10 research outputs found
Zirconium catalyzed amide formation without water scavenging
A scalable homogeneous metalâcatalyzed protocol for direct amidation of carboxylic acids is presented. The use of 2â10 mol% of the commercially available Zr(Cp)2(OTf)2·THF results in high yields of amides at moderate temperature, using an operationally convenient reaction protocol that circumvents the use of water scavenging techniques
Catalytic amide formation from non-activated carboxylic acids and amines
The amide functionality is found in a wide variety of biological and synthetic structures such as proteins, polymers, pesticides and pharmaceuticals. Due to the fact that synthetic amides are still mainly produced by the aid of coupling reagents with poor atom-economy, the direct catalytic formation of amides from carboxylic acids and amines has become a field of emerging importance. A general, efficient and selective catalytic method for this transformation would meet well with the increasing demands for green chemistry procedures. This review covers catalytic and synthetically relevant methods for direct condensation of carboxylic acids and amines. A comprehensive overview of homogeneous and heterogeneous catalytic methods is presented, covering biocatalysts, Lewis acid catalysts based on boron and metals as well an assortment of other types of catalysts.AuthorCount:4;</p
Catalytic Reductive Dehydration of Tertiary Amides to Enamines under Hydrosilylation Conditions
Tertiary
amides are efficiently reduced to their corresponding
enamines under hydrosilylation conditions, using a transition-metal-free
catalytic protocol based on <i>t</i>-BuOK (5 mol %) and
(MeO)<sub>3</sub>SiH or (EtO)<sub>3</sub>SiH as the reducing agent.
The enamines were formed with high selectivity in good-to-excellent
yields
Mechanistic Elucidation of Zirconium-Catalyzed Direct Amidation
The
mechanism of the zirconium-catalyzed condensation of carboxylic
acids and amines for direct formation of amides was studied using
kinetics, NMR spectroscopy, and DFT calculations. The reaction is
found to be first order with respect to the catalyst and has a positive
rate dependence on amine concentration. A negative rate dependence
on carboxylic acid concentration is observed along with S-shaped kinetic
profiles under certain conditions, which is consistent with the formation
of reversible off-cycle species. Kinetic experiments using reaction
progress kinetic analysis protocols demonstrate that inhibition of
the catalyst by the amide product can be avoided using a high amine
concentration. These insights led to the design of a reaction protocol
with improved yields and a decrease in catalyst loading. NMR spectroscopy
provides important details of the nature of the zirconium catalyst
and serves as the starting point for a theoretical study of the catalytic
cycle using DFT calculations. These studies indicate that a dinuclear
zirconium species can catalyze the reaction with feasible energy barriers.
The amine is proposed to perform a nucleophilic attack at a terminal
η<sup>2</sup>-carboxylate ligand of the zirconium catalyst,
followed by a CâO bond cleavage step, with an intermediate
proton transfer from nitrogen to oxygen facilitated by an additional
equivalent of amine. In addition, the DFT calculations reproduce experimentally
observed effects on reaction rate, induced by electronically different
substituents on the carboxylic acid
Catalytic Water Oxidation by a Molecular Ruthenium Complex: Unexpected Generation of a Single-Site Water Oxidation Catalyst
The increasing energy demand calls
for the development of sustainable energy conversion processes. Here,
the splitting of H<sub>2</sub>O to O<sub>2</sub> and H<sub>2</sub>, or related fuels, constitutes an excellent example of solar-to-fuel
conversion schemes. The critical component in such schemes has proven
to be the catalyst responsible for mediating the four-electron oxidation
of H<sub>2</sub>O to O<sub>2</sub>. Herein, we report on the unexpected
formation of a single-site Ru complex from a ligand envisioned to
accommodate two metal centers. Surprising NâN bond cleavage
of the designed dinuclear ligand during metal complexation resulted
in a single-site Ru complex carrying a carboxylateâamide motif.
This ligand lowered the redox potential of the Ru complex sufficiently
to permit H<sub>2</sub>O oxidation to be carried out by the mild one-electron
oxidant [RuÂ(bpy)<sub>3</sub>]<sup>3+</sup> (bpy = 2,2âČ-bipyridine).
The work thus highlights that strongly electron-donating ligands are
important elements in the design of novel, efficient H<sub>2</sub>O oxidation catalysts
Metal-Free N-Arylation of Secondary Amides at Room Temperature
The arylation of secondary acyclic amides has been achieved with diaryliodonium salts under mild and metal-free conditions. The methodology has a wide scope, allows synthesis of tertiary amides with highly congested aryl moieties, and avoids the regioselectivity problems observed in reactions with (diacetoxyiodo)benzene