J . Am. Chem. SOC.1981, 103, 6912-6915
6912
Rate and Equilibrium Constants for the Formation and Decomposition of the Tetrahedral Intermediates of the Methanolysis of Methyl Benzoates Robert A. McClelland* and Geeta Patel Contribution from the Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S I A I . Received February 25, 1981
Abstract: Rate constants and equilibrium constants have been obtained for the equilibration of the tetrahedral intermediate ArC(OMe)20H with methanol and methyl benzoate in 50% dioxane:water. Rate constants in the decomposition direction have been measured by generating the tetrahedral intermediate from an anilide acetal. Rate constants in the formation direction have been measured from the rate of exchange of CH30H with PhCOOCD3. The equilibrium constant is evaluated as the ratio of the two rate constants. For PhC(OMe)20H the formation equilibrium constant so obtained is 2.2 X M-l; this shows good agreement with a value obtained with use of a thermodynamic approach by Guthrie and Cullimore (Can. J. Chem. 1980, 58, 1281-94) for the intermediate PhC(OH),OMe. The formation (and decomposition) of ArC(OMe)20H is subject to catalysis by both general acids and general bases, and the mechanisms of these reactions are discussed. The species H2P04shows enhanced reactivity, suggested to be due to a bifunctional mode of catalysis.
Acyl transfer reactions have been recognized for sometime now proceed via discrete tetrahedral addition intermediates.’-* Although these intermediates are not normally observed in the acyl transfer r e a ~ t i o nseveral ,~ experiments have recently been reported of their observation when generated from other more to
Table I.
Rate Constants (50% Dioxane:Water, 25 “C) for 4-XC6H,C(OMe),0H Decomposition constanf
4.5 X 0.7
II ArCOMe
OH
I 4- MeOH Ar-C-OMe T I OMe
(1)
and obviously bears a close relation to the tetrahedral intermediate of the hydrolysis of the same ester. Its observation from the anilide acetal precursor provides an opportunity to directly study its kinetics of decomposition and in this paper we report the results of such a study. Rates of formation kf have also been measured with use of an appropriate isotopic tracer, and coupled with the decomposition rate constants to provide the equilibrium constant. This quantity is clearly not accessible by direct measurement since the equilibrium concentration of the adduct is so low. A method has been devised by Guthrie for estimating free energies of formation of tetrahedral intermediates, based on the measurement of free energies of stable analog~es.~,’I t is obviously of importance to
lo4
3.0 0.7
C
c
38 21
29 22
4-Br
H
x lo4 1.9 X lo4 6.6 X lo3
0.6 b b 4 X 10’ 5.3 X 10’ 4.0 X 10, 2.7 X 10’ 2.1 x 10’ 2.0 x lo2 1.9 x 10’ b b 5.5 X 10, 100 66 36
reactive precursor^.^ In the preceding paperS we reported the detection of the species A r C ( O M e ) 2 0 H as a transient intermediate in the hydrolysis of ArC(OMe)*NMeAr’. This intermediate is the tetrahedral intermediate of the degenerate methanolysis of a methyl benzoate, 0
4-Me
4-Me0
Units of M - ’ s-’, except fork, (s-l). Not significant. (I
4 13 30
0.5 b
1.6 X 10, x lo2
2.0 b
18 8 9 32
Not determined.
Table 11. Exchange of Trideuteriomethyl4-hlethoxybenzoate in 50% Dioxane Containing 1 M HCl and 2.47 M MeOH time, days
0 12 14 26 36
ArCOOCHJ ArCOOCDSu
0.0000c 0.0417 0.0471 0.0799 0.1173
kEx,b s-’ 3.9 x 10-8 3.8 X 3.5 x 10-8 3.6 X
a Ratio of peaks in mass spectra at 166: 169. Error 5 0.0005. lir In (1-ArCOOCHJArCOOCD,). No peak at 166.
