Metal-dependent pathway selection in Co(III)- and Rh(III)-catalyzed C-H activation-alkyne coupling: a DFT study

Transition-metal-catalyzed C-H activation-alkyne coupling represents a powerful strategy for the construction of strained phosphorus-containing heterocycles, yet the mechanistic origins of pathway selection and metal-dependent reactivity remain poorly understood. Herein, a comprehensive density functional theory (DFT) investigation is presented to elucidate the catalytic manifolds of Co(III)- and Rh(III)-catalyzed coupling reactions of heteroarenes with dialkynylphosphine oxides leading to four-membered 1,2-dihydrophosphete oxides. Directed C-H activation is identified as a reversible and universal entry point, while regioselectivity is established during the first alkyne migratory insertion and governed predominantly by geometric distortion effects. Importantly, the β-C(alkynyl) elimination pathway invoked in related systems is found to be kinetically inaccessible, whereas sequential alkyne migratory insertion followed by anti-elimination, cyclization, and protonolysis/catalyst regeneration constitutes the viable route to phosphacycle formation. Additional triplet-state calculations for the Co-catalyzed pathway suggest possible singlet/triplet state alternation after the first alkyne migratory insertion. Comparative analysis shows that Co(III) and Rh(III) follow closely related catalytic sequences, while rhodium lowers the key energetic spans by better accommodating sterically congested and polarized transition structures. These results provide a unified mechanistic framework for understanding metal-controlled reactivity in C-H activation-alkyne coupling reactions and offer insights relevant to the rational design of phosphorus-containing heterocycles.