Chang-Bin Yu

Pd-Catalyzed Asymmetric Hydrogenation of Unprotected Indoles Activated by Brønsted Acids

The first highly enantioselective hydrogenation of simple indoles was developed with a Brønsted acid as an activator to form the iminium intermediate in situ, which was hydrogenated using Pd(OCOCF(3))(2)/(R)-H8-BINAP catalyst system with up to 96% ee. The present method provides an efficient route to enantioenriched 2-substituted and 2,3-disubstituted indolines.

Biomimetic Asymmetric Hydrogenation: In Situ Regenerable Hantzsch Esters for Asymmetric Hydrogenation of Benzoxazinones

A catalytic amount of Hantzsch ester that could be regenerated in situ by Ru complexes under hydrogen gas has been employed in the biomimetic asymmetric hydrogenation of benzoxazinones with up to 99% ee in the presence of chiral phosphoric acid. The use of hydrogen gas as a reductant for the regeneration of Hantzsch esters makes this hydrogenation an ideal atom economic process.

Dehydration triggered asymmetric hydrogenation of 3-(α-hydroxyalkyl)indoles

Highly enantioselective hydrogenation of 3-(α-hydroxyalkyl)indoles promoted by a Bronsted acid for dehydration to form a vinylogous iminium intermediate in situ was developed with Pd(OCOCF3)2/(R)-H8-BINAP as catalyst with up to 97% ee. This methodology provides an efficient and rapid access to chiral 2,3-disubstituted indolines.

Enantioselective synthesis of cyclic sulfamidates via Pd-catalyzed hydrogenation

Using Pd(CF3CO2) 2/(S,S)-f-binaphane as the catalyst, an efficient enantioselective synthesis of cyclic sulfamidates was developed via asymmetric hydrogenation of the corresponding cyclic imines in 2,2,2-trifluoroethanol at room temperature with high enantioselectivities (up to 99% ee).

Highly Enantioselective Partial Hydrogenation of Simple Pyrroles: A Facile Access to Chiral 1-Pyrrolines

A highly enantioselective Pd-catalyzed partial hydrogenation of simple 2,5-disubstituted pyrroles with a Brønsted acid as an activator has been successfully developed, providing chiral 2,5-disubstituted 1-pyrrolines with up to 92% ee.

Homogenous Pd-Catalyzed Asymmetric Hydrogenation of Unprotected Indoles: Scope and Mechanistic Studies

An efficient palladium-catalyzed asymmetric hydrogenation of a variety of unprotected indoles has been developed that gives up to 98% ee using a strong Brønsted acid as the activator. This methodology was applied in the facile synthesis of biologically active products containing a chiral indoline skeleton. The mechanism of Pd-catalyzed asymmetric hydrogenation was investigated as well. Isotope-labeling reactions and ESI-HRMS proved that an iminium salt formed by protonation of the C═C bond of indoles was the significant intermediate in this reaction. The important proposed active catalytic Pd-H species was observed with (1)H NMR spectroscopy. It was found that proton exchange between the Pd-H active species and solvent trifluoroethanol (TFE) did not occur, although this proton exchange had been previously observed between metal hydrides and alcoholic solvents. Density functional theory calculations were also carried out to give further insight into the mechanism of Pd-catalyzed asymmetric hydrogenation of indoles. This combination of experimental and theoretical studies suggests that Pd-catalyzed hydrogenation goes through a stepwise outer-sphere and ionic hydrogenation mechanism. The activation of hydrogen gas is a heterolytic process assisted by trifluoroacetate of Pd complex via a six-membered-ring transition state. The reaction proceeds well in polar solvent TFE owing to its ability to stabilize the ionic intermediates in the Pd-H generation step. The strong Brønsted acid activator can remarkably decrease the energy barrier for both Pd-H generation and hydrogenation. The high enantioselectivity arises from a hydrogen-bonding interaction between N-H of the iminium salt and oxygen of the coordinated trifluoroacetate in the eight-membered-ring transition state for hydride transfer, while the active chiral Pd complex is a typical bifunctional catalyst, effecting both the hydrogenation and hydrogen-bonding interaction between the iminium salt and the coordinated trifluoroacetate of Pd complex. Notably, the Pd-catalyzed asymmetric hydrogenation is relatively tolerant to oxygen, acid, and water.

Enantioselective Pd-catalyzed hydrogenation of fluorinated imines: facile access to chiral fluorinated amines.

An enantioselective hydrogenation of simple fluorinated imines has been developed using Pd(OCOCF(3))(2)/(R)-Cl-MeO-BIPHEP as a catalyst, and up to 94% ee was achieved. This method provides an efficient route to enantioenriched fluorinated amines.

Highly enantioselective synthesis of sultams via Pd-catalyzed hydrogenation.

Using pd(cf(3)co(2))2/(S,S)-f-Binaphane as the catalyst, an efficient enantioselective synthesis of sultams was developed via asymmetric hydrogenation of the corresponding cyclic imines with high enantioselectivities. The hydrogenation products can be conveniently transformed to chiral homoallylic amines without loss of enantioselectivity.

Synthesis and enantioselective hydrogenation of seven-membered cyclic imines: substituted dibenzo[b,f][1,4]oxazepines

Highly enantioselective hydrogenation of seven-membered cyclic imines, substituted dibenzo[b,f][1,4]oxazepines, was achieved, with up to 94% ee, by using the [Ir(COD)Cl](2)/(S)-Xyl-C(3)*-TunePhos complex as the catalyst in the presence of morpholine-HCl.

Enantioselective Pd-catalyzed hydrogenation of enesulfonamides

Asymmetric hydrogenation of cyclic enesulfonamides affords chiral cyclic sulfonamides using Pd(OCOCF(3))(2)/diphosphine complexes as catalysts with up to 98% ee.