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.

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.

Regioselectively switchable alkyne cyclotrimerization catalyzed by the system of Ni(II)/bidentate P-ligand/Zn with ZnI2 as additive

A simple catalyst system of Ni(PPh3)2Cl2/dppb or dppm/Zn can effect an efficient and regioselectively controlled alkyne cyclotrimerization to form 1,2,4- (with no additive) or 1,3,5-regioisomers (with the use of ZnI2...

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