Air‐Stable {(C5H5)Co} Catalysts for [2+2+2] Cycloadditions

Cobalt cyclopentadienyl complexes incorporating a fumarate and a CO ligand (see picture) efficiently catalyze inter‐ and intramolecular [2+2+2] cycloadditions of alkynes, nitriles, and/or alkenes to give benzenes, pyridines, or 1,3‐cyclohexadienes. Unlike catalysts such as [CpCo(CO)₂] or [CpCo(C₂H₄)₂] (Cp=C₅H₅), they are air‐stable, easy to handle, compatible with microwave conditions, and do not necessarily require irradiation to be active.

     

Chiral Silver Amides as Effective Catalysts for Enantioselective [3+2] Cycloaddition Reactions

Asymmetric [3+2] cycloaddition of α‐aminoester Schiff bases with substituted olefins is one of the most efficient methods for the preparation of chiral pyrrolidine derivatives in optically pure form. In spite of its potential utility, applicable substrates for this method have been limited to Schiff bases that bear relatively acidic α‐hydrogen atoms. Here we report a chiral silver amide complex for asymmetric [3+2] cycloaddition reactions. A silver complex prepared from silver bis(trimethylsilyl)amide (AgHMDS) and (R)‐DTBM‐SEGPHOS worked well in asymmetric [3+2] cycloaddition reactions of α‐aminoester Schiff bases with several olefins to afford the corresponding pyrrolidine derivatives in high yields with remarkable exo‐ and enantioselectivities. Furthermore, α‐aminophosphonate Schiff bases, which have less acidic α‐hydrogen atoms, also reacted with olefins with high exo‐ and enantioselectivities. The stereoselectivities of the [3+2] cycloadditions with maleate and fumarate suggested that the reaction proceeded by means of a concerted mechanism. An NMR spectroscopic study indicated that complexation of AgHMDS with the bisphosphine ligand was not complete, and that free AgHMDS, which did not show any significant catalytic activity, existed in the catalyst solution. This means that significant ligand acceleration occurred in the current reaction system.     

[3+2] Fragmentation of a Pentaphosphido Ligand by Cyanide

The activation of white phosphorus (P₄) by transition‐metal complexes has been studied for several decades, but the functionalization and release of the resulting (organo)phosphorus ligands has rarely been achieved. Herein we describe the formation of rare diphosphan‐1‐ide anions from a P₅ ligand by treatment with cyanide. Cobalt diorganopentaphosphido complexes have been synthesized by a stepwise reaction sequence involving a low‐valent diimine cobalt complex, white phosphorus, and diorganochlorophosphanes. The reactions of the complexes with tetraalkylammonium or potassium cyanide afford a cyclotriphosphido cobaltate anion 5 and 1‐cyanodiphosphan‐1‐ide anions [R₂PPCN]^? ( 6‐R ). The molecular structure of a related product 7 suggests a novel reaction mechanism, where coordination of the cyanide anion to the cobalt center induces a ligand rearrangement. This is followed by nucleophilic attack of a second cyanide anion at a phosphorus atom and release of the P₂ fragment.     

Tuning the Stability and Reactivity of Metal‐bound Alkylperoxide by Remote Site Substitution of the Ligand

An alkylperoxonickel(II) complex with hydrotris(3,5‐diisopropyl‐4‐bromo‐1‐pyrazolyl)borate, [Ni^II(OOtBu)(Tp^iPr2,Br)] ( 3a ), is synthesized, and its chemical properties are compared with those of the prototype non‐brominated ligand derivative [Ni^II(OOtBu)(Tp^iPr2)] ( 3b ; Tp^iPr2=hydrotris(3,5‐diisopropyl‐1‐pyrazolyl)borate). Same synthetic procedures for the prototype 3b and its precursors can be employed to the synthesis of the Tp^iPr2,Br analogues. The dimeric nickel(II)‐hydroxo complex, [(Ni^IITp^iPr2,Br)₂(μ‐OH)₂] ( 2a ), can be synthesized by the base hydrolysis of the labile complexes [Ni^II(Y)(Tp^iPr2,Br)] (Y=NO₃ ( 1a ), OAc ( 1a′ )), which are obtained by the metathesis of NaTp^iPr2,Br with the corresponding nickel(II) salts, and the following dehydrative condensation of 2a with the stoichiometric amount of tert‐butylhydroperoxide yields 3a . The unique structural characteristics of the prototype 3b , that is, highly distorted geometry of the nickel center and intermediate coordination mode of the O O moiety between η¹ and η², are kept in the brominated ligand analogue 3a . The introduction of the electron‐withdrawing substitutents on the distal site of Tp^R affects the thermal stability and reactivity of the nickel(II)‐alkylperoxo species.     

