Breaking Down Barriers: The Liaison between Sigmatropic Shifts, Electrocyclic Reactions, and Three-Center Cations

Angewandte Chemie - International Edition

Volumn 42, Issue 47, 2003, Pages 5877-5882

  Hoffmann, Roald ^a   Tantillo, Dean J ^b  

^a Cornell University ^*  (United States)

^b UNIVERSITY OF CALIFORNIA   (United States)

Author keywords

Carbocations; Electrocyclic reaction; Pericyclic reaction; Proton affinity; Sigmatropic rearrangement; Transition states

Indexed keywords

ELECTRONS; POSITIVE IONS;

SIGMATROPIC SHIFTS;

PROTONS;

CATION;

ARTICLE; BINDING AFFINITY; CHEMICAL REACTION; CHEMICAL STRUCTURE; MOLECULAR INTERACTION; MOLECULAR STABILITY; PROTON TRANSPORT; REACTION ANALYSIS; STRUCTURE ANALYSIS; SURFACE PROPERTY;

EID: 0347419941     PISSN: 14337851     EISSN: None     Source Type: Journal    
DOI: 10.1002/anie.200352853     Document Type: Article

References (51)

1. From the all-cisoid reactant conformer; see: a) D. J. Tantillo, R. Hoffmann, Helv. Chim. Acta, in press;
2. b) D. J. Tantillo, C. Ranjan, R. Hoffmann, unpublished results.
3. For leading references on calculations and experiments on other related sigmatropic shifts, see: a) C. W. Spangler, Chem. Rev. 1976, 76, 187-217;
4. b) D. J. Tantillo, R. Hoffmann, J. Am. Chem. Soc. 2002, 124, 6836-6837;
5. c) I. V. Alabugin, M. Manoharan, B. Breiner, F. D. Lewis, J. Am. Chem. Soc. 2003, 125, 9329-9342;
6. d) O. Dmitrenko, R. D. Bach, R. R. Sicinski, W. Reischl, Theor. Chem. Acc. 2003, 109, 170-175;
7. e) B. A. Hess, Jr., J. Org. Chem. 2001, 66, 5897-5900;
8. f) N. J. Saettel, O. Wiest, J. Org. Chem. 2000, 65, 2331-2336;
9. g) reference [1a].
10. Cationic three-center two-electron arrays have been studied extensively; for leading references, see: a) J. E. McMurry, T. Lectka, Acc. Chem. Res. 1992, 25, 47-53;
11. b) M. Saunders, H. A. Jimenez-Vazquez, Chem. Rev. 1991, 91, 375-397;
12. c) C. A. Grob, Acc. Chem. Res. 1983, 16, 426-431;
13. d) H. C. Brown, Acc. Chem. Res. 1983, 16, 432-440.
14. a) Other researchers have made similar analogies for neutral sigmatropic shifts. See, for example, K. N. Houk, Y. Li, J. D. Evanseck, Angew. Chem. 1992, 104, 711-739; Angew. Chem. Int. Ed. Engl. 1992, 31, 682-708; and F. Jensen, J. Am. Chem. Soc. 1995, 117, 7487-7492;
15. a) Other researchers have made similar analogies for neutral sigmatropic shifts. See, for example, K. N. Houk, Y. Li, J. D. Evanseck, Angew. Chem. 1992, 104, 711-739; Angew. Chem. Int. Ed. Engl. 1992, 31, 682-708; and F. Jensen, J. Am. Chem. Soc. 1995, 117, 7487-7492;
16. a) Other researchers have made similar analogies for neutral sigmatropic shifts. See, for example, K. N. Houk, Y. Li, J. D. Evanseck, Angew. Chem. 1992, 104, 711-739; Angew. Chem. Int. Ed. Engl. 1992, 31, 682-708; and F. Jensen, J. Am. Chem. Soc. 1995, 117, 7487-7492;
17. b) it has been proposed that the nature (protonic or hydridic) of the migrating hydrogen in sigmatropic shifts can vary considerably, depending on the nature of the hydrocarbon framework over which it migrates; see: S. D. Kahn, W. J. Hehre, N. G. Rondan, K. N. Houk, J. Am. Chem. Soc. 1985, 107, 8291-8292; see also: I. V. Alabugin, M. Manoharan, B. Breiner, F. D. Lewis, J. Am. Chem. Soc. 2003, 125, 9329-9342.
18. b) it has been proposed that the nature (protonic or hydridic) of the migrating hydrogen in sigmatropic shifts can vary considerably, depending on the nature of the hydrocarbon framework over which it migrates; see: S. D. Kahn, W. J. Hehre, N. G. Rondan, K. N. Houk, J. Am. Chem. Soc. 1985, 107, 8291-8292; see also: I. V. Alabugin, M. Manoharan, B. Breiner, F. D. Lewis, J. Am. Chem. Soc. 2003, 125, 9329-9342.
19. For leading references, see: a) R. V. Williams, Chem. Rev. 2001, 101, 1185-1204;
