Trimethylsilyl‐Protected Alkynes as Selective Cross‐Coupling Partners in Titanium‐Catalyzed [2+2+1] Pyrrole Synthesis

Trimethylsilyl (TMS)‐protected alkynes served as selective alkyne cross‐coupling partners in titanium‐catalyzed [2+2+1] pyrrole synthesis. Reactions of TMS‐protected alkynes with internal alkynes and azobenzene under the catalysis of titanium imido complexes yielded pentasubstituted 2‐TMS‐pyrroles with greater than 90 % selectivity over the other nine possible pyrrole products. The steric and electronic effects of the TMS group were both identified to play key roles in this highly selective pyrrole synthesis. This strategy provides a convenient method to synthesize multisubstituted pyrroles as well as an entry point for further pyrrole diversification through facile modification of the resulting 2‐silyl pyrrole products, as demonstrated through a short formal synthesis of the marine natural product lamellarin R.     

Co(I)- versus Ru(II)-Catalyzed [2+2+2] cycloadditions involving alkynyl halides

Alkynyl chlorides, bromides, and iodides have been tested as [2 + 2 + 2] cycloaddition partners using CpCo(CO)(dimethylfumarate) and Cp*Ru(cod)Cl as precatalysts. A series of cocyclizations between diynyl dihalides and alkynes, as well as intramolecular cycloadditions of triynyl dihalides, has been carried out. While this study confirmed the versatility of the ruthenium complex with all kinds of halides, the cheap air-stable cobalt complex proved nonetheless efficient with alkynyl bromides.     

Tertiary Carbinamine Synthesis by Rhodium‐Catalyzed [3+2] Annulation of N‐Unsubstituted Aromatic Ketimines and Alkynes

A convenient and waste‐free synthesis of indene‐based tertiary carbinamines by rhodium‐catalyzed imine/alkyne [3+2] annulation is described. Under the optimized conditions of 0.5–2.5 mol % [{(cod)Rh(OH)}₂] (cod=1,5‐cyclooctadiene) catalyst, 1,3‐bis(diphenylphosphanyl)propane (DPPP) ligand, in toluene at 120 °C, N‐unsubstituted aromatic ketimines and internal alkynes were coupled in a 1:1 ratio to form tertiary 1H‐inden‐1‐amines in good yields and with high selectivities over isoquinoline products. A plausible catalytic cycle involves sequential imine‐directed aromatic C? H bond activation, alkyne insertion, and a rare example of intramolecular ketimine insertion into a Rh^I–alkenyl linkage.     

Dihydrobiphenylenes through Ruthenium‐Catalyzed [2+2+2] Cycloadditions of ortho‐Alkenylarylacetylenes with Alkynes

A new synthetic route to dihydrobiphenylenes has been developed. The process involves a mild Ru^II‐catalyzed [2+2+2] dimerization of ortho‐alkenylarylacetylenes or its more versatile variant, the Ru‐catalyzed [2+2+2] cycloaddition of ortho‐ethynylstyrenes with alkynes. Mechanistic aspects of this [2+2+2] cycloaddition are discussed.     

A Mechanistic Study of the Utilization of arachno‐Diruthenaborane [(Cp*RuCO)2B2H6] as an Active Alkyne‐Cyclotrimerization Catalyst

The reaction of nido‐[1,2‐(Cp*RuH)₂B₃H₇] ( 1 a , Cp*=η⁵‐C₅Me₅) with [Mo(CO)₃(CH₃CN)₃] under mild conditions yields the new metallaborane arachno‐[(Cp*RuCO)₂B₂H₆] ( 2 ). Compound 2 catalyzes the cyclotrimerization of a variety of internal‐ and terminal alkynes to yield mixtures of 1,3,5‐ and 1,2,4‐substituted benzenes. The reactivities of nido‐ 1 a and arachno‐ 2 with alkynes demonstrates that a change in geometry from nido to arachno drives a change in the reaction from alkyne‐insertion to catalytic cyclotrimerization, respectively. Density functional calculations have been used to evaluate the reaction pathways of the cyclotrimerization of alkynes catalyzed by compound 2 . The reaction involves the formation of a ruthenacyclic intermediate and the subsequent alkyne‐insertion step is initiated by a [2+2] cycloaddition between this intermediate and an alkyne. The experimental and quantum‐chemical results also show that the stability of the metallacyclic intermediate is strongly dependent on the nature of the substituents that are present on the alkyne.     

