2848
Ionic Reactions in Bicyclic Systems. V. Solvolysis of endo-Bicyclo[ 3.2.1] octan-2-yl (Equatorial) p-Toluenesulfonate'" Harlan L. Goering and Garry N. Fickes3 Contribution from the Department of Chemistry, The University of Wisconsin, Madison, Wisconsin 53706. Received Nocember 4, 1967 Abstract: Acetolysis and hydrolysis (80% acetone) of endo-bicyclo[3.2.1loctan-2-ylp-toluenesulfonate (IV-OTs)
are accompanied by ion-pair return which results in interconversion of enantiomers of the unsolvolyzed ester; the polarimetric rate constants (k,) are larger than the titrimetric constants ( k t )by factors of > 5 for acetolysis and > 3 for solvolysis in 80% acetone. The first-order rate of solvolysis is steady which shows that racemization of the unsolvolyzed ester (ion-pair return) is not accompanied by other structural changes. Acetolysis products derived from optically active IV-OTs, mainly the corresponding endo-acetate (IV-OAc) with small amounts of the [2.2.2] and exo-[3.2.1] isomers (I-OAc and 111-OAc), are apparently completely racemic. These results are consistent with the view that ionization (IV; arrows) gives the symmetrical nonclassical ion Va. The small amounts of racemic I-OAc and 111-OAc in the product are thought to result from isomerization (leakage) of Va to the carbonium ion system related to I and 111. Addition of lithium perchlorate does not result in a special salt effect; only the normal linear pattern is observed for both the polarimetric and titrimetric rates. This indicates that ion-pair return associated with solvolysis is largely or exclusively internal return.
T
his is the first of a group of three papers dealing with the nature of carbonium ion intermediates involved in solvolytic reactions of the epimeric bicyclo[3.2.l]octan-2-yl p-toluenesulfonates (111-OTs and IVOTs) and bicyclo[2.2.2]octan-2-y1 p-toluenesulfonate (I-OTs). The work reported in this paper involves a kinetic and stereochemical investigation of acetolysis and hydrolysis (aqueous acetone) of the endo(equatorial)-[3.2.1] isomer IV-OTs. In an earlier in~estigation~bj it was shown that the [2.2.2] and exo(axial)-[3.2.l]bicyclooctyl isomers (I and 111) are related to the same carbonium ion system. Ion-pair return results in stereospecific interconversion of I-OTs and 111-OTs and the solvolysis product is a binary mixture of [2.2.2] and exo-[3.2. I] isomers4s6containing none (aqueous acetone) or < 1 (acetolysis) of the endo-C3.2.I] isomer,' i.e., geometric configuration is preserved. As will be shown in the following paper, the isomeric p-toluenesulfonates give products having the same composition. In the earlier papers4>jevidence was presented that the carbonium ion system relating the [2.2.2] and exo[3.2.1] isomers (I and 111) involves a bridged nonclassical ion IIa instead of a rapidlys equilibrating mixture of the corresponding classical ions (IIb S 1Ic)-the rates of both isomers appear to be accelerated519 and geometric configuration is preserved. 4 , 6 , 7 Recent in(1) Presented in part at the 145th National Meeting of the American Chemical Society, New York, N. Y., Sept 1963, Abstracts, 6 4 . (2) This work was supported in part by the National Institutes of Health (R.G. 8619) and the National Science Foundation (GP-1911). (3) National Science Foundation Predoctoral Fellow, 1962-1964. (4) H. L. Goering and M. F. Sloan, J . Am. Chem. Soc., 83, 1397 (1961). (5) H. L. Goering and M. F. Sloan, ibid., 83, 1992 (1961). (6) (a) H . M. Walborsky, M. E. Baum, and A. A. Youssef, ibid., 83, 988 (1961); (b) H . M. Walborsky, J. Webb, and C. G . Pitt,J. Org. Chem., 28, 3214 (1963). (7) H. L. Goering and G. N. Fickes, J . A m . Chem. Soc., 90, 2856 (1968). (8) This alternative interpretation would require that equilibration be rapid relative to solvent capture to account for the similar product distributions for the two isomer^.^ (9) (a) P. von R. Schleyer, J . A m . Chem. SOC.,86, 1854, 1856 (1964); (b) C. S. Foote, ibid., 86, 1853 (1964).
vestigations of (a) ring expansion'O~'' and n I 2 routes to this carbonium ion system and (b) solvolysis of optically active I-OBsGbhave provided additional information supporting this view. Evidence bearing on this point is discussed in the accompanying papers7#l3in which we present the results of our stereochemical investigations of the exo-[3.2.1]-[2.2.2] system.
