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In one reported reaction, changing the reaction time changed which mirror-image form of an amine predominated: a six-minute sample favored the (S) enantiomer, while a ten-hour sample favored the (R). The result came from a specific iridium-catalyzed reaction and is a proof of principle—not a general method for making either enantiomer of any molecule by waiting longer.
What the researchers demonstrated
Shu-Li You and colleagues at the Chinese Academy of Sciences in Shanghai studied an asymmetric intermolecular allylic amination. The reaction combined 6-hydroxyisoquinoline with racemic tert-butyl carbonate, using the (S) enantiomer of a chiral iridium complex as the catalyst and methanol as the solvent. Chemistry World reported the results on 9 July 2020.
| Reaction time | Amine enantiomer favored | Reported enantiomeric excess |
|---|---|---|
| Six minutes | (S) | 94% |
| Ten hours | (R) | 98% |
These percentages are enantiomeric excess (ee), as reported by Chemistry World; they are not yields or measures of product purity. The report does not give isolated yields in its account. The figures describe samples taken at different reaction times, not a catalyst whose handedness changed during the reaction. Chemistry World’s report identifies the primary paper as H.-F. Tu et al., Nature Chemistry (2020), DOI 10.1038/s41557-020-0489-1.
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Why the favored enantiomer changed over time
The report explains the switch through competing rates of product formation and decomposition. Early in the reaction, the catalyst rapidly forms the (S)-amine from the (S) carbonate enantiomer. Over longer periods, it also catalyzes decomposition of that amine to an ether, releasing 6-hydroxyisoquinoline.
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Formation from the (R) carbonate enantiomer is slower, but the corresponding (R)-amine decomposes less effectively. As a result, it can accumulate while the initially favored (S)-amine is being depleted. The changing balance between formation and breakdown allows the later sample to favor the opposite enantiomer.
This is the explanation given for this reaction system. It should not be treated as a universal mechanism for time-dependent selectivity or as evidence that extending a reaction will generally reverse its product’s handedness.
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How this differs from the usual approach
To obtain opposite enantiomers, chemists commonly change the chirality of a component such as the catalyst or starting material. In the reported example, the catalyst configuration stayed the same while reaction time served as the control variable. That is a different strategy, not a demonstration that waiting is interchangeable with using the opposite catalyst enantiomer.
A meaningful comparison with conventional routes would need to consider catalyst configuration, starting materials, reaction time, enantiomeric excess, isolated yield, decomposition and by-products, and the range of substrates actually demonstrated. Chemistry World described related reactions as showing a similar strategy but did not provide enough detail in its account to compare their yields, selectivities, or substrate scope.
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What the result does—and does not—establish
The finding is a proof of principle: in the specific reaction reported, samples taken at different times favored opposite amine enantiomers because formation and decomposition affected them differently. The report does not establish broad applicability to other target molecules. Eric Ferreira, a synthetic organic chemist at the University of Georgia, called the work conceptually interesting and more important as a proof of principle than in a directly applicable sense. He also expressed interest in seeing the approach demonstrated in a system where only one enantiomer of the catalyst is readily available.
The study was discussed in the context of chirality’s importance in biology: living organisms are homochiral and may interact differently with two enantiomers. That motivation does not mean this synthesis produced a drug candidate or demonstrated pharmaceutical utility. Chemistry World’s account does not provide the experimental detail needed to assess full reproducibility or the reaction’s broader scope; those details would require consulting the primary paper and its supporting information.
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