Nickel-Catalyzed Cycloaddition of Vinylcyclopropanes to Imines

Cooperative Photoredox and Cobalt-Catalyzed Acceptorless Dehydrogenative Functionalization of Cyclopropylamides towards Allylic N,O-Acyl-acetal Derivatives

We disclose herein a novel photoredox and cobalt co-catalyzed ring-opening/acceptorless dehydrogenative functionalization of mono-donor cyclopropanes. This sustainable and atom-economic approach allows the rapid assembly of a wide range of allylic N,O-acyl-acetal derivatives. The starting materials are readily available and the reaction features mild conditions, broad substrate scope, and excellent functional group compatibility. The optimized conditions accommodate assorted cycloalkylamides and primary, secondary, and tertiary alcohols, with applications in late-stage functionalization of pharmaceutically relevant compounds, stimulating further utility in medicinal chemistry. Moreover, selective nucleophilic substitutions with various carbon nucleophiles were achieved in a one-pot fashion, offering a reliable avenue to access some cyclic and acyclic derivatives.

Recent updates on vinyl cyclopropanes, aziridines and oxiranes: access to heterocyclic scaffolds

This present review delineates the repertoire of vinyl cyclopropanes and their stuctural analogues to accomplish a wide array of oxa-cycles, aza-cycles, and thia-cycles under transition metal catalysis and metal-free approaches from early 2019 to the present date. The generation of electrophilic π-allyl intermediates and 1-3/1-5-dipolarophile chemistry originating from VCPs are always green, step- and atom-economical and sustainable strategies in comparsion with prefunctionalized and/or C-H activation protocols. Here, the strained ring-system extends its applicability by relieving the strain to undergo a ring-expansion reaction to accomplish 5-9 membered carbo- and heterocyclic systems. The availability of chiral ligands in the ring-expansion reaction of VCPs and their analogues has paved the way to realizing asymmetric synthetic transformations.

Stereoselective Photoredox Catalyzed (3+3) Dipolar Cycloaddition of Nitrone with Aryl Cyclopropane

By resorting to the principle of remote activation, we herein demonstrate the first photoredox catalyzed (3+3) dipolar cycloaddition of nitrones with aryl cyclopropanes. Key to the fidelity of the reaction resides in a facile manner of substrate activation by single-electron transfer (SET) oxidation with photoredox catalysis, and the reaction takes place through a stepwise cascade encompassing a three-electron-type nucleophilic substitution triggered cyclopropane ring-opening and a diastereoselective 6-endo-trig radical cyclization manifold. The reaction proceeds under mild conditions with excellent regio- and stereoselectivity, nicely complementing the well-developed Lewis acid catalyzed cycloaddition of donor-acceptor cyclopropanes. Other merits of the protocol include wide scope of aryl cyclopropanes with diversified substitution patterns and good functional-group compatibility. A mechanism involving an aryl radical cation promoted remote activation mode was also proposed and supported by mechanistic experiments.

Phosphine-Catalyzed Enantioselective (3+2) Annulation of Vinylcyclopropanes with Imines for the Synthesis of Chiral Pyrrolidines

A phosphine-catalyzed highly enantioselective and diastereoselective (up to 98 % ee and >20 : 1 dr) (3+2) annulation between vinylcyclopropanes and N-tosylaldimines has been developed, which allows facile access to a range of highly functionalized chiral pyrrolidines. Notably, this method makes use of vinylcyclopropanes as a synthon for phosphine-mediated asymmetric annulation reaction, which will offer new opportunities for potential applications of cyclopropanes substrates in phosphine-catalyzed organic transformations.

A novel type of donor-acceptor cyclopropane with fluorine as the donor: (3 + 2)-cycloadditions with carbonyls

gem-Difluorocyclopropane diester is disclosed as a new type of donor–acceptor cyclopropane, which smoothly participates in (3 + 2)-cycloadditions with various aldehydes and ketones. This work represents the first application of gem-difluorine substituents as an unconventional donor group for activating cyclopropane substrates in catalytic cycloaddition reactions. With this method, a wide variety of densely functionalized gem-difluorotetrahydrofuran skeletons, which are otherwise difficult to prepare, could be readily assembled in high yields under mild reaction conditions. Computational studies show that the cleavage of the C–C bond between the difluorine and diester moieties occurs upon a S_N2-type attack of the carbonyl oxygen.

A new type of donor–acceptor cyclopropane with gem-difluorine as an unconventional donor group undergoes (3 + 2)-cycloadditions with various aldehydes/ketones, affording densely functionalized gem-difluorotetrahydrofurans.

