Racemization of isobornyl chloride via carbocations: a non-classical look at a classic mechanism.

Henry S. Rzepa and Charlotte S. M. Allan

Interactive Table 1. B3LYP/cc-pVQZ calculated geometries for 3, 6, 8-exo, 8-endo and 9.a
Description: The Meerwein-Montefort(4) transition state 3.
Show: b
Show: c d
Relative ΔG:e 19.1; COI:f 10042/to-257 		Description: The non-classical carbocation 6
Show:
Show: g
Relative ΔG:e 0.0; COI: 10042/to-373
Description: The Houben-Pfankuch transition state 8-exo
Relative ΔG:e 11.3, COI: 10042/to-256		 Description: Stereoelectronically disfavoured 8-endo
Relative ΔG:e 19.6, COI: 10042/to-374
Description: Transition state 9
Relative ΔG:e 63.6, COI: 10042/to-375 		Description: Crystal structure of 11
Show:

aTo view the interactive components of this table, you will need a Java-enabled Web browser, and appropriate security settings to view the model rendered by the Jmol Java applet. bThe two-electron-3-center region is shown in orange. In this representation, C2-C6 appear connected by a bond, but this is only by virtue of their short internuclear separation, and implies nothing about the electronic structure of the region. cFour bond critical points are shown as light blue atoms, the single ring critical point is shown in darker blue. dThe ELF function, at an isosurface threshold of 0.659. The trisynaptic basin is shown surrounding the transferring hydrogen, colored red. eThe energy, corrected for thermal and entropic contributions relative to molecule 6, in kcal mol-1. fThe persistent object identifier resolves to a digital repository entry. This contains all the archival information necessary for viewing the molecule in a separate program, and has details of the complete calculation itself. gThe transferring methyl carbon is shown in green, along with the dihedral angle subtended to the adjacent carbocationic centre for both it and the alternative (non-migrating) methyl group.

