Two-Step Flow Synthesis of Biarylmethanes by Reductive Arylation of Tosylhydrazones

The coupling of tosylhydrazones derived from aldehydes or ketones with aryl boronic acids to yield the corresponding arylation products that was first developed in the group of Barluenga was achieved in a two-step flow protocol. Starting from the respective carbonyl compounds, tosylhydrazones were formed in the first flow step. These were directly transferred into the second reactor to be coupled with boronic acids. Remarkably, carbenes are postulated to be the highly reactive intermediates of this reaction. Both steps required heating which was managed by electromagnetic induction of a fixed bed material based on steel beads. A continuously conducted two-step flow processes over a period of almost 2 days gave the arylation product in 84% yield.