Zhaoguo Zhang

Visible-Light-Induced Decarboxylative Functionalization of Carboxylic Acids and Their Derivatives.

Visible-light-induced radical decarboxylative functionalization of carboxylic acids and their derivatives has recently received considerable attention as a novel and efficient method to create CC and CX bonds. Generally, this visible-light-promoted decarboxylation process can smoothly occur under mild reaction conditions with a broad range of substrates and an excellent functional-group tolerance. The radical species formed from the decarboxylation step can participate in not only single photocatalytic transformations, but also dual-catalytic cross-coupling reactions by combining photoredox catalysis with other catalytic processes. Recent advances in this research area are discussed herein.

CeCl3·7H2O: An Effective Additive in Ru-Catalyzed Enantioselective Hydrogenation of Aromatic α-Ketoesters

In the presence of catalytic amounts of CeCl 3.7H 2O, [RuCl(benzene)(S)-SunPhos]Cl is a highly effective catalyst for the asymmetric hydrogenation of aromatic alpha-ketoesters. A variety of ethyl alpha-hydroxy-alpha-arylacetates have been prepared in up to 98.3% ee with a TON up to 10,000. Challenging aromatic alpha-ketoesters with ortho substituents are also hydrogenated with high enantioselectivities. The addition of CeCl 3.7H 2O not only improves the enantioselectivity but also enhances the stability of the catalyst. The ratio of CeCl 3.7H 2O to [RuCl(benzene)(S)-SunPhos]Cl plays an important role in the hydrogenation reaction with a large substrate/catalyst ratio.

Highly enantioselective sequential hydrogenation of ethyl 2-oxo-4-arylbut-3-enoate to ethyl 2-hydroxy-4-arylbutyrate.

The hydrogenation of (E)-ethyl 2-oxo-4-arylbut-3-enoate with [NH2Me2](+)[{RuCl [(S)-SunPhos]}2(mu-Cl3)] gave ethyl 2-hydroxy-4-arylbutyrate with 94-96% ee. Further investigation has proved that the hydrogenation proceeded via a sequential hydrogenation of CO and CC bonds, which is sensitive to the reaction temperature. Hydrolysis of ethyl 2-hydroxy-4-phenylbutyrate (ee 93%) provided the 2-hydroxy-4-phenylbutyric acid with 81% yield at 99% ee after a single recrystallization from 1,2-dichloroethylene.

Diastereo- and enantioselective asymmetric hydrogenation of α-amido-β-keto phosphonates via dynamic kinetic resolution.

Dynamic kinetic resolution of various α-amido-β-keto phosphonates via asymmetric hydrogenation proceeded efficiently to give the corresponding β-hydroxy-α-amido phosphonates in high diastereo- and enantioselectivities (up to 99:1 syn/anti, 99.8% ee). The addition of catalytic amounts of CeCl(3)·7H(2)O is necessary to achieve both good selectivity and catalytic efficiency under mild reaction conditions.

Ru-catalyzed highly chemo- and enantioselective hydrogenation of γ-halo-γ,δ-unsaturated-β-keto esters under neutral conditions

Finely-tuned ruthenium-catalyzed highly chemoselective and enantioselective hydrogenation of γ-halo-γ,δ-unsaturated-β-keto esters at the carbonyl group was achieved under neutral reaction conditions (ee up to 97%). Both olefin and alkenyl halogen moieties, which are labile under hydrogenation conditions, remained untouched during the reaction.

Enantioselective Hydrogenation of β-Ketophosphonates with Chiral Ru(II) Catalysts

Highly effective asymmetric hydrogenation of β-ketophosphonates in the presence of Ru-(S)-SunPhos as catalyst was realized; good to excellent enantioselectivities (up to 99.9% ee) and excellent diastereoselectivities (96:4) were obtained.

Acid-labile δ-ketal-β-hydroxy esters by asymmetric hydrogenation of corresponding δ-ketal-β-keto esters in the presence of CaCO3

A series of acid-labile, optically pure ε-substituted δ-ketal-β-hydroxy esters were obtained by a Ru-SunPhos catalyzed asymmetric hydrogenation of the corresponding ε-substituted δ-ketal-β-keto esters. CaCO(3) played a dual role in the hydrogenation reaction--removing the acid generated during the formation of the catalyst and maintaining the activity of the catalyst.

Ru-Catalyzed Asymmetric Hydrogenation of γ-Heteroatom Substituted β-Keto Esters

A series of enantiomerically pure γ-heteroatom substituted β-hydroxy esters were synthesized with high enantioselectivities (up to 99.1% ee) by hydrogenation of γ-heteroatom substituted β-keto esters in the presence of Ru-(S)-SunPhos catalyst. These asymmetric hydrogenations provide key building blocks for a variety of naturally occurring and biologically active compounds.

Lewis Acid Promoted Carbon−Carbon Double-Bond Formation via Organozinc Reagents and Carbonyl Compounds

Using cheap and readily available AlCl(3) as Lewis acid, functionalized aldehydes react with organozinc reagents to give (E)-alkenes stereoselectively in high yields.

Ruthenium-Catalyzed Oxidation of Allyl Alcohols with Intermolecular Hydrogen Transfer: Synthesis of α,β-Unsaturated Carbonyl Compounds

Ruthenium-catalyzed oxidation of multisubstituted allyl alcohols in the presence of benzaldehyde gives enals or enones in good yields. Unlike the commonly reported ruthenium-catalyzed isomerization reaction of allyl alcohols to give saturated ketones, an intermolecular rather than intramolecular hydrogen transfer is involved in this transformation. This reaction offers an efficient, mild, and high-yielding method for the preparation of substituted α,β-unsaturated compounds.