Highly Modular P‐O‐P Ligands for Asymmetric Hydrogenation

Abstract

Abstract:The preparation of a library of new P-O-P ligands (phosphine-phosphites and phosphinephosphinites), easily available in two synthetic steps from enantiopure Sharpless epoxy ethers, is reported. The "lead" catalyst of the series has proven to have outstanding catalytic properties in the rhodium-catalysed asymmetric hydrogenation of a wide variety of functionalised alkenes (16 examples). The excellent performance and modular design of the catalysts makes them attractive for future applications.Keywords: asymmetric catalysis; hydrogenation; ligand design; phosphane ligands; phosphite ligands; rhodium Asymmetric hydrogenation can be considered a well established synthetic methodology that has already been incorporated into the standard "asymmetric tool-box" of the synthetic community.[1] From a practical perspective, it is one of the most efficient methodologies for the generation of new stereogenic carbon centres. There have been several significant breakthroughs in the field [1] and now a myriad of chiral Ru, Rh and Ir coordination compounds (mostly phosphorus-containing derivatives) capable of mediating the addition of H 2 to prochiral C=C, C=N and C=O bonds with very high enantioselectivities are known.[1] The development of commercial processes [2] has rendered this transformation highly desirable to both academia and industry. However, despite the remarkably advanced state of the field, many groups are still actively researching new catalytic systems that show higher activity and/or improved enantioselectivity for challenging substrates. In other cases, research efforts are directed to the development of chiral ligands with an attractive industrial profile, which ultimately means that they should induce high enantioselectivity, be easily prepared and not fall into the claims of any patent currently in force. While computational techniques are becoming increasingly important in the design of new chiral catalysts, [3] many approaches still rely to a great extent on trialand-error. Not surprisingly, combinatorial and highthroughput synthetic strategies [4] have led to the development of some highly efficient monodentate and bidentate phosphorus-containing ligands for asymmetric hydrogenation, utilising either standard covalent chemistry [4c,5] or supramolecular interactions. [6] In a complementary way, ligand tuning in asymmetric catalysis has allowed the rapid development of efficient catalytic systems. We [3a,7] and others [8] have shown that the modular nature of ligands facilitates the tuning of their performance by modifying the stereoelectronic properties of the different modular fragments (modules). This encouraged us to design a new family of P-O-P ligands 3 derived from enantiopure epoxides and the work described here details our initial efforts in this area together with their application in the asymmetric hydrogenation of functionalised alkenes.We envisaged that the chiral epoxides 1 could be converted into the P-O-P ligands 3, as shown in Scheme 1. Epoxide ring-opening with nucleophilic triv...

Synthesis of Elusive Chloropnictenium Ions

“…To establish the positive effects of the supramolecular recognition between alkali cations and the metallacrown ether catalysts on both hydrogenation activity and enantioselectivity, a series of α‐arylenamides were hydrogenated by employing the Rh/( 3a +K) complex as the catalyst under the optimized conditions. As shown in Table 3, the hydrogenation of α‐ para ‐substituted phenylenamides 6a – 6f proceeded smoothly with full conversions and excellent enantioselectivities (93–99 % ee ), which are comparable to or even better than the best results obtained for phosphine–phosphite ligands with a chiral backbone 5a,5b. For meta ‐chlorophenylenamide 6g , fairly good enantioselectivity was observed (Table 3, Entry 7).…”

“…Having realized the effective formation of chiral rhodium metallacrown ether catalysts, we then evaluated their catalytic properties in the asymmetric hydrogenation of α‐arylenamide derivatives. In general, rather few phosphine–phosphite/metal catalysts have been found to be highly enantioselective in the asymmetric hydrogenation of enamide substrates,5a,5b although such phosphorus ligands offer the advantages of easy preparation and derivatization as well as high air‐stability. Gratifyingly, 100 % conversions were achieved within 12 h for all of the reactions screened.…”

“…High efficiency and enantioselectivity have usually been achieved by utilizing such phosphine–phosphite ligands with more than one chiral element 4a. However, for enamide substrates, rather few phosphine–phosphite/metal catalysts have been found to be highly enantioselective in the asymmetric hydrogenation 5a,5b,5g,6c. Considering the widespread application of phosphine–phosphite ligands4a and as a continuation of our ongoing endeavor to develop effective chiral supramolecular catalysts based on crown ethers,2g,6h,6i herein, we report the preparation of a new class of metallacrown ether catalysts.…”

Arene–Ruthenium Chemistry and Brønsted Acid Catalysis of a Chiral Phosphane-Hydroxyl Ligand

“…9 We anticipate that POH can act as a bidentate κ 2 P,O chelate ligand through the phosphane and hydroxyl functionalities, giving rise to stable unstrained five-memberedring metallacycles (see complex 2, Scheme 2). Furthermore, the acidic properties of the OH group would be enhanced by coordination to metallic cations, resulting in potential LBA catalysts.…”

Bidentate Ligands for Enantioselective Enamide Reduction

“…In 2008, Vidal-Ferran reported that a phosphine-phosphite ligand 53 with the matched chiralities gave good enantioselectivity in this model hydrogenation [54]. Ferrocene-based 1,5-diphosphine ligands, Taniaphos, gave excellent enantioselectivities for various metal-catalyzed asymmetric reactions [55].…”