3421
METAL 10s CATALYSIS OF ACETYLACETONE ENOLTAUTOMERIXATION
The Reversible Tautomerization of Acetylacetone Enol.
I. Metal Ion Catalysis by J. E. Meany Department of Chemistry, Central Washington State College, Ellensburg, Washington 98926 (Received February 17, 1969)
The present investigation establishes that the reversible tautomerization of acetylacetone enol is sensitive toward catalysis by various divalent metal ions. The reaction was carried out in dilute acetate buffers at an ionic strength of 0.3 *If and the kinetics were studied using a spectrophotometric method. A kinetic analysis of the observed first-order kinetic rate constants, kobsd = ko f ~ O A ~ - [ O A Cf- ]~ M ~ + [ Mallowed ~ + ] the evaluation of the catalytic terms: ko, 6.1 min-'; ~ o A ~ - 875 , M-' min-'; kco2+, 260 M-' min-I; k N i 2 + , 200 M-1 min-1; kg,Z+, 110 Jf-l min-l; k ~ ~ 52~ M-l + , min-'; h2+, 20M-I min-l. A comparison is made between the order of the magnitude of metal ion catalysis for the tautomerization of acetylacetone enol and the corresponding metal ion catalysis associated with related enolizationand hydration processes. Introduction Most enolization reactions involving ketones are known to be both general acid and general base catalyzed. However, although kinetic investigations have been carried out on the base catalyzed enolization of acetylacetone, this process has been found to be insensitive toward catalysis even by the hydronium ion.' Thus, in contrast to the reversible process involving acetoacetic ester which is susceptible to catalysis by the hydronium ion and also divalent metal ions,* it has been stated that the reversible enolization of acetylacetone is independent of all acids.3 For families of reactions which are found to be subject to general acid catalysis, the sensitivity of these processes toward acid catalysis is usually inversely related to substrate acidity. Consequently, it is not surprising that the acid-catalyzed enolization of acetylactone has not been observed. The present paper reports the metal ion catalysis of the reverse of the above process, viz. the tautomerization of acetylacetone enol, a process where even the powerful hydronium ion is ineffective. There are a number of ways in which metal ions participate in catalysis. For example, there is evidence suggesting that in certain hydration and hydrolysis reactions promoted by metallo-enzymes, the metal ion sometimes binds the substrate and also may participate in the direct transfer of metal-bound water to the substratem4J It has also been found that zinc ions in the presence of imidazole buffers powerfully accelerate the hydration of acetaldehyde" Here the mobile equilibrium involving divalent zinc, imidazole, and water may account for a catalytic environment which includes a general acid, a general base, and a water molecule. Metal ions have been found effective in the catalysis of reactions involving substrates which have the capacity to act as bidentate ligands. Thus, the decarboxylations of dibasic keto acids are markedly enhanced in the pres-
ence of various heavy metal ions.' Several divalent metal ions have also been found to be very effective in the catalysis of the reversible hydration of 2-pyridinecarboxaldehydes and the dehydration of 2,2-dihydroxypropionateg for which it has been suggested that the metal ions, conveniently held in place by a basic atom, act as general acids and also participate in the intramolecular transfer of metal-bound water. Experimental Section Acetylacetone, a product of Matheson Coleman and Bell, was fractionally distilled directly prior to use, the middle fraction collected a t 128-129" (715 mm). Stock solutions of the acetylacetone (0.059 M ) were prepared in dioxane so that small quantities of substrate could be conveniently injected into the reaction mixtures. Dioxane was purchased from Eastman Organic and was doubly distilled before use. The kinetic