Electron-Deficient Alkynes as Dipolarophile in Pd-Catalyzed Enantioselective (3 + 2) Cycloaddition Reaction with Vinyl Cyclopropanes

Palladium-Catalyzed (3 + 2) Annulation of Azaborines with Vinyl Epoxides for Constructing Polycyclic Oxazaborolidines

A palladium-catalyzed (3 + 2) annulation of azaborines with vinyl epoxides has been established. By this strategy, various polycyclic oxazaborolidines with structural diversity were synthesized in generally high yields (up to 99%). The annulation can be scaled up and the polycyclic oxazaborolidines can be further functionalized through olefin metathesis and Heck reaction, which demonstrated good feasibility for downstream transformations. Moreover, the catalytic asymmetric version of this (3 + 2) annulation has been accomplished under the catalysis of palladium/chiral phosphoramidite ligand, producing chiral oxazaborolidines in overall good enantioselectivities (up to 98:2 er). This work not only represents the first catalytic asymmetric (3 + 2) annulation of 1,2-azaborines with vinyl epoxides but also offers an efficient strategy for constructing benzooxazaborolidine skeletons, particularly those in an enantioenriched fashion.

Palladium-Catalyzed Strain-Enabled [2π + 2σ] Cycloadditions of Vinyl Bicyclo[1.1.0]butanes with Methyleneindolinones

A palladium-catalyzed [2π + 2σ] cycloaddition of vinyl bicyclo[1.1.0]butanes with methyleneindolinones has been developed. The reaction enables the construction of spirobicyclo[2.1.1]hexanes bearing an all-carbon quaternary center in moderate to good yields with excellent diastereoselectivities. This method features a broad substrate scope with good functional group compatibility. The practical utility of this protocol was further demonstrated by gram-scale synthesis and postsynthetic transformations of desired product.

Copper-Catalyzed Asymmetric Nucleophilic Opening of 1,1,2,2-Tetrasubstituted Donor-Acceptor Cyclopropanes for the Synthesis of α-Tertiary Amines

Catalytic asymmetric transformation of donor-acceptor cyclopropanes (DACs) has been proven to be a highly valuable and robust strategy to construct diverse types of enantioenriched molecules. However, the use of 1,1,2,2-tetrasubstituted DACs to form products bearing quaternary stereocenters remains a long-term unsolved challenge. Here, we report the copper-catalyzed asymmetric aminative ring opening of tetrasubstituted alkynyl DACs that delivers a myriad of α-tertiary amines with high levels of enantioselectivities. The alkyne, amine, and ester moieties within the products enable diverse further applications, including the asymmetric synthesis of bioactive molecules. Mechanistic studies indicate that the zwitterionic intermediate bearing a copper-acetylide unit plays a key role in the process, which represents a new mode for achieving catalytic asymmetric transformation of DACs.

Structure-Dependent, Switchable Alder-Ene/[2π + 2σ] Cycloadditions of Vinyl Bicyclo[1.1.0]butanes with α-Ketoesters Enabled by Palladium Catalysis

A structure-dependent, palladium-catalyzed switchable alder-ene/[2π + 2σ] cycloaddition of VBCBs with α-ketoesters has been reported. A variety of cyclobutenes and 2-oxabicyclo[2.1.1]hexanes have been efficiently achieved in good to excellent yields through strain-release-driven alder-ene reactions and [2π + 2σ] cycloadditions, respectively. The potential of this method is illustrated by the scale-up reaction and diverse postsynthetic transformations of the obtained cyclic scaffolds. Additionally, the reaction mechanism and origins of the chemoselectivity have been probed by computational studies.

