Alkyne [2 + 2 + 2] Cyclotrimerization Catalyzed by a Low-Valent Titanium Reagent Derived from CpTiX3 (X = Cl, O-i-Pr), Me3SiCl, and Mg or Zn

Choosing between Ti(II) and Ti(III): selective reduction of titanocene dichloride by elemental lanthanides

The reduction of titanocene dichloride Cp2TiCl2 with lanthanide metals has led to the discovery of a surprising lanthanide effect: while with most lanthanides, a divalent [Cp2Ti] equivalent was obtained, the use of samarium or ytterbium only led to the reduction to trivalent [Cp2TiCl]-type complexes, including the structurally characterized heterobimetallic complex [Cp2Ti(μ-Cl)2SmCl2(THF)3]. These results were corroborated by reactivity studies (alkyne coupling and radical reactions), EPR spectroscopy and electrospray mass spectrometry, providing new insights into the reduction chemistry of lanthanide metals.

A Mixed-Valence Ti(II)/Ti(III) Inverted Sandwich Compound as a Regioselective Catalyst for the Uncommon 1,3,5-Alkyne Cyclotrimerization

The synthesis, structure, and catalytic activity of a Ti(II)/Ti(III) inverted sandwich compound are presented in this study. Synthesis of the arene-bridged dititanium compound begins with the preparation of the titanium(IV) precursor [TiCl2(MesPDA)(thf)2] (MesPDA = N,N'-bis(2,4,6-trimethylphenyl)-o-phenylenediamide) (2). The reduction of 2 with sodium metal results in species [{Ti(MesPDA)(thf)}2(μ-Cl)3{Na}] (3) in oxidation state III. To achieve the lower oxidation state II, 2 undergoes reduction through alkylation with lithium cyclopentyl. This alkylation approach triggers a cascade of reactions, including β-hydride abstraction/elimination, hydrogen evolution, and chemical reduction, to generate the Ti(II)/Ti(III) compound [Li(thf)4][(TiMesPDA)2(μ-η6: η6-C6H6)] (4). X-ray and EPR characterization confirms the mixed-valence states of the titanium species. Compound 4 catalyzes a mild, efficient, and regiospecific cyclotrimerization of alkynes to form 1,3,5-substituted arenes. Kinetic data support a mechanism involving a binuclear titanium arene compound, similar to compound 4, as the resting state. The active catalyst promotes the oxidative coupling of two alkynes in the rate-limiting step, followed by a rapid [4 + 2] cycloaddition to form the arene product. Computational analysis of the resting state for the cycloaddition of trimethylsilylacetylene indicates a thermodynamic preference for stabilizing the 1,3,5-arene within the space between the two [TiMesPDA] fragments, consistent with the observed regioselectivity.

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.

Low-Valent Titanium Species Stabilized with Aluminum/Boron Hydride Fragments

Low-valent titanium species were prepared by reaction of [TiCp*X₃ ] (Cp*=η⁵ -C₅ Me₅ ; X=Cl, Br, Me) with LiEH₄ (E=Al, B) or BH₃ (thf), and their structures elucidated by experimental and theoretical methods. The treatment of trihalides [TiCp*X₃ ] with LiAlH₄ in ethereal solvents (L) leads to the hydride-bridged heterometallic complexes [{TiCp*(μ-H)}₂ {(μ-H)₂ AlX(L)}₂ ] (L=thf, X=Cl, Br; L=OEt₂ , X=Cl). Density functional theory (DFT) calculations for those compounds reveal an open-shell singlet ground state with a Ti-Ti bond and can be described as titanium(II) species. The theoretical analyses also show strong interactions between the Ti-Ti bond and the empty s orbitals of the Al atom of the AlH₂ XL fragments, which behave as σ-accepting (Z-type) ligands. Analogous reactions of [TiCp*X₃ ] with LiBH₄ (2 and 3 equiv.) in tetrahydrofuran at room temperature and at 85 °C lead to the titanium(III) compounds [{TiCp*(BH₄ )(μ-X)}₂ ] (X=Cl, Br) and [{TiCp*(BH₄ )(μ-BH₄ )}₂ ], respectively. The treatment of [TiCp*Me₃ ] with 4 and 5 equiv. of BH₃ (thf) produces the diamagnetic [{TiCp*(BH₃ Me)}₂ (μ-B₂ H₆ )] and paramagnetic [{TiCp*(μ-B₂ H₆ )}₂ ] complexes, respectively.

