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

• Published in: Journal of physical organic chemistry Vol. 29; no. 2; pp. 107 - 111
• Main Authors: Okazaki, Takao, Nakagawa, Madoka, Futemma, Takeshi, Kitagawa, Toshikazu
• Format: Journal Article
• Language: English
• Published: Bognor Regis Blackwell Publishing Ltd 01.02.2016; Wiley Subscription Services, Inc
• Subjects: carbocation; DFT; NMR; polycyclic aromatic compound; superacid
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.3505

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 FSO3H‐SbF5 (4:1)/SO2ClF and 3 in FSO3H‐SbF5 (1:1)/SO2ClF 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 13C chemical shifts (Δδ13C), 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. The most stable persistent cations derived from the title compounds, 1–3, were found to be 1aH+ with protonation at C(4), 2aH+ with protonation at C(6), and 3aH+ with protonation at C(6) by experimental and theoretical methods.