Mechanism of the Aminolysis of Fischer Alkoxy and Thiocarbene Complexes: A DFT Study

Abstract

B3LYP calculations have been carried out to study the reaction mechanism of the aminolysis of Fischer carbene complexes of the type (CO)(5)Cr=C(XMe)R (X = O and S; R = Me and Ph). We have explored different possible reaction mechanisms either through neutral or zwitterionic intermediates as well as a general base catalysis assisted by an ammonia molecule. Our results show that the most favorable pathway for the aminolysis of Fischer carbene complexes is through a stepwise reaction via a zwitterionic intermediate generated by the initial nucleophilic attack. We have found that the ammonia-catalyzed mechanism entails a significantly lower barrier for the rate-determining step than the uncatalyzed one. At lower pressure gas-phase conditions, the rate-determining step corresponds to the concerted proton transfer and MeXH elimination. Thiocarbene complexes show a higher energy barrier for this rate-determining step due to the lower basicity of the MeS(-) substituent. At higher pressure or in solution, the rate-determining step corresponds to the initial nucleophilic attack. Our results indicate that the transition state of the nucleophilic attack is more advanced and has a higher barrier for alkoxycarbene than thiocarbene complexes due to the stronger pi-donor character of the alkoxy group that reduces the electrophilicity of the attacked carbene atom making the nucleophilic attack more difficult.

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

“…4,7,9 Alkoxycarbenes are readily altered in this manner, which leads to the formation of amino, thio-and aryl-carbene complexes, respectively (reaction 3, Figure 1). 10 Of these modifications, aminolysis has been used most extensively 11 because of the relative ease of the reaction and the increased stability of the products. Aminocarbenes were synthesised not long after the first stable alkoxycarbene.…”

“…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. Applications of aminocarbene complexes include polymer-bound amino chromium Fischer carbene complexes 16 and aminocarbenes are used in the Pauson-Khand cyclization reaction.…”

Iridaaromatics via Methoxy(alkenyl)carbeneiridium Complexes

“…On the other hand, reactivity with amines did not provide the expected results. It is known that primary or secondary amines can react with alkoxycarbenes to provide aminocarbenes through an aminolysis reaction [ 11 , 12 , 13 ]. However, this reaction only took place when an aqueous ammonia solution was used and the aminocarbene complex [IrCp*Cl{=C(NH 2 )CH=CPh 2 }(PPh 2 Me)]PF 6 ( 6 ) could be isolated ( Scheme 2 ).…”

Nanostructured Thin Films Obtained from Fischer Aminocarbene Complexes

“…As a consequence of the electrophilic character on carbenic carbon, α -anions were easily generated upon treatment of alkyl carbene complexes with bases [26,27,28]. Applying this efficient strategy, we synthesized four Fischer aminocarbenes’ amphiphilic character under a push-pull architecture.…”