HETEROCYCLES

Communication | Special issue | Vol. 82, No. 2, 2011, pp. 1143-1149
Received, 14th July, 2010, Accepted, 2nd September, 2010, Published online, 3rd September, 2010.
DOI: 10.3987/COM-10-S(E)96

■ McMurry Coupling of Diformyldithienylacetylene: Synthesis of [24]-, [36]-, and [48]Annulenes Composed of Thiophene, Acetylene, and Ethylene Units

Masahiko Iyoda,* Pochi Huang, Tomohiko Nishiuchi, Masayoshi Takase, and Tohru Nishinaga

Graduate School of Science, Tokyo Metropolitan University, 1-1, Minami-ohsawa, Hachioji, Tokyo 192-0364, Japan

Abstract

The reaction of diformyldithienylacetylene 4 with a McMurry reagent prepared from TiCl4, Zn, and pyridine in THF afforded 24π-dimer 1b (15%), 36π-trimer 2b (13%), and 48π-tetramer 3b (6%). From X-ray analysis, 1b adopts a twisted conformation, although the 1H NMR spectrum of 1b shows a symmetrical structure owing to a rapid conformational change in solution. Absorption and emission spectra, redox properties, and electric conductivities of 1b–3b were measured in order to study the structure–property relationship of these macrocyclic systems.

Fully conjugated macrocycles with well-defined shapes have attracted considerable attention owing to their potential applications in organic electronic and photovoltaic devices, field effect transistors, and chemical sensors.1 In this regard, π-expanded porphyrins and cyclic oligothiophenes have been extensively studied as candidates for novel electronic materials.2,3 Kawase and Oda reported the synthesis and X-ray structure of 5,6,17,18-tetradehydrotetrathia[24]annulene[2.2.2.2] (1a) using a double Fritsch-Buttenberg-Wiechell rearrangement reaction.4 In the same communication, they also reported that the McMurry coupling reaction of 5,5ʹ-diformyl-2,2ʹ-dithienylacetylene with TiCl3(DME)1.5 and Zn(Cu) in DME affords only a trace amount of 1a with macrocyclic oligothiophene 2a. Recently, we have reported the synthesis of giant macrocycles using McMurry coupling with TiCl4, Zn, and pyridine in THF.5,6 We synthesized octabutyl derivative 1b using the same reaction conditions. The latter McMurry coupling sometimes yields macrocycles in better yields than the former one can (Scheme 1).7 We report here the synthesis of tetradehydrotetrathia[24]annulene 1b, hexadehydrohexathia[36]annulene 2b, and octadehydrooctathia[48]annulene 3c together with their structure–property relationship.

Dialdehyde 4 was prepared via a four-step reaction starting from 3,4-dibutylthiophene in 37% overall yield.8,9 As reported previously, the McMurry coupling reagent, which is a low-valent titanium reagent prepared from the reaction of TiCl4 (excess amounts) with Zn in the presence of pyridine in THF, is an effective catalyst to produce cyclooligomerization products.5 Thus, cyclooligomerization of 4 under modified McMurry conditions (low-valent titanium reagent (5 equiv.), pyridine (7.5 equiv.), THF, 65 ºC, 3 h) resulted in the formation of dimer 1b, trimer 2b, and tetramer 3b in 15%, 13%, and 6.0% yields, respectively.10 The low yield of 1b as compared to that by using the previously reported cyclooligomerization may be due to the strained acetylene bonds in 1b. A similar reaction of 4 with 10 equiv. of a low-valent titanium reagent for a longer time resulted in a marked decrease in the yield of 1b owing to the reduction of the strained acetylene bonds in 1b to produce 5.11 Although 1b formed yellow crystals from hexane, 2b and 3b precipitated as gummy orange solids. All of the compounds are stable and can be stored at room temperature.

