Search

The employment of water as the solvent is crucial to substantially improve the yields with a wide range of boronic acids (II) and esters (IV) bearing various functionalities.

The dichloride ((PP{sub 3})RuCl{sub 2}) (1) is selectively transformed into ((PP{sub 3})Ru(H)({eta}{sup 1}-BH{sub 4})) (2), ((PP{sub 3})Ru(H)Cl) (3), and ((PP{sub 3})Ru(H){sub 2}) (4) by reaction with NaBH{sub 4}, LiHBEt{sub 3}, and LiAlH{sub 4}, respectively (PP{sub 3} = P(CH{sub 2}CH{sub 2}PPh{sub 2}){sub 3}). Complex 2 is fluxional on the NMR time scale in ambient-temperature solutions due to equilibration of the BH{sub 4}{sup {minus}} hydrogens through cleavage of the Ru-H{sub b}-B bridge and interconversion of H{sub t} by rotation. The hydride-tetrahydroborate complex exhibits amphoteric nature: it reacts with protic acids, yielding the cis-hydride dihydrogen complex ((PP{sub 3})Ru(H)({eta}{sup 2}-H{sub 2}))Y (Y = PF{sub 6}, 6a; BPh{sub 4}, 6b), and with bases such as PEt{sub 3} or KO{sup t}Bu, converting to the dihydride 4. The latter compound can be obtained also by thermal decomposition of 2 in refluxing toluene or benzene. Complexes 6a,b can be prepared by reaction of the dihydride 4 with protic acids. In the solid state and in solution at low temperature ((PP{sub 3})Ru(H)({eta}{sup 2}-H{sub 2})){sup +} is octahedral, and the hydride and dihydrogen ligands occupy mutually cis positions. At ambient temperature in solution the complex is trigonal-bipyramidal and the equilibration of the hydrogens might proceed through an H{sub 3} unitmore » occupying an axial position trans to the bridgehead phosphorus atom of PP{sub 3}. 36 refs., 6 figs., 2 tabs.« less

Tripodal polyphosphine ligands react with transition metal ions to form either unsaturated fragments or, in combination with suitable coligands, highly reactive complexes. Both the fragments and the complexes appear appropriately designed to interact with ethyne and 1-alkynes. By a purposeful variation of the polyphosphine ligand, metal, coligand and reaction conditions, a great variety of organometallic and organic products can be stoichiometrically and catalytically synthesized in a highly selective manner. The use of polyphosphine ligands allows the isolation and characterization of many intermediate species not normally seen in catalysis cycles.

Two equilibria were found in chloroform solutions of Bu 4 N + Br - and Br 2 , leading to tribromide and pentabromide salts. The electronic spectra and formation constants of both (K 3 =2.77 (0.13)×10 4 M -1 and K 5 =3.51 (0.35)×10 5 M -2 at 25 o C) were computed from spectrophotometric data

Publisher Summary This chapter presents results that show the first step of the interaction between olefins and Br 2 is the formation of CTCs, whose K f are highly sensitive to structural effects. Both K f ratios and reactivity ratios of olefins are scarcely affected by the solvent. An increase by two in number of alkyl substituents on the double bond increases both K f and k obsd roughly by a factor of 10 3 . Therefore, at variance with the expectation for an Ad E C1 mechanism, substituent effects are not much more influential on k obsd than on K f . This suggests that the rates of CTC ionization be actually reduced by reversal. This reversal has been demonstrated by both product and kinetic studies. In the absence of solvent nucleophilic assistance and of substituents favoring β-bromocarbonium ion intermediates, the ionization of CTC's to bromonium (poly) bromide has been shown to occur not only for congested olefins, but more generally for “normal” olefins both in aprotic chlorinated hydrocarbons and in protic solvents such as acetic acid and methanol.

A number of 6-substituted 1, 2-benzisothiazole-1, 1-dioxide alkanoic acids were synthesized and evaluated for crude rat lens aldose reductase inhibitory activity. The inhibitory potency of the acetic (6a, 10a), propionic (6b, 10b, 11b), and isopropionic (6c, 10c, 11c) derivatives was very similar and generally lower than that of the reference compound, Sorbinil. The presence of an acyl moiety on the amino group in position 6, as in the acetic and propionic derivatives 14a-f and 15a, b, respectively, resulted in a significant increase in activity. A good potency was shown by compounds 14g and 15g, in which a second carboxylic function is present on the 6-acylamino group. Also the open products 16, which contain the phenylsulfonyl fragment found in several known inhibitors of aldose reductase, were obtained and tested in the rat lens assay.

Two new sesquiterpene alcohols, β-santaldiol and α-santaldiol, have been isolated from the heartwood of Santalum insulare var. marchionense and, by means of two-dimensional NMR experiments, shown to have the β- and α-santalane skeleton, respectively.

Two new sesquiterpenoids, 6,13-dihydroxybisabola-2,10-diene and 7,13-dihydroxybisabola-2,10-diene, were isolated, together with ( E )-anceol, from the heartwood of Santalum austrocaledonicum var. austrocaledonicum . The compounds were characterized by one- and two-dimensional NMR.

