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Additions to Bicyclic Olefins. 111. Stereochemistry of the Epoxidation of Norbornene, 7,7-Dimethylnorbornene, and Related Bicyclic Olefins. Steric Effects in the 7,7-Dirnethylnorbornyl System Herbert C. Brown,* James H. Kawakami,’ and Shiro Ikegami2 Coritributioti f r o m the Richard B . Wetherill Laboratory of Purdue Unicersity, Lafrryefte, Indiana 47907. Receiced April 26, 1970 Abstract: A systematic study of the stereochemistry of epoxidation of norbornene, 7,7-dimethylnorbornene, and related bicyclic olefins was undertaken in order to establish more precisely the importance of steric effects in controlling the direction of concerted additions to such systems. The epoxidation of norbornene proceeds almost exclusively exo, 99%, whereas the corresponding epoxidation of 7,7-dimethylnorbornene takes place preferentially from the opposite direction, 88-94z endo. 1-Methylnorbornene, 2methylnorbornene, and 1,7,74rimethylnorbornene exhibit the same steric pattern of reaction. Similarly, epoxidation of the exocyclic double bond in 2-methylenenorbornane gives the em-epoxide preferentially, 86%, whereas the related 2-methylene-7,7-dimethylnorbornane gives the endo isomer preferentially, 84%. Therefore, in the case of concerted additions, such as the epoxidation reaction of the present study and the hydroboration reaction of an earlier study, the normal course of addition to olefins of the norbornene type is almost exclusively exo in the absence of 7,7-dimethyl substituents, and predominantly endo in their presence. This controlling influence of the 7,7-dimethyl substituents is exerted even in the case of these concerted additions to the exocyclic double bonds of 2-methylenenorbornane and 2-methylene-7,7-dimethy Inorbornane.
I
t has been argued t h a t t h e almost exclusive exo substitution realized in t h e solvolysis of 7,7-diniethylnorbornyl derivatives requires t h e intermediacy of a u-bridged norbornyl cation. This position was based o n the observation t h a t complex metal hydrides, such a s lithium aluminum hydride: and sodium borohydride,!’, attack norcamphor preferentially from the exo direction (l), whereas these reagents attack 7,7-dimethylnorcamphor preferentially from t h e endo direction (2). In contrast, solvolyses of both norbornyl ? j 4
2
1
(3) and 7,7-dimethylnorboinyl (4) derivatives, proceeding through the corresponding ions or ion pairs, give the exo products almost exclusiiely. It was proposed
3
4
t h a t t h e failure of t h e 7,7-dimethyl substituents t o control the stereochemistry of substitution in the cation * To M honi corrcspoiirleiice shotild be dtlrlresscd. ( I ) Gr‘irluatc research assistmt on grniits ( G 19878 and G P 6492 X) supporter1 hy the National Science Foundation. (2) Postdoctorate rcscarch associate on a grant ( G P 6492 X) supported hy t h e National Sciciice Fouiidatioii. ( 3 ) J . A . Berjon, ”Molecul,~rRearranMemciits,” P a r t I, P . d c M a ) o , Ed,, Intcrscicnce, Ne\\ York, N . Y . , 1963, Chaptcr 3. (4) S. Winstcin, et nl., J . A n i e r . Chori. Soc., 87, 376, 378, 379, 381 (1964).
( 5 ) S. Beckmann and R .
Merger, Cherii. E r r . , 89, 2738 (1956). (6) H . C. Broun and J . Murrio, J . A n i c r . Chem. Soc., 88, 2811 (1966).
(or ion pair) in t h e same manner that these methyl groups control t h e direction of attack by complex hydride required something “ ~ p e c i a l , ” B~ bridging ’~ in t h e cation (or ion pair). However, it was pointed o u t t h a t very little is known about the relative steric requirements for the reaction of complex hydrides with ketones and t h e reaction of cations or ion pairs with solvents.’ I n fact, recent studies demonstrate t h a t i n t h e base-catalyzed deuterium exchange exo substitution is favored i n both norcamphor ( 5 ) a n d camphor (6),5j9a reaction which cannot involve u-bridged intermediates.
4
H C,
,CH, exo/endo = 21
exo,’endo = 715 0
0 5
6
!t appeared desirable t o undertake a systematic study of reactions of norbornene a n d 7,7-diniethylnorbornene derivatives in order t o realize a better understanding of t h e precise factors influencing t h e direction of reaction i n the norbornyl system, and the precise influence of 7,7-diniethyl substituents o n the stereochemical course. Accordingly, w e have examined hydroboration,”’ epoxidation (reported in t h e present paper), oxyniercuration,’ hydrochlorination,12 and other addition reactions of norbornene. 7,7-diniethylnorbornene, and (7) H . C . Broun, Cherii. Brit,, 2, 199 (1966). ( 8 ) (a) A . F. Thomas and B. Willhalm, Tetrahedroiz Lett., 1309 (1965); (b) J. M. Jerkunica, S . BorEiS, and D. E. Sunko, ibid., 4465 (1965); (c) A . F. Thomas, R . A . Schneider, and J . Mcinwald, J . Anwr. Chem. Soc., 89, 68 (1967). (9) T . T . Tidwell, ibid., 92, 1448 (1970). ( I O ) H . C. Brown and J . H. Kawakami, ibid., 92, 1990 (1970). ( 1 1 ) H. C. Brown, J . H . Kawakami, and S . Ikcgami, ibid., 89, 1525 (1967).
