Titanocene Silylpropyne Complexes: Promising Intermediates en route to a Four‐Membered 1‐Metallacyclobuta‐2,3‐diene?

Abstract

Coordination of the alkyl-substituted alkynes Me SiC CH R (1: R=SiMe ; 2: R=N(SiMe ) ) to titanocene centres [Cp' Ti] (Cp'=Cp, Cp*) yields stable alkyne complexes of the type Cp' Ti(η -Me SiC CH R) (3: Cp'=Cp, R=SiMe ; 5: Cp'=Cp, R=N(SiMe ) ; 6: Cp'=Cp*, R=SiMe ) that are not prone to alkyne/allene isomerisation. When reacting alkyne 2 with Cp TiCl and Mg formation of the complex Cp* Ti(III)(η -Me SiC CH ) (7) which displays a propargylic unit coordinated to the Ti centre takes place. All complexes were fully characterised, the molecular structures for 5, 6, and 7 are discussed.

Visiting the Limits between a Highly Strained 1‐Zirconacyclobuta‐2,3‐diene and Chemically Robust Dizirconacyclooctatetraene

“…Although propargylic metalation and the structures of organolithium compounds have been well studied and understood for decades, no structural investigation of a suitable dilithioallene precursor is known, to date. Based on our experience of trimethylsilyl‐substituted substrates, and supported by the aforementioned theoretical studies of the stability of corresponding derivatives of C , we decided to focus on the synthesis of 1 . In two previous studies, the synthesis of 1 was performed by deprotonation of 1,3‐bis(trimethylsilyl)prop‐1‐yne in Et 2 O and THF with n BuLi (Scheme ).…”

“…The calculations clearly reveal the combination of the zirconocenes and tert ‐butyl substituents at the allene unit to be inappropriate due to the endergonic Gibbs free energies for the considered reactions (Table ). Upon taking a closer look at the computed values, [Cp 2 Zr(η 2 ‐Me 3 SiC 3 SiMe 3 )] appeared to be the most promising candidate for the formation of a 1‐metallacyclobuta‐2,3‐diene structure with a Gibbs free energy of formation of −8.97 kcal mol −1 , which is exergonic, but challenging, and, relative to the corresponding 1‐titanacyclobuta‐2,3‐diene (−11.8 kcal mol −1 ), this value is slightly lower …”

1-Zirconacyclobuta-2,3-dienes: synthesis of organometallic analogs of elusive 1,2-cyclobutadiene, unprecedented intramolecular C–H activation, and reactivity studies

“…Both complexes were characterised by NMR spectroscopy and their 1 H NMR spectra show informat i ve doublet resonances which correspond to the Cp proton s (2a: d 7.20, 5.35, 2b: d 7.43, 5.40 ppm). In 13 C NMR spectra, the signal s of the metal bound C atoms of the formal allene unit are observed at 164.7 (2a) and 173.2 ppm (2b), whereas the internal C atoms resonate at higher field (2a: 151.4, 2b: 147.0 ppm). Compared to the previously described Ti complex A, the metal bound C atoms of the two Zr complexes 2a and 2b resonate at much higher field (Ti-C 213.8 ppm), while the signal s of internal C atoms were found at lower field than for A (C=C= C 134.2 ppm), indicating significant differences in the electronic structures.…”

1-Titanacyclobuta-2,3-diene – an elusive four-membered cyclic allene

“…Later, our group has attempted to synthesise such structures by coupling of alkynyl and isonitrile ligands at titanocene, unfortunately this approach only resulted in redox-disproportionation of the Ti centre 15. Also, deprotonation of α-CH 2 SiMe 3 and α-CH 2 N(SiMe 3 ) 2 substituted titanocene alkyne complexes was found to be unsuccessful 16. A more promising approach appeared to be the initial construction of a C 3 framework prior to coordination to the metal.…”