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

Auraya Manaprasertsak,Alexander Roller,Sorachat Tharamak,Christina Schedl,Michael Widhalm

doi:10.3390/molecules24213844

Auraya Manaprasertsak, Alexander Roller + Show 3 more

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https://doi.org/10.3390/molecules24213844

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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.

Highlights

In many asymmetric transformations the atropisomeric 1,1’-binaphthyl moiety serves as a highly efficient chiral backbone [1]
While functionalities at C-2 and C-20 serve as sites for primary interaction in organocatalysis or coordination in transition metal catalysis, substituents at C-3 and C-30 are introduced for secondary substrate or reagent activation or to tune steric interaction
Cleavage of the N-allyl substituent is required before dibenzylation with a second 2,2’-bis(bromo)methyl-1,1’-binaphthyl moiety forms the ammonium salts of general structure

Summary

Introduction

In many asymmetric transformations the atropisomeric 1,1’-binaphthyl moiety serves as a highly efficient chiral backbone [1]. Which found application as efficient phase-transfer catalysts (PTCs) [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]. Several strategies for their synthetic access have been developed, by Maruoka’s group. Cleavage of the N-allyl substituent is required before dibenzylation with a second 2,2’-bis(bromo)methyl-1,1’-binaphthyl moiety forms the ammonium salts of general structure

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