Accelerated Metal-Free Hydration of Alkynes within Milliseconds in Microdroplets

Conversion of alkynes into ketones by hydration is one of the most fundamental organic addition reactions. Traditional methods for alkyne hydration suffer from usage of toxic and costly metals, excessive acids, harsh conditions, and long reaction times, bringing about an urgent desire to explore more efficient and green protocols to complete this transformation. A metal-free efficient protocol for the accelerated hydration of alkynes in microdroplets was successfully developed in this work to overcome these drawbacks. The hydration process was finished under room temperature using only 18 mol % sulfuric acid as the catalyst and acetonitrile as the solvent. Complete conversions were feasible on the low millisecond time scale in microdroplets for a wide range of alkynes bearing functional groups, including electron-withdrawing groups and electron-donating groups. This is in sharp contrast to trace amounts of ketones or none in 20 min in bulk phase. Compared to the bulk reaction under the same conditions, the reactions in microdroplets are accelerated by 4 to 5 orders of magnitude as seen by the increase in measured rate constants. Further, the accelerated microdroplet hydration can be scaled up to an acetophenone amount of 57.2 ± 0.1 mg min–1 (3.43 ± 0.01 g h–1) by using 2 mol L–1 phenylacetylene sprayed at 300 μL min–1. The microdroplet protocol offers several advantages including ultrahigh acceleration, complete conversion, mild reaction conditions (metal, light, and heat free), and minimal post purification, as well as a wide substrate scope, making it attractive for green and sustainable chemistry.