J. Org. Chem., Vol. 43, No. 19, 1978 3667
Structural Effects in Solvolytic Reactions
Structural Effects in Solvolytic Reactions. 27. Solvolysis of the e m - and endo-1,2-Diphenyl-2-norbornyl and - 1,2-Dimethyl-2-norbornyl p-Nitrobenzoates and Chlorides. Definitive Evidence for the Classical Nature of the 1,2-Disubstituted Tertiary 2-Norbornyl Cations and Implications for the Structure of the Parent 2-Norbornyl Cation’ Herbert C. Brown,* M. Ravindranathan,z C. Gundu Rao,2 Frank J. C h l ~ u p e kand , ~ Min-Hon Rei3 Richard B. Wetherill Laboratory, Purdue University, West Lafayette, Indiana 47907 Received March 27, 1978 exo- and endo-1,2-diphenyl- and -1,2-dimethyl-2-norbornyl p-nitrobenzoates were synthesized and their rates of solvolysis determined in 80% aqueous acetone. T h e tertiary chlorides were also synthesized and their rates of solvolysis measured in 10090ethanol. The exo/endo rate ratios for the solvolysis of 1,2-diphenyl-2-norbornylp-nitrobenzoates (350) and of 1,2-dimethyl-2-norbornyl p-nitrobenzoates (564) are similar to the ratios observed for the corresponding tertiary 2-phenyl and 2-methyl derivatives, as well as to those for the secondary 2-norbornyl tosylchlorides (178) is similar to the value ates. Similarly, the exo/endo ratio observed for the 1,2-dimethyl-2-norbornyl previously determined for the epimeric 2-norbornyl chlorides (170). Consequently, the presence of substituents a t the 2 position or a t the 1,2 positions has little effect upon the observed exoiendo rate ratios. The introduction of a 1-phenyl sulistituent into the 2-phenyl-2-norbornylp-nitrobenzoate does not increase, but decreases significantly the rate of solvolysis (by factors of 2 1 in the exo and 58 in the endo). A 1-methyl substituent, introduced into 2methyl-2-norbornylp-nitrobenzoate, increases the rate. The effect is the same in both the exo (8.5) and the endo (8.6). Similar effects were realized for the ethanolysis of the corresponding tertiary chlorides. The effects of the 1phenyl and 1-methyl substituents reveal the absence of significant charge delocalization from the 2 to the 1 position in the solvolyr,ic process. It is concluded that these tertiary derivatives must undergo solvolysis without u bridging and accompanying charge delocalization to the 1 position associated with such bridging. Yet the free-energy dip-nitrobenzoate is remarkably similar to 2-methyl-2-norbornagram for the solvolysis of 1,2-dimethyl-2-norbornyl yl p-nitrobenmate and to 2-norbornyl tosylate. It does not appear reasonable to attribute such similar behavior to the operation of totally different physical phenomena. Yet such has been claimed. Three such proposals which have been advanced are considered and refuted on the basis of available experimental data. Comparison of the rate of‘ solvolysis of 2-methyl-endo-norbornyl chloride with that for endo-norbornyl chloride reveals a relative rate of 53 000. Ignoring minor differences in the ground state energies, this yields a difference in the energies of the tertiary and secondary transition states of 6.5 kcal mol-’. This corresponds to an estimated difference in energy of the 2methyl-2-norbornyl cation and 2-norbornyl cation under stable ion conditions of 7.5 kcal mol-’ and a difference in the calorimetric heats of ionization of 2-methyl-exo-norbornylchloride and of exo-norbornyl chloride in S02ClF of 7.4 kcal mol-’. These results establish that the magnitude of the positive charge at the developing cationic center in these transition states must approach that in the intermediate ions or ion pairs, providing strong support i’or the validity of the Hammond postulate as applied to solvolytic processes. The similarity in the tertiary/secondary rate ratio for the exo isomers to the value for the endo isomers supports the absence of any significant nonclassical resonance contribution to the rate of solvolysis of exo-norbornyl derivatives. These data yield essentially identical values of 2-Me/2-H, 5.3 X lo4 for endo and 5.5 X lo4 for exo, incompatible with the presence of major nonclassical resonance contributions in the exo secondary and its reduction or absence in the exo tertiary. Other approaches for extrapolating from the tertiary derivatives to the secondary fail to support the presence of a major nonclassical resonance contribution in the exo secondary, absent in the endo Secondary and in the exo and endo tertiary derivatives, as postulated in some current proposals.
