4512
Substituent Effects and Homobenzylic Conjugation in Benzonorbornen-2 ( exo ) -yl p-Bromobenzenesulfonate Solvolyse~’-~ Hiroshi Tanida, Hiroyuki Ishitobi, Tadashi Irie, and Tadahiko Tsushima Contribution from the Shionogi Research Laboratory, Shionogi & Co., Ltd., Fukushima-ku, Osaka, Japan. Received February 25, 1969 Abstract: A series of aromatic-substituted benzonorbornen-2(exo)-yl p-bromobenzenesulfonates was prepared and the solvolysis reactions were studied. The relative rates of acetolysis of 6-CH30,H, 7-CH30,7-CH30-6-N02,and 6,7-(N0)2 derivatives at 77.60’ were 178, 1, 0.72, 1.1 X loFa,and 1.1 x loFb,respectively. The solvolyses of 6CH30, H, and 7-CH30 derivatives (le, 2e, and 3e) proceed with retention of configuration yielding only the exosubstituted products (the corresponding benzonorbornen-2(exo)-ols or their esters). However, the strongly deactivated 7-CH30-6-N02and 6,7-(N0& derivatives (4e and 5e), besides the products with retention, give the inverted endo products and the olefins. The analysis of the data indicates major participation by the aromatic ring, facilitating solvolysis and causing exo substitution in the product. When an optically active material (le-OBs, the parent system) is used, the participation brings about a racemic product. When the optically active 6,7-dinitrobenzonorbornen-2(exo)-ylbrosylate(6e-OBs)was acetolyzed,the produced exo acetate retained 4.51% of the original optical purity and the endo acetate retained 25.7%. By these data and the results of the acetolysis of labeled 6,7-dinitro3(exo)-deuteriobenzonorbornen-2(exo)-yl brosylate (7e-OBs), it was proven that the dinitrobenzene ring is migrating. Also it was shown that the endo product from 5e-OBs is partially the result of an S Nreaction ~ of the original brosylate and partially the result of the same reaction with the brosylate formed by internal return of an ion pair. Both the observed rates and the anchimerically assisted parts of the observed rates are correlated with good precision by using the modified Hammett relationship, log (k/ko)= pu+, yielding identical straight lines with p = - 3.26.
T
he participation by the benzene ring and the nonclassical structure of the carbonium ion intermediate in the solvolysis of benzonorbornenyl derivatives was first proposed by Bartlett and G i d d i n g ~ . ~The authors then discovered that, in the benzonorbornen-9(anti)yl system, the effects of the 6 substituent and the 7 substituent on the reaction rate are very substantial and are additive.6 These findings have been evaluated as one of the best pieces of evidence for the existence of participation by the benzene ring’ and for a symmetrical transition state.* As an extension of studies based principally on the same ideas, the present report shows the substituent effects on the solvolysis of benzonorbornen-2(exo)-yl brosylate and demonstrates a pa+ relationship. Since participation by the benzene ring in the solvolysis of 6-methoxybenzonorbornen-2(exo)-yl systems seems to have become a definite fact according to the recent communications submitted from three independent l a b o r a t o r i e ~ , the ~ ~ , ~major attention in this paper has been focused on the effects of strongly deactivating substituents. (1) A portion of the results of this paper appeared in preliminary communications and accounts: (a) H. Tanida. H. Ishitobi, and T. Irk, J . Amer. Chem. Soc., 90, 2688 (1968); (b) H. Tanida, Accounts Chem. Res., 1, 239 (1968). (2) Presented at the Conference on Carbonium Ions, Cleveland, Ohio, Oct 1968. - ... - - -. (3) (a) D. V. Braddon, G. A. Wiley, J. Dirlam, and S. Winstein, J . Amer. Chem. Soc., 90, 1901 (1968); (b) H.C. Brown and G. Tritle, ibid., 90, 2689 (1968); working independently, these authors have published communications of work similar to that reported in our preliminary forms.’ (4) The numbering used in this paper is shown in the charts. ( 5 ) (a) P. D. Bartlett and W. P. Giddings, J . Amer. Chem. Soc., 82, 1240 (1960); (b) W. P. Giddings and J. Dirlam, ibid., 85,3900 (1963). (6) (a) H.Tanida, ibid., 85, 1703 (1963); (b) H.Tanida, T. Tsuji, and H. Ishitobi, ibid., 86, 4904 (1964); (c) H. Tanida and H. Ishitobi, ibid., 88, 3663 (1966); (d) H. Tanida, Y. Hata, S. Ikegami, and H. Ishitobi, ibid., 89, 2928 (1967). (7) H. C.Brown and K. Takeuchi, ibid., 88, 5336 (1966). (8) B. Capon, M.J. Perkins, and C. W. Rees, “Organic Reaction Mechanisms 1967,” Interscience Publishers, London, 1968, p 30.
