Template Catalysis by Metal–Ligand Cooperation. C–C Bond Formation via Conjugate Addition of Non-activated Nitriles under Mild, Base-free Conditions Catalyzed by a Manganese Pincer Complex

Abstract

The first example of a catalytic Michael addition reaction of non-activated aliphatic nitriles to α,β-unsaturated carbonyl compounds under mild, neutral conditions is reported. A new de-aromatized pyridine-based PNP pincer complex of the Earth-abundant, first-row transition metal manganese serves as the catalyst. The reaction tolerates a variety of nitriles and Michael acceptors with different steric features and acceptor strengths. Mechanistic investigations including temperature-dependent NMR spectroscopy and DFT calculations reveal that the cooperative activation of alkyl nitriles, which leads to the generation of metalated nitrile nucleophile species (α-cyano carbanion analogues), is a key step of the mechanism. The metal center is not directly involved in the catalytic bond formation but rather serves, cooperatively with the ligand, as a template for the substrate activation. This approach of "template catalysis" expands the scope of potential donors for conjugate addition reactions.

Manganese‐Catalyzed Hydrogenation of Esters to Alcohols

“…Particularly, hydrogenation and dehydrogenation reactions catalyzed by well‐defined manganese complexes are very rare. We recently reported the first example of a catalytic Michael addition reaction of nitriles to carbonyl compounds and the first dehydrogenative coupling of alcohols and amines to form imines and H 2 catalyzed by a PNP pincer manganese complex [Mn(PNP*)(CO) 2 ] (the asterisk denotes the dearomatized ligand) . Recently Kirchner reported the coupling of alcohols and amines to form imines catalyzed by a series of manganese pincer complexes, which are stabilized by a PNP ligand based on the 2,6‐diaminopyridine scaffold and Beller reported the hydrogenation of nitriles, ketones and aldehydes with an aliphatic PNHP‐type hydride manganese pincer complex [Mn(PNHP)(H)(Br)(CO) 2 ],…”

Highly Active and Selective Manganese C=O Bond Hydrogenation Catalysts: The Importance of the Multidentate Ligand, the Ancillary Ligands, and the Oxidation State

“…This observation emphasizes an advantage and the importance of homogeneous catalysts in 3d-metal catalysis. [12] Herein, we report on the development of a highly active and selective Mn C = O bond hydrogenation catalyst. Most of the successfully applied catalysts are based on expensive noble metals, such as Ru, Rh, Ir, Pd, and Pt.…”

“…[2] The application of homogeneous hydrogenation catalysts is especially promising for the reduction of C = O bonds since a bifunctional mechanism involving the ligand can operate for efficient H 2 activation. [12] Herein, we report on the development of a highly active and selective Mn C = O bond hydrogenation catalyst. [4][5][6] We have had successfully used such ligands to design iridium catalysts [7] and observed for Co, in contrast to Ir, that only very minor alterations of the catalyst structure strongly influenced the hydrogenation activity.…”

Hydrogenation of Esters to Alcohols Catalyzed by Defined Manganese Pincer Complexes

“…[14] In general, manganese catalysts are well known for oxidation reactions whereas reductive transformations were basically unknown until very recently. Moreover,m ost compounds of this latter element show low toxicity,w hich currently makes it ah ighly attractive aspirant for the design of new catalysts.…”