Catalytic hydrogenation of styrene-g-natural rubber (ST-g-NR) in the presence of OsHCl(CO)(O2)(PCy3)2

OsHCl(CO)(O₂)(PCy₃)₂, was used as a catalyst for hydrogenation of styrene-g-natural rubber copolymer (ST-g-NR). Univariate experiments were conducted to explore the effect of variables on the rate of hydrogenation by measuring the hydrogen consumption as a function of time using a gas-uptake apparatus. From the kinetic results, the hydrogenation of ST-g-NR was observed to exhibit a first-order dependence on [CC]. The rate of hydrogenation showed a first-order dependence on the catalyst concentration and a first-order shift to zero-order dependence on hydrogen pressure with increasing hydrogen pressure. The rate of hydrogenation was also found to decrease with an increase in rubber concentration. The addition of a small amount of acid provided a beneficial effect on the hydrogenation rate of the grafted natural rubber. The hydrogenation rate of ST-g-NR was dependent on the reaction temperature and the apparent activation energy over the range of 120-160 °C was found to be 83.3 kJ/mol.     

Hydrogenation of unsaturated polymers under the action of oligoallene complexes of transition metals

The hydrogenation of isolated double bonds in cis-PB (96% cis units), trans-PB (84% trans units), and the statistical butadiene-styrene copolymer (63: 37) is performed under mild conditions (a temperature of 30–50°C and a constant hydrogen pressure of 70 kPa). Palladium, bimetallic palladium-nickel, and palladium-cobalt complexes containing oligodimethylallene ligands are used as catalysts. It is shown that the rate of hydrogenation decreases in the sequence cis-PB, trans-PB, butadiene-styrene copolymer. The highest degree of hydrogenation of cis-PB (98%) is attained in the presence of the bimetallic oligodimethylallene Pd-Ni complex ([Pd]: [Ni] = 0.8: 0.2). In the hydrogenation of the butadiene-styrene copolymer, the highest activity is exhibited by the Pd-Co catalyst (0.5: 0.5): For the butadiene component, the degree of hydrogenation is 98%, while for the styrene component, the degree of hydrogenation is 15%. The IR studies of the polymers indicate the exhaustive hydrogenation of double bonds for both polystyrene and polybutadiene components of the copolymer and the appearance of ordered sequences (-CH₂-CH₂-) _n . As evidenced by DSC measurements and X-ray analysis, a crystalline phase forms during transformation of butadiene units into polyethylene chains.     

Enantioselective reduction of benzofuryl halomethyl ketones: asymmetric synthesis of (R)-bufuralol

Enantioselective reduction of representative 2-(bromoacetyl)- and 2-(chloroacetyl)benzofurans with (−)-B-chlorodiisopinocampheylborane and by transfer hydrogenation with formic acid/triethylamine in the presence of RhCl[R,R-TsDPEN](C₅Me₅) is described. Transfer hydrogenation of the chloro ketones produced the corresponding chlorohydrins of ⩾95% ee. (R)-Bufuralol of 96% ee was prepared from the corresponding chloro ketone by transfer hydrogenation–substitution.     

A Novel Production of γ-Butyrolactone Catalyzed by Homogeneous Ruthenium Complexes

γ-Butyrolactone (hereafter abbreviated GBL) is produced by the two-stage hydrogenation of maleic anhydride(MAH) in the liquid phase: the hydrogenation of MAH to succinic anhydride(SAH) in the first stage and the subsequent hydrogenation of SAH to GBL in the second stage. A novel ruthenium catalyst system consisting of Ru salts, trialkylphosphine and p-toluene sulfonic acid (p-TsOH) was found very effective for the hydrogenation of SAH affording GBL, which exhibited excellent catalyst performance, exceeding 97% selectivity for GBL and high activity.     

