A Polymer‐Supported Chiral Fluorinated Dirhodium(II) Complex for Asymmetric Amination of Silyl Enol Ethers

Abstract

The immobilization of dirhodium(II) tetrakis[N-tetrafluorophthaloyl-(S)-tert-leucinate], [Rh 2 A C H T U N G T R E N N U N G (S-TFPTTL) 4 ], has been accomplished by copolymerization of a dirhodium(II) complex-containing monomer with styrene and 1,6-bis(4-vinylbenzyloxy)hexane as a flexible cross-linker. The polymersupported chiral fluorinated dirhodium(II) complex catalyzed the amination of silyl enol ethers with [N-(2-nitrophenylsulfonyl)imino]phenyliodinane (NsN=IPh) to provide a-amino ketones in high yields with high levels of enantioselectivity and could be used up to 20 times as the catalyst readily withstood stirring in the presence of the solid reactant.Keywords: asymmetric catalysis; heterogeneous catalysis; immobilization; nitrenes; rhodium Chiral dirhodium(II) carboxylate and carboxamidate complexes are exceptional catalysts for a wide range of enantioselective metal carbene transformations with diazocarbonyl compounds, including cyclopropanation, C À H insertion, and rearrangement or cycloaddition via ylide generation.[1] Recently, they have also been recognized as effective catalysts in metal nitrene reactions such as C À H amination and olefin aziridination. [2][3][4][5][6][7][8] Although such reactions frequently proceed in high yield and with high levels of asymmetric induction, their practical applications in pharmaceutical production are hampered by the high costs of both rhodium [9] and chiral ligands as well as the difficulty in catalyst recovery and recycling. These drawbacks have stimulated a variety of efforts toward immobilization of chiral dirhodium(II) complexes.[10-16] While considerable advances have been made in selected catalytic asymmetric carbene processes, to the best of our knowledge, there is no immobilized dirhodium(II) catalyst available for nitrene reactions. [17,18] Recently, we reported an effective immobilization of [Rh 2 A C H T U N G T R E N N U N G (S-PTTL) 4 ] (1a) [19][20][21] (Figure 1), which was achieved by preparation of a dirhodium(II) complexcontaining monomer 2a followed by copolymerization with styrene (5a) and 1,6-bis(4-vinylbenzyloxy)hexane (6) as a flexible cross-linker (Scheme 1).[22-25] The polymer-supported complex 7a with no unreacted linkers or free ligands [26] catalyzed asymmetric C À H insertions with high enantioselectivities similar to those found with the homogeneous catalyst 1a and could be used for up to 100 sequential applications with a low leaching level.[22] Very recently, a similar method proved to be beneficial for the immobilization of a chiral chlorinated dirhodium(II) complex. [27] The immobilized catalyst 7b prepared by copolymerization of dirhodium(II) complex-containing monomer 2b with 2-(trifluoromethyl)styrene (5b) and crosslinker 6 (Scheme 1) was successfully applied to intermolecular carbonyl ylide cycloaddition reactions under continuous flow conditions, where high yields as well as high levels of enantioselectivity (up to 99% ee) and turnover number (up to 11,700) were achieved.[27] Given that catalysts 7a and 7b...

RuV‐Acylimido Intermediate in [RuIV(Por)Cl2]‐Catalyzed C–N Bond Formation: Spectroscopic Characterization, Reactivity, and Catalytic Reactions

“…product yields by [Ru IV (TTP)Cl 2 ]/N 3 COR: 79 % for 8 ad , 81 % for 8 bd ). The Cu‐, [32a] Ru‐, [32b] and Rh [32c,d,e] ‐catalyzed amination of silyl enol ethers with PhI=NSO 2 Ar possibly involve N ‐sulfonyl nitrenoids. A recently reported Ir‐catalyzed N ‐alkoxycarbonyl nitrene transfer reaction of t BuMe 2 SiC(Ph)=CHMe with N 3 COOCH 2 CCl 3 gave the amination product in 49 % yield [32f] .…”

Recent advances in the synthesis of α-amino ketones

“…The use of aldehydes in the synthesis of α-amino ketones has been mainly reserved to the aza-benzoin condensation. Rovis published an enantioselective version of this transformation in 2012, utilising aliphatic aldehydes (65) and aryl-substituted boc-imines (66) (Scheme 14). 57 Aldehydes featuring oxygen, sulfur and nitrogen-containing linear tethers were well-toler-…”

“…A related transformation developed by Hashimoto uses an imidoiodinane as the nitrogen electrophile in the presence of a polymer-supported dirhodium(II) catalyst. 65 Electrophilic nitrogen was also used by Kürti in an azaderivative of the Rubottom oxidation (Scheme 19). 66 Treatment of tetrasubstituted silyl enol ethers (85) with protected hydroxylamine 86 in HFIP generated amino ketones featuring a quaternary centre (87) in up to 82% yield.…”

Dirhodium Coordination Polymers for Asymmetric Cyclopropanation of Diazooxindoles with Olefins: Synthesis and Spectroscopic Analysis

“…These heterogeneous catalysts were then successfully used in various asymmetric reactions. For example, Hashimoto et al [57][58] developed a novel approach for immobilizing chiral dirhodium catalysts such as Rh 2 (S-PTTL) 4 and Rh 2 (S-TFPTTL) 4 , by ligand exchange with bidentate ligands. Subsequent copolymerization then led to solid coordination polymers.…”