The reaction between 2,6-diazasemibullvalenes and nitroso compounds was investigated. Aza-triquinacene derivatives of interesting structural and synthetic chemistry were generated highly selectively in good to excellent isolated yields. This reaction, which was rarely found between common aziridine derivatives and nitroso compounds, could be attributed to the rigid polycyclic ring system and the substitution patterns of 2,6-diazasemibullvalenes. Δ¹-Bipyrroline derivatives were formed in excellent yields when these aza-triquinacene derivatives were treated with SmI₂.

Herein we report that 1,4-dilithio-1,3-butadienes, a type of 1,3-butadienyl dianion, can act as non-innocent ligands, taking electrons from low-valent transition metals. Dilithio reagents reacted with [{RhCl(cod)}₂] to give dilithio rhodacycle 3 a. Single-crystal X-ray structural analysis revealed the structure of 3 a with averaged bond lengths. XPS data suggested that the oxidation state of Rh in 3 a was more likely to be Rh³⁺. CDA/ECDA confirmed the electron-transfer process. ⁷Li NMR spectra of 3 a and theoretical calculations revealed a considerable aromatic character. In this process, the dilithio compounds behaved as non-innocent ligands and formal oxidants. These results demonstrated that organolithium compounds with suitable π-conjugation could be used as electron acceptor.

Herein we report that 1,4-dilithio-1,3-butadienes, a type of 1,3-butadienyl dianion, can act as non-innocent ligands, taking electrons from low-valent transition metals. Dilithio reagents reacted with [{RhCl(cod)}₂] to give dilithio rhodacycle 3 a. Single-crystal X-ray structural analysis revealed the structure of 3 a with averaged bond lengths. XPS data suggested that the oxidation state of Rh in 3 a was more likely to be Rh³⁺. CDA/ECDA confirmed the electron-transfer process. ⁷Li NMR spectra of 3 a and theoretical calculations revealed a considerable aromatic character. In this process, the dilithio compounds behaved as non-innocent ligands and formal oxidants. These results demonstrated that organolithium compounds with suitable π-conjugation could be used as electron acceptor.

Organolithium compounds can behave as reductants but never as oxidants in redox reactions. Reported herein is that 1,4-dilithio-1,3-butadienes reacted with [Ni(cod)₂] (cod=1,5-cyclooctadiene) to deliver dilithionickeloles. Single-crystal X-ray structural analysis revealed a coplanar structure of dilithionickeloles with an averaging of bond lengths. XPS data confirmed the oxidation state of Ni in dilithionickeloles was Ni²⁺. ⁷Li NMR spectra of dilithionickeloles and theoretical calculations revealed a considerable aromatic character. In this redox reaction, the dilithio dianionic compounds behaved as formal oxidants, thus oxidizing Ni⁰ into Ni²⁺. These results demonstrated that organolithium compounds with π-conjugation could be used as oxidants and could continue to accept extra electrons.

Organolithium compounds can behave as reductants but never as oxidants in redox reactions. Reported herein is that 1,4-dilithio-1,3-butadienes reacted with [Ni(cod)₂] (cod=1,5-cyclooctadiene) to deliver dilithionickeloles. Single-crystal X-ray structural analysis revealed a coplanar structure of dilithionickeloles with an averaging of bond lengths. XPS data confirmed the oxidation state of Ni in dilithionickeloles was Ni²⁺. ⁷Li NMR spectra of dilithionickeloles and theoretical calculations revealed a considerable aromatic character. In this redox reaction, the dilithio dianionic compounds behaved as formal oxidants, thus oxidizing Ni⁰ into Ni²⁺. These results demonstrated that organolithium compounds with π-conjugation could be used as oxidants and could continue to accept extra electrons.

Nucleophilic ring-opening reactions of 2,6-diazasemibullvalenes (NSBVs) were investigated. Different types of nucleophile (alcohols, phenols, thiols, carboxylic acids, water, enols, amines, indoles, metal-halide salts, sodium azide, organozinc compounds, lithium alkynethiolate, and sulfoxonium ylides) were used to afford diverse functionalized Δ¹-bipyrroline derivatives in good yields with high regio- and diastereoselectivity. Most of the reactions featured milder conditions and higher reactivity relative to those for common aziridine derivatives, probably because of the rigid ring system and substitution patterns of NSBVs.

