Enantioselective (4+2) Annulation of Donor-Acceptor Cyclobutanes by N-Heterocyclic Carbene Catalysis

DyKAT by DiCat: Stereoconvergent Dienamine-Catalyzed Claisen Rearrangements

This Claisen rearrangement establishes the feasibility of DyKAT of γ-epimeric enals via dienamine formation to afford enantioenriched products. γ-Aryl and -alkyl enals, and exocyclic enals that introduce quaternary centers, are all amenable substrates. Products are readily converted into pyrrolidines or cyclopentenols. Notably, a reactive dienamine intermediate has been isolated from a catalytic reaction, fully characterized, and converted to product upon reexposure to reaction conditions. Product configuration arises from a directing C-H···π interaction in the transition state.

Oxathiaborolium-catalyzed enantioselective [2 + 2] cycloadditions

The one-pot-prepared oxathiaborolium pentachlorostannate is an excellent Lewis acid and is successfully used to catalyze the [2 + 2] cycloadditions of N-substituted maleimides and silyl enol ethers with excellent enantioselectivities.

Friedel-Crafts-Type Reactions with Electrochemically Generated Electrophiles from Donor-Acceptor Cyclopropanes and -Butanes

We describe a general electrochemical method to functionalize donor-acceptor (D-A) cyclopropanes and -butanes with arenes utilizing Friedel-Crafts-type reactivity. The catalyst-free strategy relies on the direct anodic oxidation of the strained carbocycles, which leads after C(sp³)-C(sp³) cleavage to radical cations that act as electrophiles for the arylation reaction. Broad reaction scopes in regard to cyclopropanes, cyclobutanes, and aromatic reaction partners are presented. Additionally, a plausible electrolysis mechanism is proposed.

Electrocatalytic Activation of Donor-Acceptor Cyclopropanes and Cyclobutanes: An Alternative C(sp3 )-C(sp3 ) Cleavage Mode

We describe the first electrochemical activation of D-A cyclopropanes and D-A cyclobutanes leading after C(sp3 )-C(sp3 ) cleavage to the formation of highly reactive radical cations. This concept is utilized to formally insert molecular oxygen after direct or DDQ-assisted anodic oxidation of the strained carbocycles, delivering β- and γ-hydroxy ketones and 1,2-dioxanes electrocatalytically. Furthermore, insights into the mechanism of the oxidative process, obtained experimentally and by additional quantum-chemical calculations are presented. The synthetic potential of the reaction products is demonstrated by diverse derivatizations.

Catalytic asymmetric synthesis of spirocyclobutyl oxindoles and beyond via [2+2] cycloaddition and sequential transformations

Efficient asymmetric synthesis of a collection of small molecules with structural diversity is highly important to drug discovery. Herein, three distinct types of chiral cyclic compounds were accessible by enantioselective catalysis and sequential transformations. Highly regio- and enantioselective [2+2] cycloaddition of (E)-alkenyloxindoles with the internal C[double bond, length as m-dash]C bond of N-allenamides was achieved with N,N'-dioxide/Ni(OTf)₂ as the catalyst. Various optically active spirocyclobutyl oxindole derivatives were obtained under mild conditions. Moreover, formal [4+2] cycloaddition products occurring at the terminal C[double bond, length as m-dash]C bond of N-allenamides, dihydropyran-fused indoles, were afforded by a stereospecific sequential transformation with the assistance of a catalytic amount of Cu(OTf)₂. In contrast, performing the conversion under air led to the formation of γ-lactones via the water-involved deprotection and rearrangement process. Experimental studies and DFT calculations were performed to probe the reaction mechanism.

Carbene-catalyzed enantioselective annulation of dinucleophilic hydrazones and bromoenals for access to aryl-dihydropyridazinones and related drugs

4,5-Dihydropyridazinones bearing an aryl substituent at the C6-position are important motifs in drug molecules. Herein, we developed an efficient protocol to access aryl-dihydropyridazinone molecules via carbene-catalyzed asymmetric annulation between dinucleophilic arylidene hydrazones and bromoenals. Key steps in this reaction include polarity-inversion of aryl aldehyde-derived hydrazones followed by chemo-selective reaction with enal-derived α,β-unsaturated acyl azolium intermediates. The aryl-dihydropyridazinone products accessed by our protocol can be readily transformed into drugs and bioactive molecules.

Polarity inversion of arylidene hydrazones to react with bromoenals via carbene organic catalysis is disclosed. The reaction enantioselectively affords 6-aryl-4,5-dihydropyridazinones and related drugs with proven commercial applications.

Theoretical study of the NHC-catalyzed C–S bond cleavage and reconstruction reaction: mechanism, stereoselectivity, and role of catalysts

The mechanism of the NHC-catalyzed C–S bond activation reaction has been theoretically studied and ELF analysis along with IRC results shows that the NHC promotes the cleavage of the C–S bond via an SN2 process.

