J. Am. Chem. SOC.1995,117, zyxwvu 5983-5991 5983 The zyxwv 1-(Trimethylsily1)bicyclobutonium Ion: NMR Spectroscopy, Isotope Effects, and Quantum Chemical zy Ab Initio Calculations of a New Hypercoordinated Carbocation Hans-Ullrich Siehl,**"$ Martin FUSS: and Jiirgen Gauss* Contribution from the Institut fiir Organische Chemie der Universitat Tubingen, 0-72076 Tubingen, Germany, Institute for Fundamental Research of Organic Chemistry, Kyushu University, Fukuoka, 813 Japan, and Lehrstuhl fiir Theoretische Chemie, Institut zy fiir Physikalische Chemie, Universitat Karlsruhe, 0-76128 Karlsruhe, Germany Received February 24, 1995@ Abstract: The 1 -(trimethylsilyl)bicyclobutonium ion is generated from (1'-(trimethylsily1)cyclopropyl)methanol by reaction with SbF5. The NMR spectroscopic data in S02ClF/S02F2 solution at -128 "C are in accord with a bridged puckered bicyclobutonium structure undergoing a 3-fold rapid degenerate rearrangement that renders the two zyx p- and one y-methylene groups equivalent, leading to one averaged I3C-NMR signal for the CH2 groups at 48.9 ppm. Conformational ring inversion is slow so that two separate signals for the three averaged endo-CH2 (4.04 ppm) and three averaged exo-CH:! hydrogens (3.24 ppm) are observed. The deuterium equilibrium isotope effects for exo- and endo-CHD-labeled cations are different in sign and magnitude and are rationalized by different endo- and exo-C-H bond force constants at the pentacoordinated carbon. NMR chemical shift calculations for the 1-silylbicyclobutonium ion and the (1'-silylcyclopropy1)methyl cation were performed with the GIAO-SCF and GIAO-MP2 methods. The experimental shifts are satisfactorily reproduced by GIAO-MP2//tzp/dzcalculated shifts for the 1-silylbicyclobutonium structure. The good agreement between theory and experiment supports a fully degenerate set of interconverting 1-(trimethylsily1)bicyclobutonium ions and excludes contributions from other isomers to the observed equilibrium process. The geometric and electronic properties of the 1-(trimethylsily1)bicyclobutonium cation are intermediate between those of the parent bicyclobutonium ion and those of the methyl-substituted analogue. The cyclobutyllcyclopropylmethyl cation system has been of continuing interest for many years.' On the basis of NMR spectroscopic investigations,2 isotope effect experiment^,^ and quantum chemical calculations of geometry, chemical shifts, and vibrational frequencies? the parent system [C4H71f is now best described as a degenerate set of rapidly interconverting bicyclobutonium ions (1) with minor contributions from another degenerate set of rapidly equilibrating cyclopropylmethyl cations which are only marginally higher in energy than 1 zyxwvut (< 1 kcal mol-'). For the analogous 1-methyl-substituted cation [C4H6- ' Universitat Tubingen. Kyushu University. zyxwvutsrqpon 5 Universitat Karlsruhe. @Abstract published in Advance ACS Abstracts, May 15, 1995. (1) For recent reviews, see: (a) Olah, G. A.; Reddy, V. P.; Prakash, G. K. S. Chem. Rev. zyxwvutsrqpon 1992, 92, 69. (b) Lenoir, D.; Siehl, H.-U. In Houben- Weyl Merhoden der Orgunischen Chemie; Hanack, M., Ed.; Thieme: Stuttgart, Germany, 1990; Vol. E19c, p 413. (c) Olah, G. A,; Prakash, G. K. S.; Sommer, J. In Superucids; Wiley: New York, 1985; p 143. (d) In accord with our earlier work (refs 3, 7, 9, and 28), we use solid lines in all formulas to symbolize the connectivity and coordination of the atoms and not necessarily the nature of the bonding. (2) (a) Staral, J. S.; Yavari, I.; Roberts, J. D.; hakash, G. K. S.; Donovan, D. J.; Olah, G. A. J. Am. Chem. SOC. 1978, 100, 8016. (b) Yannoni, C. S.; Myhre, P. C.; Webb, G. G. J. Am Chem. SOC. 1990, 112, 8992. (3) Saunders, M.; Siehl, H.