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.

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.

The complexes Os(η 5-C5H5)Cl{η 2-HC⋮CC(OH)R2}(PiPr3) (R = Ph (1a), Me (1b)) react with TlPF6 to give [Os(η 5-C5H5){η 2-HC⋮CC(OH)R2}(PiPr3)]PF6 (R = Ph (2a), Me (2b)). The structures of 1a and 2a have been determined by X-ray diffraction. The comparative study of the data reveals a shortening of the Os−C(alkyne) distances on going from 1a to 2a, whereas the acetylenic bond lengths remain almost identical. Comparison of their 1H and 13C{1H} NMR spectra shows that the HC⋮ proton resonances and the chemical shifts of the acetylenic carbon atoms of 2a and 2b are substantially shifted toward lower field than are those of 1a and 1b. DFT calculations were carried out on Os(η 5-C5H5)Cl(η 2-HC⋮CR)(PH3) (R = H (A), R = CH3 (A CH3 )) and [Os(η 5-C5H5)(η 2-HC⋮CR)(PH3)]+ (R = H (B), R = CH3 (B CH3 )) model systems in order to study the differences in bonding nature of the two parent alkyne complexes, 1 and 2. Calculations give geometries very close to the X-ray-determined ones, and by using the GIAO method we succeed in qualitatively reproducing the experimental 1H and 13C chemical shifts. Both structural and spectroscopic changes can be explained by the participation of the acetylenic second π orbital (π⊥) in the metal−alkyne bonding. As we go from 1 to 2 or from A to B, the extraction of the chloride ligand transforms the 2-electron-donor alkyne ligand to a 4-electron-donor ligand, with both the π|| and the π⊥ orbitals donating to the metal and stabilizing the otherwise 16-electron unsaturated complex 2. Calculations also predict an increase of dissociation energies of the alkyne, and an enhancement in the energy of rotation of the alkyne, for complex B. Finally, Bader's atoms in molecules (AIM) analysis shows that differences in coordination nature are also reflected in the topological properties of electron density.

The catalytic isomerization of propargylic alcohols promoted by transition-metals represents a straightforward and appealing route to synthetically useful alpha,beta-unsaturated carbonyl compounds. Three different reaction pathways are known for these atom-economical transformations: (i) the so-called Meyer-Schuster and Rupe rearrangements, in which a formal 1,3- or 1,2-shift of the hydroxyl group takes place, and (ii) the redox-type isomerization, which involves a simultaneous oxidation of the alcohol unit and reduction of the C[triple bond]C bond. In this Perspective article an overview of the different metal catalysts presently available to promote these isomerization reactions, their mechanisms of action as well as relevant synthetic applications, is provided.