The proposed nonclassical structure (1) for the 2-norbornyl cation distributes positive charge from the 2 position to the 1 and 6 carbon atoms4 (eq 1).The nonclassical ion (1)
considered as significant contributors to the resonance hybrid (eq 2).
2
1’
1“
was considered to be a resonance hybrid of the three canonical structures, l’, l ” ,and 1’”. In this interpretation, the charge is not delocalized from the cationic center by hyperconjugation, but involves a specific u bridge, converting the unsymmetrical classical structure (corresponding to 1’) into the nonclassical structure (1) with a plane of symmetry. It was later suggested that this last structure (1’”) does not contribute significantly t o the resonance hybrid.5 Consequently, only canonical structures 1’ and 1’’ need now be
1’
This structure implies that significant portions of the positive charge of the carbocation are distributed equally to C - 1 and C-2. The transition state for a solvolytic process leading to such an intermediate is believed to be close to the carbocation produced.6 Consequently, in the transition state (4) for the solvolysis of an exo-norbornyl derivative (3), a significant portion of the developing positive charge a t C-2 should be delocalized to C-1 (eq 3). On the other hand, in the :3;
3
4
endo isomer (5) the 1,6-bonding pair is postulated to be stereoelectronically unfavorable for such participation (eq 4).
0022-326317811943-3667$01.00/0 0 1978 American Chemical Society
3668 J . Org. Chem., Vol. 43, No. 19, 1978
Brown et al.
(4)
5
6
Consequently, the mechanism for the delocalization of charge from C-2 to C-1, which operates in the exo isomer, cannot be effective in the transition state for the endo isomer (6). It follows that there should be far less delocalization of charge from C-2 to C-1 in 6, as compared to 4. Accordingly, a reasonable test of the nonclassical proposal for 2-norbornyl would appear to be an examination of the relative magnitudes of the charge delocalization from C-2 in the transition states for the solvolysis of appropriate exo- and endo-norbornyl derivatives. A frequently used test of this kind is the introduction of a phenyl or methyl group into the position being probed.lb These substituents provide electron density on demand to satisfy electron deficiencies at the carbon atoms to which they are attached. The 2 position in the transition state for a solvolyzing endo-norbornyl derivative should possess a far higher electron deficiency than the 2 position of the corresponding exo isomer, partially satisfied as the latter would be by electronic contributions from the 1,6-bonding pair. Therefore, 2-Me or 2-Ph should exhibit considerably greater activating effects on the endo isomers than on the corresponding exo isomers. It is not possible to test in this way for such charge delocalization at C-1. The introduction of a phenyl or methyl group at C-1 produces a species (7) which undergoes solvolysis only with rearrangement to the tertiary cation7 (8), subsequently converted to the tertiary derivatives (9) (eq 5 ) . Con-
8
7
9
sequently, such derivatives contain an internal driving force to produce in the solvolysis the more stable tertiary cation, so that it becomes difficult to utilize the effect of substituents at C-1 as a probe of the magnitude of the charge delocalization to c-1. Fortunately, these difficulties can be avoided by introducing the substituent into the related 2-substituted-2-norbornyl derivative. This structural modification converts the molecule into a system (10) which undergoes solvolysis into a tertiary cation (1 l), without. rearrangement to a more stable structure (eq 6). We are now in a position to observe whether a phenyl
11
10
12
or methyl substituent, represented by R, introduced into the exo isomer (13 10) is far more effective than the same substituent introduced into the endo isomer (14 15) (eq 7 and 8).
