J. Am. Chem. Soc. 1985, 107, 307-312
307
Kinetic Study of the Proton-Transfer Reactions Involving Kryptopyrrole and Its Cation in Aqueous Solution, Transition-State Structure Determinations F. G. Terrier,*la-bF. L. Debleds,la J. F. Verchere,Iband A. P. Chatroussela Contribution from the Laboratoire de Physicochimie des Solutions (UA CNRS 403), ENSCP, 1 1 rue Pierre et Marie Curie, 75231, Paris Cedex 05, France, and Department de Chimie, FacultP des Sciences de Rouen, 76130 Mont Saint Aignan, France. Received June 12, 1984. Revised Manuscript Received September 24, I984
Abstract: The rates of protonation (klAH)of kryptopyrrole ( l ) , Le., 2,4-dimethyl-3-ethylpyrrole, by a variety of acids and of deprotonation (klA-) of the resulting carbocation (1,H') by the conjugated bases have been directly measured by stopped-flow spectrophotometry in aqueous solution. On the basis of protonation rates by various carboxylic acid catalysts, a Brmsted aAH value of 0.54is determined. On the other hand, the ratio klH]o+/klD]o+ of the protonation rates by the solvated proton in water and deuterium oxide is equal to 2.77. Both of these results are consistent with similar data obtained in hydrogen-exchange experiments on various aromatics and with a proton transfer being about half-complete in the transition state. A significant result, however, is that an analysis of the effect of the pyrrole basicity on the protonation rates provides a similar picture of the transition state when the protonating agent is a weak acid like H2P04-but not vhen the protonating agent is H30+. The Br~nstedBp values measuring the sensitivity of the klAHvalues to the changes in the pyrrole basicity are equal to 0.63 and respectively. The latter value for H30f suggests that the proton transfer has not made very much 0.21 for H2P04- and H30+, progress in the corresponding transition state. Strong support for this view is provided by the observation of large negative entropies of activation for the protonation of 1 by H,O+ and deprotonation of 1,H+ by the solvent: ASl*(Hlo+) = -44.8 J mol-' K-' and A S - l * ( H 2 0 ) = -1 16.80 J mol-I K-'. It may be that these results simply reflect a general dependence of the structure of the transition state of the orotonation reaction of 1 w o n the nature of the catalyst or, more specifically, that solvent lagoccurs in the only case of the-protonation reaction by H;O+ The data also permit us to estimate a Marcus intrinsic barrier for the protonation of 1: AGO* = 62.9 kJ mol-'.
Because the lone pair of electrons on their nitrogen atom is an integral part of the *-electron system, pyrrole and indole derivatives have a very low nitrogen basicity.24 From a determination of the basicity of carbazoles whose protonation occurs unambiguously on nitrogen, Kresge et al. have estimated a pK, value in the vicinity of -10 for unsubstituted nitrogen-protonated pyrrole and indole.2c Accordingly, pyrroles and indoles behave preferentially as carbon bases, undergoing protonation on the a-and/or /?-carbon atoms in strongly acidic solution.2-6 Thermodynamic pK, values for the C protonation of a number of these derivatives have been determined in aqueous sulfuric or perchloric acid solutions. While unsubstituted pyrrole and indole have pK, values of -3.8 and -3.62, respectively, the presence of electron-donating group(s) appreciably increases the C basicity. Thus, a number of dialkyl and trialkylpyrroles have pK,'s 2 1 in aqueous s ~ l u t i o n . ~ , ~ Because they are accompanied by strong structural and solvent reorganization, protonation and deprotonation processes a t a carbon atom generally occur a t much lower rates than similar processes a t nitrogen or oxygen atoms and are often accessible In this context, it remains a to standard kinetic feature of pyrrole and indole chemistry that no direct study of proton transfer to or from the parent molecules or their derivatives (1) (a) Laboratoire de Physicochimie des solutions, E.S.S.C.P. (b) FacultC des Sciences de Rouen. (2) (a) Whipple, E. B.; Chiang, Y.; Hinman, R. L. J . Am. Chem. SOC. 1963.85. 26-30. (b) Chiane. Y.: Whiuule. E. B. Ibid. 1963. 85. 2763-2767: (c) Chen; H. J.; H a k a , L. E;; Hinma;,' R: L.; Kresge, A. J:; Whipple, E. B: Ibid. 1971, 93, 5102-5107. (3) Hinman, R. L.; Lang, J. J. Am. Chem. SOC.1964, 86, 3796-3806. (4) Jackson, A. H. In 'Comprehensive Organic Chemistry"; Barton, D., Ollis, W. D., Sammes, P. G., Ed; Pergamon Press: Oxford, 1979; Vol. 4, p 276. (b) Brown, R. T.; Joule, J. A,; Sammes, P. G. Ibid. p 412. (5) Jones, R. A.; Bean, G. P. In 'The Chemistry of Pyrroles"; Academic Press: London, New York, 1977; Chapter 1 1, pp 446-45 1. (6) Abraham, R. J.; Bullock, E.; Mitra, S. S. Can. J. Chem. 1959, 37, 1859-1869. (7) Buncel, E. "Carbanions: Mechanistic and Isotopic Effects"; Elsevier: Amsterdam, 1973; pp 16-20. (8) Jones, J. R. 'The Ionization of Carbon Acids"; Academic Press: London, 1973; pp 28-49. (9) Bernasconi, C. F. Pure Appl. Chem. 1982, 54, 2335-2348.
