Crystal engineering : the design of organic solids by G. R. Desiraju

Abstract

A series of host molecules derived from tartaric acid have been synthesized in optically resolved and racemic forms. Apart from two endo hydroxyl functions and four bulky aromatic groups (TADDOL-type = a,a,a',a'-tetraphenyl-l,3-dioxolane-4,5-dimethanols), they have specific polar and apolar substituents of different size, in different number, and in different positions laterally attached to the molecular framework as the characteristic features. These hosts form crystalline inclusion compounds with uncharged organic molecules ranging from protic dipolar (alcohols, amines) to apolar compounds (in all 143 different inclusion species). Inclusion formation, hostguest stoichiometric ratios, and interaction modes depend on the structural features of the host, as supported by X-ray crystallographic studies in nine cases involving MeOH, EtOH, and 2-PrOH. The optically resolved host species with Me and F substituents form linear (spiral) host-guest H-bonded chains in compounds containing MeOH guests, circular motifs of H-bonds in crystals consisting of the racemic host species, while in the presence of larger guest alcohols (2-PrOH) a finite noncyclic H-bonded cluster occurs with the chiral constituents. The crystal packings in structures involving the tetrachloro-substituted hosts and MeOH as guest are dominated by specific Cl-Cl nonbonding interactions which create interhost voids sufficiently large to accommodate a cluster of three H-bonded MeOH moieties. The packing structure of inclusion compounds containing fluoro substituted hosts are stabilized by CH (phenyl)-F interactions. Considerable interest in crystalline inclusion compounds (clathrates) has arisen in the past few years due to their practical uses in compound separation, stabilization and protection of labile species, topochemistry, or the development of new solid materials.1-7 This has stimulated development of new strategies in crystalline inclusion formation and motivated the design of novel host types.1'2 Most consistent results refer to inclusion compounds which are based on coordination-assisted clathrate formation between functionalized hosts and polar guest components.8 The formation and stability of these crystalline inclusion complexes are affected by functional as well as by topo-logical complementarity and consequently are sensitive to small structural variations.9 Among the many new types of polar host structures ,^ia®-10 the tartaric acid derived compound 1 has proved particularly successful in this respect.11 In its optically resolved form (la), it was found to be very effective in the chiral resolution of racemic guest molecules12 and quite useful for enantioselective topochemical reactions .13 On the other hand, as shown by competition experiments, both the optically resolved (la) and the ra-cemic (lb) stereoisomers exhibit preferential enclathration of secondary and primary amines, respectively, the inclusion of the tertiary amines being considerably less effective .14 The relation between these selectivity features to structure was carried out on the propylamine series of guest compounds.15 Very recently, a series of optically active derivatives of compound 1, called TADDOL's (a,-a, o', a'-tetraphenyl-l,3-dioxolane-4,5-dimethanols),16 were described to be useful as versatile auxiliaries for enan-tioselective reactions.17 However, systematic studies (1) Inclusion Weber, E.; Ddrpinghaus, N.; Franken, S. J. Chem. Soc., Perkin Trans. 2 1990, 953. (16) Beck, A. K.; Bastani, B.; Plattner, D. A.; Petter, W.; Seebach, D.; Braunschweiger, H.; Gysi, P.; La Vecchia, L.