Selective hydrogenation of nitrobenzene to aniline in dense phase carbon dioxide over Ni/gamma-Al2O3: Significance of molecular interactions

The selective hydrogenation of nitrobenzene (NB) over Ni/gamma-Al2O3 Catalysts Was investigated using different media of dense phase CO2, ethanol, and n-hexane. In dense phase CO2, the total rate of NB hydrogenation was larger than that in organic solvents under similar reaction conditions; the selectivity to the desired product, aniline, was almost 100% over the whole conversion range of 0-100%. The phase behavior of the reactant mixture in/under dense phase CO2 was examined at reaction conditions. In situ high-pressure Fourier transform infrared measurements were made to study the molecular interactions Of CO2 with the following reactant and reaction intermediates: NB, nitrosobenzene (NSB), and N-phenylhydroxylamine (PHA). Dense phase CO2 strongly interacts with NB, NSB, and PHA, modifying the reactivity of each species and contributing to positive effects on the reaction rate and the selectivity to aniline. A possible reaction pathway for the hydrogenation of NB in/under dense phase CO2 over Ni/gamma-Al2O3 is also proposed.

Extremely active catalysts for the hydrogenation of terminal alkenes

Titanocene complexes combined with nanometer-size sodium hydride are extremely active and selective catalysts for the hydrogenation of terminal alkenes under normal pressure. The initial turnover frequencies (TOFinitial) may reach 100-300 s(-1) in the hydrogenation of 1-hexene. The highest catalytic efficiency turnover (TO) reaches 1.5 x 10(5) in 2 h for the hydrogenation of styrene. These catalytic systems exhibit specific selectivity toward alkene substrates. Only terminal alkenes can be hydrogenated. No isomerization of carbon-carbon double bonds occurs during hydrogenation. A suitable substituent on the cyclopentadienyl ring of titanocene and the use of nanometric sodium hydride are key factors in the high efficiency of these catalytic systems. (C) 2002 Elsevier Science.

Preparation of lanthanide hydrides of nanometric size by the catalytic method

Rare-earth metals were hydrogenated in the presence of TiCl4 catalyst in tetrahydrofuran (THF) at 45 degreesC under normal pressure. Transmission electron micrographs showed that the re. sulting lanthanide hydrides were in the form of nanoparticles. The rate of hydrogenation decreased with increasing atomic number of the rare-earth elements.

An efficient approach to modify the catalyst activity for the hydrogenation of nitrobenzene

The addition of a suitable amount of PPh3 to PdCl2 or PdCl2(PhCN)(2) in situ can considerably increase the catalytic activity in the hydrogenation of nitrobenzene, while the catalytic activities of PdCl2 (reduced)+PPh3, PdCl2(PPh3)(2) and Pd(PPh3)(4) are very poor. The poisoning of catalyst by mercury indicates that the catalytically active species are composed of Pd(0) colloidal particles. Transmission electron micrographs show that the size of nanometric Pd(0) particles of PdCl2 with PPh3 added in situ is smaller than that of PhCl2(PPh3) or PdCl2 (reduced)+PPh3. A synergic effect of bimetallic catalysts such as PdCl2+nPPh(3)+NiCl2 (n= 0.5, 1) and PdCl2(PhCN)(2)+PPh3+FeCl3 gives rise to a further increase in the catalytic activity.

Evidence To Challenge the Universality of the Horiuti-Polanyi Mechanism for Hydrogenation in Heterogeneous Catalysis:Origin and Trend of the Preference of a Non-Horiuti-Polanyi Mechanism

The Horiuti-Polanyi mechanism has been considered to be universal for explaining the mechanisms of hydrogenation reactions in heterogeneous catalysis for several decades. In this work, we examine this mechanism for the hydrogenation of acrolein, the simplest alpha,beta-unsaturated aldehyde, in gold-based systems as well as some other metals using extensive first-principles calculations. It is found that a non-Horiuti-Polanyi mechanism is favored in some cases. Furthermore, the physical origin and trend of this mechanism are revealed and discussed regarding the geometrical and electronic effects, which will have a significant influence on current understandings on heterogeneous catalytic hydrogenation reactions and the future catalyst design for these reactions.

