Planar [6]radialenes: Structure, synthesis, and aromaticity of benzotriselenophene and benzotrithiophene

Angewandte Chemie - International Edition

Volumn 46, Issue 46, 2007, Pages 8814-8818

  Patra, Asit ^a   Wijsboom, Yair H ^a   Shimon, Linda J W ^a   Bendikov, Michael ^a  

^a WEIZMANN INSTITUTE OF SCIENCE   (Israel)

Author keywords

Aromaticity; Radialenes; Selenium; Structural properties; Sulfur heterocycles

Indexed keywords

AROMATICITY; RADIALENES; SULFUR HETEROCYCLES; X-RAY CRYSTAL STRUCTURES;

BENZENE; CRYSTAL STRUCTURE; CYCLIC VOLTAMMETRY; MOLECULAR STRUCTURE; X RAY CRYSTALLOGRAPHY;

SELENIUM COMPOUNDS;

EID: 36749014447     PISSN: 14337851     EISSN: None     Source Type: Journal    
DOI: 10.1002/anie.200703123     Document Type: Article

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33. See the Supporting Information for details.
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36. The unit cell of compound 4 consists of two very similar independent molecules. Average values for these two molecules are used in the discussion.
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39. More accurately, these bonds should be called semicyclic double bonds; however, exocyclic is the term more often used in this context.
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49. [15] revealed that the bond lengths in the central ring are similar for the two compounds. Thus, the lengths of the exo C-C bonds are 1.469 Å in 6 and 1.463 Å in 4, and the lengths of the endo C-C bonds are 1.436 Å in 6 and 1.439 Å in 4. As aromaticity and antiaromaticity effects are irrelevant with respect to the central ring in 6, which is not planar, the central ring in compound 4 must also be non-aromatic, in accordance with the NICS results.
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58. [34b] For the central rings of compounds 4 and 5, the Julg parameter is in the range 0.98-1.00, that is, close to 1, and corresponds to nearly complete bond-length equalization. This result may suggest aromaticity of the central six-membered ring. However, this equalization is a consequence of a small negative SIBL and can not be used as evidence for the aromaticity of compounds 4 and 5. a) B. Goldfuss, P. von R. Schleyer, F. Hampel, Organometallics 1996, 15, 1755-1757;
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60. 1 = 7.76 eV.
61. Part of the blue shift results from the substitution of hydrogen atoms for carbon atoms at the 3- and 4-positions of the heterocycles. Indeed, the absorption calculated at the TD-B3LYP/6-31G(d) level for 3,4-dimethylthiophene (213.6 nm) is blue shifted by 0.12 eV relative to the calculated absorption for thiophene (209.2 nm). Thus, we can estimate a conjugation of 0.37 eV between thiophene rings in 4 on the basis of the UV spectra.
62. The second calculated observable transition for compound 4 is at 256 nm. For compound 5, the corresponding calculated absorptions are at 319 and 268 nm. The calculated intensity (oscillator strength) of the second transition in 4 is 4.7 times stronger than that of the first transition, whereas for 5 the calculated intensity of the second transition is 3.7 times stronger than that of the first transition. These results are in excellent agreement with the data in Figure 2.
63. The antibonding combination contributes to the long exo C-C bonds in compounds 4 and 5.
64. 4 have an average length of 1.427, 1.451, and 1.368 Å, respectively, and are thus 0.012, 0.012, and 0.003 Å shorter than the corresponding bonds in 4.
65. [1,2]
66. [17]