Palladium-Catalyzed Regioselective and Stereospecific Ring-Opening Cross-Coupling of Aziridines: Experimental and...

Aziridines, i.e., the smallest saturated N-heterocycles, serve as useful building blocks in synthetic organic chemistry. Because of the release of the large ring strain energy accommodated in the small ring, (ca. 27 kcal/mol), aziridines undergo ring-opening reactions with a variety of nucleophiles. Therefore, among the synthetic reactions utilizing aziridines, regioselective ring-opening substitutions of aziridines with nucleophiles, such as heteroatomic nucleophiles (e.g., amines, alcohols, and thiols) and carbonaceous nucleophiles (e.g., carbanions, organometallic reagents, and electron-rich arenes), constitute a useful synthetic methodology to synthesize biologically relevant β-functionalized alkylamines. However, the regioselection in such traditional ring-opening substitutions of aziridines is highly dependent on the substrate combination, and stereochemical control is challenging to achieve, especially in the case of Lewis acid-promoted variants. Therefore, the development of robust catalytic ring-opening functionalization methods that enable precise prediction of regioselectivity and stereochemistry is desirable. In this direction, our group focused on the highly regioselective and stereospecific nature of the stoichiometric oxidative addition elementary step of 2-substituted aziridines into Pd(0) complexes in an S_N2 fashion. In conjunction with the recent advancements in transition-metal-catalyzed cross-coupling reactions of alkyl pseudohalides containing a C(sp³)-Q (Q = O, N, S, etc.) bond, aziridines can be used as nonclassical alkyl pseudohalides in regioselective and stereospecific cross-couplings.In this Account, starting from the background of transition-metal-catalyzed ring-opening functionalization of aziridines, our contributions to the palladium-catalyzed regioselective and stereoinvertive cross-couplings of aziridines with organoboron reagents to form C(sp³)-C, C(sp³)-B, and C(sp³)-Si bonds have been compiled. The developed methods allow the syntheses of medicinally important amine compounds, e.g., enantioenriched β-phenethylamines, β-amino acids, and their boron and silyl surrogates, from readily available enantiopure aziridine substrates. Notably, the regioselectivity of the ring opening can be switched by appropriate selection of the catalyst (i.e., Pd/NHC vs Pd/PR₃ systems). Computational studies rationalized the detailed mechanisms of the full catalytic cycle and the regioselectivity and stereospecificity of the reactions. The computational results suggested that the interactions operating between the Pd catalyst and aziridine substrate play important roles in determining the regioselection of the aziridine ring-opening event (i.e., oxidative addition). Also, the computational results rationalized the role of water molecules in promoting the transmetalation step through the formation of a Pd-hydroxide active intermediate. This Account evidences the benefits of synergistic collaborations between experimental and computational methods in developing novel transition-metal-catalyzed cross-coupling reactions.