I
IIa
TIb
I1c
Our earlier cursory investigation of the endo(equat0rial)-bicyclo[3.2.1]octan-2-yl system IV415showed that acetolysis of IV-OTs gives mainly IV-OAc ; the infrared spectrum of the alcohol derived from the acetate did not reveal the presence of contaminant^.^ From this it is clear that the epimeric bicyclo[3.2.l]octan-2-y1 isomers (I11 and IV) are related to different carbonium ion systems. In the following paper it will be shown that for each of these systems the presence and location of the counterion have little effect on product distributions. This means that the difference is not due to ion-pair phenomena and thus must be in the structures of the carbonium ions. To account for the stereospecificity of solvolysis and the insulation between the carbonium ion systems it was suggested that IV may give rise to a symmetrical bridged nonclassical ion Va instead of the corresponding classical (10) (a) J. A . Berson and D. Willner, ibid., 86, 609 (1964); (b) J . A. Berson and P. Reynolds-Warnhoff, ibid., 86, 595 (1964). (11) J. A. Berson and M. S. Poonian, ibid., 88, 170 (1966); R. I l 6 The difference between the polarimetric and titrimetric constants corresponds to the first-order constant for racemization, Le., k, - k, = k,,,. l ~ l 6 The steady titrimetric rate identifies the unsolvolyzed ester as IV-OTs throughout the reaction and shows that racemization is not accompanied by other structural changes-isomerization to the more reactive [2.2.2] and exo-[3.2.1] isomers (I-OTs and IIIOTs) would result in an upward drift in k,. This is also shown by the fact that ester isolated after 25 acetolysis (>70% racemization) is pure IV-OTs (isomers, if present, would not be separated by fractionation).
Table 111. Summary of Solvent and Salt Effects on Polarimetric (k,), Titrimetric (kt), and Racemization (krac)Rate Constants for Solvolysis of endo-Bicycle[ 3.2.lloctan-2-yl pToluenesulfonate (IV-OTs) at 48.86" [Salt], lO*M
Runs" 2 and 3 and 5 and 10 and 12 and 13 and
18d 19" 208 211 22J 231
A. None 5NaOAc 10NaOAc 5LiC10a 10LiC104 10LiCIOa 2 . 4 HOTS B.
16 and 25e None
+(racemic) IV-OTs
Journal of the American Chemical Society
90:II
k,
Acetolysis 5.2 1.26 5.6 1.47 5.3 1.46 4.3 3.40 3.9 5.75 3.1 6.9
80% Acetone18 3.1 0.610
--.
Re1 ratesc kt k,,,
1 1 1.15 1.07 1.15 1.13 2.8 3.5 5.0 6.7 6 . 5 10.7
1 1.17 1.16 2.7 4.5 5.5
0.58 0 . 9 7
0.48
havior is indicative of a common rate-determining step (ionization) for the three processes. The present results parallel those of related investigations 15,16, 27 and are accommodated by the Winstein solvolysis schemez7 which involves ion-pair intermediates. According to this interpretation racemization associated with solvolysis of IV-OTs results from internal return as illustrated in Chart I and summarized by eq 5 . Providing that ionization results in RZOTS
(4)
Data for the polarimetric and titrimetric experiments are compared in Table 111. This table includes k,/k, ratios, rate constants for racemization (krac), and relative magnitudes of k,,,, k,, and k, for various conditions. These data show that salt effects are similar for all three processes. This is also shown by Figure 1. The linear increase in k, is greater than that of k, and thus the gap between them (krac) also increases linearly with lithium perchlorate concentration-the reason b , < b, is that k, increases less percentagewise than k,. As has been pointed out, 15,l6 such be-
105k,,o,* sec-I
a Data for each row computed from values of k, and kt obtained from indicated experiments. * k,,, = k, - kt. Values of k,, kt, and k,,, relative to those for acetolysis in the absence of salt. d Initial value of kt obtained by extrapolation used in the calculations. e Both polarimetric and titrimetric first-order rates steady throughout the reaction. J Initial values of k, and kt used in the calculations.
krac
(active) IV-OTs
k,/kt
ka k? e [R+OTs-] d products ki
(5)
direct formation of a symmetrical intimate ion pairthis is indicated by the failure to detect capturable asymmetric intermediates-k, corresponds to the rate of ionization and k,,, is the rate constant for ion-pair return. The relationships between kt and k,,, and the constants in the scheme (and eq 5) are krac
=
k,ki/(ki
+ kz)
(7)
(27) S. Winstein, P. E. Iclinedist, Jr., and G. C. Robinson, J . A m . Chem. SOC.,83, 885 (1961), and earlier papers in that series.
/ May 22, 1968
2853
perchlorate and accumulating p-toluenesulfonic acid responsible for these drifts also involves an increase in k,lkt = (ki/kz) 1 (8) the rate of ionization and a decrease in the kl/k2ratio as the reaction progresses. which shows the relationship between the observed Under the conditions of the kinetic experiments k,/k, ratios and kl/kz(partitioning of the intermediate). acetolysis of IV-OTs in the presence of 0.05 M sodium From the data in Table I11 it is apparent that kl > acetate gives 89.4% IV-OAc, 6.6% of the exo(axia1)kz for all conditions investigated. [3.2.1] isomer 111-OAc, and 4 % of the [2.2.2] isomer I-OAc7-the latter two isomers were not detected by Chart I the analytical method (infrared analysis) used in the earlier work.4 Solvolysis in 80% acetone in the presence of a slight excess of pyridine gives 94.9% IV-OH, 4.1 % 111-OH, and 1.0% I-OH; detectable amounts of olefin are not formed in either case.7 Control experiments confirmed the earlier observation4 that under these conditions products are not isomerized. 4, As has already been mentioned, solvolysis of active PRODUCTS IV-OTs results in first-order loss of optical activity Presumably a so-called solvent-separated ion pairz7 and solutions after ten half-periods do not have detectable rotations ( + 0.003 ”). The acetolysis was ex(not included in Chart I or eq 5 ) is involved in the product-forming step-for reasons given elsewhere,15d amined more carefully than hydrolysis (80 % acetone) and control experiments showed that under conditions in systems of this type there is probably little, if of the polarimetric experiments (with and without any, direct conversion of the intimate ion pair to added sodium acetate) the resulting acetates are solvolysis product. It has been shownz7 that in optically stable. To determine the maximum rotation some cases ion-pair return involves return from the that would obtain if acetolysis proceeded with presersolvent-separated ion pair (external ion-pair return) vation of optical configuration, account must be taken in addition to that from the intimate ion pair (internal of that fraction of the substrate that racemizes (internal return). In such cases the addition of lithium perreturn) prior to solvolysis. It can be shownlja that chlorate results in a “special” salt effect ?*-presumably the fraction of the product derived from active subthe salt intercepts that fraction of the solvent-separated ion pair which otherwise would return to s ~ b s t r a t e . ~ ’ ~strate ~ ~ ~corresponds to k,/k, which for acetolysis is 0.19, or to put it another way, the average optical purity of However, if only internal return is involved only a the unsolvolyzed ester is 19% of the original value. “normal” linear positive salt effect is observed, i.e., From this it can be seen that the maximum optical lithium perchlorate has a positive salt effect on the purity of the product (complete preservation of optical rate of ionization and only a small effect on partitioning of the intimate ion-pair interinediates.16d’22 configuration) is 19% of that of the substrate. From the relative rotations it can be determined that if the From Figure 1 it is apparent that addition of lithium product were 19% as optically pure as the substrate perchlorate results in only a “normal” salt effect on the final rotations would have been as high as 0.4”. In the rate of acetolysis. Thus, according to this criterion, fact, the observed rotation was 0 + 0.003” in every exthe observed ion-pair return is largely, if not excluperiment which means that the intermediate derived sively, internal return as indicated in Chart I. It is from active substrate gives solvolysis products which interesting to note that this behavior parallels that of are at least 99% racemic. As has been mentioned, the exo-norbornyl system.22bJs this suggests that the internal return product is likewise Comparison of the lithium perchlorate normal salt completely racemic. effects on the titrimetric (k,) and polarimetric (k,) rates of acetolysis (Table I11 and Figure 1) shows that The results show that (a) ionization gives a symmetrilithium perchlorate increases the rate of ionization cal carbonium ion system (capturable intervening asymmetric species are not detected) and (b) products are (k,) and decreases internal return relative to solvolysis (kl/kz). From the k,/k, ratios it can be seen that kl/kz derived from this intermediate (by internal return and solvolysis) with nearly complete preservation of geo(eq 8) decreases from 4.2 to 2.9 as lithium perchlorate concentration increases to 0.1 M . It is this reduction metric configuration. With regard to the latter point, in internal return, relative to solvolysis, that causes k, note that for acetolysis 80% of the initially formed to be a little more sensitive than k , to changes in salt product results from internal return (complete presconcentration (b, = 53; b, = 39). These results ervation of geometric configuration) and 20 % from are qualitatively in accord with those reported for the solvolysis, i.e., kl/k2 FZ 4. Thus the 10% of the solthreo-3-p-anisyl-2-butyl systemljd and with earlier obvolysis products in which configuration is lost repservations that internal return tends to decrease with resents only 2 % of the total initial product derived increasing polarity of the mediurn.l6,I6 It should also from the intermediate. The situation is similar for be noted that the upward drifts in k , and kt, which solvolysis in 80% acetone. In this case solvolysis prodoccur throughout the acetolysis when lithium peructs represent 33% of the initial product (kl/kz = 2) chlorate is present, show similar dependencies on salt and the 5 % of the solvolysis products in which conconcentrations and again k, is more sensitive than k,. figuration is lost is < 2 % of the total. Thus in both This suggests that the synergistic salt effect of lithium cases about 98% of the initial product derived from the intermediate has the endo(equatoria1) configura(28) S. Winstein and E. Clippinger, J. A m . Chem. Soc., 78, 2784 tion. (1956). Rearrangement of eq 6 gives
+
Ik2
Goering, Fickes j Soluolysis of endo-Ricyclooctanyl p-Toluenesulfbnate
2854
The proposa14j5 that ionization of IV-OTs results rier large enough for solvent capture to compete with largely or exclusively in direct formation of the syminterconversion. 3 4 metrical nonclassical ion Va, as illustrated in Chart I, In our view, the nonclassical interpretation (Chart I) provides a simple and straightforward interpretation represents a consistent and reasonable correlation of of these results. The partial loss of geometric conthe present results. It should be noted that the apfiguration in the product-forming step(s) can be acparently small anchimeric acceleration suggests the counted for in terms of isomerization (leakage) of the possibility of simultaneous assisted (to give Va) and initially formed symmetrical ion to the bridged ion unassisted (to give Vb) ionization reactions. The reIIa related to the [2.2.2]-exo-[3.2.1] system. As will sults seem to indicate that assisted ionization is the be discussed in more detail in the following paper, it major reaction ; however, partial entry to the carbonium seems likely that the pathway for leakage is Va + ion system uia Vb cannot be ruled out. This raises Vb + IIc --c IIa. In this connection it is significant the possibility that more careful scrutiny of the products that anchimeric acceleration is small at b e ~ t . ~ ! ~ "than ' ~ ~was attempted in the present work may reveal the This suggestsgathat the energy difference for the nonpresence of an intervening asymmetric intermediate. classical Va and classical Vb ions is small relative to Experimental Section cases where large driving forces are observed and thus isomerization Diu the classical intermediate, in competiA Wycloheptenylmethyl p-Bromobenzenesulfonate (VI-OBs). A4Cqcloheptenylmethanol, bp 80-81' (1 mm), was obtained by reduction with solvent capture, seems reasonable.lO" tion (LiA1H4) of A 4-cycloheptenecarboxylic acid, mp 70.9-73.9 A rapidly equilibrating pair of classical carbonium (lit,36mp 65-67"), which in turn was prepared by the Stork-Landesions has been proposed31 as an alternate to the corman m e t h ~ d . ~ The s alcohol (shown to be homogeneous by gc17) responding nonclassical structure for similar cases. was converted to the p-bromobenzenesulfonate derivative by a previously described procedure36 for preparation of p-toluenesulThis alternative interpretation, i.e., (+)-Vb S (-)fonate derivatives. One recrystallization from ether-pentane gave Vb instead of Va, leads to several apparent difficulties VI-OBs, mp 55.0-55.6" (lit.'