Literature cited

  1. (a) For discussion of the naming convention, see Schartz, M. G.; J. Chem. Ed., 1987, 64, 92; (b) Traynham, J. G.; ibid, 1986, 63, 930. This article also reviews the historical importance of this species.
  2. Steglitz, J.; Am. Chem. J., 1899, 21, 101.
  3. Meerwein, H.; Van Emster, K. Chem. Ber., 1922, 55, 2500-2528, DOI: 10.1002/cber.19220550829.
  4. (a) The norbornyl racemisation was reported in Meerwein, H.; Wortmann, R. Ann.,1924, 435, 190-206. DOI 10.1002/jlac.19244350105 and is discussed further in the adjacent article; Meerwein, H; Montefort, E. Ann., 1924, 435, 207-218, DOI: 10.1002/jlac.19244350106. (b) For a fascinating analysis of this discovery and many related aspects, see Berson, J. "Chemical Creativity", 1999, Wiley, pp 116-136.
  5. (a) Eyring, H. Chem. Rev., 1935, 17, 65 - 77, DOI: 10.1021/cr60056a006; (b) Evans, M. G.; Polanyi, M. Trans. Faraday Soc., 1936, 32, 1333 - 1360, DOI: 10.1039/TF9363201333; (c) Glasstone, S. J. Chem. Soc., 1936, 723 - 724, DOI: 10.1039/JR9360000723.
  6. Lipp, P. Techn. Hochschule Aachen. Ber., 1920, 53B, 769-81.
  7. Houben, J.; Pfankuch, E. Ann.,1931, 489, 193; 1933, 501, 219-246, DOI 10.1002/jlac.19335010113
  8. (a) Roberts, J. D.; Yancey, J. A. J. Am. Chem. Soc., 1953, 75, 3165. DOI: 10.1021/ja01109a037 ; (b) Vaughan, W. R.; Perry, R. ibid, 3168. DOI: 10.1021/ja01109a038
  9. (a) Winstein, S.; Trifan, D. S. J. Am. Chem. Soc., 1949, 71, 2953 - 2953, DOI: 10.1021/ja01176a536. (b) Such a species was hinted at, but not explicitly discussed, in Nevell, T. P.; de Salas, E.; Wilson, C. L.J. Chem. Soc., 1939, 1188; DOI 10.1039/JR9390001188. (c) For more modern reviews, see Olah, G. A.; Surya Prakash, G. K.; Saunders, M. Acc. Chem. Res., 1983, 16, 440 - 448, DOI: 10.1021/ar00096a003; Brown, H. C. Acc. Chem. Res., 1983, 16, 432 - 440, DOI: 10.1021/ar00096a002
  10. Longuet-Higgins, H. C.; Bell, R. P. J. Chem. Soc., 1943, 250 - 255, DOI: 10.1039/JR9430000250
  11. These reactions have previously been studied by such computational techniques; (a) Haseltine, R.; Huang, E.; Ranganayakulu, K.; Sorensen, T. S. Canadian J. Chem., 1975, 53, 1056-66; (b) Schreiner, P. R.; Schleyer, P. v. R.; Schaefer, H. F.;. J.Org.Chem., 1997, 62, 4216-4228. DOI: 10.1021/jo9613388
  12. The following programs were used to perform the calculations. (a) Gaussian03 (revision E.01) was used for calculating the wavefunctions and optimizing the geometries of minima and transition states; Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A.; Gaussian, Inc., Wallingford CT, 2004. (b) Biegler-Konig F.; Schonbohm J.; J. Comp. Chem., 2002, 23, 1489-94. DOI: 10.1002/jcc.10085 The program can be downloaded at http://www.aim2000.de/. (c) Keith, T. A.; AIMAll (Version 08.05.04), 2008. URL: aim.tkgristmill.com (d) M. Kohout, DGrid, version 4.3, 2008.
  13. Born, M. and Oppenheimer, R., Annalen der Physik, 1927, 389, 457-484. DOI: 10.1002/andp.19273892002
  14. Edge protonation of cyclopropane was considered in the following articles: (a) Wolfram, K.; Bowen, L.; Schleyer, P. v. R. J. Am. Chem. Soc., 1989, 111 3479 - 3480; DOI: 10.1021/ja00191a082; (b) Lajunen, M. J. Chem. Soc., Perkin Trans. 2, 1986, 1551 - 1555, DOI: 10.1039/P29860001551; (c) Liu, K. -T. Tetrahedron Lett., 1978, 1129-32. DOI: 10.1016/S0040-4039(01)94479-X
  15. (a) Bader, R. F. W.; Atoms in Molecules: a Quantum Theory, Oxford University Press, Oxford, UK, 1990. (b) Popelier, P. L. A.; Atoms in Molecules: an Introduction, Prentice- Hall, London. (c) A concise and more complete explanation of many of the terms used here can be found in Matta, C. F.; Castillo, N.; and Boyd, R. J.; J. Phys. Chem., 2005, 109, 3669-3681. DOI: 10.1021/jp045044z
  16. (a) Bader, Richard F. W.; Johnson, S.; Tang, T.-H.; Popelier, P. L. A.. J. Phys. Chem. 1996, 100, 15398-15415. DOI: 10.1021/p961297  (b) Poater, J.; Duran, M.; Sola, M.; and Silvi, B., Chem. Rev., 2005, 105, 3911-3947. DOI: 10.1021/cr030085x  (c) Lepetit, C.; Silvi, B.; Chauvin, R., J. Phys. Chem. A, 2003, 107, 464-473. DOI: 10.1021/jp026521l
  17. Corey, E. J.; Sneen, R. A. J. Am. Chem. Soc., 1956, 78, 6269-6278. DOI: 10.1021/ja01605a013
  18. (a) Saunders, M.; Kronja, O., Carbocation Chemistry 2004, 213-235 (b) Saunders, M.; Kates, M. R. J. Am. Chem. Soc. 1983, 105, 3571-3. DOI: 10.1021/ja00349a034
  19. (a) Laube, T. Helv. Chim. Acta., 1994, 77, 943. Cambridge reference code: FITDIV01; (b) Laube, T.; Lohse, C. J. Am. Chem. Soc. 1994, 116, 9001-8. DOI: 10.1021/ja00099a018
  20. Abnormally long bonds are often artifacts of the crystal structure and its analysis, but the following appears to be the result of careful analysis of a hexaphenylethane derivative for which a central C-C length as long as 1.771(3) Å was reported; Kawai, H.; Takeda, T.; Fujiwara, K.; Inabe, T.; asnd Suzuki, T.; Cryst. Growth. Design, 2005, 5, 2256-2260. DOI: 10.1021/cg050064r
  21. Arguably, the seminal experiment which does define the very dawn of mechanistic organic chemistry is Beckmann's proposal of a planar enolate replacing a chiral center as the key intermediate in the isomerisation of menthone to isomenthone under the influence of acid, using again the polarimeter as the analytical instrument; Beckmann, E. Annalen, 1889, 250, 322, DOI: 10.1002/jlac.18892500306.