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, [Ta2 Cl6 (μ-C4 Et4)] (2), bearing a tantallacyclopentadiene moiety, was synthesized by treating [(η(2) -EtC≡CEt)TaCl3 (DME)] (1) with AlCl3 . Complex 2 and its Lewis base adducts, [Ta2 Cl6 (μ-C4 Et4 )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 [Ta2 Cl4 (μ-η(4):η(4)-C6 Et6)(μ-η(2):η(2)-EtC≡CEt)] (4), sandwiched by a two-electron reduced μ-η(4):η(4) -hexaethylbenzene and a μ-η(2):η(2)-3-hexyne ligand, as a product of an intermolecular cyclization between the metallacyclopentadiene moiety and 3-hexyne. The formation of arene complexes [Ta2 Cl4 (μ-η(4):η(4)-C6 Et4 Me2)(μ-η(2):η(2)-Me3 SiC≡CSiMe3)] (7 b) and [Ta2 Cl4 (μ-η(4):η(4)-C6 Et4 RH)(μ-η(2):η(2)-Me3 SiC≡CSiMe3)] (R=nBu (8 a), p-tolyl (8 b)) by treating [Ta2 Cl4 (μ-C4 Et4)(μ-η(2):η(2) -Me3 SiC≡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.