5183
J. Org. Chem. 1984,49, 5183-5189
Solvent Ionizing Power. Comparisons of Solvolyses of 1-Adamantyl Chlorides, Bromides, Iodides, and Tosylates in Protic Solvents T. William Bentley,* Gillian E. Carter, and Karl Roberts Department of Chemistry, University College of Swansea, Singleton Park, Swansea, S A 2 8PP, Wales, United Kingdom Received August 22, 1984
Solvolytic rate constants for 1-adamantyl iodide (1-AdI) in binary aqueous mixtures of ethanol, methanol, acetone, trifluoroethanol,and hexafluoroisopropyl alcohol and in acetic and formic acids are reported. Additional kinetic data for solvolyses of 1-adamantylhalides in 97% w/w hexafluoroisopropyl alcohol/water were obtained by using a microconductivity cell (volume, ca. 0.4 mL). Kinetic data for iodine-catalyzed solvolyses of 1-Ad1 in methanol/water mixtures are also reported. A scale of solvent ionizing power for iodides ( YI) is defined by log ( k / k o ) , . A d I = YI, where k is the rate constant for solvolysis of 1-Ad1in any solvent at 25 " c relative to 80% v/v ethanol/water (ko). Correlations of Y Iand similarly defined scales for tosylates ( YoTB)and bromides ( YBr) with data for chlorides (YC,)show variations in slopes attributed to charge delocalization in the leaving group (slopes, C1 > Br > I > OTs); acidic solvents show significant deviations from the correlation lines. The effect of iodine catalysis increases as solvent ionizing power decreases, consistent with formation of the charge delocalized leaving group I,-. YI does not correlate satisfactorily with Kosower's 2 values for aqueous and alcohol solvents, and the range of 2 values is substantially greater in energy terms than the corresponding range of Yr values. Our data provide qualified independent support for a recent proposal by Swain et al. that only two solvent properties correlate the major solvent effect on rates, equilibria and spectra. The concept of solvent ionizing power (Y) has proved useful for interpreting the mechanisms of heterolytic reactions in protic Values of Yare defined by eq 1, in which k / k o refers to solvolysis rates a t 25 "C in any solvent (k)relative to 80% v/v ethanol/water (ko). When RX (eq 1) is tert-butyl chloride, m = 1.000 defines the log
(k/kO)RX
=my
(1)
original Y values of solvent ionizing power for each solThese Y values were found to be unsuitable for correlating solvolytic rate data for t o s y l a t e ~ ,so ~ ~YOTs *~ (1) A review to the background to this work Bentley, T. W.; Schleyer, P. v. R. Adu. Phys. Org. Chem. 1977, 14, 1 (a) pp 32-34. (2) Leading references to recent work (a) Richard, J. P.; Jencks, W. P. J.Am. Chem. SOC.1984,106, 1383. (b) Allen, A. D.; Ambidge, I. C.; Che, C.; Micheal, H.; Muir, R. J.; Tidwell, T. T. Zbid. 1983,105,2343. (c) Gassman, P. G.; Tidwell, T. T. Acc. Chem. Res. 1983,16,279. (d) Allard, B.; Casadevall, E. Nouv. J. Chim. 1983, 7, 569. (e) Creary, X.; Geiger, C. C. J . Am. Chem. SOC.1982,104,4151. (f)Lee, I.; Koo, I. S.; Sohn, S. C.; Lee, H. H. Bull. Korean Chem. SOC. 1982, 3, 92. (9) Stang, P. J.; Ladika, M.; Apeloig, Y.; Stanger, A,; Schiavelli, M. D.; Hughey, M. R. J. Am. Chem. SOC.1982,104,6852. (h) Roberts, D. D. J. Org. Chem. 1984, 49, 2521. (i) Shafer, S. G.; Harris, J. M. J. Org. Chem. 1981,46, 2164. (j) VituUo, V. P.; Wilgis, F. P. J.Am. Chem. SOC. 1981,103,1982. (k) Kaspi, J.; Rappoport, Z. Ibid. 1980, 102, 3829. (1) Knier, B. L.; Jencks, W. P. Ibid. 1980,102,6789. (m) Yaks, K.; Mandrapilias, G. J. Org. Chem. 1980, 45,3892. (n) Young, P. R.; Jencks, W. P. J. Am. Chem. SOC.1979,101, 3288. (0)Harris, J. M.; Shafer, S. G.; Moffatt, J. R.; Becker, A. R. Zbid. 1979,101, 3295. (p) Hanack, M.; Collins, C. J.; Stutz, H.; Benjamin, B. M. Zbid. 1981,103, 2356. (4)Ruasse, M.-F.; Lefebvre, E. J. Org. Chem. 1984,49, 3210. (r) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. Zbid. 1984, 49, 3639. (3) (a) Bentley, T. W.; Carter, G. E. J. Org. Chem. 1983,48, 579. (b)
