J. Org. Chem. 1984,49, 3639-3641 eficial effect is opposed by a decrease in the number of water molecules.
Acknowledgment. Support of this work by the National Science Foundation (Chemical Dynamics Program) is gratefully acknowledged. Registry No. t-BuC1,507-20-0; MeOTs, 80-488; 2-adamantyl nosylate, 25665-65-0.
Solvolysis of Diphenylmethyl Chloride and Solvent Nucleophilicity Clifford A. Bunton,* Marutirao M. Mhala, and John R. Moffatt
Department of Chemistry, University of California, Santa Barbara, California 93106 Received February 28, 1984
There has been continuing discussion of the mechanism of spontaneous (solvent assisted) aliphatic nucleophilic substitution. Hughes and Ingold, postulated a duality of mechanism involving nucleophilic attack by solvent upon substrate (the s N 2 mechanism) or initial rate-limiting ionization of substrate followed by rapid capture of a carbocation (the SNmechanism).' Convincing evidence for these concepts was provided at mechanistic extremes, e.g., in bimolecular solvolysis of methyl derivatives2 and in the trapping of carbocations in solvolyses of diarylmethyl halides. A distinction was made between nucleophilic attack and electrostatic solvation of the electrophilic reaction center or the leaving group, and solvation was excluded from the definition of mechanism.'^^ This mechanistic definition therefore required that a distinction could be made between nucleophilic (covalent) interactions with an electrophilic center and solvation of that center.'^^ Solvent parameters in linear free-energy relations such as the Grunwald-Winstein equations5 depend upon solvation of the leaving group and the reaction center, and, except for limiting s N 1 reactions, also upon nucleophilic interactions with the reaction center. The original mechanistic concepts have been modified to include the intermediacy of ion pairs, for example?' and several workers have regarded the S Nand ~ s N 2 definitions as extremes and suggested that there is a mechanistic continuum with nucleophilic participation by solvent in solvolyses of many secondary alkyl substrate^.^^^ Other workers have explicitly included nucleophilic interactions a t the alkyl group and electrophilic interactions at the leaving group, as well as electrostatic solvation, in their (1)Ingold, C. K. 'Structure and Mechanism in Organic Chemistry", 2nd ed.; Come11 University Press: Ithaca, NY,1969;Chapter VII. (2)Bentley, T. W.; Bowen, C. T.; Morten, D. H.; Schleyer, P. v. R. J. Am. Chem. SOC.1981,103,5466.McGarrity, J. F.;Smyth, T. Ibid. 1980, 102,7303. (3)Schadt, F. L.; Bentley, T. W.; Schleyer, P. v. R. J.Am. Chem. SOC. 1976,98,7667. (4)Reference 3, footnote 7. (5) Grunwald, E.; Winstein, S. J. Am. Chem. SOC. 1948, 70, 846. Fainberg, A. H.; Winstein, S . Ibid. 1957,79,1597,1602,1608. (6)Swain, C. G.; MacLachlan, A. J. Am. Chem. SOC.1960,82,6095. Harris, J. M. Prog. Phys. Org. Chem. 1974,11, 89. Sneen, R. A. Acc. Chem. Res. 1973,6, 46. (7)Shiner, V. J. In "Isotope Effects in Chemical Reactions"; Collins, C. J.; Bowman, N. S., Eds.; Van Nostrand Reinhold New York, 1970; Chapter 2. Shiner, V. J.; Dowd, W.; Fisher, R.D.; Hartshorn, S. R.; Kwick, M. A,; Milakofsky, L.; Rapp, M. W. J.Am. Chem. SOC. 1969,91, 4838. (8)Bentley, T. W.; Schleyer, P. v. R. Adu. Phys. Org. Chem. 1977,14, 1.