Heterogeneously catalyzed asymmetric hydrogenation of C = C bonds directed by surface-tethered chiral modifiers

Asymmetric hydrogenation of C=C bonds is of the highest importance in organic synthesis, and such reactions are currently carried out with organometallic homogeneous catalysts. Achieving heterogeneous metal-catalyzed hydrogenation, a highly desirable goal, necessitates forcing the crucial enantiodifferentiating step to take place at the metal surface. By synthesis and application of six chiral sulfide ligands that anchor robustly to Pd nanoparticles and resist displacement, we have for the first time accomplished heterogeneous enantioselective catalytic hydrogenation of isophorone. High resolution XPS data established that ligand adsorption from solution occurred exclusively on the Pd nanoparticles and not on the carbon support. All ligands contained a pyrrolidine nitrogen to enable their interaction with the isophorone substrate while the sulfide functionality provided the required interaction with the Pd surface. Enantioselective turnover numbers of up to similar to 100 product molecules per ligand molecule were found with a very large variation in asymmetric induction between ligands: observed enantiomeric excesses increased with increasing size of the alkyl group in the sulfide. This likely reflects varying degrees of ligand dispersion on the surface: bulky substituent groups hinder close approach of ligand molecules to each other, inhibiting close-packed island formation, favoring dispersion as separate molecules, and leading to effective asymmetric induction. Conversely, small substituents favor island formation leading to very low asymmetric induction. Enantioselective reaction most likely involves initial formation of an enamine or iminium species, confirmed by use of an analogous tertiary amine, which leads to racemic product. Ligand rigidity and resistance to self-assembled monolayer formation are important attributes that should be designed into improved chiral modifiers.

Synthetic investigations on testerone and its analogues—I *1: The total synthesis of 8-isotestosterone and its anthracene analogue

The total synthesis of 8-isotestosterone (II) and the corresponding anthracene analogue (III) following the benzohydrindane route is reported. Catalytic hydrogenation of trans-1β-acetoxy-8-methyl-4,5-(3′-methyl-4′-hydroxybenzo)-hydrindane (V) followed by oxidation has furnished two isomeric tricyclic keto acetates, viz. 1β,2α-(3′-acetoxycyclopentano)-2,5-dimethyl-6-keto-1α,2,3,4,4aα,-5α,6,7,8,8aα-decahydronaphthalene (VII) and 1β,2α-(3′-acetoxycyclopentano)-2,5-dimethyl-6-keto-1α,2,3,4,4aβ,5,6,7,8,8aβ-decahydronaphthalene (IX) which are cis-non-steroid and cis-steroid configurations of the same cyclopentano-cis-decalins. A difference in the direction of enolization of the keto acetate (VII) in alkylation reaction and enol acetylation towards the methine and the methylene carbon atoms respectively has been observed.

A new synthesis of dl-3-methoxy-17β-carboxy-1,3,5(10),6,8-estrapentaene

3-Methyl-4-carboxy-2-(2′-methoxy-6′-naphthyl)cyclopenten-3-acetic acid, prepared from trans methyl 2-methyl-3-carbomethoxycyclopentanon-2-acetate and 2-methoxy-6-lithionaphthalene, on ring closure and catalytic hydrogenation gave dl-3-methoxy-17β-carboxy-1,3,5(10),6,8-estrapentaene.

Roofed polyquinanes: synthesis and electronic structure

Starting from readily available norbornenobenzoquinone 7 and employing a photothermal metathesis reaction as the main strategy, novel "roofed" polyquinane bisenones 3 and 13 have been synthesized. Among these, the former is potentially serviceable for further elaboration to dodecahedrane 1. Catalytic hydrogenation of 3 provided the dione 12, which fully inscribes the circumference of dodecahedrane sphere. The "roofed" C-16-bisenone 3 has been successfully annulated to C19-bisenone 24 and C19-trisenone 26 by employing the Greene methodology and Pauson-Khand reaction, respectively. The molecular structures of 3 and 13 were computed using molecular mechanics and semiempirical MO methods. The nonbonded distances between the double bonds vary strongly with the method employed. The interactions between the pi-MO's were, therefore, probed by means of photoelectron (PE) spectroscopy. Comparison with the PE spectra of a series of model systems with increasing complexity enabled an unambiguous assignment of the observed peaks. The symmetric and antisymmetric combinations of the pi-MO's of the enone moieties of 3 and 13 show large splittings, characteristic of propano-bridged systems in which through-space and through-bond effects act in concert.

