Phosphinous Acid-Assisted Hydration of Nitriles: Understanding the Controversial Reactivity of Osmium and Ruthenium Catalysts.

The synthesis and catalytic behavior of the osmium(II) complexes [OsCl2 (η6 -p-cymene)(PR2 OH)] [R=Me (2 a), Ph (2 b), OMe (2 c), OPh (2 d)] in nitrile hydration reactions is presented. Among them, the best catalytic results were obtained with the phosphinous acid derivative [OsCl2 (η6 -p-cymene)(PMe2 OH)] (2 a), which selectively provided the desired primary amides in excellent yields and short times at 80 °C, employing directly water as solvent, and without the assistance of any basic additive (TOF values up to 200 h-1 ). The process was successful with aromatic, heteroaromatic, aliphatic, and α,ß-unsaturated organonitriles, and showed a high functional group tolerance. Indeed, complex 2 a represents the most active and versatile osmium-based catalyst for the hydration of nitriles reported so far in the literature. In addition, it exhibits a catalytic performance similar to that of its ruthenium analogue [RuCl2 (η6 -p-cymene)(PMe2 OH)] (4). However, when compared to 4, the osmium complex 2 a turned out to be faster in the hydration of less-reactive aliphatic nitriles, whereas the opposite trend was generally observed with aromatic substrates. DFT calculations suggest that these differences in reactivity are mainly related to the ring strain associated with the key intermediate in the catalytic cycle, that is, a five-membered metallacyclic species generated by intramolecular addition of the hydroxyl group of the phosphinous acid ligand to the metal-coordinated nitrile.

Gold-Catalyzed Regio- and Stereoselective Addition of Carboxylic Acids to Iodoalkynes: Access to (Z)-ß-Iodoenol Esters and 1,4-Disubstituted (Z)-Enynyl Esters.

In the presence of catalytic amounts of the Au(I) cation [Au(PPh3)]+, a large variety of (Z)-ß-iodoenol esters (39 examples) could be synthesized under mild reaction conditions through the regio- and stereospecific intermolecular addition of carboxylic acids to iodoalkynes. Sonogashira coupling of representative (Z)-ß-iodoenol esters with terminal alkynes, alkynols, and 1,3-enynes allowed also the access to different 1,4-disubstituted (Z)-enynyl esters in excellent yields.

Broad Scope Synthesis of Ester Precursors of Nonfunctionalized Chiral Alcohols Based on the Asymmetric Hydrogenation of α,ß-Dialkyl-, α,ß-Diaryl-, and α-Alkyl-ß-aryl-vinyl Esters.

The catalytic asymmetric hydrogenation of trisubstituted enol esters using Rh catalysts bearing chiral phosphine-phosphite ligands (P-OP) has been studied. Substrates covered comprise α,ß-dialkyl, α-alkyl-ß-aryl, and α,ß-diarylvinyl esters, the corresponding hydrogenation products being suitable precursors to prepare synthetically relevant chiral nonfunctionalized alcohols. A comparison of reactivity indicates that it decreases in the order: α,ß-dialkyl > α-alkyl-ß-aryl > α,ß-diaryl. Based on the highly modular structure of P-OP ligands employed, catalyst screening identified highly enantioselective catalysts for α,ß-dialkyl (95-99% ee) and nearly all of α-alkyl-ß-aryl substrates (92-98% ee), with the exception of α-cyclohexyl-ß-phenylvinyl acetate which exhibited a low enantioselectivity (47% ee). Finally, α,ß-diarylvinyl substrates showed somewhat lower enantioselectivities (79-92% ee). In addition, some of the catalysts provided a high enantioselectivity in the hydrogenation of E/Z mixtures (ca. Z/E = 75:25) of α,ß-dialkylvinyl substrates, while a dramatic decrease on enantioselectivity was observed in the case of α-methyl-ß-anisylvinyl acetate (Z/E = 58:42). Complementary deuteration reactions are in accord with a highly enantioselective hydrogenation for both olefin isomers in the case of α,ß-dialkylvinyl esters. In contrast, deuteration shows a complex behavior for α-methyl-ß-anisylvinyl acetate derived from the participation of the E isomer in the reaction.

Gold(i)-catalyzed addition of carboxylic acids to internal alkynes in aqueous medium.

We report herein the efficient hydro-oxycarbonylation of symmetrical and unsymmetrical internal alkynes with carboxylic acids in water at 60 °C, employing the catalytic system [AuCl(PPh3)]/AgOAc (5 mol%). This simple and eco-friendly protocol allows for the synthesis of a wide variety of trisubstituted enol esters (37 examples) in high yields and with complete Z-stereoselectivity. The use of microwave irradiation as an alternative energy source has also been evaluated.

Unmasking the Action of Phosphinous Acid Ligands in Nitrile Hydration Reactions Catalyzed by Arene-Ruthenium(II) Complexes.

