https://eprints.ncl.ac.uk/file_store/production/162122/B5324BA5-F503-4BE9-947E-6F7ED01AAD04.pdf

Synthesis of cyclic carbonates from epoxides and CO2

Organic Carbonates as Solvents in Synthesis and Catalysis

Recent advances in the area of Olefin Oligomerization via metallacycles that include dimerization, trimerization, tetramerization, and beyond, are reviewed. Studies have found that metallacyclopentane decomposition to 1-butene many not be particularly facile due to the absence of metallacycle expansion. Follow-up studies concentrated on the N-H functionality and the Cr oxidation state and role of MAO show that activities and selectivities to 1-hexene are similar to the original Cr(III) complexes. Nenu and Weckhuysen prepared silica-supported triazacyclohexane complexes, by treating the reduced Phillips polymerization catalyst with triazacyclohexane ligands in dichloromethane. The influence of N-aryl functionality investigated by Killian et al. shows that the selectivity was mainly dependent upon the steric bulk attached to nitrogen, and less so on the group's basicity.

Olefin oligomerization via metallacycles: dimerization, trimerization, tetramerization, and beyond.

The development of bimetallic aluminium-salen complexes [{A1-(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.

Cyclic Carbonate Synthesis Catalysed by Bimetallic Aluminium-Salen Complexes

Asymmetric hydrogenations are increasingly being used to introduce stereogenic centres into products used in the life sciences industries. There are a number of potential pitfalls when moving from a laboratory reaction to a manufacturing process, not least of which is safety. Time-to-market pressure leads to short development times, which in the past could be a large barrier for the implementation of catalytic steps; now there are new ways to minimise this problem. The potential problems associated with impurities and other methods that can shut down the hydrogenation reactions are highlighted in this critical review (353 references).

Asymmetric homogeneous hydrogenations at scale

The use of diolefin-containing rhodium precatalysts leads to induction periods in asymmetric hydrogenation of prochiral olefins. Consequently, the reaction rate increases in the beginning. The induction period is caused by the fact that some of the catalyst is blocked by the diolefin and thus not available for hydrogenation of the prochiral olefin. Therefore, the maximum reaction rate cannot be reached initially. Due to the relatively slow hydrogenation of cyclooctadiene (cod) the share of active catalysts increases at first, and this leads to typical induction periods. The aim of this work is to quantify the hydrogenation of the diolefins cyclooctadiene (cod) and norborna-2,5-diene (nbd) for cationic complexes of the type [Rh(ligand)(diolefin)]BF(4) for the ligands Binap (1,1'-binaphthalene-2,2'-diylbis(phenylphosphine)), Me-Duphos (1,2-bis(2,5-dimethylphospholano)benzene, and Catasium in the solvents methanol, THF, and propylene carbonate. Furthermore, an approach is presented to determine the desired rate constant and the resulting respective pre-hydrogenation time from stoichiometric hydrogenations of the diolefin complexes via UV/Vis spectroscopy. This method is especially useful for very slow diolefin hydrogenations (e.g., cod hydrogenation with the ligands Me-Duphos, Et-Duphos (1,2-bis(2,5-diethylphospholano)benzene), and dppe (1,2-bis(diphenylphosphino)ethane).

Rhodium-complex-catalyzed asymmetric hydrogenation: transformation of precatalysts into active species.

By reaction of the dichloride rac-(ebthi)HfCl2 [ebthi = 1,2-ethylene-1,1′-bis(η5-tetrahydroindenyl)] with lithium in the presence of bis(trimethylsilyl)acetylene, the hafnacyclopropene rac-(ebthi)Hf(η2-Me3SiC2SiMe3) (1-Hf) was obtained. The reaction of the blue-green complex 1-Hf with an excess of ethylene at room temperature leads by insertion of the olefin to the yellow-green hafnacyclopentene 2-Hf which is only stable in solution and eliminates the alkyne at 100 °C under ethylene to form the corresponding yellow hafnacyclopentane 3-Hf, which was characterized by X-ray crystal structure analysis. The reaction of 1-Hf to give stepwise via 2-Hf the complex 3-Hf was investigated in detail and compared to the formation and stability of the corresponding zirconacyclopropene 1-Zr, zirconacyclopentene 2-Zr, and zirconacyclopentane 3-Zr. Moreover, the reaction of the titanocene alkyne complex 1-Ti with ethylene was studied. For investigating the reaction behavior of the alkyne complexes 1-M, NMR spectroscopy wa...

Combination of spectroscopic methods: in situ NMR and UV/Vis measurements to understand the formation of group 4 metallacyclopentanes from the corresponding metallacyclopropenes.

Cationic Rhodium-BINAP Complexes: Full Characterization of Solvate- and Arene-Bridged Dimeric Species

Asymmetric Hydrogenation. Dimerization of Solvate Complexes: Synthesis and Characterization of Dimeric [Rh(DIPAMP)]22+, a Valuable Catalyst Precursor

Three novel trinuclear rhodium hydride complexes of the type {[Rh(PP*)H](3)(μ(2)-H)(3)(μ(3)-H)}[BF(4)](2) containing diphosphines Tangphos, t-Bu-BisP* and Me-DuPHOS have been synthesised. The new compounds are very stable. Their structures have been characterized by X-ray analysis in the solid state and by NMR-spectroscopic investigations in solution.

Christian Fischer

Papers

Rhodium-complex-catalyzed asymmetric hydrogenation: transformation of precatalysts into active species.

Combination of spectroscopic methods: in situ NMR and UV/Vis measurements to understand the formation of group 4 metallacyclopentanes from the corresponding metallacyclopropenes.

Cationic Rhodium-BINAP Complexes: Full Characterization of Solvate- and Arene-Bridged Dimeric Species

Asymmetric Hydrogenation. Dimerization of Solvate Complexes: Synthesis and Characterization of Dimeric [Rh(DIPAMP)]22+, a Valuable Catalyst Precursor

Trinuclear rhodium hydride complexes.