NMR and DFT studies on persistent carbocations derived from benzo[ kl ]xanthene, dibenzo[ d , d ′]benzo[1,2‐ b :4,3‐ b ′]difuran, and dibenzo[ d , d ′]benzo[1,2‐ b :4,5‐ b ′]difuran in superacidic media

Persistent carbocations generated by the protonation of hetero‐polycyclic aromatic compounds with oxygen atom(s) were studied by experimental NMR and density function theory calculations. Benzo[ kl ]xanthene ( 1 ), dibenzo[ d , d ′]benzo[1,2‐ b :4,3‐ b ′]difuran ( 2 ), and dibenzo[ d , d ′]benzo[1,2‐ b :4,5‐ b ′]difuran ( 3 ) were synthesized by the annulation of arenediazonium salts. Compound 1 in FSO 3 H‐SbF 5 (4:1)/SO 2 ClF and 3 in FSO 3 H‐SbF 5 (1:1)/SO 2 ClF ionized to 1aH + with protonation at C(4) and to 3aH + with protonation at C(6), and these cations were successfully observed by NMR at low temperatures. The density function theory calculations indicated that 1aH + and 3aH + were the most stable protonated carbocations and that 2 should ionize to 2aH + with protonation at C(6). According to the changes in 13 C chemical shifts (Δδ 13 C), the positive charge was delocalized into the naphthalene unit for 1aH + , into one benzo[ b , d ]furan unit for 2aH + , and into one benzo[ b , d ]furan unit for 3aH + . Copyright © 2015 John Wiley & Sons, Ltd.