Pascale Crochet

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.

Metal-catalyzed amide bond forming reactions in an environmentally friendly aqueous medium: nitrile hydrations and beyond

Amides are versatile building blocks in synthetic organic chemistry, presenting a wide range of pharmacological applications, and are used as raw materials in industry for the large-scale production of engineering plastics, detergents and lubricants. The development of green procedures for the synthesis of this relevant class of compounds from various starting materials, which replace antiquated methods using carboxylic acid derivatives and amines, is therefore of prime interest in modern chemistry. In this review article, a survey of metal-catalyzed synthetic approaches of amides conducted in an environmentally friendly aqueous medium is given.

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.

Water-soluble ruthenium(II) catalysts [RuCl2(η6-arene)-{P(CH2OH)3}] for isomerization of allylic alcohols and alkyne hydration

The novel water-soluble ruthenium(II) complexes [RuCl(2)(eta(6)-arene)[P(CH(2)OH)(3)]]2a-c and [RuCl(eta(6)-arene)[P(CH(2)OH)(3)](2)][Cl]3a-c have been prepared in high yields by reaction of dimers [[Ru(eta(6)-arene)(micro-Cl)Cl](2)](arene = C(6)H(6)1a, p-cymene 1b, C(6)Me(6)1c) with two or four equivalents of P(CH(2)OH)(3), respectively. Complexes 2/3a-c are active catalysts in the redox isomerization of several allylic alcohols into the corresponding saturated carbonyl compounds under water/n-heptane biphasic conditions. Among them, the neutral derivatives [RuCl(2)(eta(6)-C(6)H(6))[P(CH(2)OH)(3)]]2a and [RuCl(2)(eta(6)-p-cymene)[P(CH(2)OH)(3)]]2b show the highest activities (TOF values up to 600 h(-1); TON values up to 782). Complexes 2/3a-c also catalyze the hydration of terminal alkynes.

Thiazolyl-phosphine hydrochloride salts: effective auxiliary ligands for ruthenium-catalyzed nitrile hydration reactions and related amide bond forming processes in water

A series of water-soluble N-protonated thiazolyl-phosphine hydrochloride salts have been synthesized and coordinated to the ruthenium(II) fragment [RuCl2(η6-p-cymene)]. The resulting complexes were evaluated as potential catalysts for the selective hydration of nitriles to primary amides in environmentally friendly aqueous medium. The best results in terms of activity were achieved when tris(5-(2-aminothiazolyl))phosphine trihydrochloride was used as ligand. Using the Ru(II) complex 9 derived from this salt (3 mol%), the catalytic reactions proceeded cleanly in pure water at 100 °C without the assistance of any additive, affording the desired amides in high yields (>78%) after short reaction periods (0.5–7 h). The process was operative with both aromatic, heteroaromatic, α,β-unsaturated and aliphatic nitriles, and tolerated several functional groups. The utility of 9 in promoting the formation of primary amides in water by catalytic rearrangement of aldoximes and direct coupling of aldehydes with NH2OH·HCl has also been demonstrated.

Bis(allyl)-Ruthenium(IV) Complexes: Promising Precursors for Catalytic Organic Synthesis

The present review reports on the chemistry of the bis(allyl)-ruthenium(IV) complexes ({Ru(η 3 :η 3 -C10H16)(μ- Cl)Cl}2) (C 10H16 = 2,7-dimethylo cta-2,6-diene-1,8-diyl) and (Ru(η 3 :η 2 :η 3 -C12H18)Cl2) (C 12H18 = dodeca-2,6,10 -triene- 1,12-diyl). Stoichiometric reactions allowing the preparation of a variety of organoruthenium(IV) and (II) derivatives, as well as the involvement of these species in a series of catalytic organic transformations are presented.

