195
Stable Carbocations. CXLVI.' The 1,2-Dipheny1-2-norbornylCation. Comparison of c and ?r Delocalization in Phenylnorbornyl and Related Cations George A. Olah* and Gao Liang Contribution f r o m the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106. Received December 27, 1972
Abstract: The 1,2-diphenylnorbornylcation 1-H was prepared in FS03H-S02CIFsolution at -78" and examined by proton and carbon-13 nmr spectroscopy. Ion 1-H is characterized as a rapidly equilibrating carbenium ion undergoing fast 1,2 Wagner-Meerwein shift. The degree of u delocalization in 1-H is found to be very similar to that in the 1,2-dimethy1-2-norbornylcation, 2-CH3. n delocalization in the 2-methyl-2-norbornylcation is found to be more significant than in the 2-phenyl-2-norbornyl cation in which n delocalization dominates. Reasons for differing u and A delocalization are discussed.
T
he 1,2-dianisyl-2-norbornyl cation 1-OCH3 was shown by Schleyerz to possess a rapidly equilibrat-
.A room t e m p
7
l-OCH3
ing classical ion structure. In his studies the absence of bridging was rationalized as resulting from the unfavorable nonbonded interactions which would exist in the bridged ion when one or both of the aryl groups were oriented as to allow overlap between aryl ?r orbital and the electron-deficient center. Nickon's study of the 1,2-dimethoxy-2-norbornyl cation 2-OCH3 also
S-OCH,
showed that the ion did not possess a bridged structure. More recently we have reported that the 1,Zdimethyl2-norbornyl cation 2-CH3 is a rapidly equilibrating ion
.
H,C
\
CH, 2CH3
having a low energy barrier for the 1,2 Wagner-Meerwein shift.4 Due to less severe nonbonded interactions prevailing in 2-CH3, it was observed as a partially u(1) (a) Part CXLV: G. A. Olah and G. Liang, J. Amer. Chem. Soc., 96, 189 (1974). (b) Taken in part from the Ph.D. Thesis of G. Liang, Case Western Reserve University, 1973. (2) (a) P. v. R. Schleyer, D. C. Kleinfelter, and H. G. Richey, Jr., J . Amer. Chem. Soe., 85, 479 (1963); (b) D. C.Kleinfelter and P. v. R. Schleyer, J. Org. Chem., 26, 3740 (1961). (3) A. Nickon and Y . Lin, J. Amer. Chem. Soc., 91,6861 (1969). (4) G.A. Olah, J. R. DeMember, C. Y.Lui, and R. D. Porter, ibid., 93, 1442 (1971).
delocalized carbenium ion. The 1,2-dianisyl-2-norbornyl cation was considered to be an ion in which the u delocalization was of much smaller (if of any) importance than in 2-CH3. We report now our direct observation of the parent 1,2-diphenyl-2-norbornylcation 1-H under stable ion conditions and its 'H and 13C nmr spectroscopic investigation showing that it is a rapidly equilibrating carbenium ion similar in nature to 2-CH3, having a low energy barrier for the 1,2 Wagner-Meerwein shift and a similar degree of u delocalization. Several unsymmetrically substituted 2-norbornyl cations were also examined as to gain more information on the degree of u delocalization and the trend of charge delocalizati on. 5-7
Results and Discussion The 1,2-diphenyl-2-norbornylcation 1-H was prepared from 1,2-diphenyl-endo-2-norborneol zb in FSOaH-SO2C1F (SOz) solution at -78". The ion was not stable when allowed to warm to above -50" and gave unidentified decomposition (rearrangement) products.8a Quenching of the solution of 1-H with a slurry of KOH-S02CIF at -78" gave 5 0 z of 1,2-diphenyl2-n0rbornene,~~ besides some unidentified decomposition products. The temperature-independent ( - 135 to - 60") pmr spectra (60 MHz, Figure 1) of 1-H in FS03H-SO2 solution indicate the equivalence of the two phenyl groups.8b The four equivalent methylene protons, H3 and H,, are more deshielded (6 3.10) than the corresponding protons (6 2.42) in 2-CH3. The bridgehead proton (H4) in 1-H is also slightly more deshielded than that in 2CH3. The Ce protons, however, show comparable absorption for both ions. The phenyl ring protons in 1-H are unresolved indicating only a small extent of (5) (a) G.A. Olah and A. M. White, ibid., 91, 5801 (1969); (b) G. Frankel and D. G. Farnum, "Carbonium Ions," Vol. 1, G.A. Olah and P. v. R. Schleyer, Ed., Wiley-Interscience, New York, N. Y.,1968. (6) G.A. Olah, J . Amer. Chem. SOC.,94, 808 (1972); Chem. Brit., 8, 281 (1972); Chem. Technol., 556 (1971); Science, 168, 1298 (1970). (7) (a) G.A. Olah, J. R. DeMember, C. Y . Lui, and A. M. White, J. Amer. Chem. SOC.,91, 3958 (1969);(b) G.A. Olah, A. M. White, J. R. DeMember, A. Commeyras, and C. Y. Lui, ibid., 92,4627 (1970); (c) G.A.Olah, C. L. Jeuell, D. P. Kelly, and R. D. Porter, ibid., 94, 146 (1972). (8) (a) In our hands, trifluoroacetic acid* was not a suitable solvent to obtain the stable 1,2-diphenyl-2-norbornyl cation. (b) Slightly different chemical shifts for ion 1-Hin SO2 than in SOZCIFsolutions are noticed (see Table I). Better resolution is obtained in SO1 solution.
