Reactant destabilization in the Bergman cyclization and rational design of light- and pH-activated enediynes

Journal of Physical Chemistry A

Volumn 107, Issue 18, 2003, Pages 3363-3371

  Alabugin, Igor V ^a   Manoharan, Mariappan ^a  

^a FLORIDA STATE UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

BERGMAN CYCLIZATION;

CHEMICAL BONDS; ENERGY GAP; PH EFFECTS; PHOTOCHEMICAL REACTIONS;

AROMATIC COMPOUNDS;

EID: 0037657968     PISSN: 10895639     EISSN: None     Source Type: Journal    
DOI: 10.1021/jp026754c     Document Type: Article

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61. The relative magnitudes of the changes in the in-plane and out-of-plane π-systems in the TS has been also analyzed with VB theory in ref 11. Our analysis is consistent with these results but provides more detail because it operates at the level of individual π- and σ-bonds and determines the importance of both attractive and repulsive interactions.
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63. Later Schreiner expanded the C1-C6 range to 3.4-2.9 Å using DFT computations (see ref 9).
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65. The calculated C1-C6 distance in 1 is similar to the experimental value of 4.32 Å in: McMahon, R. J.; Halter, R. J.; Fimmen, R. L.; Wilson, R. J.; Peebles, S. A.; Kuczkowski, R. L.; Stanton, J. F. J. Am. Chem. Soc. 2000, 122, 939. Small deviation is consistent with the relatively soft potential for acetylene bending.
66. This observation indicates the presence of a factor that destabilizes the TS and the product of the Bergman cyclization of enediyne 2. Most likely, the TS destabilization stems from the properties of the polymethylene bridge that locks the terminal acetylene atoms in the 10-membered cycle.
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73. Malandra, J. L.; Mills, N. S.; Kadlecek, D. E.; Lowery, J. A. J. Am. Chem. Soc. 1994, 116, 11622. Mills, N. S.; Malandra, J. L.; Burns, E. E.; Green, A.; Unruh, K. E.; Kadlecek, D, E.; Lowery, J. A. J. Org. Chem. 1997, 62, 9318. Mills, N. S.; Burns, E. B.; Hodges, J.; Gibbs, J.; Esparza, E.; Malandra, J. L.; Koch, J. J. Org. Chem. 1998, 63, 3017. Mills, N. S.; Malinky, T.; Malandra, J. L.; Burns, E, E.; Crossno, P. J. Org. Chem. 1999, 64, 511. Mills, N. S. J. Am. Chem. Soc. 1999, 121, 11690. Mills, N. S.; Benish, M. M.; Rakowitz, A. J.; Bebell, L. M. J. Org. Chem. 2002, 67, 2003-2012.
74. Malandra, J. L.; Mills, N. S.; Kadlecek, D. E.; Lowery, J. A. J. Am. Chem. Soc. 1994, 116, 11622. Mills, N. S.; Malandra, J. L.; Burns, E. E.; Green, A.; Unruh, K. E.; Kadlecek, D, E.; Lowery, J. A. J. Org. Chem. 1997, 62, 9318. Mills, N. S.; Burns, E. B.; Hodges, J.; Gibbs, J.; Esparza, E.; Malandra, J. L.; Koch, J. J. Org. Chem. 1998, 63, 3017. Mills, N. S.; Malinky, T.; Malandra, J. L.; Burns, E, E.; Crossno, P. J. Org. Chem. 1999, 64, 511. Mills, N. S. J. Am. Chem. Soc. 1999, 121, 11690. Mills, N. S.; Benish, M. M.; Rakowitz, A. J.; Bebell, L. M. J. Org. Chem. 2002, 67, 2003-2012.
75. Malandra, J. L.; Mills, N. S.; Kadlecek, D. E.; Lowery, J. A. J. Am. Chem. Soc. 1994, 116, 11622. Mills, N. S.; Malandra, J. L.; Burns, E. E.; Green, A.; Unruh, K. E.; Kadlecek, D, E.; Lowery, J. A. J. Org. Chem. 1997, 62, 9318. Mills, N. S.; Burns, E. B.; Hodges, J.; Gibbs, J.; Esparza, E.; Malandra, J. L.; Koch, J. J. Org. Chem. 1998, 63, 3017. Mills, N. S.; Malinky, T.; Malandra, J. L.; Burns, E, E.; Crossno, P. J. Org. Chem. 1999, 64, 511. Mills, N. S. J. Am. Chem. Soc. 1999, 121, 11690. Mills, N. S.; Benish, M. M.; Rakowitz, A. J.; Bebell, L. M. J. Org. Chem. 2002, 67, 2003-2012.
76. This is consistent with an earlier suggestion in ref 27.
77. i* interactions in enediyne 1 is only 0.5 kcal/mol, and thus, the decrease in this stabilizing interaction in the ground states of the cyclic enediynes 2 and 3 is only a minor component in the ground-state destabilization compared to the four-electron repulsion. Figure 5 also illustrates that the C1-C6 bond formation is of minor importance at the distances above 2.5 Å (in accord to the timing of the C1-C2/C5-C6 π-bond breaking in Figure 3).
78. Estimated by simultaneous deletion of the corresponding Fock matrix elements followed by variational recalculation of the wave function.
79. The reaction barrier at the shorter distances is mainly a result of the C1-C2 (C5-C6) π-bond breaking processes.