Bentley, T. W.; Bowen, C. T.; Morten, D. H.; Schleyer, P. v. R. J.Am. 1981,103,5466. (c) Schadt, F. L.; Bentley, T. W.; Schleyer, Chem. SOC. P. v. R. Ibid. 1976,98,7667. (d) Bentley, T. W.; Carter, G. E. Zbid. 1982, 104,5741. (e) Bentley, T. W.; Carter, G. E.; Harris, H. C. J. Chem. SOC., Chem. Commun. 1984,387. (f) Bentley, T. W.; Schleyer, P. v. R. J. Am. Chem. SOC.1976,98, 7658. (4) (a) Kevill, D. N.; Kolwyck, K. C.; Weitl, F. L. J. Am. Chem. SOC. 1970,92,7300. (b) Kevill, D. N.; Kolwyck, K. C.; Shold, D. M.; Kim, C. Zbid. 1973,95,6022. (c) K e d , D. N.; Bahari, M. S; Anderson, S. W. Zbid. 1984,106,2895. (d) Kevill, D. N.; Rissmann, T. J. J. Chem. SOC.,Perkin Trans. 2, 1984, 717. (e) Kevill, D. N.; Kamil, W. A.; Anderson, S. W. Tetrahedron Lett. 1982, 4635. ( 5 ) (a) Grunwald, E.; Winstein, S. J. Am. Chem. SOC. 1948, 70,846. (b) Fainberg, A. H.; Winstein, S. Zbid. 1956, 78, 2770. (c) Winstein, S.; Fainberg, A. H.; Grunwald, E. Ibid. 1957, 79, 4146. (6) Bentley, T. W.; Bowen, C. T.; Brown, H. C.; Chloupek, F. J. J. Org. Chem. 1981, 46, 38.
values were defined by eq 1 with R X = 2-adamantyl tosylate (11, X = OTs) and m = l.00.3c To separate the effects of nucleophilic solvent assistance (sN2 character) from those due to solvation of the leaving group, it is necessary to compare solvolyses having the same leaving g r 0 ~ p . lHowever, ~ the inadequacies of the original Y values for correlating solvolytic rate data for tosylates cannot be attributed solely to leaving group effects (chloride vs. tosylate). A comparison of solvent effects on the reactivity of tert-butyl chloride with 1-adamantyl chloride (I, X = Cl), along with other evidence, showed that solvolyses of tert-butyl chloride were sensitive to nucleophilic solvent assistance (sN2 c h a r a ~ t e r ) . ~To ~ ,remove ~ ~ ? ~ this ~ nucleophilic contribution from the original Y values and to allow for leaving group effects, a scale of solvent ionizing power for chlorides (designated YcJ was defined by eq 1with RX = 1-adamantyl chloride (I, X = Cl). An analogous scale for bromides ( YBr)was also defined.3d
(1)
(11)
Solvent effects on the reactivity of I and I1 (X = OTs) are almost identical,* and so very similar YoTavalues could be defined by eq 1with RX = 1-adamantyl tosylate (I, X = OTs) and m = 1.00. Because I (X = OTs) solvolyzes about lo5 times faster than I1 (X = OTs) and both are strongly prone to s N 1 reactions, it is possible to examine systematically the solvent effects on s N 1 reactivity for a wide range of leaving groups using the adamantyl framework as a relatively "constant" alkyl Kinetic data for reactive leaving groups (e.g. perchlorate,& t ~ s y l a t e * ~ ~ ~ ~ J ) have been obtained from solvolyses of 11, in addition to and O T S ) . ~We ~ now data for I (X = Cl,3dBr,3d78,9OMS,~" (7) Schleyer, P. v. R.; Fry, J. L.; Lam, L. K. M.; Lancelot, C. J. J.Am. Chem. SOC.1970, 92, 2542. (8) Schleyer, P. v. R.; Nicholas, R. D. J.Am. Chem. SOC.1961,83,2700. (9) (a) Delhoste, J.; Lamaty, G.; Pajanacci, P. C. R. Acad. Sci. Hebd. Seances Ser. C 1968,266,1508. (b) Raber, D. J.; Bingham, R. C.; Harris, J. M.; Fry, J. L.; Schleyer, P. v. R. J. Am. Chem. SOC.1970, 92, 5977.
