Enantiodivergent Synthesis of Tertiary α‑Aryl 1‑Indanones: Evidence Toward Disparate Mechanisms in the Palladium-Catalyzed Decarboxylative Asymmetric Protonation

Herein,
we describe a study into the scope and origin of an enantiodivergent
effect in the palladium-catalyzed decarboxylative asymmetric protonation.
By switching the achiral proton source, both enantiomers of a series
of tertiary α-aryl-1-indanones are readily accessed from the
corresponding α-aryl-β-keto allyl esters. In this example
of dual stereocontrol, enantioselectivities up to 94% (S) and 92% (R) were achieved using Meldrum’s
acid and formic acid, respectively. In an attempt to rationalize this
switch in absolute configuration an investigation of the ambiguous
mechanism of the decarboxylative asymmetric protonation was conducted.
A novel catalytic cycle for the reaction with formic acid is proposed
and subjected to a variety of experimental studies.