [40]. Some other organic compounds have recently been used as activators (ligands) of Pt complexes, e. g., unsaturated secondary and tertiary alcohols and silylated unsaturated alcohols [41], alkadiynes, cyclooctadiene [42], and vinylnorbomene as well as quinones and methylnaphthoquinones [43]. 

The host (7.15) was found by combinatorial synthesis (from a library of 100 salts) It forms host-guest complexes with several alcohols, ketones, xylenes, and others (e.g., 2 3 with methanol, 1 1 with ethanol, 2 2 with acetone, and 2 1 with acetonitrile).76... 

Huang, X. Rickman, B.H. Borhan. B. Berova. N. Nakanishi. K. Zinc porphyrin tweezer in host-guest complexation—Determination of absolute configurations of diamines, amino acids, and amino alcohols by circular dichroism. J. Am. Chem. Soc. 1998. 120. 6158. 

The data in Table II show that the a protons, and therefore the hydroxyl group, of an alcohol are furthest from the metal ion [71]. At the same time, the co protons of the alcohol are nearest the metal center. Thus, the alcohols all enter into host/guest complexation regiospecifically. Also since the OH group is at about the same distance from the metal ion in all cases, it must remain in the solvent sheath of the host molecule. Thus, the binding arises from hydrophobic relationships involving the alkyl group of the alcohol and the hydrophobic interior of the cavity. 

This lariat arm displacement in 2.25 has been used as a fluorescent handle to monitor host-guest complexation. Fluorescent titration shows that 2.25 is able to distinguish between chemically distinct alcohol guests and also between enantiomers and geometrical isomers, with 2-adamantanol giving the most stable complex because of a good size match with the cyclodextrin cavity and strong hydrophobic interactions. 

Kurtan T, Nesnas N, Koehn FE, Li Y-Q, Ntikanishi K, Berova N (2001) Chiial recognition by CD-sensitive dimeric zinc porphyrin host. 2. Structural studies of host-guest complexes with chiral alcohol and monoamine conjugates. J Am Chem Soc 123(25) 5974-5982... 

Matsui and Mochida24) have determined the thermodynamic stabilities (log 1 /Kd) for a- and P-cyclodextrin complexes with a variety of alcohols (Table 2) and analyzed the results in connection with the physicochemical properties of the guest molecules by the multivariate technique. The log 1/Kd values were plotted against log Pe, where Pe is the partition coefficient of alcohol in a diethyl ether-water system. The plots for the a- and P-cyclodextrin complexes with eight 1-alkanols gave approximately straight lines with slopes of around one. 

Examination of the steric relations in these complexes (cf. Fig. 30) suggests that the more voluminous branched alcohols cannot follow the same principle. Indeed, in the 2-butanol and also in the t-butanol inclusion compound, a different ring system is built (Fig. 17b and type I in Fig. 19). While the short-chain alcohols form twelve-membered H-bond loops, the branched butyl alcohols are embedded into a ten-membered asymmetric loop. The stoichiometry of the asymmetric unit also changes from 1 2 (host guest) ratio to 1 1. The so-built ring system of homodromic H-bonds still contains a mirror-related pair of hosts 1, but comprises only one guest molecule. 

The binaphthol 13 is different from 1 and 7 owing to the lower acidity of its functional groups. Therefore, crystalline complexes of 13 with amines (see Table 3) are not expected to have a salt character. The 13 imidazole 1 2 complex (Fig. 22)81) was studied in the light of the general interest in this guest partner and its relation to alcohol functions in biological ensembles. The host molecule adopts ideal twofold... 

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

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