studies were generally carried out in 0.002 M acetate buffers a t pH 4.66 prepared from deionized water. The ionic strength was maintained constant at 0.3 for all kinetic runs. All metal ion solutions were prepared (1) R.P. Bell and 0. M. Lidwell, Proc. Roy. SOC.,A176,88 (1940). ( 2 ) K. J. Pedereen, Acta Chem. Scand., 2,252 (1958). (3) A.A.Frost and R.G. Peareon, "Kinetics and Mechanism," 2nd ed, John Wiley & Sons, Inc., New York, N. Y.,1961, p 230. (4) H.Neurath, Enzynes, 4, 11 (1960); B.L.Vallee, J. F. Riordan, and J. E. Coleman, Proc. Nat. Acad. Sci. U . S., 49,109 (1963). (5)Y.Pocker and J. E. Meany, Biochemistry, 4,2535(1965);Y.Pocker and J. T. Stone, ibid., 6 , 668 (1967);Y.Pocker and D. G. Dickerson, ibid., 7,1995 (1968). (6) Y.Pocker and J. E. Meany, J . Phys. C h m . , 71,3113 (1967). (7) J. F.Speck, J . BioE. Chem., 178, 315 (1949);R. Steinberger and F. H. Westheimer J. Amer. Chem. Soc., 7 3 , 429 (1951); J. E. Prue, J.Chem. SOC.,2331 (1952). ( 8 ) Y.Pocker and J. E. Meany, J. Amer. Chem. Soc., 89, 631 (1967); Y .Pocker and J. E. Meany, J . Phys. Chem., 72,655(1968). (9) Y.Pocker and J. E. Memy, submitted for publication to J . Phys. Chem. Volume 73, Number 10
October 1969
3422
J. E. MEANY
from the corresponding metal nitrates in analytical or reagent grade or of comparable purity. The reactions were monitored on a Beckman Kintrac VI1 high speed recording spectrophotometer. The temperature was maintained constant at 25 0.02" by use of a Beckman thermocirculator accessory. Measurements of pH employed the use of a Beckman Century SS expanded scale pH meter. Vapor-phase chromatography, cmied out on Perkin-Elmer 811 gas chromatograph was used to check the purity of both the substrate and the dioxane used in these studies. The kinetic runs were initiated by injecting 0.005 ml of the dioxane solutions containing an equilibrated mixture of the keto and enol forms into 3 ml of the aqueous solutions under investigation. A shift in equilibrium in favor of the keto form results from the increase in the polarity of the aqueous solution*Oll'and thus the reaction is monitored by the diminution of the absorbancy of acetylacetone enol at 272 mp. The absorbancy change for each run was 0.930-0.200 optical density unit and the tautomerization was found to obey good first-order kinetics. Plots were constructed of log ( A , - A,) us. time, where A t is the absorbancy recorded as a function of time and A , the absorbancy observed after reequilibration was established between the enol and keto forms. The first-order rate constants, kobadr were calculated from the slopes of the resultant straight lines by the relation kobsd = -2.3 X slope, and were reproducible to within 2.
Results and Discussion The tautomerization of acetylacetone is a reversible process and consequently the experimentally observed rate constants, k o b s d , reported in this paper (eq 1) refer 0
OH
/I
I
CH~C=CH-CC--CH~
-
A
0
II
0
I1
CH3CCH2CCH3 (I)
to the sum of first-order rate constants for the forward, kr, and for the reverse, k,, processes, kobsd = k f k,.12 I n the present investigations, carried out at pH 4.66, the hydroxide ion concentration was maintained low enough that its catalytic contribution was imperceptible.' In addition to the general base catalyzed process, the present work establishes that the reaction is also sensitive toward catalysis by various divalent metal ions. The observed rate constants consist of a catalytic component associated with each catalyst present in the reaction media. In the presence of metal ions and acetate buffers the observed rate constants may be represented by
+
18
16 14 T-
.E3
12
9 10 8.0 6
.
0
*
0.02
.
*
-
0.04
0
*
0.06 I mote I. -1)
h2+I
*
s
0.08
,
a
0.1
Figure 1. Metal ion catalysis of the tautomerifiation of acetylacetone enol. 0, kcozt = 260 M-1 min-1; 0, k ~ =~ 200 M-' rnin-'; 0, kznw = 110 M-1 min-1; A, k , ~ ~ 2=+52 M-1 min-1; 0, kCd2+ = 20 M-1 min-1.