20. b) H. Quast, M. Seefelder, C. Becker, M. Heubes, E.-M. Peters, K. Peters, Eur. J. Org. Chem. 1999, 2763-2779;
21. c) R. V. Williams, Eur J. Org. Chem. 2001, 227-235.
22. All calculations were performed with GAUSSIAN98 (Gaussian98 (Revision A.9), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 1998.
23. Geometries were optimized (without symmetry constraints) at the B3LYP/6-31G(d) level (A. D. Becke, J. Chem. Phys. 1993, 98, 5648-5652; A. D. Becke, J. Chem. Phys. 1993, 98, 1372-1377; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785-789; P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem. 1994, 98, 11 623-11 627), whose effectiveness in describing structures and energetics for pericyclic reactions is well-documented (see, for example: O. Wiest, D. C. Montiel, K. N. Houk, J. Phys. Chem. A 1997,101, 8378-8388; K. N. Houk, B. R. Beno, M. Nendel, K. Black, H. Y. Yoo, S. Wilsey, J. K. Lee, J. Mol. Struct. 1997, 398-399, 169-179; D. A. Hrovat, B. R. Beno, H. Lange, H.-Y. Yoo, K. N. Houk, W. T. Borden, J. Am. Chem. Soc. 2000, 122, 7456-7460); all structures were characterized by frequency calculations at the B3LYP/6-31G(d) level, and zeropoint energy corrections (scaled by 0.9806 as recommended in A. P. Scott, L. Radom, J. Phys. Chem. 1996, 100, 16502-16513) from these calculations are included in the reported energies. Ball-and-stick drawings were produced with Ball & Stick (N. Müller, A. Falk, Ball & Stick V.3.7.6, molecular graphics application for MacOS computers, Johannes Kepler University Linz, 2000).
24. Geometries were optimized (without symmetry constraints) at the B3LYP/6-31G(d) level (A. D. Becke, J. Chem. Phys. 1993, 98, 5648-5652; A. D. Becke, J. Chem. Phys. 1993, 98, 1372-1377; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785-789; P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem. 1994, 98, 11 623-11 627), whose effectiveness in describing structures and energetics for pericyclic reactions is well-documented (see, for example: O. Wiest, D. C. Montiel, K. N. Houk, J. Phys. Chem. A 1997,101, 8378-8388; K. N. Houk, B. R. Beno, M. Nendel, K. Black, H. Y. Yoo, S. Wilsey, J. K. Lee, J. Mol. Struct. 1997, 398-399, 169-179; D. A. Hrovat, B. R. Beno, H. Lange, H.-Y. Yoo, K. N. Houk, W. T. Borden, J. Am. Chem. Soc. 2000, 122, 7456-7460); all structures were characterized by frequency calculations at the B3LYP/6-31G(d) level, and zeropoint energy corrections (scaled by 0.9806 as recommended in A. P. Scott, L. Radom, J. Phys. Chem. 1996, 100, 16502-16513) from these calculations are included in the reported energies. Ball-and-stick drawings were produced with Ball & Stick (N. Müller, A. Falk, Ball & Stick V.3.7.6, molecular graphics application for MacOS computers, Johannes Kepler University Linz, 2000).
25. Geometries were optimized (without symmetry constraints) at the B3LYP/6-31G(d) level (A. D. Becke, J. Chem. Phys. 1993, 98, 5648-5652; A. D. Becke, J. Chem. Phys. 1993, 98, 1372-1377; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785-789; P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem. 1994, 98, 11 623-11 627), whose effectiveness in describing structures and energetics for pericyclic reactions is well-documented (see, for example: O. Wiest, D. C. Montiel, K. N. Houk, J. Phys. Chem. A 1997,101, 8378-8388; K. N. Houk, B. R. Beno, M. Nendel, K. Black, H. Y. Yoo, S. Wilsey, J. K. Lee, J. Mol. Struct. 1997, 398-399, 169-179; D. A. Hrovat, B. R. Beno, H. Lange, H.-Y. Yoo, K. N. Houk, W. T. Borden, J. Am. Chem. Soc. 2000, 122, 7456-7460); all structures were characterized by frequency calculations at the B3LYP/6-31G(d) level, and zeropoint energy corrections (scaled by 0.9806 as recommended in A. P. Scott, L. Radom, J. Phys. Chem. 1996, 100, 16502-16513) from these calculations are included in the reported energies. Ball-and-stick drawings were produced with Ball & Stick (N. Müller, A. Falk, Ball & Stick V.3.7.6, molecular graphics application for MacOS computers, Johannes Kepler University Linz, 2000).
