Transition metal-free formal [2+2+2] cycloaddition reactions of alkynes

Photochemical Manganese-Catalyzed [2 + 2 + 2] Cycloaddition Reactions

We report the cyclotrimerization reactions of triynes using Mn(I) complexes derived from MnBr(CO)5 and phosphine ligands, such as 1,1-bis(diphenylphosphino)methane (dppm). These reactions are driven by irradiation under mild conditions (30-80 °C) without the need of additional photoinitiators. Our catalytic screening revealed that counteranions and ligands significantly influence the process. This method accommodates a broad range of functionalities in the substrates, including alkyl, aryl, Bpin, SiMe3, GeEt3, PPh2, pyridyl, and thienyl moieties, without notable interference in the transformation. Additionally, this method enables reactions with oligoalkynes-like (un)substituted hexaynes, producing 2-fold cyclization products in very good yields. Under stoichiometric conditions, the cyclization of diynes with phosphaalkynes results in the unique photochemical synthesis of phosphinines. Experimental and theoretical mechanistic studies indicate that the dissociation of the diphosphine ligand precedes the involvement of the Mn carbonyl species in the catalytic cycle. The ligand plays a crucial role in stabilizing the catalyst during the catalytic transformation and preventing the formation of unreactive cluster species.

Electrochemical Homo- and Crossannulation of Alkynes and Nitriles for the Regio- and Chemoselective Synthesis of 3,6-Diarylpyridines

We disclose a mediated electrochemical [2+2+2] annulation of alkynes with nitriles, forming substituted pyridines in a single step from low-cost, readily available starting materials. The combination of electrochemistry and a triarylamine redox mediator obviates the requirements of transition metals and additional oxidants. Besides the formation of diarylpyridine moieties via the homocoupling of two identical alkynes, the heterocoupling of two different alkynes depending on their electronic nature is possible, highlighting the unprecedented control of chemoselectivity in this catalytic [2+2+2] process. Mechanistic investigations like cyclic voltammetry and crossover experiments combined with DFT calculations indicate the initial oxidation of an alkyne as the key step leading to the formation of a vinyl radical cation intermediate. The utilization of continuous flow technology proved instrumental for an efficient process scale-up. The utility of the products is exemplified by the synthesis of π-extended molecules, being relevant for material or drug synthesis.

Palladium-Copper-Catalyzed Oxidative Intermolecular [2 + 2 + 2] Coupling-Annulation: Regioselective Synthesis of Multiaryl-Substituted Benzenes

A palladium-catalyzed intermolecular [2 + 2 + 2] oxidative coupling-annulation of terminal alkenes and alkynes using copper(II) as the oxidant has been developed through direct C-C bond formation. These reactions provide effective access to multiaryl-substituted benzenes with high regioselectivity in the absence of any ligands. The features of this protocol are broad substrate scope, and high atom and step economy. The aggregation-induced emission properties of selected products were further investigated. These synthesized multiaryl-substituted benzenes may be worth exploring for further applications in the fields of advanced functional materials or drugs.

Ni0-Catalyzed Regioselective [2 + 2 + 2] Cyclotrimerization of 1,3-Diynes: An Expeditious Synthesis of Hexasubstituted Alkynyl Benzenes

In the present work, we demonstrate a regioselective [2 + 2 + 2] cyclotrimerization of 1,3-diynes catalyzed by Ni0 to provide hexasubstituted benzenes (HSBs). HSBs have significant applications as functional materials and pharmaceuticals. The present protocol exhibited remarkable versatility, transforming 1,3-diynes with diverse alkyl, aryl, and heterocyclic groups to the corresponding HSBs. With the help of control experiments and density functional theory (DFT), the mechanism of the reaction and the origin of regioselectivity were elucidated.

Divergent Synthesis of Oxepino-Phthalides and [5,5]-Oxaspirolactones through [2 + 2 + 2]- and [2 + 3]-Annulation of Alkynyl Alcohols with α-Ynone-Esters

Unmasking the synthetic potential of alkyne functional group of alkynyl alcohols as surrogates of carbonyl compounds, herein we present the first Brønsted acid (TfOH)-catalyzed [2 + 2 + 2]-annulation of 4-pentyn-1-ols (possessing terminal alkyne) with α-ynone-esters to access tricyclic tetrahydro-oxepino-phthalides. Besides, an unprecedented synthesis of α-acetoaryl or α-alkynyl [5,5]-oxaspirolactones has been demonstrated by employing 4-pentyn-1-ols (possessing an internal alkyne) as an annulation partner, which proceeds through a divergent [2 + 3]-annulation pathway.

