S S S SiMe3 Me3Si S S S S Me3Si SiMe3 Me3Si S S SiMe3 N n n n S S S Me3Si SiMe3 n 1 2ndash;4 a n = 0 b n = 1 (42) c n = 2 (83) d n = 3 (94) e n = 4 (8) 2 n = 0, N = 0 a n = 1, N = 0 b n = 1, N = 1 a n = 2, N = 0 b n = 2, N = 1 c n = 2, N = 2 d n = 2, N = 4 e n = 2, N = 6 1 4 3 Synthesis of extended a,aA-oligo(silylthiophenes) by cerium(IV) oxidative coupling reactions Ali H. Mustafa and Michael K. Shepherd* School of Biological and Applied Sciences, University of North London, UK N7 8DB.E-mail: m.shepherd@unl.ac.uk Received (in Liverpool, UK) 21st August 1998, Accepted 9th November 1998 Cerium(IV) oxidative dimerisations of 4,4A-bis(trimethylsilyl)- 2,2A:5A,2AA-terthiophene and the quaterthienyl analogue 1c gave the corresponding sexi- and octi-thiophenes; the latter have been used to prepare a-linked hexadecithiophene and tetracosithiophene derivatives. The extreme insolubility of extended oligothiophenes renders purification and spectral analysis increasingly difficult for unsubstituted systems containing more than six a-linked thiophene units.Only two literature citations can be found for oligomers beyond octithiophene, namely decithiophene, obtained by electrochemical oxidation of quinquithiophene and characterised by absorption spectra in the solid state,1 and dodecithiophene, identified as an impurity in crude samples of octithiophene by mass spectroscopy.2 Oligo(alkylthiophenes) bearing long-chain b-substituents are much more soluble, and derivatives containing up to sixteen rings have been characterised. 3ndash;5 The longest of these, an octadodecylhexadecithiophene, was obtained in 19 yield after coupling of the corresponding lithiooctithiophene with CuCl2.5 We reasoned that silylated oligomers should be soluble enough to aid purification and, via electrophilic displacement of silicon, subsequently afford straightforward access to the unsubstituted analogues and other derivatives required for physical studies. We describe here our results to date, culminating in the isolation of the twentyfour-ring dodecakis(trimethylsilyl)tetracosithiophene 4d, the longest oligomer yet reported.We obtained b,b-disilylated thiophenes 1bndash;edagger; by the NiCl2(dppp)-catalysed couplings6 of 2 equiv. of the Grignard reagent prepared from 2-bromo-4-trimethylsilylthiopheneDagger; with 2,5-dibromothiophene, 5,5A-dibromo-2,2A-bithiophene, 5,5A-dibromo-2,2A:5A,2AA-terthiophene and 5,5AAA-dibromo- 2,2A:5A,2AA:5AA,2AAA-quaterthiophene,Dagger; respectively.The yields increase on moving from 1b to the quinquithienyl 1d, but the very low solubility of both the sexithienyl 1e and the dibromoquaterthienyl precursor rendered this route impractical for the preparation of oligomers containing more than five rings. To circumvent this problem we turned our attention to coupling the readily-soluble quaterthienyl 1c. We were encouraged by the observation that aged thin-layer chromatograms of 1c showed progressive formation of a lower Rf component on re-elution. This was attributed (and subsequently confirmed by comparison with pure samples) to photooxidation of 1c to the dimer 4a on the silica surface.A reaction of 1c with ceric ammonium nitrate gave moderate yields (ca. 15) of 4a, the major product being 3,4AAA-bis(trimethylsilyl)- 5-nitro-2,2A:5A,2AA:5AA,2AAA-quaterthiophene (43). When using ceric ammonium sulfate as the oxidising agent the octithiophene 4a was isolated in up to 66 yield,sect; with varying amounts of the dodecithiophene 4b (4ndash;7) and hexadecithiophene 4c (0.5ndash;2) as byproducts (Scheme 1).The high yield of dimer obtained under these conditions is surprising, bearing in mind that oxidation potentials for these systems should be inversely related to the size of the oligomer.6,7 Cerium(IV) sulfate oxidations were more rapid, but the relative proportions of 4b,c and other higher molecular weight products increased with respect to 4a.The oligomers 4andash;c are bright yellow crystalline solids which exhibit intense greenish fluorescence in dilute solution. The UVndash;VIS absorption spectra show only a small bathochromic shift on moving from 1c (lmax 389 nm) to 4c (lmax 419 nm), in contrast to the n-dodecyl derivatives,5 which move from yellow (lmax 379 nm) to violet (lmax 464 nm) along the same sequence. In our series, the closer proximity of the bulky silyl substituents to the point of coupling presumably leads to a higher degree of twisting about every fourth a,aAlinkage, reducing through-conjugation.This may be fortuitous mdash;levelling the oxidation potentials