Canal Inclusion. Compounds

We concentrate here on the structural aspects of helical canal inclusion compounds, primarily because this field of chemical inclusion is still at the relatively juvenile stage of establishing geometry and geometrical Variables. Comments on structure-property relationships for the chemical systems and on structure-function relationships for the biochemical systems are made wherever possible. 

In the next section we consider some general questions about the occurrence, properties and experimental investigation of helical canal inclusion compounds, in relation to inclusion compounds with different topologies. In subsequent sections we describe the properties of specific systems. 

Thiourea and selenourea both form canal inclusion compounds. The selenourea compounds, reported only briefly 37), appear to be isostructural with the rhombo-hedral thiourea compounds, although with larger dimensions which are more susceptible to the sizes of guest molecules. For urea, thiourea and selenourea the crystal densities of the compounds are significantly less than those of the hosts values (g cm - 3) for the pairs host/complex are 1.30/1.20 1.40/1.10 2.08/1.60-1.65 for urea, thiourea, selenourea, respectively37). 

A considerable amount of work has been directed towards the study of detailed molecular orientations and motions of guest molecules in urea canal inclusion compounds and structural changes such as those described above. Methods used include infrared s2) and Raman spectroscopy 52,53, esr 54 56). nqr S0,57), and nmr ( H, 2D,... 

Thiourea canal inclusion compounds 19 26) have a wider diameter than those formed by urea, such that n-alkanes are not included but that molecules of cross-section approximately 5.8-6.8 A are trapped 64). Thus many inclusion compounds have been reported between thiourea and branched alkanes or cyclic molecules. Of special interest are the inclusion compounds with cyclohexane derivatives and the recent studies carried out on the preferred conformation(s) of the ring in the restricted environment of the thiourea canal. 

DCA forms canal inclusion compounds, known as choleic acids, which most frequently have the orthorhombic space group P212121, or less frequently Pl l. In such crystals the DCA molecules hydrogen bond to each other to form an extended bilayer structure, thereby creating a hydrophobic canal between adjacent bilayers. The guest molecules present in these canals therefore tend to be non-polar or moderately polar molecules such as aromatic compounds, alkenes, ketones and certain carboxylic acids 92). Since the bilayers are held together only by van der Waals forces the canals are able to adopt different dimensions to accommodate the variety of... 

In common with the situation for DCA, the helical canal inclusion compounds of TOT have so far received little attention compared to the research activity on urea. Much interesting work has been carried out on TOT inclusion compounds with other crystal space groups including the study of monomeric carboxylic acid guests the inclusion of thermodynamically disfavoured conformers of guests and the photochemical reactivity of molecules jigse are discussed in a recent review... 

Experiments on chiral discrimination using helical canal inclusion compounds of space group P6i have so far shown only low selectivity. The inclusion compounds of TOT with racemic 2-chlorooctane, 2-bromooctane, 3-bromooctane, 2-bromononane and 2-bromododecane gave enantiomeric excesses of 4-5 % However it should... 

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