Recent topics of Cp∗RuCl-catalyzed annulation reactions

Dehydrogenative [4 + 2] Annulation of 1-Indanones with Alkynes Enabled by In-Situ-Generated Nickel Hydride

A practical and effective nickel-catalyzed dehydrogenative [4 + 2] annulation of 1-indanones with alkynes was reported. In this protocol, nickel-catalyzed desaturation of 1-indanones and nickel hydride catalyzed coupling with alkynes were first incorporated. A cyclopentadiene-type nickel hydride species was generated in situ via β-H elimination, and they subsequently reacted with a wide variety of alkynes to afford various benzo[a]fluorenone derivatives in good yields and regioselectivity.

Synthesis of 5H-chromeno[3,4-c]pyridine derivatives through ruthenium-catalyzed [2 + 2 + 2] cycloaddition

Efficient access to 5H-chromeno[3,4-c]pyridines using Ru-catalyzed [2 + 2 + 2] cycloaddition of α,ω-diynes with cyanamides was developed, providing valuable tricyclic pyridine building blocks and enabling access to a biologically relevant intermediate.

Recent Progress in Metal-Catalyzed [2+2+2] Cycloaddition Reactions

Metal-catalyzed [2+2+2] cycloaddition is a powerful tool that allows rapid construction of functionalized 6-membered carbo- and heterocycles in a single step through an atom-economical process with high functional group tolerance. The reaction is usually regio- and chemoselective although selectivity issues can still be challenging for intermolecular reactions involving the cross-[2+2+2] cycloaddition of two or three different alkynes and various strategies have been developed to attain high selectivities. Furthermore, enantioselective [2+2+2] cycloaddition is an efficient means to create central, axial, and planar chirality and a variety of chiral organometallic complexes can be used for asymmetric transition-metal-catalyzed inter- and intramolecular reactions. This review summarizes the recent advances in the field of [2+2+2] cycloaddition.

1 Introduction

2 Formation of Carbocycles

2.1 Intermolecular Reactions

2.1.1 Cyclotrimerization of Alkynes

2.1.2 [2+2+2] Cycloaddition of Two Different Alkynes

2.1.3 [2+2+2] Cycloaddition of Alkynes/Alkenes with Alkenes/Enamides

2.2 Partially Intramolecular [2+2+2] Cycloaddition Reactions

2.2.1 Rhodium-Catalyzed [2+2+2] Cycloaddition

2.2.2 Molybdenum-Catalyzed [2+2+2] Cycloaddition

2.2.3 Cobalt-Catalyzed [2+2+2] Cycloaddition

2.2.4 Ruthenium-Catalyzed [2+2+2] Cycloaddition

2.2.5 Other Metal-Catalyzed [2+2+2] Cycloaddition

2.3 Totally Intramolecular [2+2+2] Cycloaddition Reactions

3 Formation of Heterocycles

3.1 Cycloaddition of Alkynes with Nitriles

3.2 Cycloaddition of 1,6-Diynes with Cyanamides

3.3 Cycloaddition of 1,6-Diynes with Selenocyanates

3.4 Cycloaddition of Imines with Allenes or Alkenes

3.5 Cycloaddition of (Thio)Cyanates and Isocyanates

3.6 Cycloaddition of 1,3,5-Triazines with Allenes

3.7 Cycloaddition of Aldehydes with Enynes or Allenes/Alkenes

3.8 Totally Intramolecular [2+2+2] Cycloaddition Reactions

4 Conclusion

Ruthenium-Catalyzed Cycloadditions to Form Five-, Six-, and Seven-Membered Rings

Ruthenium-catalyzed cycloadditions to form five-, six-, and seven-membered rings are summarized, including applications in natural product total synthesis. Content is organized by ring size and reaction type. Coverage is limited to processes that involve formation of at least one C-C bond. Processes that are stoichiometric in ruthenium or exploit ruthenium as a Lewis acid (without intervention of organometallic intermediates), ring formations that occur through dehydrogenative condensation-reduction, σ-bond activation-initiated annulations that do not result in net reduction of bond multiplicity, and photochemically promoted ruthenium-catalyzed cycloadditions are not covered.

Divergent functionalization of terminal alkynes enabled alkynylative [5+1] benzannulation of 3-acetoxy-1,4-enynes

We here describe an alkynylative [5+1] benzannulation of 3-acetoxy-1,4-enynes with terminal alkynes, which enables both the construction of a benzene ring skeleton and intermolecular incorporation of an alkynyl group in a single reaction using Pd and Cu cooperative catalysts. The method represents efficient access to internal aryl alkynes through divergent functionalization of two terminal alkyne components: one alkyne serves as the one-carbon unit to realize the [5+1] benzannulation and the other alkyne as a nucleophile terminates the reaction.

