The synthesis of cavitand-capped porphyrin ligands, with a view towards their potential as
ligands in homogeneous catalysis, is described. The ligand apertures, one of which is outlined
in the figure below, are focal with the aim of synthesising a ligand which can control access to
the active site of the porphyrin via these apertures Synthesis of the target ligand (where R' = CH2 in the figure presented) was attempted via two
pathways. Synthesis commenced by using an in situ protocol, which used successive
functionalisation of the cavitand structure towards the required aldehyde precursor for porphyrin
formation. It was found that subsequent in situ cyclisation and porphyrin formation was
hindered by steric factors, arising directly from the short -CH2O- bridges used to link the
cavitand to the porphyrin. Ligand synthesis was thus unsuccessful.
In a second approach, the porphyrin was synthesised in isolation before being coupled with the
cavitand in a direct capping protocol, which gave more promising results. In the case of R =
C11H23 (in the figure above), preliminary UV-Vis analysis indicated a successful synthesis.
Subsequent analysis of the reaction product by NMR techniques and mass spectrometry could
not conclusively confirm the synthesis of the target ligand. The synthesis could therefore not be
deemed a success; conceivably the short bridge length being the decisive factor once more. Computational chemistry was used to investigate synthetic results, and therefore the viability of
using the -CH2O- bridges to afford limited access to the porphyrin active site. By using
molecular mechanics, -CH2O- bridges were found to be too short, giving an aperture of
insufficient size to enable only the terminus of a linear paraffin to gain access to the inner cavity
of the ligand. Further investigation using molecular dynamics indicated that a ligand bearing
bridges four or five atoms in length would afford an aperture of the desired size to accommodate
the terminus of a paraffin exclusively.
Consequently, synthesis was redesigned towards the preparation of two new ligands, bearing -
O(CH2)2O- (four atom, R' = O(CH2)2 in the figure above) and -O(CH2)3O- (five atom, R' =
O(CH2)3 in the figure above) bridges. Using 2-phenylethyl feet (R = CH2CH2C6H5 in the figure
presented) and adopting the in situ synthetic protocol, both ligands were successfully
synthesised. Characterisation using UV-Vis and NMR spectroscopic techniques, as well as
mass spectrometry confirmed that both ligands had been obtained pure. Additionally, the in situ
cyclisation (in both ligands) was performed via the use of microwave heating, a technique
hitherto unreported.
A viable synthetic route was thus established for the preparation of two new cavitand-capped
porphyrin ligands towards their use in size-selective catalysis.
In addition, a number of crystal structures of synthetic intermediates are described, five of
which are newly reported. These illustrated notable structural features regarding
resorcin[4]arene cavitands and their abilities as host molecules. In particular, the structure of
the aldehyde precursor to capped porphyrin formation following the (initial) in situ synthetic
protocol was significant in illustrating the reason as to why in situ cyclisation was unsuccessful
for the synthesis involving -CH2O- bridges. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2008.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/474 |
| Date | January 2008 |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
Page generated in 0.0017 seconds