Palladium-Mediated Annulation of Vinyl Aziridines with Michael Acceptors: Stereocontrolled Synthesis of Substituted Pyrrolidines and Its Application in a Formal Synthesis of (−)-α-Kainic Acid

Pd-Catalyzed [4 + 1] Annulation Strategy to Functionalized 4-Methyleneproline Derivatives

A Pd-catalyzed formal [4 + 1] cycloaddition reaction of sulfur ylides and in situ-generated Pd-stabilized zwitterions offers a convenient route to a series of functionalized proline derivatives. The utility of this method is demonstrated by a gram-scale synthesis and chemoselective functionalization of a proline-based derivative.

Intermolecular Carbophosphination of Alkynes with Phosphole Oxides via Ni-Al Bimetal-Catalyzed C-P Bond Activation

Intermolecular carbophosphination reaction of alkynes or alkenes with unreactive C-P bonds remains an elusive challenge. Herein, we used a Ni-Al bimetallic catalyst to realize an intermolecular carbophosphination reaction of alkynes with 5-membered phosphole oxides, providing a series of 7-membered phosphepines in up to 94 % yield.

Cooperative Isothiourea/Iridium-Catalyzed Asymmetric Annulation Reactions of Vinyl Aziridines with Pentafluorophenyl Esters

Chiral γ-lactam-containing skeletons are important motifs in bioactive natural products, pharmaceuticals, and bioactive molecules. Herein, we report a general and modular platform to access chiral γ-lactam compounds via an ITU/Ir cooperatively catalyzed [3 + 2] asymmetric annulation reaction of vinyl aziridines with pentafluorophenyl esters. Through the Lewis base and transition metal cooperative catalytic regime, a broad range of optically active γ-lactams were generated in good yields (up to 92%) with high asymmetric induction (up to 98% ee). Furthermore, the utility of this synthetic protocol was also demonstrated by the expedient preparation of diverse enantioenriched architectures.

Palladium-Catalyzed Carbonylative [5+1] Cycloaddition of N-Tosyl Vinylaziridines: Solvent-Controlled Divergent Synthesis of α,β- and β,γ-Unsaturated δ-Lactams

A palladium-catalyzed carbonylative [5+1] cycloaddition of N-tosyl vinylaziridines with CO has been developed. This protocol affords an efficient and practical approach for solvent-controlled divergent synthesis of α,β-unsaturated δ-lactams in dimethylformamide and β,γ-unsaturated δ-lactams in tetrahydrofuran in good to excellent yields. Significantly, the step- and atom-economical reactions are more regioselective toward [5+1] cycloaddition than toward [3+1] cycloaddition.

Asymmetric Synthesis of 2,2-Difluorotetrahydrofurans through Palladium-Catalyzed Formal [3+2] Cycloaddition

The asymmetric synthesis of 2,2-difluorinated tetrahydrofurans was accomplished via enantioselective formal [3+2] cycloaddition catalyzed by palladium. The asymmetric reaction between gem-difluoroalkenes and racemic vinyl epoxides or vinylethylene carbonates resulted in the formation of enantioenriched 2,2-difluorotetrahydrofurans with an enantioselectivity up to 98 %. Notably, the reaction used the readily available (R)-BINAP as the ligand at a low loading and yielded a wide variety of difluorinated products in moderate to high yields. Both chiral diastereomers could be obtained in a single sequence.

Combining palladium and ammonium halide catalysts for Morita-Baylis-Hillman carbonates of methyl vinyl ketone: from 1,4-carbodipoles to ion pairs

Here we report that Morita-Baylis-Hillman carbonates from diverse aldehydes and methyl vinyl ketones can be directly utilised as palladium-trimethylenemethane 1,4-carbodipole-type precursors, and both reactivity and enantioselectivity are finely regulated by adding a chiral ammonium halide as the ion-pair catalyst. The newly assembled intermediates, proposed to contain an electronically neutral π-allylpalladium halide complex and a reactive compact ion pair, efficiently undergo asymmetric [4 + 2] annulations with diverse activated alkenes or isatins, generally with high regio-, diastereo- and enantio-selectivity, and even switchable regiodivergent or diastereodivergent annulations can be well realised by tuning the substrate or catalyst assemblies. An array of control experiments, including UV/Vis absorption study and density functional theory calculations, are conducted to rationalise this new double activation mode combining a palladium complex and an ammonium halide as an ion-pair catalyst.

Iodide-Mediated [3 + 2]-Cycloaddition Reaction with N-Tosylaziridines and α,β-Unsaturated Ketones

The [3 + 2]-cycloaddition reaction between N-tosylaziridines and α,β-unsaturated ketones was promoted with lithium iodide. The reaction proceeded under mild conditions to provide N-tosylpyrrolidines. Quaternary carbon-possessing 3,3-disubstituted pyrrolidines including spiro compounds were afforded in high yields. A simple procedure with easy to handle reagents makes this reaction concise. The intramolecular version of this reaction was applied to synthesize tropane skeletons.