COAP-Pd Catalyzed Asymmetric Formal [3+2] Cycloaddition for Optically Active Multistereogenic Spiro Cyclopentane-Indandiones Bearing Cyclic N-Sulfonyl Ketimine Skeletons

We reported a chiral oxamide-phosphine ligand (COAP-Ph)-Pd-catalyzed asymmetric [3+2] cycloaddition reaction between vinyl cyclopropane compounds derived from 1,3-indanedione and 2-vinylcyclopropane-1,1-dicarboxylates with cyclic sulfonyl 1-azadienes. The corresponding reactions provided a series of enantiomerically active spiro cyclopentane-indandione and cyclopentane structures bearing three consecutive stereogenic centers in good yields with good diastereo- and enantioselectivity. The COAP-Pd complex serves not only to promote generation of chiral π-allyl-palladium intermediates and induce the asymmetry of the reaction, but also depress the background reaction.

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.

Asymmetric cascades of the π-allyl complex: a journey from transition-metal catalysis to metallaphotocatalysis

The enantioselective catalytic cascade involving Tsuji-Trost allylation has provided a viable strategy for the construction of multiple asymmetric C-C and C-X centres and numerous methods have been developed around it for the synthesis of various vital scaffolds. The synthetic utility of this strategy was enhanced by replacing the customary allyl acetates with ethylene diacetates/dicarbonates, vinyl epoxides, vinyl oxetanes, vinyl ethylene carbonates, vinyl cyclopropanes, enynes, and dienes using transition-metal catalysis. One more milestone was achieved when metallaphotocatalysis provided the necessary platform for these cascades by using a cheaper metal. This review will provide a summary of these enantioselective catalytic cascades from 2015.

Palladium-catalysed enantio- and regioselective (3 + 2) cycloaddition reactions of sulfamidate imine-derived 1-azadienes towards spirocyclic cyclopentanes

An enantio- and diastereoselective Pd-catalysed (3 + 2) cycloaddition of bis(trifluoroethyl) 2-vinyl-cyclopropane-1,1-dicarboxylate (VCP) with cyclic sulfamidate imine-derived 1-azadienes (SDAs) has been developed. These reactions provide highly functionalized spiroheterocycles having three contiguous stereocentres, including a tetrasubstituted carbon bearing an oxygen functionality. The two geminal trifluoroethyl ester moieties can be manipulated in a facially selective manner to afford more diversely decorated spirocycles with four contiguous stereocentres. In addition, diastereoselective reduction of the imine moiety can also afford a fourth stereocentre and exposes the important 1,2-amino alcohol functionality.

Rhodium-Catalysed Regioselective [4+2]-type Annulation of 1-H-Indazoles with Propargyl Alcohols: A direct entry to 6-alkenylindazolo[3,2-a]isoquinolines

An efficient method for the synthesis of 6-vinylindazolo[3,2-a]isoquinolines via rhodium(III)-catalysed C-H bond activation/subsequent [4+2] cyclization starting from easily available 3-aryl-1-H-indazoles and propargyl alcohols has been developed. A series of vinyl-annulated...

Gold/scandium bimetallic relay catalysis of formal [5+2]- and [4+2]-annulations: access to tetracyclic indole scaffolds

Regiodivergent formal [5+2]- and [4+2]-annulation reactions of indole derivatives with 2-(2-alkynyl)aryl cyclopropane-1,1-diesters (ACPs) have been developed. A series of tetracyclic indole derivatives were delivered in a 77% average yield with excellent regioselectivities enabled by Au(I)/Sc(III) bimetallic relay catalysis. A gram-scale reaction and further transformation of the resulting tetracyclic indoles demonstrated the practical utility of this protocol. Moreover, the photophysical properties of the obtained multicyclic compounds were also investigated.

