Synthesis of novel imine bridged macrocycles
Creators: name: Jasat, Ayub
Publisher: name: University of British Columbia
Year: 2009

description: Expanded porphyrins have recently been the focus of much synthetic attention by various groups on account of their interesting photophysical and complexation properties. A great deal of the research in this area was driven by the potential biomedical applications of these large, conjugated aromatic macrocycles, particularly in the fields of magnetic resonance imaging and photodynamic therapy. In this regard, we set out to expand the basic porphyrin framework by introducing one or more imine-like bridges between the pyrrole subunits, whilst maintaining the overall aromaticity. We thus, envisaged the synthesis of macrocycles which would exhibit strong absorption bands in the far-red region of the electromagnetic spectrum and, contain a binding cavity suitable for complexing lanthanide (III) cations in a stable ligand:metal adduct. These efforts culminated with the recent discovery of the aromatic porphocyanine macrocycle (e.g. 282). In an effort to extend this work, we began a synthetic programme which would systematically substitute the dipyrromethane subunits of porphocyanine with bipyrroles and/or, difuryl- and dithienyl-methanes. The following account presents our attempts at preparing these macrocycles. To date, we have synthesized three new analogues of 282, specifically, the difuryl compounds 334 and 340, and the dithienyl macrocycle 335. These bisimine macrocycles were prepared by acid catalysed condensation reactions of 5,5'- diformyl dipyrromethanes or bipyrroles with the 5,5'-bis(aminomethyl)- furans and thiophenes respectively. It is important to note, that, as formulated above, these macrocycle are both nonconjugated and nonaromatic, and therefore more closely resemble the porphyrinogens in terms of their spectroscopic and chemical properties. For instance, their electronic spectra contain no discernible bands beyond 400 nm, whilst their 1H NMR display no diamagnetic ring current effects associated with an extended, conjugated (4n + 2)7c-electron framework. However, unlike the porphyrinogens and the nonaromatic precursors to the tetrapyrrolic porphocyanine 282, these macrocycles do not readily oxidize to their aromatic counterparts. In fact, attempted oxidations with a range of chemical oxidants (e.g. DDQ, chloranil, l2, etc.) have all failed to effect this critical step, which we believe is essential to ensure the long-term stability of these macrocycles. Interestingly, however, compounds 334 and 335 were found to undergo partial oxidation in air to the dipyrromethene-containing macrocycles 336 and 337. However, like their precursor macrocycles (334-335) they to do not completely oxidize to the fully aromatic species. Similarly, due to the inherent instability of these bisimine macrocycles, we were unable to isolate or characterize any non-labile metal complexes. Furthermore, due to their high basicity and propensity to hydrolyse, all attempts to purify these compound have met with little success. The formation of these macrocycles in preference to linear oligomers has been tentatively inferred from the proton NMR and mass spectral data. Nevertheless, many of the precursors required for the synthesis of these and other similar macrocycles have successfully been prepared as either their N-phthaloyl derivatives, or as the biscyano compounds. Thus, we have developed efficient strategies towards these key acyclic intermediates as presented in chapter 2, and therefore anticipate further developments in this area of porphyrin chemistry in the future. A successful methodology was developed for introducing cyano groups onto the oc-positions of the difuryl and dipyrrolic intermediates required in the reductivemacrocyclization route to porphocyanines. Attempts to elucidate the mechanistic aspects of this cyclization were carried out. The introduction immediately following this section presents a comprehensive review of the expanded porphyrin literature up until the end of April 1995. It should be appreciated that whilst there is some degree of flexibility in these assignments, every attempt has been made to group similar macrocycles within the same sections.