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: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201501164
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>A dinuclear tantalum complex, [Ta<sub>2</sub>Cl<sub>6</sub>(μ‐C<sub>4</sub>Et<sub>4</sub>)] (<bold>2</bold>), bearing a tantallacyclopentadiene moiety, was synthesized by treating [(η<sup>2</sup>‐EtCCEt)TaCl<sub>3</sub>(DME)] (<bold>1</bold>) with AlCl<sub>3</sub>. Complex <bold>2</bold> and its Lewis base adducts, [Ta<sub>2</sub>Cl<sub>6</sub>(μ‐C<sub>4</sub>Et<sub>4</sub>)L] (L=THF (<bold>3 a</bold>), pyridine (<bold>3 b</bold>), THT (<bold>3 c</bold>)), served as more active catalysts for cyclotrimerization of internal alkynes than <bold>1</bold>. During the reaction of <bold>3 a</bold> with 3‐hexyne, we isolated [Ta<sub>2</sub>Cl<sub>4</sub>(μ‐η<sup>4</sup>:η<sup>4</sup>‐C<sub>6</sub>Et<sub>6</sub>)(μ‐η<sup>2</sup>:η<sup>2</sup>‐EtCCEt)] (<bold>4</bold>), sandwiched by a two‐electron reduced μ‐η<sup>4</sup>:η<sup>4</sup>‐hexaethylbenzene and a μ‐η<sup>2</sup>:η<sup>2</sup>‐3‐hexyne ligand, as a product of an intermolecular cyclization between the metallacyclopentadiene moiety and 3‐hexyne. The formation of arene complexes [Ta<sub>2</sub>Cl<sub>4</sub>(μ‐η<sup>4</sup>:η<sup>4</sup>‐C<sub>6</sub>Et<sub>4</sub>Me<sub>2</sub>)(μ‐η<sup>2</sup>:η<sup>2</sup>‐Me<sub>3</sub>SiCCSiMe<sub>3</sub>)] (<bold>7 b</bold>) and [Ta<sub>2</sub>Cl<sub>4</sub>(μ‐η<sup>4</sup>:η<sup>4</sup>‐C<sub>6</sub>Et<sub>4</sub>RH)(μ‐η<sup>2</sup>:η<sup>2</sup>‐Me<sub>3</sub>SiCCSiMe<sub>3</sub>)] (R=<italic>n</italic>Bu (<bold>8 a</bold>), <italic>p</italic>‐tolyl (<bold>8 b</bold>)) by treating [Ta<sub>2</sub>Cl<sub>4</sub>(μ‐C<sub>4</sub>Et<sub>4</sub>)(μ‐η<sup>2</sup>:η<sup>2</sup>‐Me<sub>3</sub>SiCCSiMe<sub>3</sub>)] (<bold>6</bold>) with 2‐butyne, 1‐hexyne, and <italic>p</italic>‐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.</p> </abstract>