Oxorhenium Complexes as Aldehyde-Olefination Catalysts

Reactivity of Nickel Complexes Bearing P(C=X)P Ligands (X = O, N) Toward Diazoalkanes: Evidence for Phosphorus Ylide Intermediates

Nickel carbenes are attracting attention for the development of more sustainable catalysts, among others, for cyclopropanation. Intramolecular trapping of a nickel carbene intermediate with an olefin incorporated in a P(C=C)P Ni pincer complex had previously allowed the isolation of a nickelacyclobutane intermediate and a detailed characterization of its reactivity. Herein, we report the reactivity of related nickel pincer complexes bearing a ketone P(C=O)P or an imine P(C=N)P with diazoalkanes as the carbene precursor. The observed reactivity suggests, in both cases, the reaction of the transient nickel carbene with one of the phosphine arms to form phosphorus ylides that subsequently react with the unsaturated backbone. Density functional theory (DFT) calculations are used to shed light on the mechanisms of these reactions.

Stereoselective Copper-Catalyzed Olefination of Imines

Alkenes are an important class of organic molecules found among synthetic intermediates and bioactive compounds. They are commonly synthesized through stoichiometric Wittig-type olefination of carbonyls and imines, using ylides or their equivalents. Despite the importance of Wittig-type olefination reactions, their catalytic variants remain underdeveloped. We explored the use of transition metal catalysis to form ylide equivalents from readily available starting materials. Our investigation led to a new copper-catalyzed olefination of imines with alkenyl boronate esters as coupling partners. We identified a heterobimetallic complex, obtained by hydrocupration of the alkenyl boronate esters, as the key catalytic intermediate that serves as an ylide equivalent. The high E-selectivity observed in the reaction is due to the stereoselective addition of this intermediate to an imine, followed by stereospecific anti-elimination.

Cleavage of C-O bonds in lignin model compounds catalyzed by methyldioxorhenium in homogeneous phase

Methyldioxorhenium (MDO)-catalyzed C-O bond cleavage of a variety of lignin β-O-4-model compounds yields phenolic and aldehydic compounds in homogeneous phase under mild reaction conditions. MDO is in situ generated by reduction of methyltrioxorhenium (MTO) and is remarkably stable under the applied reaction conditions allowing its reuse for least five times without significant activity loss. Based on the observed and isolated intermediates, 17 O- and 2 H-isotope labeling experiments, DFT calculations, and several spectroscopic studies, a reaction mechanism is proposed.

Bimetallic catalysis involving dipalladium(I) and diruthenium(I) complexes

Dipalladium(I) and diruthenium(I) compounds bridged by two [{(5,7-dimethyl-1,8-naphthyridin-2-yl)amino}carbonyl]ferrocene (L) ligands have been synthesized. The X-ray structures of [Pd(2)L(2)][BF(4)](2) (1) and [Ru(2)L(2)(CO)(4)][BF(4)](2) (2) reveal dinuclear structures with short metal-metal distances. In both of these structures, naphthyridine bridges the dimetal unit, and the site trans to the metal-metal bond is occupied by weakly coordinating oxygen from the amido fragment. The catalytic utilities of these bimetallic compounds are evaluated. Compound 1 is an excellent catalyst for phosphine-free, Suzuki cross-coupling reactions of aryl bromides with arylboronic acids and provides high yields in short reaction times. Compound 1 is also found to be catalytically active for aryl chlorides, although the corresponding yields are lower. A bimetallic mechanism is proposed, which involves the oxidative addition of aryl bromide across the Pd-Pd bond and the bimetallic reductive elimination of the product. Compound 1 is also an efficient catalyst for the Heck cross-coupling of aryl bromides with styrenes. The mechanism for aldehyde olefination with ethyl diazoacetate (EDA) and PPh(3), catalyzed by 2, has been fully elucidated. It is demonstrated that 2 catalyzes the formation of phosphorane utilizing EDA and PPh(3), which subsequently reacts with aldehyde to produce a new olefin and phosphine oxide. The efficacy of bimetallic complexes in catalytic organic transformations is illustrated in this work.

