Alba E. Díaz-Álvarez

Glycerol and derived solvents: new sustainable reaction media for organic synthesis.

The rapid growth of the biodiesel industry has led to a large surplus of its major byproduct, i.e. glycerol, for which new applications need to be found. Research efforts in this area have focused mainly on the development of processes for converting glycerol into value-added chemicals and its reforming for hydrogen production, but recently, in line with the increasing interest in the use of alternative greener solvents, an innovative way to revalorize glycerol and some of its derivatives has seen the light, i.e. their use as environmentally friendly reaction media for synthetic organic chemistry. The aim of the present Feature Article is to provide a comprehensive overview on the developments reached in this field.

Recent Advances in the Use of Glycerol as Green Solvent for Synthetic Organic Chemistry

Owing to its biodegradable and non-toxic nature, glycerol, the main byproduct in the production of biodiesel fuel, is being actively investigated as a green reaction medium for synthetic organic chemistry. A huge number of syn- thetic transformations have been conducted in glycerol in recent years, showing most of them having similar or even supe- rior efficiency and selectivity than those performed in conventional petroleum-based organic solvents. Herein, an over- view on the most recent advances reached in the field is presented.

Ruthenium-catalyzed one-pot synthesis of primary amides from aldehydes in water

The readily available arene–ruthenium(II) complex [RuCl2(η6-C6Me6){P(NMe2)3}] (5 mol%) proved to be an efficient catalyst for the direct synthesis of primary amides from aldehydes and hydroxylamine hydrochloride (NH2OH·HCl) in water at 100 °C. The process, which requires the presence of NaHCO3 to catch the HCl released during the formation of the key aldoxime intermediates, was operative with both aromatic, heteroaromatic, α,β-unsaturated and aliphatic aldehydes, and tolerated several functional groups. A greener approach using commercially available NH2OH solution (50 wt.% in water) is also presented.

Imination reactions of free and coordinated 2-diphenylphosphino-1-phenyl-phospholane: Access to regioisomeric ruthenium(II) complexes containing novel iminophosphorane–phosphine ligands

In this work, selective monoimination reactions of free and Ru-coordinated 2-diphenylphosphino-1-phenyl-phospholane with diphenylphosphoryl azide or 4-azido-2,3,5,6-tetrafluorobenzonitrile are described. Following this approach, a large variety of neutral and cationic mono- and dinuclear (η6-arene)–ruthenium(II) complexes containing regioisomeric iminophosphorane–phosphine ligands could be prepared and, in some cases, structurally characterized by means of X-ray diffraction methods. The catalytic activity of these ruthenium complexes, both in racemic or enantiomerically pure form, in Diels–Alder cycloaddition reactions is also presented.

Expeditious Entry to Novel 2-Methylene-2,3-dihydrofuro[3,2-c] chromen-2-ones from 6-Chloro-4-hydroxychromen-2-one and Propargylic Alcohols

A catalytic system consisting of the ruthenium(II) complex [Ru(η3-2-C3H4Me)(CO)(dppf)][SbF6] (dppf = 1,1’-bis(diphenylphosphino)ferrocene) and trifluoroacetic acid has been used to promote the coupling of secondary propargylic alcohols with 6-chloro-4-hydroxychromen-2-one. The reactions afforded unusual 2-methylene-2,3-dihydrofuro[3,2-c]chromen-2-ones in good yields.

Eugenol isomerization promoted by arene–ruthenium(II) complexes in aqueous media: influence of the pH on the catalytic activity

The catalytic activity of the arene–ruthenium(II) complexes [RuCl2(η6-C6H5OCH2CH2OH)(L)] (L = P(OMe)3 (1a), P(OEt)3 (1b), P(OiPr)3 (1c), P(OPh)3 (1d), PPh3 (1e)) in the isomerization of eugenol into isoeugenol has been evaluated. Best results in terms of activity and selectivity were observed with those catalysts containing an aliphatic P-donor ligand (1a–c). Under optimized conditions, full conversions in extremely short reaction times (5 min), and with high levels of trans-selectivity (up to 98%), could be achieved. Addition of both NaOH or H2SO4 to the aqueous media resulted in rate enhancements, suggesting two different activation pathways of the pre-catalysts. We have evidenced that sodium hydroxide promotes the release of the η6-coordinated arene ligand, while sulfuric acid favours the Ru–Cl bond dissociation and the formation of aquo-derivatives [RuCl(H2O)(η6-C6H5OCH2CH2OH)(L)][Cl].