0002-7863/85/1507-0307$01.50/0
has been reported so far.4qs Although valuable information on C-protonation rates of these rings has been derived from studies of isotopic C-H exchange a t low a c i d i t i e ~ , ' one ~ ' ~ could expect (10) Schwetlick, K.; Unverferth, K.; Mayer, R. Z. Chem. 1967, 7 , 58-62. (11) (a) Bean, G. Chem. Commun. 1971, 421-422. (b) Bean, G. P.; Wilkinson, T. J. J. Chem. Soc., Perkin Trans. 2 1978, 72-77. (12) (a) Muir, D. M.; Whiting, M. C. J . Chem. SOC.,Perkin Trans. 2 1975, 1316-1320. (b) Ibid. 1976, 388-392. (13) Alexander, R. S.; Butler, A. R. J . Chem. SOC.,Perkin Trans. 2 1980, 110-1 12. (14) (a) Challis, B. C.; Long, F. A. J . Am. Chem. SOC. 1963, 85, 2524-2525. (b) Challis, B. C.; Millar, E. M. J . Chem. SOC.,Perkin Trans. 2 1972, 1116-1119. (c) Ibid. 1972, 1618-1624. (15) (a) Kresge, A. J.; Chen, H. J.; Chiang, Y.; Murrill, E.; Payne, M. A,; Sagatys, D. S. J . Am. Chem. SOC.1971,93,413-423. (b) Kresge, A. J.; Chen, H. J. Ibid. 1972, 94, 2818-2822. (16) (a) Colapietro, J.; Long, F. A. Chem. Ind. (London)1960, 1056-1057. (b) Schulze, J.; Long, F. A. J . Am. Chem. SOC.1964, 86, 331-335. (c) Thomas, R. J.; Long, F. A. Ibid. 1964,86,4770-4773. (d) Longridge, J. L.; Long, F. A. Ibid. 1967, 89, 1292-1297. (17) (a) Kresge, A. J.; Chiang, Y. J . Am. Chem. SOC. 1961, 83, 2877-2885. (b) Ibid. 1967, 89, 441 1-4417. (c) Kresge, A. J.; Chiang, Y.; Sato, Y. Ibid. 1967, 89, 4418-4424. (d) Kresge, A. J.; Slae, S.; Taylor, D. W. Ibid. 1970, 92, 63096314. (e) Kresge, A. J.; Mylonakis, S. G.; Sato, Y.; Vitullo, V. P. Ibid. 1971, 93, 6181-6188. (18) Kresge, A. J. In "Proton Transfer Reactions"; Gold, V., Caldin, E. F., Ed.; Chapman and Hall: London, 1975; pp 179-199. (19) Kresge, A. J.; Chiang, Y.; Koeppl, G. W.; More OFerrall, R. A. J . Am. Chem. Soc. 1977, 99, 2245-2254. (20) Challis, B. C.; Long, F. A. J. Am. Chem. SOC.1965,87, 1196-1202. (21) Kresge, A. J. Chem. SOC.Reo. 1973, 2, 475-503. (22) Kresge, A. J.; Chiang, Y. J. Am. Chem. SOC.1973, 95, 803-806. (23) (a) Kresge, A. J.; Chiang, Y. J . Am. Chem. SOC.1962, 84, 3976-3977. (b) Kresge, A. J.; Sagatys, D. S.;Chen, H. L. J. Am. Chem. SOC. 1968, 90, 4174-4175. (c) Ibid. 1977, 99, 7228-7233. (24) (a) Kreevoy, M. M.; Steinwand, P. W.; Kayser, W. W. J . Am. Chem. SOC.1964, 86, 5013-5014. (b) Gold, V.; Kessick, M. A. Discuss. Faraday SOC. 1965, 39, 84-93. (25) Eliason, R.; Kreevoy, M. M. J . Am. Chem. SOC.1978, 100, 7037-704 I . (26) (a) Smith, P. J . In 'Isotopes in Organic Chemistry"; vol. 2, Buncel, E., Lee, C. C. Ed; Elsevier: S e w York, 1976; Vol. 2, pp 238-241. (b) Melander, L.; Saunders, W. H. In "Reaction Rates of Isotopic Molecules"; Wiley: S e w York, 1980; Chapter 5. (c) Schowen, R. L. Prog. Phys. Org. Chem. 1972, 9, 275-332.
