Fine-control of Helical Tubuland Inclusion Properties through the Pendant Group Approach

The helical tubulands are a family of alicyclic diols which crystallise in chiral space group P3121 and whose lattices can contain guest molecules enclosed within parallel tubes. New examples can be designed following a series of rules based on molecular structure and symmetry. In order to probe the latter requirement, and also to increase host–guest interactions, the behaviour of 2,5,8-trimethyltricyclo[5.3.1.13,9]dodecane-syn-2,syn-8-diol 4 was investigated using X-ray methods. Although molecules of 4 have no C2 symmetry they still form the helical tubuland lattice on crystallisation. In the pure apohost 4 their symmetry-breaking C5 methyl groups protrude into the tube volumes where they randomly adopt one of two equivalent orientations. Although the pendant methyl groups reduce the volume available for guest inclusion, they create irregular tube wall surfaces and interact more effectively with guest molecules. The helical tubulate inclusion compounds of 4 with diisopropyl ketone, benzene, toluene, and o-xylene show significant local methyl group ordering around a given guest molecule which performs a crucial templating role in formation of the product. In all cases, however, crystallographic C2 symmetry results in the solid through overall disorder of these diol orientations. Hence this, rather than strict molecular symmetry, is the true symmetry requirement of the P3121 helical tubuland lattice.