I was all set to write a review of an interesting study of bowtiene 1: its rearrangement to other C₁₀H₆ isomers and its dimerization. But as I was gathering my information, I wanted to prepare the images of the optimized geometries, and so I went to get the supplementary materials.

The author has a section on the supplementary materials that indicates it contains Cartesian coordinates – just what I need. (This section ends with the curious line “This material is available for free of charge via the internet at http://pubs.acs.org.”; it’s curious because the article is in a journal not published by ACS. I’ll leave for speculation just what happened here, but clearly the copy-editing done by the Canadian Journal of Chemistry is not quite up to snuff!)

So, I went to the website and clicked on the link for the supplementary material and was then told that I did not have access to this material and that either I had to become a subscriber or I had to purchase access to the article. (I should point out here that I received this article through interlibrary loan.) This is the first time that I have run into a paywall to get supporting materials! I know I am probably lucky that it took 7 years before running into this problem. But that makes this situation so frustrating – just why is the Canadian Journal of Chemistry placing supplementary material behind a paywall, especially when so few other publishers are doing this?

Well, until I get the supplementary materials, I will not write a post about this article.

New policy: I will not blog about an article unless (a) there is information on the 3-D structure of the molecules, typically in supporting materials, and (b) this information is available for free. This requirement should really be the minimum for publishing computational chemistry results. Now, I would also hope that the coordinates are readily reusable – see Henry Rzepa’s post about recent problems he’s run into!

Aromaticity and orbital symmetry rules, though seemingly of ancient origin, remain areas of active interest. This paper by Fukazawa, et al combine both issues.¹ The multiple-step electrocyclization of 1 gives 2 in a reaction that takes 9 days at 80 °C. What would be the effect of diminishing the aromatic character of the fused rings of 1? Would the reaction be faster or slower?

Before discussing the experimental results, let’s examine the B3LYP/6-31G(d) results for the reaction of 1’, 3 and 5. (Note that a slightly smaller pendant substituent is used in the computations than in the experiment.) The optimized geometries of the critical points along the reaction pathway for the cyclization of 3 are shown in Figure 1.

3 (0.0)	3-TS1 (17.9)
3-INT (10.4)	3-TS2 (13.3)
4 (-60.7)

Figure 1. B3LYP/6-31G(d) optimized geometries and relative energies (kcal mol^-1) for the critical points along the reaction 3 → 4.
Remember that all structures on my blog can be viewed interactively by clicking on the image of the molecule.

For 1’, the first barrier (for the 8π cyclization) has a barrier of about 23 kcal mol^-1, but the second step (the 4π cyclization) has an even larger barrier of 28 kcal mol^-1. However, reducing the aromaticity of one of the fused rings (compound 3) leads to lower barriers of 18 and 13 kcal mol^-1. For the cyclization of 5, only a single transition state was found – no intermediate and no second TS – with a barrier of 12 kcal mol^-1. Thus, removing these external aromatic rings reduces the barrier of the reaction, and that is exactly what is found experimentally!

References

(1) Fukazawa, A.; Oshima, H.; Shimizu, S.; Kobayashi, N.; Yamaguchi, S. "Dearomatization-Induced Transannular Cyclization: Synthesis of Electron-Accepting Thiophene-S,S-Dioxide-Fused Biphenylene," J. Am. Chem. Soc. 2014, 136, 8738-8745, DOI: 10.1021/ja503499n.

InChIs:

1: InChI=1S/C44H64S4Si4/c1-41(2,3)49(13,14)37-25-29-30-26-38(50(15,16)42(4,5)6)46-34(30)23-24-36-32(28-40(48-36)52(19,20)44(10,11)12)31-27-39(51(17,18)43(7,8)9)47-35(31)22-21-33(29)45-37/h25-28H,1-20H3/b30-29-,32-31-
InChIKey=OCNQBMWQONUVNH-IOYDOZLVSA-N

1’:InChI=1S/C32H40S4Si4/c1-37(2,3)29-17-21-22-18-30(38(4,5)6)34-26(22)15-16-28-24(20-32(36-28)40(10,11)12)23-19-31(39(7,8)9)35-27(23)14-13-25(21)33-29/h17-20H,1-12H3/b22-21-,24-23-
InChIKey=GTFPBRMBCLREPG-ICHHBZPXSA-N

2: InChI=1S/C44H64S4Si4/c1-41(2,3)49(13,14)29-21-25-26-22-30(50(15,16)42(4,5)6)46-38(26)34-33(37(25)45-29)35-36(34)40-28(24-32(48-40)52(19,20)44(10,11)12)27-23-31(47-39(27)35)51(17,18)43(7,8)9/h21-24H,1-20H3
InChIKey=OTDXAOVIIQYYNV-UHFFFAOYSA-N

