374
Structure of the Quadricyclic Carbonium Ion Sir: We wish to report new evidence in support of the structure 1 as best representing the carbonium ion intermediate derived from solvolysis of quadricyclic derivatives 2. We also wish to report evidence concerning the isomerization of this ion to the 7-norbornadienyl cation.
1
X
= (a)
2
C1,(b) OSO2CBH4CH8, (c) OS02CGH4Br
The charge-delocalized structure 1 was first proposed' to account for the remarkable reactivity of the quadricyclic derivatives investigated by Richey and Buckley2 and by ourselves.' It was found, for example, that 7quadricyclic chloride (2a) was less reactive than anti7-norbornenyl chloride (3)3 by a factor of only 100 and more reactive than 7-norbornyl chloride (4) by approximately IO9. Richey * pointed out that quadricyclic derivatives were only about 30-40 times more reactive than the corresponding tricyclic compounds (5) which contain a single cyclopropyl ring, Le., the net
noted, however, that we consider the delocalized representation, 1, to be only an operational one at this time, and it should not be considered to provide a brief against, for example, a a-complex formulation of the general type proposed by Dewar. lo We previously reported the reduction of quadricyclic brosylate (2c) with sodium borohydride in aqueous ethanol and in anhydrous diglyme using Brown's general procedure" in an attempt to determine the mode of charge delocalization in the ion, but were successful in isolating only quadricyclene and none of the tricyclic olefin (6) which should result from trapping 1 at carbon 5. We now find, however, that treatment of quadricyclic brosylate (2c) with sodium borohydride in 80 % aqueous diglyme gives a hydrocarbon product (1 5 yield, isolated) which consisted of approximately 10 % tricyclene (6) along with -89 quadricyclene and -1 norbornadiene. The tricyclene (6), of course,
H
Ha 6
6a
D 6b
could have arisen from reduction of the 7-norbornadienyl carbonium ion rather than from reduction of 1, an especially likely possibility since some norbornadiene was obtained. The source of 6 was determined by the following experiment. We have previously shown that lithium aluminum deuteride reduction of 7-norbornadienyl chloride yields 3 4 5 tricyclic olefin containing deuterium exclusively at endo-C-3 (6b) through examination of the olefin's effect of the second cyclopropyl ring was negligible, in The endo-C-3 contrast to its additive effect in other ~ystems.2,~ nuclear magnetic resonance spectrum. hydrogen of 6 was found to generate a quartet at As we have noted, however, this phenomenon appears 7 9.5 (Jab = 7.7 cps, J = 2.1 cps), and this multiplet to be the rule rather than the exception in the case of was found to be completely absent in the nmr spectrum homoallylic ions. The 7-norbornadienyl carbonium of the deuterated compound (6b). During the present ion5 and others6,' constitute examples in which a second investigation we have also treated 7-norbornadienyl double bond has little effect on the stability of the carchloride with sodium borodeuteride under the same bonium ion. Strong interaction of the cationic center conditions used for reduction of 2c and found the with more than one unsaturated center is probably pretricyclic product to consist solely of 6bI3 (within the vented by distortion of the ion into a delocalized struclimits of detection by the nmr method).I4 ture incorporating only one of the unsaturated centers.* On the other hand, sodium borodeuteride reduction The structure 1 was based, then, on Winstein's of 2c in aqueous diglyme yielded tricyclic olefin whose formulation of charge delocalization for the tricyclic nrnr spectrum contained a multiplet centered at 7 9.5, carbonium ion (the ion derived from 5).9 It should be representing approximately 10 of the product. Spin (1) P. R. Story and S. R. Fahrenholtz, J. Am. Chem. Soc., 86, 527 decoupling of the deuterium simplified this multiplet (1964). (2) H. G. Richey, Jr., and N. C. Buckley, ibid., 85, 3057 (1963). to a doublet (J = 2.1 cps), also centered at 7 9.5. (3) S. Winstein, M. Shatavsky, C. Norton, and R. B. Woodward, Consequently, this doublet was assigned to the endoibid., 77, 4183 (1955). C-3 hydrogen in 6a. The tricyclic product from so(4) It is noteworthy that the spatial relationship of the cation to the cyclopropyl rings in the quadricyclic carbonium ion appears, from dium borodeuteride reduction of 2c therefore consisted models, to be very nearly the same as it is in the preferred conformation
of the dimethylcyclopropylcarbonium ion as recently determined by Olah. See C. U. Pittman, Jr., and G. A. Olah, ibid., 87, 2998 (1965). ( 5 ) P. R. Story, L. C. Snyder, D. C. Douglas, E. W. Anderson, and R. L. Kornegay, ibid., 85, 3630 (1963). (6) 0. L. Chapman and P. Fitton, ibid., 85, 41 (1963). (7) C. H. DePuy, I. A. Ogawa, and J. C. McDaniels, ibid., 82, 2398 ( 1960).
(8) There is considerable evidence in favor of an unsymmetrical 7norbornadienylcarbonium ion structure, thereby promoting interaction of the positive center with one of the double bonds. See ref 5 and R. Hoffman, ibid., 86, 1259 (1964). (9) S. Winstein and E. M. Kosower, ibid., 81, 4399 (1959); S. Winstein, H. M. Walborsky, and I
