Theory of the Formation and Decomposition of N‐Heterocyclic Aminooxycarbenes through Metal‐Assisted [2+3]‐Dipolar Cycloaddition/Retro‐Cycloaddition

Abstract

The theoretical background of the formation of N-heterocyclic oxadiazoline carbenes through a metal-assisted [2+3]-dipolar cycloaddition (CA) reaction of nitrones R(1)CH=N(R(2))O to isocyanides C≡NR and the decomposition of these carbenes to imines R(1)CH=NR(2) and isocyanates O=C=NR is discussed. Furthermore, the reaction mechanisms and factors that govern these processes are analyzed in detail. In the absence of a metal, oxadiazoline carbenes should not be accessible due to the high activation energy of their formation and their low thermodynamic stability. The most efficient promotors that could assist the synthesis of these species should be "carbenophilic" metals that form a strong bond with the oxadiazoline heterocycle, but without significant involvement of π-back donation, namely, Au(I), Au(III), Pt(II), Pt(IV), Re(V), and Pd(II) metal centers. These metals, on the one hand, significantly facilitate the coupling of nitrones with isocyanides and, on the other hand, stabilize the derived carbene heterocycles toward decomposition. The energy of the LUMO(CNR) and the charge on the N atom of the C≡N group are principal factors that control the cycloaddition of nitrones to isocyanides. The alkyl-substituted nitrones and isocyanides are predicted to be more active in the CA reaction than the aryl-substituted species, and the N,N,C-alkyloxadiazolines are more stable toward decomposition relative to the aryl derivatives.