The Potential of Methyl Groups: Can Methyl Groups Unlock Reactivity Limitations of the Acceptorless Dehydrogenation of N‑Heterocycles?

We systematically investigated how the position of the methyl group in methylpiperidine/methylpyridine (MPI/MPY) pairs governs its acceptorless dehydrogenation reactivity through three perspectivesthermodynamics, kinetics, and adsorption. Among the MPI isomers, 2-MPI shows the highest conversion of 97.5% and a fast reaction rate of 1.39 g(H(2)) g(Pd) (-1) min(-1). Thermodynamically, a 2-positioned methyl group lowers the dehydrogenation enthalpy and raises the reaction entropy, enhancing overall spontaneity and enabling near-equilibrium conversion. Kinetically, the 2-methyl group decreases the activation energy for dehydrogenation and slows reverse hydrogenation, thereby boosting the overall reaction rate. From an adsorptive standpoint, it allows the product, MPY, to bind more weakly to the Pd catalyst, mitigating product inhibition that otherwise deactivates catalytic sites at high conversion. By contrast, 3-MPI and 4-MPI have lower thermodynamic spontaneity, higher activation energies, and stronger product inhibition, ultimately reducing their dehydrogenation reactivity. These findings reveal how a seemingly minor structural changethe repositioning of a single methyl groupcan markedly influence the enthalpy-entropy balance, activation barriers, and catalyst deactivation under solvent-free, pressurized conditions. Our integrated approach illustrates that fine-tuning the substitution position offers a powerful molecular design lever for maximizing hydrogen release and minimizing inhibitory effects in liquid organic hydrogen carrier systems.