Sustainable Knowledge-Driven Approaches in Transition-Metal-Catalyzed Transformations

ORGANOMETALLIC GAS-PHASE ION CHEMISTRY AND CATALYSIS: INSIGHTS INTO THE USE OF METAL CATALYSTS TO PROMOTE SELECTIVITY IN THE REACTIONS OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES

Carboxylic acids are valuable organic substrates as they are widely available, easy to handle, and exhibit structural and functional variety. While they are used in many standard synthetic protocols, over the past two decades numerous studies have explored new modes of metal-mediated reactivity of carboxylic acids and their derivatives. Mass spectrometry-based studies can provide fundamental mechanistic insights into these new modes of reactivity. Here gas-phase models for the following catalytic transformations of carboxylic acids and their derivatives are reviewed: protodecarboxylation; dehydration; decarbonylation; reaction as coordinated bases in C-H bond activation; remote functionalization and decarboxylative C-C bond coupling. In each case the catalytic problem is defined, insights from gas-phase studies are highlighted, comparisons with condensed-phase systems are made and perspectives are reached. Finally, the potential role for mechanistic studies that integrate both gas- and condensed-phase studies is highlighted by recent studies on the discovery of new catalysts for the selective decomposition of formic acid and the invention of the new extrusion-insertion class of reactions for the synthesis of amides, thioamides, and amidines. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Traceless Directing Groups in Sustainable Metal-Catalyzed C–H Activation

Sustainable transformations towards the production of valuable chemicals constantly attract interest, both in terms of academic and applied research. C–H activation has long been scrutinized in this regard, given that it offers a straightforward pathway to prepare compounds of great significance. In this context, directing groups (DG) have paved the way for chemical transformations that had not been achievable using traditional reactions. Few steps, high yields, selectivity and activation of inert substrates are some of the invaluable assets of directed catalysis. Additionally, the employment of traceless directing groups (TDG) greatly improves and simplifies this strategy, enabling the realization of multi-step reactions in one-pot, cascade procedures. Cheap, abundant, readily available transition metal salts and complexes can catalyze a plethora of reactions employing TDGs, usually under low catalyst loadings—rarely under stoichiometric amounts, leading in greater atom economy and milder conditions with increased yields and step-economy. This review article summarizes all the work done on TDG-assisted catalysis with manganese, iron, cobalt, nickel, or copper catalysts, and discusses the structure-activity relationships observed, by presenting the catalytic pathways and range of transformations reported thus far.

Ligand Conformational Flexibility Enables Enantioselective Tertiary C-B Bond Formation in the Phosphonate-Directed Catalytic Asymmetric Alkene Hydroboration

Conformationally flexible ancillary ligands have been widely used in transition metal catalysis. However, the benefits of using flexible ligands are often not well understood. We performed density functional theory (DFT) and experimental studies to elucidate the mechanisms and the roles of conformationally flexible α,α,α',α'-tetraaryldioxolane-4,5-dimethanol (TADDOL)-derived ligands on the reactivity and selectivity in the Rh-catalyzed asymmetric hydroboration (CAHB) of alkenes. DFT calculations and deuterium labeling studies both indicated that the most favorable reaction pathway involves an unusual tertiary C-B bond reductive elimination to give high levels of regio- and enantioselectivities. Here, the asymmetric construction of the fully substituted carbon center is promoted by the flexibility of the TADDOL backbone, which leads to two ligand conformations with distinct steric environments in different steps of the catalytic cycle. A pseudo-chair ligand conformation is preferred in the rate-determining tertiary benzylic C-B reductive elimination. The less hindered steric environment with this conformation allows the benzylic group to bind to the Rh center in an η³ fashion, which stabilizes the C-B reductive elimination transition state. On the other hand, a pseudo-boat ligand conformation is involved in the selectivity-determining alkene migratory insertion step, where the more anisotropic steric environment leads to greater ligand-substrate steric interactions to control the π-facial selectivity. Thus, using a conformationally flexible ligand is beneficial for enhancing both reactivity and enantioselectivity by controlling ligand-substrate interactions in two different elementary steps.

Microwave-Assisted Protocol for Green Functionalization of Thiophenes With a Pd/β-Cyclodextrin Cross-Linked Nanocatalyst

Microwaves (MW) are often the most efficient, in terms of heat exchange and conversion rate, of all the energy sources used to promote chemical reactions thanks to fast volumetric dielectric heating, and metal-catalyzed synthetic reactions under heterogeneous conditions are an eloquent example. We herein report a MW-assisted green protocol for the C-H arylation of thiophenes with substituted aryl halides. This sustainable protocol carried out in γ-valerolactone (GVL) is catalyzed by Pd nanoparticles embedded in cross-linked β-cyclodextrin. In view of the excellent results achieved with activated substrates, the one-pot synthesis of a 4(3H)-quinazolinone derivative has been accomplished. A pressure-resistant MW reactor, equipped with multiple gas inlets, was used for sequential (i) C-H arylation, (ii) reduction, and (iii) carbonylation in the presence of the same catalyst, but under different gas atmospheres. The robust heterogeneous Pd catalyst showed limited metal leaching in GVL, making this an efficient MW-assisted process with high atom economy.

Polymer-Anchored Bifunctional Pincer Catalysts for Chemoselective Transfer Hydrogenation and Related Reactions

A series of polymer-supported cooperative PC(sp³ )P pincer catalysts was synthesized and characterized. Their catalytic activity in the acceptorless dehydrogenative coupling of alcohols and the transfer hydrogenation of aldehydes with formic acid as a hydrogen source was investigated. This comparative study, examining homogeneous and polymer-tethered species, proved that carefully designing a link between the support and the catalytic moiety, which takes into consideration the mechanism underlying the target transformation, might lead to superior heterogeneous catalysis.