26. Geometries were optimized (without symmetry constraints) at the B3LYP/6-31G(d) level (A. D. Becke, J. Chem. Phys. 1993, 98, 5648-5652; A. D. Becke, J. Chem. Phys. 1993, 98, 1372-1377; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785-789; P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem. 1994, 98, 11 623-11 627), whose effectiveness in describing structures and energetics for pericyclic reactions is well-documented (see, for example: O. Wiest, D. C. Montiel, K. N. Houk, J. Phys. Chem. A 1997,101, 8378-8388; K. N. Houk, B. R. Beno, M. Nendel, K. Black, H. Y. Yoo, S. Wilsey, J. K. Lee, J. Mol. Struct. 1997, 398-399, 169-179; D. A. Hrovat, B. R. Beno, H. Lange, H.-Y. Yoo, K. N. Houk, W. T. Borden, J. Am. Chem. Soc. 2000, 122, 7456-7460); all structures were characterized by frequency calculations at the B3LYP/6-31G(d) level, and zeropoint energy corrections (scaled by 0.9806 as recommended in A. P. Scott, L. Radom, J. Phys. Chem. 1996, 100, 16502-16513) from these calculations are included in the reported energies. Ball-and-stick drawings were produced with Ball & Stick (N. Müller, A. Falk, Ball & Stick V.3.7.6, molecular graphics application for MacOS computers, Johannes Kepler University Linz, 2000).
27. Geometries were optimized (without symmetry constraints) at the B3LYP/6-31G(d) level (A. D. Becke, J. Chem. Phys. 1993, 98, 5648-5652; A. D. Becke, J. Chem. Phys. 1993, 98, 1372-1377; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785-789; P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem. 1994, 98, 11 623-11 627), whose effectiveness in describing structures and energetics for pericyclic reactions is well-documented (see, for example: O. Wiest, D. C. Montiel, K. N. Houk, J. Phys. Chem. A 1997,101, 8378-8388; K. N. Houk, B. R. Beno, M. Nendel, K. Black, H. Y. Yoo, S. Wilsey, J. K. Lee, J. Mol. Struct. 1997, 398-399, 169-179; D. A. Hrovat, B. R. Beno, H. Lange, H.-Y. Yoo, K. N. Houk, W. T. Borden, J. Am. Chem. Soc. 2000, 122, 7456-7460); all structures were characterized by frequency calculations at the B3LYP/6-31G(d) level, and zeropoint energy corrections (scaled by 0.9806 as recommended in A. P. Scott, L. Radom, J. Phys. Chem. 1996, 100, 16502-16513) from these calculations are included in the reported energies. Ball-and-stick drawings were produced with Ball & Stick (N. Müller, A. Falk, Ball & Stick V.3.7.6, molecular graphics application for MacOS computers, Johannes Kepler University Linz, 2000).
28. Geometries were optimized (without symmetry constraints) at the B3LYP/6-31G(d) level (A. D. Becke, J. Chem. Phys. 1993, 98, 5648-5652; A. D. Becke, J. Chem. Phys. 1993, 98, 1372-1377; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785-789; P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem. 1994, 98, 11 623-11 627), whose effectiveness in describing structures and energetics for pericyclic reactions is well-documented (see, for example: O. Wiest, D. C. Montiel, K. N. Houk, J. Phys. Chem. A 1997,101, 8378-8388; K. N. Houk, B. R. Beno, M. Nendel, K. Black, H. Y. Yoo, S. Wilsey, J. K. Lee, J. Mol. Struct. 1997, 398-399, 169-179; D. A. Hrovat, B. R. Beno, H. Lange, H.-Y. Yoo, K. N. Houk, W. T. Borden, J. Am. Chem. Soc. 2000, 122, 7456-7460); all structures were characterized by frequency calculations at the B3LYP/6-31G(d) level, and zeropoint energy corrections (scaled by 0.9806 as recommended in A. P. Scott, L. Radom, J. Phys. Chem. 1996, 100, 16502-16513) from these calculations are included in the reported energies. Ball-and-stick drawings were produced with Ball & Stick (N. Müller, A. Falk, Ball & Stick V.3.7.6, molecular graphics application for MacOS computers, Johannes Kepler University Linz, 2000).
