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.

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.

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.

Synthetic and reactivity studies of hetero-tri-anionic sodium zincates.

The synthesis and characterisation of several sodium zincate complexes are reported. The all-alkyl monomeric sodium zincate, (PMEDTA)·Na(µ-CH2SiMe3)Zn(t)Bu22, is prepared by combining equimolar quantities of (t)Bu2Zn, (n)BuNa and PMDETA (N,N,N',N'',N''-pentamethyldiethylenetriamine)]. A similar approach was used to prepare and isolate the unusual dimeric zincate [(PMEDTA)·Na(µ-(n)Bu)Zn(t)Bu2]23. When an equimolar mixture of (n)BuNa, (t)Bu2Zn and TMP(H) (2,2,6,6-tetramethylpiperidine) is combined in hexane, the hetero-tri-leptic TMP(H)-solvated zincate (TMPH)Na(µ-TMP)(µ-(n)Bu)Zn(t)Bu 4 results. Complex 4 can also be prepared using a rational approach [i.e., utilising two molar equivalents of TMP(H)]. When TMEDA is reacted with an equimolar mixture of (n)BuNa, (t)Bu2Zn and TMP(H), the monomeric sodium zincate (TMEDA)Na(µ-TMP)(µ-(n)Bu)Zn(t)Bu 5 was obtained - this complex is structurally similar to the synthetically useful relation (TMEDA)·Na(µ-TMP)(µ-(t)Bu)Zn((t)Bu) 1. By changing the sodium reagent used in the synthesis of 5, it was possible to prepare (TMEDA)Na(µ-TMP)(µ-Me3SiCH2)Zn(t)Bu 6. By reacting 5 with cis-DMP(H) (cis-2,6-dimethylpiperidine), the zincate could thermodynamically function as an amide base, to give the transamination product (TMEDA)Na(µ-cis-DMP)(µ-(n)Bu)Zn(t)Bu 7, although no crystals could be grown. However, when HMDS(H) (1,1,1,3,3,3-hexamethyldisilazane) or PEA(H) [(+)-bis[(R)-1-phenylethyl]amine] is reacted with 5, crystalline (TMEDA)Na(µ-HMDS)(µ-(n)Bu)Zn(t)Bu 8 or (TMEDA)Na(µ-PEA)(µ-(n)Bu)Zn(t)Bu 9 is isolated respectively. With PNA(H) (N-phenylnaphthalen-1-amine) the reaction took a different course and resulted in the formation of the dimeric sodium amide complex [(TMEDA)Na(PNA)]210. When reacted with benzene, it appears that a TMEDA-free variant of 5 functions thermodynamically as an (n)Bu base to yield the previously reported (TMEDA)Na(µ-TMP)((t)Bu)Zn(µ-C6H4)Zn((t)Bu)(µ-TMP)Na(TMEDA) 11. Finally when reacted with TEMPO (2,2,6,6-tetramethylpiperidinyloxy), 5 undergoes a single electron transfer reaction to form (TMEDA)Na(µ-TMP)(µ-TEMPO)Zn(n)Bu 12.

Solid state and solution studies of lithium tris(n-butyl)magnesiates stabilised by Lewis donors.

Several Lewis base adducts of the synthetically important lithium tris(n-butyl)magnesiate LiMg((n)Bu)3 have been prepared and structurally characterised. The complexes were prepared by a co-complexation approach i.e., by combining the monometallic (n)BuLi and (n)Bu2Mg reagents in hydrocarbon solution before adding a molar equivalent of a donor molecule (a bidentate amine, tridentate amine or cyclic ether). The lithium magnesiates all adopt variants of the "Weiss motif" structure, i.e., contacted ion pair dimers with a linear arrangement and metals connected by butyl anions, where tetrahedral magnesium ions are in the central positions and the lithiums occupy the outer region, solvated by a neutral Lewis donor [(donor)Li(µ-(n)Bu)2Mg(µ-(n)Bu)2Mg(µ-(n)Bu)2Li(donor)]. When TMPDA, PMDETA or (R,R)-TMCDA [TMPDA = N,N,N'N'-tetramethylpropanediamine; PMDETA = N,N,N',N'',N''-pentamethyldiethylenetriamine; and (R,R)-TMCDA = (R,R)-N,N,N',N'-tetramethylcyclohexane-1,2-diamine], are employed, dimeric tetranuclear lithium magnesiates are produced. Due to the tridentate nature of the ligand, the PMDETA-containing structure (2) has an unusual 'open'-motif. When TMEDA (TMEDA = N,N,N',N'-tetramethylethylenediamine) is employed, a n-butoxide-containing complex [(TMEDA)Li(µ-(n)Bu)(µ-O(n)Bu)Mg2((n)Bu)2(µ-(n)Bu)(µ-O(n)Bu)Li(donor)] has been serendipitously prepared and adopts a ladder conformation which is commonly observed in lithium amide chemistry. This complex has also been prepared using a rational methodology. When 1,4-dioxane is employed, the donor stitches together a polymeric array of tetranuclear dimeric units (6). The hydrocarbon solution structures of the compounds have been characterised by (1)H, (7)Li, (13)C NMR spectroscopy; 2 has been studied by variable temperature and DOSY NMR.

