Molecular and chiral recognition properties of porphyrin based receptors

The work described in this Thesis details the design and synthesis of bisporphyrin Trdger’s base receptors and the study of their ability to recognise a-amino acids, dicarboxylic acids and tartaric acids. A mixed metal dihydroxotin(IV) zinc(II) bis-porphyrin Tr6ger’s base 49 was synthesised as a host for a-amino acid recognition, while di[dihydroxotin(IV)] bisporphyrin Troger’s bases, 50 and 51, were synthesised for the recognition of diacids and tartaric acids. A mild and highly efficient method for formation of dihydroxotin(IV) porphyrins from their dichlorotin(IV) derivatives was developed which was found to be very effective for the lipophilic porphyrins used in this work. As the bis—porphyrin Trbger’s base was synthesised as a racemate, chiral resolution of these compounds needed to be achieved for chiral recognition studies. Attempts to resolve dihydroxotin(IV) zinc(II) bis-porphyrin Trbger’s 49 directly through chiral chromatography failed. Instead, enantiopure 49 was obtained indirectly from chiral resolution of dizinc(II) bis—porphyrin Trdger’s base 47 that was demetallated to give the chirally pure starting material for the synthesis of 49 in two steps. 1H NMR studies with host 49 and a series of a-amino acids showed that the host binds most of the (it-amino acids through zinc-amine and tin—carboxylate coordination as expected (with the exception of serine 88 and azetidine-2-carboxylic acid 91) and chiral recognition (de 68 — 86%) was observed for all the a-amino acids studied. The extent of the enantioselectivity was found to be dependent upon the bulkiness of the side-chain of the a-amino acid. An X-ray crystal structure of L-azetidine-Z-carboxylic acid 913 bound within the cavity of host 49 suggested that a different binding mode is in operation where zinc-amine coordination is absent and instead hydrogen-bonding and tin-carboxylate coordination drives the recognition process. The two di[dihydroxotin(IV)] bis-porphyrin Troger’s bases, 50 and 51, were shown to be good hosts for recognition of dicarboxylic acids. Binding sites are available both outside and inside the cavity of the two hosts, 50 and 51, but selective binding of the diacids within the cavity of the hosts was always observed. Only when binding inside the cavity had gone to completion does binding outside the cavity result. Chain-length recognition was also observed with the two hosts; suitable diacids (94 and 95) are bound ditopically within the cavity instantaneously but longer diacids (96 and 97) are initially bound as a mixture of monotopic and ditopic complexes within the cavity and over time the monotopic complex forms the ditopic complex. The rate of ditopic complexation was found to be dependent on both the length of the bound guests and the flexibility of the receptor. Host 50 was also shown to be a good receptor for both the natural tartaric acid (L-tartaric acid 99a) and mesa-tartaric acid 98. Molecular modelling of the receptors and their complexes was used to design the receptors and to give insight into the recognition process. X-ray crystal structures of some of the receptors and their complexes were also obtained and corresponded to the structures predicted from the NMR studies