Thermodynamic and strain effects in the competition between 5-exo-dig and 6-endo-dig cyclizations of vinyl and aryl radicals

Journal of the American Chemical Society

Volumn 127, Issue 36, 2005, Pages 12583-12594

  Alabugin, Igor V ^a   Manoharan, Mariappan ^a  

^a FLORIDA STATE UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ACTIVATION ENERGY; DATA REDUCTION; STRUCTURE (COMPOSITION);

5-EXO CYCLIZATION; AROMATIC RING; CONJUGATED SYSTEMS; POLYCYCLIC SYSTEMS; STRAIN EFFECTS;

THERMODYNAMICS;

ACETYLENE; POLYCYCLIC AROMATIC HYDROCARBON DERIVATIVE; POLYCYCLIC HYDROCARBON; RADICAL; VINYL DERIVATIVE;

AROMATIZATION; ARTICLE; CHEMICAL REACTION KINETICS; CONJUGATION; CYCLIZATION; ELECTRON; ENERGY; STEREOCHEMISTRY; SYNTHESIS; THERMODYNAMICS;

ALKYNES; CRYSTALLOGRAPHY, X-RAY; CYCLIZATION; FREE RADICALS; MODELS, CHEMICAL; MODELS, MOLECULAR; MOLECULAR STRUCTURE; THERMODYNAMICS; VINYL COMPOUNDS;

EID: 24744453294     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja052677y     Document Type: Article

References (179)

1. (a) Gleiter, R.; Kratz, D. Angew. Chem., Int. Ed. Engl. 1993, 32, 842.
2. (b) Bunz, U. H. F. Top. Curr. Chem. 1999, 201, 131.
3. (c) Hwang, G. T.; Kim, B. H. Org. Lett. 2004, 6, 2669.
4. (d) Kaafarani, B. R.; Wex, B.; Wang, F.; Catanescu, O.; Chien, L. C.; Neckers, D. C. J. Org. Chem. 2003, 68, 5377.
5. (e) Diederich, F. Chimia 2001, 55, 821.
6. (f) Xiao, J.; Yang, M.; Lauher, J. W.; Fowler, F. W. Angew. Chem., Int. Ed. 2000, 39, 2132.
7. (g) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350.
8. (h) Martin, R. E.; Mäder, T.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 817.
9. (i) Diederich, F. Chem. Commun. 2001, 219.
10. (j) Müllen, K.; Wegner, G. Electronic Materials: The Oligomeric Approach; Wiley-VCH: Weinheim, Germany, 1998.
11. (a) Scott, L. T. Angew. Chem., Int. Ed. 2004, 43, 4994 and references therein.
12. (b) Bell, M. L.; Chiechi, R. C.; Johnson, C. A.; Kimball, D. B.; Matzger, A. J.; Wan, W. B.; Weakley, T. J. R.; Haley, M. M. Tetrahedron 2001, 57, 3507.
13. (c) Brown, R. F. C.; Harrington, K. J.; McMullen, G. L. J. Chem. Soc., Chem. Commun. 1974, 123.
14. (d) Scott, L. T.; Hashemi, M. M.; Meyer, D. T.; Warren, H. B. J. Am. Chem. Soc. 1991, 113, 7082.
15. (e) Scott, L. T.; Hashemi, M. M.; Bratcher, M. S. J. Am. Chem. Soc. 1992, 114, 1920.
16. (f) Necula, A.; Scott, L. T. J. Am. Chem. Soc. 2000, 122, 1548.
17. (g) Zimmermann, G.; Nuechter, U.; Hagen, S.; Nuechter, M. Tetrahedron Lett. 1994, 35, 4747.
18. (h) Liu, C. Z.; Rabideau, P. W. Tetrahedron Lett. 1996, 37, 3437.
19. (i) Mehta, G.; Panda, G. Tetrahedron Lett. 1997, 38, 2145.