Cobalt‐Catalyzed Intramolecular Reactions between a Vinylcyclopropane and an Alkyne: Switchable [5+2] Cycloaddition and Homo‐Ene Pathways

Cobalt–diphosphine catalysts have been found to promote intramolecular reactions between a vinylcyclopropane and an alkyne to selectively afford either the [5+2] cycloaddition product or the homo‐ene reaction product under solvent control. The former product is exclusively formed in noncoordinating 1,2‐dichloroethane, whereas the latter is dominant in coordinating solvents, such as acetonitrile and dimethylacetamide. Furthermore, a highly enantioselective variant of the homo‐ene reaction afforded chiral tetrahydrofuran, pyrrolidine, and cyclopentane derivatives bearing 1,3‐diene and alkylidene substituents.     

Gold‐Catalyzed Transannular [4+3] Cycloaddition Reactions

Macrocyclic propargyl acetates containing a furan ring were prepared by using a CrCl₂‐promoted reaction. In the presence of either a Au^I or Au^III catalyst, a tandem 3,3‐rearrangement/transannular [4+3] cycloaddition reaction occurred to give propargyl acetates that are regio‐ and diastereospecific. The regiochemistry of the product is controlled by the position of the acetoxy group in the starting material and the stereochemistry of the reaction depends on the ring size.     

Calcium‐Catalyzed Formal [2+2+2] Cycloaddition

A formal intermolecular [2+2+2] cycloaddition reaction of enynes to aldehydes is presented, which can be realized in the presence of a simple and benign calcium catalyst. The reaction proceeds with excellent chemo, regio‐ and diastereoselectivity and leads to a one‐step assembly of highly interesting bicyclic building blocks containing up to three stereocenters from simple precursors via a new type of skeletal rearrangement of enynes. The observed diastereoselectivity is accounted for by two different mechanistic proposals. The first one engages mechanistic prospects arising from a gold catalyzed reaction in the absence of the stabilizing gold substituent. The second proposal involves an unprecedented cyclization–carbonyl allene ene reaction–hydroalkoxylation cascade.     

Direct Evidence for a [4+2] Cycloaddition Mechanism of Alkynes to Tantallacyclopentadiene on Dinuclear Tantalum Complexes as a Model of Alkyne Cyclotrimerization

A dinuclear tantalum complex, [Ta₂Cl₆(μ‐C₄Et₄)] ( 2 ), bearing a tantallacyclopentadiene moiety, was synthesized by treating [(η²‐EtC?CEt)TaCl₃(DME)] ( 1 ) with AlCl₃. Complex 2 and its Lewis base adducts, [Ta₂Cl₆(μ‐C₄Et₄)L] (L=THF ( 3 a ), pyridine ( 3 b ), THT ( 3 c )), served as more active catalysts for cyclotrimerization of internal alkynes than 1 . During the reaction of 3 a with 3‐hexyne, we isolated [Ta₂Cl₄(μ‐η⁴:η⁴‐C₆Et₆)(μ‐η²:η²‐EtC?CEt)] ( 4 ), sandwiched by a two‐electron reduced μ‐η⁴:η⁴‐hexaethylbenzene and a μ‐η²:η²‐3‐hexyne ligand, as a product of an intermolecular cyclization between the metallacyclopentadiene moiety and 3‐hexyne. The formation of arene complexes [Ta₂Cl₄(μ‐η⁴:η⁴‐C₆Et₄Me₂)(μ‐η²:η²‐Me₃SiC?CSiMe₃)] ( 7 b ) and [Ta₂Cl₄(μ‐η⁴:η⁴‐C₆Et₄RH)(μ‐η²:η²‐Me₃SiC?CSiMe₃)] (R=nBu ( 8 a ), p‐tolyl ( 8 b )) by treating [Ta₂Cl₄(μ‐C₄Et₄)(μ‐η²:η²‐Me₃SiC?CSiMe₃)] ( 6 ) with 2‐butyne, 1‐hexyne, and p‐tolylacetylene without any isomers, at room temperature or low temperature were key for clarifying the [4+2] cycloaddition mechanism because of the restricted rotation behavior of the two‐electron reduced arene ligands without dissociation from the dinuclear tantalum center.     

