NMR AND DFT STUDIES ON POLYCYCLIC ARENIUM IONS
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Figure 9. Structures of protonated carbocations 3aH+ and 3bH+ derived
from 3
The DFT calculations predicted that 3aH+ with protonation at C(6)
is the most stable (Fig. 9 and Table S3) and that the second most
stable cation is 3bH+ with protonation at C(3), which is higher in en-
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ions were planar. The most deshielded peaks were calculated to
be δ 9.18 for H(12), δ 187.3 for C(5a), δ 167.5 for C(4a), and δ 160.5
for C(12) in 3aH+, and δ 8.56 for H(7), δ 173.9 for C(5a), δ 172.1 for
C(10a), and δ 168.0 C(2) in 3bH+ (Figure S2). Almost all the proton
signals became deshielded by the protonation. According to the
Δδ13C values, positive charge was delocalized into three carbons in
the benzenium unit and one carbon in another benzene ring for
3aH+ and five carbons in one dibenzo[b,d]furan unit and one carbon
for 3bH+. The charge was delocalized more extensively for 3bH+.
NICS(1)zz for all the rings in 3 were calculated to be negative, indicat-
ing significant aromatic nature (Fig. 4). Protonation leads to an in-
crease of NICS for the π-rings of 3aH+ and 3bH+. Aromaticity
increases in one furan ring in 3aH+ and significantly decreases in
the central benzene ring in 3bH+. The Δδ17O values were estimated
to be large (84.1 ppm) for O(5) in 3aH+ (Table 1). The values for O(11)
in 3aH+ and the oxygens in 3bH+ are small (4 ~ 27 ppm). The O(5) in
3aH+ might have significant positive charge density.
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CONCLUSIONS
In summary, compounds 1, 2, and 3 were prepared by the intra-
molecular cyclization/dediazoniation of the diazonium salts. The
protonated cations derived from 1, 2, and 3 were characterized
by the NMR measurements and the DFT calculations. The most sta-
ble cations were found to be 1aH+ with protonation at C(4), 2aH+
with protonation at C(6), and 3aH+ with protonation at C(6). In ad-
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Acknowledgements
This work was supported in part by a Grant-in-Aid for Scientific
Research (C) (21550043 and 24550155) and a Grant-in-Aid for
Scientific Research on Innovative Areas “Reaction Integration”
(21106013) from the Ministry of Education, Culture, Sports, Science
and Technology, Japan.
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