Georgios C. Vougioukalakis

Ruthenium-Based Heterocyclic Carbene-Coordinated Olefin Metathesis Catalysts

The fascinating story of olefin (or alkene) metathesis (eq 1) began almost five decades ago, when Anderson and Merckling reported the first carbon-carbon double-bond rearrangement reaction in the titanium-catalyzed polymerization of norbornene. Nine years later, Banks and Bailey reported “a new disproportionation reaction . . . in which olefins are converted to homologues of shorter and longer carbon chains...”. In 1967, Calderon and co-workers named this metal-catalyzed redistribution of carbon-carbon double bonds olefin metathesis, from the Greek word “μeτάθeση”, which means change of position. These contributions have since served as the foundation for an amazing research field, and olefin metathesis currently represents a powerful transformation in chemical synthesis, attracting a vast amount of interest both in industry and academia.

Synthesis and Activity of Ruthenium Olefin Metathesis Catalysts Coordinated with Thiazol-2-ylidene Ligands

A new family of ruthenium-based olefin metathesis catalysts bearing a series of thiazole-2-ylidene ligands has been prepared. These complexes are readily accessible in one step from commercially available (PCy3)2Cl2Ru=CHPh or (PCy3)Cl2Ru=CH(o-iPrO-Ph) and have been fully characterized. The X-ray crystal structures of four of these complexes are disclosed. In the solid state, the aryl substituents of the thiazole-2-ylidene ligands are located above the empty coordination site of the ruthenium center. Despite the decreased steric bulk of their ligands, all of the complexes reported herein efficiently promote benchmark olefin metathesis reactions such as the ring-closing of diethyldiallyl and diethylallylmethallyl malonate and the ring-opening metathesis polymerization of 1,5-cyclooctadiene and norbornene, as well as the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene and the macrocyclic ring-closing of a 14-membered lactone. The phosphine-free catalysts of this family are more stable than their phosphine-containing counterparts, exhibiting pseudo-first-order kinetics in the ring-closing of diethyldiallyl malonate. Upon removing the steric bulk from the ortho positions of the N-aryl group of the thiazole-2-ylidene ligands, the phosphine-free catalysts lose stability, but when the substituents become too bulky the resulting catalysts show prolonged induction periods. Among five thiazole-2-ylidene ligands examined, 3-(2,4,6-trimethylphenyl)- and 3-(2,6-diethylphenyl)-4,5-dimethylthiazol-2-ylidene afforded the most efficient and stable catalysts. In the cross metathesis reaction of allyl benzene with cis-1,4-diacetoxy-2-butene increasing the steric bulk at the ortho positions of the N-aryl substituents results in catalysts that are more Z-selective.

Ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene ligands: synthesis, structure, and catalytic activity.

A series of ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene (NHC) ligands has been prepared and fully characterized. These complexes are readily accessible in one or two steps from commercially available [(PCy(3))(2)Cl(2)Ru==CHPh]. All of the complexes reported herein promote the ring-closing of diethyldiallyl and diethylallylmethallyl malonate, the ring-opening metathesis polymerization of 1,5-cyclooctadiene, and the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, in some cases surpassing in efficiency the existing second-generation catalysts. Especially in the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, all new catalysts demonstrate similar or higher activity than the second-generation ruthenium catalysts and, most importantly, afford improved E/Z ratios of the desired cross-product at conversion above 60 %. The influence of the unsymmetrical NHC ligands on the initiation rate and the activation parameters for the irreversible reaction of these ruthenium complexes with butyl vinyl ether were also studied. Finally, the synthesis of the related chlorodicarbonyl(carbene) rhodium(I) complexes allowed for the study of the electronic properties of the new unsymmetrical NHC ligands that are discussed in detail.

Removing Ruthenium Residues from Olefin Metathesis Reaction Products

Olefin metathesis has revolutionized the way chemists design and synthesize molecules, mostly due to the development of well-defined ruthenium catalysts with high oxygen-, moisture-, and functional-group tolerance. However, the complete removal of residual ruthenium after the end of a metathesis reaction often imposes significant challenges. This Minireview summarizes the strategies for the sequestration of ruthenium impurities from olefin metathesis post-reaction mixtures, thus comprising a practical guide for synthetic chemists employing ruthenium-catalyzed metathesis reactions in the synthesis of organic or polymeric materials.

Photosensitized Oxidations of Substituted Pyrroles: Unanticipated Radical-Derived Oxygenated Products

Photooxidation of pyrrole adducts 7-10 has been investigated in order to establish a general reaction pattern and mechanism for the formation of the resulting oxygenated products. The reactions were performed in several solvents utilizing both type I and type II sensitizers. In most cases, photooxidations gave complex mixture of products. Among these products, 5,5- or 6,5-bicyclic lactams (11, 15, and 19), maleimide 12 unsaturated gamma-lactams (16 and 20), 5-hydroxylactams (13, 17, and 21), and 5-methoxylactams (14, 18, and 22) were isolated and characterized. Photooxidation of 2,5-dimethyl-substituted pyrrole 10 in aprotic solvents unexpectedly afforded aldehyde 23 as the major product. Moreover, photooxidation of pyrrole adduct 10 in protic solvents exclusively gave the unprecedented solvent-trapped products 24-27. The formation of products 11-22 was rationalized by the intermediacy of a common endoperoxide intermediate, which could be formed by both type I and type II mechanisms. Compounds 23-27 were most probably formed via an electron-transfer mechanism.

