Allenylidene and higher cumulenylidene complexes [M]=C(=C)n=CR1R2 (n = 1, 2, 3) have continuously gained significance in the context of transition metal carbene chemistry. Important developments which have been disclosed during the last two years are reviewed. These include a variety of stoichiometric and catalytic reactions of allenylidene complexes and their utility in organic synthesis. The related chemistry of butatrienylidene (n = 2) and pentatetraenylidene (n = 3) complexes as well as theoretical studies are also reviewed.

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.

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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 allenylidene complexes [M(CCCR2)(η5-C9H7)L2][PF6] (M = Ru, L = PPh3, L2 = 1,2-bis(diphenylphosphino)ethane (dppe), bis(diphenylphosphino)methane (dppm), R2 = 2 Ph (1a−c), C12H8 (2,2‘-biphenyldiyl) (2a−c); M = Os, L = PPh3, R2 = 2Ph (3), C12H8 (4)) have been prepared by reaction of the complexes [MCl(η5-C9H7)L2] with HC⋮CC(OH)R2 and NaPF6 in refluxing methanol. The crystal structures of [M(CCCPh2)(η5-C9H7)(PPh3)2][PF6]·CH2Cl2 (M = Ru (1a), Os (3)) were determined by X-ray diffraction methods. In the structures the MCCC chains are nearly linear (M−C(1)−C(2) = 168.5(5)° (1a) and 169.3(4)° (3); C(1)−C(2)−C(3) = 168.2(7)° (1a) and 168.0(5)° (3)) with MC(1) distances of 1.878(5) Å (1a) and 1.895(4) Å (3). The indenyl ligand is η5-bonded to the metal with the benzo ring orientated “cis” with respect to the allenylidene group. Extended Hückel molecular orbital calculations have been used to rationalize the preferred “cis” orientation. The reaction of [RuCl(η5-C9H7)L2] (L = PPh3, L2 = dppe, dppm) with HC⋮CCMe(OH)Ph and NaPF6 in refluxing methanol leads to the formation of the allenylidene complexes [Ru{CCC(Me)Ph}(η5-C9H7)L2][PF6] (6a−c) along with the vinylvinylidene isomers [Ru{CC(H)C(Ph)CH2}(η5-C9H7)L2][PF6] (L = PPh3 (5a), L2 = dppe (5b), dppm (5c)). Only complex 6a could be isolated by chromatography (SiO2) from these mixtures along with complex 7a obtained from the deprotonation of the vinylvinylidene complex 5a. The treatment of these reaction mixtures with potassium carbonate yields the neutral σ-enynyl derivatives [Ru{C⋮CC(Ph)CH2}(η5-C9H7)L2] (7a−c). The monosubstituted allenylidene complex [Ru{CCC(H)Ph}(η5-C9H7)(PPh3)2][PF6] (9) has been prepared by the reaction of [RuCl(η5-C9H7)(PPh3)2] with HC⋮CCH(OH)Ph and NaPF6 in methanol. Under similar reaction conditions [RuCl(η5-C9H7)L2] reacts with HC⋮CCH(OH)R and NaPF6 to afford the alkenylmethoxycarbene derivatives [Ru{C(OMe)C(H)CH(R)}(η5-C9H7)L2][PF6] (L2 = dppe, R = Ph (11b); L2 = dppm, R = Ph (11c), H (13)). [RuCl(η5-C9H7)(PPh3)2] also reacts with HC⋮CC(OH)H2 to give the hydroxyvinylidene complex [Ru{CCH(CH2OH)}(η5-C9H7)(PPh3)2][PF6] (12), which is stable toward the dehydration process.

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The catalytic activity of the bis(allyl)-ruthenium(IV) dimer [[Ru(eta(3):eta(3)-C(10)H(16))(mu-Cl)Cl](2)] (C(10)H(16) = 2,7-dimethylocta-2,6-diene-1,8-diyl) (1), and that of its mononuclear derivatives [Ru(eta(3):eta(3)-C(10)H(16))Cl(2)(L)] (L = CO, PR(3), CNR, NCR) (2) and [Ru(eta(3):eta(3)-C(10)H(16))Cl(NCMe)(2)][SbF(6)] (3), in the redox isomerization of allylic alcohols into carbonyl compounds, both in tetrahydrofuran and in water, is reported. In particular, a variety of allylic alcohols have been quantitatively isomerized using [[Ru(eta(3):eta(3)-C(10)H(16))(mu-Cl)Cl](2)] (1) as catalyst, the reactions proceeding in all cases faster in water. Remarkably, complex 1 has been found to be the most efficient catalyst reported to date for this particular transformation, leading to TOF and TON values up to 62,500 h(-1) and 1 500,000, respectively. Moreover, catalyst 1 can be recycled and is capable of performing allylic alcohol isomerizations even in the presence of conjugated dienes, which are known to be strong poisons in isomerization catalysis. On the basis of both experimental data and theoretical calculations (DFT), a complete catalytic cycle for the isomerization of 2-propen-1-ol into propenal is described. The potential energy surfaces of the cycle have been explored at the B3LYP/6-311 + G(d,p)//B3LYP/6-31G(d,p) + LAN2DZ level. The proposed mechanism involves the coordination of the oxygen atom of the allylic alcohol to the metal. The DFT energy profile is consistent with the experimental observation that the reaction only proceeds under heating. Calculations predict the catalytic cycle to be strongly exergonic, in full agreement with the high yields experimentally observed.