Direct Evidence for a [4+2] Cycloaddition Mechanism of Alkynes to Tantallacyclopentadiene on Dinuclear Tantalum Complexes as a Model of Alkyne Cyclotrimerization. Issue 32 (26th June 2015)

• Record Type: Journal Article
• Authors: Yamamoto, Keishi; Tsurugi, Hayato; Mashima, Kazushi
• Is Part Of: Chemistry. Volume 21:Issue 32(2015)
• Page Start: 11369
• Page End: 11377
• Publication Date: 2015-06-26
• Subjects: alkynes -- bridging ligands -- cycloaddition -- reaction mechanisms -- tantalum; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201501164
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: 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= n Bu (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. Abstract : Let's talk about [4+2] : The reaction of a dinuclear tantalum metallacyclopentadiene complex with 2‐butyne gives a Cs ‐symmetric dinuclear tantalum complex bearing a μ‐η 4 :η 4 ‐C6 Et4 Me2 ligand as a single product. Isomeric rotation of the C6 Et4 Me2 ligand is induced by heating to form a thermodynamically favored C 1 ‐symmetric complex. These results indicate clearly that arene formation proceeds through [4+2] cycloaddition.