Teresa Varea

Aryl radicals by copper(II) oxidation of hydrazines: A new method for the oxidative and reductive arylation of alkenes

Abstract A new source of aryl radicals interesting from the preparative point of view has been found in the reaction of arylhydrazines and copper(II) sulfate. The process allows selectively both the reductive and oxidative arylation of alkenes.

Analysis of hybrid silica materials with the aid of conventional NMR and GC/MS.

Two simple and straightforward procedures for determining the organic content of hybrid silica materials by means of conventional NMR and GC/MS techniques are described. The methods involve dissolving the hybrid material either in a concentrated solution of sodium hydroxide in deuterated water containing a suitable reference or in a solution of hydrogen fluoride in water and extracting with methylene chloride. These methods constitute useful routine techniques for obtaining immediate information concerning both the amount and chemical composition of the organic compounds on the silica surface.

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 β-effect). The silica surface impedes neither the alkyl chain adopting the conformation required for the silicon β-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.

Functionalization of CnH2n+2 Alkanes: Supercritical Carbon Dioxide Enhances the Reactivity towards Primary Carbon–Hydrogen Bonds

The functionalization of the primary sites of alkanes is one of the more challenging areas in catalysis. In this context, a novel effect has been discovered that is responsible for an enhancement in the reactivity of the primary C−H bonds of alkanes in a catalytic system. The copper complex Cu(NCMe) ( =hydrotris{[3,5‐bis(trifluoromethyl)‐4‐bromo]‐pyrazol‐1‐yl}borate) catalyzes the functionalization of CnH2n+2 with ethyl diazoacetate upon inserting the CHCO2Et unit into C−H bonds. In addition, the selectivity of the reaction toward the primary sites significantly increased relative to that obtained in neat alkane upon using supercritical carbon dioxide as the reaction medium. This was attributed to the effect of the carbon dioxide molecules that withdraw electron density from the fluorine atoms of the ligand, which enhances the electrophilic nature of the metal center. DFT studies validated this proposal.

Regioselectivity in the Ligand-Assisted Addition of Vinylmagnesium Bromide: An Experimental and Theoretical Study on the γ-Alkoxycyclobutenone Model

Compounds 2 (M = Mg) obtained in the mono addition of vinylmagnesium bromide to squarates are attractive structural models to determine the influence of complexation between magnesium(II) and the alkoxide group on the regioselectivity of the 1,2- versus 1,4-addition of organomagnesium by complex induced proximity effects (CIPE). The 1,4-addition is observed almost exclusively in the case of vinylmagnesium in THF solution with formation of hydroxyketones type 5, which are always side or minor products in the known reaction of alkenyllithium derivatives. A comparative study on the reactivity of alkenyllithium and magnesiun derivatives is reported. The high regioselectivitity observed in the 1,4-addition of vinylmagnesium bromide is fully understood by computational studies of compounds 2 (M = Mg) at the DFT level with the density functional B3LYP.

Selective lipase-catalyzed acylation of epimeric α-sulfinyl alcohols: an efficient method of separation

Abstract A facile and efficient separation of mixtures of epimeric α-sulfinyl alcohols 2:3 can be accomplished by selective lipase-catalyzed acylation with neat vinyl acetate. Pseudomonas cepacia and Candida rugosa lipase showed opposite epimer differentiation in the esterification.

Enzymatic esterification of bicyclic meso-diols derived from 1,4-bis(hydroxymethyl)furan. An enantioselective Diels–Alder reaction equivalent

Abstract meso -Diols derived from the Diels–Alder adduct 1,4-bis(hydroxymethyl)furan/dimethyl acetylenedicarboxylate can be enantioselectively monoacetylated under CRL or PSL catalysis with very good yields and moderate to excellent ees. (+)-Monoacetates are always preferentially formed in the reactions catalyzed by CRL, and their (−)-enantiomers are the main products in the acetylations under PSL catalysis. Absolute configurations have been determined by X-ray analysis of a single crystal of an ( R )-α-methoxyphenylacetyl derivative.

A SIMPLE AND EFFICIENT ROUTE TO 1,4-DIKETONES FROM SQUARIC ACID

Abstract Squaric acid derivatives react with organolithium compounds at room temperature to afford with excellent yields after hydrolysis, symmetrical and unsymmetrically substituted oxygenated 1,4 diketones bearing alkyl, aryl or heteroaryl groups at the carbonyl positions. In the case of aromatic or heteroaromatic ketones the enol tautomers are also obtained.

C-C Bond Cleavage in O-Centered Mono- and Dianions Derived from α-Dicarbonyl Compounds

Abstract The reaction of organolithium compounds with oxalyl derivatives and cyclic 1,2-dicarbonyl compounds leads to pinacols or ketones derived from homolyttc C-C bond cleavage of the intermediate O,O-centered pinacol dianions depending on the ability of the substituents for the stabilisation of the resulting radical amon The homolysis is induced by electrostatic repulsion of the negatively charged oxygen atoms

Stabilization of the tris(2-thienyl)methyl cation by formation of polymethine units. A thiophene S-S intramolecular interaction

The tris(2-thienyl)methyl cation 2a+ is stabilized by delocalization over a conjugated polymethine framework; it shows restricted rotation about the C–S bond induced by S–S interaction and crystallizes as a statistically disordered racemic mixture of two enantiomeric species.