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

Abstract

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 (), dibenzo[d,d′]benzo[1,2-b:4,3-b′]difuran (), and dibenzo[d,d′]benzo[1,2-b:4,5-b′]difuran () were synthesized by the annulation of arenediazonium salts. Compound in FSO3H-SbF5 (4:1)/SO2ClF and in FSO3H-SbF5 (1:1)/SO2ClF ionized to with protonation at C(4) and to with protonation at C(6), and these cations were successfully observed by NMR at low temperatures. The density function theory calculations indicated that and were the most stable protonated carbocations and that should ionize to with protonation at C(6). According to the changes in 13C chemical shifts (Δδ13C), the positive charge was delocalized into the naphthalene unit for , into one benzo[b,d]furan unit for , and into one benzo[b,d]furan unit for . Copyright © 2015 John Wiley & Sons, Ltd.