A palladium thiolate-SNS complex-catalyst for Mizoroki–Heck cross-coupling

Abstract

Phosphine-free catalysts for the Mizoroki–Heck cross-coupling reaction offer significant advantages in terms of air and moisture stability. We synthesized two palladium(II) complexes featuring a thiolate-imine-thioether SNS pincer ligand: Pd(κ²-SNS^Me)₂ (Pd-1) and PdI(κ³-SNS^Me) (Pd-2). Their catalytic activities were compared, with Pd-2 demonstrating excellent reactivity at a low catalyst loading of 1.5 mol%, efficiently producing olefin products across various substrates within 5 hours. Conversely, Pd-1 showed no catalytic activity due to its isomerization to a Pd(II)-N₂S₂ complex via imine C–C bond formation at elevated temperatures, which restricted access to coordination sites necessary for oxidative addition and alkene coordination. Heating Pd-2 with excess NEt₃ affords a Pd dithiolate tetramer, [Pd(μ-κ³-SNS)]₄via selective C–S bond activation.