Diastereoselective [3 + 2] Cycloaddition of Quinoxalin-2(1H)-ones with Donor-Acceptor Cyclopropanes: Efficient Synthesis of Tetrahydro pyrrolo[1,2-a]quinoxalin-4(5H)-ones

A ytterbium triflate-catalyzed diastereoselective [3 + 2] cycloaddition of quinoxalinones with donor-acceptor cyclopropanes and cyclobutanes is described. A series of tetrahydropyrrolo-quinoxalinone derivatives were obtained in high yields (up to 96%) with excellent diastereoselectivities (up to 46:1). Other medicinally important heterocycles like benzoxazinone, isoquinoxalinone, and dibenzoxazepine derivatives were also suitable for the desired annulation reaction. The current method is applicable for the scale-up reaction. Further, the utility of this annulation reaction is demonstrated by the synthesis of densely functionalized proline derivatives.

Synthesis of Carbazoles and Dihydrocarbazoles by a Divergent Cascade Reaction of Donor-Acceptor Cyclopropanes

An alkylation/olefination cascade of indolecarboxaldehydes and phosphonate-functionalized donor–acceptor cyclopropanes affords functionalized dihydrocarbazoles and cyclohepta[cd]indoles in formal (3 + 3) and (4 + 3) cycloadditions. A minor modification to the reaction conditions also allows access to the fully aromatic heterocyclic scaffolds by thermal loss of an electron-rich aryl moiety.

Transition-Metal-Free [3+2] Dehydration Cycloaddition of Donor-Acceptor Cyclopropanes With 2-Naphthols

A Brønsted acid-catalyzed domino ring-opening cyclization transformation of donor-acceptor (D-A) cyclopropanes and 2-naphthols has been developed. This formal [3+2] cyclization reaction provided novel and efficient access to the naphthalene-fused cyclopentanes in the absence of any transition-metal catalysts or additives. This robust procedure was completed smoothly on a gram-scale to afford the corresponding product with comparable efficiency. Furthermore, the synthetic application of the prepared product has been demonstrated by its transformation into a variety of synthetically useful molecules.

Palladium-Catalyzed Cascade to Benzoxepins by Using Vinyl-Substituted Donor-Acceptor Cyclopropanes

A palladium-catalyzed intermolecular cascade (4+3) cyclocondensation of salicylaldehydes and vinylcyclopropanes is reported. A key feature of the reaction is the use of a phosphonate group as an acceptor moiety on the cyclopropane, exploiting its propensity to undergo olefination with aldehydes. Subsequent O-allylation enabled the formation of a range of substituted benzoxepinsWith a novel chiral ligand, the products were obtained in generally good yield and with reasonable enantioselectivity.

The Regioselective Functionalization Reaction of Unprotected Carbazoles with Donor-Acceptor Cyclopropanes

The regioselective functionalization reaction of unprotected carbazoles with donor-acceptor (D-A) cyclopropanes has been demonstrated for the first time. With Sc(OTf)₃ as Lewis acid catalyst, the N-H functionalization of carbazoles takes place through a highly selective nitrogen-initiated nucleophilic ring opening reaction. Significantly, by engaging TfOH as Brønsted acid catalyst, a straightforward C-H functionalization at the 3-position of the unprotected carbazole proceeds via Friedel-Crafts-type addition. This strategy facilitates the diversity-oriented synthesis of carbazole-containing heterocycles and expands the novel application of D-A cyclopropanes.

Co-Catalyzed Asymmetric Intramolecular [3+2] Cycloaddition of Yne-Alkylidenecyclopropanes and its Reaction Mechanism

Developing new transition metal-catalyzed asymmetric cycloadditions for the synthesis of five-membered carbocycles (FMCs) is a research frontier in reaction development due to the ubiquitous presence of chiral FMCs in various functional molecules. Reported here is our discovery of a highly enantioselective intramolecular [3+2] cycloaddition of yne-alkylidenecyclopropanes (yne-ACPs) to bicyclo[3.3.0]octadiene and bicyclo[4.3.0]nonadiene molecules using a cheap Co catalyst and commercially available chiral ligand (S)-Xyl-BINAP. This reaction avoids the use of precious Pd and Rh catalysts, which are usually the choices for [3+2] reactions with ACPs. The enantiomeric excess in the present reaction can be up to 92 %. Cationic cobalt(I) species was suggested by experiments as the catalytic species. DFT calculations showed that this [3+2] reaction starts with oxidative cyclometallation of alkyne and ACP, followed by ring opening of the cyclopropyl (CP) group and reductive elimination to form the cycloadduct. This mechanism is different from previous [3+2] reactions of ACPs, which usually start from CP cleavage, not from oxidative cyclization.

