Improved Access to Chiral Tetranaphthoazepinium-Based Organocatalysts Using Aqueous Ammonia as Nitrogen Source

Abstract

The class of 3,3′-diaryl substituted tetranaphthobisazepinium bromides has found wide application as highly efficient C2-symmetrical phase-transfer catalysts (PTCs, Maruoka type catalysts). Unfortunately, the synthesis requires a large number of steps and hampers the build-up of catalyst libraries which are often desired for screening experiments. Here, we present a more economic strategy using dinaphthoazepine 7 as the common key intermediate. Only at this stage various aryl substituents are introduced, and only two individual steps are required to access target structures. This protocol was applied to synthesize ten tetranaphthobisazepinium compounds 1a–1j. Their efficiency as PTCs was tested in the asymmetric substitution of tert-butyl 2-((diphenylmethylene)amino)acetate. Enantioselectivities up to 92% have been observed with new catalysts.

Special Issue: Development of Asymmetric Synthesis

“…In the last years, the use of organocatalysis in asymmetric synthesis has experienced growing interest due to some advantages compared to the use of metal complexes as catalysts, such as their usual higher stability, possible recyclability or lower ecological footprint due to the metal-free character of these purely organic catalysts. Thus, the use of these organocatalysts is also represented in this Special Issue, as is the case of the article by Manaprasertsak et al which presents an efficient and economic preparation of different C 2 -symmetrical 3,3 -diaryl substituted tetranaphthobisazepinium bromides; such compounds have shown wide application as highly efficient chiral phase-transfer catalysts [5]. The obtained catalysts are employed in the asymmetric substitution of a glycine-derived Schiff base under phase-transfer conditions leading to enantioenriched amino acid derivatives.…”