Tuning the photophysical and chiroptical properties of [4]helicene-diketopyrrolopyrroles.

Synthesis and functionalization of diketopyrrolo[3,4-c]pyrrole (DPP) derivatives containing chiral groups able to induce a strong chiral perturbation of the DPP core are still a challenging task. We report in this work the straightforward preparation of four bis([4]helicene)-DPP and bis([4]thiahelicene)-DPP dyes upon the condensation of 2-CN-[4](thia)helicene precursors, followed by their N-alkylation by nucleophilic substitution (compounds 9-11) or by a Mitsunobu-type strategy (compound 12). Compound 12, which contains sec-phenylethyl groups attached to the nitrogen atoms, has been obtained as (R,R) and (S,S) enantiomers. The four DPP-helicenes are luminescent in solution, while the N-benzyl (10) and N-sec-phenethyl (12) are emissive in the solid state as well. The chiroptical properties of compound 12 in solution and in the solid state indicate a strong chiral perturbation provided by the α-stereogenic centres, in spite of the stereodynamic nature of the [4]helicene flanking units.

Phosphathiahelicenes: synthesis and uses in enantioselective gold catalysis.

Enantiomerically pure thiahelicenes displaying a terminal phosphole unit and a stereogenic phosphorus center have been prepared by oxidative photocyclization of a diaryl-olefin precursor. Starting from one of these phosphathiahelicene oxides, the corresponding trivalent phosphine-Au(I) complex is obtained with complete diastereoselectivity. It affords a new, excellent precatalyst for the enantioselective cycloisomerization of N-tethered enynes (up to 96 % ee).

Enantioselective Au-Catalyzed Synthesis of Thia[5]- and Thia[6]helicenes and Their Transformation into Bowl-shaped Pleiadenes.

A series of helically shaped benzo[b]chryseno[4,3-d]thiophenes, naphtho[1,2-b]phenanthro[4,3-d]thiophenes, and chryseno[3,4-b]naphtho[1,2-d]thiophenes is synthesized via a highly enantioselective Au-catalyzed intramolecular alkyne hydroarylation reaction. The inversion barriers of the structures obtained are determined both theoretically and experimentally, and their chiroptical properties are reported. Preliminary studies on the post-synthetic functionalization of these thiahelicenes and their transformation into azahelicenes are also presented. In addition, a straightforward one-step protocol is developed, which wraps the initially obtained chryseno[3,4-b]naphtho[1,2-d]thiophenes into bowl-shaped pleiadene derivatives without erosion of the enantiopurity. The number of structurally related products that are obtained with high enantioselectivity enables the establishment of comprehensive correlations between the structure and conformational stability or (chir)optical properties.