Water‐Mediated 1,7‐Palladium Aryl‐to‐Aryl Migration in an Unusual Aryne Trimerization Enables the Synthesis of Multihelicenes with Profoundly Hindered Stereogenic Axes

The nickel(0)‐catalyzed [2 + 2 + 2] cyclotrimerization of 13,14‐picyne, a π‐extended aryne intermediate having bay regions at both sides of the aryne triple bond, has recently enabled the synthesis of an overcrowded triple [7]helicene. In contrast, it has now been found that under palladium(0) catalysis, the same π‐extended aryne intermediate trimerizes in a very different manner, notably featuring a formal 1,7‐hydrogen shift coupled with a rare 1,7‐palladium aryl‐to‐aryl migration. Remarkably, this new aryne trimerization reaction seems general and reveals possible using a further π‐extended analog of 13,14‐picyne. The two chiral multi[5]helicenes thus obtained have, in addition to helical stereogenic elements, a profoundly hindered stereogenic biaryl axis located in the fjord region of a carbo[5]helicene fragment. Both compounds are characterized by single crystal X‐ray diffraction analysis, which reveals their remarkably compact and severely distorted structures. A thorough mechanistic study, combining experiments and DFT simulations, shows that the observed 1,7‐Pd aryl‐to‐aryl migration is a two‐step 1,4‐to‐1,4’‐Pd isomerization process enabled by the presence of trace amounts of water, a proton shuttle, in the reaction mixture.