Studies on the catalytic hydrogenation of Baylis-Hillman derivatives of substituted isoxazolecarbaldehydes. Unusual retention of isoxazole ring during Pd-C-promoted hydrogenation of Baylis-Hillman adducts

Results of the catalytic hydrogenation of Baylis-Hillman adducts obtained from substituted 3-, 4- and 5-isoxazolecarbox-aldehydes and their corresponding acetates in the presence of Raney-Ni and Pd-C are presented. The hydrogenation of Baylis-Hillman adducts of substituted 5-isoxazolecarbaldehydes and 3-isoxazolecarbaldehydes in the presence of Raney-Ni furnishes diastereoselectively syn enaminones over anti and in the presence of boric acid as an additive further enhancement of diastereoselectivity in favor of syn isomer is observed. The Pd-C-promoted hydrogenation of these substrates is also diastereoselective in favor of syn isomer but occurs without the hydrogenolysis of isoxazole-ring. The presence of boric acid as an additive in this hydrogenation exhibits no pronounced effect on diastereoselectivity. The Raney-Ni-mediated hydrogenation of Baylis-Hillman adducts of substituted 4-isoxazolecarbaldehydes yield pyridone derivatives and Pd-C-promoted hydrogenation of the same substrate is diastereoselective to afford the anti isomer of the resulting products. The enaminones derived from Baylis-Hillman adducts of 3- and 5-isoxazolecarbaldehydes serve as versatile precursors for α′-hydroxy-1,3-diketones, which undergo acid-catalyzed ring-closure reaction to afford the furanone derivatives in excellent yields.     

Hydrogenation of biphenyl and isomeric terphenyls over a Pt-containing catalyst

Catalytic hydrogenation of benzene, biphenyl, and ortho-, metha-, and para-isomers of terphenyl over a 3 wt.% Pt/C at 180 °C and 70 atm was studied. The directions of hydrogenation of each substrate were revealed. Relationships between structures of the substrate and hydrogen consumption rates were found. It was shown that hydrogenation rate decreases on going from benzene to terphenyl and with increasing degree of the substrate hydrogenation. Hydrogenation rate of terphenyl isomers decreases in the following order: p-terphenyl > > m-terphenyl > o-terphenyl.     

Rhodium-catalyzed asymmetric hydrogenation of β-acetylamino acrylonitriles

The rhodium-catalyzed asymmetric hydrogenation of β-acetylamino acrylonitriles was investigated by using monophosphine and bisphosphine ligands. It was found that an Rh-QuinoxP^∗ complex exhibited high enantioselectivities for β-aryl substituted β-acetylamino acrylonitriles and the Rh-JosiPhos CyPF-t-Bu complex was proven to be effective for the hydrogenation of tetrasubstituted olefins from cyclic β-acetylamino acrylonitriles.     

Palladium-catalyzed diastereoselective hydrogenation of N-substituted 3-methyleneisoindolin-1-ones

An asymmetric synthesis of chiral N-substituted 3-methylisoindolin-1-ones was performed using the Pd⁰-catalyzed diastereoselective hydrogenation of their 3-methylene-substituted precursors, that has not been investigated earlier. The highest diastereoselectivity (67% de) was achieved when the hydrogenation was carried out in methanol mediated by a palladium catalyst (Pd/C) modified with cinchonidine.     

Ru-catalyzed enantioselective preparation of methyl (R)-o-chloromandelate and its application in the synthesis of (S)-Clopidogrel

The preparation of methyl (R)-o-chloromandelate via Ru-catalyzed asymmetric hydrogenation and transfer hydrogenation was investigated. With Ru-(R,R)-2,4,6-triisopropyl C₆H₂SO₂-DPEN as the catalyst and HCOOH-Et₃N azeotrope as the hydrogen donor, up to 92% ee was obtained in an optional condition. The synthesis of (S)-Clopidogrel was also studied.     

