Thermal Isomerization of the Buchwald Seven-Membered Zirconacyclocumulene and Its Interaction with Acetylenes. Synthesis and Structures of Novel Seven-Membered Zirconacyclocumulene Complexes

Abstract

On prolonged heating of the Buchwald seven-membered zirconacyclocumulene Cp 2 Zr[η 4 -Me 3 SiC 4 (SiMe 3 )− C(C 2 SiMe 3 )CSiMe 3 ] (1) in toluene at 100 °C, the isomerization of 1 takes place with the formation of a seven-membered zirconacyclocumulene complex, Cp 2 Zr[η 4 -Me 3 SiC 4 (SiMe 3 )−C(SiMe 3 )C(C 2 SiMe 3 )] (8), containing the Me 3 SiCC group in α-position and the SiMe 3 group in β-position with respect to the Zr atom. A remarkable peculiarity of complex 8 is its ability to undergo a rapid degenerate rearrangement at room temperature in toluene solution, as a result of which only two rather than four singlets of the Me 3 Si groups are observed in the 1 H NMR spectrum of 8 under such conditions. At temperatures below 200 K, the rate of this rearrangement decreases and, as a consequence, the 1 H NMR spectrum of 8 acquires the normal view. If 1 is heated at 100 °C in the presence of acetylenes such as 1,4-bis(tert-butyl)butadiyne ( t BuCC−CC t Bu) and tolane, the corresponding seven-membered zirconacyclocumulenes, viz., Cp 2 Zr[η 4 -t BuC 4 ( t Bu)−C(C 2 SiMe 3 )CSiMe 3 ] ( 7) and Cp 2 Zr[η 4 -Me 3 SiC 4 (SiMe 3 )−C(Ph)CPh] (14), are formed along with free Me 3 SiCC−CCSiMe 3 . The interaction of 1 with 1,4diphenylbutadiyne (PhCC−CCPh) at 100 °C leads to the formation of a mixture of products, from which a zirconacyclopentadiene metallacycle, Cp 2 Zr[η 2 -PhCC(C 2 Ph)−C(C 2 Ph)CPh] (10), as well as seven-membered zirconacyclocumulenes Cp 2 Zr[η 4 -PhC 4 (Ph)−C(C 2 SiMe 3 )CSiMe 3 ] (11), Cp 2 Zr[η 4 -Me 3 SiC 4 (SiMe 3 )−C(C 2 Ph)CPh] (12), and Cp 2 Zr[η 4 -PhC 4 (Ph)−C(C 2 Ph)CPh] (13) were isolated. The structures of 8, 9, 10, 12, and 14 have been established by X-ray crystallography. The mechanism of the reactions found is discussed.

Protolysis of Seven-Membered Zirconacyclocumulene Complexes of Zirconocene

“…The starting sevenmembered zirconacyclocumulenes 1−3 were prepared according to the published procedures. 6,12 Commercial Me 3 SiCC−CCSiMe 3 was purchased from Sigma-Aldrich and used without additional purification. A 4 M solution of HCl in dioxane was also purchased from Sigma-Aldrich.…”

“…13 C NMR (C 6 D 6 , δ, ppm): −0.8, −0.3, −0.2, 0.3, (SiMe 3 ), 102. 6,105.7 (CC),117.2,120.5 (CCH),139.3,160.3,179.7,180.7 (Cq).…”

“…This latter fact allowed one to suggest that the electronic structure of these zirconacycles can be described as a resonance hybrid of the corresponding zirconacyclocumulene resonance form and the five-membered zirconacyclopentadiene one wherein the α-alkynyl group is coordinated with zirconium (Scheme ). ,− …”

“…The aforementioned resonance description of the electronic structure of seven-membered zirconacyclocumulene complexes explains satisfactorily not only the abnormally high thermal stability of these compounds but also the peculiarities of their reactivity. Thus, it turned out that in the interaction with acetylenes, nitriles, , diarylketones, and N -benzylideneaniline, , the Buchwald zirconacycle 1 reacts in its zirconacyclocumulene resonance form, whereas in the reactions with benzil, acenaphthenequinone, and S 2 Cl 2 , , the same zirconacycle behaves as if it was in the zirconacyclopentadiene resonance form.…”

Aromaticity of 5- and 7-Membered Metallacyclocumulenes of Group 4

“…Since the pioneered works of Buchwald’s, Burlakov and Rosenthal, − the two types of metallacycles containing [3]­cumulene fragments 1 an 2 are known for group 4 metallocenes (Figure ). ,− They have not only high stability but also unique chemical behavior and electronic structure. The group 4 metal atom incorporation leads to excited mesomerism/valence tautomerism in unsaturated metallacycles and new synthetic routes of other synthetically used systems. − …”