Physical organic chemistry of transition metal carbene complexes. 2. Kinetics and mechanism of reactions of [methoxy(phenyl)carbene]pentacarbonylchromium(0) with primary aliphatic amines in aqueous acetonitrile

Abstract

A kinetic study of the aminolysis of [methoxy (phenyl)carbene]pentacarbonylchromium(0) with five primary aliphatic amines in 20% acetonitrile-80% water (v/v) at 25 °C is reported. Second-order rate constants (kA) increase with amine and OH" concentration, which indicates general base catalysis. Plots of kA vs amine or OH" concentration are curved and level off at high concentration; this implies a change in rate-limiting step and demonstrates that the reaction is stepwise. The mechanism is similar to that for ester aminolysis, i.e., the first step is nucleophilic addition of the amine to the substrate to yield a zwitterionic tetrahedral intermediate (TA*), followed by deprotonation of TA± to form TA" which, in a third step, is converted to products by general acid catalyzed methoxide expulsion. In contrast to ester aminolysis, where general base catalysis is a consequence

Synthesis, Structure, and Electrochemistry of Fischer Alkoxy- and Aminocarbene Complexes of Tungsten: The Use of DFT To Predict and Understand Oxidation and Reduction Potentials

“…These shifts are lower than those observed in literature. 38 It is interesting to note that the change from an ethoxycarbene to an aminocarbene is not reflected in the 13 C carbonyl shifts of the complexes in this study. However, other aromatic tungsten carbenes also do not show a significant difference in the carbonyl shifts when the ethoxy group is substituted for an amino substituent.…”

“…· Complexes 1-8 were characterized using 1 H, 13 C NMR and infrared spectroscopy as well as mass spectrometry. Atom-numbering schemes are shown in figures 4-9.…”

“…This indicates deshielding of the protons due to coordination of N2 to the metal, as would be expected. Furthermore, the chemical resonances noted for the 13 C NMR data of the heteroaryl ring components of complexes 7 and 8 were shifted downfield when compared to 5 and 6. This could indicate increased electron donation from the heteroaryl carbene substituent to the carbene center, however it is more likely that the changes were due to solvent shifts.…”

“…12 Aminolysis of alkoxy Fischer carbenes occurs in a similar fashion to the reaction of an ester with an amine, and is base catalysed. 13 Alternative methods for the production of aminocarbenes include the Hegedus synthesis 14 and nucleophilic attack of isocyanide metal complexes 3 as well as nucleophilic attack of a carbonyl group with an amino lithium salt, for example LDA. 15 In a recent density functional theory (DFT) study by Solá, 11 the mechanism of aminolysis was found to proceed via a zwitterionic intermediate that was generated by the nucleophilic attack at the carbene center.…”

“…The most notable change on the 13 C NMR spectra of the aminocarbene complexes compared to the ethoxycarbene analogues, is the large upfield shift (50-60 ppm) for the carbene carbon (C6) resonance. Similar decreases were noted in literature when ethoxy substituents were replaced with amines.…”