Experimental NMR and DFT Studies of Persistent Carbocations Derived from Hetero-Polycyclic Aromatic Hydrocarbons Containing Oxygen Atom: Dibenzo[b,d]furan, Benzo[b]naphtho[1,2-d]furan, Benzo[b]naphtho[2,3-d]furan, Benzo[b]naphtho[2,1-d]furan, and Dinaphtho[2,1-b:1′,2′-d]furan

Abstract

Abstract Persistent protonation carbocations generated from hetero-PAHs containing oxygen atoms in their aromatic rings, dibenzo[b,d]furan (5), benzo[b]naphtho[1,2-d]furan (6), benzo[b]naphtho[2,3-d]furan (7), benzo[b]naphtho[2,1-d]furan (8), and dinaphtho[2,1-b:1′,2′-d]furan (9), were directly observed by NMR measurements in superacid. Compound 5 was protonated mainly at C(2) in FSO3H–SbF5 (1:1) or (4:1)/SO2ClF, and 6, 8, and 9 were protonated exclusively at C(5) in CF3SO3H or FSO3H/SO2ClF, whereas 7 was protonated at C(6) and C(11) to give two species in FSO3H/SO2ClF. Surprisingly, compound 5 resists protonation in FSO3H/SO2ClF to show NMR spectra corresponding to that of the intact material. Positive charge delocalization mapping for carbocations based on experimental Δδ13C values indicates limited delocalization in these systems. The chemical shifts and charge delocalization modes derived by DFT calculations agreed with the experimental results.