The Role of Ligands and Silver(I) Additives in the\nLigand-Promoted Divergent α,β-Dehydrogenation of Aliphatic\nAcids

Herein,\nthe density functional theory method was employed to investigate\nthe mechanism of the ligand-promoted divergent α,β-dehydrogenation\nof aliphatic acids. The calculated results showed that the ligands\ncould act as an intramolecular base to activate both β-methylene\nC(sp<sup>3</sup>)-H and α-methylene C(sp<sup>3</sup>)-H bonds\nvia concerted metalation–deprotonation mechanism to generate\nα,β-unsaturated aliphatic acids and the β-C(sp<sup>3</sup>)-H activation is the rate-determining step. The origin of\nthe selective activation of different kinds of C–H bonds and\ndifferent product inhibition effects by two ligands was explained\nby the different torsional strains between the substrate and the Pd\ncatalysts. The formation of lactone includes the C–C coupling\nprocess and the cyclization process. The C–C coupling is catalyzed\nby the heterodimeric Pd(II)–Ag(I) species and reveals the pivotal\nrole of the base. The final cyclization process is catalyzed by the\nsilver additives. In addition, the formation of <i>Z</i>-products undergoes a unique rotational isomerization promoted by\nAg(I)–Ag(I) species to complete the <i>E</i>-to-<i>Z</i> isomerization. In general, this theoretical work indicates\nthat in this cascade reaction, the active catalytic species go through\nthe process from monomeric Pd species in the C–H activation\nprocess and heterodimeric Pd–Ag species in the C–C coupling\nprocess to homodimeric Ag–Ag species in the final cyclization\nprocess.