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

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.

Reaction of [TpRh(C2H4)2] with Dimethyl Acetylenedicarboxylate: Identification of Intermediates of the [2+2+2] Alkyne and Alkyne–Ethylene Cyclo(co)trimerizations

“…[4+2] pathways have been proposed not only for the cycloaddition of alkynes to alkenes, with formation of metallanorbornenes, but also for cyclotrimerization of alkynes. For instance, an interesting bridging [4+2] tantallanorbornadiene formed during the formal cyclotrimerization of 3‐hexyne has been recently described, and also a recent theoretical work proposes the participation of this type of [4+2] intermediate in TpIr‐mediated cyclotrimerization of alkynes . A further possibility for the generation of 3 from E would be the formation of a linear hexatriene with subsequent thermal electrocyclization, but this pathway seems unlikely in view of the reluctance of the organic fragment to be displaced from the rhodium center (see below).…”

Synthesis and Characterization of Heterobimetallic Tantalum–Rhodium and Tantalum–Iridium Complexes Connected by a Tantalacyclopentadiene Fragment

“…[13] Because of the high coordination number for the Ta atom in complex 3 compared with that of 2 , the bond lengths of Ta–C(1) and Ta–C(4) (2.225(11) and 2.186(12) Å) are longer than those of complex 2 , while they are consistent with other metallacyclopentadiene‐bridged dinuclear Ta complexes. [14] The bond lengths of the C‐atom linkages in the tantalacyclopentadiene, C(1)–C(2), C(2)–C(3), and C(3)–C(4) bonds, are similar, and equalization of the C–C bonds agree with other dinuclear metallacyclopentadiene complexes …”

“…Although the molecular structure of complex 5 was not clear due to difficulties obtaining single crystals, we presumed that the Ir center in complex 5 would strongly interact with the tantalacyclopentadiene moiety by deforming the flat five‐membered metallacycle, whose deformation was previously observed for a dinuclear tantalacyclopentadiene complex. [14] Loss of cyclooctadiene from the Ir center suggested the construction of a chloride‐bridged tetranuclear Ta 2 Ir 2 structure similar to complex 4 .…”

Hexaethylbenzene: A Sterically Crowded Arene and Conformationally Versatile Ligand

“…In a particularly elegant recent study by Yamamoto, Tsurugi and Mashima, the stepwise conversion of a tantalacyclopentadiene into a ditantalum–HEB complex was conclusively demonstrated . The reaction of [(η 2 ‐EtC≡CEt)TaCl 3 (DME)] with AlCl 3 in THF afforded the ditantalacyclopentadiene [(C 4 Et 4 )Ta 2 Cl 6 (THF)] ( 63 ), which can be described as a nido pentagonal bipyramidal cluster.…”

“…[91] In ap articularly elegant recent study by Yamamoto, Ts urugi and Mashima, the stepwisec onversion of at antalacyclopentadiene into ad itantalum-HEB complex was conclusively demonstrated. [92] The reactiono f[ ( h 2 -EtCCEt)TaCl 3 (DME)] with Figure 24). Unsurprisingly,i nt hese complexes, almost all the ethyl substituents occupy distal sites and therefore provide ap rotective screen that might helpt os tabilise these fascinating intermediates en route from alkyne to arene.…”