Synthesis of 1,3‐Bis(tetracyano‐2‐azulenyl‐3‐butadienyl)azulenes by the [2+2] Cycloaddition–Retroelectrocyclization of 1,3‐Bis(azulenylethynyl)azulenes with Tetracyanoethylene

1,3‐Bis(azulenylethynyl)azulene derivatives 9–14 have been prepared by palladium‐catalyzed alkynylation of 1‐ethynylazulene 8 with 1,3‐diiodoazulene 1 or 1,3‐diethynylazulene 2 with the corresponding haloazulenes 3–7 under Sonogashira–Hagihara conditions. Bis(alkynes) 9–14 reacted with tetracyanoethylene (TCNE) in a formal [2+2] cycloaddition–retroelectrocyclization reaction to afford the corresponding new bis(tetracyanobutadiene)s (bis(TCBDs)) 15–20 in excellent yields. The redox behavior of bis(TCBD)s 15–20 was examined by using CV and differential pulse voltammetry (DPV), which revealed their reversible multistage reduction properties under the electrochemical conditions. Moreover, a significant color change of alkynes 9–14 and TCBDs 15–20 was observed by visible spectroscopy under the electrochemical reduction conditions.     

Chemo‐ and Regioselective Rhodium(I)‐Catalyzed [2+2+2] Cycloaddition of Allenynes with Alkynes

A highly chemo‐ and regioselective partially intramolecular rhodium(I)‐catalyzed [2+2+2] cycloaddition of allenynes with alkynes is described. A range of diverse polysubstituted benzene derivatives could be synthesized in good to excellent yields, in which the allenynes served as synthetic equivalent to the diynes. A high regioselectivity could be observed when allenynes were treated with unsymmetrical alkynes.     

Rhodium‐Catalyzed [3+2+2] and [2+2+2] Cycloadditions of Two Alkynes with Cyclopropylideneacetamides

It has been established that a cationic rhodium(I)/H₈‐binap complex catalyzes the [3+2+2] cycloaddition of 1,6‐diynes with cyclopropylideneacetamides to produce cycloheptadiene derivatives through cleavage of cyclopropane rings. In contrast, a cationic rhodium(I)/(S)‐binap complex catalyzes the enantioselective [2+2+2] cycloaddition of terminal alkynes, acetylenedicarboxylates, and cyclopropylideneacetamides to produce spiro‐cyclohexadiene derivatives which retain the cyclopropane rings.     

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.     

Ru(II)-catalyzed [2 + 2 + 2] cycloaddition of 1,2-bis(propiolyl)benzenes with monoalkynes leading to substituted anthraquinones

[2 + 2 + 2] cycloadditions of 1,2-bis(propiolyl)benzenes with monoalkynes were effectively catalysed by Cp*RuCl(cod) under mild conditions to give substituted anthraquinones in moderate to high yields.     

