In one reported reaction, a chiral iridium catalyst produced an amine enriched in the (S) enantiomer after six minutes, but the opposite (R) enantiomer after ten hours. The change came from time-dependent differences in product formation and decomposition—not from the catalyst changing its handedness. The result is a proof of principle for one specific system, not a general way to obtain either enantiomer from any reaction by waiting.
What the researchers did
Shu-Li You and colleagues at the Chinese Academy of Sciences in Shanghai studied an asymmetric intermolecular allylic amination. They combined 6-hydroxyisoquinoline with a racemic tert-butyl carbonate, using the (S) enantiomer of a chiral iridium complex as the catalyst and methanol as the solvent.
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As reported by Chemistry World on 9 July 2020, a six-minute reaction favored the (S) amine by 94% enantiomeric excess. At ten hours, the reaction favored the (R) amine by 98% enantiomeric excess.
Those numbers describe enantiomeric excess, not yield or product purity. The news report does not give isolated yields, so the figures alone do not show how much amine was recovered at either time point.
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Why the favored enantiomer changed
The report explains the switch through a balance of formation and decomposition rates. The catalyst rapidly forms the (S) amine from the (S) carbonate enantiomer. Over longer reaction times, it also promotes decomposition of that amine to an ether, releasing 6-hydroxyisoquinoline.
Reaction with the (R) carbonate is slower, and the corresponding (R) amine decomposes less effectively. As the reaction continues, that product can accumulate while the initially favored (S) amine is depleted. In this account, reaction time changes the product distribution because the two pathways do not proceed at the same rates.
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This is the proposed explanation for the reported reaction, not a universal mechanism for time-dependent selectivity. The catalyst does not become the opposite enantiomer during the reaction; both product configurations arise from the particular substrate, catalyst, solvent, and competing reaction pathways.
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For many asymmetric syntheses, access to the opposite product enantiomer commonly involves changing the chirality of a catalyst or another chiral component. This study instead used reaction time as a control variable in its described system. It does not establish that adjusting time can generally substitute for access to both catalyst enantiomers.
Chemistry World reported that several related reactions showed a similar strategy, but its account does not enumerate their substrates or provide enough data to compare their yields and selectivities. It therefore supports a limited proof of principle, not broad substrate generality. Eric Ferreira, a synthetic organic chemist at the University of Georgia, described the work as conceptually interesting and more important as a proof of principle than as a directly applicable method.
The finding may be relevant to research on molecules with different enantiomers: living organisms are homochiral and can interact differently with the two forms. That motivation does not make this reaction a drug-development result, nor does it establish pharmaceutical utility.
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Where to find the experimental detail
The news report cites H.-F. Tu and colleagues’ paper in Nature Chemistry (2020), DOI 10.1038/s41557-020-0489-1. The primary paper and its supporting information are the appropriate sources for exact experimental conditions, analytical methods, isolated yields, and reaction scope.
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