Stereochemistry and mechanism of the hydrogenation of substituted cyclopentenes

1,2- and 2,3-disubstituted cyclopentenes were hydrogenated with several transition metal catalysts. The hydrogenation of 1,2-disubstituted cyclopentenes gave preferably the cis products with Raney Ni, whereas 2,3-disubstituted cyclopentenes gave preferably the trans isomers. The trans products were favoured in the hydrogenation of 1,2- and 2,3-dialkylcyclopentenes with Pt or Pd catalysts. A detailed product analysis of the hydrogenation of 1,2-dialkylcyclopentenes indicates that the double bond migration occurred to form 2,3-dialkylcyclopentenes in advance of the hydrogenation. In contrast, 1-methyl-2-phenylcyclopentene gave predominantly the cis product irrespective of the kind of catalyst. These results suggest the formation of a π-benzylic species during the course of the reaction. The mechanism is discussed in terms of the modified Horiuti-Polanyi mechanism.     

The influence of orientation on the electrocatalytic hydrogenation of hydroquinone chemisorbed at smooth polycrystalline platinum electrodes

The influence of orientation on the electrocatalytic hydrogenation of hydroquinone (HQ) chemisorbed at smooth polycrystalline platinum electrodes in aqueous solutions has been investigated; experimental measurements, performed in the absence of bulk (unadsorbed) HQ, were based upon thin-layer electrochemical techniques. The extent of hydrogenation was characterized by (i) n_H. the average number of hydrogen atoms reacted per chemisorbed HQ molecule, and (ii) the electrolytic charge Q_ox for oxidation of chemisorbed organic which remained on the surface after the hydrogenation reaction. The measured values of n_H indicate that the extent of HQ hydrogenation is (i) dependent upon the potential E_Hyd at which hydrogenation was earned out, and (ii) a sensitive function of its initial adsorbed orientation; at a given E_Hyd, n_His larger in the flat (η⁶) than in the edge (2,3-η²) orientation. Correlation of Q_ox with n_H, indicates that an appreciable fraction of partially hydrogenated species is desorbed from the surface; this fraction, which is a function of E_Hyd, is larger in the 2.3-η² than in the η⁶ orientation.     

FT Raman spectroscopic study of the diimide hydrogenation of cis-polybutadiene: Some evidence of cis—trans isomerization

The diimide hydrogenation of cis-polybutadiene is studied. p-Toluenesulfonylhydrazide is used to fully and partially hydrogenate cis-polybutadiene. The microstructure and hydrogenation of the samples obtained are subsequently studied by FT Raman spectroscopy. Cis—trans isomerization is found to take place during the reaction. A mechanism of isomerization and hydrogenation is also proposed.     

Enantioselective hydrogenation of (Z)- and (E)-β-arylenamides catalyzed by rhodium complexes of monodentate chiral spiro phosphorous ligands: a new access to chiral β-arylisopropylamines

A highly enantioselective rhodium-catalyzed hydrogenation of both (Z)- and (E)-β-arylenamides was developed by using monodentate chiral spiro phosphite and phosphine ligands, respectively. The hydrogenation reaction provides an efficient access to optically active β-arylisopropylamines, important building blocks for the synthesis of biologically active compounds.     

Highly enantioselective hydrogenation of N-phthaloyl enamides

Rh- or Ru-catalyzed highly enantioselective hydrogenation of N-phthaloyl enamides is presented. Electron-rich TangPhos and DuanPhos are found to be effective ligands for Rh-catalyzed hydrogenation of α-aryl enamides and up to 99% ee has been achieved. In contrast, for the hydrogenation of α-alkyl enamide, the Ru-C₃-TunePhos complex is more effective and up to 69% ee can be observed. This work is the first report of the hydrogenation of N-phthaloyl enamides.     

