Abstract

VISAGIE, Kevin J.; HODSON, Luke  and  ARNOTT, Gareth E.. C-H activation: a Critical Evaluation of a Published Method and its Application Towards Inherently Chiral Calix[4]arenes. S.Afr.j.chem. (Online) [online]. 2020, vol.73, pp.15-21. ISSN 1996-840X.  http://dx.doi.org/10.17159/0379-4350/2020/v73a3.

C-H activation offers an intriguing access into inherently chiral calix[4]arenes, but has been little explored in the literature. In this article, we report our investigation into a published C-H activation method that uses carbamates to direct a palladium catalyzed C-H activation and subsequent reaction with N-bromosuccinimide. However, we show that this report is unfortunately flawed on a number of points. An earlier reported study revealed the more likely S_EAr mechanism of the bromination reaction, which did not involve palladium catalysis. We nevertheless employed the S_EAr bromination in an attempt to form inherently chiral calix[4]arenes, using a chiral (+)-menthyl carbamate as a directing group. Unfortunately, although the reaction was high yielding, the diastereomers formed were inseparable and we were unable to quantify their ratio. Subsequent removal of the chiral (+)-menthyl carbamate, returned a small positive optical rotation, suggesting that at least a level of asymmetric induction was achieved in the bromination to afford a non-racemic product.

Keywords : C-H activation; calix[4]arene; inherent chirality; bromination; diastereoselectivity.

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