Helical tubuland diols: a synthetic and crystal engineering quest

Despite many advances in recent years, crystal engineering remains a risky venture. A successful outcome requires manipulation of the noncovalent bonding and properties such as size, shape, repulsion, attraction, polarity, and chirality. In this Account, we describe the interplay of crystal engineering and synthetic organic chemistry required to develop the family of helical tubuland diol hosts, the members of which exhibit a wide range of tube dimensions and inclusion properties. Certain alicyclic dialcohols crystallize with a hydrogen-bonded network structure, termed the helical tubuland lattice, in space group P3(1)21 (or its enantiomorph P3(2)21). Double helices of diol molecules surround parallel tubes that contain guest molecules, which are included on the basis of size and shape rather than functional group. The crystal structure of (diol)(3).(chloroacetic acid)(1.2) is illustrative. These chiral helical tubulate lattice inclusion compounds are formed when the racemic host diol is allowed to crystallize from solution. Complete enantiomer separation occurs during this process, producing a 1:1 mixture of pure (+)- and pure (-)-crystals (a conglomerate). The challenge of creating this family of compounds required the development of much synthetic chemistry, in particular new pathways to alicyclic ring systems with specific substitution patterns. It was also necessary to understand and control the supramolecular properties of the diol molecules. What makes the original compound tick, and why did it behave in this remarkable manner, when most of its structural neighbors crystallize totally differently? The synthesis of new helical tubuland diols requires not just preparation of a new molecular structure but also a transplant of the original unchanged hydrogen-bonding supramolecular synthon. Synthesis of the specific crystal space group is necessary. This was achieved by defining structural characteristics, termed molecular determinants, which are essential for the helical tubuland structure to occur. If these requirements were met, then the target molecule had a high probability of success. This investigation has close conceptual parallels with the search for pharmacophore properties of bioactive molecules. In both situations, parts of a molecule with little or no chemical reactivity may actually play vital supramolecular roles. The review illustrates how crystal engineering is based on specific supramolecular properties that can be uncovered and then exploited by synthetic chemists.

Crystal engineering of hydroxy group hydrogen bonding: design and synthesis of new diol lattice inclusion hosts

The diols 7-11 have been synthesized, and their X-ray crystal structures determined, to learn how to influence and control lattice hydroxy group hydrogen bonding using crystal engineering ideas. To obtain new lattice inclusion hosts precise structural rules can be defined which enable the necessary supramolecular interactions to be duplicated. In this manner the helical tubuland 10 and ellipsoidal clathrate 11 hosts were obtained for the first time and their chloroform inclusion compounds characterized. New synthetic routes were utilized to obtain the bicyclo[3.3.2]decane and 9-thiatricyclo[4.3.1.1(3,8)]undecane frameworks present in these compounds. The solid-state conformations of bicyclo[3.3.2]decane derivatives 9 and 10 are compared with prior predictions and studies made on this uncommon ring system.

Self-assembled helices from 2,2'-biimidazoles

2,2'-Biimidazoles were synthesized by palladium(0)-catalyzed coupling of 2-iodoimidazoles bearing an alkyl and an ester group at the 4- and 5-positions, respectively. The products were found to be fluorescent and moderately soluble in organic solvents. Three biimidazoles were subjected to single crystal X-ray diffraction analysis. In all three instances, adjacent molecules were found to be bound together in the solid state by pairs of N-H...N hydrogen bonds, forming twisted ribbon-like columns which resemble double helices. The amount of helical twist observed between neighboring biimidazole subunits in these helices varies with the identity of the alkyl and ester groups; in two cases it is approximately 60 degrees, whereas in the third it is about 90 degrees. Mass spectra of six different biimidazoles display ions with masses corresponding to dimers; this indicates that these compounds retain some affinity for each other in the gas phase. The three most soluble biimidazoles also show mass spectrometric peaks ascribable to trimers and tetramers. The solution-phase aggregation tendencies of these latter three compounds were studied by vapor pressure osmometry. In each case, the apparent molecular weight in 1,2-dichloroethane solution is higher than would be expected for free monomers.