Hydroamination and Alcoholysis of Acrylonitrile Promoted by the Pincer Complex {κ^P,κ^C,κ^P-2,6-(Ph₂PO)₂C₆H₃}Ni(OSO₂CF₃)

Abstract

This report describes the catalytic activity of the pincer-type complex {κ P ,κ C ,κ P -2,6-(Ph2PO)2C6H3}Ni(OSO2CF3) (1) in the anti-Markovnikov addition of aliphatic and aromatic amines and alcohols to acrylonitrile, crotonitrile, and methacrylonitrile. The influence of additives on the catalytic activities was investigated, and it was found that substoichiometric quantities of water promoted the C–N bond forming reactions catalyzed by 1, especially the reactions involving aromatic amines; in comparison, NEt3 had a less dramatic impact. The opposite pattern was observed for the alcoholysis of acrylonitrile promoted by 1: water had no beneficial effect on these reactions, while NEt3 proved to be a potent promoter. Another important difference between these reactions is that hydroamination works better with more nucleophilic amines, whereas the alcoholysis reactions work well with ArOH, CF3CH2OH, and ArCH2OH but not at all with the more nucleophilic aliphatic alcohols methanol, ethanol, and 2-propanol. Both hydroamination and alcoholysis proceed much better with acrylonitrile in comparison to its Me-substituted derivatives crotonitrile and methacrylonitrile. Under optimized conditions, precatalyst 1 promotes conjugate additions to acrylonitrile with catalytic turnover numbers of up to 100 (hydroamination) or higher (alcoholysis). Spectroscopic studies have established that the main Ni-containing species in the hydroamination reactions is a cationic adduct in which the olefinic substrate is bound to the Ni center via its nitrile moiety; this binding activates the double bond toward an outer-sphere nucleophilic attack by the amine (Michael addition). The solid-state structures of the cationic nitrile adducts [{κ P ,κ C ,κ P -2,6-(Ph2PO)2C6H3}Ni(NCR)][OSO2CF3] (R = Me (2a), CH2CH2N(H)Ph (2e)), which can be regarded as model complexes for the species involved in the hydroamination catalysis, have been elucidated. Also reported are the solid-state structures of the charge-neutral compound {κ P ,κ C ,κ P -2,6-(i-Pr2PO)2C6H3}Ni(OSO2CF3) and an octahedral Ni(II) species resulting from the aerobic/hydrolytic oxidation of 1.

Recent Advances on the Chemistry of POCOP–Nickel Pincer Compounds

“…Ph P-NiOTf] complexes served as precatalysts for the regioselective, antiMarkovnikov addition of aliphatic and aromatic amines and alcohols to acrylonitrile, crotonitrile and methacrylonitrile [91,92] (Fig. 21).…”

“…In contrast, the combination of this compound with water makes this system highly efficient in promoting hydroamination reactions despite the fact that this species is more prone to hydrolysis than the i Pr analogue [92] (Fig. 22).…”

Transition‐Metal‐Catalyzed Hydroamination Reactions

“…Pd and Ni catalysts are typically used for this reaction whereby the initial metal starting material in the catalytically active species can be a metal hydride that can hydrometallate the unsaturated starting materials to generate the formally cationic π-complex [115]. Typically, acid additives in combination with Pd(0) or Ni(0) starting materials are required [69,91,95,108,110,114,118] or Pd 2+ /Ni 2+ salts with weakly coordinating anions that generates a proton source in situ [93,95,96,107,111,113,[115][116][117]130] to access the reactive metal species. An alternative proposal with electron-rich catalysts involves the η 2 -coordination of neutral diene followed by direct protonation of the unsaturated ligand to generate the requisite η 3 -allyl/benzyl complex [131].…”

KOtBu‐Catalyzed Michael Addition Reactions Under Mild and Solvent‐Free Conditions

“…bond activation is very well established over the past decades. Thus far, various transition metal catalysts were employed in Michael addition reactions and notable such catalysts I – IX are summarized in Scheme . Ru and Cu catalyzed oxa‐and aza‐Michael addition reactions were reported .…”

“…Ru and Cu catalyzed oxa‐and aza‐Michael addition reactions were reported . Nickel pincer complexes were found to catalyze aza‐Michael addition reactions . Recently, Re and Mn pincer complexes were revealed to catalyze the Michael addition of alkyl cyanide derivatives to α,β ‐unsaturated esters and carbonyls .…”