β-Hydride abstraction is a well-accepted elementary step for catalytic cycles in organometallic chemistry. It is usually anticipated that alkylpalladium halides containing syn-β-hydrogen atoms will undergo β-hydride abstraction to afford the Heck-type products. However, this study discloses that the above general knowledge is only conditionally correct. Our experimental results demonstrate that the reductive elimination of alkylhalides from alkylpalladium halides containing syn-β-hydrogen atoms may surpass the β-hydride abstraction or even become exclusive in certain cases.

Metallacyclopentadienes have attracted much attention as building blocks for synthetic chemistry as well as key intermediates in many metal-mediated or metal-catalyzed reactions. However, metallacyclopentadienes of the alkaline-earth metals have not been reported, to say nothing of their structures, reaction chemistry, and synthetic applications. In this work, the first series of magnesiacyclopentadienes, spiro-dilithio magnesiacyclopentadienes, and dimagnesiabutadiene were synthesized from 1,4-dilithio 1,3-butadienes. Single-crystal X-ray structural analysis of these magnesiacycles revealed unique structural characteristics and bonding modes. Their reaction chemistry and synthetic application were preliminarily studied and efficient access to amino cyclopentadienes was established through their reaction with thioformamides. Experimental and DFT calculations demonstrated that these magnesiacyclopentadienes could be regarded as bis(Grignard) reagents wherein the two MgC(sp²) bonds have a synergetic effect when reacting with substrates.

β-Hydride abstraction is a well-accepted elementary step for catalytic cycles in organometallic chemistry. It is usually anticipated that alkylpalladium halides containing syn-β-hydrogen atoms will undergo β-hydride abstraction to afford the Heck-type products. However, this study discloses that the above general knowledge is only conditionally correct. Our experimental results demonstrate that the reductive elimination of alkylhalides from alkylpalladium halides containing syn-β-hydrogen atoms may surpass the β-hydride abstraction or even become exclusive in certain cases.

Metallacyclopentadienes have attracted much attention as building blocks for synthetic chemistry as well as key intermediates in many metal-mediated or metal-catalyzed reactions. However, metallacyclopentadienes of the alkaline-earth metals have not been reported, to say nothing of their structures, reaction chemistry, and synthetic applications. In this work, the first series of magnesiacyclopentadienes, spiro-dilithio magnesiacyclopentadienes, and dimagnesiabutadiene were synthesized from 1,4-dilithio 1,3-butadienes. Single-crystal X-ray structural analysis of these magnesiacycles revealed unique structural characteristics and bonding modes. Their reaction chemistry and synthetic application were preliminarily studied and efficient access to amino cyclopentadienes was established through their reaction with thioformamides. Experimental and DFT calculations demonstrated that these magnesiacyclopentadienes could be regarded as bis(Grignard) reagents wherein the two MgC(sp²) bonds have a synergetic effect when reacting with substrates.

An efficient synthesis of N-substituted indole derivatives was realized by combining the Pd-catalyzed one-pot multicomponent coupling approach with cleavage of the C(sp³)N bonds. Three or four components of aryl iodides, alkynes, and amines were involved in this coupling process. The cyclopentadiene–phosphine ligand showed high efficiency. A variety of aryl iodides, including cyclic and acyclic tertiary amino aryl iodides, and substituted 1-bromo-2-iodobenzene derivatives could be used. Both symmetric and unsymmetric alkynes substituted with alkyl, aryl, or trimethylsilyl groups could be applied. Cyclic secondary amines such as piperidine, morpholine, 4-methylpiperidine, 1-methylpiperazine, 2-methylpiperidine, and acyclic amines including secondary and primary amines all showed good reactivity. Further application of the resulting indole derivatives was demonstrated by the synthesis of benzosilolo[2,3-b]indole.

