Part I. The photochemistry of 2,3-diazabicyclo{2.2.2}oct-2-ene (DBO) and 11 of its derivatives has been investigated; the syntheses of two new derivatives of this class are described. Introduction of either radical stabilizing substituents at the bridgehead position or strained rings anti to the azo moiety have been found to increase the photoreactivity of the normally photoinert DBO skeleton by up to 70 fold as measured by the quantum yield of nitrogen formation. Strained rings syn to the azo function have little effect. The increase in photoreactivity with structural changes is accompanied by a corresponding decrease in fluorescence lifetimes and quantum yields. The quantum yield of nitrogen formation has been found to correlate with the activation parameters for thermal decomposition of the azoalkanes; that is, the compounds that are labile photochemically are relatively labile thermally and vice versa. This is taken as evidence that a barrier to photochemical nitrogen loss from S(,1) exists which can be lowered with suitable structural changes. Strained rings syn to the azo moiety and bridgehead methyl substitution exert little effect on photoreactivity but do affect the fluorescence quantum yield and lifetime relative to DBO; this is accounted for by slight changes in the rate of intersystem in these relatively photoinert DBO derivatives.
An investigation of products from photolysis of two DBO analogues, one with monovinyl and the other with divinyl bridgehead substitution, revealed the complete lack of formation of the corresponding bicyclo{2.2.0}hexanes, the product expected from simple collapse of the 1,4 biradical. To account for this lack of ring closure product, it is proposed that these compounds could decompose via a one bond homolysis to form a diazenyl radical as a discrete intermediate. This species then collapses to the most stable product.
The effect of temperature on the photoreactivity and photophysics of several cyclic and bicyclic azoalkanes is described. For azoalkanes whose quantum yields for nitrogen formation are less than unity at room temperature, it has been found that raising the temperature at which the photolysis is performed can be an effective means of increasing photoreactivity. A simple model in which the rate constant for excited state nitrogen loss is treated with the Arrhenius equation yields activation parameters for nitrogen loss. For the compounds investigated here, these values fall between 0-10 kcal mol('-1). At least for the 2,3-diazabicyclo{2.2.2}oct-2-ene derivatives investigated, those which are more photoreactive exhibit lower activation energies than those which are relatively photoinert.
Part II. The synthesis and thermal rearrangement of 1-vinylbicyclo{2.2.0}hexane is reported. A synthesis of this compound via a route involving a tosylhydrazone decomposition was unsuccessful but a novel tosylhydrazone fragmentation reaction was discovered in its stead. In addition, a photochemical cleavage of the strained 1,4 bond in 1-acetylbicyclo{2.2.0}hexane was discovered.
| Identifer | oai:union.ndltd.org:RICE/oai:scholarship.rice.edu:1911/15635 |
| Date | January 1981 |
| Creators | NALEPA, CHRISTOPHER JOHN |
| Source Sets | Rice University |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | application/pdf |
Page generated in 0.0022 seconds