Carbon—sulfur helicene

M. Miyasaka, A. Rajca, Synthesis of a Short Carbon—Sulfur Helicene Pd-cata-lyzed Cross-coupling at the >-Positioris of Thiophenes, Synlett 2004, 177—182. 

Fig. 15.15 Iterative synthesis and asymmetric synthesis of carbon-sulfur [7]helicene 40.

Recently, series of carbon-sulfur [n]helicenes substituted with w-octyl groups at the a-positions of the terminal thiophene rings were prepared (Fig. 15.16) [64, 87]. The helical structures of [7]helicene 44 and [ll]helicene 45 were confirmed by X-ray crystallography. Multiple short S-S contacts were found, especially for racemic [ll]helicene 45. Asymmetric synthesis of [ll]helicene 45 provided enantiomeric excess of either the (-)- or the (+)-enantiomer for the monoannelation or, unprecedented, triannelation approach (Fig. 15.16). Also, selective diannelation of octathiophene 47, followed by monoannelation of decathiophene 46, provided an efficient synthetic route to (-)-[ll]helicene 45, avoiding protection/deprotection steps [64],... 

Carbon sulfur [7]helicene (M)-(-)-40, which has a relatively low turning angle but possesses two bulky bromine groups at the ends of the inner helix, has a free energy barrier for racemization of 39.0 kcal mol-1 (half-life of 11 h at 199 °C) [85], The relatively small helical turn in-plane (based on the X-ray structures of two racemic polymorphs) is apparently offset by two bulky bromine groups at the ends of the inner helix. 

A. Rajca, M. Miyasaka, M. Pink, H. Wang, S. Rajca, Helically Annelated and Cross-conjugated Oligothiophenes Asymmetric Synthesis, Resolution and Characterization of a Carbon-Sulfur [7]Helicene, J. Am. Chem. Soc. 2004, 126, 15211-15222. 

M. Miyasaka and A. Rajca, Synthesis of functionalized carbon-sulfur [5]helicene Pd-catalyzed Negishi cross-coupling between the beta-positions of thiophenes, Synlett, 177-181 (2004). 

Carbon—sulfur [njhelicenes, that is fully thiophenic helicenes, are unique derivatives of p-oUgothiophenes. In these systems, when the number of rings is more than four, the [2,3-b] junction present between two adjacent thiophene rings leads to the formation of a twisted scaffold with all of the sulfur atoms located along the periphery of the helix. 

An asymmetric synthesis of the carbon—sulfur[7]helicene 42 makes use of (—)-sparteine as a chiral additive (2009ACIE5954, 2005JA13806). The reaction sequence consists of a double deprotonation of racemic bis(p-trithio-phene) 44 with LDA in the presence of the chiral additive followed by quenching of the dilithiated species with bis(phenylsulphonyl)sulfide to give the carbon—sulfur[7]heHcene 42 (Scheme 24). 

---