Effective Pd–NAPA precatalysts (NHC and phosphinous acid-based palladium precatalyst) enabled the KTC couplings of challenging aryl chlorides, sterically hindered reactants, and substrates with a functional-group tolerant pitfall. Pd(0)–PR₂(O^-) is proposed as an active species based on the reactivity difference with Pd-NHC and evidence from DFT and ESI-MS.

Abstract

A series of NHC and phosphinous acid-based palladium precatalysts (NAPA), (i.e., 1b–1d: SIPr-PdCl₂-PAr₂(OH)), and their efficiency in catalyzing Kumada–Tamao−Corriu (KTC) couplings are reported. The 1-catalyzed KTC couplings could reach high yields at 50 °C in 3 h. Good results were obtained for strained tetra-ortho-substituted biaryl products by raising the temperature to 80 °C and reacting for 3 h. Functional-group tolerance was achieved using dropwise addition of Grignard reagent during the reaction time course. Upon deprotonation of the ligated PAr₂(OH), density functional theory (DFT) calculations and negative-ion mode electrospray mass spectrometer (ESI-MS) revealed that SIPr tended to dissociate from the Pd(II), implying the succeeding generation of Pd(0)-P(O⁻)Ar₂ active species. Compared to Pd(0)-SPhos and Pd(0)-NHC, the oxidative addition of 4-chlorobenzonitrile to Pd(0) was computed, showing the Pd(0)–P(O⁻)Ar₂ has the lowest activation free energy.