Mechanistic Investigation of Zirconium-Catalyzed Hydroaminoalkylation of Alkynes: Substrate and Ligand Effects

The mechanism of zirconium-catalyzed hydroaminoalkylation of diphenylacetylene, and the origin of the contrasting reactivity with N-benzylaniline or N-(trimethylsilyl)benzylamine substrates, have been investigated. Isolated intermediates have revealed that the nature of the C–H activation and C–C bond-forming steps in alkyne hydroaminoalkylation are analogous to those of the alkene variant, regardless of the amine substrate. In the zirconium-catalyzed hydroaminoalkylation of alkynes, the open coordination sphere at the zirconium center, supported by the bis(ureate) ligand, enables the coordination of neutral protic donors. This is essential for promoting catalytic turnover in these reactions. Under catalytic conditions, dimethylamine acts as a proton source for releasing the allylic amine products while minimizing the formation of side products. Additionally, we identified the formation of a homoleptic tetra(ureate) zirconium complex as the main catalyst decomposition pathway in catalytic alkyne hydroaminoalkylation. The formation of similar homoleptic structures is further favored when employing smaller bis(urea) proligands, thus explaining the poor performance of some ligands in catalysis. However, further increasing the bis(urea) proligand size to minimize such catalyst decomposition favors isomerization side-products resulting in reduced yields of the desired product. This study provides ligand design principles that guide the development of coordinatively flexible catalysts to achieve catalytic turnover in systems that rely upon protonolysis steps, as in hydroaminoalkylation.