Javier Francos

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.

Bis(allyl)ruthenium(IV) Complexes Containing Water‐Soluble Phosphane Ligands: Synthesis, Structure, and Application as Catalysts in the Selective Hydration of Organonitriles into Amides

The novel mononuclear ruthenium(IV) complexes [RuCl(2)(eta(3):eta(3)-C(10)H(16))(L)] [L=(meta-sulfonatophenyl)diphenylphosphane sodium salt (TPPMS) (2a), 1,3,5-triaza-7-phosphatricyclo[3.3.1.1(3, 7)]decane (PTA) (2b), 1-benzyl-3,5-diaza-1-azonia-7-phosphatricyclo[3.3.1.1(3, 7)]decane chloride (PTA-Bn) (2c), 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA) (2d), and 2,4,10-trimethyl-1,2,4,5,7,10-hexaaza-3-phosphatricyclo[3.3.1.1(3, 7)]decane (THPA) (2e)] have been synthesized by treatment of the dimeric precursor [{RuCl(mu-Cl)(eta(3):eta(3)-C(10)H(16))}(2)] (C(10)H(16)=2,7-dimethylocta-2,6-diene-1,8-diyl) (1) with two equivalents of the corresponding water-soluble phosphane. Reaction of 1 with one equivalent of the cage-type diphosphane ligand 2,3,5,6,7,8-hexamethyl-2,3,5,6,7,8-hexaaza-1,4-diphosphabicyclo[2.2.2]octane (THDP) allowed also the high-yield preparation of the dinuclear derivative [{RuCl(2)(eta(3):eta(3)-C(10)H(16))}(2)(mu-THDP)] (2f). All these new complexes have been analytically and spectroscopically (IR and multinuclear NMR) characterized. In addition, the structure of 2b, 2c, 2d, and 2f was unequivocally confirmed by X-ray diffraction methods. Complexes 2a-f are active catalysts for the selective hydration of nitriles to amides in pure aqueous medium under neutral conditions. The wide scope of this catalytic transformation has been evaluated by using the most active catalysts [RuCl(2)(eta(3):eta(3)-C(10)H(16))(THPA)] (2e) and [{RuCl(2)(eta(3):eta(3)-C(10)H(16))}(2)(mu-THDP)] (2f). Advantages of using MW versus conventional thermal heating are also discussed.

Ruthenium-catalyzed redox isomerization/transfer hydrogenation in organic and aqueous media: A one-pot tandem process for the reduction of allylic alcohols

The hexamethylbenzene-ruthenium(II) dimer [{RuCl(μ-Cl)(η6-C6Me6)}2] 1 and the mononuclear bis(allyl)-ruthenium(IV) complex [RuCl2(η3:η2:η3-C12H18)] 2, associated with base and a hydrogen donor, were found to be active catalysts for the selective reduction of the CC bond of allylic alcohols both in organic and aqueous media. The process, which proceeds in a one-pot manner, involves a sequence of two independent reactions: (i) the initial redox-isomerization of the allylic alcohol, and (ii) subsequent transfer hydrogenation of the resulting carbonyl compound. The highly efficient transformation reported herein represents, not only an illustrative example of auto-tandem catalysis, but also an appealing alternative to the classical transition-metal catalyzed CC hydrogenations of allylic alcohols. The process has been successfully applied to aromatic as well as aliphatic substrates affording the corresponding saturated alcohols in 45–100% yields after 1.5–24 h. The best performances were reached using (i) 1–5 mol% of 1 or 2, 2–10 mol% of Cs2CO3, and propan-2-ol or (ii) 1–5 mol% of 1 or 2, 10–15 equivalents of NaO2CH, and water. The catalytic efficiency is strongly related to the structure of the allylic alcohol employed. Thus, in propan-2-ol, the reaction rate essentially depends on the steric requirement around the CC bond, therefore decreasing with the increasing number of substituents. On other hand, in water the transformation is favoured for primary allylic alcohols vs. secondary ones.