29. Geometries were optimized (without symmetry constraints) at the B3LYP/6-31G(d) level (A. D. Becke, J. Chem. Phys. 1993, 98, 5648-5652; A. D. Becke, J. Chem. Phys. 1993, 98, 1372-1377; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785-789; P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem. 1994, 98, 11 623-11 627), whose effectiveness in describing structures and energetics for pericyclic reactions is well-documented (see, for example: O. Wiest, D. C. Montiel, K. N. Houk, J. Phys. Chem. A 1997,101, 8378-8388; K. N. Houk, B. R. Beno, M. Nendel, K. Black, H. Y. Yoo, S. Wilsey, J. K. Lee, J. Mol. Struct. 1997, 398-399, 169-179; D. A. Hrovat, B. R. Beno, H. Lange, H.-Y. Yoo, K. N. Houk, W. T. Borden, J. Am. Chem. Soc. 2000, 122, 7456-7460); all structures were characterized by frequency calculations at the B3LYP/6-31G(d) level, and zeropoint energy corrections (scaled by 0.9806 as recommended in A. P. Scott, L. Radom, J. Phys. Chem. 1996, 100, 16502-16513) from these calculations are included in the reported energies. Ball-and-stick drawings were produced with Ball & Stick (N. Müller, A. Falk, Ball & Stick V.3.7.6, molecular graphics application for MacOS computers, Johannes Kepler University Linz, 2000).
30. Geometries were optimized (without symmetry constraints) at the B3LYP/6-31G(d) level (A. D. Becke, J. Chem. Phys. 1993, 98, 5648-5652; A. D. Becke, J. Chem. Phys. 1993, 98, 1372-1377; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785-789; P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem. 1994, 98, 11 623-11 627), whose effectiveness in describing structures and energetics for pericyclic reactions is well-documented (see, for example: O. Wiest, D. C. Montiel, K. N. Houk, J. Phys. Chem. A 1997,101, 8378-8388; K. N. Houk, B. R. Beno, M. Nendel, K. Black, H. Y. Yoo, S. Wilsey, J. K. Lee, J. Mol. Struct. 1997, 398-399, 169-179; D. A. Hrovat, B. R. Beno, H. Lange, H.-Y. Yoo, K. N. Houk, W. T. Borden, J. Am. Chem. Soc. 2000, 122, 7456-7460); all structures were characterized by frequency calculations at the B3LYP/6-31G(d) level, and zeropoint energy corrections (scaled by 0.9806 as recommended in A. P. Scott, L. Radom, J. Phys. Chem. 1996, 100, 16502-16513) from these calculations are included in the reported energies. Ball-and-stick drawings were produced with Ball & Stick (N. Müller, A. Falk, Ball & Stick V.3.7.6, molecular graphics application for MacOS computers, Johannes Kepler University Linz, 2000).
31. Geometries were optimized (without symmetry constraints) at the B3LYP/6-31G(d) level (A. D. Becke, J. Chem. Phys. 1993, 98, 5648-5652; A. D. Becke, J. Chem. Phys. 1993, 98, 1372-1377; C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785-789; P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem. 1994, 98, 11 623-11 627), whose effectiveness in describing structures and energetics for pericyclic reactions is well-documented (see, for example: O. Wiest, D. C. Montiel, K. N. Houk, J. Phys. Chem. A 1997,101, 8378-8388; K. N. Houk, B. R. Beno, M. Nendel, K. Black, H. Y. Yoo, S. Wilsey, J. K. Lee, J. Mol. Struct. 1997, 398-399, 169-179; D. A. Hrovat, B. R. Beno, H. Lange, H.-Y. Yoo, K. N. Houk, W. T. Borden, J. Am. Chem. Soc. 2000, 122, 7456-7460); all structures were characterized by frequency calculations at the B3LYP/6-31G(d) level, and zeropoint energy corrections (scaled by 0.9806 as recommended in A. P. Scott, L. Radom, J. Phys. Chem. 1996, 100, 16502-16513) from these calculations are included in the reported energies. Ball-and-stick drawings were produced with Ball & Stick (N. Müller, A. Falk, Ball & Stick V.3.7.6, molecular graphics application for MacOS computers, Johannes Kepler University Linz, 2000).