0022-3263/84/ 1949-3639$01.50/0
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Table I. Rate Constants of Solvolysis of Diphenylmethyl Chloride" solventb 103k, s-1 solventb 103k, 8-1 60% MeCN 5.00 60% Me2COc 3.25 50% MeCN 17.7 50% Me2COc 96.6 65.7 70% EtOHC 6.90 40% MeCN 35% MeCN 82.4 60% EtOHc 62.6 194 40% EtOHc 226 30% MeCN 27.5% MeCN 255 97% TFE 1050' 440d 70% TFE 153d 25% MeCN 159 60% MeOHc Percent of organic solvent by weight unless speca At 25.0 OC. ified. CPercentby volume. dFrom k = 0.0044,0.078& a t 5.0 "C and 10.0 OC, respectively. 'From k = 0.0205, 0.0325,and 0.062 at -10.0,-5.0,and 0 OC, respectively. fFrom k = 0.0264 and 0.0491 s-l a t -10.0 and -5.0 OC, respectively.
mechanistic definitions or quantitative treatment^.^ Solvolysis of tert-butyl halides was long considered to be a limiting s N 1 reaction, with no nucleophilic attack on the substrate by sol~ent,',~~',~ although solvation of the leaving halide ion was recognized as important.'"Jl This view has recently been challenged, and relative reactivities of tert-butyl and 1-adamantyl chloride are different in nucleophilic solvents and in weakly nucleophilic solvents such as trifluoroacetic acid (TFA) or hexafluoroisopropyl alcohol (HFIP) or trifluoroethanol (TFE)where tert-butyl chloride is abnormally unreactive.12 The cage structure of 1-adamantyl substrates precludes nucleophilic attack by solvents from the rear. These observations may have another explanation, which is that electrostatic solvation of a forming carbocation is very important;cf. ref 10 and 13. This interaction will depend upon the distance between solvent molecules and a forming carbocation, and solvent molecules can be close to the positive end of a dipolar transition state in solvolysis of tert-butyl halides, or similar open-chain substrates. Such solvent-dipole interactions should be less important in solvolysis of 1-adamantyl substrates. This explanation implies that one cannot distinguish kinetically between nucleophilic participation by solvent and electrostatic solvation (solvent-dipole interaction) at the carbocationic reaction center.'^^ It should be possible to make such a distinction by comparing solvent effects upon solvolyses of open-chain substrates such as tert-butyl and diphenylmethyl halides. The alkyl groups in these substrates are open to solvent molecules from the rear, but the common-ion inhibition of solvolyses of diphenylmethyl halides is compelling evidence for formation of a free carbocation.'J4J5 Values of Y based on the rates of solvolysis of tert-butyl chloride5 differ from those of YOT: and YC1,l2especially for weakly nucleophilic solvents. Solvolysis of diphenylmethyl chloride has been followed in a variety of solvents, but most of them were basic or nucleophilic, e.g., mixtures of water with alcohols, acetone, or dioxane. Logarithmic plots of rate constants against Yare linear, with slopes, m, close to unity, but there is dispersion of the plots." Our (9)Swain, C. G.; Mosely, R. B.; Bown, D. E. J. Am. Chem. SOC.1955, 77,3731.Swain, C. G.;Swain, M. S.; Power, A. L.; Alumini, S. Ibid. 1983, 105,502.Peterson, P. E.; Vidrine, D. W.; Waller, F. J.; Henrichs, P. M.; Magaha, S.; Stevens, B. Ibid. 1977,99,7968. (10)Abraham, M. H.; Taft, R. W.; Kamlet, M. J. J. Org. Chem. 1981, 46, 3053. (11)Kevill, D. N.; Kamil, W. A.; Anderson, S. W. Tetrahedron Lett. 1982,23,4635. (12)Bentley, T.W.; Carter, G. E. J. Am. Chem. SOC.1982,104,5741. (13)Mohanty, R. K.; Robertson, R. E. Can. J. Chem. 1977,55,1319. (14)Bailey, J. H.;Fox, J. R.; Jackson, E.; Kohnstam, G.; Queen A. Chem. Commun. 1966,122. (15)This inhibition is not observed in all solvents. In some the carbocation has a very short life, in others there is return from an ion pair.16
0 1984 American Chemical Society