20. (j) Seiders, T. J.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1996, 118, 2754.
21. (k) Scott, L. T.; Cheng, P.-C.; Hashemi, M. M.; Bratcher, M. S.; Meyer, D. T.; Warren, H. B. J. Am. Chem. Soc. 1997, 119, 10963.
22. (l) Yamaguchi, S.; Xu, C.; Tamao, K. J. Am. Chem. Soc. 2003, 125, 13662.
23. (m) Xu, C.; Wakamiya, A.; Yamaguchi, S. J. Am. Chem. Soc. 2005, 127, 1638.
24. For the most recent examples and leading references, see: (a) Han, X.; Zhang, Y.; Wang, K. K. J. Org. Chem. 2005, 7, 2406.
25. For a general discussion of synthetic utility of radical reactions, see: (b) Giese, B. Radicals in Organic Synthesis: Formation of Carbon-Carbon Bonds; Pergamon: Oxford, U.K., 1986.
26. (c) Curran, D. P. Synthesis 1988, 417 and 489.
27. (d) Jasperse, C. P.; Curran, D. P.; Fevig, T. L. Chem. Rev. 1991, 91, 1237.
28. (e) Melikyan, G. G. Synthesis 1993, 833.
29. (f) Iqbal, J.; Bhatia, B.; Nayyar, N. K. Chem. Rev. 1994, 94, 519.
30. (g) Snider, B. B. Chem. Rev. 1996, 96, 339.
31. (h) Curran, D. P.; Porter, N. A.; Giese, B. Stereochemistry of Radical Reactions; VCH: Weinheim, Germany, 1996.
32. (i) Gansauer, A.; Bluhm, H. Chem. Rev. 2000, 100, 2771.
33. (j) Radicals in Organic Synthesis; Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim, Germany, 2001.
34. (k) Sibi, M. P.; Manyem, S.; Zimmerman, J. Chem. Rev. 2003, 103, 3263.
35. 2-centered radicals attracted continuous attention due to their involvement into DNA-damaging processes responsible for the biological activity of enediyne antibiotics, in polymerization of acetylenes and in organic synthesis, (a) Enediyne Antibiotics as Antitumor Agents; Borders, D. B., Doyle, T. W., Eds.; Marcel Dekker: New York, 1995.
36. (b) Chen, X.; Tolbert, L. M.; Hess, D. W.; Henderson, C. Macromolecules 2001, 34, 4104.
37. (c) John, J. A.; Tour, J. M. J. Am. Chem. Soc. 1994, 116, 5011.
38. (d) Mifsud, N.; Mellon, V.; Perera, K. P. U.; Smith, D. W., Jr.; Echegoyen, L. J. Org. Chem. 2004, 69, 6124.
39. (e) Smith, D. W., Jr.; Babb, D. A.; Snelgrove, R. V.; Townsend, P. H., III; Martin, S. J. J. Am. Chem. Soc. 1998, 120, 9078.
40. Baldwin, J. E. J. Chem. Soc., Chem. Commun. 1976, 734.
41. (a) Jones, R. R.; Bergman, R. G. J. Am. Chem. Soc. 1972, 94, 660.
42. (b) Bergman, R. G. Acc. Chem. Res. 1973, 6, 25.
43. (c) Prall, M.; Wittkopp, A.; Schreiner, P. R. J. Phys. Chem. A 2001, 105, 9265.
44. (d) Schmittel, M.; Strittmatter, M.; Kiau, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 1843.
45. (e) Schmittel, M.; Vavilala, C. J. Org. Chem. 2005, 70, 4865 and references therein.
46. (f) Nagata, R.; Yamanaka, H.; Okazaki, E.; Saito, I. Tetrahedron Lett. 1989, 30, 4995.
47. (g) Myers, A. G.; Kuo, E. Y.; Finney, N. S. J. Am. Chem. Soc. 1989, 111, 8057.
48. (h) Saito, K.; Nagata, R.; Yamanaka, H.; Murahashi, E. Tetrahedron Lett. 1990, 31, 2907.
