Rossella Mello

The photolysis at 254 nm of lithium iodide and olefins 1 carrying an electron-withdrawing Z-substituent in CO2-saturated (1 bar) anhydrous acetonitrile at room temperature produces the atom efficient and transition metal-free photoiodocarboxylation of the C=C double bond. The reaction proceeds well for terminal olefins 1 to form the new C-I and C-C σ-bonds at the α and β-positions of the Z-substituent, respectively, and is strongly inhibited by polar protic solvents or additives. The experimental results suggest that the reaction channels through the radical anion [CO2·-] in acetonitrile, yet involves different intermediates in aqueous medium. The stabilizing ion-quadrupole and electron donor-acceptor interactions of CO2 with the iodide anion play a crucial role in the reaction course as they allow CO2 to penetrate the solvation shell of the anion in acetonitrile, but not in water. The reaction paths and the reactive intermediates involved under different conditions are discussed.

The photolysis of triethylamine (1a) in the presence of carbon dioxide leads to the hydrogenation of CO₂, the α-C-C coupling of triethylamine (1a), and the CO₂–insertion into the α-C-H σ-bond of amine 1a. The reaction is proposed to proceed through the radical ion pair [R₃N^·+·CO₂^·-] generated by the photoionization of amine 1a and the electron capture by CO₂. The presence of lithium tetrafluoroborate in the reaction medium promotes the efficient and stereoselective α-C-C coupling of 1a by enhancing the production of α-dialkylamino radicals and the isomerization of N,N,N’,N’-tetraethylbutane-2,3-diamine (4a).

Lithium β-ketocarboxylates 1(COOLi), prepared by reaction of lithium enolates 2(Li⁺) with carbon dioxide, readily decarboxylate in THF solution unless in the presence of lithium salts, in which case they are indefinitely stable at room temperature under inert atmosphere. The availability of stable THF solutions of lithium β-ketocarboxylates 1(COOLi) in the absence of carbon dioxide allowed for reactions with nitrogen bases and alkyl halides 3 to give α-alkyl ketones 1(R) after acidic hydrolysis. The sequence then represents the use of carbon dioxide as a removable directing group for the selective monoalkylation of lithium enolates 2(Li⁺). The roles of lithium salts in preventing the decarboxylation and disproportionation reactions of lithium β-ketocarboxylates 1(COOLi), and in determining the course of the reaction with bases and alkyl halides 3, are discussed.

The photolysis of iodide anions promotes the reaction of carbon dioxide with hydrogen sulfide or thiols to quantitatively yield formic acid and sulfur or disulfides. The reaction proceeds in acetonitrile and aqueous solutions, at atmospheric pressure and room temperature by irradiation using a low‐pressure mercury lamp. This transition‐metal‐free photocatalytic process for CO₂ capture coupled with H₂S removal may have been relevant as a prebiotic carbon dioxide fixation.

No solvent, no reaction: ethanol inhibits S _N 1 reactions of alkyl halides in sc CO ₂ by competing with the substrate for solvation.

ScCO₂, a non-polar solvent with a dielectric constant lower than n-pentane, promotes the electrophilic bromination of aromatics as efficiently as strongly ionizing solvents such as aqueous acetic and trifluoroacetic acids.

The development of catalytic methods for the effective functionalization of methane yet remains a challenge. The best system known to date is the so‐called Catalytica Process based on the use of platinum catalysts to convert methane into methyl bisulfate with a TOF rate of 10⁻³ s. In this contribution, we report a series of silver complexes containing perfluorinated tris(indazolyl)borate ligands that catalyze the functionalization of methane into ethyl propionate upon reaction with ethyl diazoacetate (EDA) by using supercritical carbon dioxide (scCO₂) as the reaction medium. The employment of this reaction medium has also allowed the functionalization of ethane, propane, butane, and isobutane.

The rate constants for the epoxidation of cis-2-heptene with [2-percarboxyethyl]-functionalized silica (1a) and meta-chloroperbenzoic acid (mCPBA) (1b) in different solvents have been determined at temperatures in the −10 to 40 °C range. The heterogeneous epoxidation exhibits a dependence of the reaction rate on solvent polarity opposite to its homogeneous counterpart and anomalous activation parameters in n-hexane, which are interpreted in terms of the surface-promoted solvent structure at the solid–liquid interface. The results show that highly polar solvents can strongly inhibit heterogeneous reactions performed with silica-supported reagents or catalysts.

Anhydrous [2-percarboxyethyl] functionalized silica (2a) is an advantageous oxidant for performing the epoxidation of olefins 1. Epoxides 3 do not undergo the ring-opening reactions catalyzed by the acidic silica surface, except for particularly activated cases such as styrene oxide. The hydrophilic and acidic character of the silica surface does not interfere with the directing effects exerted by allylic H-bond acceptor substituents. The alkenes 1 carrying hydroxyl groups react with silica-supported peracid 2a faster than unsubstituted alkenes, thus reversing the trend known for reactions with soluble peracids. These results are attributed to the H-bond interactions of substrate 1 with the silanol and carboxylic acid groups bonded to the silica surface.