Extended Barbaralanes: Sigmatropic Shiftamers or σ-Polyacenes?

Journal of the American Chemical Society

Volumn 126, Issue 13, 2004, Pages 4256-4263

  Tantillo, Dean J ^a,b   Hoffmann, Roald ^a   Houk, Kendall N ^c   Warner, Philip M ^d   Brown, Eric C ^e   Henze, Daven K ^e  

^a Department of Obstetrics Gynecology   (United States)

^b UNIVERSITY OF CALIFORNIA   (United States)

^c UNIVERSITY OF CALIFORNIA   (United States)

^d NORTHEASTERN UNIVERSITY   (United States)

^e UNIVERSITY OF WASHINGTON   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

AROMATIC POLYMERS; BONDING; CHEMICAL BONDS; FREE RADICALS; MOLECULAR STRUCTURE;

DELOCALIZED MOLECULES; FUSED BARBARALANES; POLYENYL RADICAL CHAINS;

MOLECULAR DYNAMICS;

AROMATIC COMPOUND; BARBARALANE DERIVATIVE; POLYCYCLIC HYDROCARBON; RADICAL; SIGMA POLYACENE DERIVATIVE; UNCLASSIFIED DRUG;

ARTICLE; CHEMICAL ANALYSIS; CHEMICAL BINDING; CHEMICAL INTERACTION; COPE REARRANGEMENT; STRUCTURE ANALYSIS;

EID: 1842420497     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja0392364     Document Type: Article

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46. This value was also calculated at the same level in ref 3a.
47. 7′ (n = 1) was also optimized with both MP2/6-31G(d) and CASSCF-(10,10)/6-31G(d). In both cases, delocalized structures resulted that are extremely similar to that found with B3LYP/6-31G(d) (see Figure 1).
48. For two similarly tethered infinite polyacetylene chains, a second set of orbitals is also present, with coefficients on the other carbons of the polyene chains. The nature of the linking framework would keep these carbons quite a bit farther apart than the bridgehead carbons, however, leading to only very small through-space interactions and a pair of nearly degenerate orbitals at the orbital frontier. It is possible, therefore, that the infinite system might have a triplet ground state, albeit of a rather different nature than the oligomers we study.
49. One system with fused semibullvalene units was also examined (selected distances shown in Å): (diagram presented) This molecule is a highly strained (note in particular the 134° angle at the central carbon) and curved molecule (now with a direction of curvature opposite that of 8), but still contains close interpolyene contacts. This molecule can also be considered a member of the class of molecules known as fenestranes: molecules containing a central quaternary carbon that serves as a corner for four fused rings that are arrayed like the panes of a window or the "four comers" states of the American southwest. For leading references on fenestranes, see: (a) Georgian, V.; Saltzman, M. Tetrahedron Lett. 1972, 4315-4317.
50. (b) Thommen, M.; Keese, R. Synlett 1997, 231, and references therein. It should be noted that a structure with a localized inter-polyene single bond at one end is slightly more stable than this structure (by ∼2 kcal/mol, based on electronic energies).
51. A "side view" of structure 23: (diagram presented)
52. We have not yet studied the seemingly less constrained way to hold two polyenes in proximity to each other that has been explored by W. von. E. Doering and co-workers and Hopf and co-workers (Doering, W. von E.; He, J.; Shao, L. J. Am. Chem. Soc. 2001, 123, 9153-9151; Schüll, V.; Hopf, H. Tetrahedron Lett. 1981, 22, 3439-3442), who examined derivatives of bis-polyenyl cyclobutanes: (diagram presented)
53. We have not yet studied the seemingly less constrained way to hold two polyenes in proximity to each other that has been explored by W. von. E. Doering and co-workers and Hopf and co-workers (Doering, W. von E.; He, J.; Shao, L. J. Am. Chem. Soc. 2001, 123, 9153-9151; Schüll, V.; Hopf, H. Tetrahedron Lett. 1981, 22, 3439-3442), who examined derivatives of bis-polyenyl cyclobutanes: (diagram presented)
54. σ-Bond cleavage in certain polyenes to produce two polyenyl radicals has been shown previously to be facile. See, for example: Beno, B. R.; Fennen, J.; Houk, K. N.; Lindner, H. J.; Hafner, K. J. Am. Chem. Soc. 1998, 120, 10490-10493, and ref 19.
55. CASSCF(14,14)/6-31G(d) and CASPT2/6-31G(d) single-point calculations agree that the fully delocalized 7' (n = 2) is more stable than the end-closed form (by ∼-1 kcal/mol with CASSCF and by ∼-6 kcal/mol with CASPT2).
56. B3LYP was also previously shown to give energy differences for 7 and 7′ (n = 0, barbaralane) that were quite similar to those computed with CASPT2. See ref 3c.
57. Structures for 7′ (n = 1) with localized terminal inter-polyene bonds were also explored using the B3LYP/6-31G(d)//B3LYP/6-31G(d), CASSCF-(10,10)/6-31G(d)//B 3LYP/6-31G(d), CASPT2/6-31G(d)//B3LYP/6-31G(d), CASSCF(10,10)/6-31 G(d)//CASSCF(10,10)/6-31G(d), and CASPT2/6-31G-(d)//CASSCF(10, 10)/6-31G(d) levels. The results are shown in Table 5. B3LYP and CASPT2 energy calculations agree that the end-closed and open (fully delocalized) structures are very similar in energy-irrespective of the method used for the geometry optimization-while CASSCF consistently predicts that the end-closed form is considerably more stable than the open form (by 5-6 kcal/mol). Generally, CASPT2 energy calculations are considered to be more reliable than CASSCF energy calculations, and B3LYP has been shown to give similar results to CASPT2 for various reactions.
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68. Application of the NICS technique to the polyacenes is described in: Schleyer, P. v. R.; Manoharan, M.; Jiao, H.; Stahl, F. Org. Lett. 2001, 3, 3643-3646. For a related study, see: Poater, J.; Fradera, X.; Duran, M.; Solà M. Chem. Eur. J. 2003, 9, 1113-1122.
69. Application of the NICS technique to the polyacenes is described in: Schleyer, P. v. R.; Manoharan, M.; Jiao, H.; Stahl, F. Org. Lett. 2001, 3, 3643-3646. For a related study, see: Poater, J.; Fradera, X.; Duran, M.; Solà M. Chem. Eur. J. 2003, 9, 1113-1122.
70. The NICS values in Figure 5 were computed at the GIAO-B3LYP/6-31G-(d) level. These are similar to (and consistently ∼-1 ppm more negative than) those computed previously at the IGLO/DZ2P//B3LYP/6-311+G** level: -9.7 for benzene, -8.9 for naphthalene, -7.6 and -11.5 for anthracene, and -6.6 and -11.4 for tetracene.