Photolysis of Tertiary Amines in the Presence of CO2: The Paths to Formic Acid, α-Amino Acids, and 1,2-Diamines.

The photolysis of triethylamine (1a) in the presence of carbon dioxide leads to the hydrogenation of CO2, the α-C-C coupling of 1a, and the CO2 insertion into the α-C-H σ-bond of amine 1a. This 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).

SN1 reactions in supercritical carbon dioxide in the presence of alcohols: the role of preferential solvation.

Ethanol () inhibits SN1 reactions of alkyl halides in supercritical carbon dioxide (scCO2) and gives no ethers as products. The unexpected behaviour of alcohols in the reaction of alkyl halides with 1,3-dimethoxybenzene () in scCO2 under different conditions is rationalised in terms of Brønsted and Lewis acid-base equilibria of reagents, intermediates, additives and products in a singular solvent characterised by: (i) the strong quadrupole and Lewis acid character of carbon dioxide, which hinders SN2 paths by strongly solvating basic solutes; (ii) the weak Lewis base character of carbon dioxide, which prevents it from behaving as a proton sink; (iii) the compressible nature of scCO2, which enhances the impact of preferential solvation on carbon dioxide availability for the solvent-demanding rate determining step.

Inverse solvent effects in the heterogeneous and homogeneous epoxidation of cis-2-heptene with [2-percarboxyethyl]-functionalized silica and meta-chloroperbenzoic acid.

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.

Reactions at interfaces: oxygenation of n-butyl ligands anchored on silica surfaces with methyl(trifluoromethyl)dioxirane.

The oxygenation of n-butyl and n-butoxy chains bonded to silica with methyl(trifluoromethyl)dioxirane (1) revealed the ability of the silica matrix to release electron density toward the reacting C(2)-H σ-bond through the Si-C(1) and Si-O(1) σ-bonds connecting the alkyl chain to the surface (silicon ß-effect). The silica surface impedes neither the alkyl chain adopting the conformation required for the silicon ß-effect nor dioxirane 1 approaching the reactive C(2) methylene group. Reaction regioselectivity is insensitive to changes in the solvation of the reacting system, the location of organic ligands on the silica surface, and the H-bonding character of the silica surface. Reaction rates are faster for those organic ligands either within the silica pores or bonded to hydrophilic silica surfaces, which evidence the enhanced molecular dynamics of confined dioxirane 1 and the impact of surface phenomena on the reaction kinetics. The oxygenation of n-butyl and n-butoxy chains carrying trimethylsilyl, trimethoxysilyl, and tert-butyl groups with dioxirane 1 under homogeneous conditions confirms the electronic effects of the silyl substituents and the consequences of steric hindrance on the reaction rate and regioselectivity. Orthosilicic acid esters react preferentially at the methylene group adjacent to the oxygen atom in clear contrast with the reactivity of the carboxylic or sulfonic acid alkyl esters, which efficiently protect this position toward oxidation with 1.

Oxidation of alcohols to carbonyl compounds with CrO3.SiO2 in supercritical carbon dioxide.

Supercritical carbon dioxide (scCO2) is an effective reaction medium to perform the oxidation of primary and secondary aliphatic alcohols to the corresponding carbonyl compounds with chromium trioxide supported on silica. These reactions were performed by flowing a solution of the alcohol in scCO2 through a column containing the supported reagent and recovering the product by depressurization. This method avoids the use of organic solvents and the contamination of the products with chromium species.

Structural, magnetic, and spectroscopic comparative studies on four new derivatives of DIMMAL (2-di1H-2-imidazolylmethylmalonate): a novel generator of multidimensional networks.

This paper reports the synthesis, structure solution, and magnetic characterization of four new DIMMAL-containing compounds (H2DIMMAL = 2-di1H-2-imidazolylmethylmalonic acid), H2DIMMAL x H2O (1), Na2(DIMMAL) x 5H2O (2), [Cu(HDIMMAL)2] (3), and [Cu2(DIMMAL)2(H2O)2] x 2H2O (4). Compound 1, containing two carboxylates and two protonated imidazole rings, adopts the dizwitterion configuration. These monohydrate MBBs pack together into a 3D array driven, as in the other three cases herein reported, by a combination of multiple-path H-bonds and aromatic-aromatic interactions. Compound 2 consists of centrosymmetric Na+ tetramers in which four NaO6 distorted octahedra are interconnected by carboxylate and water bridges. Compound 3 consists of mononuclear [Cu(HDIMMAL)2] units in which HDIMMAL- acts as a tridentate ligand through two imidazole N atoms and the deprotonated O from a carboxylate. Compound 4 consists of centrosymmetric cyclic dinuclear [Cu2(DIMMAL)2(H2O)2] x 2H2O units involving propionate-arm bridges. The building-block units described above, in each case, are interconnected into 3D networks by multiple H-bonding paths and aromatic-aromatic interactions. The EPR spectra are indicative of an essentially d(x2-y2) ground state for the copper(II) ions in 3 and 4 (CuN4O2 and CuN2O2O' chromophores, respectively). Magnetic susceptibility measurements in the range of 1.8-200 K for compound 4 show weak antiferromagnetic exchange between the copper(II) ions (2J = -1.6(1) cm(-1)). The effectiveness of the propionate-arm bridges, involving C-C sigma bonds, in propagating magnetic exchange between the copper(II) ions is discussed.