1,2,3,4-Tetrasubstituted cyclopentadienes and indene derivatives with identical or different substituents were obtained in good to excellent isolated yields through a zirconocene- and CuCl-mediated intermolecular coupling process. This synthetic procedure involved three organic partners, including one CH₂I₂, and two different or identical alkynes. Two alkynes or one diyne undergo Cp₂Zr^II-mediated (Cp=η⁵-C₅H₅) pair-selective reductive coupling to afford the corresponding zirconacyclopentadiene derivatives, which react, in the presence of CuCl and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1 H)-pyrimidinone (DMPU), with CH₂I₂ through intermolecular followed by intramolecular coupling to afford the cyclopentadiene derivatives. An application of the prepared tetrasubstituted cyclopentadiene derivatives was demonstrated by the facile synthesis of the corresponding zirconocene complexes [(^4RCp)₂ZrCl₂] and [(^4RCp)₂ZrR′₂] (R′=Me, Et, or nBu). The unique 1,2,3,4-tetrasubstituted cyclopentadiene ligands and the corresponding metallocenes are expected to have further applications in organometallic chemistry and organic synthesis.

A variety of ester-substituted cyclopentadiene derivatives have been synthesized by one-pot reactions of 1,4-dilithio-1,3-butadienes, CO, and acid chlorides. Direct deprotonation of the ester-substituted cyclopentadienes with Ae[N(SiMe₃)₂]₂ (Ae=Ca, Sr, Ba) efficiently generated members of a new class of heavier alkaline earth (Ca, Sr, Ba) metallocenes in good to excellent yields. Single-crystal X-ray structural analysis demonstrated that these heavier alkaline earth metallocenes incorporated two intramolecularly coordinated ester pendants and multiply-substituted cyclopentadienyl ligands. The corresponding transition metal metallocenes, such as ferrocene derivatives and half-sandwich cyclopentadienyl tricarbonylrhenium complexes, could be generated highly efficiently by metathesis reactions. The multiply-substituted cyclopentadiene ligands bearing an ester pendant, and the corresponding heavier alkaline earth and transition-metal metallocenes, may have further applications in coordination chemistry, organometallic chemistry, and organic synthesis.

5-Azaindoles either with three different substituents at their 2-, 4-, and 6-positions or with two identical substituents at their 2- and 6-positions and a different one at the 4-position, were obtained in good to excellent isolated yields by a zirconocene-mediated multicomponent process. Each reaction involved four organic partners, comprising a Si-tethered diyne, one tBuCN component, and two (either different or identical) nitriles. All these four components were combined through the action of a Cp₂Zr^II species into a three-ring fused Zr/Si-containing organometallic complex in a perfectly chemo- and regioselective manner. This multicomponent reaction process consisted of three reaction steps, all of which were made clear through the isolation and characterization of their corresponding organometallic intermediates: the zirconacyclopropene-azasilacyclopentadienes 2, the allenyl-aza-zirconacycles 3, and the three-ring fused complexes 6. X-ray single-crystal structural analyses of two three-ring fused Zr/Si-containing intermediates and two 5-azaindoles unambiguously showed the positions of the different substituents and the regioselectivity. Iminopyrrole derivatives could be also highly selectively prepared from a Si-tethered diyne and two different nitriles.

An efficient multicomponent synthesis of 5-azaindoles and dihydropyrrolo[3,2-c]azepines was achieved by zirconocene-mediated coupling of silicon-tethered diynes, nitriles, and isocyanides. The synthesis, structures, and intramolecular cyclization of mono- and bis(iminoacyl)Zr intermediates were investigated to elucidate the reaction process. Upon hydrolysis, the isolated mono(iminoacyl)Zr intermediates underwent intramolecular cyclization to afford tetrasubstituted 5-azaindoles, whereas intramolecular cyclization of bis(iminoacyl)Zr intermediates led to the formation of dihydropyrrolo[3,2-c]azepines. The structure of a bis(iminoacyl)Zr intermediate, formed through insertion of two molecules of CyNC into the ZrC bond, and structures of two dihydropyrrolo[3,2-c]azepines were characterized by single-crystal X-ray structural analysis.

A full account of a useful transformation from silylated 1,4-dilithio-1,3-butadienes to α-lithio siloles is described. These lithio siloles formed by this procedure are general, in terms of substitution patterns and synthetic methods, affording diversified silole derivatives. Notably, some structurally complex molecules, such as bridged bis-silole compounds, have been synthesized easily and successfully by applying our protocol. The structure of the α-lithio silole, which adopts a dimeric fashion through two lithium bridges, was confirmed by X-ray analysis. Furthermore, a possible mechanism of the skeleton rearrangements via E/Z isomerization of 1-silyl-1-lithio alkene and nucleophilic attack on silicon is proposed, and is also proved by experimental investigations.