Optimisation of a lithium magnesiate for use in the non-cryogenic asymmetric deprotonation of prochiral ketones.

A study has been conducted to determine whether lithium magnesiates are feasible candidates for the enantioselective deprotonation of 4-alkylcyclohexanones. The commercially available chiral amine (+)-bis[(R)-1-phenylethyl]amine (2-H) was utilised to induce enantioselection. When transformed to its lithium salt and combined with (n)Bu2Mg, improved enantioselective deprotonation of 4-tert-butylcyclohexanone (with respect to the monometallic lithium amide) at 20 °C was observed. In an attempt to optimise the reaction further, different additives were added to the lithium amide. The best performing deprotonations at 0 °C were those in which (Me3SiCH2)2Mg (er pro-S 74 : 26) and (Me3SiCH2)2Mn (er pro-S 72 : 28) were added, hence the lithium magnesiate "LiMg(2)(CH2SiMe3)2" was used in the remainder of the study. The optimum solvent for the reaction was found to be THF. NMR spectroscopic studies of a D8-THF solution of "LiMg(2)(CH2SiMe3)2" appear to show that this mono-amide bis-alkyl species is in equilibrium with a bis-amide mono-alkyl compound (and a tris-alkyl lithium magnesiate). When a genuine bis-amide lithium magnesiate solution is used, the deprotonation results were essentially identical to those obtained for "LiMg(2)(CH2SiMe3)2". By adding LiCl to "LiMg(2)(CH2SiMe3)2" the er at 0 °C improved to 81 : 19. At -78 °C good yields and an er of 93 : 7 were obtained. This LiCl-containing base was used to successfully deprotonate other 4-alkylcyclohexanones.

Axially chiral triazoloisoquinolin-3-ylidene ligands in gold(I)-catalyzed asymmetric intermolecular (4 + 2) cycloadditions of allenamides and dienes.

The first highly enantioselective intermolecular (4 + 2) cycloaddition between allenes and dienes is reported. The reaction provides good yields of optically active cyclohexenes featuring diverse substitution patterns and up to three stereocenters. Key to the success of the process is the use of newly designed axially chiral N-heterocyclic carbene-gold catalysts.

Access to unusual polycyclic spiro-enones from 2,2'-bis(allyloxy)-1,1'-binaphthyls using Grubbs' catalysts: an unprecedented one-pot RCM/Claisen sequence.

Treatment of 2,2'-bis(allyloxy)-1,1'-binaphthyls with the first-generation Grubbs' carbene under MW-irradiation results in the formation of new polycyclic spiro-enones through an unprecedented RCM/Claisen sequence.

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.

Chemistry by Nanocatalysis: First example of a solid-supported RAPTA complex for organic reactions in aqueous medium.

A ruthenium-arene-PTA (RAPTA) complex has been supported for the first time on an inorganic solid, that is, silica-coated ferrite nanoparticles. The resulting magnetic material proved to be a general, very efficient and easily reusable catalyst for three synthetically useful organic transformations; selective nitrile hydration, redox isomerization of allylic alcohols, and heteroannulation of (Z)-enynols. The use of low metal concentration, environmentally friendly water as a reaction medium, with no use at all of organic solvent during or after the reactions, and microwaves as an alternative energy source renders the synthetic processes reported herein "truly" green and sustainable.

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/TFA-catalyzed regioselective C-3-alkylation of indoles with terminal alkynes in water: efficient and unprecedented access to 3-(1-methylalkyl)-1H-indoles.

An unprecedented C-3-alkylation reaction of indoles with terminal alkynes in aqueous medium has been developed using catalytic amounts of ruthenium and trifluoroacetic acid.

1,4-Butanediol as a reducing agent in transfer hydrogenation reactions.

1,4-Butanediol is able to deliver two equivalents of H(2) in hydrogen-transfer reactions to ketones, imines, and alkenes. Unlike simple alcohols, which establish equilibrium in the reduction of ketones, 1,4-butanediol acts essentially irreversibly owing to the formation of butyrolactone, which acts as a thermodynamic sink. It is therefore not necessary to use 1,4-butanediol in great excess in order to achieve reduction reactions. In addition, allylic alcohols are reduced to saturated alcohols through an isomerization/reduction sequence using a ruthenium catalyst with 1,4-butanediol as the reducing agent. Imines and alkenes are also reduced under similar conditions.

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].