( R )‐BINOL‐6,6’‐bistriflone: Shortened Synthesis, Characterization, and Enantioselective Catalytic Applications
Résumé
The title compound, characterized by X-ray crystallography, was accessed in 4 steps with 92 % ee. and 25 % yield from an O-protected (R)-BINOL precursor. This revised synthetic route relied on a chlorosulfonylation reaction, as a shortcut to a previously developed sequence requiring the use of toxic SO2 gas and bromine. The strongly electron-impoverished (R)-6,6′-Tf2-BINOL proved an effective ligand in metal-catalyzed enantioselective transformations such as a Zr-based Mannich-type reaction. The catalytic species was characterized by X-ray crystallography as a unique tetrameric metal cluster. The 6,6′-bistriflone groups also allowed to exalt the H-bond donor capacity of the BINOL moiety, as illustrated in an organocatalyzed Morita-Baylis-Hillman transformation. Theoretical study indicated that the 6,6′-bistriflone groups induce a drop of the phenol acidity of 5 pKa units in DMSO. Overall, this work simplified the access, completed the characterization, and confirmed the potential of (R)-6,6′-Tf2-BINOL as a promising platform to further elaborate activated chiral metal ligands or organocatalysts.
Domaines
Chimie
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