32. -1 in terms of electronic energies, and disappears when zero-point energy corrections are included.
33. For a discussion of a different sort of connection between the properties of sigmatropic and electrocyclic transition structures (specifically in transition structures for certain rearrangments of α-lactams), see: D. J. Tantillo, K. N. Houk, R. V. Hoffman, J. Tao, J. Org. Chem. 1999, 64, 3830-3837.
34. Only anionic and neutral electrocyclic transition structures and neutral and cationic sigmatropic transition structures are shown, since protonation of cationic electrocyclic transition structures would lead to dicationic sigmatropic transition structures and deprotonation of anionic sigmatropic transition structures would lead to dianionic electrocyclic transition structures.
35. [4a]
36. Calculated nucleus-independent chemical-shift (NICS) values (GIAO-B3LYP/6-31G(d); for details on the NICS method, see: P. von R. Schleyer, C. Maerker, A. Dransfeld, H. J. Jiao, N. J. R. von E. Hommes, J. Am. Chem. Soc. 1996, 118, 6317-6318; P. von R. Schleyer, M. Manoharan, Z.-X. Wang, B. Kiran, H. Jiao, R. Puchta, N. J. R. von E. Hommes, Org. Lett. 2001, 3, 2465-2468) for all of these transition structures are negative (ranging from - 13.6 to-7.2). For small systems, there is no clear connection between the NICS calculated for the electrocyclic and sigmatropic transition structures, but for larger systems, nearly identical NICS values were calculated for the electrocyclic and corresponding sigmatropic transition structures (-11.0 for the anionic 8π electrocyclic transition structure and -11.1 for the neutral [1,7] transition structure, and -9.6 for the neutral 8π electrocyclic transition structure and -9.9 for the cationic [1,8] transition structure). This reflects comparable degrees of delocalization in these structures, consistent with the similarities of their geometries. Deviations in the smaller systems may be due to differences in ring size for the electrocyclic and sigmatropic transition structures affecting the proximity of the points at which NICS values were calculated to the carbon-carbon bonds; such problems should be minimized in the larger systems.
37. Calculated nucleus-independent chemical-shift (NICS) values (GIAO-B3LYP/6-31G(d); for details on the NICS method, see: P. von R. Schleyer, C. Maerker, A. Dransfeld, H. J. Jiao, N. J. R. von E. Hommes, J. Am. Chem. Soc. 1996, 118, 6317-6318; P. von R. Schleyer, M. Manoharan, Z.-X. Wang, B. Kiran, H. Jiao, R. Puchta, N. J. R. von E. Hommes, Org. Lett. 2001, 3, 2465-2468) for all of these transition structures are negative (ranging from - 13.6 to-7.2). For small systems, there is no clear connection between the NICS calculated for the electrocyclic and sigmatropic transition structures, but for larger systems, nearly identical NICS values were calculated for the electrocyclic and corresponding sigmatropic transition structures (-11.0 for the anionic 8π electrocyclic transition structure and -11.1 for the neutral [1,7] transition structure, and -9.6 for the neutral 8π electrocyclic transition structure and -9.9 for the cationic [1,8] transition structure). This reflects comparable degrees of delocalization in these structures, consistent with the similarities of their geometries. Deviations in the smaller systems may be due to differences in ring size for the electrocyclic and sigmatropic transition structures affecting the proximity of the points at which NICS values were calculated to the carbon-carbon bonds; such problems should be minimized in the larger systems.
