Albertus J. Sandee

A robust, environmentally benign catalyst for highly selective hydroformylation

By a sol-gel process a rhodium complex containing a diphosphane with a large natural P-Rh-P bite angle is covalently anchored in a silica matrix (see picture). The immobilized catalyst is a very selective hydroformylation catalyst that is completely and conveniently separated from the product and can be reused in numerous cycles.

Bidentate ligands by supramolecular chemistry—the future for catalysis?

In this Frontier Article we give our view on recent developments in transition-metal catalyst development that evolve from a combination of supramolecular strategies and traditional ligand design and development.

UREAphos : supramolecular bidentate ligands for asymmetric hydrogenation

Supramolecular bidentate phosphite ligands are presented as a new class of ligands for rhodium catalysed asymmetric hydrogenation.

Application of a Supramolecular‐Ligand Library for the Automated Search for Catalysts for the Asymmetric Hydrogenation of Industrially Relevant Substrates

A procedure is described for the automated screening and lead optimization of a supramolecular-ligand library for the rhodium-catalyzed asymmetric hydrogenation of five challenging substrates relevant to industry. Each catalyst is (self-) assembled from two urea-functionalized ligands and a transition-metal center through hydrogen-bonding interactions. The modular ligand structure consists of three distinctive fragments: the urea binding motif, the spacer, and the ligand backbone, which carries the phosphorus donor atom. The building blocks for the ligand synthesis are widely available on a commercial basis, thus enabling access to a large number of ligands of high structural diversity. The simple synthetic steps enabled the scale-up of the ligand synthesis to multigram quantities. For the catalyst screening, a library of twelve new chiral ligands was prepared that comprised substantial variation in electronic and steric properties. The automated procedures employed ensured the fast catalyst assembly, screening, and direct acquisition of samples for analysis. It appeared that the most selective catalyst was different for every substrate investigated and that small variations in the building blocks had a major impact on the catalyst performance. For two substrates, a catalyst was found that provided the product with outstanding enantioselectivity. The subsequent automated optimization of these two leads showed that an increase of catalyst loading, dihydrogen pressure, and temperature had a positive effect on the catalyst activity without affecting the catalyst selectivity.

Solid-Phase Synthesis of Homogeneous Ruthenium Catalysts on Silica for the Continuous Asymmetric Transfer Hydrogenation Reaction

The solid-phase synthesis of new asymmetric transfer hydrogenation catalysts as well as the use of these silica supported systems in batch and flow reactors is reported. The ruthenium complex of NH-benzyl-(1R,2S)-(-)-norephedrine covalently tethered to silica showed a high activity and enantioselectivity in the reduction of acetophenone. In three consecutive batchwise catalytic runs, we obtained ee values of 88%. In a continuous flow reactor, a very constant catalytic activity was observed; no catalyst deactivation occurred over a period of one week. This has been ascribed to successful site isolation. Using optimized conditions in this flow reactor, the ee was as high as 90% at 95% conversion. The supported catalysts generally show the same trend in catalyst performance as in solution. The viability of our approach was further shown in one example, the ruthenium(II) complex of (1S,2R)-(+)-2-amino-1,2-diphenylethanol, for which an enantiomeric excess of 58% was observed, which is nearly three times higher than its homogeneous analogue.

Continuous, selective hydroformylation in supercritical carbon dioxide using an immobilised homogeneous catalyst

A continuous process for the selective hydroformylation of higher olefins in supercritical carbon dioxide (scCO2) is presented; the catalyst shows high selectivity and activity over several hours and no decrease in performance was observed over several days.

Xantphos-based, silica-supported, selective, and recyclable hydroformylation catalysts: a review.

Immobilization on silica of selective homogeneous hydroformylation catalysts based on xanthene ligands is reviewed. Various immobilized catalysts are compared, such as SAPC, sol-gel-based catalysts, silica-anchored catalysts used both in an organic phase as well as in supercritical carbon dioxide (scCO 2 ), and chemical modifications of silica-anchored catalysts. In all instances, the high selectivity of the homogeneous Xantphos ligands is retained and linear to branched ratios are 20 or higher. Formation of 2-octene from 1-octene via isomerization also remains low (<5%) as in the homogeneous phase. The rates expressed in turnover frequencies drop considerably except for the experiments in scCO 2 , which are only half of those in the homogeneous phase. Leaching of rhodium to the product is in all cases below the detection limit of ICP-AES ( 1 ppm).

Rhodium-P,O-bidentate coordinated ureaphosphine ligands for asymmetric hydrogenation reactions

We present new ureaphosphine ligands that coordinate in a P,O-bidentate fashion to rhodium(i). The ureaphosphine-Rh(i)-complexes were effectively used in the asymmetric hydrogenation of cyclic enamides giving high conversions and enantioselectivity.

ROTACAT: a rotating device containing a designed catalyst for highly selective hydroformylation.

A novel concept is presented for the immobilization of a homogeneous catalyst. A hydroformylation catalyst was covalently anchored to monoliths that were constructed as the blades of a mechanical stirrer and used in a batch process. The catalyst was effective in the hydroformylation of both higher and lower alkenes and showed a high regioselectivity for the linear aldehyde. The concept was proven to be useful in a liquid organic and aqueous phase as well as in the gas phase and the catalyst could be used numerous times without catalyst deterioration. No catalyst deactivation was observed in a period over half a year.

Diversity in Complexation of [RhI(cod)]+ and [IrI(cod)]+ by Pyridine-Amine-Pyrrole Ligands

Complexation of [RhI(cod)]+ and [IrI(cod)]+ by the new pyridine-amine-pyrrole ligands Py–CH2–N(R)–CH2–Pyr–H (HLR; R = H, Bzl, Bu) and the corresponding pyridine-amine-pyrrolate ligands [Py–CH2–N(R)–CH2–Pyr]− (LR−; R = H, Bzl, Bu, CH2Py) has been investigated. The neutral ligands HLR (R = H, Bu, Bzl) give [(HLR)MI(cod)]+ (M = Rh, Ir) in which HLR acts as a didentate ligand via the pyridine nitrogen (NPy) and the amine nitrogen (NRamine). The crystal structures of [(HLH)MI(cod)]PF6 (M = Rh: [1]PF6 and M = Ir: [2]PF6) have been determined. Deprotonation of [(HLR)MI(cod)]+ (M = Rh, Ir; R = H, Bzl, Bu) results in the neutral complexes [(LR)MI(cod)] (M = Rh, Ir) of the mono-anionic ligands LR− (R = H, Bzl, Bu). In square-planar [(LH)MI(cod)] (M = Rh: 3, M = Ir: 4), LH− is didentate via NHamine and the pyrrolate nitrogen (NPyr). The X-ray structures of 3 and 4 reveal that in both cases the uncoordinated NPy accepts a hydrogen bond from NHamine. The X-ray structures of [(LBzl)MI(cod)] (M = Rh: 5, M = Ir: 6), show that LBzl− is didentate via Namine and NPyr for M = Rh and tridentate for M = Ir. In solution LBzl− is tridentate for both M = Rh and M = Ir. The neutral complexes [{Py–CH2–N(R)–CH2–Pyr}MI(cod)] (M = Rh, Ir) cannot be oxidised selectively with H2O2. This is in marked contrast to the previously observed selective oxidation of the corresponding cationic complexes [{Py–CH2–N(R)–CH2–Py}RhI(cod)]+. Rhodium complex 5 is an active catalyst for the stereoregular polymerisation of phenylacetylene, whereas iridium complex 6 is inactive.