| Abstract |
This dissertation describes the singlet oxygen (1Ο2, 1Δg) oxidations of organic unsaturated substrates. The main findings of this work are: (a) the electronic and solvent effects in the regioselectivity of 1Ο2 or triazolinedione (RTAD) ene reaction with tri- and tetra-substituted alkenes; (b) the stereochemistry of allylic oxidation; (c) the mechanisms of 1Ο2 or RTAD ene reaction, [2+2] and [4+4] cycloadditions with cyclopropyl substituted substrates; (d) the incorporation and photooxidation of alkenes in Nafion-Na+ membranes and (e) the photooxidation of N-hydroxyalkyl substituted pyrroles.
In the First Chapter, the electronic effect in the regioselectivity of 1Ο2 ene reaction with stereoselective deuterium labeled isobutenylarenes was investigated. These studies revealed that the relative product distribution depends on the electronic nature of para-substituents. For comparison purpose, the same reaction was also carried out with MTAD. Moreover, the “push-pull” electronic effect for the allylic oxidation was also investigated. To this end, the stereoselectively deuterium labeled 1,1-diaryl-2-methyl-1-propene was prepared. A small but systematic regioselectivity was found to operate in the allylic oxidation in protic solvents, while no regioselectivity was observed in aprotic solvents. Finally, the stereochemistry of allylic oxidation was studied with an alkene bearing two equivalent double bond faces. The conclusion drawn from this study is that the reaction: (1) proceeds through an intermediate perepoxide, and (2) is a suprafacial transformation.
In the Second Chapter, the mechanisms of 1Ο2 or RTAD ene reaction, [2+2] and [4+4] cycloadditions with cyclopropyl substituted substrates were investigated. In the 1Ο2 ene reaction no rearranged products were detected and the established perepoxide was confirmed as the intermediate. However, the same reaction with RTAD in MeOH gave rearranged products. Rearranged products were also detected in the [4+2] cycloaddition of 1Ο2 or RTAD with cyclopropyl substituted dienes. The formation of these products is consistent with a dipolar or diradical intermediate (τ ≥ 2 psec). Similar products were also detected in the [2+2] cycloaddition of 1Ο2 to a cyclopropyl substituted enolether in aprotic solvents. Also, an unexpected epoxide was isolated, when this reaction was performed in MeOH. Finally, the [2+2] cycloaddition of RTAD to the above mentioned enolether, in CHCl3, resulted in a non-stereospecific formation of diazetidines.
In the Third Chapter, the incorporation and photooxidation of organic substrates in Nafion-Na+ membranes (Nf-Na+, type 117) were investigated. a-Pinene was successfully incorporated into this fluoropolymer. Further studies on the photooxidation of this substrate were precluded due to its low-affinity with Nf-Na+. Subsequently, the incorporation of Methylene Blue into Nf-Na+ resulted in a system which is able to produce 1Ο2, in the presence of molecular oxygen and the appropriate light source. Finally, trans-stilbene was also incorporated into Nf-Na+ in order to study its photooxidation. In contrast to previously reported results, the trans-epoxide was the only product detected.
In the Fourth Chapter, the photooxidations of N-hydroxyalkyl substituted pyrroles in several solvents were investigated. Seventeen new products were isolated and characterized using NMR spectroscopy. In most cases the bicyclic lactams were the minor products observed, in contrast to previously reported studies. The photooxidation mechanisms (Type I or II) that lead to the formation of these products, are also proposed. Furthermore, the photooxidation of 1-(2-hydroxyethyl)-2,5-dimethylpyrrole yielded three unexpected products, that are reported for the first time.
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