Nickel–PNN catalysed sustainable synthesis of polysubstituted quinolines under microwave irradiation†

Abstract

In this manuscript, the synthesis of a series of cationic Ni^II complexes based on mixed donor triazolyl-pyridine-based aminophosphine, [(NiCl){^R′P^RNN}-κ³-P,N,N]OTf (R = H or Me; R′ = Ph or ^tBu), is described. The neutral dearomatized complexes [(NiCl){^R′PNN}-κ³-P,N,N] (R′ = Ph or ^tBu) were prepared by deprotonation of [(NiCl){P^PhN(H)N}-κ³-P,N,N]OTf (Ni1) and [(NiCl){(^tBu₂P)N(H)N}-κ³-P,N,N]OTf (Ni3) with 1 equiv. of ^tBuOK. These complexes (0.5 mol%) were employed for the synthesis of substituted quinolines at 110 °C under microwave irradiation with 20 mol% of KOH. Among these, complexes [(NiCl){^{t Bu}P^HNN-κ³-P,N,N}]OTf (Ni3) and [(NiCl){^{t Bu}PNN}-κ³-P,N,N] (Ni5) were found to be highly efficient. A wide range of derivatives, including aryl, aliphatic, acyclic ketones, primary alcohols and aminobenzyl alcohols, yielded the corresponding quinolines in good to excellent yields (62 examples). A series of control experiments were carried out to establish the reaction mechanism. HR-MS spectral studies were investigated to characterize the nickel-hydride intermediate. Mechanistic studies confirmed that the reaction takes an acceptorless dehydrogenative coupling pathway.