Synthesis Of (+/-)-2-Isopropyl-5,6-Dimethylbicyclo[3.2.1]Oct-6-En-8-One

Methanolic hydrogen chloride cyclization of the triketone 8, prepared from the Mannich base 7 and 2-methylcyclopentane-1,3-dione, gives ketones 9 and 10. NaBH4 reduction of 9 followed by Grignard reaction with CH3MgI affords the diol 12. Catalytic hydrogenation of 12 followed by PCC oxidation yields the ketoalcohol 13. Dehydration of 13 with SOCl2/pyridine results in a 1:1 mixture of the endo-14 and exo-15 olefins, separated by chromatography.

Total synthesis of substituted (±)-6,6-dimethyl-B-norestra-1,3,5(10)-trien-17?-ols and their 9?-isomers

Total syntheses of (±)-1,4-dimethoxy-6,6-dimethyl-B-norestra-1,3,5(10)-trien-17?-ol(11a), (±)-2,3-dimethoxy-6,6-dimethyl-B-norestra-1,3,5(10)-trien-17?-ol (11b), and (±)-3-methoxy-6,6-dimethyl-B-norestra-1,3,5(10)trien-17?-ol (11c), have been carried out starting from 4,7-dimethoxy-3,3-dimethylindan-1-one (1), 5,6-dimethoxy-3,3-dimethylindan-1-one (2), and 4?-methoxy-3-methylbut-2-enophenone (4), respectively. Generally, it is found that the intermediate 6,6-dimethyl-B-norestra-1,3,5(10),8-tetraen-17?-ols (10), on lithium�liquid ammonia reduction, yield a mixture of 8?,9?- and 8?,9?-trienols, (11) and (12) respectively, in the ratio 1 : 1. This is due to the comparable stabilities of these two isomers. However, the reduction carried out in presence of aniline affords a higher percentage of the 8?,9?-trienol (11). The assignment of configurations is made by chemical and 1H n.m.r. analysis. Catalytic hydrogenation of the tetraenols (10) is shown to proceed via initial isomerisation to the corresponding 6,6-dimethyl-B-norestra-1,3,5(10),9(11)-tetraen-17?-ols (26), followed by hydrogenation from the ?-side to give, exclusively, the 8?,9?-trienols (12).

A Biodegradable Diblcok Copolymer Poly(ethylene glycol)-block-poly(L-lactide-co-2-methyl-2-carboxyl-propylene carbonate): Docetaxel and RGD Conjugation

A diblcok copolymer monomethoxy poly (ethylene glycol)-block-poly(L-lactide-co-2-methyl-2-carboxyl-propylene carbonate) (MPEG-b-P(LA-co-MCC)) was obtained by copolymerization of L-lactide (LA) and 2-methyl-2-benzoxycarbonyl-propylene carbonate (MBC) and subsequent catalytic hydrogenation. The pendant carboxyl groups of the copolymer MPEG-b-P(LA-co-MCC) were conjugated with antitumor drug docetaxel and tripeptide arginine-glycine-aspartic acid (RGD), respectively.

Aliphatic Poly(ester-carbonate)s Bearing Amino Groups and Its RGD Peptide Grafting

This article deals with (1) synthesis of novel cyclic carbonate monomer (2-oxo [1,3]dioxan-5-yl)carbamic acid benzyl ester (CAB) containing protected amino groups; (2) ring-opening copolymerization of the cyclic monomer with L-lactide (LA) to provide novel degradable poly(ester-carbonate)s with functional groups; (3) removal of the protective benzyloxycarbonyl (Cbz) groups by catalytic hydrogenation to afford the corresponding poly(ester-co-carbonate)s with free amino groups; (4) grafting of oligopeptide Gly-Arg-Gly-Asp-Ser-Tyr (GRGDSY, abbreviated as RGD) onto the copolymer pendant amino groups in the presence of 1,1'-carbonyldiimidazole (CDI).