Solvent- and Electrolyte-Dependent Highly Selective Electrochemical Transfer Hydrogenation of Pyrazolo[1,5- <i>a</i> ]pyrimidines

Electrochemical transfer hydrogenation using non-H₂ hydrogen sources, such as H₂O, demonstrates excellent selectivity and supports an environmentally friendly and sustainable process. This method offers a promising alternative to traditional hydrogenation techniques that utilize alkali metal hydrides or molecular hydrogen gas. In this work, we present, for the first time, electrochemical protocols for the selective transfer hydrogenation of polynitrogen-containing aromatic heterocycles. These protocols can be fine-tuned by adjusting the solvent and supporting electrolyte. For instance, using a mixture of MeCN and H₂O, along with NH₄Cl as an electrolyte, facilitates the regioselective transfer hydrogenation of the C6-C7 double bond in pyrazolo[1,5-<i>a</i>]pyrimidines at 30 °C. Conversely, employing DMSO combined with H₂O and ^<i>n</i>Bu₄NBF₄ allows for the reduction of both the C6-C7 double bond and the N4-C5 imine bond. Importantly, the unsaturated C-C and C-N bonds within the pyrazole ring of pyrazolo[1,5-<i>a</i>]pyrimidines remain intact during these transfer hydrogenation reactions. The observed selectivities are influenced by two main factors: (a) the stronger reducing ability of DMSO compared to MeCN, and (b) the more robust interaction between ammonium cations (from NH₄Cl) and the nitrogen's electron lone pair in pyrazolo[1,5-<i>a</i>]pyrimidines, which differs from the interaction with tetrabutylammonium cations (from ^<i>n</i>Bu₄NBF₄).