Ruthenium(IV) catalysts for the selective estragole to trans-anethole isomerization in environmentally friendly media

Several ruthenium(IV) complexes have been tested as potential catalysts for the isomerization of estragole into anethole using water and glycerol as alternative green reaction media. Best results in terms of activity and E-selectivity were obtained with the dimeric species [{RuCl(μ-Cl)(η3:η3-C10H16)}2] (C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) and the mononuclear derivative [RuCl2(η3:η3-C10H16){P(OMe)3}]. In particular, using a ruthenium loading of 1 mol%, almost quantitative and stereoselective formation of trans-anethole (trans/cis ratios = 99 : 1) could be reached at 80 °C in short times (5–30 min) employing water–MeOH (EtOH) or glycerol–MeOH (EtOH) mixtures (1 : 1 v/v) as solvent. Recyclability issues have also been addressed.

Ruthenium-catalyzed synthesis of β-oxo esters in aqueous medium: Scope and limitations

The ability of the hydrosoluble ruthenium(II) complexes [RuCl2(η6-arene)(PTA)] 3a–d, [RuCl2(η6-arene)(PTA-Bn)] 4a–d, [RuCl2(η6-arene)(DAPTA)] 5a–d, [RuCl2(η6-arene)(TPPMS)] 6a–d (arene = C6H6, p-cymene, 1,3,5-C6H3Me3, C6Me6) to promote the atom-economic formation of β-oxo esters, by addition of carboxylic acids to terminal propargylic alcohols in water has been explored. Scope, limitations and catalyst recycling have been evaluated using the most active catalyst [RuCl2(η6-C6H6)(TPPMS)], 6a.

Palladium-catalyzed cycloisomerization of (Z)-enynols into furans using green solvents: glycerol vs. water

Heteroannulation reactions of (Z)-2-en-4-yn-1-ol derivatives into furans can be conveniently performed in water and glycerol using cis-[PdCl2(DAPTA)2] as catalyst. Higher activities were observed in an aqueous medium, but catalyst recycling was much more effective in glycerol.

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.

Investigation of binap-based hydroxyphosphine arene–ruthenium(II) complexes as catalysts for nitrile hydration

The binap-based hydroxyphosphine-(η6-arene)–ruthenium(II) complexes [RuX{η6:κ1(P)-PPh2-binaphthyl}{PPh2(OH)}][OTf] (X = OTf (4), Cl (5)) have been evaluated as potential catalysts for the selective hydration of nitriles to primary amides. The triflate derivative 4 proved to be the most active, being able to hydrate a large variety of aromatic, heteroaromatic, α,β-unsaturated and aliphatic nitriles in pure water at 100 °C. The utility of complex 4 to promote the catalytic rearrangement of aldoximes has also been demonstrated. In addition, insights about the role played by the hydroxyphosphine ligand PPh2(OH) during the catalytic reactions are given.

Ruthenium-catalyzed intermolecular [2+2+2] alkyne cyclotrimerization in aqueous media under microwave irradiation

Abstract The ability of the bis(allyl)-ruthenium(IV) complex [{RuCl(µ-Cl)(η3: η3-C10H16)}2] (C10H16=2,7-dimethylocta-2,6-diene-1,8-diyl) to promote intermolecular [2+2+2] alkyne cyclotrimerization reactions in aqueous media under microwave (MW) irradiation has been evaluated. Advantages and disadvantages of using MW vs. conventional thermal heating are discussed.

Palladium(II) complexes with a phosphino-oxime ligand: synthesis, structure and applications to the catalytic rearrangement and dehydration of aldoximes

The treatment of [PdCl2(COD)] (COD = 1,5-cyclooctadiene) with 1 and 2 equivalents of 2-(diphenylphosphino)benzaldehyde oxime in dichloromethane at room temperature led to the selective formation of [PdCl2{κ2-(P,N)-2-Ph2PC6H4CHNOH}] (1) and [Pd{κ2-(P,N)-2-Ph2PC6H4CHNOH}2][Cl]2 (2), respectively, which represent the first examples of Pd(II) complexes containing a phosphino-oxime ligand. These compounds, whose structures were fully confirmed by X-ray diffraction methods, were active in the catalytic rearrangement of aldoximes. In particular, using 5 mol% complex 1, a large variety of aldoximes could be cleanly converted into the corresponding primary amides at 100 °C, employing water as solvent and without the assistance of any cocatalyst. Palladium nanoparticles are the active species in the rearrangement process. In addition, when the same reactions were performed employing acetonitrile as solvent, selective dehydration of the aldoximes to form the respective nitriles was observed. For comparative purposes, the catalytic behaviour of an oxime-derived palladacyclic complex has also been briefly evaluated.