Highly water-soluble arene-ruthenium(II) complexes: application to catalytic isomerization of allylic alcohols in aqueous medium

Arene-ruthenium(II) derivatives [RuCl2(η6-C6H5OCH2CH2OH)(L)] (L = P(OMe)3 (2a), P(OEt)3 (2b), P(OiPr)3 (2c), P(OPh)3 (2d), PPh3 (2e)) have been prepared from the dimer [{RuCl(μ-Cl)(η6-C6H5OCH2CH2OH)}2] and the appropriate P-donor ligand. The hydroxyethoxy substituent on the arene induces water-solubility of the resulting complexes (up to 755 g L−1); in particular derivative 2a being one hundred times more soluble in water than its p-cymene congener [RuCl2(η6-p-cymene){P(OMe)3}]. Compounds 2a–e are active catalysts for isomerization of allylic alcohols into the corresponding ketones in aqueous medium. The best performances are obtained with derivatives 2a–c which have shown the highest activity reported to date for the isomerization of aromatic or disubstituted substrates in water.

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.

Neutral and cationic (η6-arene)-ruthenium(II) complexes containing the iminophosphorane-phosphine ligand Ph2PCH2P(=N-p-C5F4N)Ph2: influence of the arene ring in catalytic transfer hydrogenation of cyclohexanone

Abstract Ruthenium(II) dimers [{Ru(η 6 -arene)(μ-Cl)Cl} 2 ] ( 1a – f ) readily react with the iminophosphorane–phosphine ligand Ph 2 PCH 2 P(N- p -C 5 F 4 N)Ph 2 ( 2 ), in dichloromethane at room temperature, to afford the neutral derivatives [Ru(η 6 -arene)Cl 2 { k 1 - P -Ph 2 PCH 2 P(N- p -C 5 F 4 N)Ph 2 }] (arene=C 6 H 6 ( 3a ), 1- i Pr-4-C 6 H 4 Me ( 3b ), 1,3,5-C 6 H 3 Me 3 ( 3c ), 1,2,3,4-C 6 H 2 Me 4 ( 3d ), 1,2,4,5-C 6 H 2 Me 4 ( 3e ), C 6 Me 6 ( 3f )). Treatment of 3a – f with AgSbF 6 in dichloromethane yields the cationic species [Ru(η 6 -arene)Cl{ k 2 - P , N -Ph 2 PCH 2 P(N- p -C 5 F 4 N)Ph 2 }][SbF 6 ] ( 4a – f ). The catalytic activity of complexes 3 and 4 in transfer hydrogenation of cyclohexanone by propan-2-ol has been studied. Among them, the cationic derivative [Ru(η 6 -C 6 Me 6 )Cl{ k 2 - P , N -Ph 2 PCH 2 P(N- p -C 5 F 4 N)Ph 2 }][SbF 6 ] ( 4f ) shows the highest activity. Electrochemical data for 3 and 4 are also reported.

Novel ruthenium(II) complexes containing imino- or aminophosphine ligands for catalytic transfer hydrogenation

Five- and six-coordinate ruthenium(II) complexes containing imino- and aminophosphines have been prepared by ligand exchange processes. Thus, reactions of [RuCl2(PPh3)3] with 2-Ph2PC6H4CHNR (R = Ph (1a); 2′,6′-C6H3Me2 (1b); 2′-C6H4OMe (1c)) lead to the chelate iminophosphine complexes [RuCl2(κ2-P,N-2-Ph2PC6H4CHNR)(PPh3)] (R = Ph (3a); 2′,6′-C6H3Me2 (3b)) and [RuCl2(κ3-P,N,O-2-Ph2PC6H4CHN-2′-C6H4OMe)(PPh3)] (3c), respectively. Similarly, reactions with aminophosphine ligands 2-Ph2PC6H4CH2NHR (R = Ph (2a); iPr (2d); (S)-CHMeCy (2e)) afford the 16-electron complexes [RuCl2(κ2-P,N-2-Ph2PC6H4CH2NHR)(PPh3)] (R = Ph (5a); iPr (5d); (S)-CHMeCy (5e)). The iminophosphines 2-Ph2PC6H4CHNR (R = iPr (1d); (S)-CHMeCy (1e)) react with [RuCl2(DMSO)4] to lead to the bis-iminophosphine complexes [RuCl2(κ2-P,N-2-Ph2PC6H4CHNR)2] (R = iPr (4d); (S)-CHMeCy (4e)). The crystal structure of 4d has been determined by X-ray diffraction. Complexes 3a–c, 4d,e and 5a,d,e are active in catalytic transfer hydrogenation of acetophenone. All of them are more efficient than the precursor [RuCl2(PPh3)3].