H-bond directed self-assembly of oligomeric molecular strands and hydrogen mediated rhodium-catalyzed reductive cyclization of 1,6-enynes
Key structural and interactional features of the components for hydrogen bond mediated self-assembly are reviewed, with emphasis on the assembly of synthetic oligomers to form duplexes and tubular structures, as well as related applications in the design of functional materials. A strategy for the preparation of molecular strands that self-assemble through the action of interstrand H-bonds to form duplex superstructures is described. Specifically, duplex oligomers based on the 3,6-diaminopyridazine hydrogen-bonding motif were designed and prepared. The mode of assembly and the thermodynamic parameters of duplex aggregation are established by X-ray crystallographic analysis, 1 H NMR dilution experiments, isothermal titration calorimetry (ITC) and vapor pressure osmometry (VPO). ITC analysis indicates a strong positive cooperative effect upon strand extension from monomer to trimer. In addition, studies toward the design of molecular strands that assemble to form tubular structures are described. Here, alkyl chains decorated with aminopyrazolone moieties were examined. In the solid state, aminopyrazolones aggregate to form either linear H-bonded tapes or discrete cyclic tetramers, as established by single crystal X-ray diffraction analysis. Evidence for cyclic aggregation in solution, though not conclusive, led us to investigate bis(aminopyrazolone) systems, whereby the energy bias between the linear and cyclic aggregation modes could potentially be magnified to favor the latter. However, single crystal X-ray diffraction analysis of S,S-dihexylpropyl bis(aminopyrazolone) reveals a double H-bonded tape. The mode of assembly in solution for the bis(aminopyrazolone) could not be established unambiguously. Finally, the use of elemental hydrogen as a terminal reductant in the rhodiumcatalyzed enantioselective reductive cyclization of 1,6-enynes is described. Whereas 1,6- enynes containing 1,2-substituted alkenes fail to provide reductive cyclization products due to competitive cycloisomerization, related alkenes in the form of conjugated enones afford reductive cyclization products in good to excellent yield and enantioselection.