Note that the predicted effects of introducing such groups at C-2 are opposite to those for introducing the groups at C-1 for systems involving nonclassical resonance. At C-2 the rate-enhancing effect of the groups should be larger for the endo isomer. At C-1 the rate-enhancing effects of the groups should be larger for the exo isomer. I n the absence of nonclassical resonance, similar effects of such groups would be anticipated for both epimers. There was still another question we wished to answer. It has been argued that resonance is possible in the 2-norbornyl cation because the two canonical structures (1’ and 1”)are equivalent with identical energies (eq 2 ) . On the other hand, in a tertiary cation, such as 2-phenyl- or 2-methyl-2-norbornyl (16), the two canonical structures (16 and 17) differ greatly in energies. Whatever the level of resonance stabilization in a nonclassical 2-norbornyl system (1 = 1’ l”),it should be much less8 in 16 (eq 9). However, in symmetrically disubsti-
-
17
16
tuted 1,2-norbornyl cations, the two canonical structures (18’ and 18”)are again equivalent (eq 10). Will nonclassical reso-
la
18’
18”
nance, of the kind postulated to be present in 2-norbornyl (eq 2 ) , now return? If so, the 1-R substituent should exhibit an enormously greater effect in the exo isomer (eq 7 ) compared with the endo isomer (eq 8). Accordingly, we undertook to synthesize and to determine the rates of solvolysis of exo- and endo- 1,2-diphenyl-2-norp -nitrobenzoates in 80% bornyl and 1,2-dimethyl-2-norbornyl aqueous acetone and the corresponding chlorides for solvolysis in 100% ethanol. Finally, attention is called to the study of the related 1,2di-p-anisyl-2-norborny1,g 1,2-dipheny1-2-norbornyl,l0 and 1,2-dimethy1-2-n0rbornyl~~ cations under stable ion conditions, and to the highly pertinent studies on the solvolysis of optically active 1,2-dimethyl-2-norbornylderivative^.^^^^^ These studies will be incorporated into the discussion of the present results. Results Synthesis. l-Phenyl-exo-norbornan01~~ (19) was oxidized to 1-phenylnorbornanone (20) by aqueous chromic acid, utilizing the two-phase oxidation p r 0 ~ e d u r e . l Addition ~ of phenyllithium in ether provided 1,2-diphenyl-endo-norbornanolI4 (21),converted to the p-nitrobenzoate (22) by treatment with n-butyllithium and p-nitrobenzoyl chloride in THF16 (eq 11).
+
--+
(7)
PhLi 13
1,4
10
15
22
OPNB
I
ether
OH
J . Org. Chem., Vol. 43, No. 19, 1978 3669
Structural Effects in Solvolytic Reactions Major problems were encountered in the synthesis of the exo isomer. The standard procedure to obtain the exo isomer through solvolysis of the exo chloride (23) or the endo OPNB (22) failed. Only the corresponding olefin, 1,2-diphenylnorbornene (24), was produced (eq 12). Attempts to hydrate 24
24
hPh
by oxymercuration-demercuration17 or by synthesis of the epoxide18 followed by reduction19 failed. However, hydroboration of 24 by HBBr2/SMez20 followed by oxidation with alkaline hydrogen peroxide gave a mixture of approximately 10%of the desired 1,2-diphenyl-exo-norbornanol(25) with the isomeric secondary alcohol (26) (eq 13). The alcohol
35
36
31
to be no method now available to prepare 2-methyl-endonorbornyl chloride (38).
CI 38
Fortunately, treatment of 1,2-dimethyl-exo-norbornyl chloride (32) with hydrogen chloride takes another course. Here, isomerization to the secondary chloride is not feasible. Instead, there is an isomerization to another isomer, identified as 1,2-dimethyl-endo-norbornyl chloride (39) (eq 17).