J. Org. Chem., Vol. 49, No. 19, 1984
3640
Notes THFIP
1 CF~COZH
AH20
o,
-2 -,0 -
w*50m50
090
x
w
-
O70
oioo
70%;60 060
.so m70
-
070
me0
)IO0
0 EO
-2.so IS0
I
I
I
I
I
Quantitative Solvent Effects. Figure 1 shows the variation of log k for solvolysis of diphenylmethyl chloride with Y. The new data are listed in Table I, and the other data are from ref 12 and 17. The slopes, m,differ for the various solvent pairs, as noted earlier,17 but there are striking discrepancies for the fluorinated solvents, where reaction is much faster than estimated from Y. For example, Y values for CF3C02Hand Me2CO:H20(4060) are similar, as are values for HFIP and MeCN:HzO (25:75 w/w) but k for diphenylmethyl chloride must be >10 s-' in the fluorinated solvents at 25 "C, because reactions are too fast to follow at -10 "C. However, if solvolysis of diphenylmethyl and 1-adamantyl chloride have similar solvent dependence our observations would be understandable because for CF,C02H and HFIP Ycl = 4.6 and 5.8, respectively,lZand they are larger than the value for water, which is 4.57. One might argue that electrostatic stabilization of the cationic center is relatively unimportant in solvolyses of diphenylmethyl chloride because charge is highly delocalized. This argument seems unsatisfactory because solvent effects are similar for reactions of arylalkyl substrates of different reactivities and therefore different extents of charge delo~alization.~~ Internal ion-pair return has been postulated for solvolyses of diarylmethyl halides in solvents of low p ~ l a r i t y . ~ ? ~ J ~ Such ion pairs, if present, could react readily with nucleophilic solvents, such as methanol or ethanol, but not with the less nucleophilic solvents (cf. N value^)^ so that this return should retard reaction in the fluorinated solvents. We therefore assume that internal return cannot explain the deviations of log k for diphenylmethyl chloride from the Y scale in solvents of high polarity. Another possibility is that elimination is a significant r e a ~ t i o n , ' ~ but then the overall reaction of tert-butyl chloride in the less nucleophilic solvents would be faster than expected for a substitution process.'Jg Reactions of both tert-butyl and diphenylmethyl chloride could involve the intermediacy of ion pairs which are attacked by nucleophile in a rate-limiting step.6,8J6The transition state would then include nucleophile and elements of the substrate and it is not obvious how this mechanism could be distinguished from nucleophilic attack on substrate with extensive C-C1 bond breaking, unless trapping of the ion pair was observed and was consistent with the kinetics.20 There is considerable evidence that relatively stable and bulky carbocations are not strongly solvated'0*22 and that solvent assistance in limiting SN1reactions is due more to solvation of the leaving group than the forming carbocation." Our results are consistent with this view and with the conclusion that solvents such as water, methanol, and ethanol interact nucleophilically in the transition state for solvolysis of tert-butyl chloride rather than by dipoledipole interactions."J2 Dispersion in Y Plots. There is considerable dispersion in the plots in Figure 1even for nucleophilic solvents, and the pattern is similar for plots of log k / k o against Yc1,12 based on solvolysis of 1-adamantyl chloride, for solvents of low water content and Y < 0. In both sets of plots values (19) Cocivera, M.; Winstein, S. J. Am. Chem. SOC.1963, 85, 1702. (20) Observation of common-ion inhibition shows that this mechanism
(16) Winstein, S.; Clippinger, E.; Fainberg, A. H.; Heck, R.; Robinson, G. C. J. Am. Chem. SOC.1956, 78, 328. Goering, M. L.; Hopf, M. Zbid. 1971,93, 1224. (17) Winstein, S.; Fainberg, A. H.; Grunwald, E. J. Am. Chem. SOC. 1957, 79, 4146.
