Chiral Catalyst Design: Cyclopentadiene‐Based Brønsted Acids

Abstract

The combination of -I and -M effects and the aromaticity of the conjugate base renders the chiral pentacarboxy-substituted cyclopentadienes (PCCPs) acidic. Their convenient synthesis together with substrates derived from the chiral pool and their performance in the tested reactions renders them prodigious candidates to become a new and highly competitive class of chiral Brønsted acid catalysts.

Pentacarboxycyclopentadienes in Organic Synthesis

“…PCCPs 1, 12, and 42 are strong CH acids existing generally as fulvenes [27,43,44], and their deprotonation gives highly stable Cp anions. The aromatic stabilization of these anions, as well as the presence of 5 electron-acceptor alkoxycarbonyl groups signifi cantly increase their acidity, making it comparable with the acidity of strong mineral acids and increasing the effi ciency of such catalysts, the enantioselectivity of which is associated with the chiral alkoxy substituents (Scheme 11) [30,[51][52][53].…”

Promotion of Organic Reactions by Non‐Benzenoid Carbocyclic Aromatic Ions

“…[33] It was estimated that compounds similar to 70 possess strong acidity (pK a % 8. [34] It offers avery versatile alternative to the BINOL/VAPOL/SPINOL-based frameworks for asymmetric Brønsted acid catalysis.A part from the studies discussed in this section, however, the synthetic role of cyclopentadienide remains poorly investigated in organic chemistry. With as imple stereogenic moiety such as (À)menthyl on the ester groups,the catalyst 70 could give rise to excellent yields and very high enantioselectivities of up to 97 %a nd 85 %, for the Mukaiyama-Mannich and the oxocarbenium aldol reactions,r espectively.H igh efficiency and enantioselectivity were still achieved at av ery low catalyst loading of 0.01 mol %.…”

“…This is alandmark study in chiral catalyst design as well as in the utilization of the cyclopentadienide framework in organic reactions. [34] It offers avery versatile alternative to the BINOL/VAPOL/SPINOL-based frameworks for asymmetric Brønsted acid catalysis.A part from the studies discussed in this section, however, the synthetic role of cyclopentadienide remains poorly investigated in organic chemistry.…”

“…This is a landmark study in chiral catalyst design as well as in the utilization of the cyclopentadienide framework in organic reactions . It offers a very versatile alternative to the BINOL/VAPOL/SPINOL‐based frameworks for asymmetric Brønsted acid catalysis.…”