Negishi's Reagent Versus Rosenthal's Reagent in the Formation of Zirconacyclopentadienes

Abstract

Zirconacyclopentadienes are versatile precursors for a large number of heteroles, which are accessible by Zr-element exchange reactions. The vast majority of reports describe their preparation by the use of Negishi's reagent, which is a species that is formed in situ. The zirconacyclopentadiene is then formed by the addition of one equivalent of a diyne or two equivalents of a monoyne moiety to this Negishi species. Another route involves Rosenthal's reagent (Cp₂ Zr(py)Me₃ SiC≡CSiMe₃ ), which then reacts with a diyne or monoyne moiety. In this work, the efficiency of both routes was compared in terms of reaction time, stability of the product in the reaction mixture, and yield. The synthetic implications of using both routes are evaluated. Novel zirconacyclopentadienes were synthesized, characterized directly from the reaction mixture, and crystal structures could be obtained in most cases.

Keywords: Negishi′s reagent; Rosenthal′s reagent; metallacycles; substituent effects; zirconium.

Scheme 1

Transformation of zirconacyclopentadienes into metalloles and extended diene systems.9b–9d Use of polyzirconacyclopentadienes for the synthesis of stable polymers.10, 11

Scheme 2

General synthesis of zirconacyclopentadienes of type 1.

Scheme 3

Formation of the “Cp₂Zr” Negishi's reagent 8.12

Figure 1

Structure of the Rosenthal's reagent 9.

Scheme 4

Synthetic path to form the zirconacyclopentadienes 11 a–11 h by using two different sources of “Cp₂Zr” and eight types of dialkynes 10 a–10 h.

Scheme 5

Synthetic path to form the zirconacyclopentadienes 11 i–11 l by using two different sources of “Cp₂Zr” and four alkynes 10 i–10 l.

Figure 2

¹H NMR spectra (recorded at 300 K, 200 MHz in C₆D₆) of the reaction monitoring for the synthesis of zirconacyclopentadiene 11 a (Negishi's reagent) with naphthalene as standard (1 equiv). a) Starting material Cp₂ZrCl₂, b) naphthalene, c) starting material 10 a at t=0 min. Reaction monitoring after d) t=10 min, e) t=30 min, f) t=1 h, g) t=3 h, and h) t=22 h. i) Zirconacyclopentadiene 11 a previously isolated.23

Figure 3

¹H NMR spectra (recorded at 300 K, 200 MHz in C₆D₆) of the reaction monitoring for the synthesis of zirconacyclopentadiene 11 c (Negishi's reagent) with naphthalene as standard (1 equiv). a) Starting material Cp₂ZrCl₂, b) naphthalene, c) starting material 10 c. Reaction monitoring after d) t=10 min, e) t=30 min, f) t=1 h, g) t=3 h, and h) t=22 h. i) Zirconacyclopentadiene 11 c that was previously isolated.

Figure 4

¹H NMR spectra (recorded at 300 K, 600 MHz in C₆D₆) of the reaction monitoring of the formation of zirconacyclopentadiene 11 e at 22 °C with naphthalene as an internal standard. a) Starting material Cp₂ZrCl₂, b) naphthalene, c) starting material 10 e at t=0 min. Reaction monitoring after d) t=10 min, e) t=30 min, f) t=1 h, g) t=3 h, and h) t=22 h. i) Zirconacyclopentadiene 11 e previously isolated.

Figure 5

¹H NMR spectra (recorded at 300 K, 200 MHz in C₆D₆) of the reaction monitoring of the formation of zirconacyclopentadiene 11 a at 22 °C with naphthalene as an internal standard. a) Rosenthal's reagent 9, b) naphthalene, c) starting material 10 a, d) monitoring after 10 min. e) Zirconacyclopentadiene 11 a.23

Figure 6

¹H NMR spectra (recorded at 300 K, 600 MHz in C₆D₆) of the reaction monitoring of the stability test of zirconacyclopentadiene 11 e with naphthalene as standard (1 equiv). a) Naphthalene, b) starting material 10 e. Reaction monitoring after c) 10 min, d) 30 min, e) 1 h, f) 3 h, g) 22 h, and h) isolated zirconacyclopentadiene 11 e.

Figure 7

Molecular structures of 11 d, 11 f, 11 h–11 l showing 50 % probability ellipsoids and the crystallographic numbering scheme. Only the major parts of the disordered molecule of 11 f are shown for clarity. For 11 h, only one independent molecule is shown. Only the major parts of the disordered molecule of 11 i are shown for clarity.