26
mixture was converted into the mixed p-nitrobenzoates and most of the secondary isomer was removed by fractional crystallization. The resulting product, containing approximately 40% of the p-nitrobenzoate (27) of 25, proved satisfactory for the solvolytic study. The secondary p -nitrobenzoate showed no evidence of undergoing solvolysis under the conditions used to solvolyze the desired tertiary isomer. 2-Methyl-endo-norbornanol (28) was converted to 1methylnorbornanone (29) by means of a combined isomerization-oxidation technique. The ketone (29) was treated with methylmagnesium iodide to form 1,2-dimethyl-endo-norbornano121(30). The alcohol was then converted into the p nitrobenzoate (31) by the usual procedurelc (eq 14). The endo
Me 32
CI 39
The formation of the second isomer can be followed by t;ie appearance of two new methyl peaks in the 'H NMR spectrum and by the formation of an isomer which undergoes solvolysis at a rate 1/178 that of the original chloride. Both isomeric chlorides, which were not separated, undergo hydrolysis to the same alcohol (33). At equilibrium the two chlorides are present in the ratio 70% 32/30% 39. Equilibration of 1,2-Dimethyl-2-norbornanols.The exo and endo alcohols, 30 and 33, were dissolved in cyclohexane (0.2 M solutions) and isomerized under the influence of 6 M sulfuric acid a t room temperature. GC analysis of aliquots revealed an equilibrium distribution of 72% exo and 28Y0 endo, similar to the equilibrium distribution for the chlorides, 70% 32 and 30% 39. Rates of Solvolysis. The rates of solvolysis of the p-nitrobenzoates were determined in 80% aqueous acetone by the MeMgl (14) titrimetric procedure.le In our earlier studies,la we had utilized 60% aqueous dioxane as the solvolytic medium, following Bartlett and Stiles.25However, we observed that this medium was not a satisfactory solvent for the slower compounds. Competitive reaction with oxygen produced acid, resulting in erratic, somewhat high values for the slower endo derivatives. Accordingly, we shifted to aqueous acetone on the recalcohol (30) was converted into the exo chloride (32) by ommendation of R. C. Fort and P. v. R. Schleyer and this has treatment with hydrogen chloride in an automatic hydroproven to be a far superior medium. Accordingly, we redechlorinator.22The chloride was then hydrolyzed in buffered termined the kinetic data for the isomeric 1,2-dimethyl-2aqueous acetone to yield 1,2-dimethyl-exo-norbornanol(33), norbornyl p-nitrobenzoates utilizing aqueous acetone and which was converted into the p-nitrobenzoate16 (34) (eq only these data are here reported. The pertinent rate data are 15). summarized in Table I. Treatment of 2-methyl-exo-norbornanol(35) with hydroThe rates of solvolysis of the tertiary chlorides were detergen chloride yields 2-methyl-exo-norbornyl ~ h l o r i d e (36). ~ ~ s ~ ~ mined in 100% ethanol. The differential method was employed for determining the rate constants for 1,2-dimethyl-exo-and On further contact with hydrogen chloride rearrangement occurs to the secondary chloridez4(37) (eq 16).There appears -endo-norbornyl chlorides.Z6The available rate data for the
I
Brown et al.
3670 J. Org. Chem.,,Vol. 43, No. 19, 1978
Table I. Rates of Solvolysis of 1,2-Dimethyl-2-norbornyl and 1,2-Diphenyl-2-norbornylp-Nitrobenzoates and Related Derivatives in 80% Aaueous Acetone
AH*, OPNB
k l X lo6 s - ~ Tz, "C
isomer
T1,"C
exo endo exoe
94.6 (100) 54.7 (150) 40.0 (75)
6.94 (75) 5.41 (125) 1.91 (50)
endo,' exo endo exog endo"
41.4 (125)
3.11 (100) 179 (50) 30.2 (75) 9.86 (50) 17.9 (100)
2-Phd 1,2-Ph2
364 (100) 168 (75) 204 (125)
25 "C 0.010b 1.13 x 10-5 * 5.45 x 10-2 b (8.44 X 10-2)c 9.67 x 10-5 b 7.56 0.059 0.36b 1.03 x 10-3 b
kcal mol-'
AS*, eu
exo/ endo, 25 "C
26.3
-7.0
885
30.1
-7.5 -2.5
26.6 30.0 23.6 25.1 24.8 28.2
564 (875)c
-3.8
re1 rate, 25 "C 1.00
1.oo
5.45 (8.5)' 8.6
-2.7 -7.4
127
1.oo
-4.9 -5.2
350
0.048
1.00 0.0174
Values in parentheses are k , values, k , / k t = 1.55 in 90% aqueous a Reference 47. Calculated from data at higher temperatures. acetone at 25 "(2 (ref 12), assumed to be the same in 80% aqueous acetone. d Reference 46. e Registry no.: 13351-32-1.f Registry no.: 13351-31-0.g Registry no.: 67162-93-0. Registry no.: 67162-94-1.
"
___-
system 2-norbornyl
2-methyl-2-norbornyl 1,2-dimethyl-2-norbornyl
2-phenyl-2-norbornyl 1,2-dipheny1-2-norbornyl 2 - p -anisyl-2-norbornyl
Table 11. Rates of Ethanolysis for Norbornyl Chlorides isomer
registry no.