Synthesis of Tricyclic Fused 3‐Aminopyridines through Intramolecular CoI‐Catalyzed [2+2+2] Cycloaddition between Ynamides,Nitriles, and Alkynes

Three‐ring circus : An expedient route to tricyclic fused 2‐trimethylsilyl‐3‐aminopyridines exhibiting unprecedented skeletons is described. The key step is a very efficient cobalt‐catalyzed [2+2+2] cycloaddition of a polyunsaturated compound displaying an ynamide, an alkyne, and a nitrile functionality (see picture).

     

[2+2]环加成反应机理的理论研究

[2+2]环加成反应是有机化学中非常重要的一类反应,其机理的研究一直是实验和理论工作者关注的课题之一。本文从理论的角度综述了三类[2+2]环加成反应的反应机理,即简单烯烃或炔烃参与的环加成反应、累积双键体系参与的环加成反应以及稀土钍化合物参与的环加成反应, 得出对于简单的烯烃或炔烃之间的环加成反应一般是按双自由基机理进行,而其他两类反应主要按协同或两性离子方式进行,并且从前线分子轨道作用理论角度分析了产生不同反应机理的原因。     

[2+1] Cycloaddition Affording Methylene‐ and Vinylidenecyclopropane Derivatives: A Journey around the Reactivity of Metal‐Phosphinito–Phosphinous Acid Complexes

Metal–phosphinito–phosphinous acid complexes are interesting catalysts exhibiting unique reactivities. In this account, we intend to provide a clear overview of palladium– and platinum–phosphinito–phosphinous acid complexes, their preparation from secondary phosphine oxides, and their applications in catalysis. They have been mainly used to develop [2+1] cycloadditions to afford methylenecyclopropane derivatives using norbornenes and various alkynes as partners. As a function of the catalyst, the reaction conditions, or the nature of the reagents, different synthetic transformations have been observed: [2+1] cycloadditions, giving rise to either alkylidenecyclopropanes or vinylidenecyclopropanes; tandem [2+1]/[3+2] cycloadditions, and so forth. The mechanisms of these reactions have been studied to rationalize the different reactivities observed.     

Enantio‐ and Diastereoselective Synthesis of Chiral Allenes by Palladium‐Catalyzed Asymmetric [3+2] Cycloaddition Reactions

A protocol for the asymmetric synthesis of highly substituted chiral allenes with control of point and axial chirality has been developed. A palladium‐catalyzed [3+2] cycloaddition using readily available racemic allenes gives access to densely functionalized chiral allenes with excellent yields and functional group tolerance. The catalytic asymmetric protocol utilizes a broad range of allenyl TMM (trimethylenemethane) donors to form cyclopentanes, pyrrolidines, and spirocycles with very good control of regio‐, enantio‐, and diastereoselectivity. The chiral allene moiety is shown to be a valuable functional group for rapid elaboration towards complex targets.     

Enantioselective Synthesis of [9]‐ and [11]Helicene‐like Molecules: Double Intramolecular [2+2+2] Cycloaddition

The enantioselective synthesis of completely ortho‐fused [9]‐ and [11]helicene‐like molecules has been achieved through a rhodium‐mediated, intramolecular, double [2+2+2] cycloaddition of phenol‐ or 2‐naphthol‐linked hexaynes. Crystal structures and photophysical properties of these [9]‐ and [11]helicene‐like molecules have also been disclosed.     

Rhodium‐Catalyzed Asymmetric Synthesis of Silicon‐Stereogenic Dibenzosiloles by Enantioselective [2+2+2] Cycloaddition

A rhodium‐catalyzed asymmetric synthesis of silicon‐stereogenic dibenzosiloles has been developed through a [2+2+2] cycloaddition of silicon‐containing prochiral triynes with internal alkynes. High yields and enantioselectivities have been achieved by employing an axially chiral monophosphine ligand, and the present catalysis is also applicable to the asymmetric synthesis of a germanium‐stereogenic dibenzogermole. Preliminary studies on the optical properties of these compounds are also described.     

钌催化二环烯烃与炔烃的[2＋2]环加成反应的研究进展

综述了近年来钌催化二环烯烃与炔烃发生[2＋2]环加成反应的研究进展, 重点讨论了钌催化二环烯烃和炔烃的 [2＋2]环加成反应中反应物结构、反应物比例、溶剂及温度等因素对反应的影响.