Catalytic Enantioselective [2+2] Cycloaddition of α-Halo Acroleins: Construction of Cyclobutanes Containing Two Tetrasubstituted Stereocenters

A catalytic enantioselective formal [2+2] cycloaddition between α-halo acroleins and electronically diverse arylalkenes is described. In the presence of (S)-oxazaborolidinium cation as the catalyst, densely functionalized cyclobutanes containing two vicinal tetrasubstituted stereocenters were produced in high yields and high diastereoselectivities with excellent enantioselectivities. Mechanistic studies revealed that the cis isomer could be transformed into the trans isomer via an enantiocontrolled process. A gram-scale reaction of this catalytic method was used to demonstrate its synthetic potential.

NHC/Copper-Cocatalyzed [4 + 3] Annulations of Salicylaldehydes with Aziridines for the Synthesis of 1,4-Benzoxazepinones

N-heterocyclic carbene/copper-cocatalyzed [4 + 3] annulation of salicylaldehydes with aziridines was developed, giving the corresponding 1,4-benzoxazepinones in good yields with exclusive regioselectivity. Copper serves as a Lewis acid to activate the small strained aziridines, and the formation of NHC-salicylaldehyde adduct plays an important role in improving the regioselectivity.

Synthesis of Hexahydropyridazines by [4 + 2] Cycloaddition of Donor-Acceptor Cyclobutanes and cis-Diazenes

The GaCl₃-catalyzed [4 + 2] cycloaddition between alkoxy- and aryl-activated donor-acceptor cyclobutane diesters and cis-diazene 1a (4-phenyl-1,2,4-triazoline-3,5-dione, PTAD) is disclosed. The reaction provides hexahydropyridazine derivatives as single diastereomers in good to excellent yields in most cases. The structural assignment of the cycloadduct 3b was unambiguously established by single-crystal X-ray diffraction.

Ring-Opening Reactions of Donor-Acceptor Cyclobutanes with Electron-Rich Arenes, Thiols, and Selenols

Donor-acceptor (D-A) cyclobutanes with two geminal ester groups as acceptors are reacted with electron-rich arenes as nucleophiles to afford ring-opened products. AlCl₃ mediates this Friedel-Crafts-type reaction. A variety of donors and electron-rich arenes are used. Nucleophilic thiols and selenols also trigger this ring-opening reaction. Furthermore, a comparison of various physical parameters has been carried out for several D-A cyclobutanes.

Highly enantioselective synthesis of functionalized azepino[1,2-a]indolesviaNHC-catalyzed [3+4] annulation

The enantioselective [3+4] annulation of 3-formylindol-2-methylmalonates with 2-bromoenals catalyzed by NHCs is described to efficiently synthesise functionalized azepino[1,2-a]indoles.

Sulfinate and Carbene Co-catalyzed Rauhut-Currier Reaction for Enantioselective Access to Azepino[1,2-a]indoles

A carbene and sulfinate co-catalyzed intermolecular Rauhut-Currier reaction between enals and nitrovinyl indoles is disclosed. The carbene catalyst activates the enal and the sulfinate co-catalyst activates the nitrovinyl indole. Both activation processes are realized via the formation of covalent bonds between the catalysts and substrates to generate catalyst-bound intermediates. The dual catalytic reaction affords azepino[1,2-a]indole products with excellent stereoselectivity. Our study demonstrates the unique involvement of sulfinate as an effective nucleophilic catalyst in activating electron-deficient alkenes for asymmetric reactions. This dual catalytic approach should also encourage future explorations of both sulfinate and carbene catalysts for new reactions.

Asymmetric synthesis of polysubstituted methylenecyclobutanes via catalytic [2+2] cycloaddition reactions of N-allenamides

A highly enantioselective [2+2] cycloaddition reaction of alkylidene malonates with the internal C[double bond, length as m-dash]C bond of N-allenamides was developed with a MgII/N,N'-dioxide complex as a catalyst. Various polysubstituted methylenecyclobutanes were afforded in good yields (up to 99%) and excellent enantioselectivities (up to 96% ee) under mild conditions. The utility of the donor-acceptor cyclobutane product was demonstrated as a masked 1,4-dipole in the formal [4+2] annulation reaction with a silyl enol ether.

Construction of Fused Pyrrolidines and β-Lactones by Carbene-Catalyzed C-N, C-C, and C-O Bond Formations

A carbene-catalyzed intermolecular C-N bond formation, which initiates a highly selective cascade reaction for the synthesis of pyrrolidine fused β-lactones, is disclosed. The nitrogen-containing bicyclic β-lactone products are obtained with good yields and excellent stereoselectivities. Synthetic transformations of the reaction products into useful functional molecules, such as amino catalysts, can be efficiently realized under mild reaction conditions. Mechanistically, this study provides insights into modulating the reactivities of heteroatoms, such as nitrogen atoms, in challenging carbene-catalyzed asymmetric carbon-heteroatom bond-forming reactions.

Enantioselective intermolecular all-carbon [4+2] annulation via N-heterocyclic carbene organocatalysis

The challenge of asymmetric intermolecular all-carbon [4+2] annulation via α,β-unsaturated acyl azoliums was addressed with 2-acyloxy-3-butenones as suitable dienolate precursors, and a variety of highly functionalized cyclohexene products were delivered at a high level of enantioselectivity.