Complex Os(η 5 -C 5 H 5 )Cl(P i Pr 3 ) 2 (1) reacts with equimolecular mixtures of TlPF 6 and alkynes such as phenylacetylene and cyclohexylacetylene to give [OsH(η 5 -C 5 H 5 )(CtCR)(P i Pr 3 ) 2 ]PF 6 (R ) Ph (2), Cy (3)). The structure of 2 in the solid state has been determined by X-ray diffraction analysis. The distribution of ligands around the metallic center can be described as a four-legged piano-stool geometry with the hydride and alkynyl ligands mutually transoid. The reaction of 1 with 2-phenyl-3-butyn-2-ol and TlPF 6 leads to [OsH(η 5 -C 5 H 5 ){CtCC(OH)-MePh}(P i Pr 3 ) 2 ]PF 6 (4), which evolves into the hydride-enynyl complex [OsH(η 5 -C 5 H 5 ){Ct CC(Ph)dCH 2 }(P i Pr 3 ) 2 ]PF 6 (5) in solution of chloroform. Treatment of 1 with 1,1-diphenyl-2-propyn-1-ol and TlPF 6 affords [OsH(η 5 -C 5 H 5 ){CtCC(OH)Ph 2 }(P i Pr 3 ) 2 ]PF 6 (6), which reacts with KOH in methanol to give the neutral compound Os(η 5 -C 5 H 5 ){CtCC(OH)Ph 2 }(P i Pr 3 ) 2 ( 7) by extraction of the hydride ligand. The addition of 1 equiv of HPF 6 to the solutions of 7 leads to the allenylidene, [Os(η 5 -C 5 H 5 )(CdCdCPh 2 )(P i Pr 3 ) 2 ]PF 6 (8), which affords the dicationic carbyne derivative [Os(η 5 -C 5 H 5 )(CCHdCPh 2 )(P i Pr 3 ) 2 ](PF 6 ) 2 (9) by reaction with HPF 6 . The structure of 9 in the solid state has been also determined by X-ray diffraction analysis. In this case, the geometry around the osmium center is close to octahedral with the triisopropylphosphine ligands mutually cis disposed (P-Os-P ) 105.12(8)°). Complex 8 also reacts with nucleophilic reagents; the reaction with CH 3 Li gives Os(η 5 -C 5 H 5 ){CtCC-(CH 3 )Ph 2 }(P i Pr 3 ) 2 (10), whereas the reactions with acetone and methanol solutions of KOH afford Os(η 5 -C 5 H 5 ){CtCC[CH 2 C(O)CH 3 ]Ph 2 }(P i Pr 3 ) 2 (11) and Os(η 5 -C 5 H 5 ){CtCC(OCH 3 )Ph 2 }-(P i Pr 3 ) 2 (12), respectively. To understand the chemical behavior of 8, EHT-MO calculations on the model compounds Os(η 5 -C 5 H 5 )Cl(CdCdCH 2 )(PH 3 ) (13) and [Os(η 5 -C 5 H 5 )(CdCdCH 2 )L-(PH 3 )] + (L ) PH 3 (14), CO ( 15)) have been also carried out. The results suggest that the behavior of 8 as nucleophile is a consequence of the high electron density of the allenylidene ligand, while the behavior as γ-electrophile is due to its cationic nature. In addition, we have determined by ab initio calculations the energies of stabilization by protonation of 13-15 with a naked proton. In the three cases the formation of the corresponding carbyne derivatives [Os(η 5 -C 5 H 5 )Cl(CCHdCH 2 )(PH 3 )] + (16; 267 kcal‚mol -1 ), [Os(η 5 -C 5 H 5 )(CCHdCH 2 )-(PH 3 ) 2 ] 2+ (17; 180 kcal‚mol -1 ), and [Os(η 5 -C 5 H 5 )(CCHdCH 2 )(CO)(PH 3 )] 2+ (18; 157 kcal‚mol -1 ) involves a stabilization of the system.

An optimized synthesis of the 2,4,10-trimethyl-1,2,4,5,7,10-hexaaza-3-phosphatricyclo-[3.3.1.1 3,7 ]decane ligand (THPA) is described. It

The addition of 1,1-diphenyl-2-propyn-1-ol to pentane solutions of the cyclopentadienyl compound Os(η5-C5H5)Cl(PiPr3)2 (1) produces the displacement of a phosphine ligand from 1 and the formation of the π-alkyne complex Os(η5-C5H5)Cl{η2-HC⋮C−C(OH)Ph2}(PiPr3) (2), which affords the allenylidene derivative Os(η5-C5H5)Cl(CCCPh2)(PiPr3) (3) in toluene at 85 °C. The structure of 3 has been determined by X-ray diffraction. The Os−Cα, Cα−Cβ, and Cβ−Cγ bond lengths are 1.875(6), 1.222(9), and 1.344(9) Å, respectively, while the Os−Cα−Cβ and Cα−Cβ−Cγ angles are 171.6(6)° and 172.0(7)°, respectively. Protonation of 3 with HBF4·OEt2 leads to the α,β-unsaturated carbyne [Os(η5-C5H5)Cl(⋮C−CHCPh2)(PiPr3)]BF4 (4), as a result of the attack of the proton from the acid at the Cβ carbon atom of the allenylidene. The nucleophilicity of this atom is also revealed by the reaction of 3 with dimethyl acetylenedicarboxylate, which leads to the allenylvinylidene Os(η5-C5H5)Cl{CC(CO2Me)C(CO2Me)CCPh2}(PiPr3) (5). A second C3 + C2 coupling process is the formation of the pentatrienyl complex Os(η5-C5H5){(3−5-η)CH2CHCCCPh2}(PiPr3) (6) by reaction of 3 with CH2CHMgBr. Complex 3 also reacts with KI to give Os(η5-C5H5)I(CCCPh2)(PiPr3) (7). The reduction of the Cβ−Cγ double bond of the allenylidene ligand of 3, to form the vinylidene complex Os(η5-C5H5)Cl(CCH−CHPh2)(PiPr3) (8), has been carried out in the presence of NaBH4 and methanol.

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.