2’: InChI=1S/C32H40S4Si4/c1-37(2,3)21-13-17-18-14-22(38(4,5)6)34-30(18)26-25(29(17)33-21)27-28(26)32-20(16-24(36-32)40(10,11)12)19-15-23(35-31(19)27)39(7,8)9/h13-16H,1-12H3
InChIKey=IYZNCPPDTHWWCO-UHFFFAOYSA-N

3: InChI=1S/C32H40O2S4Si4/c1-39(2,3)29-17-21-22-18-30(40(4,5)6)37-27(22)15-16-28-24(20-32(38(28,33)34)42(10,11)12)23-19-31(41(7,8)9)36-26(23)14-13-25(21)35-29/h17-20H,1-12H3/b22-21-,24-23-
InChIKey=ZJBDGDJVLGNVOD-ICHHBZPXSA-N

4: InChI=1S/C32H40O2S4Si4/c1-39(2,3)21-13-17-18-14-22(40(4,5)6)36-30(18)26-25(29(17)35-21)27-28(26)32-20(16-24(38(32,33)34)42(10,11)12)19-15-23(37-31(19)27)41(7,8)9/h13-16H,1-12H3
InChIKey=QUSJUOMZBJUGON-UHFFFAOYSA-N

5: InChI=1S/C32H40O8S4Si4/c1-45(2,3)29-17-21-22-18-30(46(4,5)6)42(35,36)26(22)15-16-28-24(20-32(44(28,39)40)48(10,11)12)23-19-31(47(7,8)9)43(37,38)27(23)14-13-25(21)41(29,33)34/h17-20H,1-12H3/b22-21-,24-23-
InChIKey=NNZTUSIYEPMHMP-ICHHBZPXSA-N

6: InChI=1S/C32H40O8S4Si4/c1-45(2,3)21-13-17-18-14-22(46(4,5)6)42(35,36)30(18)26-25(29(17)41(21,33)34)27-28(26)32-20(16-24(44(32,39)40)48(10,11)12)19-15-23(47(7,8)9)43(37,38)31(19)27/h13-16H,1-12H3
InChIKey=JZHQQYXUIQXWLQ-UHFFFAOYSA-N

Macrocycles composed of aromatic subunits, like polycycloparaphenylenes, are of interest as components of nanotubes and for possible interesting optical properties. Tremendous advances have occurred over the past decade in preparing these rings ; see for examples these posts. Yamago now reports on the synthesis, optical properties and structure of [4]cyclo-2,7-pyrenylene 1, made by joining four pyrene units together.¹

B3LYP/6-31G(d) optimization of the structure of 1 reveals a D₂ geometry (Figure 1). This structure shows a very distorted pyrene unit. The strain energy of 1 is estimated as 392 kJ mol^-1 (though how this was arrived at is not mentioned!), which is much larger than the strain energy of [8]-cycloparaphenylene.

Figure 1. B3LYP/6-31G(d) optimized structure of 1
This is another molecule to be sure to click on and rotate using JMol.

The nature of the HOMO and LUMO of 1 is very different than that of linear tetra-2,7-pyrene. The degenerate HOMOs and degenerate LUMOs of the linear compound have a node at the 2 and 7 positions and are localized to the terminal and central pyrene units, respectively. The HOMO and LUMO of 1 are fully delocalized. The implications of this are seen in the spectroscopy and electrochemistry of 1.

References

(1) Iwamoto, T.; Kayahara, E.; Yasuda, N.; Suzuki, T.; Yamago, S. "Synthesis, Characterization, and Properties of [4]Cyclo-2,7-pyrenylene: Effects of Cyclic Structure on the Electronic Properties of Pyrene Oligomers," Angew. Chem. Int. Ed. 2014, 53, 6430-6434, DOI: http://dx.doi.org/10.1002/anie.201403624.

InChIs

1: InChI=1S/C64H32/c1-2-34-18-50-20-36-4-3-35-19-49(17-33(1)57(35)58(34)36)51-21-37-5-7-41-25-53(26-42-8-6-38(22-51)59(37)61(41)42)55-29-45-13-15-47-31-56(32-48-16-14-46(30-55)63(45)64(47)48)54-27-43-11-9-39-23-52(50)24-40-10-12-44(28-54)62(43)60(39)40/h1-32H/b51-49-,52-50-,55-53-,56-54-
InChIKey=AWPUJIHMRRFHHU-GYSOBDCPSA-N