Cycloadditions of Donor-Acceptor Cyclopropanes and -butanes using S=N-Containing Reagents: Access to Cyclic Sulfinamides, Sulfonamides, and Sulfinamidines

We present (3+2)‐ and (4+2)‐cycloadditions of donor–acceptor (D–A) cyclopropanes and cyclobutanes with N‐sulfinylamines and a sulfur diimide, along with a one‐pot, two‐step strategy for the formal insertion of HNSO₂ into D–A cyclopropanes. These are rare examples of cycloadditions with D–A cyclopropanes and cyclobutanes whereby the 2π component consists of two different heteroatoms, thus leading to five‐ and six‐membered rings containing adjacent heteroatoms.

Visible-Light-Induced Dearomatization of Indoles/Pyrroles with Vinylcyclopropanes: Expedient Synthesis of Structurally Diverse Polycyclic Indolines/Pyrrolines

Visible-light-induced cycloaddition reactions initiated via energy-transfer processes have recently evolved as powerful methods for the construction of strained cyclic molecules that are not easily accessed using known ground-state synthetic methods. Particularly, the reactions initiated by the excitation of aromatic rings provide an alternative solution to the direct transformations of aromatic feedstocks under the scheme of dearomatization. Vinylcyclopropanes (VCPs) are well-known reagents in radical clock experiments, working as a probe to detect transient radical intermediates. However, the synthetic applications in this regard still remain limited due to uncontrollable selectivities. Herein, we report visible-light-induced dearomatization of indole- or pyrrole-tethered VCPs, in which several competitive reaction pathways, including [5 + 2], [2 + 2], interrupted [5 + 2], and [5 + 4] cycloadditions, can be well regulated by engineering substrate structures and tuning reaction conditions. The reaction mechanism has been explored by combined experimental and computational investigations. These reactions provide a convenient method to synthesize structurally diverse polycyclic molecules with high efficiency and good selectivity.

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.

Visible-light-induced, catalyst and additive-free cycloaddition of vinylcyclopropanes: access to sulfur-containing seven-membered heterocycles

The first visible-light-induced, catalyst and additive-free protocol for [5 + 2] cycloadditions of vinylcyclopropanes (VCPs) has been achieved.

Synergistic Palladium-Phosphoric Acid Catalysis in (3 + 2) Cycloaddition Reactions between Vinylcyclopropanes and Imines

The palladium-catalyzed (3 + 2) cycloaddition reaction between vinylcyclopropanes (VCPs) bearing geminal EWG’s and imines represents a straightforward and flexible entry to polysubstituted pyrrolidine derivatives. In this paper, we demonstrate that using a synergistic catalysis approach, based on the combination of phosphoric acid and palladium catalysts, it is possible to engage for the first time N-aryl and N-benzyl imines in this cycloaddition reaction. A range of polysubstituted pyrrolidines is obtained with moderate to good yields and diastereoselectivities, using a simple palladium species (Pd(PPh3)4) and an archetypical phosphoric acid as catalyst combination. A two-step scheme which exploits the same palladium catalyst for two consecutive and mechanistically distinct reactions (the cycloaddition and a Suzuki–Miyaura cross-coupling) is also presented. This synergistic catalysis approach is well posited for the development of the enantioselective version of this reaction. A screening of common BINOL-derived chiral phosphoric acids as catalyst component identified a species giving the product with moderate, yet promising, enantioselectivity (64% ee).

Stereocontrolled [3+2] Cycloaddition of Donor-Acceptor Cyclopropanes to Iminooxindoles: Access to Spiro[oxindole-3,2'-pyrrolidines]

A novel stereocontrolled assembly of spiro[oxindole-3,2'-pyrrolidines] via [3+2]-cycloaddition of donor-acceptor cyclopropanes to electron-poor ketimines, iminooxindoles, was developed. The method allows for efficient employment of common readily available donor-acceptor cyclopropanes, functionalized with ester, keto, nitro, cyano etc. groups, and N-unprotected iminooxindoles. The stereospecificity of the initial S_N2-like imine attack on a cyclopropane molecule together with a high diastereoselectivity of further C-C bond formation facilitate a rapid access to spiro[oxindole-3,2'-pyrrolidines] in their optically active forms. Preliminary in vitro testing of the synthesized compounds against LNCaP (p53+) and PC-3 (p53-) cells revealed good antiproliferative activities and p53-selectivity indices for several compounds that are intriguing in terms of their further investigation as inhibitors of MDM2-p53 interaction.

Chemoselective asymmetric dearomative [3 + 2] cycloaddition reactions of purines with aminocyclopropanes

Chemoselective asymmetric [3 + 2] cycloaddition reactions of purines with aminocyclopropanes for the dearomatization of purines have been successfully developed.

Synthetic applications of vinyl cyclopropane opening

Vinyl cyclopropanes are amongst the most useful building blocks in organic synthesis. Their easy opening and capacity to generate dipoles have been exploited for the synthesis of cyclopentanes with good yields and sometimes excellent stereoselectivities. In this review we give an overview of their applications, focusing on the present century.