(18)Bunton, C. A., Mhala, M. M.; Moffatt, J. R. J. Org. Chem., pre-
vious note in this issue.
is not followed in solvolyses of diarylalkyl halides in solvents of low water content. If a free carbocation is formed in these solvents it should also be formed in wetter solvents, although there it probably has a very short lifetime.21 (21) Jencks, W. P. Ace. Chem. Res. 1980, 13, 161. Ta-Shma, R.; RaDDODOrt. Z. J. Am. Chem. soc. 1983.105.6082. r c (22) Staiely, R. H.; Wieting, R. D.; Beauchamp, J. L. J. Am. Chem.
.
SOC.1977, 99, 5964.
J. Org. Chem. 1984,49,3641-3643
of log k / k o for solvolysis of diphenylmethyl chloride in aqueous acetone are lower than those expected from the behavior in other solvents. Return of a free carbocation to reactant is not responsible for these differences, because there is insufficient chloride ion to trap a carbocation. Return of an ion pair is p ~ s s i b l e , ~but J ~ it cannot fully explain the dispersion because ion-pairs derived from tert-butyl or diphenylmethyl chloride can be attacked from the rear, but that from 1-adamantyl chloride cannot. Initial-state interactions could be playing a role, although they are neglected in the formulation of the GrunwaldWinstein and similar equations, despite evidence for their importance in nucleophilic s u b ~ t i t u t i o n . ~Initial-state ~ interactions should be very different for alkyl and arylalkyl substrates. To some extent there will be cancellations of these effects in going from initial states, but they should be least complete in solvolysesof arylalkyl substrates where charge delocalization in the transition state imposes conformational constraints and substrate structure, as well as mechanism, plays a role in kinetic polarity scales. It is therefore probably not feasible to devise generally applicable kinetic solvent scales for SN1reactions.
Experimental Section Materials. The substrates were commercial materials purified by distillation or recrystallization. Solvents were purified by standard m e t h o d ~ ~and * ~ mixed ~ J ~ solvents were generally made u p by weight. Kinetics. Reactions of diphenylmethyl chloride were generally followed spectrophotometrically at 255 nm, on a Gilford or a Hewlett Packard 8450 spectrophotometer. Some reactions were followed conductimetrically or by monitoring the protonation of methyl orange spectrophotometrically. Most reactions were followed at 25.0 "C except for the faster reactions which were followed at lower temperatures. [Substrate] was 3 X M for reactions followed spectroM for reactions followed conducphotometrically and 5 X timetrically or by use of methyl orange. Reactions in TFE and aqueous MeCN were followed spectrophotometrically and k in 60 w t % MeCN determined conductimetrically and spectrophotometrically agreed. Reactions in aqueous MeOH, EtOH, and MezCO were followed by use of methyl orange. Our choice of temperature was limited by freezing of the solvent or low solubility of the substrate at low temperatures, and we were unable t o follow reactions in HFIP or trifluoroacetic acid. The first-order rate constants, k, are in reciprocal seconds.
Acknowledgment. Support by the National Science Foundation (chemical Dynamics) is gratefully acknowledged. Registry No. Diphenylmethyl chloride, 90-99-3. (23) Robertson, R. E. h o g . Phys. Org. Chem. 1967,4,213. Shaskus, J.; Haake, P. J. Org. Chem. 1983, 48, 2036.
Hard Acid and Soft Nucleophile Systems. 8.' Reductive Dehalogenation of o - and p -Halophenols and Their Derivatives Manabu Node, Takeo Kawabata, Keiichiro Ohta, Mayumi Fujimoto, Eiichi Fujita, and Kaoru Fuji*
Institute for Chemical Research, Kyoto University, Uji Kyoto 611, Japan Received August 15, 1983
Combination systems consisting of a hard acid and a soft nucleophile have been effectively applied for cleavage re0022-32631841 1949-3641$01.50/0
3641
Table I. Reductive Dehalogenation of o-Halophenol Derivatives X.
run X R1 1 Br Me 2 Br H 3 Br Ac 4 Br H 5 1 H 6 I Me
AlC13, time, molar equiv h H 2.5 1.5 Me 1.5 0.5 Me 1.5 4.0 CH2C02Et 1.5 17.0 H 1.5 0.15 COzMe 5.0 6.5
Isolated yield.