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

CpTiCl2 -Catalyzed Cross-Coupling between Internal Alkynes and Ketones: A Novel Concept in the Synthesis of Halogenated, Conjugated Dienes

A novel concept for the synthesis of halogenated, conjugated dienes is disclosed: the CpTiCl₂ -catalyzed coupling of keto-alkynes, in the presence of Me₃ SiBr/Et₃ N⋅HBr. This reaction provided five-, six-, and seven-membered carbocycles, nitrogenated heterocycles, as well as six-membered oxygenated heterocycles leading to a brominated conjugate diene. These products showed high reactivity in the Diels-Alder, Suzuki, and Sonogashira reactions, giving complex chemical structures in only three steps from the corresponding acyclic keto-alkyne. Hopefully, this strategy will pave the way towards the synthesis of bioactive natural products and new materials.

Preparation of Dimeric Monopentamethylcyclopentadienyltitanium(III) Dihalides and Related Derivatives

The synthesis, crystal structure, and reactivity of a series of half-sandwich titanium(III) dihalide complexes [Ti(η⁵-C₅Me₅)X₂] (X = Cl, Br, I) and several of its Lewis base derivatives were investigated. The reaction of the trihalides [Ti(η⁵-C₅Me₅)X₃] (X = Cl (1), Br (2), I (3)) with LiAlH₄ (≥1 equiv) in toluene at room temperature results in the formation of the halide-bridged dimers [{Ti(η⁵-C₅Me₅)X(μ-X)}₂] (X = Cl (4), Br (5), I (6)). The treatment of 4 with [Li{N(SiMe₃)₂}] (≥2 equiv) at room temperature affords the precipitation of the amido titanium(III) complex [{Ti(η⁵-C₅Me₅)(μ-Cl){N(SiMe₃)₂}}₂] (7), but analogous reactions of 4 with other lithium reagents [LiR] (R = Me, CH₂SiMe₃, NMe₂) lead to disproportionation into titanium(IV) [Ti(η⁵-C₅Me₅)R₃] and presumably titanium(II) derivatives. Similarly, complex 4 in solution at temperatures higher than 100 °C undergoes disproportionation as demonstrated by its reactions with cobaltocene and N-(4-methylbenzylidene)aniline yielding the ionic paramagnetic compound [Co(η⁵-C₅H₅)₂][Ti(η⁵-C₅Me₅)Cl₃] (8) and the diamagnetic diazatitanacyclopentane [Ti(η⁵-C₅Me₅)Cl{N(Ph)CH(p-tolyl)}₂], respectively. Treatment of complex 4 with 2 equiv of 2,6-dimethylphenylisocyanide or tert-butylisocyanide in toluene at room temperature affords the paramagnetic titanium(III) dinuclear adducts [{Ti(η⁵-C₅Me₅)Cl(μ-Cl)(CNR)}₂] (R = 2,6-Me₂C₆H₃ (9), tBu (10)). Magnetic studies for polycrystalline 9 show that it displays a weak intramolecular antiferromagnetic coupling between the Ti ions, which is consistent with the long Ti-Ti distance of 3.857(1) Å determined by X-ray diffraction. The isocyanide ligands in complex 10 undergo a reductive coupling reaction in toluene to give the titanium(IV) iminoacyl derivative [{Ti(η⁵-C₅Me₅)Cl₂}₂(μ-η²:η²-tBuN═C-C═NtBu)] (11). Whereas an analogous dinuclear structure was found in the aqua titanium(III) complex [{Ti(η⁵-C₅Me₅)Cl(μ-Cl)(OH₂)}₂] (12), resulting from the reaction of 4 with adventitious amounts of water, compound 4 reacts with excess ammonia to give a mononuclear adduct [Ti(η⁵-C₅Me₅)Cl₂(NH₃)₂] (13) with a robust layered pattern in the solid state.

