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Catalyst as colour indicator for endpoint detection to enable selective alkyne trans-hydrogenation with ethanol


The stereoselective semi-hydrogenation of internal alkynes to E-alkenes is an important, but challenging, transformation, partly because of over-reduction to undesired alkanes. A stop criterion that enables determination of the endpoint of the semi-hydrogenation is thus required to eliminate this over-hydrogenation. Despite its widespread applications in analytic chemistry, the strategy of using colour change for endpoint detection is very rarely applied in catalytic organic transformations. Here we report that an iridium complex catalyses the semi-hydrogenation of internal alkynes using ethanol as the hydrogen donor to afford E-alkenes and ethyl acetate. Importantly, issues of over-reduction and stereoselection have been successfully addressed by using a colour change effect due to the shift of the catalyst resting states, thereby precisely detecting the endpoint of the reaction. This catalytic system is applicable to a wide variety of internal alkynes bearing many auxiliary functional groups, and its utility for synthesis of biologically relevant molecules has been demonstrated.

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Fig. 1: Development of catalytic methods for the selective hydrogenation of alkynes to E-alkenes with EtOH.
Fig. 2: Iridium-catalysed TH of diphenylacetylene with EtOH.
Fig. 3: Reaction profile of catalytic TH of 1a with EtOH and key catalyst resting states.
Fig. 4: E-selective TH of 1a with EtOH using the colour change to detect the endpoint.
Fig. 5: Scope of TH of alkynes to E-alkenes with EtOH.
Fig. 6: Transfer hydrogenation of alkynes containing biologically relevant skeletons.
Fig. 7: Proposed catalytic cycle.

Data availability

X-ray crystallographic data for compounds A and 8-PhOH have been deposited at the Cambridge Crystallographic Data Centre under deposition numbers 1877722 and 1877723, respectively. These data can be obtained from the Cambridge Crystallographic Data Centre via All other data supporting the findings of this study are available within the Article (and Supplementary Information files) or from the corresponding author on reasonable request.


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The authors acknowledge support from the National Key R&D Program of China (2016YFA0202900 and 2015CB856600), the National Natural Science Foundation of China (21825109, 21821002 and 21432011), the Chinese Academy of Sciences (XDB20000000 and QYZDB-SSW-SLH016) and the Science and Technology Commission of Shanghai Municipality (17JC1401200).

Author information

Authors and Affiliations



Z.H. and Y.W. developed the concept of using the transition metal catalyst as a colour indicator for the selective semi-hydrogenation of alkynes to E-alkenes with EtOH. Y.W. and Z.-D.H. identified the substrate scope. Y.W. and Z.-D.H. conducted the mechanistic investigations. Z.H. conceived and supervised the project and prepared the manuscript.

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Correspondence to Zheng Huang.

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Supplementary information

Supplementary Information

Supplementary methods, Supplementary Figs. 1–79, Supplementary Tables 1–6, Supplementary references

Complex A

Crystallographic data for complex A

Complex 8-PhOH

Crystallographic data for complex 8-PhOH

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Wang, Y., Huang, Z. & Huang, Z. Catalyst as colour indicator for endpoint detection to enable selective alkyne trans-hydrogenation with ethanol. Nat Catal 2, 529–536 (2019).

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