Artificial metalloenzymes derived from bovine β-lactoglobulin for the asymmetric transfer hydrogenation of an aryl ketone – synthesis, characterization and catalytic activity

Abstract

A series of diimines derived from saturated and unsaturated fatty acids and including a dipyridylamine (dpa) or a bispyridylmethane (bpm) scaffold as a chelating moiety were synthesized and characterized spectroscopically. Complexation by [LM(μ-Cl)Cl]2 (M = Ru, L = p-cymene; M = Rh, L = Cp*) afforded the monocationic, mononuclear complexes of general formula [LM(N^N)Cl]Cl with N^N being the diimine ligand. Unsurprisingly, these new complexes catalysed the transfer hydrogenation of an activated aromatic ketone, namely 2,2,2-trifluoroacetophenone (TFACP), in water at neutral pH and mild temperature in the presence of formate as a hydrogen donor. The catalytic activity of the complexes expressed as TOF was shown to depend not only on the metal (Ru or Rh) but also on the chelating entity (dpa or bpm) and the length and nature of the lipidic chain tethered to it. Incorporation of the complexes within bovine β-lactoglobulin (βLG) as the protein host was studied by circular dichroism and fluorescence spectroscopy and again noticeable differences were observed between the saturated and unsaturated fatty acid derivatives. Eventually, the ability of the protein hybrids to catalyse the transfer hydrogenation of TFACP was demonstrated. Good-to-quantitative conversions in the corresponding alcohol were reached within 72 h with the rhodium(iii) hybrids and the best enantioselectivities (up to 32% ee for the (R)-enantiomer) were measured with the Rh(iii) cofactors derived from palmitic and stearic acids once incorporated into the isoform A of βLG.

The Golden Age of Transfer Hydrogenation

“…387 The past few years have witnessed an extensive exploration of pyridine-based ruthenium catalysts for TH of a wide variety of substrates. 403 Compound 88 containing a chiral pyridyl-based 1H-pyrazolyl− oxazolinyl ligand was shown to be a robust and productive catalyst for ATH of ketones under mild conditions, and a series of alcohols were obtained with good to excellent conversions and enantioselectivities within short times down to 5 min. 394 Furthermore, the activity of 88 was much higher than that of ruthenium pyridyl−pyrazolyl−oxazolinyl complexes featuring no NH functionality, which confirmed an outer-sphere mechanism for the involved "N−H" effect.…”

Supramolecular Anchoring of NCN‐Pincer Palladium Complexes into a β‐Barrel Protein Host: Molecular‐Docking and Reactivity Insights

“…Therefore, a displacement experiment was carried out as follows. The CD spectrum of a stoichiometric mixture of β‐LG and N , N ‐dipyridin‐2‐hexadecanamide ( Ref ), whose insertion within the β‐LG cavity was previously asserted by CD spectroscopy,[13b] was first recorded. A broad positive band centered at 270 nm was observed on the CD spectrum of the mixture of β‐LG and Ref .…”

“…The cavity generated by this folding can host various hydrophobic ligands, including fatty acids . We previously took advantage of this feature to incorporate half‐sandwich dipyridylamine Rh III complexes within β‐LG, which conferred transfer hydrogenase properties to this protein …”

“…This could, in turn, bring the chiral center(s) closer to the substrate and eventually enhance the enantioselectivity of the catalyzed reaction. As a proof‐of‐concept, we chose the β‐lactoglobulin (β‐LG)/fatty‐acid supramolecular assembly, from which we previously built artificial transfer hydrogenases with promising activity and selectivity for the reduction of a prochiral ketone . Palladium(II) complexes of prochiral hemilabile NCN‐pincer ligands carrying a fatty‐acid side chain were synthesized and characterized spectroscopically.…”

Systematic Engineering of Artificial Metalloenzymes for New-to-Nature Reactions

“…Therefore, they have a great potential to enable sustainable synthetic routes to various compounds of interest 3,4 and, if functional in a cellular environment, open up new possibilities for metabolic engineering and synthetic biology 5 . ArMs have been constructed by repurposing natural metalloenzymes 6,7 , designing binding sites for metal ions [8][9][10][11] and incorporating organometallic cofactors into protein scaffolds [12][13][14][15][16][17][18][19][20] . Moreover, unnatural amino acids 21 and photoexitation 22,23 have been used to unlock new reactivity in enzymes.…”