The catalytic hydration of benzonitrile and acetonitrile has been studied by employing different arene-ruthenium(II) complexes with phosphinous (PR2OH) and phosphorous acid (P(OR)2OH) ligands as catalysts. Marked differences in activity were found, depending on the nature of both the P-donor and η(6)-coordinated arene ligand. Faster transformations were always observed with the phosphinous acids. DFT computations unveiled the intriguing mechanism of acetonitrile hydration catalyzed by these arene-ruthenium(II) complexes. The process starts with attack on the nitrile carbon atom of the hydroxyl group of the P-donor ligand instead of on a solvent water molecule, as previously suggested. The experimental results presented herein for acetonitrile and benzonitrile hydration catalyzed by different arene-ruthenium(II) complexes could be rationalized in terms of such a mechanism.

Highly enantioselective hydrogenation of 1-alkylvinyl benzoates: a simple, nonenzymatic access to chiral 2-alkanols.

Going chiral! Highly enantioselective catalytic hydrogenations of enol esters 1 by using a Rh catalyst bearing a P-OP ligand are described (see scheme; NBD=norbornadiene). The catalytic system has a broad scope and allows the preparation of a wide range of chiral esters 2 bearing diverse alkyls or a benzyl group with high enantioselectivities. These esters can easily be converted in highly enantioenriched 2-alkanols.

Ruthenium(IV)-catalyzed isomerization of the C=C bond of o-allylic substrates: a theoretical and experimental study.

A general mechanism to rationalize Ru(IV) -catalyzed isomerization of the C=C bond in O-allylic substrates is proposed. Calculations supporting the proposed mechanism were performed at the MPWB1K/6-311+G(d,p)+SDD level of theory. All experimental observations in different solvents (water and THF) and under different pH conditions (neutral and basic) can be interpreted in terms of the new mechanism. Theoretical analysis of the transformation from precatalyst to catalyst led to structural identification of the active species in different media. The experimentally observed induction period is related to the magnitudes of the energy barriers computed for that process. The theoretical energy profile for the catalytic cycle requires application of relatively high temperatures, as is experimentally observed. Participation of a water molecule in the reaction coordinate is mechanistically essential when the reaction is carried out in aqueous medium. The new mechanistic proposal helped to develop a new experimental procedure for isomerization of allyl ethers to 1-propenyl ethers under neutral aqueous conditions. This process is an unique example of efficient and selective catalytic isomerization of allyl ethers in aqueous medium.

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.

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.

Half-sandwich ruthenium(ii) complexes with tethered arene-phosphinite ligands: synthesis, structure and application in catalytic cross dehydrogenative coupling reactions of silanes and alcohols.

The preparation of the tethered arene-ruthenium(ii) complexes [RuCl2{η6:κ1(P)-C6H5(CH2)nOPR2}] (R = Ph, n = 1 (9a), 2 (9b), 3 (9c); R = iPr, n = 1 (10a), 2 (10b), 3 (10c)) from the corresponding phosphinite ligands R2PO(CH2)nPh (R = Ph, n = 1 (1a), 2 (1b), 3 (1c); R = iPr, n = 1 (2a), 2 (2b), 3 (2c)) is presented. Thus, in a first step, the treatment at room temperature of tetrahydrofuran solutions of dimers [{RuCl(µ-Cl)(η6-arene)}2] (arene = p-cymene (3), benzene (4)) with 1-2a-c led to the clean formation of the corresponding mononuclear derivatives [RuCl2(η6-p-cymene){R2PO(CH2)nPh}] (5-6a-c) and [RuCl2(η6-benzene){R2PO(CH2)nPh}] (7-8a-c), which were isolated in 66-99% yield. The subsequent heating of 1,2-dichloroethane solutions of these compounds at 120 °C allowed the exchange of the coordinated arene. The substitution process proceeded faster with the benzene derivatives 7-8a-c, from which complexes 9-10a-c were generated in 61-82% yield after 0.5-10 h of heating. The molecular structures of [RuCl2(η6-p-cymene){iPr2PO(CH2)3Ph}] (6c) and [RuCl2{η6:κ1(P)-C6H5(CH2)nOPiPr2}] (n = 1 (10a), 2 (10b), 3 (10c)) were unequivocally confirmed by X-ray diffraction methods. In addition, complexes [RuCl2{η6:κ1(P)-C6H5(CH2)nOPR2}] (9-10a-c) proved to be active catalysts for the dehydrogenative coupling of hydrosilanes and alcohols under mild conditions (r.t.). The best results were obtained with [RuCl2{η6:κ1(P)-C6H5(CH2)3OPiPr2}] (10c), which reached TOF and TON values up to 117 600 h-1 and 57 000, respectively.

The chemistry of guanidinate complexes of the platinum group metals.

In this Perspective article, recent advances in the chemistry of platinum group metal complexes containing mono- and dianionic guanidinate ligands, i.e. [(RN)2C-NR2]- and [(RN)2C[double bond, length as m-dash]NR]2-, respectively, are presented. Synthetic and structural aspects, reactivity studies, and applications of these compounds are discussed.

Double asymmetric hydrogenation of conjugated dienes: a self-breeding chirality route for C2 symmetric 1,4-diols.

