Iridium-Catalyzed Sustainable Access to Functionalized Julolidines through Hydrogen Autotransfer
Résumé
The straightforward and ecofriendly preparation of functionalized julolidines starting from tetrahydroquinoline, diols, and aldehydes, for which water is produced as the only side product was investigated. To achieve this task, several well-defined ruthenium and iridium complexes including three new complexes were prepared from the corresponding phosphine–sulfonates, phosphine–carboxylates, and phosphine–phosphonates. The first transformation involved in situ generation of enaminoiminium intermediates, which allowed the formation of the julolidines through formal N,C(sp2)-cyclization of tetrahydroquinoline and the propane-1,3-diols. The influence of the chelate acidity points out that [Cp*IrIII]-based catalysts (Cp*=C5Me5) featuring phosphine–carboxylate and phosphine–sulfonate ligands were suitable for the cyclization, whereas the acidic phosphinophosphonate-containing complex favored the formation of reduced N-alkylated tetrahydroquinoline. We found that substitution of the propane-1,3-diols was crucial for the generation of enaminoiminium ions, which accounts for the efficiency and selectivity of the reaction. Applying another hydrogen autotransfer process, the prepared julolidines were easily functionalized at the C2 position.
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