A synthetic sequence for the preparation of fully conjugated 2,7-disubstituted fluorazone (9H-pyrrolo[1,2--a]indol-9-one) derivatives has been developed comprising an Elming–Clauson–Kaas-type pyrrole formation, POCl3-mediated ring closure, selective halogenation and elongation of the conjugated backbone through cross-coupling reactions. As a proof of principle, this methodology was used to prepare for the first time two organic D–π–A dyes containing the fluorazone moiety. The new compounds display broad absorption bands in the visible-light region when adsorbed on nanocrystalline TiO2 and electrochemical properties compatible with their employment as photosensitizers in dye-sensitized solar cells. Small-scale photovoltaic devices fabricated with the fluorazone dyes yielded power conversion efficiencies in the range of 2.1–2.4 %, which correspond to approximately 70 % of the efficiency obtained with the reference organic dye DF15 under the same conditions
An efficient synthesis has been developed for the preparation of 9H-pyrrolo[1,2-a]indol-9-ones (fluorazones) from readily available anthranilic acid derivatives via a one-pot amide- and pyrrole-formation step, followed by an intramolecular cyclodehydration. The cyclodehydration process is mediated by the activation of aromatic tertiary amides by triflic anhydride (TfO). Comparison of various benzo-substituents is shown to demonstrate the high functional group tolerance of this transformation. In addition, study of the reaction mechanism is also presented to unfold the exact role of the applied base additive. Herein, as a first example, we report our findings that TfO-mediated amide activation is obstructed by the easy protonation of amides by the formed triflic acid during the activation step. Additionally, it has been also proven that the base additive is not involved in the transformation of O-triflyliminium triflates into reactive species (e.g., nitrilium triflates) and is only responsible to neutralize the superacid to avoid the protonation of both the secondary or tertiary amides.
Syntheses of two regioisomeric series of atropisomeric amino alcohols and a comparative study on their application in the enantioselective addition of diethylzinc to benzaldehyde are reported. Systematic modification of the electronic and steric properties of the functional groups resulted in highly efficient catalyst ligands in both series. Quantum-chemical calculations agreed well with the experimental results of this first systematic comparative study on regioisomeric atropisomeric ligands.
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