Olah, Liang / 1,2-Diphenyl-2-norbornyl Cation
1%
s 7
8
s
6
4
2
3
Figure 1. Pmr spectrum (60 MHz) of the 1,2-diphenyl-2-norbornylcation in FS03H-S02 solution at -70".
Table I. Pmr Parameters of 1,2-Diphenyl-2-norbornyl Cation and Model Cationsa 1-OH 2-CH3 3-CH3 3-CeHs
4.64(d, 6.5) 4.83 (d, 6.5)
4
4.09 3.81 4.30d
5 6
3.35 3.10" 2.42 2.70 3.49
3.35 3.10 2.86 2.70 3.20
2.10 1.82 1.52 1.47 1.41 4.09
3.02 2.75 2.86 3.28,1.09 2.00
3.35 3.10 2.42 1.71 2.04
2.42
2.20
2.20
3.81 2.42
4.30
7.6-8.2 7.3-7.9 2.43 3.00 8.66 8.71 8.70 9.00
7.90 7.87 7.89 8.16
8.44 8.46 8.50 8.78
= Chemical shifts are in 6 (ppm) relative to external TMS. Cationic solutions are obtained in FSOaH-SOzCIF unless otherwise indicated Aromatic carbons are indicated by o = ortho carbons, m = meta carbons, and p = paracarbons. In FSO3H-SO2. d In FS03H-SbF5SOzClF.
6
FS0,H-SO,
1-H
charge delocalization into the aromatic ring which is believed to be the result of unfavorable nonbonded interaction between the two phenyl rings that forces them out of plane for T-p overlap. To prove this latter point, several model phenylcycloalkylcarbenium ions (3-C6H6,4,$&5,9aand 69b) were studied and their pmr
4
5
6
(9) (a) D.G.Farnum and G. Mehta, J. Amer. Chem. Soc., 91, 3256 (1969). (b) This work, see Experimental Section.
Journal of the American Chemical Society
96:I
parameters are summarized in Table I. All these ions show similar characteristic phenyl ring patterns in their pmr spectra indicating substantial charge delocalization into the neighboring phenyl group. We therefore conclude that ion 1-H is a classical carbenium ion undergoing rapid degenerate 1,2 Wagner-Meerwein shift with little charge delocalization into the phenyl rings. The complete Fourier transform cmr spectrum of ion 1-H was also obtained and compared with those of the 2-phenyl-2-norbornyl cationlo* as well of the 1,2-dimethy1-'Ob and -2-methyl-2-norbornyl cations. loa Carbon shifts and corresponding coupling constants ( J c H ) for these ions are summarized in Table I1 with their assignments. The average cmr shifts for the CI and Ca carbons in ion 1-H ( 6 ~ cf22.5) is very close to that in the model ion 2-CH3 ( 6 ~ c+26.0) indicating that the former also is a rapidly equilibrating ion. The average of the cmr shifts of the C1 and Cz carbons in the dimethyl-tert-butylcarbenium ion 7 ( 6 1 8 ~ - 1 1)58 could be considered a model for a rapidly equilibrating structure of ion 1, without charge delocalization. The observed average cmr shift of 22.5 for 1-H apparently indicates that the ion is a tertiary rapidly equilibrating structure with limited charge delocalization. Cp in the 2-phenyl2-norbornyl cation 3-C6H6has been shown to have a (10) (a) Only partial cmr spectra were previously given for these (b) Minor corrections of cmr assignments are made: C3 in ion~.~*'*
