Synthetic and crystallographic studies of bicyclo[3.3.1]nonane derivatives: from strong to weak hydrogen bonds and the stereochemistry of network formation

The syntheses and crystal structures of four unsaturated bicyclo[3.3.1] nonane derivatives containing various functionalities are presented and their intermolecular interactions examined. The impact of unsaturation on crystal structures and intermolecular networks of the six membered rings was found to be significant compared to the saturated analogues. Thus, unsaturated diol rac-1, in striking contrast to its saturated analogue rac-6, does not crystallise with spontaneous resolution. The hydrogen bonds in the starting bicyclononane diene diol rac-1, and the weaker hydrogen bonds in the dienone rac-2, and the bromo and nitrile derivatives, rac-3, rac-4 and (+)-4, respectively, were found significant for the overall structure. The two bromine atoms in rac-3 have significant halogen-halogen interactions. In several structures 2D nets can be identified and overall structures can be interpreted as close packing of these layers. The crystal structures were also subject to independent analysis by Hirshfeld surfaces, and this method provided additional insights, especially for the structural role of the unsaturation. The possible relation between chiral networks and conglomerate formation is discussed.