Catalytic Enantioselective Reaction of Allenylnitriles with Imines Using Chiral Bis(imidazoline)s Palladium(II) Pincer Complexes

A Hydrogen Evolution Catalyst [Co2O2] Metallacycle Enables Regioselective Allene C(sp2)-H Functionalization

Selective functionalization of allenic C(sp2)-H is an ideal approach to upgrading simple allenes to synthetically useful allenes, albeit suffering from challenges associated with inert reactivity and inferior selectivity. Inspired by energy chemistry, a catalytic hydrogen evolution reaction (HER) strategy was leveraged to selectively activate weakly acidic allene C(sp2)-H bonds in a reductive mode. An array of [Co2O2] metallacycle complexes were readily devised starting from amino acids, and they were demonstrated as robust HER catalysts, which would selectively break allenic C(sp2)-H bonds to release hydrogen. With the newly developed HER catalyst, regioselective electrochemical functionalization of allenic C(sp2)-H with alcoholic α C(sp3)-H was unprecedentedly achieved. This strategy features excellent regioselectivity, unconventional chemoselectivity, good functional-group tolerance (62 examples), and mild conditions. Mechanism experiments revealed a reactive hydroxy-coordinated cobalt(II) species in the reaction. Density functional theory (DFT) calculations were also conducted to rationalize the regioselectivity observed in the reaction.

Synthesis of 1,1-Disubstituted Allenylic Silyl Ethers Through Iron-Catalyzed Regioselective C(sp2)─H Functionalization of Allenes

We report a synthesis of allenylic silyl ethers through iron-catalyzed functionalization of the C(sp2)─H bonds of monosubstituted alkylallenes. In the presence of a cyclopentadienyliron dicarbonyl based catalyst and triisopropylsilyl triflate as a silylation agent, a variety of aryl aldehydes were suitable coupling partners in this transformation, furnishing a collection of 1,1-disubstituted allenylic triisopropylsilyl ethers as products in moderate to excellent yields as a single regioisomer. Lithium bistriflimide was identified as a critical additive in this transformation. The optimized protocol was scalable, and the products were amenable to further transformation to give a number of unsaturated, polyfunctional derivatives.

Photo and copper dual catalysis for allene syntheses from propargylic derivatives via one-electron process

Different from the traditional two-electron oxidative addition-transmetalation-reductive elimination coupling strategy, visible light has been successfully integrated into transition metal-catalyzed coupling reaction of propargylic alcohol derivatives highly selectively forming allenenitriles: specifically speaking, visible light-mediated Cu-catalyzed cyanation of propargylic oxalates has been realized for the general, efficient, and exclusive syntheses of di-, tri, and tetra-substituted allenenitriles bearing various synthetically versatile functional groups. A set of mechanistic studies, including fluorescence quenching experiments, cyclic voltammetric measurements, radical trapping experiments, control experiments with different photocatalyst, and DFT calculation studies have proven that the current reaction proceeds via visible light-induced redox-neutral reductive quenching radical mechanism, which is a completely different approach as compared to the traditional transition metal-catalyzed two-electron oxidative addition processes.

Diastereoselective Direct Catalytic Asymmetric Mannich-Type Reactions of Alkylnitriles with a Ni(II)-Carbene Complex

Despite recent advances in reactions using alkylnitriles as carbon nucleophiles, diastereoselective direct catalytic asymmetric reactions, in which two consecutive chiral centers could be controlled, remain largely unexplored. Herein, we report the addition of alkylnitriles (such as propionitrile) to imines in the presence of a catalytic amount of a chiral pincer-type Ni-carbene complex and potassium 2,6-di-tert-butyl-4-methylphenoxide (KBHT). BHT and molecular sieves were used as additives to improve the yields, diastereoselectivity, and enantiomeric ratio up to >99%, 90:10 anti/syn, and 97.5:2.5 er, respectively. The Mannich adducts can be readily converted to the corresponding β-amino acids.

Enantioselective reaction of N-cyano imines: decarboxylative Mannich-type reaction with malonic acid half thioesters

The enantioselective reaction of imines bearing a cyano group as an activating group with malonic acid half thioesters gave chiral cyanamide derivatives with high enantioselectivity. The density functional theory (DFT) calculation clarified the stereochemical outcome and importance of the N-cyano group for imines.

Chiral (phosphine)-(imidazoline) PCN pincer palladium(II) complexes: synthesis and application in asymmetric hydrophosphination of 2-alkenoylpyridines with diphenylphosphine

A series of new chiral PCN pincer Pd(II) complexes 3a−l with aryl-based (phosphine)-(imidazoline) ligands were conveniently synthesized from readily available starting materials with the key step being phosphination/C−H palladation reaction....

Contrasteric coupling of allenes and tetrahydroisoquinolines by iron-catalysed allenic C(sp2)-H functionalisation

An iron-catalysed C-H functionalisation of simple monosubstituted allenes for the synthesis of 1-tetrahydroisoquinolinyl 1,1-disubstituted allenes is reported. This transformation represents the first example of a direct conversion of allenic C-H bonds to C-C bonds through cross dehydrogenative coupling. The optimized protocol features broad scope and employs mild, functional group tolerant conditions.