were found to be linear where the slope of the straight line is defined as k ~ % + Figure . 1shows this linear relationship arising from the catalysis by several divalent ions where the values kM2+associated with Go2+, Ni2+, Zn2+, 1L/In2+,and Cd2+were deduced as 260 M-I min-l, 200 M-l min-', 110 M-l min-l, 52 M-I min-l, and 20 M-' min-', respectively. Pedersen2 has observed in the corresponding enolization of acetoacetic ester that the value of an additional ternary term which includes both acetate anions and metal ions is substantial. For example
k = 1.14
+ 490[OAc-] + 160[Cu2+]+ 6.84 x lo4[Cu2+ ] [OAc-]
(3) I n the present work, the acetate ion concentration was varied up to 0.004 M but no perceptible increases in the values h2+ were observed. Further increases in acetate concentration were precluded due to the background catalysis by the acetate ion. was deduced from a series of 10 runs The value in the absence of metal ion in which the concentrations of acetic acid and its conjugate base were varied simultaneously while maintaining a constant pH of 4.66. The slope of the straight line arising from plotting kobsdus. [OAc-] gives the value k O ~ c = - 875 M-l min-l and the intercept was used in the evaluation of the spontaneous rate constant, ko = 6.1 min-'. The reaction
(10) The ultraviolet spectra reflect the dependency of the degree of enolization upon solvent polarity. For example, for acetylacetone XH*O 274, t 2060; X1eobutane 272, c 12,000 (ref 11). It was spectrophotometrically determined that in the dioxane solutions used in these investigations 80% of the starting material existed in the enol form. A t the end of the reaction in the aqueous solutions tested, the keto form represents about 85% (ref 1, 11). kobsd = ~oAc-[OAC-] $- k M 2 + (2) (11) R. M.Silverstein and G. C. Bassler, "Spectrometric Identification of Organic Compounds," 2nd ed, John Wiley & Sons, Inc., New where ko is the spontaneous rate constant. The conYork, N. Y., 1967, p 159. centrations of both acetic acid and the acetate anion (12) These experimentally observed rate constants, hobad, are related to the forward rate constant kf,by the fraction of tautomerization, x; were maintained constant a t 0.001 M throughout each kf = Xkobsd. Since in aqueous solutions the keto form accounts for series of runs and accordingly, plots of kobgd us. [&I2+] about 85% of the equilibrated mixture, x = 0.85.
+
The Journal of Phvaical Chemistrg
[n/2f]
~
+
3423
METALIONCATALYSIS OF ACETYLACETONE ENOLTAUTOMERIZATION ~~~
~
Table I : Metal Ion Catalysis of the Tautomerieation of Aceylacetone Enol, Hydration of 2-Pyridinecarboxaldehyde, and the Dehydration of 2,2-Dihydroxypropionate COZ
Tautomerization of acetylacetone enol Hydration of 2-pyridinecarboxaldehydea Dehydration of 2,2-di hydro~ypropionate~ 0
f
260 33
x
108
1.05 X loa
Nia
k ~ " , M - 1 min-' znr+
Mnz
200
110
52
31 X 10* 1.58 X 10'
+
Cda
f
20
28 X 108
0.46 X lo8
4 . 0 X IOs
3.02 X IOQ
0.150 X lo8
0.185 X IO3
Refer to rate coefficients for the forward process. Investigations8 were carried out in 0.01 M diethylmalonate buffers a t pH 7.2. were carried out in 0.10 M diethylamalonate buffers a t pH 6.6.
b Investigations9
was also tested in acidic solutions up to 0.3 N in HC1 and no enhancement in rate due to the hydronium ion was observed. The metal ion catalysis in the forward and reverse processes may be envisaged
tion of 2-pyridinecarboxaldehyde follows almost an identical trend as that exhibited by these ions in the tautomerization of acetylacetone enol. I n addition, Pedersenla has found the order Ni2+> Zn2+> Mn2+> Cd2+in the ability of these metal ions to catalyze the
H20
It is instructive to compare the order of effectiveness with which various metal ions catalyze the tautomerization of acetylacetone enol to corresponding orders of magnitude of other metal ion catalyzed processes. Table I shows the catalytic rate coefficients for Go2+, Ni2+, Zn2+, Mn2+, and Cd2+associated with the reaction presently under consideration as well as the hydration of 2-pyridinecarbo~aldehyde~ and the dehydration From the data it will be of 2,2-dihydro~ypropionate.~ noted that, although the order of catalytic effectiveness is not identical for each reaction, there exist similarities between the relative magnitude of metal ion catalysis in each case. It is interesting to note that the ratios of the values listed for reaction 2 to those for reaction 3 have values one-tenth the corresponding catalytic rate coefficients for reaction 1 with the exception of Cd2+. The ability of these metal ions to catalyze the hydra-
enolization 'of 2-carbethoxycyclopentanone. It should be noted that whereas the former hydration reaction involves the transfer of a water molecule, the latter tautomerization reactions do not. The relative magnitude of metal ion catalysis for each process listed in Table I parallels the predicted order of stabilities of complexes involving these divalent metal ions.14 The catalyses by these ions in the processes presently under consideration may be attributed to the ability of the metal ions to act as general acids. For the hydration reactions, the added role of the metal ion in the direct intramolecular transfer of metal-bound water to the carbonyl carbon cannot be ruled out. (13) K. J. Pedersen, Acta Chem. Scand., 2,385 (1948). (14) F. L.Eplattenier, I. Murase, and A. E. Martell, J . Amer. Chem. SOC.,89,837 (1987).
Volume 73,Number 10 October 1989
