Synthesis and Characterization of Bimetallic Nanoclusters Stabilized by Chiral and Achiral Polyvinylpyrrolidinones. Catalytic C(sp³)–H Oxidation

Abstract

Second-generation chiral-substituted poly-N-vinylpyrrolidinones (CSPVPs) (−)-1R and (+)-1S were synthesized by free-radical polymerization of (3aR,6aR)-and (3aS,6aS)-5-ethenyltetrahydro-2,2-dimethyl-4H-1,3-dioxolo [4,5-c]pyrrol-4-one, respectively, using thermal and photochemical reactions. They were produced from respective D-isoascorbic acid and D-ribose. In addition, chiral polymer (−)-2 was also synthesized from the polymerization of (S)-3-(methoxymethoxy)-1-vinylpyrrolidin-2one. Molecular weights of these chiral polymers were measured using HRMS, and the polymer chain tacticity was studied using 13 C NMR spectroscopy. Chiral polymers (−)-1R, (+)-1S, and (−)-2 along with poly-N-vinylpyrrolidinone (PVP, MW 40K) were separately used in the stabilization of Cu/Au or Pd/Au nanoclusters. CD spectra of the bimetallic nanoclusters stabilized by (−)-1R and (+)-1S showed close to mirror-imaged CD absorption bands at wavelengths 200−300 nm, revealing that bimetallic nanoclusters' chiroptical responses are derived from chiral polymer-encapsulated nanomaterials. Chemo-, regio-, and stereoselectivity was found in the catalytic C−H group oxidation reactions of complex bioactive natural products, such as ambroxide, menthofuran, boldine, estrone, dehydroabietylamine, 9-allogibberic acid, and sclareolide, and substituted adamantane molecules, when catalyst Cu/Au (3:1) or Pd/Au (3:1) stabilized by CSPVPs or PVP and oxidant H 2 O 2 or t-BuOOH were applied. Oxidation of (+)-boldine N-oxide 23 using NMO as an oxidant yielded 4,5-dehydroboldine 27, and oxidation of (−)-9-allogibberic acid yielded C6,15 lactone 47 and C6-ketone 48.

Indication of Pd–C or Cu–C Intermediates in Bimetallic Nanoclusters During Pd/Au-PVP- or Cu/Au-PVP-Catalyzed Oxidations of endo-4-Oxatricyclo[5.2.1.02,6]-8-decene and Tetrahydro-γ-carbolines

“…After testing different lengths, PVP having a molecular weight of ~40,000 was found to be suitable for the stabilization of Cu/Au or Pd/Au bimetallic nanoclusters. 2a Consequently, it was selected for our studies in search of bimolecular coupling reactions. Bimetallic Cu/Au (3:1)-PVP nanoclusters were prepared by treatment of a mixture of 3 equivalents of CuCl, 1 equivalent of HAuCl 4 ·3H 2 O, and 0.11 equivalents of PVP (MW 40,000) in H 2 O with 7 equivalents of NaBH 4 at 25 °C for 2 hours.…”

“…Monometallic Cu, Pd, or Au nanoclusters did not provide the same reactivity as those derived from bimetallic nanoclusters. 2a…”

“…Bimetallic Cu/Au (3:1)-PVP nanoclusters were prepared by treatment of a mixture of 3 equivalents of CuCl, 1 equivalent of HAuCl 4 ·3H 2 O, and 0.11 equivalents of PVP (MW 40,000) in H 2 O with 7 equivalents of NaBH 4 at 25 °C for 2 hours. 2a Similarly, Pd/Au (3:1)-PVP was prepared from the reaction of 3 equivalents of Na 2 PdCl 4 , 1 equivalent of HAuCl 4 ·3H 2 O, 0.11 equivalents of PVP (MW 40,000) and 10 equivalents of NaBH 4 in H 2 O at 25 °C. 2c Different concentrations can be used in the preparations, and a final concentration of either 10 mM or 25 mM was prepared.…”

“…Alternatively, reductive elimination in complex III provides product 2 and Cu/Au-PVP. The formation of II or III may also involve a (HO)Cu II (OH)/Au-(PVP) reactive species, 2a which undergoes hydrogen atom abstraction from the C–H adjacent to the ether group. The resulting carbon intermediate then extracts a hydroxy group from H 2 O 2 or Cu I (OH)/Au-(PVP).…”

Copper Sulfate (CuSO4): An Efficient Reagent in Organic Synthesis

“…9 (G) Huafang Fan et al synthesized vinyl lactams using a tree-steps reaction involving oxidative cleavage of Disoascorbic acid, followed by acetonide formation of the resulting D-erythronolactone catalyzed by copper sulfate and, per last, ring opening of the lactone with sodium azide, with 39% yield. 14 (H) Jeffrey R. Groch, Nicholas R. Lauta and Jon T. Njardarson synthesized two chiral sulfonimines using the copper sulfate to drive imine formation in the final product with varying yields of 60-89%. 15 (I) Alex A. Hunt-Painter et al proposed a synthetic rout for the synthesis of an iminosugar using a method of protection of isopropylidene groups using acetone, sulfuric acid and copper sulfate for a subsequent iodination with 78% yield over two steps.…”