Nickel‐Catalyzed Synthesis of N‐Aryl‐1,2‐dihydropyridines by [2+2+2] Cycloaddition of Imines with Alkynes through T‐Shaped 14‐Electron Aza‐Nickelacycle Key Intermediates

Despite there being a straightforward approach for the synthesis of 1,2‐dihydropyridines, the transition‐metal‐catalyzed [2+2+2] cycloaddition reaction of imines with alkynes has been achieved only with imines containing an N‐sulfonyl or ‐pyridyl group. Considering the importance of 1,2‐dihydropyridines as useful intermediates in the preparation of a wide range of valuable organic molecules, it would be very worthwhile to provide novel strategies to expand the scope of imines. Herein we report a successful expansion of the scope of imines in nickel‐catalyzed [2+2+2] cycloaddition reactions with alkynes. In the presence of a nickel(0)/PCy₃ catalyst, a reaction with N‐benzylidene‐P,P‐diphenylphosphinic amide was developed. Moreover, an application of N‐aryl imines to the reaction was also achieved by adopting N‐heterocyclic carbene ligands. The isolation of an (η²‐N‐aryl imine)nickel(0) complex containing a 14‐electron nickel(0) center and a T‐shaped 14‐electron five‐membered aza‐nickelacycle is shown. These would be considered as key intermediates of the reaction. The structure of these complexes was unambiguously determined by NMR spectroscopy and X‐ray analyses.     

Copper‐Catalyzed [2+2+2] Modular Synthesis of Multisubstituted Pyridines: Alkenylation of Nitriles with Vinyliodonium Salts

A [2+2+2] modular synthesis of multisubstituted pyridines, with excellent regioselectivity, has been realized by copper catalysisand involves three distinct components: vinyliodonium salts, nitriles, and alkynes. The reactions proceeded with the facile formation of an aza‐butadienylium intermediate by alkenylation of the nitrile with a vinyliodonium salt. Moreover, the alkynes in the reaction were extended to alkenes, which are an advantage of expense and relative scarceness of alkynes.     

Rhodium(III)‐Catalyzed [3+2]/[5+2] Annulation of 4‐Aryl 1,2,3‐Triazoles with Internal Alkynes through Dual C(sp2)H Functionalization

A rhodium(III)‐catalyzed [3+2]/[5+2] annulation of 4‐aryl 1‐tosyl‐1,2,3‐triazoles with internal alkynes is presented. This transformation provides straightforward access to indeno[1,7‐cd]azepine architectures through a sequence involving the formation of a rhodium(III) azavinyl carbene, dual C(sp²)? H functionalization, and [3+2]/[5+2] annulation.     

Cobalt‐Mediated Linear 2:1 Co‐oligomerization of Alkynes with Enol Ethers to Give 1‐Alkoxy‐1,3,5‐Trienes: A Missing Mode of Reactivity

A variety of 1,6‐heptadiynes and certain borylalkynes co‐oligomerize with enol ethers in the presence of [CpCo(C₂H₄)₂] (Cp=cyclopentadienyl) to furnish the hitherto elusive acyclic 2:1 products, 1,3,5‐trien‐1‐ol ethers, in preference to or in competition with the alternative pathway that leads to the standard [2+2+2] cycloadducts, 5‐alkoxy‐1,3‐cyclohexadienes. Minor variations, such as lengthening the diyne tether, cause reversion to the standard mechanism. The trienes, including synthetically potent borylated derivatives, are generated with excellent levels of chemo‐, regio‐, and diastereoselectivity, and are obtained directly by decomplexation of the crude mixtures during chromatography. The cyclohexadienes are isolated as the corresponding dehydroalkoxylated arenes. In one example, even ethene functions as a linear cotrimerization partner. The alkoxytrienes are thermally labile with respect to 6π‐electrocyclization–elimination to give the same arenes that are the products of cycloaddition. The latter, regardless of the mechanism of their formation, can be viewed as the result of a formal [2+2+2] cyclization of the starting alkynes with acetylene. One‐pot conditions for the exclusive formation of arenes are developed. DFT computations indicate that cyclohexadiene and triene formation share a common intermediate, a cobaltacycloheptadiene, from which reductive elimination and β‐hydride elimination compete.     