0 1985 American Chemical Society
308 J. Am. Chem. SOC.,Vol. 107, No. 2, 1985
Terrier et al.
OD
1
Figure 1. Ultraviolet absorption spectra of 1 X M solutions of 1 in ( 1 ) water, ( 2 ) pH 4.18, acetic acid buffer, ( 3 ) pH 3.66, formic acid buffer, (4) pH 3.18, formic acid buffer, (5) 2 X IO-' M HCI, and (6) 0.1
M HC1.
that a direct kinetic study of the equilibrium protonation of some pyrrole and indole derivatives would provide much interesting information on the process. In looking a t the behavior of some of the most basic of these derivatives, we have found that kryptopyrrole, Le., 2,4-dimethy-3-ethylpyrrole,was a suitable compound for such a study in aqueous solution. The data obtained add to the understanding of electrophilic addition to these heterocycles.
Results Protonation of kryptopyrrole 1 is known to occur exclusively at the unsubstituted C-a position to give the cation 1,H' according to eq 1 The p H dependence of the equilibrium 1 has been spectrophotometrically studied using dilute HC1 solutions as well as .*q5q6
(27) Kresge, A. J.; Chiang, Y. J . Chem. SOC.,B 1967, 58-61. (28) Baliga, B. T.; Bourns, A. N. Can. J . Chem. 1966, 44, 379-386. (29) Chwang, W. K.; Eliason, R.; Kresge, A. J. J . Am. Chem. SOC.1977, 99, 805-808. (30) Bernasconi, C. F.; Hibdon, S. A. J . Am. Chem. SOC.1983, 105, 4343-4348. (31) Alexander, R. S.;Butler, A. R. J . Chem. SOC.,Perkin Truns. 2 1976, 696-701. (32) Kreevoy, M. M.; Kretchner, R. A. J . Am. Chem. SOC.1964, 86, 2435-2439 and references therein. (33) Bordwell, F. G.; Bartmess, J. E.; Hautala, J. A. J . Org. Chem. 1978, 43, 3 107-3 1 13 and references therein. (34) Bell, R. P.; Grainger, S. J. Chem. SOC.,Perkin Trans. 2 1976, 1367-1370. (35) (a) Marcus, R. A. J . Phys. Chem. 1968, 72, 891-899. (b) Cohen, A. C.; Marcus, R. A. Ibid. 1968, 72, 4249-4256. (36) Kresge, A. J. Acc. Chem. Res. 1975, 8, 354-360. (37) Note that the intrinsic kinetic barrier in the Marcus sense35is the sum of the work done to bring the reactants together and the intrinsic barrier for the actual proton transfer, Le., the W, and AGO*terms indicated in ref 36. (38) Organic syntheses, Coll. Vol. 3, ed. by John Wiley and Sons, New York, 1955 p 513. (39) Halle, J. C.; Pouet, M. J.; Simonnin, M. P.; Terrier, F. Can. J . Chem. 1982, 60, 1988-1995.