Radical Reactivity of Aza[60]fullerene: Preparation of Monoadducts and Limitations

Six aza[60]fullerene monoadducts were synthesized by the thermal reaction between the azafullerene radical C(59)N* and 9-alkyl-substituted fluorenes, 9,10-dihydroanthracene, or xanthene. Unlike fluorenes, dihydroanthracene, and xanthene, the structurally related substituted diphenylmethanes, ethylbenzene, cumene, 1,2-diphenylethane, 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene, 10,11-dihydro-5H-dibenzo[a,d]cycloheptene, 9-methylanthracene, and 9-benzylanthracene do not lead to the isolation of azafullerene monoadducts. Moreover, 1,2-dichlorobenzene, the most commonly utilized solvent for azafullerene reactions, reacts slowly with the azafullerenyl radical C(59)N* affording the corresponding aza[60]fullerene monoadduct.

Novel Ru(II) sensitizers bearing an unsymmetrical pyridine-quinoline hybrid ligand with extended π-conjugation: synthesis and application in dye-sensitized solar cells

Heteroleptic ruthenium(II) sensitizers DV42 and DV51, encompassing a novel unsymmetrical pyridine-quinoline hybrid ligand with extended π-conjugation, were synthesized, characterized, and utilized in nanocrystalline dye-sensitized solar cells. Due to the extended conjugation of DV42 and DV51, the absorption of the corresponding sensitized TiO2 films extends into the red spectral range, shifted by 30-40 nm relative to the absorption of TiO2 films sensitized with the standard Z907 ruthenium(II) dye. Contact angle measurements of DV42- and DV51-sensitized TiO2 films suggest that these films are hydrophilic with contact angle values commonly observed upon sensitization with the standard N3 ruthenium(II) dye. Electrochemical studies of the novel ruthenium(II) dyes show that their first oxidation potentials lie well below the I(-)/I3(-) redox potential allowing easy regeneration. The excited-state oxidation potentials of both dyes lie above the TiO2 conduction band, permitting efficient electron injection from the excited dye molecules into the semiconductor conduction band. Liquid electrolyte dye-sensitized solar cells incorporating DV42- or DV51-sensitized TiO2 photoelectrodes afford overall power conversion efficiencies of 3.24 or 4.36% respectively. These efficiencies are up to 56% of the power conversion efficiencies attained by TiO2 photoelectrodes sensitized by the benchmark Z907 ruthenium(II) dye under similar experimental conditions.

Solvent‐Dependent Changes in the Triazolinedione–Alkene Ene Reaction Mechanism

The influence of the solvent on the triazolinedione-alkene ene reaction mechanism has been investigated. Both inter- and intramolecular kinetic isotope effects with tetramethylethylenes and 2,2,2-(trideuterio)methyl-7-methyl-2,6-octadiene-[D3]-1,1,1 provide, for the first time, strong evidence for changes in the mechanism of the reaction on going from non-protic to polar protic solvents. In non-protic polar or apolar solvents, an aziridinium imide that equilibrates to an insignificant extent with an open intermediate (a dipolar or a polarized biradical) is formed irreversibly in the first, rate-determining step of the reaction, which is followed by fast hydrogen abstraction. On the contrary, in polar protic solvents, hydrogen abstraction is rate limiting, allowing the main dipolar intermediate to equilibrate with its open intermediate(s) as well as with the starting reagents.

Fullerene C60 supported on silica and γ-alumina catalyzed photooxidations of alkenes

Deposition of fullerene C60 (2% w/w) on silica and γ-alumina provokes a two orders-of-magnitude increase of its activity for the liquid-phase photooxidation of 2-methyl-2-heptene. Kinetic studies concerning the above photooxidation showed a first-order dependence of the reaction rate on the alkene concentration. The corresponding reaction-rate constant was found to be higher in the case where γ-alumina was used as carrier. The nature of the carrier does not influence the mechanism and the selectivity of the reaction. High dispersion of the supported fullerene is achieved on the surface of the carriers, which increase the fullerene light absorbance especially in the visible range.

Green asymmetric synthesis: β-amino alcohol-catalyzed direct asymmetric aldol reactions in aqueous micelles.

The ability of chiral β-amino alcohols to catalyze the direct asymmetric aldol reaction was evaluated for the first time in aqueous micellar media. A family of cheap and easily accessible β-amino alcohols, obtained in one step from naturally occurring amino acids, was shown to successfully catalyze the asymmetric aldol reaction between a series of ketones and aromatic aldehydes. These aldol reactions furnished the corresponding β-hydroxy ketones with up to 93% isolated yield and 89% ee. (S)-2-phenylglycinol and Triton X-100 proved to be the best organocatalyst and surfactant, respectively.