Homogeneous and heterogeneous asymmetric reactions: Part 11. Sonochemical enantioselective hydrogenation of trifluoromethyl ketones over platinum catalysts

Enantioselective sonochemical hydrogenation of alpha,alpha,alpha-trifluoromethyl ketones, namely, 1,1,1-trifluoroacetophenone and 1,1,1-trifluoro-phenylacetone was carried out over various platinum catalysts modified by cinchonidine in different solvents. Both compounds yielded the (R)-alcohol as major product. The reaction rates and the enantiomeric excesses were determined over Pt/C, Pt/SiO2, Pt/K-10 and Pt/Al2O3 catalysts under conventional conditions. Since Pt/Al2O3 exhibited the best catalytic performance the effect of ultrasound on the catalytic activity and enantioselectivity was tested using this catalyst applying different sonochemical pretreatments. These methods included a pretreatment before the reaction in hydrogen and oxygen or both. The ultrasonic irradiation was found to be highly advantageous in the case of trifluoroacetophenone, whereas only moderate changes were observed using trifluoro-phenylacetone. After insonation of the catalyst, the enantioselectivity was considerably improved. Both the aerobic and anaerobic sonochemical pretreatments resulted in significant improvement in optical yield (up to 49% and 17% ee, at room temperature under 10 bar in 1,2-dichlorobenzene). In parallel, the hydrogenation rates increased to a small extent (1.1-1.2-fold increase) after hydrogenative ultrasonic pretreatment, whereas the rates decreased by a factor of 1.5-2 after aerobic insonation. To obtain more insight into the process, the effect of insonation time on the activity and enantioselectivity and actual changes of the catalyst were also studied.     

Selective,catalytic carbon-carbon bond activation and functionalization promoted by late transition metal catalysts

The selective catalytic activation and functionalization of carbon-carbon bonds in a series of substituted cyclopropane substrates has been developed using commercially available transition metal catalysts. Catalytic hydrogenation and olefination procedures, tolerant of a range of functional groups, have been discovered. Introduction of a chelate-assisting substituent such as [PPh2] is effective in altering the kinetic selectivity and lowering the activation barrier for the catalytic processes.     

4-(Tetrahydro-4H-thiopyran-1-oxide-4-ylidene)-cyclohexanone oxime in the solid-state. A two-dimensional network of enantiomorphous chains interconnected by weak hydrogen bonds

From a saturated C6H6 solution of racemic 4-(tetrahydro-4H-thiopyran-1-oxide-4-ylidene)-cyclohexanone oxime [1(1-R/1-S)] the co-crystal is crystallized. Single crystal X-ray analysis showed that (1)4.C6H6 (P1 space group) in the solid-state consists of enantiomorphous, non-covalent polymer-like chains that contain, in an alternating fashion, the crystallographically independent enantiomers 1-R and 1-R' or 1-S and 1-S', respectively. Within each chain the enantiomers are linked by 'head-to-tail' intermolecular oxime-sulfoxide hydrogen bonding [D(2) motif]. Neighbouring chains consist of enantiomers with opposite configuration and possess opposite molecular 'head-to-tail' alignments. The enantiomorphous chains are interconnected by weak intermolecular C-HO hydrogen bonds involving centrosymmetric C-Hoxime [R(12)] and C-Hsulfoxide [R(8)] motifs between the 1-R and 1-S molecules in neighbouring chains; a nearly planar two-dimensional hydrogen bonding network motif is obtained. In the crystallographic direction [1 0 0] the layers stack in such a fashion that chains occupying successive layers with an identical 'head-to-tail' alignment are positioned on top of each other. Concomitantly, channels with areas of ca. 25 Angstroms(2) are obtained, which are occupied by C6H6 solvent molecules. A comparison of the IR and Raman spectra of with those obtained for native 1 that does not contain C6H6, indicates that intermolecular oxime-sulfoxide hydrogen bonding [D(2) motif] also occurs for native 1 in the solid-state.     

Molecular printboards: monolayers of beta-cyclodextrins on silicon oxide surfaces

Monolayers of beta-cyclodextrin host molecules have been prepared on SiO2 surfaces. An ordered and stable cyano-terminated monolayer was modified in three consecutive surface reactions. First, the cyanide groups were reduced to their corresponding free amines using Red Al as a reducing agent. Second, 1,4-phenylene diisothiocyanate was used to react with the amine monolayer where it acts as a linking molecule, exposing isothiocyanates that can be derivatized further. Finally, per-6-amino beta-cyclodextrin was reacted with these isothiocyanate functions to yield a monolayer exposing beta-cyclodextrin. All monolayers were characterized by contact angle measurements, ellipsometric thickness measurements, Brewster angle Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry, which indicate the formation of a densely packed cyclodextrin surface. It was demonstrated that the beta-cyclodextrin monolayer could bind suitable guest molecules in a reversible manner. A fluorescent molecule (1), equipped with two adamantyl groups for complexation, was adsorbed onto the host monolayer from solution to form a monolayer of guest molecules. Subsequently, the guest molecules were desorbed from the surface by competition with increasing beta-cyclodextrin concentration in solution. The data were fitted using a model. An intrinsic binding constant of 3.3 +/- 1 x 10(5) M(-1) was obtained, which corresponds well to previously obtained results with a divalent guest molecule on beta-cyclodextrin monolayers on gold. In addition, the number of guest molecules bound to the host surface was determined, and a surface coverage of ca. 30% was found.     

Das Phasendiagramm des Systems TeO2–MoO3

Phase Diagram of the System TeO₂? MoO₃ The phase equilibrium diagram of the system TeO₂? MoO₃ has been studied using differential thermal analysis. It is shown that a new congruently melting phase, Te₂MoO₇, is formed (m. p. 551°C). The eutectics were found at 526°C near 55.5 mole% TeO₂ and at 543%C near 67.5 mole% TeO₂. The compound Te₂MoO₇ shows glass-forming tendency.     

Mechanism of hydrogenolysis and isomerization of oxacycloalkanes on metals, XVI. Transformation of tetrahydrofuran on platinum catalysts

The hydrogenolysis of tetrahydrofuran (THF) on TiO₂, SiO₂ and Al₂O₃ supported Pt catalysts has been investigated in the gas phase at 423–623 K in a pulse reactor. The main reactions are hydrodeoxygenation to butane and decarbonylation to propane. The supports had a marked effect on the selectivity of conversion. On the Pt−TiO₂ catalyst mainly butane was formed while on the other two catalysts propane formation was dominant. Based on the results of earlier studies on oxacycloalkanes as well as on data in the literature and the experimental results presented here, a new reaction scheme is outlined for the interpretation of the chemical processes discussed. Part XV.: J. Mol. Catal. (in press)