[5]Ferrocenophane based ligands for stereoselective Rh-catalyzed hydrogenation and Cu-catalyzed Michael addition

A series of homochiral [5]ferrocenophane based N/P, N/S, N/Se, Se/P and P/P ligands was prepared from (R)-N,N-dimethylamino[5]ferrocenophane. These ligands were tested in the Rh-catalyzed hydrogenation of dimethyl itaconate and in Cu-catalyzed Michael addition of Et₂Zn to cyclohex-2-enone. The best results in terms of conversion and enantioselectivity in the Rh-catalyzed hydrogenation provided bis(diphenylphosphine) ligand 2h (100% conversion and 95% ee) and aminophosphine 2a in the Cu-catalyzed conjugate addition (100% conversion 84% ee). The enantioselectivity of the Rh-catalyzed hydrogenation of methyl 2-acetamidoacrylate was lower (41% ee).     

Stereoselective synthesis of two potential metabolites of cis-metconazole

The stereoselective synthesis of compounds 5 and 6, potential hydroxylated metabolites of the agricultural fungicide cis-metconazole, is reported. In a key step of the initially surveyed synthetic route, hydrodechlorination of 12 was competitive with hydrogenation of the trisubstituted olefin. Application of a Miyaura borylation/hydrogenation/boron-to-halogen exchange reaction sequence solved the chemoselectivity issue.     

Stereoselective synthesis of rac-(8R,13S,14S)-7-oxa-estra-4,9-diene-3,17-dione

The first synthesis of a novel oxa-steroid, rac-(8R,13S,14S)-7-oxa-estra-4,9-diene-3,17-dione has been achieved via stereoselective catalytic hydrogenation of the tetra-substituted indene intermediate, whose structure was confirmed by X-ray crystallography. In contrast to the previous reports of similar indene systems, it was found that catalytic hydrogenation of indenes with a large substituent at the C-2 position and a bulky β-oriented protective group at the C-6 position resulted in cis-indanes instead of trans-indanes.     

Facile access to chiral 4-substituted chromanes through Rh-catalyzed asymmetric hydrogenation

Rh/ZhaoPhos-catalyzed asymmetric hydrogenation of a series of (E)-2-(chroman-4-ylidene)acetates was successfully developed to prepare various chiral 4-substituted chromanes with high yields and excellent enantioselectivities (up to 99% yield, 98% ee). Moreover, the gram-scale hydrogenation could be performed well in the presence of 0.02 mol% catalyst loading (TON = 5000), the hydrogenation product was easily converted to access other important compounds, which demonstrated the synthetic utility of this asymmetric catalytic methodology.     

The use of new chiral phosphite and amidophosphite ligands in the Rh-catalyzed hydrogenation of dehydro-??-amino acid derivatives

New chiral phosphite-type ligands were synthesized. The tests of these ligands in the asymmetric Rh-catalyzed hydrogenation of N-acetyl derivatives of dehydro-??-amino acid esters showed their high enantioselectivity (up to 75% ee). The reaction of nucleophilic addition of phthalimide to disubstituted alkynes offering access to esters of N-phthaloyldehydro-??-amino acids was discovered. Higher conversion and enantioselectivity in the hydrogenation of N-acetyl and N-phtaloyl derivatives of dehydro-??-amino acids were observed in fluorinated alcohols as compared to common organic solvents.     

Readily available chiral phosphine–aminophosphine ligands derived from 1-phenylethylamine for Rh-catalyzed enantioselective hydrogenations

A series of new chiral phosphine–aminophosphine ligands have been prepared via a two- or three-step transformation from commercially available and inexpensive (S)-1-phenylethylamine, and successfully used in the rhodium-catalyzed asymmetric hydrogenation of various enamides, β-dehydroamino acid esters, and dimethyl itaconate. The results show that the ligand structure plays an important influence on both the reactivity and enantioselectivity. Ligand 2d bearing a N–H proton and two F-atoms on the 3,5-positions of the phenyl ring of the aminophosphino moiety was most effective for the hydrogenation of enamides and (Z)-β-aryl-β-(acylamino)acrylates, whereas ligand 1b showed the highest enantioselectivities in the hydrogenation of (Z)-β-alkyl-β-(acylamino)acrylates and dimethyl itaconate.