Efficient Cluster‐Based Catalysts for Asymmetric Hydrogenation of α‐Unsaturated Carboxylic Acids

Abstract

The new clusters [H(4)Ru(4)(CO)(10)(μ-1,2-P-P)], [H(4)Ru(4)(CO)(10) (1,1-P-P)] and [H(4)Ru(4)(CO)(11)(P-P)] (P-P=chiral diphosphine of the ferrocene-based Josiphos or Walphos ligand families) have been synthesised and characterised. The crystal and molecular structures of eleven clusters reveal that the coordination modes of the diphosphine in the [H(4)Ru(4)(CO)(10)(μ-1,2-P-P)] clusters are different for the Josiphos and the Walphos ligands. The Josiphos ligands bridge a metal-metal bond of the ruthenium tetrahedron in the "conventional" manner, that is, with both phosphine moieties coordinated in equatorial positions relative to a triangular face of the tetrahedron, whereas the phosphine moieties of the Walphos ligands coordinate in one axial and one equatorial position. The differences in the ligand size and the coordination mode between the two types of ligands appear to be reflected in a relative propensity for isomerisation; in solution, the [H(4)Ru(4)(CO)(10)(1,1-Walphos)] clusters isomerise to the corresponding [H(4)Ru(4)(CO)(10)(μ-1,2-Walphos)] clusters, whereas the Josiphos-containing clusters show no tendency to isomerisation in solution. The clusters have been tested as catalysts for asymmetric hydrogenation of four prochiral α-unsaturated carboxylic acids and the prochiral methyl ester (E)-methyl 2-methylbut-2-enoate. High conversion rates (>94%) and selectivities of product formation were observed for almost all catalysts/catalyst precursors. The observed enantioselectivities were low or nonexistent for the Josiphos-containing clusters and catalyst (cluster) recovery was low, suggesting that cluster fragmentation takes place. On the other hand, excellent conversion rates (99-100%), product selectivities (99-100% in most cases) and good enantioselectivities, reaching 90% enantiomeric excess (ee) in certain cases, were observed for the Walphos-containing clusters, and the clusters could be recovered in good yield after completed catalysis. Results from high-pressure NMR and IR studies, catalyst poisoning tests and comparison of catalytic properties of two [H(4)Ru(4)(CO)(10)(μ-1,2-P-P)] clusters (P-P=Walphos ligands) with the analogous mononuclear catalysts [Ru(P-P)(carboxylato)(2)] suggest that these clusters may be the active catalytic species, or direct precursors of an active catalytic cluster species.

Asymmetric hydrogenation of an α-unsaturated carboxylic acid catalyzed by intact chiral transition metal carbonyl clusters – diastereomeric control of enantioselectivity

“…The diphosphine ligands were found to exclusively coordinate in a bridging fashion, giving rise to nine-membered "dimetallacycles". As previously observed for related tetraruthenium tetrahydrido clusters, 22 the Walphos ligand coordinates in an axial-equatorial mode. In the case of 3, P1 is coordinated in an axial position to Ru1, the metal that is coordinated to both of the hydrides, while P2 is coordinated in an equatorial position to Ru2, which is coordinated by one hydride.…”

“…We have previously observed exactly the same types of coordination modes for Josiphos and Walphos ligands coordinated to tetraruthenium tetrahydride clusters. 22 The two metal-metal bonds in 20 that are bridged by hydrides are slightly elongated in comparison to the non-bridged Ru1-Ru3 edge (cf caption, Fig. 6).…”

“…19,20 In previous studies, we have investigated asymmetric hydrogenation of α-unsaturated carboxylic acids using catalytic systems based on [(µ-H) 4 Ru 4 (CO) 12 ] clusters derivatised with chiral diphosphines. We have shown that such diphosphine ligands may effect strong chiral induction, 21,22 and that good conversions and enantioselectivities may be achieved. While catalysis by fragmentation products cannot be excluded, the recovery of unaltered starting clusters after catalytic reactions, recycling of catalysts/catalyst precursors with identical catalytic results, and mercury poisoning tests all implicate the presence of an active cluster catalyst.…”

Homogeneous Catalysis by Organometallic Polynuclear Clusters

“…A series of tetraruthenium hydrido clusters was substituted with Josiphos-(32h, i and 33f, g) and Walphos phosphines (32d, e, f, g and 33d, e) to assess the steric and electronic influence of the ligand on the hydrogenation of 29a [84].…”

“…Asymmetric hydrogenation of 29a, 29b, and 31a using tetraruthenium clusters 32 or 33 having chiral ligands as catalysts. Terminal CO molecules have been omitted for clarity,[81][82][83][84].…”