No abstract

Cholic acid (CA) forms inclusion crystals that have a sandwich-type lamellar structure constructed by the alternative stacking of host bilayers and guest layers. Five disubstituted benzenes, o-toluidine, m-fluoroaniline, o-chlorotoluene, o-bromotoluene, and indene, are accommodated in the two-dimensional void space between the host bilayers at 1:2 host-guest stoichiometries. Thermal gravimetric analysis of the inclusion crystals revealed that all the guest molecules, except o-toluidine, are released in two separate steps, indicating the formation of intermediate crystals after the first guest release. Adequate heat treatment of the four inclusion crystals induces release of half or three quarters of the guest molecules. X-ray diffraction patterns of the intermediate crystals revealed that the crystals have a bilayer structure the same as those of the common CA inclusion crystals. They have one-dimensional cavities, in which the guest molecules are included at a 1:1 or 2:1 host-guest stoichiometry. These facts indicate that the host bilayers move 1.6-4.5 A perpendicular to the layer direction by desorption of the guest molecules. Furthermore, a reverse structural change is also achieved by absorption of the guest molecules to regenerate the starting sandwich-type inclusion crystals. This reversible change in the host bilayer by the guest sorption and desorption is a novel example of organic intercalation materials.

Continuous flow of the substrate solution and hydrogen gas through a tube reactor packed with Pd/C catalyst brings about a highly reactive and efficient hydrogenation system, which converts 4-cyanobenzaldehyde to the benzyl alcohol derivatives at 25 degrees C, and at 90 degrees C, the cyano group becomes reduced to give the corresponding amine and toluene derivatives within 2 min.

Fifty eight inclusion crystals of cholamide (CAM) with aliphatic alcohols have been systematically investigated by X‐ray crystallography. The host frameworks of the inclusion crystals can be categorized into three structural types: bilayer, herringbone, and crossing structures. Most of the guests are included in bilayer structures, which can be further divided into four sub‐types. The host frameworks isomerize depending on the size and shape of the guest molecules. This dependence is clearly evaluated by the relationship between the volume of the guest molecules and the packing coefficient of the void space in the host frameworks. Although the host frameworks with the cavities and the structural isomerization are very similar to those of the inclusion crystals of cholic acid (CA), the inclusion behavior of aliphatic alcohols in the two hosts is completely different: fifty four alcohols can be included in the bilayer‐type structures of CAM, but only two in those of CA. This is attributed to their different functional groups, which lead to different hydrogen‐bond networks with the guest alcohols. The result in the two hosts is a good example to understand the effect of host–guest hydrogen bonds on guest inclusion in cavities that are identical in size and shape. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Steroidal cholamide (CAM) has been found to form inclusion crystals with 23 aromatic compounds in 1:1 and 2:1 host-to-guest molar ratios. The 1:1 crystals have guest-dependent host frameworks, termed β-trans-type, where weak hydrogen bonds such as N-H • • • π, C-H • • • π, and C-H • • • O play a key role in linking the host and guest molecules. The steroidal sidechains involving methyl, methylene, and amide groups serve as the hydrogen bond donors, and aromatic guest molecules serve as the acceptors. Three kinds of such weak hydrogen bonds are visualized by the Hirshfeld surfaces of the guest molecules. Comparisons of the inclusion crystals of CAM and cholic acid (CA) with the same guests clarify a profound effect of the weak hydrogen bonds.In the case of the 1:1 crystals, the N-H • • • π hydrogen bonds explain the fact that CAM always employs the β-trans-type framework, while CA employs either an R-gaucheor β-trans-type framework. On the other hand, the 2:1 crystals, termed DCA-type, have only C-H • • • π hydrogen bonds. The guest-dependent isomerization of these frameworks is examined in terms of the weak hydrogen bonds as well as compatibilities in size and shape.