Theoretical Insight into Palladium(II)-Counterion-Ligand Cooperative Regiodivergent Syntheses of Indolo[3,2-c]coumarins and Benzofuro[3,2-c]quinolinones from Diphenylethyne Derivatives.
2020
With
two distinct active sites, 2-hydroxy-2′-amino-diphenylethyne
derivatives can offer benzofuro[3,2-c]quinolinones
via the O-attack/N-carbonylation cyclization or indolo[3,2-c]coumarins via the N-attack/O-carbonylation
cyclization. This work presents a density functional theory-based
computational study to understand the mechanism and origin of the
palladium(II)-catalyzed regiodivergent reactivity of diphenylethyne
derivatives. It is indicated that the reaction features a palladium(II)–counterion–ligand
cooperative catalysis. The O-attack/N-carbonylation cyclization mainly
benefits from the inductive effect of the rigid electron-withdrawing
bidentate nitrogen ligand and the stabilization of the 3c-4e bond
between the trifluoroacetate (TFA) anion and the hydroxyl group in
the substrate for the precursor and transition state, while the viability
of the N-attack/O-carbonylation cyclization stems intrinsically from
the stronger nucleophilicity of the N atom as well as the important
π–π interaction between the flexible electron-rich
bidentate phosphine ligand and the substrate. Moreover, these calculations
propose an unconventional reductive elimination mechanism for the
transformation from Pd(II) to Pd(0), where the intramolecular nucleophilic
attack of the N/O atom on the carbonyl C atom results in the formal
reductive elimination product. The calculated overall barriers of
14.8 kcal/mol for Pd(TFA)2 with the bidentate nitrogen
ligand and 23.9 kcal/mol for Pd(OTf)2 with the bidentate
phosphine ligand are qualitatively consistent with the mild experimental
conditions.
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