Base-controlled product switch in the ruthenium-catalyzed protodecarbonylation of phthalimides: a mechanistic study

Abstract

The whole reaction mechanism of the ruthenium-catalyzed protodecarbonylation of N-substituted phthalimides into secondary amides was unravelled by a combined experimental and theoretical study. The chemoselectivity of the reaction, which is catalyzed by para-cymene coordinated Ru(II) species all over the catalytic cycle, is exclusively controlled by the unique roles of the bases. Meanwhile, in the presence of K₂CO₃ or KOH at high temperatures, the same product (benzamide) is mainly formed, whereas at low temperatures, KOH led to an unexpected side-product (phthalamic acid) and no reactivity was observed with K₂CO₃. The non-covalent interactions between the potassium cations and the different carbonyl groups in the molecules are key to providing a thermodynamically favourable pathway with energetically accessible transition states. The unexpected formation of carbon dioxide (CO₂) in the course of the reaction originates from the phthalimide substrate and the base K₂CO₃ in two different elementary steps, respectively.