Development of Dipolarophiles for Catalytic Asymmetric Cycloadditions through Pd-π-Allyl Zwitterions

The development of new and efficient methodology for the construction of optically active molecules is of great interest in both synthetic organic and medicinal chemistry fields. To this end, the personal account summarizes our studies on the development of electron-deficient alkenes, allenes, and alkynes containing single activator as new dipolarophiles for Pd-catalyzed asymmetric cycloaddition reactions. These new dipolarophiles can participate in Pd-catalyzed asymmetric [3+2] and [4+2] cycloadditions through Pd-π-allyl 1,3- and 1,4-zwitterions in-situ generated by the reaction of Pd(0) catalyst with vinyl aziridines, vinyl epoxides, vinyl cyclopropanes, 4-vinyl-1,3-dioxan-2-ones, and vinyl benzoxazinanones. These [3+2] and [4+2] cycloadditions provide efficient approaches to a wide range of enantiomerically enriched five- and six-membered ring compounds containing contiguous chiral centers with high to excellent chemo-, diastereo-, and enantioselectivities. The utilities of these protocols are demonstrated by transformation of the cycloadducts into other useful chiral building blocks. DFT calculations reveal the dissimilar reactivity of different electron deficient alkenes and rationalize the mechanism and stereo-control of the reaction. A Pd-catalyzed inverse [3+2] cycloaddition is disclosed.

Palladium-catalyzed asymmetric [3 + 2] cycloaddition of vinyl aziridines and α,β-unsaturated imines generated in situ from aryl sulfonyl indoles

The first palladium-catalyzed asymmetric formal [3 + 2] cycloaddition of vinyl aziridines and in situ generated unsaturated imines has been established.

Photoinduced triiodide-mediated [3 + 2] cycloaddition of N-tosyl aziridines and alkenes

A photoinduced triiodide-mediated [3 + 2] cycloaddition of N-Ts aziridines and alkenes is described herein. This operationally simple protocol enables regioselective access to a wide range of substituted pyrrolidines under mild-free conditions.

Visible-Light Photoredox-Catalyzed Formal [5 + 1] Cycloaddition of N-Tosyl Vinylaziridines with Difluoroalkyl Halides

A visible-light photoredox-catalyzed formal [5 + 1] cycloaddition of N-tosyl vinylaziridines with difluoroalkyl halides as unique C1 synthons was developed. The procedure provides an efficient and practical method to synthesize diverse pyridines in moderate to good yields. The reaction underwent a radical-initiated kinetically controlled ring-opening of vinylaziridines and involved a key α,β-unsaturated imine intermediate, followed by an E2 elimination, a 6π electrocyclization, and defluorinated aromatization.

Sulfinate-Engaged Nucleophilic Addition Induced Allylic Alkylation of Allenoates

A strategically novel Pd-catalyzed nucleophilic addition induced allylic alkylation reaction (NAAA) of allenoates has been successfully accomplished. By judiciously integrating ZnCl₂-promoted Michael addition with Pd-catalyzed allylic alkylation, allenoates readily undergo allyl-sunfonylation at the internal double bond, thus providing a straightforward avenue for the rapid assembly of a host of structurally diversified α-allyl-β-sufonylbut-3-enoate derivatives. The success of this transformation profits from a delicate control of the reaction kinetic of each elementary step, thanks to the synergistic interaction of Pd/Zn bimetallic system, thus suppressing either direct allylic sulfonylation or premature quenching of therein in situ generated ester enolate intermediate. Furthermore, by expanding the scope of workable Michael acceptor beyond those previously required doubly activated ones, such as methylenemalononitrile, the present work substantially enriches the repertoire of NAAA reactions.

Synergetic iridium and amine catalysis enables asymmetric [4+2] cycloadditions of vinyl aminoalcohols with carbonyls

Catalytic asymmetric cycloadditions via transition-metal-containing dipolar intermediates are a powerful tool for synthesizing chiral heterocycles. However, within the field of palladium catalysis, compared with the well-developed normal electron-demand cycloadditions with electrophilic dipolarophiles, a general strategy for inverse electron-demand ones with nucleophilic dipolarophiles remains elusive, due to the inherent linear selectivity in the key palladium-catalyzed intermolecular allylations. Herein, based on the switched regioselectivity of iridium-catalyzed allylations, we achieved two asymmetric [4+2] cycloadditions of vinyl aminoalcohols with aldehydes and β,γ-unsaturated ketones through synergetic iridium and amine catalysis. The activation of vinyl aminoalcohols by iridium catalysts and carbonyls by amine catalysts provide a foundation for the subsequent asymmetric [4+2] cycloadditions of the resulting iridium-containing 1,4-dipoles and (di)enamine dipolarophiles. The former provides a straightforward route to a diverse set of enantio-enriched hydroquinolines bearing chiral quaternary stereocenters, and the later represent an enantio- and diastereodivergent synthesis of chiral hydroquinolines.