80. Note that the NBOs can still be clearly matched to the original π- and σ-orbitals even in the TS. This is in a sharp contrast to Pipek-Mezey localized Orbitals used for dissected NICS computations in ref 13. The Pipek-Mezey localized orbitals mix to the extent that the in-plane π- and σ-systems become indistinguishable.
81. Jafri, J. A.; Newton, M. D. J Am. Chem. Soc. 1978, 100, 5012.
82. Zimmerman, H. E. J. Am. Chem. Soc. 1966, 88, 1564, Zimmerman, H. E. Tetrahedron 1982, 38, 753. See also Bernardi, F.; Celani, P.; Olivucci, M.; Robb, M. A.; Suzzivalli, G. J. Am. Chem. Soc. 1995, 117, 10531.
83. Zimmerman, H. E. J. Am. Chem. Soc. 1966, 88, 1564, Zimmerman, H. E. Tetrahedron 1982, 38, 753. See also Bernardi, F.; Celani, P.; Olivucci, M.; Robb, M. A.; Suzzivalli, G. J. Am. Chem. Soc. 1995, 117, 10531.
84. Zimmerman, H. E. J. Am. Chem. Soc. 1966, 88, 1564, Zimmerman, H. E. Tetrahedron 1982, 38, 753. See also Bernardi, F.; Celani, P.; Olivucci, M.; Robb, M. A.; Suzzivalli, G. J. Am. Chem. Soc. 1995, 117, 10531.
85. One can argue that the presence of the in-plane C3-C4 bond and the six-electron out-of-plane π-system renders the whole molecule nonantiaromatic in the strictest sense (and this is consistent with the results of our NICS computations, see the Supporting Information). Therefore, we use the quotation marks for the term "antiaromatic region". However, use of this term is justified because the lack of the π-π* stabilization along the strong π-π repulsion provides an appealing analogy to antiaromatic molecules.
86. Kagan, J.; Wang, X.; Chen, X.; Lau, K. Y.; Batac, I. V.; Tuveson, R. W.; Hudson. J. B J. Photochem. Photobiol., B: Biol. 1993, 21, 135-142.
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93. Note also several reports of indirect, photochemically triggered, but thermal Bergman cycloaromatizations. (a) Wender, P. A.; Zercher, C. K.; Beckham, S.; Haubold, E.-M. J. Org. Chem. 1993, 58, 5867-5869. Wender, P. A.; Beckham, S.; O'Leary, J. G. Synthesis 1994, 1278.
94. Note also several reports of indirect, photochemically triggered, but thermal Bergman cycloaromatizations. (a) Wender, P. A.; Zercher, C. K.; Beckham, S.; Haubold, E.-M. J. Org. Chem. 1993, 58, 5867-5869. Wender, P. A.; Beckham, S.; O'Leary, J. G. Synthesis 1994, 1278.
95. (b) Nicolauo, K. C.; Dai, W. M.; Wendeborn, S. V.; Smith, A. L.; Torisawa, Y.; Maligres, P.; Hwang, C. K. Angew. Chem., Int. Ed. Engl. 1991, 30, 1032.
96. (c) Nakatani, K.; Isoe, S.; Maekawa, S.; Saito, I. Tetrahedron Lett. 1994, 35, 605. Nakatani, K.; Maekawa, S.; Tanabe, K.; Saito, I. J. Am. Chem. Soc. 1995, 117, 10635.
97. (c) Nakatani, K.; Isoe, S.; Maekawa, S.; Saito, I. Tetrahedron Lett. 1994, 35, 605. Nakatani, K.; Maekawa, S.; Tanabe, K.; Saito, I. J. Am. Chem. Soc. 1995, 117, 10635.
98. (d) Basak, A.; Moh'd Bdour, H.; Shain, J. S.; Mandal, S.; Rudra, K. R.; Nag, S. Bioorg. Med. Chem. Lett. 2000, 10, 1321. Two attempts resulted in only cis-trans isomerization (UV, 366 nm).
99. (e) Reference 38.
100. (f) Konig, B.; Schofield, E.; Bubenitschek, P.; Jones, P. G. J. Org. Chem. 1994, 59, 7142. Photosensitized electron-transfer reactions of aromatic enediynes are known, but they proceed via mechanisms different from the Bergman cyclization. Cyclization of radical cations:
101. (g) Ramkumar, D.; Kalpana, M.; Varghese, B.; Sankararaman, S. J. Org. Chem. 1996, 61, 2247. Cyclization of radical anions:
102. (h) Alabugin, I. V.; Kovalenko, S. V. J. Am. Chem. Soc. 2002, 124, 9052.
103. The thermal Bergman cyclization is also a symmetry-allowed process as a result of the through-bond coupling of the two radical centers. See, for example, ref 11 for a discussion.
104. Clark, A. E.; Davidson, E. R.; Zaleski, J. M. J. Am. Chem. Soc. 2001, 123, 2650.
105. The MO energies were estimated from the B3LYP/6-31G** computations for the reactant and B3LYP/6-31G**//UB3LYP/6-31G** for the TS and the product. Because of the ongoing discussion about the physical meaning of "DFT MOs", we would like to stress that we restrict ourselves in use of this correlation diagram only to the qualitative discussions.
106. 2 rather than to an sp atom.
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114. C5C6 in the protonated aniline 6 is significantly larger than that in 1,2-diethynyl benzene indicating than the π → π* bonding interactions are also increased in the TS for the Bergman cyclization of 6.
115. Alabugin, I. V.; Manoharan, M., manuscript in preparation.