Application of Asymmetric Gold-Catalyzed Hydroarylation in the Enantioselective Synthesis of Helicenes

Abstract

A novel family of electron deficient, α-cationic ligands was synthesized, combining a chiral TADDOL-backbone with an electron withdrawing 1,2,3-triazolium moiety. Coordination to Au(I) gave the respective catalysts. Both ligands and catalysts were characterized with the relevant analytical techniques. First application of the catalyst system in the efficient, enantioselective synthesis of [6]helicenes (as already described by the ALCARAZO group) encouraged the development of further helicene synthesis. Suitable alkyne-precursors for the synthesis of C2-symmetrically and non-C2-symmetrically substituted [4]helicenes were prepared. While the former gave exceptional yields and selectivities, dependent on the precursors substitution pattern the latter lacked in selectivity. Further investigation is required to increase the control over regio- and enantioselectivity of the reaction. Encouraged by the results, a synthesis of C2-symmetrically substituted [5]helicenes was developed. When subjecting phenanthrene-based precursors to the hydroarylation, the obtained reactivities and selectivities were promising. Substrates based on picene were attempted but posed a challenge as their limited solubility made purification impossible. In collaboration with P. REDERO, MSc. a number of non-C2-symmetrically substituted [5]helicenes were synthesized utilizing BINOL-based α-cationic phosphonite ligands. A range of substitution patterns was tolerated by the reaction giving [5]helicenes with high regio- and enantioselectivities. All obtained helicenes were completely characterized and their absolute stereochemistry determined by X-ray crystallography and ECD-measurements. The analysis was supplemented by further measurements of their electrooptical properties. In all cases substrates with electron-withdrawing groups or substituents leading to expanded π-system limited the efficiency of the reaction. With the ongoing development of chiral, α-cationic catalysts by the ALCARAZO group, the described problematic reactions should be revisited, to check for improved selectivities even with problematic substitution patterns.