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Photochemistry of tetrahydro-1, 4-naphthoquinones in the solid state Dzakpasu, Alice Afi


Previous investigations involving the behaviour of cis-4a,5,8,8a-tetrahydro-1,4-naphthoquinone and its derivatives in solution under UV irradiation raised the question of what role, if any, the ground state conformations of these molecules play in a) the type of photochemical behaviour the substrate exhibits and b) the product distribution in cases where more than one product is formed. In addition, it has never been established experimentally just what the geometric requirements for the various reactions encountered in this series are. Such requirements usually provide insights into the geometry of the transition state itself. In the solid state, the initial conformation of molecules of any given substrate can be accurately determined by single crystal X-ray diffraction methods. Furthermore, since the crystal lattice usually resists any gross changes in conformation during the course of a reaction, it follows that most reactions in the crystalline state will occur from the ground state conformation of the substrate. By studying these reactions in the solid state and correlating the results with the X-ray data, it was hoped that the questions raised as well as others e.g. concentration effects could be answered. Eleven substrates were chosen for the investigation. They were all prepared by the Diels Alder addition of a diene to a quinone. By slow crystallization all substrates with the exception of 6,7-dimethyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 2 and 2,3,4aβ,5α,8α,8aβ-hexamethyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 10, gave crystals suitable for single crystal X-ray structure determination. Relevant X-ray data of nine of these substrates and five of the solid state photoproducts are reported. In the solid state, 5α,8α-dimethyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 1, 6,7-dimethyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 2, and the parent compound 4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 3, did not undergo the photochemical intramolecular hydrogen abstraction they undergo in solution. Instead, they dimerized stereospecifically to their respective centrosymmetric dimers when irradiated with UV light below their respective eutectic temperatures. This is rationalized in terms of a parallel alignment of the C(2), C(3) double bonds of adjacent monomers and short intermolecular centre-to-centre separation (≤4.040Å) of these double bonds. Five of the substrates, namely, 6,7-diphenyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 4, 2,3-dimethyl-1, 4-4aβ, 9aβ~tetrahydro-9,10-anthraquinone, 6,4aβ,8aβ-dicyano-6,7-dimethyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 7, 2,3,4aβ,6,7,8aβ-hexamethyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 9 and 2,3,4aβ,5β,8β,8aβ- hexamethyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 11 have intermolecular double bond contacts exceeding 4.1Å and did not undergo dimerization in the solid state. Instead, upon UV irradiation, they formed tricyclic enone alcohols derived from intramolecular abstraction by oxygen of the C(8) hydrogen which lies in the plane of the C(1)=0(1) carbonyl group. Substrates 9 and 11, in addition, each gave a tricyclic diketone resulting from the abstraction of one of the C5 hydrogens by C2. The geometric requirements, the geometries of the transition states and intermediates and the factors governing the modes of closure of the biradical intermediates in both of these hydrogen abstraction reactions and their implications for other hydrogen abstractions such as the Norrish Type II are discussed. One substrate, namely, 4aβ,8aβ-dicyano-5α,8α-dimethyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 8, which lacks short intermolecular double bond contacts and has its C(8) hydrogen out of plane and far removed from the C(1)=0(1) group neither dimerized nor gave hydrogen abstraction products when irradiated in the solid state. Instead, it gave an oxetane resulting from an intramolecular [π2+ π2] addition of C(1)=0(1) to the C(6)=C(7) double bond. The intermolecular double bond contact here was 3.20Å. Lastly, 2,3,6,7-tetramethyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 5, and 2,3,4aβ5α,8α,8aβ-hexamethyl-4aβ,5,8,8aβ-tetrahydro-1,4-naphthoquinone, 10, failed to react when irradiated in the solid state. The reason for this is not clear but the possibility of deexcitation via excimer formation and subsequent dissociation is raised.

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