Knölker-Type Iron Complexes Bearing an N-Heterocyclic Carbene Ligand: Synthesis, Characterization, and Catalytic Dehydration of Primary Amides

Abstract

A family of iron Knölker-type complexes bearing an N-heterocyclic carbene (NHC) ligand has been synthesized from the corresponding tricarbonyl­(η4-cyclopentadienone)iron complex by reaction with the in situ generated NHC ligand, under UV irradiation (350 nm) at room temperature. The six complexes, obtained in good yields (64–84%) after purification on column chromatography, were fully characterized, including X-ray diffraction and electrochemical studies. The activity of these complexes in catalysis has been demonstrated for the dehydration of primary benzamides into benzonitriles under hydrosilylation conditions using inexpensive polymethylhydrosiloxane (PMHS) as the hydrosilane at 100 °C.

Recent Catalytic Applications of (Cyclopentadienone)iron Complexes

“…While the first catalytic applications and most further developments of CICs were in reactions involving HT steps, in the last years these iron complexes have been reported to promote also different types of reactivity. In 2015, Darcel, Sortais and co‐workers described the synthesis and characterization of several new CICs in which one of the CO ligands is replaced by an NHC . These derivatives (some of which are shown in Scheme ) were able to promote the dehydration of primary amides to nitriles in fair to good yields, whereas the corresponding CIC 1a – i.e., the Knölker's complex – was catalytically inactive.…”

“…Some of the NHC‐substituted CICs reported by Sortais, Darcel and co‐workers, and their application in the dehydration of primary amides. PMHS = polymethylhydrosiloxane.…”

Metal‐Catalyzed Dehydration of Primary Amides to Nitriles

“…Darcel and coworkers reported the synthesis and the activity of similar NHC containing Knolker‐type iron complexes toward the dehydration of primary amides . These complexes ( 12 ) successfully performed dehydration in the presence of inexpensive PMHS (Scheme ).…”

Sulfuryl Fluoride Mediated Conversion of Aldehydes to Nitriles

“…These syntheses require highly toxic metal cyanides, elevated temperature, and introduce an additional carbon atom from the cyanide nucleophile. Nitriles can also be obtained from alcohols, amines, amides, and aldoximes . However, these methods often require high temperature, metal catalysts, and strong oxidizing reagents.…”

“…Established methods for the preparation of aliphatic nitriles take advantage of the highly nucleophilic character of the cyanide anion.A romatic nitriles are commonly prepared using the Sandmeyer [11] and the Rosenmund-von Braun reactions. [12] These syntheses require highly toxic metal cyanides, elevated temperature, and introduce an additional carbon atom from the cyanide nucleophile.N itriles can also be obtained from alcohols, [13][14][15][16] amines, [17,18] amides, [19][20][21] and aldoximes. [22][23][24] However,these methods often requireh igh temperature, metal catalysts,a nd strong oxidizingr eagents.…”