Inverse electron-demand cycloadditions in palladium catalysis are inherently limited by the linear selectivity in the allylic substitution step. To overcome this issue, the authors report a synergetic iridium (branched-selective) and amine catalysis in [4+2] cycloadditions of vinyl aminoalcohols with carbonyls.

Synthetic approach to skeletally diverse nitrogen heterocycles from dicyano-2-methylenebut-3-enoates

Divergent synthesis of three types of azaheterocycles, including vinylaziridines, pyrroline and 2-aminopyridines, has been achieved through [2 + 1], formal-[4 + 1] and [5 + 1] annulations.

Catalytic C-N and C-H Bond Activation: ortho-Allylation of Benzoic Acids with Allyl Amines

A facile insertion of ruthenium into aromatic C-H and allylic C-N bonds are the key steps in a [Ru( p-cymene)Cl₂]₂-catalyzed ortho-C-H allylation of benzoic acids. This protocol allows drawing on the large pool of allylic amines for state-of-the-art ortho-functionalizations of arenes, turning neutral amines into leaving groups. Concise syntheses of biologically active compounds provide further evidence of the synthetic potential of this methodology.

Nucleophilic ring opening reactions of aziridines

Aziridine ring opening reactions have gained tremendous importance in the synthesis of nitrogen containing biologically active molecules. During recent years, a great effort has been put forward by scientists toward unique bond construction methodologies via ring opening of aziridines. In this regard, a wide range of chiral metal- and organo-catalyzed desymmetrization reactions of aziridines have been reported with carbon, sulfur, oxygen, nitrogen, halogen, and other nucleophiles. In this review, an outline of methodologies adopted by a number of scientists during 2013-2017 for aziridine ring opening reactions as well as their synthetic applications is described.

Enantioselective total synthesis of (-)-kainic acid and (+)-acromelic acid C via Rh(i)-catalyzed asymmetric enyne cycloisomerization

A diversity-oriented synthetic strategy was developed for the total synthesis of kainoid amino acids, which led to the enantioselective synthesis of (-)-kainic acid and the first total synthesis of (+)-acromelic acid C. Rh(i)-catalyzed asymmetric enyne cycloisomerization served as the key reaction in this strategy for the rapid construction of highly functionalized lactam, and the resulting vinyl acetate moiety was further utilized as a versatile building block for the installation of both isopropylidene and 2-pyridone units existing in natural kainoids.

Trisubstituted alkenes with a single activator as dipolarophiles in a highly diastereo- and enantioselective [3+2] cycloaddition with vinyl epoxides under Pd-catalysis

1,1,2-Trisubstituted alkenes with a single activator were used in the Pd-catalyzed asymmetric cycloaddition of vinyl epoxides with high diastereo- and enantioselectivities.

Cycloaddition/annulation strategies for the construction of multisubstituted pyrrolidines and their applications in natural product synthesis

Pyrrolidines are privileged substructures of numerous bioactive natural products and drugs.

Alkene functionalization for the stereospecific synthesis of substituted aziridines by visible-light photoredox catalysis

A novel strategy involving visible-light-induced functionalization of alkenes for the stereospecific synthesis of substituted aziridines was developed.

Divergent synthesis of functionalized pyrrolidines and γ-amino ketones via rhodium-catalyzed switchable reactions of vinyl aziridines and silyl enol ethers

Rhodium-catalyzed enantiospecific intermolecular [3+2] cycloadditions and ring-opening reactions of vinylaziridines with silyl enol ethers have been developed to synthesize functionalized pyrrolidines and γ-amino ketones, respectively.

Lewis acid-promoted cascade reaction for the synthesis of Michael acceptors and its application in a dimerization reaction

An efficient Lewis acid-promoted cascade reaction with dimethyl sulfoxide as a methylene source for the synthesis of Michael acceptors is reported. The key to developing this procedure is the selection of a mild base to modulate the equilibrium of various intermediates in order to drive the reaction forward to the formation of Michael acceptor and dimeric compound products. Extensive studies were performed to gain insight into a possible reaction mechanism.

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An efficient Lewis acid-promoted cascade reaction with dimethyl sulfoxide as a methylene source for the synthesis of Michael acceptors is reported.

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The selection of a mild base is the key for this reaction.

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Extensive studies were performed to gain insight into a possible reaction mechanism.

Mechanism and origins of selectivity in rhodium-catalyzed intermolecular [3 + 2] cycloadditions of vinylaziridines with allenes

DFT calculations were performed to investigate the rhodium-catalyzed intermolecular [3 + 2] cycloadditions of vinylaziridines with allenes.

Rhodium-catalyzed intermolecular [3+3] cycloaddition of vinyl aziridines with C,N-cyclic azomethine imines: stereospecific synthesis of chiral fused tricyclic 1,2,4-hexahydrotriazines

A new rhodium-catalyzed intermolecular [3+3] cycloaddition of vinylaziridines with C,N-cyclic azomethine imines has been described to construct enantioenriched 1,2,4-hexahydrotriazines.