Enantioselective CH Amination Using Cationic Ruthenium(II)–pybox Catalysts

Abstract

C À H bond amination has emerged as a powerful tool for the synthesis of complex nitrogen-containing molecules. Following the early discoveries by Breslow and Gellman, [1] Du Bois and co-workers revolutionized this area of chemistry by developing protocols for practical, efficient, and predictable reactions for oxidative C À H amination.[2] Dirhodium(II) tetracarboxylate catalysts were shown to be particularly effective. Manganese-and ruthenium-porphyrin complexes, [3] silver complexes, [4] and preoxidized nitrogen sources have also been developed as catalysts to carry out this important reaction. [5] Despite these recent advances, general methods for both enantioselective and intermolecular C À H amination remain elusive. Although chiral dirhodium(II) complexes have been developed as catalysts for highly enantioselective metallocarbene reactions, [6] their application to CÀH amination chemistry is yet to produce the same spectacular results. [7] To date, the most effective protocol for asymmetric CÀH amination requires the combination of enantioenriched sulfoxamines as chiral auxiliaries and a chiral dirhodium(II) catalyst.[8] To address the challenge of catalytic asymmetric CÀH amination, we chose to study ruthenium(II)-pybox (pybox = pyridine bisoxazoline) complexes (Scheme 1). Despite reports that show complex 1 exhibited limited reactivity and selectivity in C À H amination reactions, [3f] we felt that the modular nature of the ligand, and the fact that the anionic ligands were independent of the chiral pybox ligand, offered us an opportunity which had not been possible by using either dirhodium(II)-or porphyrin-based catalyst systems.Ruthenium(II)-pybox complexes 1-4 were readily prepared by using the method developed by Nishiyama et al. (Scheme 1).[9] In our initial study, the challenging test substrate sulfamate ester 5[10] was treated with 1.1 equivalents of the oxidant bis(acetoxy)iodobenzene and 5 mol % catalyst 2 to give the desired product of CÀH insertion 6, albeit in modest yield and enantiomeric excess (Table 1, entries 1-3).Based on the study by Fiori and Du Bois, which demonstrates that rhodium-catalyzed C À H amination involves formation of an electrophilic metallonitrene and a build up of positive charge on the carbon center during the insertion process, we rationalized that a cationic catalyst would be more reactive than its neutral analogue.[2e] Halide abstraction from the Scheme 1. Synthesis of ruthenium(II)-pybox complexes. [b] ee [%] [b]

Mechanism and Enantioselectivity of Dirhodium-Catalyzed Intramolecular C–H Amination of Sulfamate

“…The substrate lies "above" the Rh plane defined by Rh and the ligand N and O atoms in TS2 C R, which is somewhat similar to that for the Ru-promoted reaction proposed by Blakey and co-workers. 37 The conformation of OSS TS2 C S is similar to those in the diruthenium-promoted reaction proposed by Du Bois 7a and the Rh-mediated reaction proposed by Liu et al, 15 with the substrate "standing" on the Rh plane. Therefore, the conformations of OSS TS2 C R and OSS TS2 C S adopted in the present paper show that the substrate avoids a direct approach to the five moieties in Rh 2 (S-nap) 4 and can be predicted to be the most stable C−N formation transition states toward the R and S enantiomers of the product, respectively.…”

Ruthenium‐Catalyzed Oxidation for Organic Synthesis

“…Intramolecular CÀH amination proceeds in the presence of ruthenium pybox catalyst with bis(acyloxy)iodobenzene (Eq. (7.88)) [145]. …”

Oxidations of Amines, Amides, Ethers, Sulfides, Phosphines, Arsines, Stibines and Miscellaneous Substrates

“…Oxidation of amines R 3 /PhIO/PMS/ CH 2 Cl 2 , while benzylamine was oxidised to benzaldehyde and benzhydrylamine to diphenylketone [16]. With RuX 2 (R 2 -pybox)(C 2 H 4 )/PhI(OAc) 2 /MgO/CH 2 Cl 2 sulfamate esters gave cyclic imines, thus effecting enantioselective C-H amination [17].…”