Structural Aspects of the Solid-State Photochemistry of Tetrahydronaphthoquinones

Article abstract of DOI:10.1107/S0108768183002566

Crystal structure and photochemical studies of a series of cis-4a,5,8,8a-tetrahydro-1,4-naphthoquinone derivatives have revealed the detailed course of the photoreactions in the solid state and in solution.The most favourable reaction involves (2 + 2) intermolecular cycloaddition in the solid state for those crystals with parallel C=C bonds in adjacent molecules with separations of < 4.1 Angstroem.In the absence of such a molecular-packing arrangement unimolecular processes occur, the most favoured involving β-H abstraction by O or γ-H abstraction by C.The H...O or C abstraction distances are slightly smaller than normal van der Waals contacts, the orientations of the groups are favourable, and new bonds are formed with a minimum of atomic movement in the solid state.In solution the same photoproducts are formed, with additional products which require conformational changes which are topochemically forbidden in the crystal lattice.Reduction of the tetrahydronaphthoquinones yields 4aβ,5,8,8aβ-tetrahydro-1-naphthoquin-4α-ols and -4β-ols.The two series have different molecular conformations, with the hydroxyl groups occupying more sterically favoured pseudo-equatorial positions.As a consequence, solid-state photochemical reaction in the 4α-ol series proceeds via abstraction of a C(5) hydrogen atom by C(3) followed by C(2)-C(5) bonding, while in the 4β-series a C(8) hydrogen atom is abstracted by C(3) followed by C(2)-C(8) bonding.Distances and orientations are again favourable for H abstraction and new bond formation with minimum atomic movement.In solution, reaction proceeds via a kinetically favourable (2+2) intramolecular cycloaddition, involving a higher-energy conformer which is topochemically forbbiden in the solid state.Lattice control of the photochemical process (crystal engineering) has been exploited by designing a 4α-methyl-4β-ol derivative, in which the bulkier 4α-methyl substituent constrains the molecule to adopt the conformation typical of the 4α-ol series and to give the appropriate 4α-ol photoproduct.