Efficient Access to Chiral Benzhydrols via Asymmetric Transfer Hydrogenation of Unsymmetrical Benzophenones with Bifunctional Oxo-Tethered Ruthenium Catalysts

Abstract

A concise asymmetric transfer hydrogenation of diaryl ketones, promoted by bifunctional Ru complexes with an etherial linkage between 1,2-diphenylethylenediamine (DPEN) and η(6)-arene ligands, was successfully developed. Because of the effective discrimination of substituents at the ortho position on the aryl group, unsymmetrical benzophenones were smoothly reduced in a 5:2 mixture of formic acid and triethylamine with an unprecedented level of excellent enantioselectivity. For the non-ortho-substituted benzophenones, the oxo-tethered catalyst electronically discerned biased substrates, resulting in attractive performance yielding chiral diarylmethanols with >99% ee.

{Ni4O4} Cluster Complex to Enhance the Reductive Photocurrent Response on Silicon Nanowire Photocathodes

“…As an alternative to fossil fuels, H 2 will play an important role for the future because it is storable, transportable, can be converted to hydrocarbon fuels using the Fischer-Tropsch or Sabatier process and can be converted into electrical energy using fuel cells [ 2 ]. Following the work by Fujishima and Honda [ 1 ], there has been enormous interest in the field of semiconductor materials for photocatalysis and photoelectrolysis, specifically on the mechanisms that are involved in photoelectrochemical (PEC) water splitting [ 3 , 4 , 5 , 6 , 7 , 8 ]. The photocatalytic approach for water splitting can be divided into two types.…”

Organocatalytic Redox Deracemization of Cyclic Benzylic Ethers Enabled by An Acetal Pool Strategy

“…[12] However,c atalytic enantioselective methods to access the optically pure compounds remain elusive. [13] 6H-Benzo [c]chromene 5a was initially selected as am odel substrate,a nd the combination of Hantzsch ester 6 and chiral phosphoric acid 7a was chosen as the asymmetric transfer hydrogenation system for optimization (Table 1). [14,15] To clearly understand the details of both oxidation and reduction steps,w ep erformed the oxidation prior to the addition of 6 and 7a.Asexpected, the oxidation of secondary ether 5a proved to be challenging.Common oxidants that are effective in oxidizing primary benzylic ethers,such as TBHP, Ph 3 CBF 4 ,N a 2 S 2 O 8 ,a nd PhI(OAc) 2 ,e xhibited no reactivity toward 5a ( Table 1, entry 1).…”

“…Ther eaction without molecular sieves proceeded in inferior yield and enantioselectivity,p resumably due to the influence of moisture (entries 7, 8, and 12). Chiral phosphoric acid catalysts were screened extensively,a nd it was found that the C 2symmetric imidodiphosphoric acid 8d gave the highest level of enantiofacial discrimination (entries 8a nd [13][14][15][16][17]. [14d] Solvent optimization revealed that the reaction in am ixture of methyl tert-butyl ether (MTBE) and CH 2 Cl 2 was most efficient, giving 5a in 93 %y ield with 93 % ee ( Table 1, entry 18).…”

Organocatalytic Redox Deracemization of Cyclic Benzylic Ethers Enabled by An Acetal Pool Strategy

“…[10] Herein, we report the first redox deracemization of awide array of cyclic benzylic ethers on the basis of this "acetal pool" strategy. [13] 6H-Benzo [c]chromene 5a was initially selected as am odel substrate,a nd the combination of Hantzsch ester 6 and chiral phosphoric acid 7a was chosen as the asymmetric transfer hydrogenation system for optimization (Table 1). [12] However,c atalytic enantioselective methods to access the optically pure compounds remain elusive.…”

“…To our delight, these additives effected complete oxidation with 1.05 equivalents of the oxidant. Chiral phosphoric acid catalysts were screened extensively,a nd it was found that the C 2symmetric imidodiphosphoric acid 8d gave the highest level of enantiofacial discrimination (entries 8a nd [13][14][15][16][17]. Ther eaction without molecular sieves proceeded in inferior yield and enantioselectivity,p resumably due to the influence of moisture (entries 7, 8, and 12).…”