Strategies for the Synthesis of Chiral Carbon‐Bridged Group IV ansa‐Metallocenes

Abstract

This minireview provides a survey of the various synthetic approaches to chiral ansa‐metallocenes of Ti, Zr, and Hf containing a carbon‐based bridge. The individual strategies to install substitution patterns at either the cyclopentadienyl framework or the bridging unit are highlighted with focus on the progress made towards a direct preparation of single complex stereoisomers. The review further includes the discussion of potential problems such as the formation of undesired diastereomers, the threat of racemization of enantiopure material, and synthetic challenges originating from the synthesis, purification, and isolation of the target complexes. The review has been written with the goal in mind to facilitate the design and synthesis of new chiral ansa‐metallocene derivatives for emerging research areas in asymmetric catalysis and organometallic chemistry.

Main‐Group Metallocenophanes

“…This synthetic route was first realized in 1997b yB urger et al in the preparation of Me 2 C[1]magnesocenophane 1a, [26] and has since been adopted for severale xamples of [1]-and [2]magnesocenophanes ( Figure 3). It is worth mentioning, that the carba [1]b ridging motif is rarely found in transitionm etal metallocenophanes, [15,17] as it makes the ligand system quite small and the resulting ring strain in such metallocenophanes is considered to be extremely high. For instance, this seems to be the reason that C [1]ferrocenophanes are unknown.…”

“…For instance, this seems to be the reason that C [1]ferrocenophanes are unknown. [15][16][17] An exception are metallocenophanes of group 4m etals with d 0 electron configuration, in which the d-orbitalst hat are raised in energy upon tilting, are unoccupied. Due to the high ionic character of the alkaline earth metal Cp bonds, the ring strain in C [1]metallocenophanes of group 2m etals is probablyr elativelys mall.…”

“…This structural motif was first realizedf or iron, ferrocenophanes, respectively,b ut has since then been studied extensively for various transition metals for over 50 years. [15][16][17][18][19] Different transitionm etal metallocenophanes have seen appli-cations for example in homogenousc atalysis [20,21] and ringopeningp olymerizations. [22,23] Furthermore, it is worth mentioningt hat this concept has since also been applied to systems with other p ligands such as phenylo rc ycloheptatrienyl.…”

“…One such modification that has, in many cases, proven to greatly affect the overall reactivity of the corresponding compound, is interlinkage of the two cyclopentadienyl moieties, resulting in ansa ‐metallocenes, also referred to as metallocenophanes (Figure 2). This structural motif was first realized for iron, ferrocenophanes, respectively, but has since then been studied extensively for various transition metals for over 50 years [15–19] . Different transition metal metallocenophanes have seen applications for example in homogenous catalysis [20, 21] and ring‐opening polymerizations [22, 23] .…”

Widening the Scope of “Inherently Chiral” Electrodes: Enantiodiscrimination of Chiral Electroactive Probes with Planar Stereogenicity

“…Still missing is the fourth case, i. e. planar stereogenicity (C), including for example paracyclophanes, [47][48][49] ansa compounds, [50] chiral tetrathiafulvalenes, [51] pillar[n]arenes [52] and metallocenes with at least two different substituents on one of the aromatic rings. [53][54][55] Actually, the above four stereogenic elements can be fascinatingly related through a common fil rouge rationale, starting from the stereocenter, as shown in Figure 2.…”