(12) H. C. Brown and K.-T. Liu, ibid., 89,466,3898,3900(1967).
J o i ~ m n lof rhe Alllerican Clietiiical Socier,, , 92:23 / Nocertiber 18, 1970
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related 01efins.I~ These studies have led us t o a new, promising interpretation of the steric influence of the 7,7-diniethyl substituents in reactions of the norbornyl system14 a n d has provided a considerable a m o u n t of new information which will have t o b e accounted for in a complete theory of the behavior of the norbornyl cation. This paper presents the results of our study of the stereochemistry of epoxidation of norbornene, 7,7-diniethylnorbornene, a n d related olefins.
Results and Discussions The epoxidation of norbornene has been previously described in the literature, but no unequivocal data for the precise aniount of exo a n d e n d o isomers in the epoxide product has been reported. In part, this deficiency arises from the tendency of these labile epoxides t o rearrange or deconipose while being analyzed by gas chromatography. l 6 It might b e considered that this difficulty could b e overconie by reduction of the epoxides t o the isomeric alcohols, followed by glpc analysis of the latter. However, the usual lithium aluminuni hydride reduction of norbornene oxide to form the alcohol is very slow a n d is often accompanied by rearrangement.I7 Fortunately, we were able t o circumvent these difficulties. First, we established glpc conditions which permitted us to analyze norbornene oxide and many related bicyclic epoxides, without the coniplicating rearrangements or deconipositions that had troubled earlier workers.Ifi Second, we developed a new convenient procedure for the facile reduction of such epoxides without rearrangement, utilizing lithium in ethylenediamine.” These developments permitted us t o determine precisely the stereochemistry of epoxidation of norbornene, 7,7-dimethylnorbornene, and related bicyclic olefins. The epoxidation o f norbornene with rn-chloroperbenzoic acid in methylene chloride at 25” for 1 hr, followed by reduction of the product with lithium in ethylene diamine (eq l), gives an 8 7 x yield of 99.0% exonorbornanol (7), 0.3 endo-norbornanol (8), 0.5 % nortricyclanol (9), and 0.2 7-norbornanol (lo), indicating almost exclusive exo addition. In contrast,
x
7
,OH
I
OH
--
9
10
the epoxidation of 7,7-dimethylnorbornene at 25 O for 27 hr, followed by reduction (eq 2), gives a n 87 % yield of 6 exo- (11) a n d 94 endo-7,7-dimethylnorbornanol (12), indicating a strong preference for e n d o addition.
x
Hx[ZO]
[Li]
I
OH 12
Analysis of the norbornene epoxide by both glpc a n d pmr indicated the presence of the exo isomer in a yield in the range of 98-99.5%:. Consequently, the three methods indicate almost exclusive exo attack by the reagent ( ~ 9 9 % ) . Similarly, b o t h glpc a n d p m r examination of the product from epoxidation of 7,7dimethylnorbornene indicated the presence of 12 of the exo isomer. This is higher t h a n the 6 x indicated by the lithium ethylenediamine method. However, t h e results clearly indicate that the epoxidation of this olefin occurs preferentially e n d o (88-94 endo). Competitive epoxidation experiments indicated that norbornene reacts with rn-chloroperbenzoic acid at a rate approximately 100 times that of 7,7-dimethylnorbornene. If we assign kendofor norbornene, a n arbitrary value of 1.00, then k,,, in norbornene will have a value of 100-200, k,,, in 7,7-diniethylnorbornene will have a value of roughly 0.1, a n d k e n d o in 7,7-dimethylnorbornene will also have a value of approximately one, very similar t o that for kendo norbornene. Consequently, the presence of t h e 7,7-dimethyl substituents decreases the rate of exo attack of the reagent by a factor of approximately 1000, causing the reaction to proceed predominately endo, a n d causing the overall rate t o b e slower by a factor of approximately 100. Similar stereochemical results were realized with 1-methylnorbornene (99 % exo), 2-methylnorbornene (99.5 % exo), a n d 1,7,7-trimethylnorbornene (5 % exo). Consequently, we appear to be observing a consistent pattern of behavior. T h e stereochemical influence of the 7,7-diinethyl substituents is exerted in this reaction even when the double bond is exocyclic, a t t h e corner of the bicyclo[2.2. llheptane structure. Thus, 2-niethylenenorbornane (13) gives 86 % exo-epoxide, whereas 2-methylene-7,7diniethylnorbornane (14) gives 84 e n d o product.
z
x
8
(13) For detailed reviews of electrophilic additions to olefins, with pertinent literature refercnces, see R . C. Fahey, Top. Stereochcm., 3, 237 (1968); P. B. D. de In Mare and R. Bolton, “Electrophilic Additions to Unsaturated Systems,” Elsevier, Amsterdam, 1966; T. G. Traylor, Accounts Chem. Res., 2 , 152 (1969). (14) H. C. Brown and J. H. Kawakami, J . Amer. Chem. Soc., 9 2 , 201 (1970). (15) (a) H. M . Walborsky and D. F. Loncini, ibid., 76, 5396 (1954); (b) H . Kwart and W. G. Vosburgh, ibid., 76, 5400 (1954); (c) S. B. Soloway and S . J. Cristol, J . Org. Chem., 25, 327 (1960); (d) H. ]