The allylation of 1,3-dicarbonyl compounds and malononitrile with aliphatic allylic substrates is achieved under mild conditions in the presence of new ruthenium catalysts. The ruthenium complex [Ru(C₅Me₅)(2-quinolinecarboxylato)(CH₂CHCH-n-Pr)] [BF₄] as a precatalyst, allows the synthesis of mono-allylated branched derivatives. On the other hand, the parent complex [Ru(C₅Me₅)(MeCN)₃] [PF₆] as a precatalyst, straightforwardly favours the bis-allylation of the procarbonucleophiles leading to bis-allylated bis-linear products. The involvement of the two precatalysts provides a sequential synthesis of unsymmetrical mixed linear-branched bis-allylated derivatives.

A zirconocene-mediated one-pot multicomponent synthesis process leading to the synthesis of pyrrolo[3,2-c]pyridine (5-azaindoles) and pyrrole derivatives has been developed starting from a silicon-tethered diyne and three or two different or identical organonitriles. Pyrrolo[3,2-c]pyridine and pyrrole derivatives with diverse structures and substitution patterns have been highly selectively prepared by this protocol. A wide variety of organonitriles and silicon-tethered diynes, either aliphatic or aromatic with both electron-withdrawing and -donating groups, have been used to afford 5-azaindoles and/or pyrroles in good-to-excellent isolated yields. A key intermediate formed in the Cp₂Zr^II-mediated reaction of one (PhC≡C)₂SiMe₂, two molecules of iPrCN and one p-tolylCN was characterised by X-ray single-crystal structural analysis, which has allowed us to gain a good understanding of the reaction process.

Stable [ruthenium(R-substituted-tetramethylcyclopentadiene)(2-quinolinecarboxylato)(1-R′-substituted-allyl) hexafluorophosphate (R=Me, R′=H, Me, n-Pr, Ph; R=t-Bu, R′=Me) and [ruthenium(pentamethylcyclopentadiene)(2-quinolinecarboxylato)(1-n-propylallyl)] tetrafluoroborate (4′a), as allylruthenium(IV) complexes, have been synthesized in one step, starting from [ruthenium(R-substituted-tetramethylcyclopentadiene)tris(acetonitrile) hexafluorophosphate or tetrafluoroborate complexes, quinaldic acid, and allylic alcohols. Single stereoisomers are obtained and the X-ray single crystal structure determinations of 3b (R=t-Bu, R′=Me) and 4′a allow one to specify the preferred arrangement. Complexes 3a (R=R′=Me) and 3b are involved as precatalysts favoring the formation of branched products in regioselective nucleophilic allylic substitution reactions, starting from ethyl 2-(E)-hexen-1-yl carbonate and chlorohexene as unsymmetrical aliphatic allylic substrates. Phenols, dimethyl malonate, and primary (aniline) and secondary (pyrrolidine, piperidine) amines have been used as nucleophiles under mild basic conditions. For the first time, the regioselectivity in favor of the branched product obtained from purely aliphatic allylic substrates is close to the high regioselectivity previously reached starting from cinnamyl-type substrates in the presence of ruthenium catalysts.