49. (i) Myers, A. G.; Drapovich, P. S.; Kuo, E. Y. J. Am. Chem. Soc. 1992, 114, 9369.
50. (a) Brunton, S. A.; Jones, K. J. Chem. Soc., Perkin Trans. 1 2000, 763.
51. (b) Boger, D. L.; Coleman, R. S. J. Am. Chem. Soc. 1988, 110, 4796.
52. (c) Dittami, J. P.; Ramanathan, H. Tetrahedron Lett. 1988, 29, 45.
53. (d) Grissom, J. W.; Klingberg, D.; Meyenburg, S.; Stallman, B. L. J. Org. Chem. 1994, 59, 7876.
54. (e) Benati, L.; Leardini, R.; Minozzi, M.; Nanni, D.; Spagnolo, P.; Zanardi, G. J. Org. Chem. 2000, 65, 8669.
55. (f) Marion, F.; Courillon, C.; Malacria, M. Org. Lett. 2003, 5, 5095.
56. For the role of PAH in the formation of soot particles see: (a) Haynes, B. S.; Wagner, H. G. Prog. Energy Combust. Sci. 1981, 7, 229.
57. (b) Calcote, H. F. Combust. Flame 1981, 42, 215.
58. For mutagenic and carcinogenic properties of PAH, see: (c) Longwell, J. P. In Soot in Combustion System and its Toxic Properties; Lahaye, J., Prado, G., Eds.; Plenum: New York, 1983; p 37.
59. (d) Thilly, W. G. In Soot in Combustion System and its Toxic Properties; Lahaye, J., Prado, G., Eds.; Plenum: New York, 1983; p 1.
60. (e) Lowe, J. P.; Silverman, B. D. Acc. Chem. Res. 1984, 17, 332.
61. (f) Ball, L. M.; Warren, S. H.; Sangaiah, R.; Nesnow, S.; Gold, A. Mutat. Res. 1989, 224, 115.
62. (g) Frenklach, M.; Clary, D. W.; Gardiner, W. C.; Stein, S. E. Symp. Int. Combust., Proc. 1984, 20, 887.
63. (h) Kazakov, A.; Frenklach, M. Combust. Flame 1998, 112, 270.
64. (i) Kern, R. D.; Wu, C. H.; Yong, J. N.; Pamidimukkala, K. M.; Singh, H. J. Energy Fuels 1988, 2, 454.
65. (j) Miller, J. A.; Volponi, J. V.; Pauwels, J.-F. Combust. Flame 1996, 105, 451.
66. (k) Morter, C.; Farhat, S.; Adamson, J.; Glass, G.; Curl, R. J. Phys. Chem. 1994, 98, 7029.
67. (l) Wu, C.; Kern, R. J. Chem. Phys. 1987, 91, 6291.
68. (m) Homann, K. Angew. Chem., Int. Ed. 1998, 37, 2435.
69. (n) Gonzales, J. M.; Barden, C. J.; Brown, S. T.; Schleyer, P. v. R.; Schaefer, H. F., III; Li, Q.-S. J. Am. Chem. Soc. 2003, 125, 1064.
70. (a) Walch, S. P. J. Chem. Phys. 1995, 103, 8544.
71. (b) Dewar, M. J. S.; Gardiner, W. C., Jr.; Frenklach, M.; Oref, I. J. Am. Chem. Soc. 1987, 109, 4456.
72. (c) Madden, L. K.; Moskaleva, L. V.; Lin, M. C. J. Phys. Chem. A 1997, 101, 6790.
73. (d) Mebel, A. M.; Morokuma, K.; Lin, M. C. J. Chem. Phys. 1995, 103, 7414.
74. Olivella, S.; Solé, A. J. Am. Chem. Soc. 2000, 122, 11416.
75. The difference is about 2.5-3 kcal/mol: (a) Denisova, T. G.; Denisov, E. T. Russ. Chem. Bull. 2002, 51, 949.
76. (b) Wilt, J. W.; Lusztyk, J.; Peeran, M.; Ingold, K. U. J. Am. Chem. Soc. 1988, 110, 281.
