Rhodium-catalyzed asymmetric hydrogenation with self-assembling catalysts in propylene carbonate

Coupling Reactions of Carbon Dioxide with Epoxides Catalyzed by Vanadium Aminophenolate Complexes

A series of vanadium compounds supported by tetradentate aminobis(phenolate) ligands were screened for catalytic reactivity in the reaction of propylene oxide (PO) with CO₂ : [VO(OMe)(O₂ NO^BuMeMeth )], [VO(OMe)(ON₂ O^BuMe )], [VO(OMe)(O₂ NN^BuBuPy )], and [VO(OMe)(O₂ NO^BuBuFurf )]. They showed similar reactivities, but reaction rates were higher for [VO(OMe)(ON₂ O^BuMe )], which was studied in more detail. Turnover frequencies for conversion of PO over 500 h^-1 were observed. Activation energies were determined experimentally through in situ IR spectroscopy for propylene carbonate (48.2 kJ mol^-1 ), styrene carbonate (45.6 kJ mol^-1 ), and cyclohexene carbonate (54.7 kJ mol^-1 ) formation.

Ru coordinated with BINAP in knitting aryl network polymers for heterogeneous asymmetric hydrogenation of methyl acetoacetate

The aryl network polymers formed by Friedel–Crafts reaction with no-modified BINAP chiral ligand, and its application in asymmetric hydrogenation.

Kinetics and mechanism of vanadium catalysed asymmetric cyanohydrin synthesis in propylene carbonate

Propylene carbonate can be used as a green solvent for the asymmetric synthesis of cyanohydrin trimethylsilyl ethers from aldehydes and trimethylsilyl cyanide catalysed by VO(salen)NCS, though reactions are slower in this solvent than the corresponding reactions carried out in dichloromethane. A mechanistic study has been undertaken, comparing the catalytic activity of VO(salen)NCS in propylene carbonate and dichloromethane. Reactions in both solvents obey overall second-order kinetics, the rate of reaction being dependent on the concentration of both the aldehyde and trimethylsilyl cyanide. The order with respect to VO(salen)NCS was determined and found to decrease from 1.2 in dichloromethane to 1.0 in propylene carbonate, indicating that in propylene carbonate, VO(salen)NCS is present only as a mononuclear species, whereas in dichloromethane dinuclear species are present which have previously been shown to be responsible for most of the catalytic activity. Evidence from ⁵¹V NMR spectroscopy suggested that propylene carbonate coordinates to VO(salen)NCS, blocking the free coordination site, thus inhibiting its Lewis acidity and accounting for the reduction in catalytic activity. This explanation was further supported by a Hammett analysis study, which indicated that Lewis base catalysis made a much greater contribution to the overall catalytic activity of VO(salen)NCS in propylene carbonate than in dichloromethane.

Cyclic carbonate synthesis catalysed by bimetallic aluminium-salen complexes

The development of bimetallic aluminium-salen complexes [{Al(salen)}(2)O] as catalysts for the synthesis of cyclic carbonates (including the commercially important ethylene and propylene carbonates) from a wide range of terminal epoxides in the presence of tetrabutylammonium bromide as a cocatalyst is reported. The bimetallic structure of one complex was confirmed by X-ray crystallography. The bimetallic complexes displayed exceptionally high catalytic activity and in the presence of tetrabutylammonium bromide could catalyse cyclic carbonate synthesis at atmospheric pressure and room temperature. Catalyst-reuse experiments demonstrated that one bimetallic complex was stable for over 60 reactions, though the tetrabutylammonium bromide decomposed in situ by a retro-Menschutkin reaction to form tributylamine and had to be regularly replaced. The mild reaction conditions allowed a full analysis of the reaction kinetics to be carried out and this showed that the reaction was first order in aluminium complex concentration, first order in epoxide concentration, first order in carbon dioxide concentration (except when used in excess) and unexpectedly second order in tetrabutylammonium bromide concentration. Further kinetic experiments demonstrated that the tributylamine formed in situ was involved in the catalysis and that addition of butyl bromide to reconvert the tributylamine into tetrabutylammonium bromide resulted in inhibition of the reaction. The reaction kinetics also indicated that no kinetic resolution of racemic epoxides was possible with this class of catalysts, even when the catalyst was derived from a chiral salen ligand. However, it was shown that if enantiomerically pure styrene oxide was used as substrate, then enantiomerically pure styrene carbonate was formed. On the basis of the kinetic and other experimental data, a catalytic cycle that explains why the bimetallic complexes display such high catalytic activity has been developed.

Rh-catalyzed asymmetric hydrogenation of unsaturated lactate precursors in propylene carbonate

The asymmetric hydrogenation of alpha-acetoxy acrylates to O-acetyl lactates with Rh catalysts based on chiral bisphospholane ligands was investigated in propylene carbonate (PC) as "green" solvent. In contrast to DuPHOS-type ligands, catASium M ligands lead to full conversion of the substrate in PC and induce excellent enantioselectivities for ethyl ester and methyl ester substrates (>98 %). Moreover, the undesired opening of the maleic anhydride moiety of the catASium M ligand observed in MeOH can be prevented under these conditions. The chiral product can be easily separated from the carbonate solvent by distillation. In this way, an ecologically benign process for the production of enantiopure lactic acid derivatives was established which offers a highly efficient catalytic transformation in a green solvent under mild conditions (1-10 bar H(2)).

Organic carbonates as alternative solvents for palladium-catalyzed substitution reactions

Organic carbonates, such as propylene carbonate, butylene carbonate, and diethyl carbonate, were tested in the Pd-catalyzed asymmetric allylic substitution reactions of rac-1,3-diphenyl-3-acetoxy-prop-1-ene with dimethyl malonate or benzylamine as nucleophiles. Bidentate diphosphanes were used as chiral ligands. The application of monodentate phosphanes capable of self-assembling with the metal was likewise tested. In the substitution reaction with dimethyl malonate, enantioselectivities up to 98% were achieved. In the amination reaction, the chiral product was obtained with up to 83% ee. The results confirm that these "green solvents" can be advantageously used for this catalytic transformation as an alternative to those solvents usually employed which run some risk of being harmful to the environment.