A number of new chiral bis(ferrocenyl)−triphosphine ligands have been synthesized by the reaction of cyclohexylphosphine with different chiral ferrocenyl aminophosphines in hot acetic acid. The thioether ligand bis{(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl} sulfide has been similarly prepared by the reaction of (S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl acetate with either (S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl mercaptan or (S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl mercaptan sodium salt. All the above reactions proceed with retention of configuration on the side-chain stereocenters of the starting materials. Monomeric Ru(II) complexes containing either chloride or acetonitrile coligands have been prepared with all the chiral tridentate ligands. Both the new ligands and the ruthenium complexes have been characterized by multinuclear NMR spectroscopy. The structure of [(S)-(R)-Pigiphos]RuCl2·2CH2Cl2 ((S)-(R)-Pigiphos = bis{(S)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyl}cycloh...

Publisher Summary This chapter discusses the activation of dioxygen by the cobalt-group metal complexes. The activation of dioxygen by transition metal complexes may appear at first to be both oxymoronic and redundant; common sense describes dioxygen as a powerful oxidant and a reactive species. Dioxygen could overcome the kinetic barrier of its unpaired electrons and triplet ground state by excitation to its first excited state, in which all electrons are paired. However, dioxygen complexation to a transition metal can also result in activation and create stable complexes that can be studied, modified, and used in further reactions in a controlled manner. The activation of dioxygen encompasses a broad spectrum of research fields, from biology/biochemistry to chemistry, and hence to catalysis, industrial processes, and theoretical analyses. Although a wide variety of nitrogen donor ligands exist in transitionmetal complexes, relatively few general types have been employed in those complexes that have been found to activate dioxygen.

A new sesquiterpenoid, campherene-2,13-diol, has been isolated and characterized from the heartwood of Santalum austrocaledonicum var austrocaledonicum . Its structure has been established by the use of 1D and 2D NMR spectral techniques and shown to contain the campherenane skeleton.

Two new eudesmane-type sesquiterpenoids (7-epi-γ-eudesmol and 7-epi-β-eudesmol) were isolated and characterized along with five other sesquiterpene compounds (β-selinene, 7-epi-α-eudesmol, isointermedeol, juniper camphor and β-dihydroagarofuran) from the essential oil of Laggera alata var. alata grown in Madagascar. The assigned structures were based on their spectral data.

Displacement of THF by dihydrobenzo[b]thiophene (DHBT) in [(triphos)Ir(H)2(THF)]BPh4 yields the η1-S-DHBT adduct [(triphos)Ir(H)2(η1-DHBT)]BPh4 which reacts in THF at room temperature with KOBut to give the neutral 2-vinylthiophenolate derivative (triphos)Ir(H)2{η1-o-S(C6H4)CHCH2} via C2–S bond cleavage; the latter compound is hydrogenated under mild conditions (2 bar H2, 80 °C) to the 2-ethylthiophenolate derivative (triphos)Ir(H)2{o-S(C6H4)Et} and under harsh conditions (30 bar H2, 160 °C) to free 2-ethylthiophenol and (triphos)IrH3.

A micellar environment enables catalytic, diastereoselective and enantioselective Baeyer–Villiger oxidation of cyclobutanones (ee up to 90%) with H2O2 as oxidant using Co(Salen) catalyst 1, while the same catalytic system is inactive in organic solvents.

Direct alcohol fuel cells (DAFCs) are attracting increasing interest as power sources for portable applications due to some unquestionable advantages over analogous devices fed with hydrogen.1 Alcohols, such as methanol, ethanol, ethylene glycol, and glycerol, exhibit high volumetric energy density, and their storage and transport are much easier as compared to hydrogen. On the other hand, the oxidation kinetics of any alcohol are much slower and still H2-fueled polymer electrolyte fuel cells (PEMFCs) exhibit superior electrical performance as compared to DAFCs with comparable electroactive surface areas.2,3 Increasing research efforts are therefore being carried out to design and develop more efficient anode electrocatalysts for DAFCs.

A novel one-pot two-step procedure has been developed to synthesize highly substituted 2-arylpropionic and arylacetic acids, by treatment with aqueous HI, from cyanohydrins. The hydrogenolytic reduction of α-hydroxy-2-arylpropionic acids was the key step of the process and the optimization of the reaction conditions led to identify aqueous HI as an appropriate and selective reagent for the reductive deoxygenation of cyanohydrins. The synthetic route described a general and efficient strategy for the preparation of large libraries of phenylacetic and phenylpropionic acids derivatives.

New chiral P,N-ferrocenyl imino-phosphine ligands have been synthesized and the absolute configuration of the stereocenters in each molecule has been determined by a single-crystal X-ray analysis of a common intermediate. PdII-allyl complexes of the new ligands have been isolated and tested as catalyst precursors in the asymmetric allylic alkylation of 1,3-diphenylprop-2-en-1-yl acetate with dimethyl malonate in different solvents. Quantitative yields and enantiomeric excesses as high as 80% have been obtained.

Isomerization of allylic and homoallylic alcohols is catalyzed by the zwitterionic Rh(I) complex (sulphos)Rh(cod) in water–n-octane to give the corresponding aldehyde or ketone in high yields and chemoselectivity. A π-allyl metal hydride mechanism is proposed on the basis of various independent experiments in both homogeneous and biphasic systems [sulphos=−O3S(C6H4)CH2C(CH2PPh2)3].

Recent legislation directed at reducing nitrogen levels in fossil fuels has led to numerous studies of the hydrodenitrogenation (HDN) process, whereby nitrogen is removed from organic compounds in petroleum feedstocks. In this review a comprehensive study of the coordination, activation, hydrogenation, hydrogenolysis and denitrogenation of N-heterocycles assisted by transition metal complexes in either fluid solution or single-site systems, is reported. Furthermore, similarities to the reactions occurring in the HDN process over commercial heterogeneous catalysts are also discussed.