A [2+2+2]‐Cyclotrimerization Approach to Selectively Substituted Fluorenes and Fluorenols,and Their Conversion to 9,9′‐Spirobifluorenes

Synthesis of selectively substituted fluorenes and fluorenols was achieved by using catalytic [2+2+2]cyclotrimerization. Various starting diynes were reacted with different alkynes in the presence of a catalytic amount of Wilkinson’s catalyst (RhCl(PPh₃)₃) providing the compounds possessing the fluorene scaffold in good isolated yields. A set of four regioselectively substituted fluorenols was converted to the corresponding 9,9′‐spirobifluorenes and their spectral characteristics were measured.     

Nickel-catalyzed intermolecular [3 + 2 + 2] cocyclization of ethyl cyclopropylideneacetate and alkynes. Synthesis of seven-membered carbocycles

The [3 + 2 + 2] cocyclization of ethyl cyclopropylideneacetate (1a) and various alkynes proceeded smoothly in the presence of Ni(cod)2-PPh3. The cycloheptadiene derivatives were synthesized in highly selective manners. The unique reactivity of 1a was essential for the progress of the reaction. The observed regioselectivity of the product formation and the mechanism of the reaction are discussed.     

Stereoselective Nickel‐Catalyzed [2+2] Cycloadditions of Ene‐Allenes

A stereoselective nickel‐catalyzed [2+2] cycloaddition of ene‐allenes is reported. This transformation encompasses a broad range of ene‐allene substrates, thus providing efficient access to fused cyclobutanes from easily accessed π‐components. A simple and inexpensive first‐row catalytic system comprised of [Ni(cod)₂] and dppf was used in this process, thus constituting an attractive approach to synthetically challenging cyclobutane frameworks under mild reaction conditions.     

Ni‐Catalyzed Synthesis of Fluoroarenes via [2+2+2] Cycloaddition Involving α‐Fluorine Elimination

A method for direct synthesis of tetrasubstituted fluoroarenes via nickel‐catalyzed [2+2+2] cycloaddition is presented. The reaction combines one molecule of 1,1‐difluoroethylene with two molecules of alkynes and involves sequential cleavage of the C?F and C?H bonds in difluoroethylene. The catalytic cycle is established by reduction of the intermediary Ni^II fluoride with a triethylborane‐based borate.     

Nickel-catalyzed intermolecular [3 + 2 + 2] cocyclization of ethyl cyclopropylideneacetate and alkynes

The [3 + 2 + 2] cocyclization of ethyl cyclopropylideneacetate (1a) and terminal alkynes (2) proceeded smoothly in the presence of 10 mol % "Ni(PPh3)2", which was prepared in situ from Ni(cod)2 and PPh3. The high reactivity of 1a, which was induced by the introduction of an electron-withdrawing group, is very important for the progress of this reaction. The cycloheptadiene derivatives were synthesized in highly selective manner in good yields.     

Valence‐to‐Core X‐Ray Emission Spectroscopy of Iron–Carbonyl Complexes: Implications for the Examination of Catalytic Intermediates

Valence‐to‐core X‐ray emission spectroscopy (V2C XES) has been applied to a series of compounds relevant to both homogeneous catalysts and intermediates in heterogeneous reactions, namely [Fe(CO)₅], [Fe₂(CO)₉], [Fe₃(CO)₁₂], [Fe(CO)₃(cod)] (cod=cyclo‐octadienyl), [Fe₂Cp₂(CO)₄] (Cp=cyclo‐pentadienyl), [Fe₂Cp*₂(CO)₄] (Cp*=tetramethylcyclopentadienyl), and [FeCp(CO)₂(thf)][B(ArF)₄] (ArF=pentafluorophenyl). DFT calculations of the V2C XES spectra show very good agreement with experiment, which allows for an in depth analysis of the origins of the observed spectral signatures. It is demonstrated that the observed spectral features can be broken down into specific ligand and metal fragment contributions. The relative intensities of the observed features are further explained through a quantitative investigation of the metal 3p and 4p contributions to the spectra. The ability to use V2C XES to separate carbonyl, hydrocarbon, and solvent contributions is highlighted.