TI, "C
exon
765-91-3
175 (99.7)b
endo"
2999-06-6
exo endo exo endo exo exo exol
19138-54-6 6196- 86- 7 35033-23-9 6564-96-1 16166-72-6 67162-95-2
x
AHS
AS*
exo/ endo
0.0236d 26.6 (5.42 x 10-4)f 0.56 (85)c 1.40 X g (3.2 x 10-6)f 0.754 (0) 30.0 23.3 0.168" 6.03 (0) 210 22.6 1.18 9080 (0) 158000 17.9 542(0) 11300 19.1 2.55 X lo8
-4.8
170e
kl
10-6 s-1
Tzt "C 38.8 (85)c
25 "C
-1.2 178 -0.6 178 -2.2 -3.5
a In 80% ethanol. Reference 27b. Reference 27a. Calculated from data at higher temperatures. e Calculated from the exo/endo rate ratio at 85 "C (70) assuming constant entropy. f Rate constant in ethanol. Calculated from the rate of chloride in 80% ethanol using the factor for the tosylates, 121, 80% EtOH/kl, EtOH = 43.5; M.-H. Rei, Ph.D. Thesis, Purdue University, 1967. Calculated from chloride, assuming the effect of the exo/endo rate ratio of 170 at 25 "C. Calculated from the rate of 1,2-dimethyl-endo-norbornyl the 1-methyl substituent to be the same as in the exo isomers (kl,2.dimethyl.exo./kz.~e~hyl-exo-= 7.0). This assumption appears to be valid: see text. H. C. Brown and K. Takeuchi, J . Am. Chem. SOC.,88,5336 (1966).
ethanolysis of the tertiary and secondary 2-norbornyl chlorides are summarized in Table 11. We wished to compare the rates of ethanolysis of exo- and endo-norbornyl chlorides a t 25 "C with our values for the tertiary chlorides. Data were available for the solvolysis in 80% ethanol of exo-norbornyl chloride at 85 and a t 99.7 "C and of endo-norbornyl chloride a t 85 0C.27The exo/endo rate ratio of 70 a t 85 "C calculates to be 170 a t 25 "C, in good agreement with the value of 178 for 1,2-dimethyl-2-norbornyl chloride. The rate constant for the unknown 2-methyl-endo-norbornyl chloride was calculated from the rate for 1,2-dimethylendo- norbornyl chloride by assuming that the effect of the 1-methyl substituent was the same as in the exo isomers ( k 1,2.dimethyl.exl,./kP.methyl.exo. = 7.0). These values are included in Table 11. Alternatively, we could have estimated the rates for exoand endo -norbornyl chlorides by proceeding from the tosylates, correcting from tosylate to chloride.lb Similarly, we could have proceeded from the rate constant for 2-methyl-endonorbornyl p-nitrobenzoate (Table I) to the value for the chloride by correcting for the leaving group.28 In fact, the values are very similar. However, the procedure adopted involves much smaller corrections. I t also avoids the uncertainty involved in the usual assumption of a constancy in the correction factors for different leaving groups.29rs" Discussion As was pointed out earlier, the nonclassical 2-norbornyl cation (2) is stabilized by resonance involving two equivalent
canonical structures (1' and 1"). Such resonance should be considerably lower in tertiary 2-norbornyl cations where the two canonical structures (16 and 17) differ considerably in their energies.* It was a major objective of the present study to examine the possibility that the introduction of a substituent a t C-1, identical with that a t C-2 (18), would again provide two equivalent canonical structures (18' and IS"), resulting in an increase of the nonclassical resonance in the system. Such an increase could readily be detected in the transition state by comparing the effect of the substitutent a t C-1 in the exo isomer as compared to that produced in the endo isomer. The rate constant for ethanolysis of 2-methyl-endo-norbornyl chloride is 53 000 times that for endo- norbornyl chloride (Table 11). Ignoring small differences in the ground-state energies, this means that the transition state for the solvolysis of the tertiary chloride must be 6.5 kcal mol-' more stable than that for the corresponding secondary chloride. T o the extent that solvent participation contributes to the transition state for the secondary chloride, the difference will be even larger. However, recent studies have revealed that solvent participation is not significant in the solvolysis of both exo- and endo- norbornyl derivati~es.3~23~ Consequently, it appears reasonable to ignore what can only be quite minor contributions of this kind by the usual solvolytic media. According to the Hammond postulate, the transition state for solvolysis should be close to the first intermediate, the corresponding free ion or ion pair.6 Therefore, the secondary
J . Org. Chem., Vol. 43, No. 19, 1978 3671
