Dinuclear Zn-Catalytic System as Brønsted Base and Lewis Acid for Enantioselectivity in Same Chiral Environment

Zinc (Zn) is a crucial element with remarkable significance in organic transformations. The profusion of harmless zinc salts in the Earth's outer layer qualifies zinc as a noteworthy contender for inexpensive and eco-friendly reagents and catalysts. Recently, widely recognized uses of organo-Zn compounds in the field of organic synthesis have undergone extensive expansion toward asymmetric transformations. The ProPhenol ligand, a member of the chiral nitrogenous-crown family, exhibits the spontaneous formation of a dual-metal complex when reacted with alkyl metal (R-M) reagents, e.g., ZnEt2. The afforded Zn complex possesses two active sites, one Lewis acid and the other Brønsted base, thereby facilitating the activation of nucleophiles and electrophiles simultaneously within the same chiral pocket. In this comprehensive analysis, we provide a thorough account of the advancement and synthetic potential of these diverse catalysts in organic synthesis, while emphasizing the reactivity and selectivities, i.e., dr and ee due to the design/structure of the ligands employed.

Ruthenium-Catalyzed 1,6-Hydroalkylation of para-Quinone Methides with Ketones via the in Situ Activation of C(sp3)-H Bonds

A simple and efficient method for the ruthenium-catalyzed 1,6-hydroalkylation of para-quinone methides (p-QMs) with ketones via the in situ activation of C(sp3)-H bonds has been disclosed. Without the need for preactivation of the substrates and oxidant, a broad range of p-QMs and ketones are well tolerated, producing the expected 1,6-hydroalkylation products with moderate to good yields. Step-by-step control experiments and DFT calculation were conducted systematically to gain insights for the plausible reaction mechanism. This finding may have potential application in the selective diarylmethylation of ketones at the α-C position in organic synthesis.

Dinuclear Zinc-Catalyzed Asymmetric Desymmetrization of Cyclopentendiones: Access to Functional Cyclopentanediones Bearing an All-carbon Quaternary Stereocenter

The success in the identification of the two enantioisomeric surfaces of electrophiles by dinuclear zinc catalysts is disclosed. This protocol realizes a dinuclear zinc-cocatalyzed desymmetrization of cyclopentendiones using α-hydroxy aryl ketones as nucleophiles through Michael addition reaction. Under mild conditions, a series of functional cyclopentanediones bearing multiple stereogenic centers including an all-carbon quaternary stereocenter, were obtained in moderate to good yields with excellent stereoselectivities.

Catalytic Asymmetric Umpolung Tandem Reactions of Hemiacetals via Dinuclear Zinc Cooperative Catalysis

The umpolung activity of hemiacetals serving as α-carbon nucleophiles has been reported via dinuclear zinc cooperative catalysis. This umpolung strategy has been applied to catalytic asymmetric tandem reactions of 1-tosylindoline-2,3-diols with β,γ-unsaturated-α-keto compounds, providing a broad series of structurally diverse tetrahydrofuran spirooxindoles and dihydrofurans, respectively. In addition, products could be transformed to quinazoline and quinoline derivatives.

Iron catalyzed tandem ring opening/1,6-conjugate addition of cyclopropanols with p-quinone methides: new access to γ,γ-diaryl ketones

An iron(III) catalyzed tandem ring opening/1,6-conjugate addition of cyclopropanols to p-quinone methides leading to γ,γ-diaryl ketones has been described. This catalytic protocol provides a novel and efficient method to access γ,γ-diaryl ketone derivatives in good to excellent yields with high functional group tolerance. Importantly, γ,γ-diaryl ketone can be further functionalized to give a versatile set of useful products.

