Ag2O versus Cu2O in the Catalytic Isomerization of Coordinated Diaminocarbenes to Formamidines: A Theoretical Study.

Juan F Van Der Maelen,Javier Ruiz

doi:10.3390/ma15020491

Abstract

DFT theoretical calculations for the Ag2O-induced isomerization process of diaminocarbenes to formamidines, coordinated to Mn(I), have been carried out. The reaction mechanism found involves metalation of an N-H residue of the carbene ligand by the catalyst Ag2O and the formation of a key transition state showing a μ-η2:η2 coordination of the formamidinyl ligand between manganese and silver, which allows a translocation process of Mn(I) and silver(I) ions between the carbene carbon atom and the nitrogen atom, before the formation of the formamidine ligand is completed. Calculations carried out using Cu2O as a catalyst instead of Ag2O show a similar reaction mechanism that is thermodynamically possible, but highly unfavorable kinetically and very unlikely to be observed, which fully agrees with experimental results.

Highlights

For many years silver(I) oxide has been widely used as a catalyst in different organic transformations, leading to the synthesis of a variety of organic molecules such as pyrroles, imidazolines, β-keto esthers, or benzothiophenes [1,2,3,4,5], as well as in many stoichiometric reactions, such as the synthesis of N-heterocyclic carbenes (NHC) based on Ag2O [6,7,8]
Density functional theory (DFT) computations have been performed with the GAUSSIAN09 and ADF2012 program packages [20,21], starting from our experimental geometry for cation 3, here included, and from the one of a derivative of cation 1, obtained from X-ray single-crystal diffraction data and previously published [22], using the B3P86-D3(BJ) Becke’s three-parameter exchange functional with the non-local Perdew correlation functional, as well as the M06-D3 hybrid functional of Truhlar and Zhao [23,24,25,26], which are both well known for obtaining reliable results with transition-metal organometallic compounds [27,28,29,30]
We have carried out DFT theoretical calculations that support a plausible mechanism for the experimentally observed Ag2O-catalyzed isomerization of diaminocarbenes to formamidines, coordinated to Mn(I)

Summary

Introduction

For many years silver(I) oxide has been widely used as a catalyst in different organic transformations, leading to the synthesis of a variety of organic molecules such as pyrroles, imidazolines, β-keto esthers, or benzothiophenes [1,2,3,4,5], as well as in many stoichiometric reactions, such as the synthesis of N-heterocyclic carbenes (NHC) based on Ag2O [6,7,8]. We have recently described an efficient experimental procedure to transform isocyanides into formamidines by reaction with primary amines [13,14], involving a key step tautomerization of diaminocarbenes coordinated to Mn(I), mediated by Ag2O, into formamidines This behavior is apparently exclusive to Ag2O, as isostructural Cu2O does not show any activity in the same reaction, nor do other metallic oxides of variable nature. In order to shed light on the mechanism of this intriguing reaction and to make a contribution to the rather scarcely developed field of mechanistic studies of organic and organometallic transformations assisted by Ag2O, we have carried out DFT theoretical calculations supporting the finding that the exclusive behavior of Ag2O in the catalytic isomerization of diaminocarbenes to formamidines is essentially based on an Materials 2022, 15, 491.

Experimental Section

Computational Details

Cu2O Mechanism M2

Ag2O Mechanism M3

Conclusions