Synthesis of Functionalized Carbon-Sulfur [5]Helicene: Pd-Catalyzed Negishi Cross-Coupling Between the β-Positions of Thiophenes

Abstract

Octyl-and bromo-substituted carbon-sulfur [5]helicene was prepared in several steps starting from either thiophene or 3-bromothiophene. Pd-catalyzed Negishi cross-coupling between the b-positions of thiophenes was one of the key steps in the synthesis.Helicenes, molecules with annelated and helical p-conjugated systems, have been known for about 50 years. 1-3 In the past few years, there has been a resurgence of interest in synthesis of helicenes. 4-10 Because of the strong chiral, especially chirooptical, properties, helicenes are attractive building blocks for chiral materials. 11 Our own interest is the development of amorphous thin films with extraordinary high optical rotatory powers for optical waveguides and magneto-conductive materials (e.g., chiral metals). 12,13 Carbon-sulfur helicenes may be viewed as novel annelated oligothiophenes and as fragments of the intriguing (C 2 S) n carbon-sulfur helix ( Figure 1). 14-17 C 2 -symmetric [7]helicene 1 is the only helicene of this class known to date. 18 [7]Helicene 1 can be obtained in one enantiomeric form. 19 Short helicenes, which are adequately functionalized for metal-catalyzed cross-couplings and possess relatively low barriers for racemization, are desirable for efficient syntheses of extended helicenes. Well-soluble alkyl-substituted helicenes, compatible with a wide range of redox potentials, are among the targets. We now report the synthesis of C 1 -symmetric carbon-sulfur [5]helicene 2 ( Figure 1).The previously reported iterative synthesis of C 2 -symmetric 1 was based upon Cu II -mediated CC-bond forming homocouplings between the b-positions and CS-bond forming annelations at the a-positions of thiophenes. 18 Although the b-positions of thiophenes, especially when sterically hindered, may be considered as 'difficult' for CC-bond forming cross-couplings, the development of suitable conditions may allow for an efficient synthesis of less symmetric helicenes, such as 2.4-Bromo-2-octylthiophene 7 is prepared according to two routes (Scheme 1). 20 In route A, 3-bromothiophene 3 is treated with LDA, followed by addition of trimethylchlorosilane (TMSCl), to give TMS-protected product 4. 21 Treatment with LDA, followed by addition of N-methoxy-N-methyloctanamide provides ketone 5. 22,23 Subsequent reduction and deprotection of the TMS group gives thiophene 7 in isolated 45% yield for four steps. 24 Route B offers a shorter, three-step synthesis of 7, starting from inexpensive thiophene 8. Monoalkylation of metallated thiophene provides 2-octylthiophene 9, which is brominated with NBS to yield 10. [25][26][27] Lithiation with LDA in THF, followed by MeOH-quench results in rearranged product 7 in isolated 36% yield for three steps. 28 The reaction conditions for the Negishi coupling between 7 (1.0 equiv) and 12 (0.84 equiv) are optimized as follows Figure 1 Scheme 1 Synthesis of 4-bromo-2-octylthiophene 7. S Br S Br TMS X S Br X C 8 H 17

Synthesis and Physicochemical Properties of Piceno[4,3-b:9,10-b′]dithiophene Derivatives and Their Application in Organic Field-Effect Transistors

“…2-Octylthiophene ( 2b ), 22 2-decylthiophene ( 2c ), 23 2-bromo-5-octylthiophene ( 3b ), 24 2-bromo-5-decylthiophene ( 3c ), 23 4-bromo-2-octylthiophene ( 4b ), 25 3-thiophenecarboxaldehyde ( 5a ), 26 and 2,7-dibromophenanthrene ( 6 ) 12 showed identical spectroscopic data to that reported in the literature.…”

Construction and Phosphorescence Behavior of S/Se-Heteroaromatics/Phenanthrene-Fused Hetero[9]helicenes

“…They have been widely applied in multiple fields, such as asymmetric catalysis, molecular recognition, and circularly polarized luminescence (CPL) materials. − Thia-helicenes are a subclass of the helicene family, with at least one benzene unit replaced by thiophene, changing distinct electronic properties and chiroptical properties from their parent molecules. The typical carbon–sulfur helicenes, bearing all sulfur atoms along the outer edge of the molecular skeletons, have been systematically studied by Rajca. − Wang and co-workers have developed this chemistry, and exhibited double thiahelicenes, , triple thiahelicenes, and thiophene/selenophene-based helicenes. , …”