Bromine-lithium exchange on gem-dibromoalkenes part 1: batch vs microflow conditions

The generation and reactivity of two model vinyl carbenoids from gem-dibromoalkenes 1 were studied in microflow systems. From substrate 1a (R = Ph, CF3), the lithium-bromine exchange could be simply performed within 30 ms at 20 degrees C with very good E-selectivity whereas the reaction was unselective under batch conditions, even at -78 degrees C. Moreover, the unstable carbenoid generated from 1b (R = Ph, H) could be trapped as the major product while only the Fritsch-Buttemberg-Wiechell rearrangement product was obtained in a flask under cryogenic conditions.