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Geometric Requirements for Hydrogen Abstractability and 1,4-Biradical Reactivity in the Norrish/Yang Type II Reaction:  Studies Based on the Solid State Photochemistry and X-ray Crystallography of Medium-Sized Ring and Macrocyclic Diketones

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journal contribution
posted on 1996-07-03, 00:00 authored by Anna D. Gudmundsdottir, Thillairaj J. Lewis, Leslie H. Randall, John R. Scheffer, Steven J. Rettig, James Trotter, Chung-Hsi Wu
The Norrish/Yang type II photochemistry of ten even-numbered cyclic diketones ranging in ring size from 10-membered to 26-membered has been studied in the crystalline state as well as in solution. In the solid state, the diketones undergo stereoselective cyclobutanol formation in which the cis or trans ring fusion stereochemistry of the photoproducts is governed by the conformation of the diketone present in the crystal as determined by X-ray crystallography. The reactive γ-hydrogen atoms are identified and the distance and angular parameters associated with their abstraction are derived from the crystallographic data. For the most part, the abstractions occur through boatlike rather than chairlike six-atom geometries, and the average value of d, the CO···H abstraction distance, for 16 reactive γ-hydrogens was found to be 2.74 ± 0.04 Å; the average values of the angular parameters ω (the γ-hydrogen out-of-plane angle), Δ (the CO···Hγ angle), and θ (the C−Hγ···O angle) are 53 ± 5°, 83 ± 4°, and 115 ± 2°, respectively. In a similar manner, the geometric parameters associated with the ring closure reactions of the intermediate 1,4-hydroxy biradicals were estimated from the crystallographic data. This indicates that both the pre-cis and pre-trans biradicals are poorly aligned for cleavage but are well oriented for closure, with radical separations of 3.1−3.2 Å. For four of the diketones, the solid state photoproduct ratios were found to be temperature dependent as a result of enantiotropic phase transitions. For two of the diketones, the high-temperature, metastable phases were characterized by solid state 13C and 2H NMR spectroscopy. Crystals of 1,14-cyclohexacosanedione were found to be dimorphic, and the conformation adopted by the macrocycle is very different in the two dimorphs. As a result, irradiation of one of the dimorphs leads to cis-cyclobutanol and photolysis of the other gives trans, a particularly clear demonstration of the effect of conformational polymorphism on solid state chemical reactivity. Overall, the solid state results indicate that the product distribution is determined by the relative rates of forward rather than reverse hydrogen atom transfer and that the biradicals react in a least-motion, conformation-specific manner. In solution, on the other hand, the photoreaction is ring size-dependent, resembling the solid state results for the 12- and 14-membered-ring diketones, and consisting mainly of type II elimination accompanied by reduced amounts of nonstereoselective cyclobutanol formation for the 16−26-membered-ring compounds. It is suggested that this ring size dependence stems from the relative conformational freedom of the intermediate 1,4-biradicals in the fluid mediummotions that are slow compared to closure for the 12- and 14-membered rings, but that permit alignment for biradical cleavage and alternative modes of closure in the “floppier” 16-membered and larger rings. The one exception to the above generalizations is 1,6-cyclodecanedione, which was found to be photochemically inert both in the solid state and solution despite having a crystal conformation favorable for type II photochemistry. Possible reasons for this behavior are presented and discussed.