77. (c) Beckwith, A. L. J. Tetrahedron 1981, 37, 3073.
78. (d) Beckwith, A. L. J.; Blair, I. A.; Phillipou, G. Tetrahedron Lett. 1974, 26, 2251.
79. (e) Lal, D.; Griller, G.; Husband, S.; Ingold, K. U. J. Am. Chem. Soc. 1974, 96, 6355.
80. König, B.; Pitsch, W.; Klein, M.; Vasold, R.; Prall, M. Schreiner, P. R. J. Org. Chem. 2001, 66, 1742.
81. Kovalenko, S. V.; Peabody, S.; Manoharan, M.; Clark, R. J.; Alabugin, I. V. Org. Lett. 2004, 6, 2457.
82. Peabody, S.; Breiner, B.; Kovalenko, S. V.; Paul, S.; Alabugin, I. V. Org. Biomol. Chem. 2005, 3, 218.
83. Schmittel, M.; Rodriguez, D.; Steffen, J.-P. Angew. Chem., Int. Ed. 2000, 39, 2152.
84. For a formal 6-endo-dig radical cyclization of 2-isocyanobiphenyl radical, see: Lenoir, I.; Smith, M. L. J. Chem. Soc., Perkin Trans. 1 2000, 641.
85. Bowles, D. M.; Palmer, G. J.; Landis, C. A.; Scott, J. L.; Anthony, J. E. Tetrahedron 2001, 57, 3753.
86. Scott, J. L.; Parkin, S. R.; Anthony, J. E. Synlett 2004, 161.
87. Lewis, K. D.; Rowe, M. P.; Matzger, A. J. Tetrahedron 2004, 60, 7191.
88. (a) Rainier, J. D.; Kennedy, A. R. J. Org. Chem. 2000, 65, 6213.
89. (b) Rainier, J. D.; Kennedy, A. R.; Chase, E. Tetrahedron Lett. 1999, 40, 6325.
90. Bharucha, K. N.; Marsh, R. M.; Minto, R. E.; Bergman, R. G. J. Am. Chem. Soc. 1992, 114, 3120.
91. Matzger, A. J.; Vollhardt, K. P. C. Chem. Commun. 1997, 1415.
92. (a) Lin, C.-F.; Wu, M.-J. J. Org. Chem. 1997, 62, 4546.
93. (b) Wu, H. J.; Lin, C. F.; Lee, J. L.; Lu, W. D.; Lee, C. Y.; Chen, C. C.; Wu, M. J. Tetrahedron 2004, 60, 3927.
94. (a) Marcus, R. A. J. Chem. Phys. 1956, 24, 966.
95. (b) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155.
96. (c) Marcus, R. A. J. Phys. Chem. 1968, 72, 891.
97. (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098.
98. (b) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
99. (c) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623.
100. (a) Frisch, M. J. et al. Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh, PA, 1998.
101. (b) Frisch, M. J. et al. Gaussian 03, revision C.01; Gaussian, Inc.: Wallingford CT, 2004. See SI for details.
102. Schreiner, P. R.; Navarro-Vazquez, A.; Prall, M. Acc. Chem. Res. 2005, 38, 29 and references therein.
103. (a) Crawford, T. D.; Kraka, E.; Stanton, J. F.; Cremer, D. J. Chem. Phys. 2001, 114, 10638.
104. (b) Cramer, C. J. J. Am. Chem. Soc. 1998, 120, 6261.
105. Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO 4.0: University of Wisconsin: Madison, WI, 1996.
106. (a) Shaik, S. S.; Schlegel, H. B.; Wolfe, S. Theoretical Aspects of Physical Organic Chemistry; Wiley & Sons: New York, 1992; pp 33-44.
