A passage from pincer complexes to rationally designed phosphine-free Co(iii) catalysts supported by a pentadentate ligand for activation of alcohols: studies on sp3 C–H alkylation of 9H-fluorene and quinoline synthesis

Abstract

In this study, a pentadentate carboxamide ligand (BPAPA-H = (2-(bis(pyridin-2-ylmethyl)amino)-N′-phenyl-N′-(pyridin-2-yl)acetohydrazide) is introduced and utilized for the synthesis of cobalt(III) catalysts [Co(III)(BPAPA)Cl]ClO₄ (C1) and [Co(III)(BPAPA)Br]ClO₄ (C2). These cobalt(III) catalysts are used for selective mono sp³ C–H alkylation of 9H-fluorene with cheap and abundant alcohols via the borrowing hydrogen (BH) approach, with the generation of ecologically benign water as a side product. Employing the existing methodology, 41 derivatives of mono sp³ C–H alkylated fluorene were produced utilizing different aromatic and aliphatic alcohols as alkylating agents, with isolated yields reaching as high as 97%. To investigate the catalytic potential in the synthesis of heterocycles, our optimized cobalt(III) catalyst facilitated the acceptorless dehydrogenative (AD) coupling of 2-aminobenzyl alcohol with aromatic ketones, resulting in the formation of 25 quinoline derivatives with yields reaching as high as 96%. The current methodology was also explored in the gram-scale synthesis of sp³ C–H alkylation of fluorene and quinoline synthesis for large-scale applications. A series of control experiments were carried out to explain and reveal the possible reaction mechanism and intermediates. The key intermediates involved in the catalytic cycle were characterized with the help of HRMS studies.