Structural Effects in Solvolytic Reactions and tertiary 2-norbornyl cations (or ion pairs) should differ in energy by a quantity somewhat larger than 6.5 kcal mol-'. Under stable ion conditions, the tertiary 2-methyl-2-norbornyl cation is estimated to be some 7.5 kcal mol-' more stable than the secondary 2-norbornyl cation.33Similarly, the difference in the calorimetric heats of ionization of 2methyl-exo-norbornyl and exo-norbornyl chlorides in S02CIF These has recently been determined to be 7.4 kcal values are in excellent agreement with the value derived from the relative rates and the Hammond postulate.6 Resonance involving canonical structures which differ in energies by 6.5-7.5 kcal mol-' should not be large. In the same way, the two canonical structures for 2-phenyl-2-norbornyl cation (16, R = Ph) can be estimated to differ in energies by somewhat more than 1 2 kcal mol-'. The corresponding structures for the 2-p-anisyl-2-norbornylcation should differ in energies by some 15 kcal mol-'. Consider the consequences of introducing a substituent a t C-1, identical with the substituent a t C-2 (eq 10). In the endo isomer, the substituent will have little effect, since u bridging is postulated not to occur in the endo isomer (eq 8). However, in the exo isomer (eq :'), return of all or part of the resonance energy should bring about rate increases of 103-106 or even greater. Thus we should observe large unambiguous effects of the 1-R substituent in systems where nonclassical resonance returns or increases. The 1,2-Di-p-anisyl-2-norbornyl System. We did not undertake an examination of this interesting system. I t had been examined under stable ion conditions and unambiguous evidence had been obtained for its classical natureg (eq 18).
jugation in 41.35,36(However, such steric difficulties do not prevent the three p-anisyl groups in the more crowded trip -anisylmethyl cation from stabilizing the system: ~ K for R p-An3C+ 0.82; p-An2CH+ -1.24).37 Second, Winstein has suggested that highly stable 2-norbornyl cations, stabilized by extreme groups, such as 2-p-anisyl, should be classical.38 However, he implied that less stabilizing groups, such as phenyl and methyl, could provide nonclassical cations. Accordingly, these were the focus of our studies. The 1,2-Diphenyl-2-norbornylSystem. The introduction of a phenyl group a t the 2 position of norbornyl increases the rate of hydrolysis over the parent compound by a factor of approximately lo9 (Table 11).The introduction of a phenyl group into the 1 position (43)in 2-phenyl-endo-norbornyl p-nitrobenzoate (42)does not increase the relative rate (RR), but decreases it by a factor of 58 (eq 19). Evidently, the com-
RR(25'C)
40
1158
bined inductive and steric effects of the 1-Ph substituent are responsible; no significant electronic contribution is anticipated for this isomer. The critical case is the exo isomer. The presence of a 1-Ph substituent in 1,2-diphenyl-exonorbornyl p-nitrobenzoate (45) does not result in any enhanced rate. Indeed, there is an actual decrease by a factor of 21 over the parent compound (44) (eq 20).
RR(25'C)
Me
1.00
1.00
1/21
In the corresponding chlorides (46 and 47)!the factor is similar, 16 (eq 21).
40'
The ion was generated in concentrated sulfuric acid. The UV spectrum was similar to that of the 2-p-anisyl-2-norbornyl cation, without the changes anticipated for extended conjugation in a symmetrical cr-bridged cation (41).
Ph RR(25'C)
1.00
1/16
The exo/endo rate ratio for 45/43 is 350, slightly larger than the value of 127 realized for the parent system 44/42. This increase in the exo/endo rate ratio arises not from any increase in the rate of the exo isomer (45), but from a comparative decrease in the rate of the endo isomer (43).Possibly, the decrease arises from the enhanced steric difficulties afforded by the 1-Ph substituent to the departure of the endo leaving The 2-p-anisyl-2-norbornylcation is half formed from the carbinol in 41% sulfuric acid. To form the ion 40 * 40' requires more (51%),not less concentrated sulfuric acid. This is not in accord with greater stabilization of the cation by the cumulative effect of two p-anisyl groups, as in 41. The 2-p-anisyl-2-norbornyl cation does not react easily with bromine, evidently because the p -anisyl group is conjugated with the cationic center. The 1,2-di-p-anisyl cation takes up bromine in
