• Home
  • Search Results
  • Direct Evidence for a [4+2] Cycloaddition Mechanism of Alkynes to Tantallacyclopentadiene on Dinuclear Tantalum Complexes as a Model of Alkyne Cyclotrimerization.

Direct Evidence for a [4+2] Cycloaddition Mechanism of Alkynes to Tantallacyclopentadiene on Dinuclear Tantalum Complexes as a Model of Alkyne Cyclotrimerization.

Chemistry (Weinheim an der Bergstrasse, Germany) (2015-06-30)
Keishi Yamamoto, Hayato Tsurugi, Kazushi Mashima
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=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.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Tetrahydrofuran, anhydrous, ≥99.9%, inhibitor-free
Sigma-Aldrich
Toluene, anhydrous, 99.8%
Sigma-Aldrich
Tetrahydrofuran, anhydrous, contains 250 ppm BHT as inhibitor, ≥99.9%
Sigma-Aldrich
Hexane, anhydrous, 95%
Sigma-Aldrich
Anthracene, ReagentPlus®, 99%
Sigma-Aldrich
4-Ethynyltoluene, 97%
Sigma-Aldrich
Anthracene, reagent grade, 97%
Sigma-Aldrich
Hexamethylbenzene, 99%
Sigma-Aldrich
Anthracene, suitable for scintillation, ≥99.0% (GC)
Sigma-Aldrich
Hexamethylbenzene, purified by sublimation, ≥99%
Sigma-Aldrich
Anthracene, sublimed grade, ≥99%
Supelco
Tetrahydrofuran, HPLC grade, ≥99.9%, inhibitor-free