The 2-Oxocyclobutanecarboxylic Acid Keto-Enol System
that we have studied,2-6 by hydration of the corresponding
acylketene (3), eq 2. The ketene, in turn, was produced by a
photo-Wolff reaction of 2-diazocyclopentane-1,3-dione (4) eq
reacting solutions was controlled at 25.0 ( 0.05 °C, and reactions
were followed by monitoring changes in UV absorption in the
region 265-290 nm. Most of the kinetic data obtained conformed
to the first-order rate law well, and observed first-order rate
constants were calculated by least-squares fitting of a single-
exponential function. In some of the ketene hydrations, however,
a minor amount of a competing reaction, perhaps hydration of a
ketocarbene intermediate,14 appeared to be taking place. This
additional reaction produced minor deviations from first-order
behavior, and in these cases the fitting was done by using either a
single-exponential plus linear expression or a double exponential
function.
3
. The reactions involved in these transformations are fast, and
Rates of enolization of 2-oxocyclobutanecarboxylic acid were
measured by bromine scavenging under first-order conditions, using
an excess of bromine in HBr and NaOH solutions. The measure-
ments in HBr were done in the presence of NaBr where bromine
-
is complexed as the Br
3
ion, and the reaction was followed by
monitoring the absorbance of this ion at λ ) 268 nm. A Cary 2200
flash photolytic methods were therefore used to monitor their
progress.
â-Ketocarboxylic acids, such as that investigated here, can,
in principle, form either ketone enols, such as that shown in
eqs 1 and 2, or carboxylic acid enols, such as 5. High-level ab
spectrometer, whose cell compartment was thermostated at 25.0 (
0.05 °C, was used for this purpose. The measurements in NaOH
solutions were done under conditions where bromine exists as the
-
OBr ion, and these reactions were followed by monitoring the
absorbance of this ion at λ ) 330 nm. A stopped-flow spectrometer
operating at 25.0 ( 0.05 °C was used for this purpose.
Rates of hydrolysis of 2-oxocyclobutanecarboxylic acid were also
measured spectrophotometrically, again with the Cary spectrometer
operating at 25.0 ( 0.05 °C. The reactions were followed by
monitoring decay of the absorbance of this acid at λ ) 200 nm.
Acidity Constant Determination. The acid ionization constant
of the carboxylic acid group of 2-oxocyclobutanecarboxylic acid
was determined spectrophotometrically by monitoring the increase
in absorbance at λ ) 210 nm, which occurred as this acid ionized.
The data so obtained were analyzed by using the titration curve
expression shown in eq 4, in which QaK is the acid ionization
initio calculations have shown, however, that ketone enols are
considerably more stable than the corresponding carboxylic acid
7
enols, in keeping with much greater enol content of acetone
8
9
(
pKE ) 8) than that of acetic acid (pKE ) 20). It seems safe
to conclude, therefore, that the substance studied here is the
ketone enol, as shown in eq 1.
+
AHA[H ] + A Q
B
aK
A )
(4)
+
[
H ] + Q
aK
Experimental Section
Materials. 2-Diazocyclopentane-1,3-dione was prepared from
cyclopenetane-1,3-dione by diazo transfer, using tosyl azide as the
diazo transfer agent and triethylamine as the base.10
constant at the ionic strength of the measurement (0.10 M), AHA
and A are the limiting absorbances of the acidic and basic forms
of the substrate, respectively, and A is the absorbance of a solution
B
2
-Oxocyclobutanecarboxylic acid was prepared by photolyzing
-diazocyclopentane-1,3-dione in the presence of water. A solution
of 300 mg of the diazo compound in 15 mL of acetonitrile to which
00 µL of water had been added was irradiated with 254 nm Hg-
in which both forms of the substrate are present.
2
Results
5
lamps (Rayonet) in a quartz test tube for 1 h. The reaction mixture
was then dried over magnesium sulfate and the solvent was removed
in vacuuo to provide 240 mg (87% yield) of 2-oxocyclobutanecar-
Reaction Identification. Irradiation of 2-diazo-1,3-diketones
is known to give a photo-Wolff reaction producing acylketenes,
which, in the presence of water, undergo hydration to 2-oxo-
1
boxylic acid, whose H NMR spectrum agreed with a published
2-6,15
carboxylic acids.
This was verified in the present case by
report.11
showing that irradiation of 2-diazocyclopentane-1,3-dione in
acetonitrile solution containing 3% water produced a 90% yield
All other materials were best available commercial grades.
Kinetics. Rates of ketene hydration and enol ketonization were
measured with microsecond12 and nanosecond (eximer laser
1
of 2-oxocyclobutanecarboxylic acid, whose H NMR spectrum
13
1
1
agreed with a published report.
operating at λ ) 248 nm) flash photolysis systems that have already
been described.12,13 With both systems, the temperature of the
Flash photolysis of aqueous solutions of 2-diazocyclopentane-
,3-dione produced a rapid, microsecond rise in absorbance at
1
λ ) 260-300 nm followed by a much slower decay. These
changes were assigned, on the basis of the above-described
product study, as well as by analogy with the behavior of our
previously examined 2-diazo-1,3-diketone systems,3 to hydra-
tion of 2-oxocyclobutylideneketene, itself formed within the time
of the laser pulse; this gave the enol of 2-oxocyclobutanecar-
boxylic acid (eq 2), followed by ketonization of the enol to
(
(
7) Hoz, S.; Kresge, A. J. J. Phys. Org. Chem. 1997, 10, 182-186.
8) Chiang, Y.; Kresge, A. J.; Schepp, N. P. J. Am. Chem. Soc. 1989,
1
11, 3977-3980.
9) Guthrie, J. P. Can. J. Chem. 1993, 71, 2123-2128. Guthrie, J. P.;
Liu, Z. Can. J. Chem. 1995, 73, 1395-1398.
10) Oda, M.; Masai, M.; Kitahara, Y. Chem. Lett. 1977, 307-310.
Korobitsyna, I. K.; Nikolaev, V. A. J. Org. Chem. USSR (Engl. Transl.)
976, 12, 1245-1251. Nikolaev, V. N. Dissertation, Leningrad State
University, 1974.
(
,5,6
(
1
(
11) Amice, P.; Conia, J. M. Tetrahedron Lett. 1974, 479-482.
(12) Chiang, Y.; Hojatti, M.; Keeffe, J. R.; Kresge, A. J.; Schepp, N. P.;
(14) Chiang, Y.; Jefferson, E. A.; Kresge, A. J.; Popik, V. V.; Xie, R.-
Q. J. Phys. Org. Chem. 1998, 11, 610-613.
(15) Korobitsyna, I. K.; Nikolaev, V. N. J. Org. Chem. USSR (Engl.
Transl.) 1976, 12, 1252-1260.
Wirz, J. J. Am. Chem. Soc. 1987, 109, 4000-4009.
13) Andraos, J.; Chiang, Y.; Huang, C. G.; Kresge, A. J.; Scaiano, J.
C. J. Am. Chem. Soc. 1993, 115, 10605-10610.
(
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