The synthesis and double asymmetric hydrogenation of (Z,Z)-1,3-diene-1,4-diyl diacetates is described. In this reaction C2/meso ratios up to 85 : 15 and enantioselectivities up to 97% ee have been achieved. As the hydrogenation products can be converted into chiral 1,4-diols, key starting materials for the preparation of the best catalysts used, this catalytic system enables a self-breeding chirality process.

Ruthenium/TFA-catalyzed coupling of activated secondary propargylic alcohols with cyclic 1,3-diones: furan versus pyran ring formation.

A catalytic system consisting of the 16-electron allyl-ruthenium(II) complex [Ru(eta(3)-2-C3H4Me)(CO)(dppf)][SbF6] (dppf = 1,1'-bis(diphenylphosphino)ferrocene) and trifluoroacetic acid (TFA) has been used to promote the coupling between secondary propargylic alcohols and cyclic 1,3-diketones. The nature of the resulting products was found to be dependent on the ring size of the dicarbonyl compound employed. Thus, whereas 6,7-dihydro-5H-benzofuran-4-ones have been selectively obtained starting from 1,3-cyclohexanediones, via furan-ring formation, the use of 1,3-cyclopentanedione leads instead to 6,7-dihydro-4H-cyclopenta[b]pyran-5-ones via a pyran-ring formation process.

Ruthenium-catalyzed reduction of allylic alcohols: an efficient isomerization/transfer hydrogenation tandem process.

A simple and highly efficient method for the selective reduction of the C=C bond in allylic alcohols has been developed using the ruthenium(II) catalyst [{RuCl(mu-Cl)(eta(6)-C(6)Me(6)}2].

Ruthenium-Catalyzed Synthesis of ß-Hydroxyamides from ß-Ketonitriles in Water.

An unprecedented hydration/transfer hydrogenation tandem process for the catalytic conversion of ß-ketonitriles into synthetically useful ß-hydroxyamides in water has been developed, making use of the ruthenium(II) complex [RuCl2(η6-p-cymene){P(4-C6H4F)2Cl}] in combination with sodium formate.

Bis(allyl)-ruthenium(iv) complexes with phosphinous acid ligands as catalysts for nitrile hydration reactions.

Several mononuclear ruthenium(iv) complexes with phosphinous acid ligands [RuCl2(η(3):η(3)-C10H16)(PR2OH)] have been synthesized (78-86% yield) by treatment of the dimeric precursor [{RuCl(µ-Cl)(η(3):η(3)-C10H16)}2] (C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) with 2 equivalents of different aromatic, heteroaromatic and aliphatic secondary phosphine oxides R2P([double bond, length as m-dash]O)H. The compounds [RuCl2(η(3):η(3)-C10H16)(PR2OH)] could also be prepared, in similar yields, by hydrolysis of the P-Cl bond in the corresponding chlorophosphine-Ru(iv) derivatives [RuCl2(η(3):η(3)-C10H16)(PR2Cl)]. In addition to NMR and IR data, the X-ray crystal structures of representative examples are discussed. Moreover, the catalytic behaviour of complexes [RuCl2(η(3):η(3)-C10H16)(PR2OH)] has been investigated for the selective hydration of organonitriles in water. The best results were achieved with the complex [RuCl2(η(3):η(3)-C10H16)(PMe2OH)], which proved to be active under mild conditions (60 °C), with low metal loadings (1 mol%), and showing good functional group tolerance.

Ru(IV)-catalyzed isomerization of allylamines in water: a highly efficient procedure for the deprotection of N-allylic amines.

A general and efficient method for the deprotection of N-allylic substrates in aqueous media, using catalytic amounts of the bis(allyl)-ruthenium(IV) complexes [Ru(eta3:eta2:eta3-C12H18)Cl2] and [{Ru(eta3:eta3-C10H16)(micro-Cl)Cl}2], has been developed.

Catalytic synthesis of amides via aldoximes rearrangement.

Amide bond formation reactions are among the most important transformations in organic chemistry because of the widespread occurrence of amides in pharmaceuticals, natural products and biologically active compounds. The Beckmann rearrangement is a well-known method to generate secondary amides from ketoximes. However, under the acidic conditions commonly employed, aldoximes RHC=NOH rarely rearrange into the corresponding primary amides RC(=O)NH2. In recent years, it was demonstrated that this atom-economical transformation can be carried out efficiently and selectively with the help of metal catalysts. Several homogeneous and heterogenous systems have been described. In addition, protocols offering the option to generate the aldoximes in situ from the corresponding aldehydes and hydroxylamine, or even from alcohols, have also been developed, as well as a series of tandem processes allowing the access to N-substituted amide products. In this Feature article a comprehensive overview of the advances achieved in this particular research area is presented.

Dichloro(dodeca-2,6,10-triene-1,12-diyl)ruthenium(IV): a highly efficient catalyst for the isomerization of allylic alcohols into carbonyl compounds in organic and aqueous media.

The catalytic activity of the bis(allyl)-ruthenium(iv) complex [Ru([small eta](3):[small eta](2):[small eta](3)-C(12)H(18))Cl(2)] in the transposition of allylic alcohols into carbonyl compounds, both in THF and H(2)O as solvent, is reported.