Rhodium-catalyzed cycloisomerization of ester-tethered 1,6-diynes with cyclopropanol moiety leading to tetralone/exocyclic diene hybrid molecules

The rhodium-catalyzed cycloisomerization of ester-tethered 1,6-diynes bearing a cyclopropanol moiety produced tetralone/exocyclic diene hybrid molecules with thermodynamically unfavorable alkene geometry. The results of control experiments and density functional theory calculations suggest that the ester tether plays an important role in the efficiency of E/Z isomerization processes.

CpCo(i) precatalysts for [2 + 2 + 2] cycloaddition reactions: synthesis and reactivity

Synthesis of CpCo complexes containing olefins and phosphite/phosphoramidite ligands give access to a reactive class of precatalysts for cyclotrimerisation reactions.

Mechanistic Insights into the Ru(II)-Catalyzed Intramolecular Formal [3 + 2] Cycloaddition of (E)-1,6-Enynes

Design of a unique reaction pathway in transition-metal-catalyzed 1,6-enynes cyclization to construct valuable synthetic motifs is a significant challenge in organic chemistry. Herein, we report a Ru(II)-catalyzed formal [3 + 2] cycloaddition as an efficient method to prepare unprecedented bicyclo[3.3.0]octenes from readily available (E)-1,6-enynes. Mechanistic studies based on the deuterium labeling experiments and the DFT calculation disclose a reasonable mechanistic pathway, where a ruthenacyclopentene generated by an ene-yne oxidative cyclization undergoes a sequential ß-hydride elimination and intramolecular hydroruthenation to form a ruthenacyclohexene, producing the desirable bicyclo[3.3.0]octenes.

Benzene construction via Pd-catalyzed cyclization of 2,7-alkadiynylic carbonates in the presence of alkynes

A palladium-catalyzed highly regio- and chemo-selective cyclization of 2,7-alkadiynylic carbonates with functionalized alkynes to construct 1,3-dihydroisobenzofuran and isoindoline derivatives under mild conditions has been developed. Functional groups such as alcohol, sulfonamide, and indoles could be well tolerated. After careful mechanistic studies, a mechanism involving oxidative addition and regioselectivity-defined double alkyne insertions has been proposed.

Ruthenium Catalyzed Rearrangement of Ene-cyclopropenes. Divergent Reaction Pathways

The reaction of ene-cyclopropenes with Cp*RuCl(cod) leads to alkenyl bicyclo[3.1.0]hexanes, bicyclo[4.1.0]heptanes, and bicyclo[5.1.0]octanes. This reaction involves a reverse regioselectivity in the cyclopropene opening than with gold chlorides. With gem-disubstituted cyclopropenes, a novel cycloisomerization based on ring-opening nucleophilic attack and rearrangement is observed. Alternatively, some gem-disubstituted cyclopropenes give dimerizations of the intermediate carbene.

Synthesis of Fluorescent Azafluorenones and Derivatives via a Ruthenium-Catalyzed [2 + 2 + 2] Cycloaddition

An original and mild synthetic route for the preparation of novel azafluorenones and derivatives via a ruthenium-mediated [2 + 2 + 2] cycloaddition of α,ω-diynes and cyanamides has been developed. This atom-economical catalytic process demonstrated remarkable regioselectivities to access fluorescent azafluorenone derivatives. The photophysical properties of azafluorenone derivatives have been evaluated, and photoluminescence phenomena at solid and liquid states have been highlighted.

A Readily Accessible Class of Chiral Cp Ligands and their Application in RuII -Catalyzed Enantioselective Syntheses of Dihydrobenzoindoles

Chiral cyclopentadienyl (Cp^x ) ligands have a large application potential in enantioselective transition-metal catalysis. However, the development of concise and practical routes to such ligands remains in its infancy. We present a convenient and efficient two-step synthesis of a novel class of chiral Cp^x ligands with tunable steric properties that can be readily used for complexation, giving Cp^x Rh^I , Cp^x Ir^I , and Cp^x Ru^II complexes. The potential of this ligand class is demonstrated with the latter in the enantioselective cyclization of azabenzonorbornadienes with alkynes, affording dihydrobenzoindoles in up to 98:2 e.r., significantly outperforming existing binaphthyl-derived Cp^x ligands.