Palladium-Catalyzed Formal (5 + 6) Cycloaddition of Vinylethylene Carbonates with Isatoic Anhydrides for the Synthesis of Medium-Sized N,O-Containing Heterocycles

Under the reaction conditions of Pd(PPh₃)₄ (2.5 mol %) and PPh₃ (10 mol %) in EtOAc at 60 °C, the formal (5 + 6) cycloaddition of vinylethylene carbonates with isatoic anhydrides proceeded smoothly and furnished medium-sized N,O-containing heterocycles in reasonable chemical yields. The chemical structures of the title products were clearly identified by X-ray diffraction analysis.

Conjugated ynones in catalytic enantioselective reactions

Conjugated ynones are easily accessible feedstock and the existence of an alkyne bond endows ynones with different attractive reactivities, thus making them unique substrates for catalytic asymmetric reactions. Their compatibility under organocatalytic, metal-catalyzed as well as cooperative catalytic conditions has resulted in numerous enantioselective transformations. Importantly, conjugated ynones can act as nucleophiles or electrophiles, and serve as easily accessed synthons for different cyclization pathways. This review summarizes the recent literature examples of the catalytic reactions of conjugated ynones and related compounds such as alkyne conjugated α-ketoesters, and classifies these reaction types alongside mechanistic insights whenever possible. We aim to trigger more intensive research in the future to render the asymmetric transformation of ynones as a common and reliable tool for asymmetric synthesis.

Exploiting Heavier Organochalcogen Compounds in Donor-Acceptor Cyclopropane Chemistry

Donor-acceptor (D-A) cyclopropanes have gained increased momentum over the past two decades. The use of these highly strained three-membered entities paved the way to innovative and original transformations yielding complex cyclic and acyclic architectures that otherwise might be difficult to address. Since the fundamentals were laid by Wenkert and Reissig in the late 1970s, the field has flourished impressively including asymmetric transformations as well as elegant synthetic applications in the construction of natural occurring products. In this Account, we aim to highlight especially our efforts in the context of an efficient access to sulfur- and selenium-containing compounds, of either cyclic or open-chain nature, by exploiting D-A cyclopropane chemistry. Light will be shed on the three fundamental transformations: ring-opening reactions, cycloadditions, and rearrangements.Our synthetic endeavors started back in 2011 guided by quantum chemical studies to obtain 3,3'-linked bisthiophenes along with an unprecedented rearrangement delivering sulfur- and selenium-containing cagelike scaffolds. Inspired by these surprising results, we further deepened our efforts to the construction of new sulfur-carbon and selenium-carbon bonds within the context of D-A cyclopropane chemistry. In the first instance, we capitalized on the great versatility of organosulfur and organoselenium compounds regarding their amphiphilic character to act either as nucleophilic or as electrophilic species. By such an approach, ring-openings via a nucleophilic attack of sulfenyl and selenyl halides furnished 1,3-bishalochalcogenated products. A similar protocol led us to a desymmetrization reaction of meso-cyclopropyl carbaldehydes employing novel chiral imidazolidinone organocatalysts. In contrast, electrophilic sulfur was supplied by N-(arylthio)succinimide substrates to access thiolated γ-amino acid derivatives and their selenium equivalents.Combining the highly reactive thiocarbonyl compounds and vicinal donor-acceptor substituted cyclopropanes opened new vistas in the field of atom-economic cycloaddition reactions to build up sulfur-containing heterocycles of various sizes. The first systematic study of such transformations was made by our group in 2017 leading to highly decorated thiolanes, whereas an intramolecular approach furnished thia-[n.2.1]bicyclic ring systems. Our investigations were then successfully extended to the synthesis of tetrahydroselenophenes by using capricious selenoketones. Recently, we were able to yield the unsaturated analogues, selenophenes, by a (3 + 2)-cycloaddition of D-A cyclopropanes with ammonium selenocyanates followed by oxidation. The formal insertion of thioketenes was realized by employing 3-thioxocyclobutanones as surrogates for disubstituted thioketenes to obtain 2-substituted tetrahydrothiophenes bearing a semicyclic double bond via a (3 + 2) spiroannulation/(2 + 2) cycloreversion sequence. Even the formation of seven-membered S-heterocycles was realized by (4 + 3)-cycloaddition processes. In 2016, we demonstrated the synthesis of benzo-fused dithiepines from in situ generated ortho-bisthioquinones, whereas the utilization of thia-Michael systems as a hetero-4π-component delivered tetrahydrothiepine derivatives containing just one sulfur atom embedded in the ring system.