The structure of 1b was determined by using X-ray structural analysis (Figure 1).12 Single crystals of 1b were prepared from ether‒methanol. As shown in Figure 1, 1b has a crystallographic C2 symmetry axis passing through the midpoints of the two acetylene bonds (C5-C5* and C11-C11*). The sulfur atom S1 is located in the center of the neighboring thiophene ring, and the S1···S2 distance is 3.262(1) Å, which is 12% shorter than the sum of van der Waals radii (3.7 Å). Although the X-ray structure of 1a has been reported to be planar, 1b has an extremely twisted structure. The 1H NMR spectrum of 1b at room temperature was consistent with a symmetrical structure having two different butyl groups and an olefinic singlet (δ 6.61). In other words, 1b undergoes a rapid conformational change in solution. Furthermore, the 1H NMR spectra of all-Z-2b (two different butyl groups and an olefinic singlet (δ 6.57)) exhibited no conformational change in solution, whereas and E,Z,E,Z-3b (two different butyl groups and two different olefinic singlet (δ 6.90 and 6.61)) showed a rapid rotation of the two E-olefins at room temperature.13

As shown in Figure 2, the absorption maxima of 1b–3b (λmax: 1b, 355 nm; 2b, 369 nm; 3b, 436 nm) were red-shifted in relation to the size of the π system, whereas in the fluorescence spectra of 1b–3b, no red shift was observed (λem: 1b, 582 nm, ΦF = 0.0003; 2b, 587 nm, ΦF = 0.001; 3b, 589 nm, ΦF = 0.03).14 The low quantum yield of 1b may be due to the rapid conformational change in solution.

All of the compounds had relatively low oxidation potentials, which were determined by using cyclic voltammetry: 1b, Eox1/2 = 0.37 V (2e); 2b, Eox1/2 = 0.33 V (2e); 3b, Eox11/2 = 0.28 V (1e); and Eox21/2 = 0.56 V (1e) vs. Fc/Fc+.15 These macrocycles can be oxidized to produce aromatic 22π and 34π dications (NICS(0) = –17.123 and –17.02, respectively). However, 1b2+ and 2b2+ were not stable enough to observe via 1H NMR spectroscopy, and we could only acquire their UV-vis-NIR spectra.

The most interesting feature in these 24π, 36π, and 48π annulenes is the structure of 1b. Although Kawase and Oda have reported that 1a has a planar π-π stacked structure, MO calculations indicate that 1a should have a twisted conformation like the crystal structure of 1b. As shown in Figure 3, there is strong S···S overlap in HOMO-6 and HOMO-7.4,16 In other words, the S atoms interact favorably.

In summary, we synthesized octabutyltetradehydrotetrathia[24]annulene[2.2.2.2] (1b) and the higher homologues 2b and 3b by using a modified McMurry coupling reaction. From X-ray analysis, 1b adopts a twisted conformation with a C2 axis passing through the midpoints of the two acetylene bonds. On the basis of the spectroscopic characterization of 1b and the MO calculations on tetradehydrotetrathia[24]annulene[2.2.2.2] (1a), a twisted structure appears to be the most stable conformation of 1a.

ACKNOWLEDGEMENTS
This work was partially supported by a grant-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. The authors are thankful to Prof. Takeshi Kawase (University of Hyogo) for acquiring the X-ray diffraction data for 1a. The authors are greatly indebted to Mr. Tomoshi Tomizawa, Mr. Jun Yamakawa, Dr. Eigo Isomura, and Dr. Mohammad Jalilur Rahman (Tokyo Metropolitan University) for experimental assistance and helpful discussions.

