Gold(I)‐Catalyzed Intramolecular Hydroarylation of o‐Ethynylarylphosphonium Salt Leading to the Formation of Seven‐ and Six‐membered Phosphacycles

Abstract

The gold(I)-catalyzed intramolecular hydroarylation of o-ethynylarylphosphonium salt 1 gave phosphepinium 2 or phosphinium salt 3. The use of methylphosphonium salt instead of phosphine chalcogenide is key to success for the intramolecular hydroarylation. The substrate with 2-methoxyphenyl group at the alkyne position preferentially provided seven-membered phosphacycle 2, while that with a hydrogen did six-membered phosphacycle 3. Theoretical calculations revealed that the activation energy of 6-exo-dig cyclization in the substrate with the aryl group increased due to the steric repulsion of the two aryl rings. Furthermore, the Wittig-type reaction of phosphepinium salt 2 a afforded methyl-substituted phosphepin oxide 4 a via the elimination of an aryl group.