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 |
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Authors: | Takao Okazaki Madoka Nakagawa Takeshi Futemma Toshikazu Kitagawa |
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Institution: | Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie, Japan |
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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. Benzokl]xanthene ( 1 ), dibenzod,d′]benzo1,2‐b:4,3‐b′]difuran ( 2 ), and dibenzod,d′]benzo1,2‐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 benzob,d]furan unit for 2aH+ , and into one benzob,d]furan unit for 3aH+ . Copyright © 2015 John Wiley & Sons, Ltd. |
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Keywords: | carbocation DFT NMR polycyclic aromatic compound superacid |
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