Titanocene-Catalyzed Pinacol Couplings: Reagent-ControlledTransition-Metal-Catalyzed Radical Reactions

Abstract

The titanocene‐catalyzed pinacol coupling of aromatic aldehydes proceeding in good yields and with high diastereoselectivity under reagent control is described.

4‐exo‐Cyclisierungen durch Templatkatalyse

“…Die von Oltra et al [10] eingeführte Reagenskombination aus Me 3 SiCl und Collidin führt zu guten Umsätzen. Diese Methode der Regenerierung der katalytisch aktiven Komplexe ist bei Epoxidöffnungen der Verwendung von Me 3 SiCl, [11] die Fürstner et al in ihren wegweisenden Katalysen eingeführt haben, [12] [13] Diese Struktur stützt unsere Hypothese der Produktinhibierung. Die Hydroxy-und die Amidgruppe sind ideal positioniert, um die Cyclobutane zu exzellenten zweizähnigen Liganden zu machen.…”

Titanocene‐Promoted Eliminations on Epoxy Alcohols and Epoxy Esters

“…After the formation of Ti III –alkoxide, as an initial step, the interconversion of 31A into 35A would occur through a six‐membered Ti IV intermediate (see Scheme ), followed by regioselective oxirane cleavage to 31B , and elimination of titanocene epoxide to give 31C . From 35A , regioselective oxirane cleavage would afford 35B , which could evolve, by reduction with Ti III , to give 35C , or by elimination of CH 2 OTi IV in a similar manner as the diradical B in Scheme , to give 35D 15. The 31A / 35A equilibrium was displaced to 35A because the ratio of products 33 + 34 was always higher than 32 .…”

The McMurry Coupling and Related Reactions

“…These results are encouraging for further investigation of asymmetric induction by using enantiomerically pure metallocene catalysts [248]. Recent studies have revealed substantial effects of the structure of the titanocene-type catalyst on the chemo-and stereoselectivity of the pinacolization reaction [249][250][251][252]. The nature of the active species and intermediates in these pinacol coupling reactions, which are performed with the aid of rather complicated systems, is not known.…”