Theoretical insight into the zinc(ii)-catalyzed synthesis of 2-indolyltetrahydroquinolines from N-propargylanilines and indoles: cross-dehydrogenative coupling with temporally separated catalytic activity

Abstract

A DFT study has been carried out to provide insight into the molecular mechanism of the zinc(II)-catalyzed cross-dehydrogenative coupling of N-propargylanilines with indoles developed by Nakamura et al. (Angew. Chem., Int. Ed., 2016, 55, 6758–6761). The proposed catalytic cycle consists of two subcycles, through which three C–H bonds (two sp² and one sp³) are activated sequentially. One subcycle carries out the transformation of N-propargylaniline to the intermediate product (IMP), N-benzyl-1,4-dihydroquinoline, and another realizes the transformation of IMP with indole into the final product. Different from general knowledge, the activation of C(sp³)–H is found to be easier than C(sp²)–H activation. The reaction is demonstrated to be an abnormal catalytic cascade reaction, which features the temporal separation of the multiple catalytic activities of the zinc(II) catalyst. The cascade characteristic illuminated in the present work is instructive for developing controlled and efficient syntheses of various products from catalytic cascade reactions. A cascade reaction can be designed so well that the catalyst can sequentially show its multiple catalytic activities, allowing for the controlled efficient synthesis of desired intermediates and products. The present work also rationalizes the experimental observation from a deuterium labeling study.