107. (b) Pross, A. Theoretical and Physical Principles of Organic Reactivity; Wiley: New York, 1995.
108. Selected examples: (a) Gas-phase proton transfer: Magnoli, D. E.; Murdoch, J. R. J. Am. Chem. Soc. 1981, 103, 7465.
109. (b) Theoretical study on group transfer reactions: Murdoch, J. R.; Magnoli, D. E. J. Am. Chem. Soc. 1982, 104, 3792.
110. N-2 reactions: Wolfe, S.; Mitchell, J.; Schlegel, H. B. J. Am. Chem. Soc. 1981, 103, 7694.
111. (d) Electron/proton transfer reactions and nucleophilic substitutions: Murdoch, J. R. J. Am. Chem. Soc. 1983, 105, 2660.
112. (e) Cyclopropylcarbinyl rearrangements: Newcomb, M.; Johnson, C. C.; Makek, M. B.; Varick, T. R. J. Am. Chem. Soc. 1992, 114, 10915.
113. (f) Martinesker, A. A.; Johnson, C. C.; Horner, J. H.; Newcomb, M. J. Am. Chem. Soc. 1994, 116, 9174.
114. (g) Reactions of carbonyl compounds: Guthrie, J. P.; Guo, J. N. J. Am. Chem. Soc. 1996, 118, 11472.
115. (h) Guthrie, J. P. J. Am. Chem. Soc. 2000, 122, 5529.
116. (i) Reactions of trialkyl phosphites: Canestrari, S.; Maŕin, A.; Sgarabotto, P.; Righi, L.; Greci, L. J. Chem. Soc., Perkin Trans. 2 2000, 833.
117. (j) Electron-transfer dynamics in synthetic DNA hairpins: Lewis, F. D.; Kalgutkar, R. S.; Wu, Y.; Liu, X.; Liu, J.; Hayes, R. T.; Miller, S. E.; Wasielewksi, M. R. J. Am. Chem. Soc. 2000, 122, 12346.
118. (k) Group transfer reactions involving radicals: Newcomb, M.; Makek, M. B.; Glenn, A. G. J. Am. Chem. Soc. 1991, 113, 949.
119. For the extension of Marcus theory to describe processes that have no identity reactions such as internal rotation and conformational rearrangements, see: Chen, M. Y.; Murdoch, J. R. J. Am. Chem. Soc. 1984, 106, 4735.
120. Pericyclic reactions: (a) Yoo, H. Y.; Houk, K. N. J. Am. Chem. Soc. 1997, 119, 2877.
121. (b) Aviyente, V.; Yoo, H. Y.; Houk, K. N. J. Org. Chem. 1997, 62, 6121.
122. (c) Aviyente, V.; Houk, K. N. J. Phys. Chem. A 2001, 105, 383.
123. (d) Alabugin, I. V.; Manoharan, B. B.; Lewis, F. J. Am. Chem. Soc. 2003, 125, 9329.
124. Pericyclic reactions in transition metal complexes: (e) Kristjansdottir, S. S.; Norton, J. R. J. Am. Chem. Soc. 1991, 113, 4366.
125. (f) Gisdakis, P.; Rösch, N. J. Am. Chem. Soc. 2001, 123, 697.
126. a in the IUPAC nomenclature.
127. For a more advanced but also more complicated parabolic model of radical addition reactions, see: (b) Denisov, E. T. Russ. Chem. Rev. 2000, 69, 153.
128. (c) Denisov, E. T.; Denisova, T. G. Russ. Chem. Rev. 2000, 71, 417.
129. The "near-attack" conformation was taken as the reactant to facilitate the comparison with conjugated radicals discussed in the rest of the article. As a result, the experimentally observed barriers for these flexible systems will be different from the computed values.
130. Schleyer, P. v. R.; Puhlhofer, F. Org. Lett. 2002, 4, 2873.
131. Experimental precedents for analogues of 1 will be discussed in detail in the following sections. Cyclizations of analogues of radical 2 show strong 5-exo-selectivity: (a) Journet, M.; Malacria, M. J. Org. Chem. 1994, 59, 6885.
