Rhenium Alkyne Catalysis: Sterics Control the Reactivity.

Metathesis reactions, including alkane, alkene, and alkyne metatheses, have their origins in the fundamental understanding of chemical reactions and the development of specialized catalysts. These reactions stand as transformative pillars in organic chemistry, providing efficient rearrangement of carbon-carbon bonds and enabling synthetic access to diverse and complex compounds. Their impact spans industries such as petrochemicals, pharmaceuticals, and materials science. In this work, we present a detailed mechanistic study of the Re(V) catalyzed alkyne metathesis through density functional theory calculations. Our findings are in agreement with the experimental evidence from Jia and co-workers and unveil critical factors governing catalyst performance. Our work not only enhances our understanding of alkyne metathesis but also contributes to the broader landscape of catalytic processes, facilitating the design of more efficient and selective transformations in organic synthesis.

Chiral NHC Ligands for Enantioselective Gold(I) Catalysis Under Aerobic Conditions: the Importance of Conformational Flexibility and Steric Hindrance of NHC Ligand on Reactivity.

Gold(I) catalysis has been recognized as a valuable tool for the unique transformation of multiple carbon-carbon bonds. Enantioselective π-catalysis based on gold(I) complexes is, however, still underdeveloped due to lack of privileged ligands. Herein, we present an accessible method to a new family of stable yet catalytically active chiral NHC-Au(I)-Cl complexes. The key to preserving a simultaneous fine balance between reactivity and stability in this newly developed family appears to be sterically hindered, but conformationally flexible NHC ligands. These could be easily accessed on a multigram scale by merging sterically hindered anilines with commercially available amino alcohols and amines via a four-steps synthetic sequence without the need for chromatographic purification. Further investigations of the catalytic activity of NHC-Au-Cl complexes identified the OH functionality incorporated into the NHC core as crucial for the level of enantioselectivity as well as the TsO- anion responsible for the activation of NHC-Au(I)-Cl. Finally, NMR studies and X-ray investigations revealed for the first time that the widely accepted ion metathesis (NHC-Au-Cl to NHC-Au-OSO2 R) responsible for the activation of NHC-Au-Cl complexes does not take place (or it is very slow) in commonly used MeNO2 in contrast to DCM.

Suzuki-Miyaura Cross-Coupling of Amides by N-C Cleavage Mediated by Air-Stable, Well-Defined [Pd(NHC)(sulfide)Cl2] Catalysts: Reaction Development, Scope, and Mechanism.

The Suzuki-Miyaura cross-coupling of amides by selective N-C acyl bond cleavage represents a powerful tool for constructing biaryl ketones from historically inert amide bonds. These amide bond activation reactions hinge upon efficient oxidative addition of the N-C acyl bond to Pd(0). However, in contrast to the well-researched activation of aryl halides by C(sp2)-X oxidative addition, very few studies on the mechanism of C(acyl)-N bond oxidative addition and catalyst effect have been reported. Herein, we report a study on [Pd(NHC)(sulfide)Cl2] catalysts in amide N-C bond activation. These readily prepared, well-defined, air- and moisture-stable Pd(II)-NHC catalysts feature SMe2 (DMS = dimethylsulfide) or S(CH2CH2)2 (THT = tetrahydrothiophene) as ancillary ligands. The reaction development, kinetic studies, and reaction scope are presented. Extensive DFT studies were conducted to gain insight into the mechanism of C(acyl)-N bond oxidative addition and catalyst activation. We expect that [Pd(NHC)(sulfide)Cl2] precatalysts featuring sulfides as well-defined, readily accessible ancillary ligands will find application in C(acyl)-X bond activation in organic synthesis and catalysis.

Experimental and theoretical studies on the acetaldehyde reaction with (+)-catechin.

Acetaldehyde plays a key role in determining some wine properties. Interesting is the reaction of acetaldehyde with flavonoids, as the ensuing products can alter wine color, astringency, colloidal stability. Many studies reported on the formation of ethylidene-bridged flavan-3-ols as products of the reaction between acetaldehyde and either (+)-catechin or (-)-epicatechin. In white wines after one year of incubation with acetaldehyde only vinyl-(+)-catechin and vinyl-(-)-epicatechin were observed, while no ethylidene linked oligomers were detected. This observation prompted us to study the reaction of (+)-catechin with acetaldehyde in wine model solution through an experimental and theoretical approach, with the purpose of exploring the nature of the species involved along with the mechanisms leading to them. The products of the reaction were observed over 38 days. The results showed that ethylidene-bridged catechins are the first products to be formed but over time the dissociation of these dimers causes vinyl-catechins to accumulate.

A predictive journey towards trans-thioamides/amides.

The cis-trans isomerization of (thio)amides was studied by DFT calculations to get the model for the higher preference for the cis conformation by guided predictive chemistry, suggesting how to select the alkyl/aryl substituents on the C/N atoms that lead to the trans isomer. Multilinear analysis, together with cross-validation analysis, helped to select the best fitting parameters to achieve the energy barriers of the cis to trans interconversion, as well as the relative stability between both isomers. Double experimental check led to the synthesis of the best trans candidate with sterically demanding t-butyl substituents, confirming the utility of predictive chemistry, bridging organic and computational chemistry.

Unravelling Enzymatic Features in a Supramolecular Iridium Catalyst by Computational Calculations.

Non-biological catalysts following the governing principles of enzymes are attractive systems to disclose unprecedented reactivities. Most of those existing catalysts feature an adaptable molecular recognition site for substrate binding that are prone to undergo conformational selection pathways. Herein, we present a non-biological catalyst that is able to bind substrates via the induced fit model according to in-depth computational calculations. The system, which is constituted by an inflexible substrate-recognition site derived from a zinc-porphyrin in the second coordination sphere, features destabilization of ground states as well as stabilization of transition states for the relevant iridium-catalyzed C-H bond borylation of pyridine. In addition, this catalyst appears to be most suited to tightly bind the transition state rather than the substrate. Besides these features, which are reminiscent of the action modes of enzymes, new elementary catalytic steps (i. e. C-B bond formation and catalyst regeneration) have been disclosed owing to the unique distortions encountered in the different intermediates and transition states.

Deciphering the Reactive Pathways of Competitive Reactions inside Carbon Nanotubes.

Nanoscale control of chemical reactivity, manipulation of reaction pathways, and ultimately driving the outcome of chemical reactions are quickly becoming reality. A variety of tools are concurring to establish such capability. The confinement of guest molecules inside nanoreactors, such as the hollow nanostructures of carbon nanotubes (CNTs), is a straightforward and highly fascinating approach. It mechanically hinders some molecular movements but also decreases the free energy of translation of the system with respect to that of a macroscopic solution. Here, we examined, at the quantum mechanics/molecular mechanics (QM/MM) level, the effect of confinement inside CNTs on nucleophilic substitution (SN2) and elimination (syn-E2 and anti-E2) using as a model system the reaction between ethyl chloride and chloride. Our results show that the three reaction mechanisms are kinetically and thermodynamically affected by the CNT host. The size of the nanoreactor, i.e., the CNT diameter, represents the key factor to control the energy profiles of the reactions. A careful analysis of the interactions between the CNTs and the reactive system allowed us to identify the driving force of the catalytic process. The electrostatic term controls the reaction kinetics in the SN2 and syn/anti-E2 reactions. The van der Waals interactions play an important role in the stabilization of the product of the elimination process.