Pd-Catalyzed diastereoselective [3 + 2] cycloaddition of vinylcyclopropanes with sulfamate-derived cyclic imines

The palladium-catalyzed diastereoselective [3 + 2] cycloaddition reaction of vinylcyclopropanes with sulfamate-derived cyclic imines worked efficiently under mild conditions, providing sulfamate-fused pyrrolidines in good to excellent yields.

Dearomatization of electron poor six-membered N-heterocycles through [3 + 2] annulation with aminocyclopropanes

Dearomative [3 + 2] annulation of N-heterocycles with donor–acceptor aminocyclopropanes: fast access to indolizidine building blocks.

Many abundant and highly bioactive natural alkaloids contain an indolizidine skeleton. A simple, high yielding method to synthesize this scaffold from N-heterocycles was developed. A wide range of pyridines, quinolines and isoquinolines reacted with donor–acceptor (DA)-aminocyclopropanes via an ytterbium(iii) catalyzed [3 + 2] annulation reaction to give tetrahydroindolizine derivatives. The products were obtained with high diastereoselectivities (dr > 20 : 1) as anti-isomers. Additionally, the formed aminals could be easily converted into secondary and tertiary amines through iminium formation followed by reduction or nucleophile addition. This transformation constitutes the first example of dearomatization of electron-poor six-membered heterocycles via [3 + 2] annulation with DA cyclopropanes.

Recent Methodologies That Exploit C-C Single-Bond Cleavage of Strained Ring Systems by Transition Metal Complexes

In this review, synthetic and mechanistic aspects of key methodologies that exploit C-C single-bond cleavage of strained ring systems are highlighted. The focus is on transition-metal-catalyzed processes that are triggered by C-C bond activation and β-carbon elimination, with the review concentrating on developments from mid-2009 to mid-2016.

Asymmetric [3 + 2] Cycloaddition of Vinylcyclopropanes and α,β-Unsaturated Aldehydes by Synergistic Palladium and Organocatalysis

The stereoselective [3 + 2] cycloaddition between vinylcyclopropanes and α,β-unsaturated aldehydes promoted by combined palladium and organocatalysis is disclosed. The unique synergistic catalytic system allows for the stereoselective formation of highly substituted cyclopentanes with up to four stereocenters in high yields and selectivities. Vinylcyclopropanes with two different geminal substituents facilitate the formation of cyclopentanes containing a quaternary stereocenter. Furthermore, the developed reaction performs well on gram scale, and a number of transformations are demonstrated.

Transition-metal-free coupling reaction of vinylcyclopropanes with aldehydes catalyzed by tin hydride

Donor-acceptor cyclopropanes are useful building blocks for catalytic cycloaddition reactions with a range of electrophiles to give various cyclic products. In contrast, relatively few methods are available for the synthesis of homoallylic alcohols through coupling of vinylcyclopropanes (VCPs) with aldehydes, even with transition-metal catalysts. Here, we report that the hydrostannation of vinylcyclopropanes (VCPs) was effectively promoted by dibutyliodotin hydride (Bu2 SnIH). The resultant allylic tin compounds reacted easily with aldehydes. Furthermore, the use of Bu2 SnIH was effectively catalytic in the presence of hydrosilane as a hydride source, which established a coupling reaction of VCPs with aldehydes for the synthesis of homoallylic alcohols without the use of transition-metal catalysts. In contrast to conventional catalytic reactions of VCPs, the presented method allowed the use of several VCPs in addition to conventional donor-acceptor cyclopropanes.

Diversity-oriented synthesis of acyclic nucleosides via ring-opening of vinyl cyclopropanes with purines

The diversity-oriented synthesis of acyclic nucleosides has been achieved via ring-opening of vinyl cyclopropanes with purines.

Highly enantioselective [3+2] cycloaddition of vinylcyclopropane with nitroalkenes catalyzed by palladium(0) with a chiral bis(tert-amine) ligand

An enantioselective [3+2] cycloaddition of vinyl cyclopropane derived from 1,3-indanedione with nitroalkenes catalyzed by palladium(0) with a chiral bis(tert-amine) ligand was developed in high yields with good diastereoselectivities and excellent enantioselectivities. The resulting bis(tert-amine)-palladium complex proved to be a highly efficient catalyst for this cycloaddition.

1,3-Carbon D–A strategy for [3 + 2] cycloadditions/annulations with imines: synthesis of functionalized pyrrolidines and related alkaloids

[3 + 2] Cycloaddition/annulation: this article summarize the recent developments in synthesis of pyrrolidines and related alkaloids through complementary [3 + 2] cycloaddition/annulation of 1,3-carbon D–A precursors with imines.