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.

Regio- and enantioselective ring-opening reaction of vinylcyclopropanes with indoles under cooperative catalysis

The title reaction has been established under the cooperative bimetallic catalysis of iridium and copper catalysts, which afforded indole C3-allylation products with branched selectivity in moderate yields and good enantioselectivities.

Ring Opening of Donor-Acceptor Cyclopropanes with Acyclic 1,3-Diketones for the Synthesis of 1,6-Dicarbonyl Compounds

1,6-Dicarbonyl compounds, representing the formal addition products of the α-position of acetophenone derivatives to donor-acceptor cyclopropanes, were synthesized in two steps via first ring opening of donor-acceptor cyclopropanes with acyclic 1,3-diketones followed by DBU catalyzed retro-Claisen-type C-C bond cleavage reactions. In the first step, acyclic 1,3-diketones selectively worked as C-nucleophiles to add to donor-acceptor cyclopropanes. In the second step, the alkyl ketone part of the ring-opening products resulting from unsymmetrical 1,3-diketones was selectively cleaved in the presence of DBU in methanol.

Highly diastereoselective synthesis of 3-methylenetetrahydropyrans by palladium-catalyzed oxa-[4 + 2] cycloaddition of 2-alkenylbenzothiazoles

A highly diastereoselective synthesis of 3-methylenetetrahydropyrans via palladium-catalyzed oxa-[4 + 2] cycloaddition of 2-alkenylbenzothiazoles with allyl carbonates bearing a nucleophilic alcohol side chain is presented. This synthetic methodology tolerates a wide variety of 2-alkenylbenzothiazoles and afforded the desired 3-methylenetetrahydropyrans in good yields and excellent dr. In addition, further derivatizations resulted in new scaffolds, making them useful synthetic precursors.

Recent advances in the catalytic synthesis of 3-aminooxindoles: an update

3-Substituted-3-aminooxindoles are versatile scaffolds and these motifs constitute the core structure of number of natural products and biologically active compounds.

Chameleon-Like Activating Nature of the Spirooxindole Group in Donor-Acceptor Cyclopropanes

The concept of a chameleon activating group is considered in the context of donor-acceptor cyclopropane chemistry. When spiro-conjugated with cyclopropane, oxindole can act as an acceptor or a donor depending on the electronic nature of vicinal substituents. This dichotomy is reflected in the alteration of chemoselectivity of spiro[oxindole-1,3'-cyclopropane] ring opening with nucleophiles.

Palladium-catalyzed asymmetric formal [3+2] cycloaddition of α-N-heterocyclic acrylates with vinyl epoxides for construction of isonucleoside analogs

A highly efficient Pd-catalyzed asymmetric formal [3+2] cycloaddition of α-N-heterocyclic acrylates with vinyl epoxides has been developed for the first time.