Photochemical Reactions of Optically Active Chiral and Achiral Organic Materials

Author

Nicie Conley Murphy

Date of Award

12-2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Richard M. Pagni

Committee Members

Robert N. Compton, George W. Kabalka, Gary S. Sayler

Abstract

Sodium chlorate, NaClO₃, which crystallizes in the space group P2₁3, forms optically active crystals upon crystallization although the molecule itself is achiral. NaClO₃, has been shown to produce an asymmetric distribution of (+) and (-) crystals when crystallized from water while stirring. It has been discovered that beta or positron radiation can also induce asymmetric crystallization in NaClO₃ solutions. Like NaClO₃, achiral 4,4’-dimethylchalcone crystallizes in enantiomeric forms in the space group P2₁3. Kondepudi et al. provided evidence that stirred crystallizations of 4,4’-dimethylchalcone can induce a bimodal distribution of (+) and (-) crystals. The research presented in this dissertation offers results which show that beta radiolysis and linearly polarized and right- and left-circularly light also affect the crystallization of 4,4’-dimethylchalcone from ethyl acetate. These influences have been found to induce asymmetry in the crystals in unusual ways. The chirality of the crystals was determined via their reaction in water with pyridinium tribromide to yield the chiral compound 1,2-dibromo-4,4’-dimethylchalcone, thus allowing the chirality of the crystal to be propagated to molecular chirality. Benzil, and N,N-diisopropylbenzoylformamide are also achiral molecules that crystallize in the chiral space groups P3₁2₁, and P2₁2₁2₁, respectively. It is possible with suitable reagents in water to convert the molecules in the crystals directly into chiral molecules. Only in the case of the chalcone, however, is asymmetric induction unequivocally observed.

Photoreactions initiated with circularly polarized light have been used for asymmetric synthesis, with mixed results. Reactions going through radical or ion pairs may offer a means to generate large enantiomeric excesses without degradation of the substrate. The photochemistry of racemic and resolved 2-iodooctane was previously studied by Gao et al. to give information on the dynamics of radical pair and ion pair intermediates as a function of solvent polarity and viscosity. By comparing the disappearance of the optically active substrate with its loss of optical activity, F, the fraction of the initially formed radical pair (RP) or ion pair (IP) resulting in product was determined for three solvents with differing polarities and viscosities. Gao et al. studied this effect in methanol, cyclopentane, 2-methyl-2-propanol and the gas phase. These F values will be valuable in assessing the photochemistry of 2-iodooctane in the same media with circularly polarized light.

So as to determine the full effect of polarity and viscosity on the photochemistry of 2-iodooctane, the results of an extension to this study are offered here. 2-iodooctane was examined in acetonitrile and in aqueous acetonitrile solutions. The photochemistry of 2-deutrerio-2-iodoctane was also studied. The photoreaction in acetonitrile occurred almost exclusively through heterolytic reaction with less than 1.6% occurring through hemolytic reactions. A comparison of the disappearance of the optically active substrate with its loss of optical activity was done and F, the fraction of the initially formed radical pair or ion pair resulting in product was determined for solvent mixtures. It was determined that reactions in non-polar solvents proceed via radical pairs while reactions in more polar solvents proceed via either a mixture of ion pair and radical pair intermediates or exclusively through ion pairs. Reactions going through radical pairs have a greater tendency to escape the cage and react, thus larger F values are observed in these reactions. Viscosity does not correlate with the value of F, however. If the viscosity is high, fewer escape products are formed. Reactions in which the solvent polarity is high generate fewer escape products, have more intermediate returning to reactant, and have lower F values.

Recommended Citation

Murphy, Nicie Conley, "Photochemical Reactions of Optically Active Chiral and Achiral Organic Materials. " PhD diss., University of Tennessee, 2005.
https://trace.tennessee.edu/utk_graddiss/4363