§ 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
147–149 °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).
(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 At-
tempts 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 cer-
ium(IV) sulfate in 1,2-dichloroethane at room temperature to
give the sexadecithiophene 4c (35% yield), the tetracosithio-
phene 4d (11%), and a mixture of higher oligomers (ca. 20%)
after column chromatography on silica. The dotriacontithio-
phene 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.
¶ 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. Bäuerle, 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. Bäuerle, U. Segelbacher, A. Maier and M. Mehring, J. Am. Chem. Soc.,
1993, 115, 10217.
Notes and references
† New compounds have been characterised by spectroscopic and micro-
8 G. Engelmann, G. Kossmehl, J. Heinze, P. Tschuncky, W. Jugelt and
H-P. Welzel, J. Chem. Soc., Perkin Trans. 2, 1998, 169.
analytical data.
‡ Prepared by bromination of the appropriate disilyl derivative: NBS, aq.
THF, 0 °C.
Communication 8/06600K
2744
Chem. Commun., 1998, 2743–2744