Neue Entwicklungen der Pauson-Khand-Reaktion

Abstract

Ethylen/CO (310-360 bar) 0.22 Mol-% [Co 2 (CO) 8 ] 150 °C, 16 h Schema 3. Katalytische Pauson-Khand-Reaktion nach Rautenstrauch.

Nickel‐Catalyzed Reductive Cyclization of Unactivated 1,6‐Enynes in the Presence of Organozinc Reagents

“…[25][26][27][28][29] To further explore this cyclization chemistry, we have now investigated a Ni-based system with the aim of uncovering new reactivity and selectivity in a Ni-mediated Pauson-Khand-type cyclization under a reductive atmosphere, such as CO gas. [30][31][32][33][34] Unlike Ni II species, most Ni 0 complexes are extremely airsensitive and are thus rather difficult to manipulate. [35] Consequently, many known processes have resorted to the generation of these compounds in situ from air-stable Ni II precursors in the presence of an appropriate reducing reagent, such as diisobutylaluminum hydride or a dialkyl zinc reagent.…”

CC Bond‐Forming Reactions

“…The Pauson-Khand reaction [329,330,331,332] has been applied in combinatorial chemistry for some years [333,334,335]. Its mild reaction conditions have allowed application in SPOS.…”

Asymmetric Synthesis of Bicyclic Ketones Having an Angular Substituent via Ti(II) Alkoxide-Mediated Tandem Cyclization of Trisubstituted Olefinic Substrates

“…The increasing importance of asymmetric synthesis leads to a greater demand for chiral building blocks; however, the preparation of optically active ketones 1 − 5 has not yet been reported in the literature. In addition to the conventional methods, the recently introduced transition metal-catalyzed or -mediated carbonylative cyclization of enynes provides an attractive method for preparing a variety of bicyclic ketones, − including optically active ones. 4a,g,6b However, this process does not appear to be suitable for the preparation of the above ketones, because the cyclization of 1,1-disubstituted olefinic substrates, which are precursors for the angularly substituted ketones, usually requires the presence of substituents to the tether portion (i.e., steric promotion; for example, the assistance by the Thorpe−Ingold effect) to achieve good yields. 4d, To find a viable solution, we reexamined our tandem cyclization of 2,7-enynoates with a titanium(II) alkoxide as shown in eq 1, but this time, we chose substrates having a trisubstituted olefin. More importantly, the replacement of the ester portion of the substrate with an appropriate chiral auxiliary led us to develop an asymmetric synthesis of 1 − 5 with high enantiomeric purity.…”