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 β-positions of thiophenes was one of the key steps in the synthesis.

Key words

helical structures - cross-coupling - alkylthiophene - ­helicene - oligothiophene

References

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23

Attempts at direct alkylation of α-metallated 4 with 1-bromooctane resulted in either negligible extent of conversion and/or migration of the TMS group with alkylation at the α-position adjacent to the bromine.

30

LDA is added to 13 in Et₂O.

31

Upon the resolution enhancement (exponential line broadening, LB = -1.20 Hz, and Gaussian broadening, GB = +0.30 Hz) of the ¹H NMR spectra for Li ₁ , the upfield (δ = 6.763 ppm) and the downfield (δ = 6.673 ppm) resonances appear as a doublet (J = 1.5 Hz, resolved nearly to the baseline) and a broad singlet (with multiple shoulders), respectively. For Li ₂ , the ¹H resonance at δ = 7.231 ppm appears as a broadened triplet (a singlet with two shoulders).

32

In the presence of (-)-sparteine, yields of 15, as determined by ¹H NMR spectra of crude mixtures, are in the 40-50% range.

34

The center part of the multiplet is somewhat broadened.