1Ht
various methoxyacetic, formic, and acetic acid buffer solutions. The spectral changes are reversible, and there is a good isobestic point at 240 nm (Figure 1). From the variations at 260 nm of the optical density obtained at the equilibrium as a function of pH, a pK, value of 3.75 was determined at 25 "C ( I = 0.1 M KCl). Interestingly, the acidity of 1,H' is essentially temperature independent; measurements at 15 and 35 "C led to pK, values of 3.78 and 3.76, respectively. Reaction 1 was kinetically studied at 25 "C in a stopped-flow apparatus. All experiments were carried out under pseudofirst-order conditions in aqueous solutions at I = 0.1 M (KC1) by monitoring the appearance or disappearance of the absorption , 1 was apof 1,H' a t 270 nm. At pH C P K , ~ . ~ ' equilibrium proached from left to right by mixing an aqueous solution of 1 (about 2 X M ) with HC1 solutions or appropriate buffer solutions, Le., cyanoacetic, chloroacetic, methoxyacetic, and formic acid buffers. At pH > PK,',~', equilibrium 1 was approached from right to left from pH-jump experiments. These were carried out by mixing a 0.01 M HC1 solution of 1,H+ with methoxyacetic, formic, acetic, succinic, cacodylic, and dihydrogen phosphate buffers made up so as to have the desired final pH. In the buffers where eq 1 could be approached from both reactant and product sides, the rate data obtained in the two series of experiments were similar within experimental error. A strong catalysis of the reactions was observed and studied in detail. Typically, experiments were conducted a t three different buffer ratios with kobsd being determined at any given pH at six to eight different buffer concentrations. Most of the results have been summarized in Table The observed first-order rate constant of approach to eq 1, is given by
kobsd=
k-lH20
kabsd,
reflecting the rate
+ klH30+[H30'] + k I A H [ A H ]+ k l A - [ A - ]
(2)
While klH3'+ and k-lH20 are the rate constants defined by eq 1 , kIAHand klAare the rate constants referring to protonation of 1by A H and deprotonation of 1,H' by A-, respectively, according to eq 3. The various rate constants were determined as follows.
1
kAH + AH & 1,H' + Ak_,A'
(3)
In HC1 solutions, eq 2 simplifies to
kohd = k-lH20 + klH30+[H30f]
(4)
and klHjO+and k-lH20 were obtained from the slope and intercept of the plot of kobsd vs. [ H 3 0 + ] which was linear. In solutions of the two strongest buffer acids used, Le., cyanoacetic and chloroacetic acids, the catalysis by A- was negligible at the concentrations employed but the hydrogen ion concentration did not remain constant as the buffer concentration was varied, even at a constant buffer ratio. Such buffer failure was taken into account by calculating the hydrogen ion concentration of all buffer solutions15and determining the klAHvalues from the slopes of the koM-klH30+[H30+]vs. [AH] plots which were linear (Figure SI). In methoxyacetic, formic, and acetic buffers, such buffer failure was negligible but both the catalysis by A- and A H contributed significantly to kobsd. This is illustrated by Figure 2 which shows that slopes and intercepts of linear plots of k&sd vs. the formic acid concentration were dependent on pH. A standard treatment of this p H dependence leads to klAH= 141 M-' and k-lA' = 56 M-I s-l for methoxyacetic acid, klAH= 132 M-' s-I and kIA= 90 M-' s-I for formic acid and, klAH= 22 M-' s-' and kIA= 196 M-' s-' for acetic acid. To be noted is that the corre(40) See paragraph at end of paper regarding supplementary material.
J. Am. Chem. SOC.,Vol. 107, No. 2, 1985 309
Reactions InGoluing Kryptopyrrole Table I. Kinetic and Thermodynamic Parameters for Protonation of Kryptopyrrole According to Equation 1 in Aqueous Solution a t f = 25 oCa,b
'obsqr -1
f, o c ~~
~
25 5690 1.01 5630 3.75, 3.15c 38.20 -44.8 38.15 -1 16.80 0 12
15
klH30+L mol-' s-' k_lH20, s-' K,; L mol-' PK, A H , * , kJ mol-' AS,*, J mol-' K-' AH-,*, kJ mol-, AS-'*, J mol-' K-' AH,', kJ mol-, ASl', J mol-' K-'
3155 0.56 5635 3.75, 3.78c
35 9600 1.71 5615 3.75, 3.76'
25 pH.3.60
/
20
a I = 0.1 M KCI. bEstimated errors: rate constants *3%; AH* values 3~ 1.5 kJ mol-'; A S * values i 5 J mol-' K-I. 'Spectrophotometric determination.
15
Table 11. Rate Constants for Catalysis by Buffer Speciesa
klAH,L
k_lA-,L mol-' s-' -1.74 0.01 8* H30+ 17c cyanoacetic acid 2.37 chloroacetic acid 2.71 24' 3.45 methoxyacetic acid 56 formic acid 3.60 90 acetic acid 4.64 196 succinate ion 5.60 8 50 6.15 cacodylic acid 5500 H,POa6.70 6000 O f = 25 OC, I = 0.1 M KCI. Determined potentiometrically at I = 0.1 M KCI.
