Ligand-Controlled Chemoselectivity in Manganese-Catalyzed Coupling of Benzyl Alcohol with Phosphorus Ylide: A Theoretical Study

The selective formation of C–C and CC bonds is fundamentally important in pharmaceutical synthesis and materials science. Herein, the ligand-controlled chemoselectivity of the Mn-catalyzed coupling of benzyl alcohol (BnOH) with a phosphorus ylide for the C–C or CC product was theoretically investigated through DFT and DLPNO–CCSD­(T) calculations. For the PNP-supported catalyst 1A ([Mn­(CO)2(N­(C2H4PiPr2)2)]), BnOH undergoes dehydrogenation via Mn–N cooperation (ΔG≠ = 25.5 kcal/mol) to yield benzaldehyde and the Mn-hydride intermediate 4A. Benzaldehyde subsequently reacts with the ylide via Wittig olefination to generate an E-alkene, which is selectively hydrogenated by 4A (ΔG≠ = 29.5 kcal/mol) to afford the C–C product. In contrast, the NNP-ligated catalyst favors CC bond formation, as the corresponding Mn-hydride 4B undergoes H2 evolution (ΔG≠ = 31.2 kcal/mol) rather than alkene hydrogenation (ΔG≠ = 33.1 kcal/mol). This divergent reactivity originates from the lower hydride dissociation energy and greater steric hindrance of the NNP ligand, which collectively suppress alkene hydrogenation and promote H2 release. Guided by these insights, catalyst 1A was modified by replacing iPr with bulkier tBu groups to construct 1C, which shifts the chemoselectivity toward the CC product exclusively. These findings emphasize the crucial role of ligand design in steering the chemoselectivity in base-metal-catalyzed transformations.