2764
J. A m . Chem. SOC.1985, 107, 2764-2772
Bridgehead Adamantyl, Diamantyl, and Related Cations and Dications a George A. Olah,*lb G. K. Surya Prakash,lb Joseph G. Shih,lb V. V. Krishnamurthy,lb Gheorge D. Mateescu,lCGao Liang,lCGyorgy Sipos,ld Volker Buss,le Tamara M. Gund,le and Paul v. RaguC Schleyer*“ Contribution from the Donald P . and Katherine B. Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, California 90089, and the Institute fur Organische Chemie, Friedrich- Alexander Universitat, Erlangen-Nurnburg, 8520 Erlangen. West Germany. Received July 23, 1984
Abstract: The bridgehead 1-adamantyl cation 3a and a series of methyl-substituted adamantyl cations 3b-d have been prepared in superacid medium and characterized by IH and 13CNMR spectroscopy. The relative deshielding of bridgehead y carbons with respect to @-methyleneshas been rationalized in terms of a C-C hyperconjugative effect. The cation generated from the ionization of exo-5,6-trimethylene-2-norbornyl derivatives has been shown to be the equilibrating 2,3-trimethylene-2-norbornyl cation 12. The temperature-dependent 13CNMR spectroscopic studies on 12 indicate the barrier for the degenerate exo2,3-hydrogen shift, AG’ = 7.2 kcal/mol. The cation 12 in a solution of SOzCIFis stable even at room temperature; however, in a solution of SO,, it smoothly and irreversibly rearranges to bridgehead 1-adamantyl cation 3a. On the other hand, 2-methyl-5,6-exo-trimethylene-2-norbornyl cation 17 was found to be quite stable. The 1-adamantyl cation was also studied as its isolated SbF6- salt by solid-state CPMAS ”C NMR spectroscopy. The bridgehead 1- and 4-diamantyl cations 4 and 27 were also prepared and studied. Heteronuclear-correlated 2D NMR spectra were also obtained for 3a and 4 and used in many assignments of the NMR shifts. Cation 27 slowly and irreversibly rearranges to 4. The cation 4 was also generated by the ionization of 3-diamantyl derivatives. The above rearrangements to the thermodynamically more stable ion 4 has been rationalized in terms of facile intermolecular hydrogen shifts rather than intramolecular shifts. The bridgehead diamantyl cations 4 and 27 also derive their stability from C-C hyperconjugative effects similar to their adamantyl analogues. In contrast to the instability of secondary 3-diamantyl cation the corresponding 3-methyl- and 3-phenyl-substituted analogues were found to be rather stable. 3,3’-(1,l’-Biadamantyl) dication 6 and 4,9-diamantyl dication 5 were also generated and studied. The C-C hyperconjugative stabilizing interaction which is found “operative” in all of the bridgehead cations including dication 6 is absent in the 4,9-diamantyl dication 5. Charge-charge repulsion in 5 seems to diminish such C-C hyperconjugative interaction. All attempts to prepare the 1,3-adamantyl dication 8 or the 1,6-diamantyl dication 7 were unsuccessful.
Over the past 2 decades, extensive research has been carried out pertaining to the chemistry of adamantane, diamantane, and other so-called diamondoid m o l e c ~ l e s .The ~ ~ ~unique structural features of adamantane (1) and diamantane ( 2 ) are also ideal for a systematic study of their cations, relating to both structure and chemical reactivity. The tight interlocking of cyclohexane rings
They were characterized by ‘H NMR ~ p e c t r o s c o p y . W ~ ~e~now wish to describe in detail the preparation and ‘H and 13CN M R spectroscopic characterization of bridgehead adamantyl, substituted adamantyl, related bridgehead diamantyl, and 3-substituted diamantyl cations. Studies relating to the formation of 3a from trimethylenenorbornyl derivatives have been included along with the cross-polarization magic angle spinning I3C N M R spectroscopic study of 3a in its crystalline state (as its SbF,- salt). The I3C and ‘H N M R chemical shifts in 3a and 4 were also crosscorrelated by using heteronuclear correlated two-dimensional (2D N M R ) spectra. We also describe the preparation and spectroscopic characterization of the related 4,9-diamantyl dication 5
2
into rigid, relatively strain-free chair conformations allows no easy formation of double bonds and no back-side (electrophilic or nucleophilic) attack. In our preliminary work, the preparation and study of the 1-adamantyl cation 3a and the 1-diamantyl cation 4 were reported. +
8 7
(1) (a) Stable Carbocations. 258. For part 257, see: Krishnamurthy, V. V.; Iyer, P. S.; Prakash, G. K. S . ; Olah, G. A. J . Am. Chem. SOC.1984, 106, 7068. (b) University of Southern California. (c) Case Western Reserve University. (d) Visiting Scientist from the University of Szeged, Hungary. (e) Princeton University. (f) Friedrich-Alexander Universitat, Erlangen, Nurnberg. (2) (a) Fort, R. C., Jr. “Adamantane, The Chemistry of Diamond Molecules”; Marcel Dekker: New York, 1976. (b) Balaban, A. T.; Schleyer, P. v. R. Tetrahedron 1978, 34, 3599. (c) Schleyer, P. v. R. J . A m . Chem. SOC.1957, 79, 3292. (3) McKervey, M. A. Tetrahedron Lett. 1980, 36, 971.
