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. The reduction of alkynes to alkenes is complicated by the potential for over-reduction to the alkane. Here, for the iridium-catalysed semi-hydrogenation of alkynes, the endpoint of the reaction is clearly identified by a colour change of the metal complex.