Enantiopure Assembly of M4L6 Supramolecular Clusters with Different Sizes and their Application to Asymmetric Organic Reactions
Final Report Abstract
In summary, a C2 symmetry ligand with extended length was synthesized by incorporating the enantiopure amino acid L-Alanine. An important motivation of this project is the possibility of obtaining a larger and enantiopure host due to the embedded chiral information from the L-Alanine and thus allowing for host-mediated enantioselective catalysis of larger substrates. Unfortunately, the formation of the desired M4L6 host was not successful. Instead, we infer that a self-assembly with lower symmetry M2L3 might be formed. However, all attempts to characterize the isolated product failed to provide enough information about its structure. Within my postdoctoral research stay, the influence of the charge on catalytical efficiencies and the chemistry of the tetrahedron host were investigated. Therefore, Ge4L6 and Si4L6 with reduced charge of -8 were synthesized. While Ge4L6 is already literatureknown, [Si4L6]8- was synthesized and characterized to be the first ‘empty’ tetrahedron cluster with tetravalent metals. This breakthrough result allowed us to explore the catalytic properties of the -8 hosts and compare them with those of the well-established [Ga4L6]12-. The results of those studies can be summarized as follows: 1. There is no significant difference in the confined geometry inside the cavity. This is evident by the similar reaction rate of the aza-Cope rearrangement, where formation of chairlike intermediate is key for the enhanced reaction rate. 2. [Si4L6]8- exhibits different pKa shift that do not enable the encapsulation of amines such as TMEDA in basic solution. This resulted in slowing down the Nazarov cyclization which requires a protonation of the hydroxyl group of the substrate inside the cavity. In addition to those two projects, I established a collaboration between our group in Berkeley and the laboratory of Professor Havenith at Ruhr University Bochum. With the help of Terahertz spectroscopy (THz) we were able to obtain primarily results of the behavior of water molecules in the presence of [Ga4L6]12-. The sharpness of the peak at ~200 cm^-1 might be due to the retarded water motions inside the cavity. Further studies are currently underway.
