Chiral-Substituted Poly-N-vinylpyrrolidinones and Bimetallic Nanoclusters in Catalytic Asymmetric Oxidation Reactions

Abstract

A new class of poly-N-vinylpyrrolidinones containing an asymmetric center at C5 of the pyrrolidinone ring were synthesized from l-amino acids. The polymers, particularly 17, were used to stabilize nanoclusters such as Pd/Au for the catalytic asymmetric oxidations of 1,3- and 1,2-cycloalkanediols and alkenes, and Cu/Au was used for C-H oxidation of cycloalkanes. It was found that the bulkier the C5 substituent in the pyrrolidinone ring, the greater the optical yields produced. Both oxidative kinetic resolution of (±)-1,3- and 1,2-trans-cycloalkanediols and desymmetrization of meso cis-diols took place with 0.15 mol % Pd/Au (3:1)-17 under oxygen atmosphere in water to give excellent chemical and optical yields of (S)-hydroxy ketones. Various alkenes were oxidized with 0.5 mol % Pd/Au (3:1)-17 under 30 psi of oxygen in water to give the dihydroxylated products in >93% ee. Oxidation of (R)-limonene at 25 °C occurred at the C-1,2-cyclic alkene function yielding (1S,2R,4R)-dihydroxylimonene 49 in 92% yield. Importantly, cycloalkanes were oxidized with 1 mol % Cu/Au (3:1)-17 and 30% HO in acetonitrile to afford chiral ketones in very good to excellent chemical and optical yields. Alkene function was not oxidized under the reaction conditions. Mechanisms were proposed for the oxidation reactions, and observed stereo- and regio-chemistry were summarized.

Highly Enantioselective C−H Oxidation of Arylalkanes with H2O2 in the Presence of Chiral Mn‐Aminopyridine Complexes

“…The oxidant efficiency was moderate, 70 μmol of H 2 O 2 affording 10–20 μmol of products (alcohol+ketone, Table ). Nevertheless, this efficiency is comparable with that of the system of Costas et al., which requires 3.5 equivalents of H 2 O 2 , and is much better than for the Cu/Au nanoparticles‐based system of Hua and co‐workers, which requires approximately 15 equivalents of H 2 O 2 …”

Enantioselective Dihydroxylation of Alkenes Catalyzed by 1,4‐Bis(9‐O‐dihydroquinidinyl)phthalazine‐Modified Binaphthyl–Osmium Nanoparticles

“…trans ‐β‐Methylstyrene mainly gave syn adduct (1 R ,2 R )‐ 2 g in 93 % yield with 97.1 % ee , whereas cis ‐β‐methylstyrene gave syn adduct (1 R ,2 S )‐ 2 h in 81 % yield with 96.5 % ee . The results suggest a Si‐attack process from the C1 position in this system . Cinnamates, including those with methyl ester, ethyl ester, and isopropyl ester groups, performed well in the system, and products 2 i – k were obtained with excellent ee values over 98 %.…”

“…In addition to ligand stabilization, polymer‐stabilized nanoparticles, mainly benefiting from steric hindrance, have also been developed for asymmetric reactions (Figure b). Among these studies, leading scientists, including Kobayashi, Toste, Glorius, Morris, and Hua, have expanded the catalytic applications into asymmetric 1,4‐addition, asymmetric intermolecular cyclopropanation, asymmetric arylation, asymmetric transfer hydrogenation, and asymmetric oxidation, respectively.…”

“…However, recovery and recycling of the osmium catalyst remains unsatisfactory, and these reactions have limited substrate scope and result in poor enantioselectivity; moreover, metal leaching is observed mostly as a result of weak interaction of Os with the support. Most importantly, metal nanoparticles have not been well investigated in AD . We envisioned that the prevalent metal–carbon bond stabilization might be an alternative approach to enable efficient nanocatalytic asymmetric dihydroxylation.…”

Molybdenum-catalyzed asymmetric anti-dihydroxylation of allylic alcohols

“…However, the toxicity, volatility, and high cost of OsO 4 urge organic chemists to establish alternative catalytic systems for asymmetric dihydroxylation utilizing inexpensive and less toxic catalysts. In the past two decades, a number of Os-free asymmetric syn-dihydroxylations have been developed 11 , including the examples using chiral Mn- 12,13 or Fe- 14,15 complexes, bimetallic nanoclusters 16 and organic phase transfers as catalysts 17,18 . In the field of anti-dihydroxylation, highly enantioselective variants are scarce.…”