-U. J. Am. Chem. SOC. 1980, 102, 6868. (4) (a) Koch, W.; Liu, B.; DeFrees, D. J. J. Am. Chem. SOC. 1988, 110, 7325. (b) McKee, M. L. J. Phys. Chem. 1986, 90, 4908. (c) Saunders, M.; Laidig, K. E.; Wiberg, K. B.; Schleyer, P. v. R. J. Am. Chem. SOC. 1988, 110, 7652. (d) VanEik, H.; Gabelica, V.; Sunko, D. E.; Buzek, P.; Schleyer, P. v. R. J. Phys. Org. Chem. 1993, 6, 427. (5) Olah, G. A.: Kelly, D. P.; Jeuell, C. L.; Porter, R. D. J. Am. Chem. SOC. 1970, 92, 2544. (6) (a) Saunders, M.; Rosenfeld, J. J. Am. Chem. SOC. 1970, 92, 2548. (b) Olah, G. A.; Jeuell, C. L.; Kelly, D. P.; Porter, A. D. J. Am. Chem. SOC. 1972, 94, 146. (c) Olah, G. A.; Spear, R. J.; Hiberty, P. C.; Hehre, W. J. J. Am. Chem. SOC. 1976, 98, 7470. (d) Kirchen, R. P.; Sorensen, T. S. J. Am. Chem. SOC. 1977, 99, 6687. (e) Olah, G. A,; Prakash, G. K. S.; Donovan, D. J.; Yavari, J. J. Am. Chem. SOC. 1978, 100, 7085. 0002-7863/95/1517-5983$09.00/0 CH3]+,6the investigations of equilibrium isotope effects on the NMR spectra of and dideuteratedcationsss9 and quantum chemical ab initio calculationsI0 agree that this cation system can be adequately described by considering only one degenerate set of hypercoordinated puckered methylbicyclobutoniumion structures (2) without contributions from (1'-methylcyclopropy1)- methyl cation structures. In contrast, the isotope effects observed for the corresponding trideuterio-substituted cation have been interpreted by invoking the presence of a minor species in equilibrium with the major isomer.'' The nature of the postulated minor species, however, is not clear.' A trimethylsilyl group in the a-position to the positive charge in carbocations is destabilizing compared to an a-methyl group but has a stabilizing effect compared to an a - h y d r ~ g e n . ' ~ . ' ~ This has been shown by Apeloig and Stanger on the basis of solvolytic studies and quantum chemical calculation^'^ and has recently been confirmed by NMR spectroscopic investigations of a-silyl-substituted benzyl cations in s01ution.I~ (7) Siehl, H.-U. J. Am. Chem. SOC. 1985, 107, 3390. (8) Prakash, G. K. S.; Arvanaghi, M.; Olah, G. A. J. Am. Chem. SOC. (9) Siehl, H.-U. In Physical Organic Chemistry 1986; Kobayashi, M., (10) (a) Schleyer, P. v. R.; Gauss, J.; Buzek, P. Private communication. (11) Saunders, M.; Krause, N.; J. Am. Chem. SOC. 1988, 110, 8050. (12) (a) In The chemistry of organic silicon compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 2, p 193. (b) A recent review: Lambert, J. B. Tetrahedron 1990, 46, 2677. For the effect of an a-silyl group in carbanions and radicals, see: Zhang, S.; Zhang, X.- M.; Bordwell, F. G. J. Am. Chem. Sac. 1995, 117, 602. (13)Bausch. M. J.; Gong, Y. J. Am. Chem. SOC. 1994, 116, 5963. (14) (a) Apeloig, Y.; Stanger, A. J. Am. Chem. SOC. 1985, 107, 2806. (b) Stang, P. J.; Ladika, M.; Apeloig, Y.; Stanger, A,; Schiavelli, M. D.; Hughey, M. R. J. Am. Chem. SOC. 1982, 104, 6852. (15) Kaufmann, F.-P. Ph.D. Thesis, University of Tubingen, 1992. 1985, 107, 6017. Ed.; Elsevier: Amsterdam, 1987; p 25. (b) Buzek, P. Ph.D. Thesis, University of Erlangen, 1993. 0 1995 American Chemical Society