Synthesis of (s-trans-η⁴-Butadiene)tantalocene Cation and Its CC-Coupling Reactions with Ketones, Nitriles, and Alkynes

Abstract

η 4 -Butadiene)(η 5 -cyclopentadienyl)tantalum dichloride (6), synthesized by treatment of CpTaCl 4 (5) with (butadiene)magnesium (7), was treated with 2 molar equiv of sodium cyclopentadienide to yield Cp 3 Ta(butadiene) (9). The spectroscopic analysis revealed the (η 5 -Cp) 2 (η 1 -Cp)(η 2 -C 4 H 6 )Ta structure of 9. Treatment of 9 with 1 molar equiv of the salt [(Cp 2 Zr-CH 3 ) + (CH 3 B(C 6 F 5 ) 3 ) -] (11) resulted in the clean transfer of a cyclopentadienide ligand from tantalum to zirconium with formation of the neutral zirconium compound Cp 3 -Zr-CH 3 (12) and the cation Cp 2 Ta(butadiene) + (10; with CH 3 B(C 6 F 5 ) 3 -counteranion). The spectroscopic characterization and an X-ray crystal structure analysis has revealed that 10 is a cationic (s-trans-η 4 -butadiene)bis(η 5 -cyclopentadienyl)tantalum complex. It reacts cleanly with a variety of ketones at 60°C to form the respective 7,7-disubstituted tetrahydrotantalaoxepines 13. These cationic seven-membered metallacyclic σ-allyl complexes undergo a rapid topomerization process on the NMR time scale (∆G q top ≈ 12 kcal/mol -1 ). Organic nitriles add similarly at 60°C to the (s-trans-η 4 -butadiene)tantalocene cation (10) to yield the respective 7-substituted 3,4-dihydrotantala-6H-azepines (14) that exhibit a similar dynamic NMR behavior due to conformational equilibration. Two examples of the seven-membered σ-allyl metallacyclic cation complexes 14 were characterized by X-ray crystal structure analyses (14a, R ) CH 3 , and 14d, R ) CH 2 -p-C 6 H 4 CH 3 ). In contrast, 2-butyne adds to the (s-trans-η 4 -butadiene)tantalocene cation in a 1:1 ratio to yield the corresponding metallacyclic (π-allyl)metallocene cation complex 15a (characterized by X-ray diffraction). The addition of 1-pentyne gave a 60:40 mixture of the respective regioisomers 15b and 15c, exhibiting the n-C 3 H 7 substituent at the ring position C5 and C6, respectively.

Reactions of (Butadiene)tantalocene Cation with Alkyl Isocyanides

“…Thermally Induced Reactions of (Butadiene)tantalocene with Alkyl Isocyanides. (η 4 - s - trans -Butadiene)tantalocene cation ( 3 ) was prepared according to the synthetic route that we have recently developed, starting from (η 4 - s - cis -butadiene)(η 5 -cyclopentadienyl)tantalum dichloride ( 1 ) . Treatment with 2 molar equiv of sodium cyclopentadienide gave [(η 5 -Cp) 2 (η 1 -Cp)(η 2 -butadiene)Ta] ( 2 ), which was subsequently treated with [Cp 2 ZrCH 3 + ][CH 3 B(C 6 F 5 ) 3 - ] .…”

“…We have shown that only the [(η 4 - s - trans -C 4 H 6 )TaCp 2 ] + isomer is stable and is observed at ambient temperature. It inserts a variety of unsaturated reagents (ketones, nitriles, alkynes) to form the respective metallacyclic (π-allyl)- or the seven-membered (σ-allyl)metallocene complexes, similar to the way their neutral group 4 metallocene analogues react with these reagents . We have now treated (butadiene) tantalocene cation ( 3 ) with a small series of isonitrile reagents RN⋮C under various reaction conditions and observed a quite different behavior and outcome of these reactions.…”

Diene complexes of early transition metals: ideas and progresses at Osaka University

“…Recently, Erker reported that a cationic diene complex of tantalocene [Cp 2 Ta(s-trans-g 4 -butadiene)] + (19), which is isoelectronic and isolobal to that of hafnocene and exhibits similar structural feature and reactivity [40][41][42][43].…”

(Alkyl)(monocyclopentadienyl)niobium and ‐tantalum Complexes in Insertion Processes

“…This is the most relevant detail of the molecular structure ( Figure 6) because, frequently, in diene mono(cyclopentadienyl)niobium and -tantalum complexes [57a,57d,58] the diene ligand forms a metallacyclopent-3-ene system with a preferred supino conformation, although there are examples with a prono conformation. Furthermore, the diene ligand can adopt an s-trans conformation, as observed in bis(cyclopentadienyl)zirconium, [59] mono(cyclopentadienyl)tantalum [60] and cationic bis(cyclopentadienyl)tantalum [61] complexes. In our case, the crystallographic data do not correspond to the same bonding system and, therefore, we propose a moiety consisting of a ''diene'' C31ϪC32ϪC33ϪC34 fragment linked to the metal atom through the C33ϪC34 atoms, forming a niobacyclopropane ring, and the donor vinyl substituent C31ϪC32.…”