Organocatalytic Asymmetric Synthesis of Biologically Relevant 3,3- Dihydroxyphenyloxindoles via para-Quinone Methides Derived from Isatins

The first asymmetric synthetic approach to biologically relevant 3,3-diphenyloloxindoles was developed using para-quinone methides derived from isatins and phenols. Chiral phosphoric acid efficiently catalyzed the reaction and delivered 3,3-diphenyloloxindoles under mild conditions with up to an equivalent yield and excellent enantioselectivity (up to >99% ee). The chirality was maintained in further synthesis.

A new asymmetric activation strategy for hydrazones as acyl anion equivalents in the bimetallic catalyzed carbonyl-ene reaction

A new asymmetric activation strategy for hydrazones as acyl anion equivalents is developed in the bimetallic catalyzed carbonyl-ene reaction of isatins and hydrazones. Under mild conditions, optically active functionalized 3-hydroxy-2-oxindoles were furnished in up to 98% yield with up to 97% enantioselectivity. In this process, formaldehyde tert-butylhydrazone which is seldom employed in asymmetric carbonyl-ene reactions accelerated by a metallic catalyst can be activated well by a Brønsted base. A possible catalytic cycle is proposed.

Catalyst-free 1,6-conjugate addition of indoles and 4-hydroxycoumarins to para-quinone methides: synthesis of unsymmetrical triarylmethanes

The catalyst-free 1,6-conjugate addition of indoles and 4-hydroxycoumarins to para-quinone methides is reported. This protocol allowed us to access a range of unsymmetrical triarylmethanes in good to excellent yields. The outlined procedure is operationally simple, efficient, atom and step economical. The synthesized heterocyclic triarylmethanes were further converted into highly substituted indoloisoquinolines and pyranochromenones via metal-catalyzed C-H activation/annulation.

CF3-Containing para-Quinone Methides for Organic Synthesis

A new family of CF3-containing para-quinone methides (CF3-QMs) was systematically investigated for its suitability in organic synthesis. Addition of different nucleophiles gives access to target molecules with a benzylic CF3-containing stereogenic center straightforwardly. The electrophilicity parameter E of the prototypical CF3-QM 2,6-di-tert-butyl-4-(2,2,2-trifluoroethylidene)cyclohexa-2,5-dien-1-one was determined to be -11.68 according to the Mayr scale, making it one of the most reactive quinone methides known so far.

Enantioselective synthesis of indanone spiro-isochromanone derivatives via a dinuclear zinc-catalyzed Michael/transesterification tandem reaction

An enantioselective Michael/transesterification tandem reaction of α-hydroxy indanones with ortho-ester chalcones was realized using dinuclear zinc catalysts. A series of enantiomerically pure spiro[indanone-2,3'-isochromane-1-one] derivatives were obtained in good yields with excellent stereoselectivities (up to >20 : 1 dr, up to >99% ee). This protocol could be conducted on a gram scale without affecting its stereoselectivities. In addition, the absolute stereochemistry of the products was determined by X-ray crystallographic analysis of 3ac, and a positive nonlinear effect was observed. Finally, a possible catalytic cycle was proposed to explain the origin of the enantioselectivity.

Brønsted Base and Lewis Acid Cooperatively Catalyzed Asymmetric exo'-Selective [3 + 2] Cycloaddition of Trifluoromethylated Azomethine Ylides and Methyleneindolinones

A Brønsted base and Lewis acid cooperatively catalyzed 1,3-dipolar cycloaddition is reported through chiral dinuclear zinc catalysts. An asymmetric exo'-selective [3 + 2] cycloaddition of CF₃-containing N-unprotected isatin-derived azomethine ylides is realized. In the presence of 10 mol % of catalyst, azomethine ylides react efficiently with methyleneindolinones, giving a series of trifluoromethyl-substituted 2,3-pyrrolidinyl dispirooxindoles with highly enantio- (up to 99% ee) and exo'-selectivity (>20:1 dr). Up to four contiguous stereogenic centers, including two adjacent spiro quaternary stereocenters, are constructed in one step.

