Salen Ligands Revisited: Synthesis and Application of a Planar Chiral “Ferro‐Salen” Ligand

Abstract

Condensation of the O‐protected planar chiral hydroxyferrocene carbaldehyde (Sp)‐1 with ethylenediamine, followed by deprotection gave rise to the “ferro‐salen” ligand (Sp,Sp)‐3 in good yield. This scaffold was used for the preparation of a series of metal complexes [(Sp,Sp)‐4/5/6], which were subsequently applied for the Lewis acid catalyzed asymmetric trimethylsilylcyanation of benzaldehyde.

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

“…The preparation of such ligands implicated the use of highly air-sensitive hydroxyferrocenes. 26,47,48 Ferrocene-saliminato ligands 81 were prepared from aldehydes 23c by condensation with arylamines and removal of the silyl-protecting group. 26 Reaction of 81 with LDA gave the corresponding lithiated complex 82 characterized by X-ray diffraction.…”

Salen-ligands based on a planar-chiral hydroxyferrocene moiety: Synthesis, coordination chemistry and use in asymmetric silylcyanation

“…The C 1 -symmetry observed for this isomer in solution would be in agreement with a cis-b type structure in either L-or D-configuration. This hypothesis is supported by an X-ray crystal structure analysis of a partial hydrolysis product of (S p ,S p )-4a, which features a Ti 2 O 2 -core ligated by two intact ferro-salen units in a (cis-b)-(S p , S p ,D)-configuration (as depicted in an earlier communication [13] and not shown again within this account). The hydrolysis product is thus presumably formed by partial hydrolysis of the C 1 -symmetric major isomer in solution, which in turn can tentatively be assigned as the (cis-b)-(S p ,S p ,D)-isomer of 4a.…”

“…The synthesis of the ethylene-bridged ferrosalen ligand (S p ,S p )-3a was carried out as previously described by us, namely by condensation of (S p )-1 with ethylenediamine and subsequent fluoride-induced deprotection of the O-silyl proctected hydroxyferrocene units [13]. In analogous fashion, the acid-catalyzed condensation of (S p )-1 with 0.5 equivalents of the respective enantiomers of trans-1,2-cyclohexanediamine [16] in toluene solution gave the corresponding O-protected bisimines (S,S,S p ,S p )-2b and (R,R,S p ,S p )-2c, respectively (see Scheme 2).…”

“…Unless otherwise noted, all starting materials were commercially available and were used without further purification. Compounds (S p )-1 [14], (R p )-1 [14], (S p ,S p )-2a [13], (S p ,S p )-3a [13], (S p ,S p )-4a [13], (S p ,S p )-5a [13] and (S p ,S p )-6a [13] were prepared according to literature procedures. For the analytical data of (S p ,S p )-2a and (S p ,S p )-3a see the Supporting Information.…”

“…We have recently described an improved synthesis of this "ferro-salen" ligand [(S p ,S p )-3a, see Scheme 1], plus some first examples of its coordination chemistry and use in catalysis [13]. Our synthesis was based on an enantiomerically pure (S p )-isomer of the O-protected 2-hydroxyferrocene carbaldehyde derivative (S p )-1.…”