Synthetic Routes toward Asymmetrically Substituted (Functionalized) 4H-Cyclopenta[2,1-b:3,4-b′]dithiophenes

Abstract

A two-step synthetic protocol involving (i) a Wittig-type carbonyl olefination, and (ii) regioselective alkylation of the exocyclic double bond with LiAlH₄ and an alkyl bromide, was developed as an alternative to the recently reported three-step synthetic approach toward asymmetrically substituted/functionalized 4H-cyclopenta[2,1-b:3,4-b′]dithiophenes. The two routes are rather complementary, with specific advantages depending on the desired substitution pattern, and are of particular appeal for the construction of semiconducting materials to be applied in organic photovoltaics.

Key words

cyclopentadithiophene - Friedel–Crafts cyclization - Wittig reaction - reduction - organic photovoltaics

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• 18 4-(Octylidene)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (8a): n-BuLi (4.0 mL of a 2.5 M solution in n-hexane, 10.0 mmol) was added to a solution of octyltriphenylphosphonium bromide (4.55 g, 10.0 mmol) in anhyd THF (30 mL) at –82 °C under N₂ atmosphere. The mixture was stirred for 30 min at –82 °C and then a solution of CPDT-4-one 7 (1.48 g, 7.69 mmol) in anhyd THF (30 mL) was added. After stirring for an additional 30 min at this temperature, the mixture was allowed to reach r.t. The reaction was quenched with H₂O and the aqueous layer was extracted with Et₂O. After drying the combined organic layers over MgSO₄, removal of the solvents in vacuo, and purification by column chromatography (SiO₂, petroleum ether), a yellow oil was obtained (1.64 g, 74%). ¹H NMR (300 MHz, CDCl₃): δ = 7.27 (d, J = 4.7 Hz, 1 H), 7.13 (d, J = 4.8 Hz, 1 H), 7.12 (d, J = 4.8 Hz, 1 H), 7.08 (d, J = 4.9 Hz, 1 H), 6.44 (t, J = 7.8 Hz, 1 H), 2.68 (q, J = 7.5 Hz, 2 H), 1.62 (quint, J = 7.4 Hz, 2 H), 1.23–1.48 (m, 8 H), 0.89 (t, J = 6.8 Hz, 3 H).
• 19 4-(2-Ethylhexyl)-4-octyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (10a; see ref. 9a): A solution of 2-ethylhexyl bromide (1.40 g, 7.25 mmol) and CPDT 8a (2.09 g, 7.25 mmol) in anhyd THF (25 mL) was added dropwise to a suspension of LiAlH₄ (0.275 g, 7.25 mmol) in anhyd THF (10 mL) under N₂ atmosphere at r.t. After 15 h, Et₂O (50 mL) and aq HCl (1.0 M, 30 mL) were added carefully. The organic layer was separated and washed with aq NaHCO₃ and H₂O, followed by drying over MgSO₄. Purification by column chromatography (SiO₂, petroleum ether) and removal of the solvent under reduced pressure yielded a colorless oil (2.28 g, 78%).

A few functionalized asymmetrically dialkylated CPDTs have already been reported (see also ref. 13):
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