Results Preparations. A number of aromatic-substituted 2benzonorbornenyl derivatives were synthesized as outlined in Chart I. The hydrochlorination of benzonorbornadiene was reported to give solely benzonorbornen-2(exo)-yl chloride (le-Cl).Q Treatment of 6-methoxybenzonorbornadiene with concentrated hydrochloric acid yielded an 8 : 2 mixture, by vpc analysis, of 6-methoxy- and 7methoxybenzonorbornen-2(exo)-yl chlorides (2e-C1 and 3e-C1) (homo-para and homo-meta chlorides, eq a). lo The reactivity difference in solvolysis between the homo-para and homo-meta-exo derivatives is very large (eq c and d). It was, therefore, possible to separate these chlorides easily by hydrolysis of only the reactive 2e-C1 into an alcohol (2e-OH), as was described in the communication1a and, in full, in the Experimental Section.” Oxidation of 2e-OH and 3e-OH by the Oppenauer method or with chromic anhydride in pyridine led to 6-methoxybenzonorbornen-2-one (2-0) and 7-methoxybenzonorbornen-2-one (3-0), respectively. Comparison of the characteristic ‘La bands in 238 mp (e 8660)) the ultraviolet spectra of 2 - 0 and 3-0 (in isooctane, shoulder at 228 mp ( E -5620)) evidences the homo-para and homo-meta assignm e n t ~ . ~In~ addition, , ~ ~ the nmr patterns of aromatic (9) S. J. Cristol and R. Caple, J . Org. Chem., 31, 2741 (1966). (10) The 7:3 ratio previously reported’* was the relative yields of isolated chlorides. The reason for the preferred formation of 2e-C1 is, in substance, the same for its greater solvolytic reactivity, namely homopara stabilization of the intermediate, as discussed later. We are presently investigating the substituent effects on the addition reactions of benzonorbornadienes, which will be reported later. (11) exo and endo configurations in the alcohols dealt in this paper are assigned by the nature of infrared OH stretching bands as described in H. Tanida, T. Tsuji, and S. Teratake, J . Org. Chem., 32, 4121 (1967). (12) H.H. Jaffe and M. Orchin, “Theory and Applications of Ultraviolet Spectroscopy,” John Wiley & Sons, Inc., New York, N. Y., 1962, p 260.
Journal of the American Chemical Society / 91:16 / July 30, 1969
4513
Chart I
-&
hX-0;8
Z
8
z
2
\
0
V
A
X= Cl,OH,OAc, OBs etc, le-X,2 =H &-X, Z = 6-CH30 3e-X,Z = 7-CH30 4e-X,Z =7-CH30,6-N00 5e-X, 2 = 6,7-(N0&
1-0 20 3-0 40 5-0
In-X
2n-X 3n-X 4n-X 5n-X
6
homo-meta
3e-a
3e-OH
3eOAc
----+
CH30 * 2 0 N
OAc
+ 4eOH
4eOAc
protons in 2-0 and 3-0 (AMX type, in acetone-d6 at 100 Mc) are in agreement with the assignments. It is noted that, owing to its location in the deshielding area of the carbonyl group, the proton at C-5 in 2 - 0 (the M part) appears at a lower field than does the corresponding proton at C-8 in 3-0, whereas, owing to their location in the shielding area, the protons at
I
I
C-7 and C-8 in 2 - 0 (the AX part) appear at a higher field than the corresponding protons at C-6 and C-5 in 3-0, respectively (see Experimental Section). Electrophilic aromatic substitution reactions of the benzonorbornene derivatives show an unusually strong /3 orientation.I4 Thus, nitration of 3e-OAc with fuming nitric acid in acetic anhydride gave 7-methoxy-6nitrobenzonorbornen-2(exo)-yl acetate (4e-OAc) in 96% yield, which was hydrolyzed with very dilute hydrochloric acid to obtain 4e-OH (eq e). Mononitration of le-OAc with fuming nitric acid in acetic anhydride followed by introduction of a second nitro group with fuming nitric acid and concentrated sulfuric acid afforded 6,7-dinitrobenzonorbornen-2(exo)-ylacetate (5e-OAc) in an over-all yield of about 49% (eq f). Hydrolysis gave 5e-OH. All exo brosylates were prepared by treatment of the alcohols with p-bromo-
(13) The ultraviolet spectrum of 6-methoxy-l,2,3,4-tetrahydro-1,4ethanonaphthalen-2-one in n-heptane shows a maximum at 234 mp ( e 8850) and that of 7-methoxy-l,2,3,4-tetrahydro-l,4-ethanonaphthalen-2one in n-heptane shows a shoulder at 220 mp (e -8700) (a private com(14) H. Tanida and R. Muneyuki, J . Amer. Chem. SOC.,87, 4794 munication from Dr. K. Takeda). See footnote 7 in ref la. (1965).