of these systems and hence suppressing their tendency to polymerise. Coupling of the terthiophene 1b under conditions as described above gave the sexithiophene 3a (48 yield) and the nonithiophene 3b (11) as major products,para; along with higher oligomers. This reaction proved more sluggish than that above, probably as a consequence of the higher energy of the terthienyl cation radical intermediate.A reaction of 4-trimethylsilyl- 2,2A:5A,2AA-terthiophene resulted in coupling in even lower yield Scheme 1 Chem. Commun., 1998, 2743ndash;2744 2743(8), between the two a-positions adjacent to the b-silyl group; the sexithiophene 1e and its 4,3AAA-disilyl isomer were not detected. These observations are in line with a recent study on the stabilising effect of progressive b-methylation on cation radicals derived from 5,5A-dimethyl-2,2A-bithiophene.8 Attempts to dimerise 4,4A-bis(trimethylsilyl)-2,2A-bithiophene 1a to 2 in like manner resulted in quantitative recovery of the starting material.The octithiophene 4a oligomerised on reaction with cerium( IV) sulfate in 1,2-dichloroethane at room temperature to give the sexadecithiophene 4c (35 yield), the tetracosithiophene 4d (11), and a mixture of higher oligomers (ca. 20) after column chromatography on silica. The dotriacontithiophene 4e appears to be the major constituent of the residue, corroborated by TLC studies of dimerisation attempts using 4c, but we have not yet been able to isolate pure samples.The relative insolubility of the higher oligomers of 4 has prevented preparative separation by chromatography, while the more soluble analogues 3 are not satisfactorily resolved beyond 3b. We anticipate that replacement of either or both trimethylsilyl substituents in 1c by more lipophilic alkylsilyl groups should circumvent this problem and ultimately, via protodesilylation, enable preparation of the parent systems. Preliminary studies indicate that compounds 3a and 4a give quantitative yields of analytically pure sexithiophene and octithiophene respectively, on treatment with TFA in CH2Cl2.Notes and references dagger; New compounds have been characterised by spectroscopic and microanalytical data. Dagger; Prepared by bromination of the appropriate disilyl derivative: NBS, aq.THF, 0 deg;C. sect; Preparation of 4a: Finely ground cerium ammonium sulfate (10.5 g) was added in portions over ca. 6 h to a stirred solution of 1c (3.0 g) in C6H6 (40 cm3). The resulting suspension was stirred at room temperature for ca. 30 h, the solvent removed under reduced pressure, and the solid residue extracted with pentane (100 cm3). The extract was chromatographed directly on silica (the column should be shielded from light) eluting with pentane to give unreacted 1c (0.72 g).Further eulution with 4 CH2Cl2 in pentane gave 4a (1.87 g, 62, 82 based on consumed 1c), bright yellow crystals, mp 147ndash;149 deg;C (Found: C, 55.9; H, 5.3. C44H50S8Si4 requires C, 55.8; H, 5.3); dH(250 MHz, CDCl3) 7.29 (2 H, d, a-H, A,H-rings), 7.20 (2 H, d, b- H, A,H-rings), 7.13 (2 H, s, D,E-rings), 7.08 (8 H, m, B,C,F,G-rings), 0.29 (18 H, s, SiMe3, A,H-rings), 0.17 (18 H, s, SiMe3, D,E-rings); dC 130.8, 129.3, 128.3, 124.7, 124.6, 124.3, 124.2 (C-H), 143.7, 143.2, 140.6, 137.9, 137.8, 136.4, 136.1, 135.8, 135.6 (quaternary carbons), 0.6, 20.7 (CH3); m/z (LSI) 947 (M + 1)+ (100).Further gradient elution up to 25 CH2Cl2 in pentane gave 4b (125 mg, 4), followed by 4c (14 mg, 0.5) and a mixture of higher oligomers (15 mg). para; The yields for coupling reactions of 1b and 4a have not been optimised, and do not take into account recovery of the precursors. 1 Z. Xu, D. Fichou, G. Horowitz and F. Garnier, J. Electroanal. Chem. Intrafacial Electrochem., 1989, 267, 339. 2 D. Fichou, M. Teulade-Fichou, G. Horowitz and F. Demanze, Adv. Mater., 1997, 9, 75. 3 W. ten Hoeve, H. Wynberg, E. E. Havinga and E. W. Meijer, J. Am. Chem. Soc., 1991, 113, 5887. 4 M. Sato and M. Hiroi, Chem. Lett., 1994, 985. 5 P. Bauml;uerle, T. Fischer, B. Bidlingmeir, A. Stabel and J. P. Rabe, Angew. Chem., Int. Ed. Engl., 1995, 34, 303. 6 D. D. Cunningham, L. Laguren-Davidson, H. B. Mark, C. Van Pham and H. Zimmer, J. Chem. Soc., Chem. Commun., 1987, 1021. 7 P. Bauml;uerle, U. Segelbacher, A. Maier and M. Mehring, J. Am. Chem. Soc., 1993, 115, 10217. 8 G. Engelmann, G. Kossmehl, J. Heinze, P. Tschuncky, W. Jugelt and H-P. Welzel, J. Chem. Soc., Perkin Trans. 2, 1998, 169. Communication 8/06600K 2744 Chem. Commun., 1998, 2743ndash;2744
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