Iron-Catalyzed Contrasteric Functionalization of Allenic C(sp2)-H Bonds: Synthesis of α-Aminoalkyl 1,1-Disubstituted Allenes

An iron-catalyzed C-H functionalization of simple monosubstituted allenes is reported. An efficient protocol for this process was made possible by the use of a newly developed electron-rich and sterically hindered cationic cyclopentadienyliron dicarbonyl complex as the catalyst and N-sulfonyl hemiaminal ether reagents as precursors to iminium ion electrophiles. Under optimized conditions, the use of a mild, functional-group-tolerant base enabled the conversion of a range of monoalkyl allenes to their allenylic sulfonamido 1,1-disubstituted derivatives, a previously unreported and contrasteric regiochemical outcome for the C-H functionalization of electronically unbiased and directing-group-free allenes.

Access to Allene-Containing Molecules via Enantioselective Reactions of Azolium Cumulenolate Intermediates

Azolium cumulenolates are a special type of intermediates in N-heterocyclic carbene catalysis. They contain elongated linear structures with three contiguous C=C bonds and sterically unhindered α-carbon atoms. These structural features make it difficult to develop enantioselective reactions for these intermediates. Here we disclose the first carbene-catalyzed highly enantioselective addition reactions of azolium cumulenolates. The reaction starts with alkynals as the precursors for azolium cumulenolate intermediates that undergo enantioselective addition to activated ketones. From the same set of substrates, both allene and spirooxindole products can be obtained with high yields and excellent enantioselectivities. The allene moieties in our optically enriched products carry rich reactivities and can be transformed to diverse molecules. The spirooxindole scaffolds in our products are important structural motifs in natural products and medicines.

Direct Catalytic Asymmetric Addition of Alkylnitriles to Aldehydes with Designed Nickel-Carbene Complexes

A direct catalytic asymmetric addition of acetonitrile to aldehydes that realizes over 90 % ee is the ultimate challenge in alkylnitrile addition chemistry. Herein, we report achieving high enantioselectivity by the strategic use of a sterically demanding Ni^II pincer carbene complex, which afforded highly enantioenriched β-hydroxynitriles. This highly atom-economical process paves the way for exploiting inexpensive acetonitrile as a promising C2 building block in a practical synthetic toolbox for asymmetric catalysis.

Synthesis and application in asymmetric catalysis of P-stereogenic pincer–metal complexes

P-stereogenic pincer: synthesis and application in asymmetric catalysis.

Chiral palladium pincer complexes for asymmetric catalytic reactions

Palladium pincer complexes, containing a monoanionic terdentate ligand composed of an anionic aryl carbon atom and two mutually compatible donor sites, have aroused considerable interest since their first reports in the late 1970s. The high stability of the Pd pincer complexes and particularly their high modularity make these species ideal candidates for catalysis. Furthermore, the nature of the meridional coordination of the pincer ligands, and along with this their ability to enforce a stereo-specific environment around the Pd center, provide a good opportunity for developing chiral Pd pincer catalysts. Thus, a broad variety of chiral Pd pincer complexes have been prepared by the introduction of various stereochemical centers in the pincer skeletons. These chiral Pd pincer complexes have been successfully applied to many asymmetric catalytic reactions such as hydrophosphination reactions, allylation of aldehydes and imines, Michael and aldol reactions, Suzuki-Miyaura reactions as well as reactions of nitrile compounds with imines. This review focuses on the synthetic methods and the applications of chiral Pd pincer complexes in asymmetric catalysis.

Catalytic Asymmetric Mannich Reaction of α-Fluoronitriles with Ketimines: Enantioselective and Diastereodivergent Construction of Vicinal Tetrasubstituted Stereocenters

Diastereodivergent and enantioselective conversion of isatin ketimines to α-fluoro-β-aminonitriles with vicinal tetrasubstituted stereocenters is achieved by a chiral copper complex/guanidine base catalyzed Mannich reaction with proper choice of the bisphosphine ligand. The reaction is broad in scope, scalable, and provides efficient access to a series of 3-aminoindolinones exhibiting a quaternary carbon-fluorine stereocenter with high yields and stereoselectivities. Selective transformations of the Mannich reaction products into multifunctional 3-aminooxindoles without erosion of enantiomeric and diastereomeric purity highlight the synthetic utility.

Enantioselective conjugate addition of an α,α-dithioacetonitrile with nitroalkenes using chiral bis(imidazoline)–Pd complexes

The first highly enantioselective reaction of α,α-dithioacetonitriles with nitroolefins as electron deficient olefins using chiral Phebim–Pd catalysts was developed.

Intramolecular alkene hydroamination and degradation of amidines: divergent behavior of rare earth metal amidinate intermediates

The methods for catalytic intramolecular alkene hydroamination and degradation of amidines have been established. Furthermore, a tandem reconstruction/cyclization of amidines has also been developed.

Enantioselective aza-Friedel–Crafts reaction of cyclic ketimines with indoles using chiral imidazoline–phosphoric acid catalysts

A highly enantioselective aza-Friedel Crafts reaction of cyclic 4-aryl-3-oxo-1,2,5-thiadiazol-1,1-oxides as cyclic ketimines with indoles was developed.

Direct catalytic enantioselective Mannich-type reaction of α,α-dithioacetonitriles with imines using chiral bis(imidazoline)–Pd complexes

The first highly enantioselective reaction of α,α-dithioacetonitriles with imines has been developed.