Laser flash photolysis of 9-hydroxy-9-fluorenecarboxylic acid in
hexafluoro-2-propanol (HFIP) generates
a transient with λmax at 495 nm that reacts with
nucleophiles such as methanol and bromide and is insensitive
to
oxygen. In the presence of 0.15 M trifluoroacetic acid, a
different transient with λmax at 560 nm that also reacts
with
nucleophiles is formed. The 495 and 560 nm species are identified
as a zwitterion (the 9-carboxylate-9-fluorenyl
cation) and its conjugate acid (the 9-carboxy-9-fluorenyl cation),
respectively. The assignment of the zwitterion is
supported by the observation of infrared absorptions in the carboxylate
region that are quenched by nucleophiles
with rate constants similar to those obtained by UV−visible
detection. Both the zwitterion and its protonated
form
are observed at intermediate acid concentrations, leading to an
estimate of 1 × 10-2
M-1 for the equilibrium constant
for the ionization of the 9-carboxy-9-fluorenyl cation in HFIP.
Absolute rate constants for reaction of the
zwitterion
with alcohols, anionic nucleophiles, and substituted aromatics are
reported and compared to data for other 9-fluorenyl
cations.
A Novel Photochemical Synthesis of Dideoxyfuranosyl Disaccharides.-Irradiation of the chiral ketone results in rearrangement to a furan-derived carbene, which is trapped by monosaccharides to yield products from OH insertion of the carbene. -(ANGELINI, M. P.; LEE-RUFF, E.; Tetrahedron Lett. 39 (1998) 48, 8783-8786; Dep. Chem., York Univ., North York, Toronto, Ont. M3J 1P3, Can.; EN)
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