Improving C=N Bond Reductions with (Cyclopentadienone)iron Complexes: Scope and Limitations

Abstract

Herein, we broaden the application scope of (cyclopentadienone)iron complexes 1 in C=N bond reduction. The catalytic scope of pre‐catalyst 1b, which is more active than the “Knölker complex” (1a) and other members of its family, has been expanded to the catalytic transfer hydrogenation (CTH) of a wider range of aldimines and ketimines, either pre‐isolated or generated in situ. The kinetics of 1b‐promoted CTH of ketimine S1 were assessed, showing a pseudo‐first order profile, with TOF = 6.07 h–1 at 50 % conversion. Moreover, the chiral complex 1c and its analog 1d were employed in the enantioselective reduction of ketimines and reductive amination of ketones, giving fair to good yields and moderate enantioselectivity.

Chiral (cyclopentadienone)iron complexes with a stereogenic plane as pre-catalysts for the asymmetric hydrogenation of polar double bonds

“…The catalytic activity of the newly synthesized complexes (±)-1a-f, activated in situ with Me3NO, in the hydrogenation of polar double bonds was tested using acetophenone (S1) and (E)-N-(4methoxyphenyl)-1-phenylethan-1-imine (S2) as model substrates (Table 1). Pre-catalysts (±)-1a and (±)-1b gave high conversion of S1 and S2 at both 5 mol% and 2 mol% loading ( Table 1, entries [1][2][3][4]. Along the series (±)-1c → (±)-1f, conversions were found to decrease with the increasing size of the "large" substituent R L (Table 1, entries 5-12): while (±)-1c (R L = TMS) and (±)-1d (R L = TES) gave full conversion of S1 and acceptable conversions of S2 (entries 5-8), the activity of (±)-1e (R L = TIPS) and (±)-1f (R L = CPh3) decreased dramatically (entries 9-12).…”

Tris(acetylacetonato) Iron(III): Recent Developments and Synthetic Applications

“…Considering the relatively young area of this research, one can anticipate even more relevant contributions in the future. Scheme 32 (a) Selective reduction of nitroarenes to anilines by Lemaire, 100 (b) asymmetric reduction using chiral (cyclopentadienone)iron complexes by Gennari, 101 and (c) imination of sulfoxides and sulfides by Bolm; 103 TMDS = 1,1,3,3-tetramethyldisilazane, Ns = nosyl, o2s = yield over two steps.…”

“…100 This iron-mediated reduction in the presence of TMDS tolerates various functional groups, such as esters, aldehydes, carboxylic acids, bromides, etc., and affords the corresponding anilines 236 as the hydrochloride salts in excellent yields (Scheme 32, a). In 2018, Gennari and co-workers 101 published an asymmetric reduction of imine 237 using a modified chiral Knölker-type catalyst 102 and obtained, in the presence of Fe(acac) 3 , high conversions with moderate selectivities (Scheme 32, b). Another Fe(acac) 3 -mediated reaction is Bolm's imination of sulfoxide 239 which proceeds with complete retention of configuration at the sulfur center and affords sulfoximine 241.…”

Synthesis and Catalytic Application of Knölker-Type Iron Complexes with a Novel Asymmetric Cyclopentadienone Ligand Design

“…In later years, this problem was circumvented with the use of ligands featuring a C2-rotational symmetry axis along the catalytically functional carbonyl bond, because for such a design coordination of the metal to either face of the ligand will lead to the same complex. As such, higher catalytic enantioselectivities could be achieved [42,43], with the best example featuring BINOLderived cyclopentadienone ligand provided by Gajewski et al [44][45][46]. Alternatively, racemic mixtures of asymmetric ruthenium [47] and iron catalysts [48] with different 2,5-substituents were separated using preparative chiral HPLC chromatography, yielding batches of both pure enantiomers.…”

“…By in-depth study of all available literature on asymmetric (cyclopentadienone)metal catalysts, we deduced that desirable structural aspects are C2-rotary symmetry of the ligand, and having the chirality-inducing moieties closer located to the active site of the catalyst, in order to envision In later years, this problem was circumvented with the use of ligands featuring a C 2 -rotational symmetry axis along the catalytically functional carbonyl bond, because for such a design coordination of the metal to either face of the ligand will lead to the same complex. As such, higher catalytic enantioselectivities could be achieved [42,43], with the best example featuring BINOL-derived cyclopentadienone ligand provided by Gajewski et al [44][45][46].…”