Abstract
Reaction of 1,1-dilithio-2,3,4,5-tetraphenylsilole (*5*) with diphenylcyclopropeneone proceeds with loss of silicon to yield the dilithium salt of 2,3,4,5-tetraphenyl-1,4-hysroquinone (*8*), which was trapped with trimethylchlorosilane to yield 1,4-bis(trimethylsiloxy)-2,3,5,6-tetraphenylbenzene, *9*. A possible pathway for the formation of *8* and *9* is proposed.
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References
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Experimental. Diphenylcyclopropenone and lithium were purchased from Aldrich and used without further purification. Solvents were distilled from sodium benzophenone ketyl. All procedure with air- and moisture-sensitive compounds were carried out using a Schlenk line under a nitrogen or argon atmosphere. NMR spectra were recorded at 25°C on a Varian UNITY-500 spectrometer at 499.625 MHz for 1H, 125.714 MHz for 13C, 99.326 MHz for 29Si. Reaction mixtures were purified by column chromatography using Silica gel 60.
Preparation of a dilithiotetraphenylsilole (5). A solution of 1,1-dichloro-2,3,4,5-tetraphenylsilole9 (500 mg, 1.10 mmol) in 10 mL of THF with Li (110 mg, 15.9 mmol) was stirred at 60 °C for 24 h gave a dark red solution of 5. The solution was filtered and used for further reactions.
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Preparation of 8 and 9. A solution of diphenylcyclopropenone (453 mg, 2.20 mmol) in 10 mL of THF was slowly added to a dark red solution of 5 (1.10 mmol) in THF prepared as described above at −78°C and stirred for 3 h. The solution was warmed to room temperature and stirred for an additional 3 h to give a red solution of a silaspirotropylidenyl dianion 7 or 2,3,5,6-tetramethylbenzene hydroquinone dianion 8 (Scheme 3). A solution of Me3SiCl (0.28 mL, 2.20 mmol) in 5 mL of THF was added to the red solution at -78 oC and the mixture was stirred at room temperature for 40 h. After evaporation of solvents, the NMR spectra of the yellow-brown solid residue indicated a ∼65% yield of 9. The 29Si NMR of the crude product showed a strong signal at 19.06 ppm (84%, due to 9) and weaker dignals at 17.57 (11%) and 17.17 ppm (5%). The product 9 was purified by column chromatography (eluent hexanes – benzene, 0 to 50% benzene). and obtained as pale-yellow crystals (yield 37%, based on starting diphenylcyclopropenone). 1H NMR (CDCl3): δ = 0.22 (s, 18H, OSiMe3), 6.60–6.97 (m, 20H, Ar). 13C NMR (CDCl3): δ = 0.68 (s, OSiMe3), 125.1–132.4 (m, Ar). 29Si NMR (CDCl3): δ = 19.06. X-ray crystal data for C36H38O2Si2: monoclinic P21/n, a = 11.6748(7) Å, b = 6.7311(4) Å, c = 19.9599(13) Å, α = 90°, β = 97.487(1)°, γ = 90°, V = 1555.16(17) Å3, Z = 2, T = 100(2) K, D calc = 1.193 Mg/m3. All non-hydrogen atoms were refined with anisotropic displacement coefficients. All hydrogen atoms were included in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients. The molecule resides on a crystallographic inversion center. The final least-squares refinement of 184 parameters against 3183 data resulted in residuals R (based on F 2 for I ≥ 2σ) and wR (based on F 2 for all data) of 0.0456 and 0.1295, respectively. The final difference Fourier map was featureless. All software used is contained in the Bruker-AXS software suite: Bruker-AXS. (2000-2003) SADABS V.2.05, SAINT V.6.22, SHELXTL V.6.10 & SMART 5.622 Software Reference Manuals. Bruker-AXS, Madison, Wisconsin, USA.
Reaction of diphenylcyclopropenone with strong reducing agents (Mg/MgI2, CpMgBr, Na/Hg) produces some 8. See Toshima, N., Moritani, I. and Nishida, S., 1967, Bull. Chem. Soc. Japan 40, 1245. However this does not seem to account for our results since the amounts of 8 formed are very small (1–5%) and are always accompanied by much larger amounts of the ortho isomer.
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Timokhin, V.I., Guzei, I.A. & West, R. An unusual reaction of 1,1-dilithio-2,3,4,5-tetraphenyl-1-silacyclopentadiene with diphenylcyclopropenone: synthesis of 1,4-Bis(trimethylsiloxy)-2,3,5,6-tetraphenylbenzene. Silicon Chem 3, 75–78 (2006). https://doi.org/10.1007/s11201-005-9004-1
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DOI: https://doi.org/10.1007/s11201-005-9004-1