$ mp 53.5-55"), in 93% yield. The including accounting for the observed symmetry propsample used for product studies7 and determination of the rate of erties and stereoselectivity of chemical capture of the acetolysis (48.86') was recrystallized three additional times. endo-Bicyclo[3.2.1]octan-2-ol (IV-OH). A. By Acetolysis of intermediate. In this connection, it is pertinent that VI-OBs. Acetolysis of VI-OBs at 60" for 48 hr followed by isolaevidence has been presented that the classical [3.2.1] tion and saponification of the resulting acetates by a previously ion Vb (generated from endo-2-aminomethylnorbornane described procedure36agave an 80 % yield of endo-bicyclo[3.2.1]by the ring-expansion route) is intercepted in part in octan-2-01 (IV-OH), mp 174.5-177.7" (lit.36a mp 175.7-176.7'). aqueous acetic acid and gives an endolexo product The infrared spectrum of this material was indistinguishable from that of a sample prepared by a different route.36n However, gc17 ratio of 4 or less.lob In the present case there is no showed that the alcohol contained 3.0% of the [2.2.2] isomer indication of capturable initially formed asymmetric I-OH and 3.6% of the exo-[3.2.1] isomer 111-OH. The acetate species and the endolexo product ratio is 23 for hyfrom which the alcohol was derived had the same composition. drolysis ( S O X acetone) and 13 for acetolysis. For B. By Hydrolysis of A4-Cycloheptenylmethyl p-Bromobenzenesulfonate (VI-OBs) in 80% Acetone. A solution of 36.6 g (0.106 the carbonium ion derived from the epimer, 11-OTs, mol) of VI-OBs and 9.3 g (0.13 mol) of pyridine in 400 ml of 80% this ratio is -0.01 for acetolysis and zero for hyacetone's was sealed in a heavy-walled ampoule and heated at 48.86" drolysis.7 Evidence that the location of the counterion, for 8 days. Most of the acetone was removed by distillation with respect to the endo-[3.2.l]carbonium ion V, has (steam bath), and the resulting solution, from which alcohol had only a small effect on product distributions is presented precipitated, was diluted with 350 ml of water and continuously extracted with pentane for 24 hr. The cold extract, to which ether in the following paper. Thus, the stereoselectivity was added for complete solution of the alcohol, was washed with cannot be attributed to ion-pair phenomena (as has one 25-1111 and two 15-ml portions of cold 2 % hydrochloric acid been suggested for the [2.2.2]-exo-[3.2.1] system32)and and then with three IO-ml portions of saturated potassium carbonate must be a consequence of structural features of the solution and dried over magnesium sulfate. Removal of solvent carbonium ion itself. The bridged structure Va acby distillation left 12.1 g (91 yield) of endo-bicyclo[3.2.l]octan-2-ol (IV-OH), mp 170.3-173.6". Analysis by gc17 showed that this counts for the stereoselectivity in an obvious way. material contained 1.4% I-OH, 1.1 111-OH, and 0.9% A4-cycloA corollary of the equilibrating classical ion hyheptenylmethanol (VI-OH). pothesis is that the partners are separated by an acResolution of the endo-Bicyclo[3.2.l]octan-2-ylSystem IV. In a tivation barrier. Evidently this barrier is assumed3I typical experiment, IV-OH was converted to endo-bicyclo[3.2.1]octan-2-yl acid phthalate (IV-AP), mp 117.3-119.2" (lit.6amp 114to reflect the energy difference between the classical and 117"), in 90% yield by a previously described procedure.6& This nonclassical structures. As has been pointed out for a material was purified by two recrystallizations from a methylene similar case,33 failure to intercept initially formed asymchloride-pentane mixture and contained 1.3 of the isomeric metric classical ions requires interconversion of the acid phthalates I-AP and III-AP.'$ The acid phthalate IV-AP was resolved as follows.6a A soluenantiomeric classical ions Vb at a frequency cortion of 22.7 g (0.0830 mol) of IV-AP in 20 ml of dry acetone was responding to little, if any, activation energy. It would added to a refluxing solution of 32.8 g (0.0830 mol) of brucine in be expected that the required reorganization of the 450 ml of dry acetone. After standing overnight in a refrigerator solvation shell for such a process would result in a bar46.6 g of brucine salt was collected, mp ca. 123-125" dec, [cY]*~D O
z
z
(29) For acetolysis (49") k t for IV-OTs is 2.2 times that (obtained by extrapolation of rate data at 75' 30) for trans(equatorial)-4-t-butylcyclohexyl p-toluenesulfonate. The rate of ionization of IV-OTs is 11 times that of solvolysis of trans-4-t-butylcyclohexylp-toluenesulfonate. (30) S. Winstein and J. Holness, J . Am. Chem. SOC.,77, 5562 (1955). (31) (a) H. C. Brown, "The Transition State," Special Publication No. 16, The Chemical Society, London, 1962, pp 140-158, 174-178; P. S. Skell and R. J. Maxwell, J . A m . Chem. Soc., 84, 3963 (1962); H. C. Brown, I
