Photochemistry of medium and large membered ring diketones in both the solution and crystalline state
Creators: name: Lewis, Thillairaj Johnathan
Publisher: name: University of British Columbia
Year: 2008

description: The type II photochemistry of nine diametric cyclic diketones (ten, twelve, fourteen, sixteen, eighteen, twenty, twenty-two, twenty-four and twenty-six membered rings), two non-diametric cyclic diketones (sixteen and seventeen membered rings) and two cyclic keto-alcohols (sixteen and eighteen membered rings) was studied in both solid and solution media in order to investigate the relationship between conformation and photo reactivity. Upon irradiation, most compounds investigated underwent smooth γ-hydrogen abstraction. The stereoelectronic requirements for the γ-hydrogen atom abstraction and the reactivity differences observed in the solid and solution reactions have been investigated with the help of X-ray crystallography and molecular mechanics calculations. Stereo selective cyclization was observed during the solid state photoreactions of all diketones investigated, with the exception of the ten, fourteen and seventeen membered ring compounds. The stereochemistry of the major solid state product correlated well with the hydrogen abstraction geometry found in the solid state conformation. Upon irradiation in solution, in rings larger than fourteen membered, the stereoselectivity is largely lost, but a slight preference for "trans" over "cis" cyclobutanol formation together with fairly large amounts of cleavage product formation were observed. In smaller rings, however, the product distributions in solution were quite similar to those observed in the solid state. In the above series of diketones, two groups of γ-hydrogen atoms can be distinguished with respect to their stereoelectronic dispositions in the solid state conformations. The γ-hydrogens having close O‧‧‧H contacts (&lt;2.78 Å) with the carbonyl oxygen atoms make a boat-like abstraction geometry, whereas the other type of γ-hydrogens with longer O‧‧‧H contact distances (close to 3 Å) make a chair-like abstraction geometry. The geometric parameters for abstraction of all closest γ-hydrogen atoms on each diketone remain close to their average values of d = 2.72 Å, ω = 52.4°, Δ = 82.4°, Θ = 114.7°. In non-diametric diketones the chemically non-equivalent γ-hydrogen atoms lead to the possibility of forming regioisomeric pairs of cyclization and cleavage products. In the solid state conformation of the sixteen membered non-diametric diketone, the non-equivalent γ-hydrogen atoms differ further in their stereoelectronic dispositions. The regioselectivity observed in this diketone reveals an efficient abstraction of the γ-hydrogen atoms having close O‧‧‧H contacts (boat-like abstraction geometry), and almost no abstraction for the γ-hydrogen atoms with O‧‧‧H contacts close to 3 Å (chair-like abstraction geometry). Interestingly, four diketones exhibit solid-solid phase transitions at elevated temperatures. In an attempt to investigate such solid state behavior, photochemical reactions were performed at various temperatures above and below the transition temperature, together with solid state NMR (¹³C CPMAS, deuterium wide line NMR techniques), solid state FTIR and X-ray powder diffraction studies. The phase transitions of three of the four diketones are reversible in nature, and above the transition points the molecular behaviour with regard to the photochemistry resembles that in isotropic fluid media. The X-ray crystal structure analysis of the two crystal modifications of the twenty six membered ring diketone, cyclohexacosane-1,14-dione, reveals the existence of "conformational polymorphism". The solid state photochemistry of these dimorphs shows a divergent photochemical behaviour, and such differences have been shown (with the help of the X-ray crystal structures), to be the result of conformational factors rather than packing effects. Investigations also reveal an irreversible solid-solid phase transformation of one of these two crystal modifications into its dimorph. In diketones, substitution of at least one of the α-hydrogen atoms with a methyl group (one methyl group at each α-carbon atom) completely changes the photoreaction pathway to yield type I products exclusively. Photolysis of three diametric diketones (sixteen, eighteen and twenty membered rings) included in zeolites afforded both Norrish type I and type II photoproducts.