Synthesis and photochemistry of 9-acetoxy-9-benzoylfluorene
LE3 .A278 2009
Bachelor of Science
9-benzoyl-9-fluorenol (24) and 9-acetoxy-9-benzoylfluorene (25) were synthesized by means of a procedure involving a Grignard reaction on a trimethylsilylfluorenone cyanohydrin intermediate product. Confirmation of the synthesis was achieved through mass spectrometry, 1H NMR and 13C NMR. An x-ray crystallographic structure was obtained for 25. The photochemistry of both 24 and 25 was investigated to determine the potential use of the 9-benzoylfluorenyl group as a photocage. GC-MS and UV-Vis studies indicate that 24 undergoes homolytic α-cleavage upon irradiation. However, analysis using UV-Vis, GC-MS, and 1H NMR suggest that 25 releases the acetate target molecule through the desired cationic pathway when exposed to light, rather than following the unwanted radical pathway. The quantum yield for this photoreaction was measured in an aqueous-methanol solution and was determined to be 0.38. This yield was acquired by observing the growth of photoproducts 24 and 36 with 1H NMR. The 9-benzoylfluorenyl group is advantageous for photocaging applications in several ways. Primarily, its synthesis is straightforward and clean. Also, the release of carboxylic acids by heterolytic cleavage occurs rapidly and with a moderate quantum yield. This yield can easily be improved by the addition of electron-donating substituents. Furthermore, this system is conveniently achiral due to the symmetry of the fluorene ring system. Finally, unlike some well known photocages, no highly absorbing cyclic byproducts that compete for the photolysing light are formed. The 9-benzoylfluorenyl system shows great promise in becoming an efficient new photoremovable protecting group.
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