Chiral 1,1′-Diphosphetanylferrocenes: New Ligands for Asymmetric Catalytic Hydrogenation of Itaconate Derivatives

Abstract

The attainment of both high catalytic efficiency and high enantioselectivity remains a formidable challenge in asymmetric catalysis. [1] Bidentate ligands composed of trans-2,5disubstituted phospholane groups have been shown to be useful in asymmetric catalytic hydrogenation reactions. [2] Despite high enantioselectivities, practical application of this technology frequently requires enhancement of catalyst activity and productivity. To augment the turnover frequencies of catalysts bearing bis(phospholane) ligands, we introduced more flexible backbones (for example, 1,3-propandiyl and 1,1'-ferrocene bridges). [3] Whereas catalytic rates were greatly improved in these systems, enantioselectivities were found to plummet. We now have found that the combination of efficiency and selectivity may be realized through use of phosphetanes. Here we outline the synthesis of new 1,1'diphosphetanylferrocene ligands (1; FerroTANE) [4] and demonstrate the superiority of these ligands over known systems in the highly efficient and enantioselective Rh-catalyzed hydrogenation of itaconate derivatives.The first optically active phosphetanes previously were described in seminal reports by Marinetti and Ricard. [5] More recently, we [6] and the group of Marinetti and Gene à t [7] have independently prepared and examined enantiomerically pure 2,4-disubstituted phosphetanes for use as ligands in asymmetric catalysis. The chiral 2,4-disubstituted phosphetane moiety may be constructed from readily available enantiomerically pure 1,3-diols. The requisite 1,3-diols were prepared conveniently through asymmetric hydrogenation of 1,3-diketones using well-documented procedures involving biaryldiphosphane ± Ru catalysts. [8] Subsequently, the diols were converted to 1,3-diol cyclic sulfates 3 through treatment with thionyl chloride followed by Ru-catalyzed oxidation with sodium periodate. [6, 7] As shown in Scheme 1, the reaction between the cyclic sulfates 3 and the known diphosphanylferrocene 2 [3b] provided facile access to the desired ligands 1 a ± e, which were isolated as yellow to orange crystalline solids in moderate to good overall yields. A wide range of different 2,4-disubstituted FerroTANE ligands may be obtained through this procedure. The facility with which ligand 1 e (R tBu) was formed is particularly surprising considering

Synthesis of Chiral Heterocyclic Phosphines for Application in Asymmetric Catalysis

“…One year later Burk and co-workers broadened this small set of ligands by preparation of bis(phosphetanes) 4c-e by a slightly modified procedure and summerized the whole class of ligands under the name FerroTANE [29]. In the Rhcatalyzed hydrogenation of dimethyl itaconate best enantioselectivity was obtained with 4c as a chiral ligand (98 %ee).…”

A Flexible Approach to Different Families of Bidentate P,P Ligands as Highly Efficient Ligands for Asymmetric Catalysis

“…Hydrogenation of dimethyl itaconate ( 22 ) with the precatalyst derived from ligand 13 gave the ( S ) product in 94.8 % ee and also the “inverse itaconate”18a 23 was hydrogenated rapidly with the same catalyst to give the product in 98.7 % ee (Scheme , Table 1). Hydrogenation of 24 with this precatalyst was sluggish even at 40 bar at S/C = 100 and the product had an ee of only 34 %.…”

Enantioselective Hydrogenation of Alkenes with Ferrocene‐Based Ligands

“…5) have recently been described and tested, with sometimes quite impressive results. Interesting examples are f-binaphane [11], ferrotane [12], L2 with only planar chirality [13], bisphosphonite L3 [14], the sugar-based phospholane L4 [15] and the P-chiral phosphines L5 [16]. Rh complexes of ferrotanes showed very good performance for various amido itaconates, and achieved very high TONs and TOFs for substrate 4 (Table 25.3, entry 3.1).…”