In this review, we outline the first-row transition metal-catalyzed hydroelementation of pyridines and quinolines with dihydrogen (from H₃N⋅BH₃), hydrosilanes, or hydroboranes to selectively provide the 1,2- or 1,4-dihydro products. We also describe the regioselectivity and working mode of the catalytic systems on the basis of experimental and/or computational mechanistic observations and insights.

Abstract

Catalytic partial reduction of N-heteroarenes with H₂ or H[E] (E=Si, B-based) has been a useful and general method for synthesis of a broad range of dihydropyridines (DHP) and dihydroquinolines (DHQ). In recent seven years, one of the most notable advances in this context is being able to utilize earth-abundant and inexpensive first-row transition metal-based catalytic systems. These catalytic procedures are generally considered more environmentally benign and sustainable when compared to conventional catalytic systems relying on precious metals. This Review describes 20 molecular catalytic systems based on first-row transition metals for selective single hydroelementation of pyridines and quinolines with H₂ surrogate, hydrosilanes, and hydroboranes providing 1,2- or 1,4-dihydropyridines and -dihydroquinolines. The observed reaction profiles such as scope and activity are briefly presented, while the proposed working modes over a series of elemental steps – H−[E] bond cleavage, hydride (H⁻) or hydrogen atom (H⋅) transfer, and product release, are discussed in detail on the basis of experimental and/or computational mechanistic observations and insights.