(congressane dication) and the 3,3’-( 1,l’-biadamantyl) dication 6 . Attempted preparation of the 1,6-diamantyl dication 7 and the 1,3-adamantyI dication 8 were, however, unsuccessful.
Results and Discussion 1-AdamantylCation 3a and Related Methyl-Substituted Cations (3b-d). The bridgehead 1-adamantyl cation 3a can be readily prepared by treating adamantane ( 1 ) with superacids (4) Schleyer, P. v. R.; Fort, R. C., Jr.; Watts, W. E.; Comisarow, M . 9.; Olah, G. A. J . A m . Chem. SOC.1964, 86, 4195. ( 5 ) Olah, G. A,; Lukas, J. J . A m . Chem. SOC.1968, 90, 933.
0002-7863/85/ 1507-2764$01.50/0 0 1985 American Chemical Society
J . Am. Chem. Soc., Vol. 107, No. 9, 1985 2165
Adamantyl and Diamantyl Cations and Dications
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Of particular interest is the generation of 3a starting from
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Figure 1. ’H N M R spectrum of I-adamantyl cation 3a at (a) 14.09 kG (60 MHz), (b) 23.48 kG (100 MHz), and (c) 58.70 kG (250 MHz).
(FS03H/SbF5or HF/SbF5) with or without low nucleophilicity solvents (SO2or S0,ClF). The ion 3a can also be generated from a variety of precursors including 1- and 2-substituted adamantanes.6-8 The mechanism of the rearrangement of the 2NSO
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2-exo-(chloro)hydroxy-exo-trimethylenenorbornanein SbF5/S02 solution at temperatures above -30 OC (vide infra).’ The 60-MHz ‘H N M R spectrum of the 1-adamantyl cation 3a (Figure 1) consists of three unresolved multiplets at 6 2.30 (6 H), 4.19 (6 H) and 5.19 (3 H ) (Table I). The relative intensities of the signals require assignment of the lowest field peak to the y (bridgehead) protons. This indicates that /3 protons which are nearest to the (formal) positive charge are not as deshielded as the y protons. A better resolved spectrum is obtained at 100 MHz, \.,herein the 6 protons are observed as an AB pattern. At 250 MHz, this AB pattern is well resolved and JAB= 15.6 Hz. The unusual fact that in the 1-adamantyl cation 3a the y protons absorb at a lower field than the @ ones has been explained in terms of a cage e f f e ~ t .Originally, ~ this effect was interpreted4 in terms of an interaction of the “empty” p orbital of the trivalent carbocation center with the back lobes of three sp3 bridgehead C-H orbitals which converge toward the (tetrahedral) center of the molecule. This geometrically favored overlap would account
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for selective sharing of the positive charge by the bridge-head positions. Indeed extended Hiickel calculations suggest stabilization to the extent of 1.2 kcal/mole for such interactions.l0JI However, subsequent M O calculations by Hoffmann, Gleiter, and Heilbronner12-14indicated that such interactions are unlikely in adamantyl systems wherein the overlapping orbitals are intervened by only two bonds. More recent work by Sunko and Hehre15 has pointed out that resonance structures such as 3a 3a’ are responsible for the y-deshielding effect. In a way this can
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adamantyl cation formed as an intermediate in the latter case has been discussed earlier.8.9
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(6) Mateescu, G. D. Ph.D. Dissertation, Case Western Reserve University, 1971. ( 7 ) Liang, G. Ph.D. Dissertation, Case Western Reserve University, 1973. (8) Olah, G . 4.;Liang, G.; Mateescu, G. D. J. Org. Chem. 1974,39, 3750. (9) A rapid intermolecular hydride exchange specifically involving bridgehead hydrogens has been ruled out since no change in the ‘H NMR spectrum occurred over a temperature range of -90 to +120 ‘C. Exceptionally high barriers have been estimated for such a process; see: Vogel, P.; Saunders, M.; Thielecke, W.; Schleyer, P. v. R. Tetrahedron Lett. 1971, 18, 1429. Also, see: Schleyer, P. v. R.; Lam, L. K. M.; Raber, D. J.; Fry, J. L.; McKervey, M. A.; Alford, G. R.; Cuddy, B. D.; Keizer, V. G.; Geluk, A. W.; Schlatman, J. L. M. A. J. A m . Chem. SOC.1970, 92, 5246.
3a‘
(10) Shimizu, K.; Miyamichi, K.; Kato, H.; Yonezawa, T. Nippon Kagaku Zasshi 1969, 90, 1206; Chem. Abstr. 1970, 72, 7080a.
(1 1) Fujimoto, H.; Kitagawa, Y.; Hao, H.; Fukui, K. Bull. Chem. SOC. Jpn. 1970, 43, 52. (12) Hoffmann, R. Acc. Chem. Res. 1971, 4, 1. (13) Gleiter, R.; Hoffmann, R.; Stohrer, W.-D. Chem. Ber. 1972, 105, 8. (14) Hoffmann, R.; Mollere, P. D.; Heilbronner, E. J. A m . Chem. SOC. 1973, 95, 4860.
(15) Sunko, D. E.; Starcevic, S . H.-; Pollack, S . K.; Hehre, W. J. J . A m . Chem. SOC.1979, 101, 6163.
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J . Am. Chem. SOC.,Vol. 107, No. 9, 1985
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