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UBC Theses and Dissertations
Novel structures and unusual reactivity powered by tautomerism and electron delocalization in salicylimines Moosavi Mehr, Seyed Hessam
A family of tris(salicylaldimine) (TSAN) analogues was prepared by condensation of 2,4,6-triformylphloroglucinol (TFP) and various nitrogen-containing bases. Characterization of these compounds by ¹H NMR spectroscopy and single-crystal X-ray diffraction (SCXRD) revealed that some of them adopt the previously unknown enol-imine tautomeric form. Experimental data and ab initio modelling were used to establish which factors govern the keto-enamine/enol-imine tautomeric equilibrium, culminating in a simple structural model of TSAN behaviour. According to this model, π electron-withdrawing X groups in the NH₂X starting material stabilize the keto-enamine tautomeric form, whereas σ electron withdrawing X, e.g. electronegative heteroatoms, lead to the enol-imine form. The same tautomeric equilibrium has also been leveraged in a family of hydroxysalicylaldehyde Schiff bases to bring about facile exchange of specific aromatic CH hydrogen atoms when these compounds are dissolved in CD₃OD or D₂O under ambient conditions. The mechanism of this surprising isotopic exchange reaction has been investigated experimentally using ¹H NMR kinetic experiments on these Schiff bases and a number of control compounds as well as ab initio modelling. Both sources point to the involvement of the minor keto-enamine tautomeric form of the salicylimines, which facilitates electrophilic aromatic substitution of hydrogen with deuterium by stabilizing the sp³-hybridized Wheland intermediate formed in the course of the reaction. The impact of the keto-enamine tautomeric form on the electronic structure of TSANs has been studied in isolation by preparing a TSAN permanently “locked” in keto-enamine connectivity. This was achieved by replacing the labile proton present in regular TSANs with a non-labile methyl group. The resulting compound assumes the expected keto-enamine structure; however, a large degree of strain is introduced since coplanarity of the peripheral enamine arms with the central ring, required for electronic coupling between the two, results in steric repulsion between the methyl groups and the carbonyl oxygen atoms. The central ring of the product is consequently highly contorted. The unsatisfactory preparation of many of the salicylaldehyde precursors to the SANs above has been improved through a mild and efficient procedure using formamidine acetate and acetic anhydride. This method allows one to install up to three formyl group on various phenolic substrates without the harsh conditions encountered with common formylation techniques such as the Vilsmeier–Haack reaction.
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