R2
yield," tempb rt 0 "C 0 "C 0 "C rt 0 "C
--
%
rt rt
95 94 89 98
rt
95
87
r t = room temperature.
Table 11. Reductive Dehalogenation of p -Halophenol Derivatives
-
-
"ex - R'o-o EtSH/CHzCI2
AlC13, time, tempd run X R molar equiv min 1 Br H 1.5 15 0 ° C 2 Br Me 2.6 70 r t 1.5 40 0 ° C 3 Br Et 4 5 6
Br Ph 1 H I Me
1.5 1.5 1.5
0 "C 10 rt 25 0 ° C
120
-
product (yield, %)= R' = H (89)bvc R' = H (86) R' = H (64),bR' = Et (25)b rt R' = Ph (98) R' = H (82) R' = H (72)
Isolated yield. bBy GLC analysis. 10% starting material was recovered. rt = room temperature.
actions of variety of chemical bonds. Chemoselective cleavage can be achieved by proper choice of an acid and a nucleophile. A system involving aluminum chloride and ethanethiol cleaves C-0 bonds in ethers2 and esters: CNO2 bonds: activated C=C bonds,5 and Ar-SR bonds.lP6 Those reactions, except for reductive cleavage of Ar-SR bonds, proceed through a push-pull mechanism associated with hard acids and soft nucleophiles. Intervention of a radical cation has been suggested for the reductive removal of -SR group on polyaromatic ring.6 In our studies on dealkylation of bromoanisoles with the aluminum chloride and ethanethiol system, we encountered the fact that 0- and p-bromoanisoles suffered debromination along with the normal demethylation to afford phenol, while m-bromoanisole was smoothly dealkylated to give m-bromophenol. This paper describes the scope and limitations of this reductive cleavage of the carbonhalogen bond in 0- and p-halophenols and their derivatives with a combination system of aluminum chloride and ethanethiol.' (1) For part 7, see: Node, M.; Nishide, K.; Kawabata, T.; Ohta, K.; Watanabe, K.; Fuji, K.; Fujita, E. Chem. Pharm. Bull. 1983,31, 4306. (2) (a) Node, M.; Hori, H.; Fujita, E. J. Chem. SOC.Perkin Trans. 1 1976, 2237. (b) Fuji, K.; Ichikawa, K.; Node, M.; Fujita, E. J. Org. Chem. 1979,44,1661. (c) Node, M.; Nishide, K.; Sai, M.; Ichikawa, K.; Fuji, K.; Fujita, E. Chem. Lett. 1979, 97. (d) Node, M.; Nishide, K.; Fuji, K.; Fujita, E. J. Org. Chem. 1980,45,4275. (3) (a) Node, M.; Nishide, K.; Sai, M.; Fujita, E. Tetrahedron Lett. 1978,5211. (b) Node, M.; Nishide, K.; Sai, M.; Fuji, K.; Fujita, E. J. Org. Chem. 1981, 46, 1991. (c) Node, M.; Nishide, K.; Ochiai, M.; Fuji, K.; Fujita, E. Ibid. 1981, 46, 5163. (4) Node, M.; Kawabata, T.; Ueda, M.; Fujimoto, M.; Fuji, K.; Fujita, E. Tetrahedron Lett. 1982,23,4047. (5) Fuji, K.; Kawabata, T.; Node, M.; Fujita, E. Tetrahedron Lett. 1981, 22, 875. (6) Node, M.; Nishide, K.; Ohta, M.; Fujita, E. Tetrahedron Lett. 1982, 23, 689. (7) For a preliminary communication, see: Node, M.; Kawabata, T.; Ohta, K.; Watanabe, K.; Fuji, K.; Fujita, E. Chem. Pharm. Bull. 1983,31, 749.
0 1984 American Chemical Society