References

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7. Z. Hu, J. L. Atwood, and M. P. Cava, J. Org. Chem., 1994, 59, 8071. CrossRef
8. Dialdehyde 4 was prepared in 75% yield by successive treatments of bis(3,4-dibutyl-2- thienyl)acetylene9 with n-BuLi (2.75 equiv.) at 0 ºC and DMF (2.9 equiv.) in the temperature range from 0 ºC to room temperature.
9. C. Ringenbach, A. De Nicola, and R. Ziessel, J. Org. Chem., 2003, 68, 4708. CrossRef
10. Spectral data of new compounds: 1b, yellow crystal, mp 116 ºC (DSC), 1H NMR (500 MHz, CDCl3)
δ 6.61 (s, 4H), 2.61 (t, J = 7.8 Hz, 8H), 2.47 (t, J = 7.8 Hz, 8H), 1.33–1.58 (m, 32H), 0.87–0.94 (m, 24H); 13C NMR (125 MHz, CDCl3) δ 144.80, 141.4, 134.6, 123.6, 120.7, 92.5, 32.9, 32.7, 28.4, 27.3, 22.8, 22.7, 13.9; MS (EI) m/z 877 (M+); 2b, red amorphous waxy solid, 1H NMR (500 MHz, CDCl3) δ 6.57 (s, 6H), 2.64 (t, J = 7.8 Hz, 12H), 2.49 (t, J = 7.8 Hz, 12H), 1.34–1.59 (m, 48H), 0.90–0.94 (m, 36H); 13C NMR (125 MHz, CDCl3) δ 146.3, 141.2, 134.0, 122.6, 119.3, 89.7, 32.8, 32.5, 28.3, 27.3, 22.7, 22.7, 14.0, 13.8; LDI-TOF-MS m/z 1314.70 (M+); 3b, red amorphous waxy solid, 1H NMR (CDCl3) δ 6.90 (s, 4H), 6.61 (s, 4H), 2.67 (t, J = 7.8 Hz, 8H), 2.63 (t, J = 7.8 Hz, 8H), 2.56 (t, J = 7.8 Hz, 8H), 2.51 (t, J = 7.8 Hz, 8H), 1.35–1.61 (m, 64H), 0.94 (m, 48H); 13C NMR (125 MHz, CDCl3) δ 147.7, 146.2, 141.6, 140.1, 137.7, 134.2 , 122.7, 120.2, 119.3, 117.1, 90.5, 90.2, 33.2, 33.0, 32.64, 32.56, 28.41, 28.36, 27.4, 27.1, 22.81, 22.78, 22.75, 22.73, 14.1, 14.03, 13.95, 13.93; LDI-TOF-MS m/z 1753.10 (M+); 4, red viscous oil, 1H NMR (500 MHz, CDCl3) δ 10.00 (s, 2H), 2.89 (t, J = 8.0 Hz, 4H), 2.71 (t, J = 7.8 Hz, 4H), 1.56–1.62 (m, 8H), 1.40–1.46 (m, 8H), 0.98-0.95 (m, 12H); 13C NMR (125 MHz, CDCl3) δ 180.9, 150.0, 148.0, 137.6, 125.8, 90.9, 33.3, 31.5, 26.8, 26.0, 21.7 (2C), 13.0, 12.8; MS (EI) m/z 470 (M+)
.
11. 5 was obtained in 10% yield based on 4. Yellow viscous oil, 1H NMR (500 MHz, CDCl3) δ 6.46 (s, 8H), 2.46 (t, J = 7.8 Hz, 16H), 1.27–1.43 (m, 32H), 0.89 (t, J = 7.8 Hz, 24H); 13C NMR (125 MHz, CDCl3) δ 144.80, 141.4, 134.6, 123.6, 120.7, 92.5, 32.9, 32.7, 28.4, 27.3, 22.8, 22.7, 13.9; MS (EI) m/z 880 (M+).
12. X-Ray analysis of 1b: C56H76S4, Mw 877.41, monoclinic, space group C2/c (#15), a = 24.271(6) Å, b = 10.824(3) Å, c = 22.530(6) Å, β = 118.543(5)º, V = 5200(2) Å3, Z = 4, Dc = 1.121 g cm−3, R1 = 0.0796, Rw = 0.1854, GOF = 1.060. Among a total of 11150 reflections measured, 3752 were unique, and the observed (I > 2.00σ(I )) 2948 reflections were used for the refinement. The crystal structure was solved by using the SHELXS-97 program and refined by using the full matrix least-squares method in the SHELXS-97 software package. Cambridge Crystallographic Data Centre deposition No. CCDC‒752997.
13. The most stable diastereomers of 2b and 3b are all-Z- and E,Z,E,Z-isomers, respectively, on the basis of MO calculations at the B3LYP/6-31G(d) level.
14. Fluorescence quantum yields (ΦF) were determined via a comparison with quinine sulfate in 0.5 M H2SO4 (ΦF = 0.546).
15. CV analyses were carried out using 0.5 M solutions of the compounds in CH2Cl2 at 23 °C using Pt working and counter electrodes, Ag/Ag+ as a reference electrode, and Bun4NPF6 as a supporting electrolyte.
16. Kawase and Oda have reported a planar X-ray structure for 1a.4 Since the S···S interatomic distances are 3.014 and 3.024 Å, which are 18.3% and 18.5% shorter than the sum of van der Waals radii (3.7 Å) respectively, the distances are too short to stabilize the structure. However, the π-π stacking interactions in the crystal may stabilize the planar structure.
17. HOMOs of 1a on the basis of MO calculations at the B3LYP/6-31G(d) level are as follows: