Graphene Oxide-Arginine Composites: Efficient Dual Function Materials for Integrated CO2 Capture and Conversion.

The "on-demand" capture and utilization of CO2 is effectively realized with a readily accessible dual function organic composite. The covalent and controlled derivatization of graphene oxide (GO) surface with naturally occurring arginine led to a "smart" material capable of capturing (chemisorption) CO2 from high-purity flue-gas as well as low-concentration streams (i. e. direct air capture) and concomitant chemical activation toward the incorporation into cyclic carbonates. The overall integrated CO2 capture and conversion (ICCC) strategy has been fully elucidated mechanistically via dedicated computational, spectroscopic and thermal analyses.

Reductive cyclodimerization of chalcones: exploring the "self-adaptability" of galvanostatic electrosynthesis.

The "self-adaptability" of galvanostatic electrolysis was shown to assist a multistage unprecedented chemo- and diastereoselective electrochemically promoted cyclodimerization of chalcones. The process, all involving the reductive events, delivered densely functionalized cyclopentanes featuring five contiguous stereocenters (25 examples, yields of up to 95%, dr values up to >20 : 1). Dedicated and combined experimental as well as electrochemical investigation revealed the key role of a dynamic kinetic resolution of the aldol intermediate for the reaction mechanism.

On the Number of π-Electrons Involved in Stepwise Cycloaddition Reactions.

The development of higher-order cycloadditions has mainly been restricted by the requisite usage of highly conjugated and reactive π-systems. Recent years have witnessed organocatalysis as a potent mediator for several of the challenges associated herein, rendering higher-order cycloadditions a legitimate option for achieving the selective construction of specific molecular scaffolds. These developments reinvigorate the efforts to try to understand the underlying principles for cycloadditions involving a higher number of π-electrons than the "classical" cycloadditions; how do we properly address the impact which the addition of further π-electrons have on the reactivity of a system? Herein, computational investigations of two model higher-order cycloaddition systems have been performed to try to provide insights on changes in energetic barriers induced by the presence of benzofusions in a position which is unobstructive to the reactivity. With experimental substantiation as support, these studies might open up for a discussion on whether the π-electrons of benzofused systems simply act as spectator electrons, or play a tangible role on the observed reactivity to an extent where a distinct nomenclature is meritable.

Nickel Catalyzed Carbonylation/Carboxylation Sequence via Double CO2 Incorporation.

A carbonylation-carboxylation synthetic sequence, via double CO2 fixation, is described. The productive merger of a Ni-catalyzed cross-electrophile coupling manifold, with the use of AlCl3, triggered a cascade reaction with the formation of three consecutive C-C bonds in a single operation. This strategy traces an unprecedented synthetic route to ketones under Lewis acid assisted carbon dioxide valorization. Computational insights revealed a unique double function of AlCl3, and labeling (13CO2) experiments validate the genuine incorporation of CO2 in both functional groups.

Electrochemical C(sp3)-H functionalization of ethers via hydrogen-atom transfer by means of cathodic reduction.

The chemo- and stereoselective electrochemical allylation/alkylation of ethers is presented via a C(sp3)-H activation event. The electrosynthetic protocol enables the realization of a large library of functionalized ethers (35 examples) in high yields (up to 84%) via cathodic activation of a new type of redox-active carbonate (RAC), capable of triggering HAT (Hydrogen-Atom-Transfer) events through the generation of electrophilic oxy radicals. The process displayed high functional group tolerance and mild reaction conditions. A mechanistic elucidation via voltammetric analysis completes the study.

Recyclable GO-Arginine Hybrids for CO2 Fixation into Cyclic Carbonates.

New covalently modified GO-guanidine materials have been realized in a gram-scale synthesis and purified by an innovative microfiltration. The use of these composites in the fixation of CO2 into cyclic carbonates is demonstrated. Mild operating conditions, high yields (up to 85 %), wide scope (15 examples) and recoverability/reusability (up to 5 cycles) of the material account for the efficiency of the protocol. Dedicated control experiments shed light on the activation modes exerted by GO-l-arginine during the ring-opening/closing synthetic sequence.

Direct Synthesis of α-Aryl-α-Trifluoromethyl Alcohols via Nickel Catalyzed Cross-Electrophile Coupling.

A nickel-catalyzed reductive cross-electrophile coupling between the redox-active N-trifluoroethoxyphthalimide and iodoarenes is documented. The protocol reproduces a formal arylation of trifluoroacetaldehyde under mild conditions in high yields (up to 88 %) and with large functional group tolerance (30 examples). A combined computational and experimental investigation revealed a pivotal solvent assisted 1,2-Hydrogen Atom Transfer (HAT) process to generate a nucleophilic α-hydroxy-α-trifluoromethyl C-centered radical for the Csp2 -Csp3 bond forming process.

Convenient synthesis of tricyclic N(1)-C(2)-fused oxazino-indolones via [Au(I)] catalyzed hydrocarboxylation of allenes.

A new [Au(I)] catalyzed intramolecular hydrocarboxylation of allenes is presented as a valuable synthetic route to oxazino-indolones. The use of 3,5-(CF3)2-C6H3-ImPyAuSbF6 as the optimal catalyst (5 mol%) was necessary to guarantee (i) wide tolerance of functional groups, (ii) mild reaction conditions (r.t., 16 h), and (iii) high yields (up to 90%). Preliminary attempts towards an enantioselective version (81 : 19 er) are also documented by means of a new family of chiral C1-symmetric ImPyAuCl complexes.

Regio- and Stereoselective Electrochemical Alkylation of Morita-Baylis-Hillman Adducts.

Electrosynthesis is effectively employed in a general regio- and stereoselective alkylation of Morita-Baylis-Hillman compounds. The exposition of N-acyloxyphthalimides (redox-active esters) to galvanostatic electroreductive conditions, following the sacrificial-anode strategy, is proved an efficient and practical method to access densely functionalized cinnamate and oxindole derivatives. High yields (up to 80%) and wide functional group tolerance characterized the methodology. A tentative mechanistic sketch is proposed based on dedicated control experiments.

Visible-Light Assisted Covalent Surface Functionalization of Reduced Graphene Oxide Nanosheets with Arylazo Sulfones.

We present an environmentally benign methodology for the covalent functionalization (arylation) of reduced graphene oxide (rGO) nanosheets with arylazo sulfones. A variety of tagged aryl units were conveniently accommodated at the rGO surface via visible-light irradiation of suspensions of carbon nanostructured materials in aqueous media. Mild reaction conditions, absence of photosensitizers, functional group tolerance and high atomic fractions (XPS analysis) represent some of the salient features characterizing the present methodology. Control experiments for the mechanistic elucidation (Raman analysis) and chemical nanomanipulation of the tagged rGO surfaces are also reported.

Merging C-C σ-bond activation of cyclobutanones with CO2 fixation via Ni-catalysis.

A carboxylative Ni-catalyzed tandem C-C σ-bond activation of cyclobutanones followed by CO2-electrophilic trapping is documented as a direct route to synthetically valuable 3-indanone-1-acetic acids. The protocol shows an adequate functional group tolerance and useful chemical outcomes (yield up to 76%) when AlCl3 is adopted as an additive. Manipulations of the targeted cyclic scaffolds and a mechanistic proposal based on experimental evidence complete the investigation.

Organocatalytic Enantioselective Construction of Conformationally Stable C(sp2)-C(sp3) Atropisomers.

Nonbiaryl atropisomers are molecules defined by a stereogenic axis featuring at least one nonarene moiety. Among these, scaffolds bearing a conformationally stable C(sp2)-C(sp3) stereogenic axis have been observed in natural compounds; however, their enantioselective synthesis remains almost completely unexplored. Herein we disclose a new class of chiral C(sp2)-C(sp3) atropisomers obtained with high levels of stereoselectivity (up to 99% ee) by means of an organocatalytic asymmetric methodology. Multiple molecular motifs could be embedded in this class of C(sp2)-C(sp3) atropisomers, showing a broad and general protocol. Experimental data provide strong evidence of the conformational stability of the C(sp2)-C(sp3) stereogenic axis (up to t1/225 °C >1000 y) in the obtained compounds and show kinetic control over this rare stereogenic element. This, coupled with density functional theory calculations, suggests that the observed stereoselectivity arises from a Curtin-Hammett scenario establishing an equilibrium of intermediates. Furthermore, the experimental investigation led to evidence of the operating principle of central-to-axial chirality conversions.

Aminocatalytic [8+2] Cycloaddition Reactions toward Chiral Cyclazines.

An efficient and exceptionally stereoselective synthesis of chiral cycl[3.2.2]azines has been realized by means of the rational design and utilization of novel (E)-3-benzylidene-3H-pyrrolizines in iminium-ion-catalyzed [8+2] cycloaddition reactions. The presented protocol allows for the incorporation of diverse enals, including cinnamaldehydes, enolizable aldehydes, and substrates of extended conjugation. The obtained products contain both an electron-rich alkenyl pyrrole moiety and an electron-deficient carbaldehyde substituent, and both moieties can undergo selective transformations with retention of the stereochemical information established in the [8+2] cycloaddition.

Enantioselective Construction of the Cycl[3.2.2]azine Core via Organocatalytic [12 + 2] Cycloadditions.

The first enantioselective [12 + 2] cycloaddition has been developed for the construction of a chiral cycl[3.2.2]azine core, a tricyclic moiety with a central ring-junction nitrogen atom, by an operationally simple one-step organocatalytic process. The reaction concept builds upon aminocatalytically generated 12π-components derived from 5H-benzo[a]pyrrolizine-3-carbaldehydes reacting with different electron-deficient 2π-components and affording the complex scaffold of benzo[a]cycl[3.2.2]azine (indolizino[3,4,5-ab]isoindole) with excellent enantio- and diastereoselectivity in good yields. The developed reaction is robust toward diverse substituent patterns and has been extended to different classes of electron-deficient 2π-components by minor variations in reaction setup. The obtained [12 + 2] cycloadducts are electron-deficient in nature, and their reaction with nucleophiles have been demonstrated. The enantioselective [12 + 2] cycloaddition with α,ß-unsaturated aldehydes as the electron-deficient 2π-components relies upon an unconventional, simple aminocatalyst. In order to understand the high stereoselectivity of the [12 + 2] cycloaddition for this simple catalyst, combined experimental and computational investigations were performed. The investigations point to activation of both the 5H-benzo[a]pyrrolizine-3-carbaldehyde and the α,ß-unsaturated aldehyde by the aminocatalyst and that the reaction proceeds by a stepwise cycloaddition, where especially the ring-closure is crucial for the stereochemical outcome. For other electron-deficient 2π-components, such as α,ß-unsaturated ketoesters and nitroolefins, a more sterically demanding aminocatalyst has been applied and the corresponding [12 + 2] cycloadducts are obtained with excellent stereoselectivity. The [12 + 2] cycloaddition with vinyl sulfones afforded fully unsaturated systems, which display photoluminescence properties and for which quantum yields have been evaluated.

Organocatalytic Enantioselective 1,3-Dipolar [6+4] Cycloadditions of Tropone.

A highly stereoselective 1,3-dipolar [6+4] cycloaddition towards bridged azabicyclo[4.3.1]decane scaffolds has been developed, reacting aldehydes, 2-aminomalonates and tropone under mild conditions in the presence of a chiral phosphoric acid catalyst. The scope is demonstrated for a series of aldehydes and 2-aminomalonates, and the reaction proceeds in high yields, >95:5 d.r. and up to 99 % ee. A series of transformations, as well as a mechanistic proposal, are presented.

Central-to-Axial Chirality Conversion Approach Designed on Organocatalytic Enantioselective Povarov Cycloadditions: First Access to Configurationally Stable Indole-Quinoline Atropisomers.

The first stereoselective synthesis of enantioenriched axially chiral indole-quinoline systems is presented. The strategy takes advantage of an organocatalytic enantioselective Povarov cycloaddition of 3-alkenylindoles and N-arylimines, followed by an oxidative central-to-axial chirality conversion process, allowing for access to previously unreported axially chiral indole-quinoline biaryls. The methodology is also implemented for the design and the preparation of challenging compounds exhibiting two stereogenic axes. DFT calculations shed light on the stereoselectivity of the central-to-axial chirality conversion, showing unconventional behavior.

Catalytic Enantioselective Hetero-[6+4] and -[6+2] Cycloadditions for the Construction of Condensed Polycyclic Pyrroles, Imidazoles, and Pyrazoles.

The development of the first chemo-, regio-, and stereoselective hetero-[6+4] and -[6+2] cycloadditions of heteroaromatic compounds via amino aza- and diazafulvenes is presented. Pyrroles, imidazoles, and pyrazoles substituted with a formyl group react with an aminocatalyst to generate an electron-rich hetero-6π-component that reacts in a chemo-, regio-, and stereoselective manner with electron-deficient dienes and olefins. For the hetero-[6+4] cycloaddition of the pyrrole system with dienes, a wide variation of both reaction partners is possible, providing attractive pyrrolo-azepine products in high yields and excellent enantioselectivities (99% ee). The hetero-[6+4] cycloaddition reaction concept is extended to include imidazoles and pyrazoles, giving imidazolo- and pyrazolo-azepines. The same activation concept is successfully employed to include hetero-[6+2] cycloadditions of the pyrrole system with nitroolefins, giving important pyrrolizidine-alkaloid scaffolds. Experimental NMR and mechanistic studies allowed for the identification of two different types of intermediates in the reaction. The first intermediate is the result of a rapid formation of an iminium ion, which generates a hetero-6π aminofulvene intermediate as a mixture of two isomers. Density functional theory calculations were used to determine the mechanism and sources of asymmetric induction in the hetero-[6+4] and -[6+2] cycloadditions. After formation of the reactive hetero-6π-components, a stepwise addition occurs with the diene or olefin, leading to a zwitterionic intermediate that undergoes cyclization to afford the cycloadduct, followed by eliminative catalyst release. The stereoselectivity is controlled by the second step, and computations elaborate on the various substrate and catalyst effects that alter the experimentally observed enantioselectivities. The computational studies provided a basis for improving the enantioselectivity of the hetero-[6+2] cycloaddition.

Catalytic Enantioselective [10+4]†Cycloadditions.

The first peri- and stereoselective [10+4] cycloaddition between catalytically generated amino isobenzofulvenes and electron-deficient dienes is described. The highly stereoselective catalytic [10+4] cycloaddition exhibits a broad scope with high yields, reflecting a robust higher-order cycloaddition. Experimental and computational investigations support a kinetic distribution of intermediate rotamers dictating the enantioselectivity, which relies heavily on additive effects.

Quinone-Fused Pyrazoles through 1,3-Dipolar Cycloadditions: Synthesis of Tricyclic Scaffolds and in vitro Cytotoxic Activity Evaluation on Glioblastoma Cancer Cells.

A novel and straightforward synthesis of highly substituted isoquinoline-5,8-dione fused tricyclic pyrazoles is reported. The key step of the synthetic sequence is a regioselective, Ag2 CO3 promoted, 1,3-dipolar cycloaddition of C-heteroaryl-N-aryl nitrilimines and substituted isoquinoline-5,8-diones. The broad functional group tolerability and mild reaction conditions were found to be suitable for the preparation of a small library of compounds. These scaffolds were designed to interact with multiple biological residues, and two of them, after brief synthetic elaborations, were analyzed by molecular docking studies as potential anticancer drugs. In vitro studies confirmed the potent anticancer effects, showing promising IC50 values as low as 2.5 µm against three different glioblastoma cell lines. Their cytotoxic activity was finally positively correlated to their ability to inhibit PI3K/mTOR kinases, which are responsible for the regulation of diverse cellular processes in human cancer cells.

An organocatalytic enantioselective direct α-heteroarylation of aldehydes with isoquinoline N-oxides.

A new protocol for the enantioselective direct α-heteroarylation of aldehydes with isoquinoline N-oxides, via chiral enamine catalysis, has been successfully developed. High enantiomeric excesses and moderate to good yields were achieved for a variety of α-heteroarylated aldehydes.