132. (b) Sha, C.-K.; Zhan, Z.-P.; Wang, F.-S. Org. Lett. 2000, 2, 2011.
133. For examples of using unsaturation in the bridge to alleviate unfavorable enthropy contributions to cyclization reactions, see: Byrne, L. A.; Gilheany, D. G. Synlett 2004, 6, 933.
134. The Z-fulvene radical obtained from 5 is 0.1 kcal/mol more stable than the E-isomer formed from 6.
135. (a) Leffler, J. E. Science 1953, 117, 340.
136. (b) Hammond, G. S. J. Am. Chem. Soc. 1955, 77, 334.
137. These interactions lead to increased bond order between the radical carbon and adjacent atoms and decreased bonding in the vicinal bonds. Interestingly, the elongation of vicinal C-C bond is more pronounced in the fulvene radical where the only C-C bond is available to delocalize the spin density in contrast to the naphthyl radical where two C-C bonds are involved.
138. Moreover, Olivella and Sole had shown that 5-exo product can rearrange in the 6-endo product under conditions when the former radical is not trapped by H-abstraction. Even though the TS for the rearrangement is rather high in energy relative to the 5-exo product, it is still lower in energy than the barrier for direct 6-endo cyclization.
139. For a seminal contribution, see: (a) Bent, H. A. Chem. Rev. 1961, 67, 275.
140. For a detailed discussion of the role of hybridization in chemical properties of nonbonding orbitals see: (b) Alabugin, I. V.; Manoharan, M.; Zeidan, T. A. J. Am. Chem. Soc. 2003, 125, 14014.
141. For the effect of hybridization in X-H bonds, see: (c) Alabugin, I. V.; Manoharan, M.; Peabody, S.; Weinhold, F. J. Am. Chem. Soc. 2003, 125, 5973.
142. (d) Alabugin, I. V.; Manoharan, M.; Weinhold, F. J. Phys. Chem. A 2004, 108, 4720.
143. Interestingly, 5-endo-dig cyclization of an isomeric hex-1,3-diene-5-yne- 2-yl radical is also highly sensitive to strain effects. Alabugin, I. V.; Manoharan, M. J. Am. Chem. Soc. 2005, 127, 9534.
144. For a rare example of 6-endo-dig selectivity in reactions of alkyl radicals in polycyclic systems, see: (a) Breithor, M.; Herden, U.; Hoffmann, H. M. R. Tetrahedron 1997, 53, 8401.
145. (b) Hoffmann, H. M. R.; Herden, U.; Breithor, M.; Rhode, O. Tetrahedron 1997, 53, 8383.
146. The 6-endo-dig cyclizations of vinyl radicals have also been enforced in some cascade reactions where polycyclic strain slowed the alternative 5-exo-dig mode. (c) Albrecht, U.; Wartchow, R.; Hoffmann, H. M. R. Angew. Chem., Int. Ed. Engl. 1992, 31, 910.
147. This observation is not limited to dig cyclizations. For selected examples of similar strain effects on the competition between radical 6-endo-trig and 5-exo-trig cyclizations, see: (a) Kametani, T.; Honda, T. Heterocycles 1982, 19, 1861.
148. (b) Kano, S.; Yuasa, Y.; Asami, K.; Shibuya, S. Heterocycles 1988, 27, 1437.
149. For the effect on Bergman cyclization, see: (c) Jones, G. B.; Wright, J. M.; Plourde, G., II; Purohit, A. D.; Wyatt, J. K.; Hynd, G.; Fouad, F. J. Am. Chem. Soc. 2000, 122, 9872.
150. (a) Evans, M. G. Discuss. Faraday Soc. 1947, 2, 271.
151. (b) Evans, M. G.; Gergely, J.; Seaman, E. C. J. Polym. Sci. 1948, 3, 866.
