engleski

"Backdoor induction" of chirality: Trans-1, 2-cyclohexanediamine as a key building block for asymmetric hydrogenation catalysts

In asymmetric catalysis the source of chirality is often close to the catalytic metal. A different approach is used in catalytic systems based on "backdoor induction" of chirality. In these systems, the chiral information is relayed through non-covalent interactions from distant chiral groups to the metal coordination sphere, thus inducing chirality at the reaction centre.1 In our laboratory, maximum selectivity achieved by utilising this principle has been 84 % e.e. with amino acid based phosphine ligands in rhodium catalysed hydrogenation reactions on model substrates.2 Herein, we present a series of newly synthesised chiral ligands and their Rh(I) complexes for the application in enantioselective hydrogenation reactions. Pt(II) complexes were synthetized as model systems for comparison. We have synthesised 21 new chiral ligands, consisting of three variable building blocks ; a metal binding triphenylphosphine, a central chiral 1, 2-diamine and an auxiliary moiety for fine tuning properties. We have varied all of the building blocks: the triphenyphosphine is introduced as an acid in three substitution patterns, meta, para and dimeta, while the chiral diamine was cyclohexane or cyclopentane. The auxiliary group was varied in order to evaluate its effect on selectivity: bulky substituents for steric influence and electron donating or accepting substituents for electronic influence. The major feature of this work is the formal isomerisation of the central amide building block, utilizing diamines instead of amino acids used in the literature. This change has resulted in increased selectivity, affording up to 97 % e.e. in rhodium catalysed hydrogenation on model substrates, acetamidoacrylate and acetamidocinnamate. Our best ligand consists of mtriphenylphosphine acid for metal binding, chiral 1, 2-cyclohexanediamine and triphenylacetic acid as a bulky auxiliary. References: 1a) A. C. Laungani, B. Breit, Chem. Commun., 2008, 844 ; 1b) A. Desmarchelier, X. Caumes, M. Raynal, A. Vidal-Ferran, P. W. N. M. van Leeuwen, L. Bouteiller, J. Am. Chem. Soc., 2016, 138, 4908. 2) Z. Kokan, S. I. Kirin, Eur. J. Org. Chem., 2013, 8154.

catalysis ; chirality ; triphenyphosphine ; hydrogenation ; rhodium(I)

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