see how our kinetically based numbers compare. Experimental Section
(1) Bender, M. L. Chem. Rev. 1960, 60, 53-113. (2) Jencks, W. P. “Catalysis in Chemistry and Enzymology”; McGrawHill: New York, 1968. (3) A number of stable molecules with a tetrahedral intermediate structure are known, but these all have some special feature which results in the stabilization of the adduct form and/or destabilization of the carbonyl form. A summary of these examples can be found in reference 4b. (4) (a) Capon, B.; Gall, J. H.; Grieve, D. M. A. J . Chem. SOC.,Chem. Commun. 1976 1034-1035. (b) Capon, B.; Grieve, D. M. A. J . Chem. SOC., Perkin Trans. 2 1980, 30&302. (c) Capon, B.; Ghosh, A. K., J . Am. Chem. SOC.1981,103, 1765-1768. (d) Ahmad, M.: Bergstrom, R. G.; Cashen, M. J.; Kresge, A. J.; McClelland, R. A,; Powell, M. F. J . Am. Chem. SOC.1977, 99, 4827-4829. (e) Ahmad, M.; Bergstrom, R. G.; Cashen, M. J.; Chiang, Y.; Kresge, A. J.; McClelland, R. A,; Powell, M. F. ibid. 1979, 101, 2669-2677. ( f ) McClelland, R. A.; Ahmad, M. J . Org. Chem. 1979, 44, 1855-1860. (9) McClelland, R. A.; Ahmad, M.; Bohonek, J.; Gedge, S.Can. J . Chem. 1979, 57, 1531-1540. (h) McClelland, R. A,; Alibhai, M. Ibid. 1981, 59, 1169-1 176. (5) McClelland, R. A.; Patel, G. J . Am. Chem. SOC.,preceding paper.
0002-7863/81/1503-6912$01.25/0
The preparation of the anilide acetals and the kinetic methods used to study the hemiorthoester decomposition are described in the preceding paper.5 Trideuteriomethyl benzoates were prepared from the appropriate benzoyl chloride and perdeuteriomethanol (Aldrich). The labeled ester (OS g) was dissolved in 500 mL of a solution made up by placing in a volumetric flask 50% by volume dioxane, a known amount of CH,OH, sufficient HCI to make the final HCI concentration 1 M, and water. The solution was placed in a water bath thermostated at 25 OC, and at appropriate times, a 100-mL portion was removed and extracted with ether, the ether portion washed with saturated sodium bicarbonate and dried ~
(6) (a) Guthrie, J. P. J . Am. Chem. SOC.,1973, 95, 6999-7003. (b) Guthrie, J. P. Ibid. 1974, 96, 3608-3615. (c) Guthrie, J. P. Can. J . Chem. 1975,53, 898-906. (d) Guthrie, J. P. Ibid. 1976,54, 3562-3573. (e)Guthrie, J. P.; Cullimore, P. A. Can. J . Chem. 1980, 58, 1281-1294. (7) See also: Fastrez, J. J . Am. Chem. SOC.1977, 99, 7004-7013.
0 1981 American Chemical Society
Rate and Equilibrium Constants for ArC(OMe)20H
J . Am. Chem. SOC.,Vol. 103, No. 23, 1981 6913 Table III. Rate Constants for Methanol Exchange of Methyl Benzoates in 50% Dioxane Containing 1 M HC1
20
ester C,H,COOCD, C,H,COOCD, C,H,COOCD, 4-MeOC6H,COOCD, 4-BrC6H,COOCD,
15 a
[MeOH]
k
1.23 2.47 4.95 2.47 2.47
2.7 5.2 1.2 3.7 5.7
~s-l ~ k ~, + f , 'M" s-' X
lo-*
3.8
x 10-7 X X
X
4.2 X l o - @ 5.0 x 10-8 3.0 X lo-' 4.6 X lo-@
X
lo-' lo-''
2 k ~ ~ / [ M e o [H+]. H]
COOCH3J/[ArCOOCD3]. The result of a typical run is shown in Table 11. With both methanol and water present, a hydrolysis reaction can also occur, and the following equation can be derived on rigorously solving the differential equations for the system.
10
kobrd
5
I
I
.10
.2 0 [HA] + [A-1
Figure 1. Observed rate constants for PhC(OMe),OH decomposition in chloroacetic acid buffers. The insert plots the slope of the lines of the main figure vs. the fraction of the buffer in the acid form.
(MgS04), and the ether removed. The remaining liquid was analyzed directly on an AEI MS9 mass spectrometer. The quantity measured, using a procedure previously described,8 was the intensity ratio of peaks corresponding to ArCOOCH3 and ArCOOCD3.
Results Kinetics of Tetrahedral Intermediate Decomposition. As discussed in the previous paper,5 the kinetic behavior associated with product formation from ArC(OMe)2NMeAr' in acid solution (pH