Polymer-Supported-Cobalt-Catalyzed Regioselective Cyclotrimerization of Aryl Alkynes

A convoluted poly(4-vinylpyridine) cobalt(II) (P4VP-CoCl2) system was developed as a stable and reusable heterogeneous catalyst. The local structure near the Co atom was determined on the basis of experimental data and theoretical calculations. This immobilized cobalt catalyst showed high selectivity and catalytic activity in the [2 + 2 + 2] cyclotrimerization of terminal aryl alkynes. With 0.033 mol % P4VP-CoCl2, the regioselective formation of 1,3,5-triarylbenzene was realized without 1,2,4-triarylbenzene formation. Further, a multigram-scale (11 g) reaction proceeded efficiently. In addition, the polymer-supported catalyst was successfully recovered and used three times. X-ray photoelectron spectroscopy analysis of the recovered catalyst suggested that cobalt was in the +2 oxidation state. The 1,3,5-triarylbenzene derivatives were applied to the synthesis of a molecular beam electron resist and a polycyclic aromatic hydrocarbon.

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

Computational prediction on the catalytic activity of heterobimetallic complex featuring MM' triple bond in acetylene cyclotrimerization: Mechanistic study

A detailed reaction mechanism of acetylene cyclotrimerization catalyzed by V(ⁱ PrNPMe₂ )₃ Fe-PMe₃ (denote as CAT), a heterobimetallic complex featuring V-Fe triple bond, was computationally investigated using density functional theory. The calculated results show that the first acetylene firstly attaches to the V atom of CAT to get a four-membered ring structure through [2 + 2] cycloaddition reaction. For the second acetylene addition, there are two cyclotrimerization mechanisms, outer sphere mechanism and inner mechanism. The inner sphere reaction pathway is the main reaction pathway. By replacing the V with Nb and Ta, Fe with Ru and Os, a series of new catalysts are screened computationally. The calculated results show that, all of the nine heterobimetallic complexes show high activity at mild condition. The energy barrier of the rate determining step is related to the natural population analysis (NPA) charge of M' and the Wiberg bond index (WBI) of M-M' bond. The more negative NPA charge of M' and the smaller WBI of M-M' bond, the lower energy barrier is.

Diels–Alder cycloaddition polymerization of highly aromatic polyimides and their multiblock copolymers

A novel route to highly aromatic polyimides is presented and is used to form multiblock copolymers which is inherently difficult to achieve via traditional routes for this important polymer family.

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.

Synthesis and Utility of 2,2-Dimethyl-2 H-pyrans: Dienes for Sequential Diels-Alder/Retro-Diels-Alder Reactions

The practical use of 2,2-dimethyl-2 H-pyrans as electron-rich dienes in sequential Diels-Alder/retro-Diels-Alder (DA/rDA) domino processes to generate aromatic platforms has been demonstrated. Different polysubstituted alkyl 2-naphthoates have been synthesized by the DA/rDA reaction of benzynes and 2,2-dimethyl-2 H-pyrans. The use of other activated alkynes allows the access of substituted alkyl benzoate derivatives.

Formal Bimolecular [2 + 2 + 2] Cycloaddition Strategy for the Synthesis of Pyridines: Intramolecular Propargylic Ene Reaction/Aza Diels-Alder Reaction Cascades

Two methods for the synthesis of multisubstituted pyridines are described. In each strategy, a highly reactive vinylallene is generated via an intramolecular propargylic ene reaction in the presence of an azadienophile. Reactions employing ethyl N-(tosyl)iminoacetate furnish an intermediate that undergoes elimination and isomerization upon the addition of DBU. The reaction of the intermediate vinylallene with TsCN leads to the isolation of a 2-sulfonylpyridine that serves as a versatile intermediate undergoing substitution reactions with oxygen and carbon nucleophiles.

2-Picoline catalyst-triggered [2 + 2 + 2] cycloaddition-type reaction of acetylenedicarboxylates, aldehydes and alkenes

2-Picoline efficiently catalyzes the formation of α,β-enones from acetylenedicarboxylates and aldehydes in the presence of alkenes, thereby leading to pyrans in moderate to good yields with complete regioselectivities. This synthetic method of pyrans is represented as a first example of catalytic and direct [2 + 2 + 2] cycloaddition of three different components under the metal-free conditions.

FeCl3-Assisted Niobium-Catalyzed Cycloaddition of Nitriles and Alkynes: Synthesis of Alkyl- and Arylpyrimidines Based on Independent Functions of NbCl5 and FeCl3 Lewis Acids

NbCl₅-catalyzed [2 + 2 + 2] cycloaddition of nitriles with alkynes was used to synthesize pyrimidine derivatives. In this reaction, the use of individual Lewis acids, namely NbCl₅ and FeCl₃, is a key strategy for achieving the reaction using a catalytic amount of NbCl₅. The roles of the two Lewis acids were investigated using FT-IR spectroscopy. The results showed that NbCl₅ served as an efficient Lewis acid catalyst for nitrile activation, whereas FeCl₃ showed stronger Lewis acidity toward pyrimidines, releasing NbCl₅ into the catalytic cycle.