38. + model system, a bent structure with a C⋯C distance of 2.1 Å is preferred.
39. "Transition-state acidity" has been discussed previously; for leading references, see: J. L. Kurz, Acc. Chem. Res. 1972, 5, 1-9 and A. B. Watts, H. Patel, J. Theor. Biol. 2001, 209, 417-429. This concept does deal with the differences in rate for reactions involving protonated and unprotonated transition states, but is often (although not always) cast in terms of the protonation state of a catalytic molecule, rather than that of a transition structure itself. To our knowledge, this concept has not previously been applied to the analysis of pericyclic reactions.
40. "Transition-state acidity" has been discussed previously; for leading references, see: J. L. Kurz, Acc. Chem. Res. 1972, 5, 1-9 and A. B. Watts, H. Patel, J. Theor. Biol. 2001, 209, 417-429. This concept does deal with the differences in rate for reactions involving protonated and unprotonated transition states, but is often (although not always) cast in terms of the protonation state of a catalytic molecule, rather than that of a transition structure itself. To our knowledge, this concept has not previously been applied to the analysis of pericyclic reactions.
41. a) F. A. Carroll, Perspectives on Structure and Mechanism in Organic Chemistry, Brooks/Cole, Pacific Grove, 1998, and references therein;
42. b) http://webbook.nist.gov/chemistry/;
43. c) based on B3LYP6-31G(d) calculations on the reactants for the reactions corresponding to the transition structures in Figure 5.
44. It was previously reported that the disrotatory "transition state" for the cyclobutene-butadiene interconversion is actually a second-order saddle point; see: J. Breulet, H. F. Schaefer III, J. Am. Chem. Soc. 1984, 106, 1221-1226.
45. It was shown previously that formally forbidden electrocyclic transition structures can be stabilized significantly through complexation to certain transition metals; see: D. J. Tantillo, R. Hoffmann, Helv. Chim. Acta 2001, 84, 1396-1404; D. J. Tantillo, R. Hoffmann, J. Am. Chem. Soc. 2001, 123, 9855-9859, and references therein.
46. It was shown previously that formally forbidden electrocyclic transition structures can be stabilized significantly through complexation to certain transition metals; see: D. J. Tantillo, R. Hoffmann, Helv. Chim. Acta 2001, 84, 1396-1404; D. J. Tantillo, R. Hoffmann, J. Am. Chem. Soc. 2001, 123, 9855-9859, and references therein.
47. We say "partially" because the Jahn-Teller-like situation in the forbidden transition structure is replaced upon protonation by a second-order Jahn-Teller system.
48. High-energy, but real, transition structures for formally forbidden suprafacial [1,7] shifts over cycloheptatriene were previously described; see: T. Okajima, K. Imafuku, J. Org. Chem. 2002, 67, 625-632.
49. The effects of substituents on the stereochemical preferences of sigmatropic shifts have been discussed previously; see: M. T. Zoeckler, B. K. Carpenter, J. Am. Chem. Soc. 1981, 103, 7661-7663 and N. D. Epiotis, J. Am. Chem. Soc. 1973, 95, 1206-1214.
50. The effects of substituents on the stereochemical preferences of sigmatropic shifts have been discussed previously; see: M. T. Zoeckler, B. K. Carpenter, J. Am. Chem. Soc. 1981, 103, 7661-7663 and N. D. Epiotis, J. Am. Chem. Soc. 1973, 95, 1206-1214.
51. R. B. Woodward, R. Hoffmann, The Conservation of Orbital Symmetry, Verlag Chemie, Weinheim, Germany, 1970.