152. (c) Semenov, N. N. Some Problems in Chemical Kinetics and Reactivity (Engl. Transl.); Princeton Press: Princeton, NJ, 1958; p 29.
153. (d) Fischer, H.; Radom, L. Angew. Chem., Int. Ed. 2001, 40, 1340 and references therein.
154. (e) Shaik, S. S.; Canadell, E. J. Am. Chem. Soc. 1990, 112, 1446.
155. (f) Coote, M. L.; Wood, G. P. F.; Radom, L. J. Phys. Chem. A 2002, 106, 12124.
156. (g) Gomez-Balderas, R.; Coote, M. L.; Henry, D. J.; Radom, L. J. Phys. Chem. A 2004, 108, 2874.
157. (h) Henry, D. J.; Coote, M. L.; Gomez-Balderas, R.; Radom, L. J. Am. Chem. Soc. 2004, 126, 1732.
158. (i) Henry, D. J.; Parkinson, C. J.; Mayer, P. M.; Radom, L. J. Phys. Chem. A 2001, 105, 6750.
159. (j) Gómez-Balderas, R.; Coote, M. L.; Henry, D. J.; Fischer, H.; Radom, L. J. Phys. Chem. A 2003, 107, 6082.
160. (k) Wong, M. W.; Pross, A.; Radom, L. J. Am. Chem. Soc. 1993, 115, 11050.
161. (l) Wong, M. W.; Pross, A.; Radom, L. J. Am. Chem. Soc. 1994, 116, 6284.
162. (m) Wong, M. W.; Pross, A.; Radom, L. J. Am. Chem. Soc. 1994, 116, 11938.
163. (n) Batchelor, B. N.; Fischer, H. J. Phys. Chem. 1996, 100, 9794.
164. Note, however, that the systematic error caused by the first factor can be decreased by limiting the discussion to the relative trends in similar reactions as it is done in this article.
165. Note that these barriers are likely to be different from those determined experimentally due to the presence of other conformers of the conformationally flexible reactants. However, the relative barriers for the 5-exo and 6-endo processes can be directly compared with experimental differences because both cyclizations proceed from the same "near-attack" conformer.
166. The lower yield in the case of the bis-TFP (tetrafluoropyridinyl) enediyne is due to the formation of acyclic products rather than the competition with 6-endo cyclization.
167. Formation of products resulting from addition of the initiating Sn radicals to one of the triple bonds is also possible.
168. Logan, C. F.; Chen, P. J. Am. Chem. Soc. 1996, 118, 2113.
169. (a) Squires, R. R.; Cramer, C. J. J. Phys. Chem. A 1998, 102, 9072.
170. (b) Davico, G. E.; Bierbaum, V. M.; DePuy, C. H.; Ellison, G. B.; Squires, R. R. J. Am. Chem. Soc. 1995, 117, 2590.
171. (c) Logan, C. F.; Chen, P. J. Am. Chem. Soc. 1996, 118, 2113.
172. (d) Schottelius, M. J.; Chen, P. J. Am. Chem. Soc. 1996, 118, 4896.
173. (e) Hoffner, J.; Schottelius, J.; Feichtinger, D.; Chen, P. J. Am. Chem. Soc. 1998, 120, 376.
174. (f) Kraka, E.; Cremer, D. J. Am. Chem. Soc. 2000, 122, 8245.
175. (g) Kraka, E.; Cremer, D. J. Comput. Chem. 2001, 22, 216.
176. (h) Stahl, F.; Moran, D.; Schleyer, P. v. R.; Prall, M.; Schreiner, P. R. J. Org. Chem. 2002, 67, 1453.
177. (i) Winkler, M. J. Phys. Chem. A 2005, 109, 1240.
178. (j) Alabugin, I. V.; Manoharan, M. J. Am. Chem. Soc. 2003, 125, 4495.
179. Zhang, D. Y.; Borden, W. T. J. Org. Chem. 2002, 67, 3989.