Titanium catalysis for the synthesis of fine chemicals – development and trends

Atlas as a Titan(ium) is holding the earth-abundant chemistry world. Titanium is the second most abundant transition metal, is a key player in important industrial processes (e.g. polyethylene) and shows much promise for diverse applications in the future.

Group 4 Diarylmetallocenes as Bespoke Aryne Precursors for Titanium-Catalyzed [2 + 2 + 2] Cycloaddition of Arynes and Alkynes

Despite the ubiquity of reports describing titanium (Ti)-catalyzed [2 + 2 + 2] cyclotrimerization of alkynes, the incorporation of arynes into this potent manifold has never been reported. The in situ generation of arynes often requires fluoride, which instead will react with the highly fluorophilic Ti center, suppressing productive catalysis. Herein, we describe the use of group 4 diarylmetallocenes, Cp^R₂MAr₂ (Cp^R = C₅H₅, C₅Me₅; M = Ti, Zr), as aryne precursors for the Ti-catalyzed synthesis of substituted naphthalenes via coupling with 2 equiv of an alkyne. Fair-to-good yields of the desired naphthalene products could be obtained with 1% catalyst loadings, which is roughly an order of magnitude lower than similar reactions catalyzed by palladium or nickel. Additionally, naphthalenes find broad applications in the electronics, photovoltaics, and pharmaceutical industries, urging the discovery of more economic syntheses. These results indicate that aryne transfer from a Cp^R₂M(η²-aryne) complex to another metal is a viable route for the introduction of aryne fragments into organometallic catalytic processes.

The Puzzling Monopentamethylcyclopentadienyltitanium(III) Dichloride Reagent: Structure and Properties

Following the track of the useful titanocene [Ti(η⁵-C₅H₅)₂Cl] reagent in organic synthesis, the related half-sandwich titanium(III) derivatives [Ti(η⁵-C₅R₅)Cl₂] are receiving increasing attention in radical chemistry of many catalyzed transformations. However, the structure of the active titanium(III) species remains unknown in the literature. Herein, we describe the synthesis, crystal structure, and electronic structure of titanium(III) aggregates of composition [{Ti(η⁵-C₅Me₅)Cl₂} _n]. The thermolysis of [Ti(η⁵-C₅Me₅)Cl₂Me] (1) in benzene or hexane at 180 °C results in the clean formation of [{Ti(η⁵-C₅Me₅)Cl(μ-Cl)}₂] (2), methane, and ethene. The treatment of 1 with excess pinacolborane in hexane at 65 °C leads to a mixture of 2 and the paramagnetic trimer [{Ti(η⁵-C₅Me₅)(μ-Cl)₂}₃] (3). The X-ray crystal structures of compounds 2 and 3 show Ti-Ti distances of 3.267(1) and 3.219(12) Å, respectively. Computational studies (CASPT2//CASSCF and BS DFT methods) for dimer 2 reveal a singlet ground state and a relatively large singlet-triplet energy gap. Nuclear magnetic resonance spectroscopy of 2 in aromatic hydrocarbon solutions and DFT calculations for several [{Ti(η⁵-C₅Me₅)Cl₂} _n] aggregates are consistent with the existence of an equilibrium between the diamagnetic dimer [{Ti(η⁵-C₅Me₅)Cl(μ-Cl)}₂] and a paramagnetic tetramer [{Ti(η⁵-C₅Me₅)(μ-Cl)₂}₄] in solution. In contrast, complex 2 readily dissolves in tetrahydrofuran to give a green-blue solution from which blue crystals of the mononuclear adduct [Ti(η⁵-C₅Me₅)Cl₂(thf)] (4) were grown.

Cyclotrimerization of alkynes catalyzed by a self-supported cyclic tri-nuclear nickel(0) complex with α-diimine ligands

The cyclic tri-nickel(0) complex [{Ni(μ-L^Me-2,4)}₃] (L^Me-2,4 = [(2,4-Me₂C₆H₃)NC(Me)]₂) self-supported by the α-diimines catalyzes cyclotrimerization of alkynes to form substituted benzenes.