Intramolecular inclusion complexes

X-ray diffraction analysis show that these receptors can bind neutral and anionic bidentate guests species, e.g. 1,4-diazabicyclo[2.2.2]octane, isonicotin-ate, or terephthalate to form 1 1 intramolecular inclusion complexes.415... 

The goal of much activity in synthesis of host-guest complexes is to mimic the selectivity and catalytic activity of enzymes. In Chapter 31, Schneider et al. examine the use of intramolecular inclusion complexes for catalysis of nucleophilic displacement reactions. The main emphasis here is on macrocyclic ammonium ions as hosts. 

The first examples reported of totally synthetic uncharged host-uncharged guest intramolecular inclusion complexes elucidated by x-ray crystal structures were the calixarenes (125,126). This class of macrocyclic cyclophanes contains phenolic residues in a cyclic array linked by methylene groups all ortho to the hydroxy groups (Figure 2.53). 

Fig. 2. Classification/nomenclature of host—guest type inclusion compounds, definitions and relations (/) coordinative interaction, (2) lattice barrier interaction, (J) monomolecular shielding interaction (I) coordination-type inclusion compound (inclusion complex), (II) lattice-type inclusion compound (multimolecular/extramolecular inclusion compound, clathrate), (III) cavitate-type inclusion compound (monomolecular/intramolecular inclusion...

It is not easy to control the steric course of photoreactions in solution. Since molelcules are ordered regularly in a crystal, it is rather easy to control the reaction by carrying out the photoreaction in a crystal. However, molecules are not always arranged at an appropriate position for efficient and stereoselective reaction in their crystals. In these cases inclusion chemistry is a useful technique, as it can be employed to position molecules appropriately in the host-guest structure. Chiral host compounds are especially useful in placing prochiral and achiral molecules in suitable positions to yield the desired product upon photoirradiation. Some controls of the steric course of intramolecular and intermolelcular photoreactions in inclusion complexes with a host compound are described. 

Aliphatic ketones (R = alkyl) usually undergo preferential a-cleavage on excitation nevertheless, cyclobutanol formation via intramolecular H-abstraction and cyclization is favoured (a) by an a-fluoro substituent, (b) by the rigidity of the group R (e.g. for R = cycloalkyl or bicycloalkyl), or by irradiation in either (c) inclusion complexes or (d) the crystal itself (solid state irradiation). 

Intramolecular and intermolecular interactions between C—H groups and the 71 system135 of carbon double bonds of olefins and of aromatic rings are of interest in the fields of molecular recognition and inclusion complexes in both organic chemistry and biochemistry. 

B. Enantioselective Intramolecular Photocyclization Reactions of Achiral Molecules in Inclusion Complex With a Chiral Host Compound... 

Intramolecular [2 + 2] photocyclization reactions of 2- /V-(2-propcnyl)amino]cy-clohex-2-enones (67) are also controlled enantioselectively by carrying out irradiation in inclusion complex with a chiral host compound. When inclusion crystals of 67 with 12 are irradiated in the solid state, optically active 9-azatricyclo[5.2.1.0]-decan-2-ones (68) were obtained in the chemical and optical yields indicated in Table 3 [30],... 

Hilgenfeld and R. Saenger, W. (1982) Stetter s Complexes are no Intramolecular Inclusion Compounds, Angew. Chem. Int. Ed. Engl. 21, 1690-1701. 

The keto-enol equilibrium of the 1,3-diketones has been the subject of intensive studies using various physical techniques and theoretical calculations [78-80], Recently, X-ray crystal analysis of acetylacetone (83) was carried out at 110 K, and it was found that it exists as an equilibrium mixture of the two enol forms 83b and 83c [81]. Room-temperature studies show an acetylacetone molecule with the enolic H-atom centrally positioned, which can be attributed to the dynamically averaged structure 83d. Application of a crystal engineering technique showed that a 1 1 inclusion complex of83 can be formed with l,l/-binaphthyl-2,2/-dicarboxylic acid in which the enol form is stabilized by a notably short intramolecular hydrogen bond [82],... 

The magnetic circular dichroism spectrum of thioxanthone and the circular dichroism spectrum of its inclusion complex with cyclodextrin have been measured and interpreted with the aid of PPP and CNDO/S calculations. The first jtJt state exhibits intramolecular charge transfer characteristics <1994JPC10432>. 

The tense and relaxed forms of a-cyclodextrin mimic the induced-fit model proposed for enzymes. In the inclusion complexes of a-cyclodextrin, the macrocycle adopts a round, relaxed shape with all the 0(2),0(3) hydroxyls engaged in intramolecular 0(2)- (3 ) hydrogen bonds (Fig. 18.8) [577]. The average O - - O distance of around 3.0 A is consistent with an H 0 separation of about 2.0 A, indicating relatively weak interactions. This form of a-cyclodextrin persists even when an included guest molecule, such as krypton (3.8 A diameter) is too small to fully occupy the 5.0 A cavity and is disordered over several sites [578], analogous to the gas hydrate structures discussed in Part IV, Chapter 21. 

Detailed analyses of intramolecular structures are possible. Comparison of NMR and fluorescence data shows meso- and racemic diastereoisomers are found from 2,4-di(2-pyrenyl)pentane 24 jhe polarization of monomer and excimer of 4,9, disubstituted pyrenes have been measured in nematic liquid crystals 25 Quenching of pyrene fluorescence by alcohols in cyclodextrin inclusion complexes has also been studied in detail 26 Solvent effects on the photophysical properties of pyrene-3-carboxylic acid has been used to measure the pJJ, in different solvents 27 Geminate recombination in excited state proton transfer reactions has been studied with... 

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