This Review compiles the evolution,
mechanistic understanding,
and more recent advances in enantioselective Pd-catalyzed allylic
substitution and decarboxylative and oxidative allylic substitutions.
For each reaction, the catalytic data, as well as examples of their
application to the synthesis of more complex molecules, are collected.
Sections in which we discuss key mechanistic aspects for high selectivity
and a comparison with other metals (with advantages and disadvantages)
are also included. For Pd-catalyzed asymmetric allylic substitution,
the catalytic data are grouped according to the type of nucleophile
employed. Because of the prominent position of the use of stabilized
carbon nucleophiles and heteronucleophiles, many chiral ligands have
been developed. To better compare the results, they are presented
grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions
are mainly promoted by PHOX or Trost ligands, which justifies organizing
this section in chronological order. For asymmetric oxidative allylic
substitution the results are grouped according to the type of nucleophile
used.
Optically pure alcohols are abundant in nature and attractive as feedstock for organic synthesis but challenging for further transformation using atom efficient and sustainable methodologies, particularly when there is a desire to conserve the chirality. Usually, substitution of the OH group of stereogenic alcohols with conservation of chirality requires derivatization as part of a complex, stoichiometric procedure. We herein demonstrate that a simple, inexpensive, and environmentally benign iron(III) catalyst promotes the direct intramolecular substitution of enantiomerically enriched secondary and tertiary alcohols with
O
-,
N
-, and
S
-centered nucleophiles to generate valuable 5-membered, 6-membered and aryl-fused 6-membered heterocyclic compounds with chirality transfer and water as the only byproduct. The power of the methodology is demonstrated in the total synthesis of (+)-lentiginosine from D-glucose where iron-catalysis is used in a key step. Adoption of this methodology will contribute towards the transition to sustainable and bio-based processes in the pharmaceutical and agrochemical industries.
The Ir-MaxPHOX-type catalysts demonstrated high catalytic
performance
in the hydrogenation of a wide range of nonchelating olefins with
different geometries, substitution patterns, and degrees of functionalization.
These air-stable and readily available catalysts have been successfully
applied in the asymmetric hydrogenation of di-, tri-, and tetrasubstituted
olefins (ee′s up to 99%). The combination of theoretical calculations
and deuterium labeling experiments led to the uncovering of the factors
responsible for the enantioselectivity observed in the reaction, allowing
the rationalization of the most suitable substrates for these Ir-catalysts.
Phosphite-thioether ligands with a simple modular architecture, derived from inexpensive l-(+)-tartaric acid and d-mannitol, have been for the first time successfully applied (ee values up to 99 %) in the synthesis of 2-aminotetralines and 3-aminochromanes by metal-catalyzed asymmetric hydrogenation of cyclic β-enamides. The ligands have the advantages of the robustness of the thioether/phosphite moieties and the extra control provided by the flexibility of the chiral pocket through the presence of a biaryl phosphite group and a modular carbohydrate-derived backbone. Moreover, they are solid and stable to air and they are therefore easy to handle, manipulate, and store. Usefully, both enantiomers of the hydrogenated products were obtained by simply switching from Rh to Ir. Low hydrogen pressure and environmentally friendly propylene carbonate can be used, with no loss of selectivity.
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