[Ru(η⁵-C₅Me₄R)(MeCN)₃][PF₆] (R = CH₂tBu, iPr, tBu, and CF₃; 2–5) complexes were synthesized in two steps starting from the appropriate cyclopentadienes and RuCl₃·3H₂O. The fully substituted ruthenocenes [Ru(C₅Me₅)(C₅Me₄R*)], [Ru(C₅Me₅)(C₅nPr₄R*)] {R* = (1R,5S)-6,6-dimethylbicyclo[3.1.1]hept-2-en-2-yl} and [Ru(C₅Me₅)(C₅nPr₅)] were obtained by treating [Ru(C₅Me₅)Cl]₄ with the corresponding cyclopentadienyllithium salts. Complexes 2–5 were evaluated as catalyst precursors for nucleophilic allylic substitution reactions, and the results were compared to those obtained with the [Ru(C₅Me₅)(MeCN)₃][PF₆] (1) precatalyst. The etherification of p-methoxyphenol with the typical aliphatic chlorohexene allylic substrate shows that the introduction of the bulky tert-butyl and trifluoromethyl groups into the tetramethylcyclopentadienyl ring results in a valuable enhancement in regioselectivity in favour of the branched allyl aryl ether.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The one-pot salt-metathesis reaction of ScCl₃, cyclopentadienyl lithium salts, and allylmagnesium chlorides afforded with ease the corresponding base-free half-sandwich scandium di(η³-allyl) complexes [(C₅Me₄SiMe₃)Sc(C₃H₅)₂] (1 a), [(C₅Me₅)Sc(C₃H₅)₂] (1 b), and [(C₅Me₅)Sc(2-MeC₃H₄)₂] (1 c) in high yields. Reaction of 1 a with 1 equivalent of [PhNMe₂H][B(C₆F₅)₄] in toluene gave rapidly the N,N-dimethylaniline-coordinated cationic mono(η³-allyl) complex [(C₅Me₄SiMe₃)Sc(η³-C₃H₅)(η⁶-PhNMe₂)][B(C₆F₅)₄] (2). The similar reaction of 1 a with [Ph₃C][B(C₆F₅)₄] yielded the analogous toluene-separated ion pair [(C₅Me₄SiMe₃)Sc(η³-C₃H₅)(η⁶-PhMe)][B(C₆F₅)₄] (3). When [PhNMe₂H][BPh₄] was treated with 1 a, the contact ion pair [(C₅Me₄SiMe₃)Sc(η³-C₃H₅)( μ,η⁶-Ph)BPh₃] (4) was obtained. Recrystallization of 2, 3, and 4 in THF yielded the corresponding thf-separated ion pair complexes [(C₅Me₄SiMe₃)Sc(η³-C₃H₅)(thf)₂][B(C₆F₅)₄] (5) and [(C₅Me₄SiMe₃)Sc(η³-C₃H₅)(thf)₂][BPh₄] (6). The N,N-dimethylaniline-coordinated cationic scandium allyl complex 2 and the toluene-coordinated analogue 3 showed high activity (activity: 3>2) toward the polymerization and copolymerization of isoprene and norbornene to afford random copolymers with a broad range of isoprene content (33–86 mol %). The tight ion pair 4 and the thf-coordinated complexes 5 and 6 showed no activity under the same conditions. These results offer unprecedented insight into the structure–activity relationship of a cationic metal polymerization-catalyst system.

Addition cyclization of 1,2,3,4-tetrasubstituted 1,4-dilithio-1,3-dienes (Type I) with four equivalents of various aromatic nitriles in the presence of hexamethylphosphoramide (HMPA) gives exclusively fully substituted pyridines in moderate to good yields. Similarly, trisubstituted pyridines can be prepared by the reaction of 2,3-dialkyl- or diaryl-substituted 1,4-dilithio-1,3-dienes (Type II) with nitriles. However, five- or six-membered-ring fused 2,3-disubstituted 1,4-dilithio-1,3-dienes (Type III) reacted with various aromatic and aliphatic nitriles without α-hydrogen atoms to afford tricyclic Δ¹-bipyrrolines in high yields. The reaction of six-membered-ring fused 2,3-disubstituted 1,4-dilithio-1,3-diene (Type III) with 2-cyanopyridine afforded the corresponding pyridine, and no tricyclic Δ¹-bipyrroline was observed. Seven-membered-ring fused dilithiodienes reacted with PhCN or trimethylacetonitrile to afford the corresponding pyridines in good yield. When 1,2,3,4-tetrasubstituted dilithio reagents (Type I) were treated with Me₃SiCN, a tandem silylation/intramolecular substitution process readily occurred to yield siloles, whereas the reaction of 2,3-disubstituted dilithio reagents (Types II and III) with Me₃SiCN gave rise to (Z,Z)-dienylsilanes with high stereoselectivity. These results revealed that the formation of tricyclic Δ¹-bipyrrolines, pyridines, siloles, and (Z,Z)-dienylsilanes are strongly dependent on the substitution patterns of the dilithio butadienes and the nature of the nitriles employed.