0022-3263/84/ 1949-5183$01.50/0 0 1984 American Chemical Society
5184
Bentley, Carter, and Roberts
J. Org. Chem., Vol. 49, No. 26, 1984
Table I. Rate Constants for Solvolyses of 1-Adamantyl Iodide (I, X = I)" AH*, AS*, solvent temp, "C k , s-l kcal/mol eu 80% EtOH 100.1 (3.52 f 0.04) X (3.28 f 0.04) X 23.7 -6.7 75.0 9.0 x 10-7 25.0' 23.2 -8.6 25.0c3d 8.45 x 10-7 25.0C,e (5.8 f 0.2) X lo-' (3.55 0.05) x 10-5 25.0 50% EtOH (2.79 f 0.04) X 20% EtOH 25.0 (1.01 f 0.02) x 10-2 25.0 H2@ (7.03 f 0.15) X 100% MeOH 75.09 27.1 0.0 (1.15 f 0.02) x 10-5 60.09 60.3e*h (1.4 f 0.1)x 10-5 50.09~~ (3.14 f 0.01) X lo4 8.4 x 10-8 25.OC (8.99 0.11) x 10-5 50.0 80% MeOH -1.4 (1.40 f 0.01) x 10-5 24.5 35.0 (3.31 f 0.01) X lo4 25.0h (3.4 f 0.1) x 104 25.0' (1.17 f 0.01) x 10-3 50.0 60% MeOH -1.2 22.9 (5.4 f 0.1)x 10-5 25.W (6.72 0.02) x 10-3 45.0 40% MeOH 22.0 0.6 (6.08 0.11)x 10-4 25.0 (3.41 f 0.05) X 20% MeOH 250' (1.22 0.02) x 10-4 75.09 80% (CH3)ZCO 23.1 -10.5 (8.61 f 0.11) X lo4 50.09 3.9 x 10-7 25.0' (1.79 f 0.01) x 10-4 50.0 22.1 -7.5 (9.28 f 0.12) X lo4 25.0 (9.1 f 0.2) X lo4 25.0k (2.69 f 0.01) x 10-3 50.0 40% (CH&CO 21.2 -4.7 (1.56 f 0.03) X 25.0 (1.76 f 0.08) X 25.0 20%(CH3)2CO (1.82 0.02) x 10-3 25.0k (3.54 f 0.25) x 10-3 25.0' 10%(CH3)2CO (5.4 0.8) x 10-3 25.0' 5% (CH3)ZCO (9.6 f 0.1) x 10-5 25.0 97 % CFXHVOH - 25.0 (2.38 f 0.04) X lo-' 70% CF3CH20H (4.3 f 0.1)x 10-4 25.0 50%
*
* *
HCOZH
100.5m*" (2.7 f 0.3) X (3.9 f 0.2) x 104 81.1 3.6 x 10-9 25.0c (2.5 f 0.5) X lo-' 25.0"
25.5O
-11.6
" Determined conductimetrically in duplicate except where otherwise noted; errors shown are average deviations. Percent EtOH, MeOH, and (CH3)&0 refer to ethanol/water (v/v), methanol/water (v/v), and acetone/water (v/v) respectively; percent CF3CH20H and percent (CF3)&HOH refer to trifluoroethanol/ water (w/w) and hexafluoroisopropyl alcohol/water (w/w), respectively. Calculated from data a t other temperatures. Data from ref 8. e Determined titrimetrically; solution buffered with 0.015 M 2,6-lutidine. f Containing