Organocatalytic asymmetric cascade 1,6-addition/hemiketalization/retro-Henry reaction of ortho-hydroxyphenyl-substituted p-QMs with α-nitroketones

Novel organocatalytic asymmetric cascade 1,6-addition/hemiketalization/retro-Henry reaction is developed to give new chiral 2-(1-(4-hydroxyphenyl)ethyl)phenols with good yields and excellent enantioselectivities under mild reaction conditions.

Regioselective catalytic asymmetric N-alkylation of isoxazol-5-ones with para-quinone methides

A highly regioselective and enantioselective N-alkylation of isoxazol-5-ones with para-quinone methides has been developed to obtain enantioenriched N-diarylmethane substituted isoxazolinones with high yields and enantioselectivities.

Dinuclear zinc synergistic catalytic asymmetric phospha-Michael/Michael cascade reaction: synthesis of 1,2,3-trisubstituted indanes bearing phosphoryl groups

A new dinuclear zinc synergistic catalytic asymmetric phospha-Michael/Michael cascade reaction of o-dienones and dialkyl phosphates is reported. This method has been proven to be general and efficient for the formation of a range of chiral 1,2,3-trisubstituted indane compounds containing phosphorus groups in good yields (up to 92%) with excellent stereoselectivities (up to >99% ee and up to >99 : 1 dr). The relative configuration of the product was identified as having a trans,trans substitution pattern via two-dimensional (2D) nuclear Overhauser effect spectroscopy 1H-1H NMR experiments. A possible mechanism was proposed.

Dinuclear Zinc-AzePhenol Catalyzed Asymmetric Aza-Henry Reaction of N-Boc Imines and Nitroalkanes under Ambient Conditions

The asymmetric aza-Henry reaction of N-Boc imines and nitroalkanes was realized in the presence of 10 mol % dinuclear zinc-AzePhenol catalysts under ambient conditions. A variety of nitroamines were obtained in good yields (up to 97%) with excellent enantioselectivities (up to 99% ee) and high diasteroselectivity (up to 14:1 dr). Our protocol combined the operational simplicity and mild reaction conditions, thus making the process amenable for technical applications.

Dinuclear zinc cooperative catalytic three-component reactions for highly enantioselective synthesis of 3,3′-dihydrofuran spirooxindoles

A new dinuclear zinc cooperative catalytic enantioselective three-component reaction via a domino Knoevenagel/Michael/cyclization sequence has been developed.

Base-mediated 1,6-conjugate addition of the Seyferth-Gilbert reagent to para-quinone methides

A 1,6-conjugate addition reaction of the Seyferth-Gilbert reagent (SGR) to p-quinone methides is reported. This base-mediated protocol allows rapid access to diarylmethylated diazomethylphosphonates. The reaction proceeds under mild basic conditions, making it a practical approach for the synthesis of diarylmethylated diazomethylphosphonates with a broad substrate scope. Interestingly, the treatment of the conjugate adduct with a catalytic amount of rhodium acetate resulted in the 1,2-aryl migration of the rhodium carbenoid intermediate to generate the corresponding 1,2-diaryl alkenylphosphonates in excellent yields.

A homodinuclear cobalt complex for the catalytic asymmetric Michael reaction of β-ketoesters to nitroolefins

A homodinuclear Co₂/aminophenol sulfonamide complex has been developed for the asymmetric Michael reaction.

Catalytic asymmetric 1,6-conjugate addition of in situ generated para-quinone methides with tritylthiol

An asymmetric phase-transfer catalyzed 1,6-conjugate addition of in situ generated para-quinone methides with tritylthiol has been developed.

The [4 + 2] cyclization/retro-Mannich reaction cascade of para-quinone methide derivatives with Pd-containing 1,4-dipoles

The first [4 + 2] cyclization/retro-Mannich reaction cascade of para-quinone methides with Pd-containing 1,4-dipoles generated in situ from vinyl benzoxazinanones has been established. The corresponding rearrangement products were obtained in generally good yields.