Asymmetric Bromination/Semipinacol Rearrangement Enabled by Brønsted Acids of Stereogenic-at-Cobalt(III) Complexes.

On the basis of the chiral ion pair between catalytic anionic stereogenic-at-cobalt(III) complexes and halonium ion intermediates, an asymmetric halogenation/semipinacol rearrangement reaction has been established using N-bromosuccinimide as the halogen source. This protocol provides an alternative approach for accessing a diverse set of chiral ß-bromocycloketones in good yields with high enantioselectivities (≤96% yield, 95.5:4.5 er).

Chiral Carborane Acids Decorated with Binol-Based Phosphonates: Synthesis, Characterization, and Application.

The syntheses of hexabrominated closo-carborates decorated with different chiral Binol-derived phosphonates and their conjugate acids are described. X-ray diffraction analysis reveals a polymeric structure for the sodium salt with the anionic units connected by [B-Br-Na-O═P]+ linkages. For the acid, coordination of the proton to the phosphonate's P═O oxygen atom is assumed. The pKa value was estimated by combining experiments and computations. Application of these Brønsted acids as chiral catalysts in an imino-ene and a Mukaiyama-Mannich reaction was moderately successful.

Enantioselective Access to Chiral 2,5-Diketopiperazines via Stereogenic-at-Cobalt(III)-Catalyzed Ugi-4CRs/Cyclization Sequences.

An asymmetric synthesis of chiral 2,5-diketopiperazines by the Ugi-4CR/cyclization is exhibited. The employment of catalytic anionic chiral Co(III) complexes delivered α-propiolyl aminoamides in high yields with excellent enantioselectivities (31 examples, up to 95% ee). The following treatment of Ugi-adducts with PPh3 leads to chiral 2,5-DKPs without significant loss of enantioselectivities (26 examples, up to 91% ee).

Atroposelective Silylation of 1,1'-Biaryl-2,6-diols by a Chiral Counteranion Directed Desymmetrization Enhanced by a Subsequent Kinetic Resolution.

A desymmetrizing silylation of aromatic diols is reported. The previously unknown asymmetric silyl ether formation of phenol derivatives is achieved by applying List's counteranion directed silylation technique. A silylium-ion-like silicon electrophile generated from an allylic silane paired with an imidodiphosphorimidate (IDPi) enables enantioselective discrimination of achiral 1,1'-biaryl-2,6-diols. The enantioselectivity of that desymmetrization is further improved by a downstream kinetic resolution, converting the monosilylated minor enantiomer into the corresponding, again achiral bissilylated diol.

Hybrid Palladium Catalyst Assembled from Chiral Phosphoric Acid and Thioamide for Enantioselective ß-C(sp3 )-H Arylation.

A hybrid palladium catalyst assembled from a chiral phosphoric acid (CPA) and thioamide enables a highly efficient and enantioselective ß-C(sp3 )-H functionalization of thioamides (up to 99 % yield, 97 % ee). A kinetic resolution of unsymmetrical thioamides by intermolecular C(sp3 )-H arylation can be achieved with high s-factors. Mechanistic investigations have revealed that stereocontrol is achieved by embedding the substrate in a robust chiral cavity defined by the bulky CPA and a neutral thioamide ligand.

Assembling a Hybrid Pd Catalyst from a Chiral Anionic CoIII Complex and Ligand for Asymmetric C(sp3 )-H Functionalization.

An unusual hybrid palladium catalyst containing an anionic chiral CoIII complex and a chiral phosphoramidite ligand shows a high capacity for catalyzing asymmetric thioamide-directed C(sp3 )-H arylation and delivers excellent yield and enantioselectivity (up to 99 % yield, 99 % ee). Significant synergy between the chiral ligand and the anion in terms of stereochemical control was observed. Mechanistic investigations have revealed both the nature of the C-H activation and the origin of the enantioselectivity.

Enantioselective Bromocyclization of Tryptamines Induced by Chiral Co(III)-Complex-Templated Brønsted Acids under an Air Atmosphere.

The chiral Co(III)-complex-templated Brønsted acids were found to be efficient bifunctional phase-transfer catalysts for the highly enantioselective bromocyclization of protected tryptamines with readily available N-bromosuccinimide (NBS) under an air atmosphere. The 3-bromohexahydropyrrolo[2,3- b]indoles, which are key building blocks of cyclotryptamine alkaloids, were thus obtained in up to 95% yield and 93.5:6.5 er.

Brønsted acids of anionic chiral Co(iii) complexes as catalysts for the stereoselective synthesis of cis-4-aminofuranobenzopyrans.

A highly enantioselective interrupted Povarov reaction of salicylaldimines and 2,3-dihydrofuran was developed, through the elegant Brønsted acid catalysis of anionic chiral Co(iii) complexes. This reaction affords the cis-4-aminofuranobenzopyran derivatives with up to 95% yield, >20 : 1 dr and 96 : 4 er. Moreover, a one-pot three-component procedure of salicylaldehydes, anilines, and 2,3-dihydrofuran proves to be successful, with higher reaction efficiency.

Switchable Stereoselectivity in Bromoaminocyclization of Olefins: Using Brønsted Acids of Anionic Chiral Cobalt(III) Complexes.

Brønsted acids of anionic chiral CoIII complexes act as bifunctional phase-transfer catalysts to shuttle the substrates across the solvent interface and control stereoselectivity. The diastereomeric chiral CoIII -templated Brønsted acids, with the same chiral ligands, enabled a switch in the enantioselective bromoaminocyclization of olefins to afford the two enantiomers of 2-substituted pyrrolidines with high enantioselectivities (up to 99:1 e.r.).

Sodium salts of anionic chiral cobalt(III) complexes as catalysts of the enantioselective Povarov reaction.

The sodium salts of anionic chiral cobalt(III) complexes (CCC(-) Na(+) ) have been found to be efficient catalysts of the asymmetric Povarov reaction of easily accessible dienophiles, such as 2,3-dihydrofuran, ethyl vinyl ether, and an N-protected 2,3-dihydropyrrole, with 2-azadienes. Ring-fused tetrahydroquinolines with up to three contiguous stereogenic centers were thus obtained in high yields, excellent diastereoselectivities (endo/exo up to >20:1), and high enantioselectivities (up to 95:5 e.r.).

Enantioselective Organocatalyzed Cascade Dearomatizing Spirocycloaddition Reactions of Indole-Ynones.

An intramolecular organocatalytic cascade dearomatizing spirocycloaddition reaction of indole-ynone compounds containing O-silyl-naphthol substituents has been developed with the use of a chiral bifunctional thiourea. This process was able to provide various structurally diverse polycyclic spiroindolines in high yields (up to 98%) with excellent stereoselectivities (>20:1 dr, up to 98% ee) involving the formation of carbonylvinylidene ortho-quinone methide intermediates.

Enantioselective synthesis of 1-aminoindene derivatives via asymmetric Brønsted acid catalysis.

We describe a catalytic asymmetric iminium ion cyclization reaction of simple 2-alkenylbenzaldimines using a BINOL-derived chiral N-triflyl phosphoramide. The corresponding 1-aminoindenes and tetracyclic 1-aminoindanes are formed in good yields and high enantioselectivities. Further, the chemical utility of the obtained enantiopure 1-aminoindene is demonstrated for the asymmetric synthesis of (S)-rasagiline.