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Direct Evidence for a [4+2] Cycloaddition Mechanism of Alkynes to Tantallacyclopentadiene on Dinuclear Tantalum Complexes as a Model of Alkyne Cyclotrimerization
Author(s) -
Yamamoto Keishi,
Tsurugi Hayato,
Mashima Kazushi
Publication year - 2015
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201501164
Subject(s) - moiety , chemistry , cycloaddition , alkyne , pyridine , stereochemistry , adduct , tantalum , lewis acids and bases , ligand (biochemistry) , medicinal chemistry , catalysis , organic chemistry , biochemistry , receptor
A dinuclear tantalum complex, [Ta 2 Cl 6 (μ‐C 4 Et 4 )] ( 2 ), bearing a tantallacyclopentadiene moiety, was synthesized by treating [(η 2 ‐EtCCEt)TaCl 3 (DME)] ( 1 ) with AlCl 3 . Complex 2 and its Lewis base adducts, [Ta 2 Cl 6 (μ‐C 4 Et 4 )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 [Ta 2 Cl 4 (μ‐η 4 :η 4 ‐C 6 Et 6 )(μ‐η 2 :η 2 ‐EtCCEt)] ( 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 [Ta 2 Cl 4 (μ‐η 4 :η 4 ‐C 6 Et 4 Me 2 )(μ‐η 2 :η 2 ‐Me 3 SiCCSiMe 3 )] ( 7 b ) and [Ta 2 Cl 4 (μ‐η 4 :η 4 ‐C 6 Et 4 RH)(μ‐η 2 :η 2 ‐Me 3 SiCCSiMe 3 )] (R= n Bu ( 8 a ), p ‐tolyl ( 8 b )) by treating [Ta 2 Cl 4 (μ‐C 4 Et 4 )(μ‐η 2 :η 2 ‐Me 3 SiCCSiMe 3 )] ( 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.