Asymmetric Dihydroxylation of 2‐Substituted 1‐Vinylferrocenes: The First Non‐Enzymatic Kinetic Resolution of Planar‐Chiral Ferrocenes

Abstract

The first non-enzymatic kinetic resolution of planar chiral ferrocenes has been achieved by the Sharpless catalytic asymmetric dihydroxylation (AD) of a set of racemic 2-substituted 1-ethenylferrocenes 1a-d. The enantioselectivity factor k rel varies from 20 to 62 [for (DHQD) 2 PYR ligands], and from 5 to 27 [for (DHQ) 2 PYR ligands]. The stereochemical outcome of the resolution can be easily predicted by the mnemonic device for AD, with the additional hypothesis that in the preferred transition state the olefin group and the upper cyclopentadiene ring of vinylferrocenes exhibit an essentially coplanar geometry.Keywords: asymmetric catalysis; dihydroxylation; ferrocene ligands; kinetic resolution; metallocenes More than fifty years after its discovery, ferrocene and its derivatives are still the object of unabated interest in several areas of science.[1] In particular, much effort has been devoted to the enantioselective preparation of planar-chiral 1,2-disubstituted ferrocenes, which have played a key role in the development of efficient ligands for asymmetric catalysis.[2] Up to date, the vast majority of synthetic approaches towards these compounds rely on the directed metallation of suitablely functionalized ferrocene precursors. The metallation of enantiopure ferrocenes bearing chiral, ortho-directing auxiliary groups such as alkylamines, [3] oxazolines, [4] sulfoxides, [5] or acetals [6] is the most widely used strategy, together with Snieckus enantioselective ortho-metallation of achiral ferrocenecarboxamides [7] by the butyllithium-sparteine system.Recent research from our laboratory has been focused on the synthesis of b-ferrocenyl-b-amino alcohols, a new class of central chiral ferrocene derivatives, and on the study of their synthetic applications.[8] In the course of these studies, we have made extensive use of the Sharpless asymmetric dihydroxylation [9] (AD) of 1-ferrocenylethenes. [10] We have found that in most instances this reaction takes place with good to excellent enantioselectivity, and that the ferrocenyl moiety has a strong affinity for the SW binding pocket of PYR-based ligands. In spite of the considerable controversy about the detailed mechanism of the AD process, [9] there is a general agreement on the importance of p-stacking interactions in the stabilization of the transition state (TS) complex for aryl-substituted alkenes. Theoretical calculations on the AD of styrene and of stilbene show that in the TS the phenyl and the alkene moieties present a coplanar geometry.[11] We reasoned that a similar coplanarity requirement should be operative in the AD of vinylferrocenes. If this is the case, the presence of a bulky substituent adjacent to the vinyl group would strongly destabilize one of the two limiting planar conformations of the molecule, so that for a given enantiomer of a 2-substituted 1-vinylferrocene one face of the olefin would be much more reactive than the other (see Figure 1).Thus, for the (R p )-enantiomer of a 2-substituted 1-vinylferrocene, dihydroxylation woul...

The diastereoselective ortho-lithiation of Kagan’s ferrocenyl acetal. Generation and reactivity of chiral 2-substituted ferrocenecarboxaldehydes

“…This was obtained from aldehyde 23a by a Wittig reaction and was involved in asymmetric dihydroxylation to give compound 101 in high ee (Scheme 27). 56 Ferrocenyl nucleoside analogues with apoptosis-inducing activity were prepared from aldehyde 23g (R = CO 2 Me). 57 CuBr-SMe 2 afforded product 103 with high diastereoselectivity.…”

Catalytic Non‐Enzymatic Kinetic Resolution

“…The first example of a highly enantioselective metal‐catalyzed KR of planar‐chiral ferrocene substrates was not published until 2006, when Moyano et al. reported the KR of 2‐substituted 1‐vinylferrocenes via asymmetric dihydroxylation 281. The employment of chiral ligands, such as (DHQD) 2 PYR, allowed high enantioselectivity factors of up to 62.3 to be reached for the KR of ferrocenylethenes with bulky substituents.…”

Stereoselective Synthesis of Planar Chiral Ferrocenes

“…However, chiral catalyst mediated kinetic resolution, a potential alternative to enzymatic methods, was first applied to the synthesis of planar chiral ferrocenes by Moyano in 2006 [108]. In this work (Scheme 2.32), the Sharpless asymmetric dihydroxylation (AD) reaction was employed on racemic 2-substituted-1-vinylferrocene 128a-d to give products 129 with stereochemical outcomes in accordance with the Sharpless mnemonic rule.…”