Acid-Induced Intramolecular Transformations in Mixed Phosphonium–Iodonium Ylides with a Dibenzothiaphosphonium Core

Mixed phosphonium–iodonium ylides with a cyclic dibenzothiaphosphonium fragment undergo acid-induced intramolecular rearrangement with loss of iodobenzene and incorporation of water, affording sulfonium salts in up to 93% yield. This reactivity contrasts with the ring expansion observed for oxygen-bridged analogues. Control experiments support an intramolecular pathway. The initially formed sulfonium diastereomer slowly equilibrates to the more stable diastereomer by S-inversion and ring inversion. Kinetic NMR at different temperatures together with DFT calculations identify S-inversion as the rate-limiting step of isomerization.