Contrasting Reactivity of 2-Mesityl-1,8-Naphthyridine (Mes-NP) with Singly-Bonded [RhII–RhII] and [RuI–RuI] Compounds

Reaction of cis-[Rh2(CH3COO)2(CH3CN)6](BF4)2 with two equivalents of 2-mesityl-1,8-naphthyridine (Mes-NP) affords trans-[Rh2(CH3COO)2(Mes-NP)2](BF4)2 (1). X-ray structure reveals weak Rh–C(ipso) interaction, and a short Rh–Rh distance. The same ligand, in contrast, oxidatively cleaves the Ru–Ru bond in cis-[Ru2(CO)4(CH3CN)6](BF4)2 and results in trans-[Ru(Mes-NP)2(CH3CN)2](BF4)2 (2). Both compounds adopt trans geometry to relieve the steric strain. Compound 2 exhibits moderate activity for the alcohol oxidation and aldehyde olefination reactions.

Copper−Carbene Complexes as Catalysts in the Synthesis of Functionalized Styrenes and Aliphatic Alkenes

(NHC)-Cu (NHC = N-heterocyclic carbene) complexes efficiently catalyzed the methylenation of a variety of aliphatic and aromatic aldehydes and ketones in the presence of trimethylsilyldiazomethane, triphenylphosphine, and 2-propanol. The copper catalysts are not only inexpensive compared to rhodium complexes, but they also exhibit better functional group compatibility with aromatic aldehydes and ketones. Indeed very high yields were obtained for the formation of styrenes containing nitro, trifluoromethyl, amino, and ester groups, as well as for pyridine-, pyrrole-, and indole-substituted alkenes.

Heterogenization of an organorhenium(vii) oxide on a modified mesoporous molecular sieve

A novel route to heterogenize an organorhenium(VII) oxide, derived from the well examined methyltrioxorhenium(VII) (MTO), on the surface of an iodosilane-modified MCM-41 is applied. The successful grafting of the -CH(2)-ReO3 moiety on the surface was evidenced by 1H CP MAS NMR, IR spectroscopy, TG-MS, and elemental analysis. XRD and TEM analyses confirm the retaining of long-range ordering throughout the grafting process. The rhenium loading of the mesoporous material after heterogenization of MTO is found to be 1.25 wt%. Despite containing formally a derivative of the very sensitive benzyltrioxorhenium(VII) the material is stable at room temperature and applicable as a heterogeneous catalyst for aldehyde olefination reactions.

Synthesis, Structure, and Transformation of Novel Osmium Azine and Ylide Complexes

Three novel triosmium complexes with unusual coordination characteristics are reported. Treatment of the hydridotriosmium cluster (mu-H)2Os3(CO)10 with CNNPPh3 in CH2Cl2 gave complexes (mu-H)Os3(CO)(10)(mu2-eta2-C(H)NNPPh3) (1) and (mu-H)Os3(CO)10(mu2-eta1-CHPPh3) (2). Complex 1 represents the first example of the existence of a coordinated phosphinazine ligand. An in-situ 1H NMR study showed that the reaction of (mu-H)2Os3(CO)10 with CNNPPh3 produced complex 1 as the initial product in 100% conversion. The latter is not stable in solution and slowly eliminates nitrogen to form an unusual ylide complex 2 in quantitative yield. The thermolysis of 2 in refluxing toluene afforded (mu-H)3Os3(CO)9(mu3-eta1-CCO2CH2Ph) (3) as a colorless compound. Complexes 1-3 were characterized by spectroscopic methods and single-crystal X-ray diffraction analysis. The interesting feature of structure 3 is the presence of a mu3-alkylidyne ligand where the symmetrically triply bridged CCO2CH2Ph fragment lies perpendicular to and above the triosmium triangle.