Coordination Behavior and Reactivity of α‐Heterosubstituted Trimethylsilylalkynes at Titano‐ and Zirconocene

Abstract

Keywords: Metallocenes / Zirconium / Titanium / Alkynes / RegioselectivityThe coordination behavior of the α-donor-atom-substituted (trimethylsilyl)alkynes OC 4 H 8 N-CϵC-SiMe 3 (2), EtO-CϵC-SiMe 3 (3), and Me 2 P-CϵC-SiMe 3 (4) towards group 4 metallocene generators [Cp 2 Ti(η 2 -btmsa)] (1a), [Cp 2 Zr(py)-(η 2 -btmsa)] (1b), and [Cp* 2 Ti(η 2 -btmsa)] (1c) [Cp = η 5 -cyclopentadienyl; btmsa = bis(trimethylsilyl)acetylene; py = pyridine; Cp* = η 5 -pentamethylcyclopentadienyl] was investigated. By using 1a and 1b, metallacyclopropenes were ob-Scheme 1. Different types of four-membered metallacycles.[a] Leibniz-Scheme 2. Possible formation of four-membered heterometallacycles.www.eurjic.org FULL PAPER alkynes are, even in their free form, very reactive [10] and engaged in side-reactions that afford C alkyne -heteroatom cleavage products. [9] Our way of choice to make more stable compounds and still have the α-heteroatom at the alkyne was to substitute one heteroatom-bearing group with the trimethylsilyl group. These α-heterosubstituted (trimethylsilyl)alkynes retain the possibility of direct influence on the triple bond of the alkyne and should still be able to induce the formation of the four-membered metallacycle. Dynamic behavior in terms of the aforementioned flip-flop mechanism should be discouraged as well since there is now just one donor heteroatom (D) available. Furthermore, the triple bond is strongly polarized for the period 2 heteroatoms N and O (Scheme 3), which is expressed in the 13 C NMR spectroscopic shifts of their alkyne moiety (D-CϵC/CϵC-Si): δ = 109.3/62.5 (2) and 109.9/36.9 ppm (3), respectively. This effect is nearly nullified for the period 3 heteroatom phosphorus, with the corresponding signals between δ = 110 and 112 ppm (4). Scheme 3. Polarization of alkynes by donor atoms in the α position.We used the alkynes OC 4 H 8 N-CϵC-SiMe 3 (2), EtO-CϵC-SiMe 3 (3), and Me 2 P-CϵC-SiMe 3 (4) in reactions with the metallocene generators [Cp 2 Ti(η 2 -btmsa)] (1a), [Cp 2 Zr(py)(η 2 -btmsa)] (1b), and [Cp* 2 Ti(η 2 -btmsa)] (1c) [Cp = η 5 -cyclopentadienyl; btmsa = bis(trimethylsilyl)acetylene; py = pyridine; Cp* = η 5 -pentamethylcyclopentadienyl]. Additionally, we explored the reactivity of selected complexes towards the small carbonyl molecules CO 2 and acetone. Results and DiscussionReactions of OC 4 H 8 N-CϵC-SiMe 3 (2) with the metallocene generators 1a and 1b resulted in the formation of the metallacyclopropenes 5a and 5b, respectively, in moderate to good yields (Scheme 4). Whereas 5a is stable without an additional ligand, the pyridine ligand in 5b is still attached, which is a common behavior for these types of complexes. [1] Scheme 4. Alkyne replacement with nitrogen-substituted alkyne 2.

Recent Synthetic and Catalytic Applications of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes

“…As outlined above, in some cases starting from the metallacyclopropenes these three membered metallacycles yield five‐membered metallacyclopentadienes . (Scheme ).…”

“…Reactions of the unsubstituted Cp complexes with the O‐substituted silylalkyne Me 3 Si‐C≡C‐OEt gave no metallacyclopropenes but with Cp* complexes such mononuclear complexes were obtained (Scheme ). [44a]…”

“…In the reactions of Cp* 2 Ti(η 2 ‐btmsa) and rac ‐(ebthi)Ti(η 2 ‐btmsa) with the alkyne pinBC 2 Bpin stable titanacyclopropenes were formed (Scheme ), whereas with Cp 2 Ti(η 2 ‐btmsa) no such product is obtained. The reason for this is the steric influence of Cp* and rac ‐(ebthi) which prevents the coordination of a second alkyne, which was believed to be the reason for the presence of ill‐defined mixtures in the case of the Cp system.…”

Reactions of [Cp2Ti(η2‐Me3SiC2SiMe3)] with 1,4‐Bis(diphenylphosphanyl)but‐2‐yne: Coupling and Isomerization versus Phosphorylation

“…[7,9a] NMR spectra: Bruker AV300 and AV400 spectrometers. 1 Diffraction data for 4 and 5 were collected with Bruker APEX-II CCD and STOE-IPDS II diffractometers, respectively, using graphite-monochromated Mo-K α radiation. The structures were solved by direct methods and refined by full-matrix least-squares procedures on F 2 with the SHELXTL software package.…”

“…More recently, we extended these investigations to unsymmetrically dihetero-substituted alkynes YCϵCSiMe 3 , for example, the α-donor-substituted trimethylsilylalkynes R 2 NCϵCSiMe 3 , EtOCϵCSiMe 3 , and Me 2 PCϵCSiMe 3 . [5] In these investigations we aimed to use donor heteroatoms to supply an additional coordination site directly attached to the alkyne moiety, which could promote the formation of highly strained, four-membered metallacycles of the phosphorus atoms. Another complex, [Cp 2 Ti{-C-(CH 2 PPh 2 )=C(CH 2 PPh 2 )P(Ph 2 )H-}] (5), was identified to be the result of a formal hydrophosphorylation of the substrate 2 by HPPh 2 , and features a Ti-H-P bridge.…”