Tanida, et al.
1 Solvolysis Reactions of Brosylates
4514 benzenesulfonyl chloride in pyridine. The endo alcohols (2n-OH, 3n-OH, 4n-OH, and 5n-OH), needed as authentic samples in the products study, were prepared by oxidation of the corresponding exo alcohols to the respective ketones followed by reduction with diborane or lithium aluminum hydride. In order t o investigate the intermediate(s) involved in the solvolysis of Se-OBs, the optically active brosylate (6e-OBs) and the em-3 deuterium-substituted brosylate (7e-OBs) were prepared (Chart 11). Essentially using
optical purities of these compounds. Oxidation of 6e-OH gave optically active 6,7-dinitrobenzonorbornen2-one (6-0), which led to 6n-OAc by treatment with diborane followed by acetylation. The origin of optical activities of these compounds is that the inactive materials are a mixture of equal amounts of the two enantiomers (for example, A and B) and in the active
Chart I1
A
--
&,,, be-OAc
x
NO2 NO2* 6e-X
6nOAc
6-0
B
materials either one predominates. Since the origin is not affected by these chemical conversions, the optical purities in 6-0 and 6n-OAc are considered to be identical with that in 6e-OH, unless purification by crystallization is performed. The optical activities of the compounds are described in the Experimental Section, Deuterioboration of benzonorbornadiene with sodium borodeuteride and boron trifluoride proceeds to give exclusively the cis-em addition product, exo-3deuteriobenzonorbornen-2(exo)-ol(ge-OH), l7 which was subsequently transformed into exo-3-deuterio-6,7-dinitrobenzonorbornen-2(exo)-ol and its brosylate (7eOH and 7e-OBs). Mass spectral analysis of 7e-OAc indicated deuteration of 0.90 * 0.02 atom. Since, in the nmr spectrum, the exo-3 proton in 7e-OH and 7e-OAc overlaps with the C-9 protons, it is not possible to determine whether or not the deuterium locates at a position other than the exo-3. However, it is reasonable to assume that the synthesis does not involve rearrangement of the deuterium from the original C-2 to other carbons. Solvolysis Rates. The rates determined by titration of forming p-bromobenzenesulfonic acid are summarized in Table I, together with the derived activation parameters. The acetolyses were carried out in glacial acetic acid containing equivalent sodium acetate by the standard Because of the great reactivity of the 2e-OH system, the rate of solvolysis of 2e-C1 in 70 % aqueous acetone was determined and compared with that of the parent le-C1. For discussions, le-OBs and Se-OBs were solvolyzed in various solvents such as acetic acid, ethanol, aqueous ethanol, and aqueous dioxane, and the data obtained are listed in Table 11. Good first-order kinetics were observed in all the solvolyses. Theoretical infinity titers were obrained in the acetolyses, but infinity titers in some cases of the ethanolyses and hydrolyses were to some extent less than the theoretical values. In these cases, the observed infinity titers were used for calculation. The rate measurements of the change in the optical activities of 8e-OBs and 6e-OBs were carried out in the same buffered acetic acid as used for the titration rate. Linear first-order plots were found for both the brosylates. More than 99.9% racemization in the acetolysis of 8e-OBs was reported,5b whereas the optical activity in the acetolysis of 6e-OBs increased with the reaction time. The rate constants (k,) observed were 6.94 X sec-l at 50" and 2.67 X 10-5 sec-I at 25" for 8eOBs, and 1.08 X sec-I at 140" for 6e-OBs. Com-
&.-&--\
\
I
H 9e-OH
7e.X
the same method reported recently, l5 benzonorbornadiene was allowed to react with ( -)-diisocampheylborane16 obtained from ca. 86z optically pure (+)-apinene in diglyme. Oxidation with hydrogen peroxide in aqueous sodium hydroxide and acetylation gave the optically active benzonorbornen-2(exo)-yl acetate (8eOAc), which was purified by fractionation at a spinningband column. Hydrolysis in ethanolic potassium hydroxide gave 8e-OH. Subsequent transformations leading to 6e-OAc were the same as those described for Se-OAc. When 6e-OAc was hydrolyzed by treatment with lithium borohydride and then purified by recrystalliza~ (c 1.014, chloroform), was tion, 6e-OH, [ a I z 3+31.3 obtained. For an unambiguous interpretation of the experimental results, it is necessary that the following compounds have the same optical purity as this 6e-OH. Treatment of 6e-OH with acetic anhydride in pyridine and p-bromobenzenesulfonyl chloride in pyridine gave the materials for acetolysis, 6e-OAc and 6e-OBs, respectively. Yields of both of these esters were almost quantitative, so that there were no changes in the (15) D. J. Sandman, I
