Unraveling the mechanism of Pd-catalyzed hydrocyanation of methylenecyclopropane or cyclopropene with Me2C(OH)CN: a DFT study

Abstract

The multifaceted reactivities of methylenecyclopropane and cyclopropene have attracted broad interest. Density functional theory (DFT) calculations have been employed to elucidate the mechanism of the Pd-catalyzed hydrocyanation of Me₂C(OH)CN with methylenecyclopropane or cyclopropene, and to identify two distinct pathways. The first is a revised ring-opening pathway available to both substrates, beginning with oxidative addition of Pd(0) to the C–C bond of cyclopropene or methylenecyclopropane, followed by protonation by Me₂C(OH)CN, then β-C elimination, and reductive elimination. Conversely, the Lewis acid Al(OⁱPr)₃-promoted ring-retentive pathway for cyclopropene proceeds through a different initiation step in which Pd(0) reacts with Me₂C(OH)CN via a concerted transition state that features simultaneous C–C bond oxidative addition and oxygen-assisted hydrogen transfer from Al(OⁱPr)₃. This process generates a zwitterionic intermediate that subsequently undergoes hydrogen transfer to cyclopropene and reductive elimination to afford the ring-retentive product. The reaction sequence of the substrate is regulated by Al(OⁱPr)₃ through two factors: its coordination to the nitrogen atoms of Me₂C(OH)CN, which facilitates electron transfer from the Pd center, and the electron-rich nature of its oxygen atom.