Experimental NMR and DFT studies of persistent carbocations derived from hetero-polycyclic aromatic hydrocarbons containing oxygen atom: Dibenzo[b,d]furan, Benzo[b]naphtho[1,2-d]furan, Benzo[b]naphtho[2,3-d]furan, Benzo[b]naphtho[2,1-d]furan, and Dinaphtho[2,1-b: 1′,2′-d]furan

Article abstract of DOI:10.1246/bcsj.20140182

Persistent protonation carbocations generated from hetero-PAHs containing oxygen atoms in their aromatic rings, dibenzo[b,d]furan (5), benzo[b]naphtho[1,2-d]furan (6), benzo[b]naphtho[2,3-d]furan (7), benzo[b]naphtho[2,1-d]furan (8), and dinaphtho[2,1-b: 1′,2′-d]furan (9), were directly observed by NMR measurements in superacid. Compound 5 was protonated mainly at C(2) in FSO₃H-SbF₅ (1: 1) or (4: 1)/SO₂ClF, and 6, 8, and 9 were protonated exclusively at C(5) in CF₃SO₃H or FSO₃H/SO₂ClF, whereas 7 was protonated at C(6) and C(11) to give two species in FSO₃H/SO₂ClF. Surprisingly, compound 5 resists protonation in FSO₃H/SO₂ClF to show NMR spectra corresponding to that of the intact material. Positive charge delocalization mapping for carbocations based on experimental Δδ¹³C values indicates limited delocalization in these systems. The chemical shifts and charge delocalization modes derived by DFT calculations agreed with the experimental results.

Full text of DOI:10.1246/bcsj.20140182

Experimental NMR and DFT Studies of Persistent Carbocations Derived
from Hetero-Polycyclic Aromatic Hydrocarbons Containing Oxygen
Atom: Dibenzo[b,d]furan, Benzo[b]naphtho[1,2-d]furan,
Benzo[b]naphtho[2,3-d]furan, Benzo[b]naphtho[2,1-d]furan,
and Dinaphtho[2,1-b:1¤,2¤-d]furan
1
1
1
2
Takao Okazaki,* Madoka Nakagawa, Toshikazu Kitagawa,* and Kenneth K. Laali*
1Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 514-8507
2Department of Chemistry, University of North Florida, 1 UNF Drive, Jacksonville, Florida 32224, USA
E-mail: okazaki@chem.mie-u.ac.jp
Received: June 24, 2014; Accepted: July 25, 2014; Web Released: July 31, 2014
Persistent protonation carbocations generated from hetero-PAHs containing oxygen atoms in their aromatic rings,
dibenzo[b,d]furan (5), benzo[b]naphtho[1,2-d]furan (6), benzo[b]naphtho[2,3-d]furan (7), benzo[b]naphtho[2,1-d]furan
(
8), and dinaphtho[2,1-b:1¤,2¤-d]furan (9), were directly observed by NMR measurements in superacid. Compound 5 was
protonated mainly at C(2) in FSO3H­SbF5 (1:1) or (4:1)/SO2ClF, and 6, 8, and 9 were protonated exclusively at C(5) in
CF3SO3H or FSO3H/SO2ClF, whereas 7 was protonated at C(6) and C(11) to give two species in FSO3H/SO2ClF.
Surprisingly, compound 5 resists protonation in FSO3H/SO2ClF to show NMR spectra corresponding to that of the intact
13
material. Positive charge delocalization mapping for carbocations based on experimental ¦¤ C values indicates limited
delocalization in these systems. The chemical shifts and charge delocalization modes derived by DFT calculations agreed
with the experimental results.
Several classes of polycyclic aromatic compounds (PAHs)
and hetero-PAHs are recognized as interesting materials in their
synthesis, aromaticity, biological activity, and application to
organic semiconductor materials.1 Reaction of these PAHs
with electrophiles proceeds through carbocation intermediates.
Direct studies of the electronic structures of cationic inter-
mediates are relevant to the understanding of the reaction
mechanisms and further developments in synthetic method-
nated at the C(2) position (Figure 1). Their reactions with
common electrophiles resulted in the formation of sulfonium
salts by S-alkylation. Reactions of 2 with MeI and EtI/AgBF
in 1,2-dichloroethane gave 2Me and 2Et .
4
­4
+
+
Oxygen has higher electronegativity than carbon and sulfur.
Furan is widely accepted to be less aromatic than benzene and
1
4,15
thiophene, and shows some diene character.
However the
charge delocalization modes for carbocations derived from
oxygen-containing hetero-PAHs remain uncertain. To explore
the charge delocalization of protonated polycyclic aromatic
compounds, the present work focused on the benzofuran family
shown in Figure 2. Dibenzo[b,d]furan (5) reacts with electro-
philic reagents to give 2-substituted derivatives. Keumi et al.
reported on the selectivity of products in Friedel­Crafts
5
,6
ology, in particular for the construction of PAHs and hetero-
PAHs via carbocation chemistry.7
,8
Protonation carbocations derived from PAHs can be gen-
erated in superacid media under stable ion conditions, and have
been extensively studied by experimental NMR measurement
and theoretical method. Positive charge delocalization modes
have been analyzed on the basis of the difference in their NMR
chemical shifts with those of parent neutral compounds. Over
the past decades, a significant body of literature data on the
generation and structure­reactivity of carbocations derived
1
6
reactions and nitration of 5. Dinaphtho[2,1-b:1¤,2¤-d]furan
(9) rearranges to dinaphtho[1,2-b:1¤,2¤-d]furan and 1-hydroxy-
1
7
benzo[ j]fluoranthene on reaction with aluminum chloride.
The formylation of benzo[b]naphtho[1,2-d]furan (6) gives
9
­13
18
from alternant and nonalternant PAHs have been reported.
benzo[b]naphtho[1,2-d]furan-5-carboxaldehyde.
oxidation of 5 and 6 leads to the dihydrodiols.
Biological
1
9
Observation of protonation carbocations derived from hereto-
PAHs has been relatively limited. Carbocations derived from
sulfur-containing hetero-PAHs were generated in superacid
Herein we report persistent carbocations generated from
oxygen-containing hetero-PAHs 5­9 by protonation in super-
acids at low temperature (Figure 2) and their NMR character-
istics. The resulting carbocations were also studied by DFT
calculations. Their positive charge delocalization modes on
1
2
and analyzed by NMR and DFT (density functional theory).
Benzo[b]naphtho[2,1-d]thiophene (2) was protonated at the
C(5) position in FSO H/SO ClF, whereas benzo[a]fluorene (1)
3
2
1
3
13
was protonated at the C(9) position. Both phenanthro[4,3-b]-
thiophene (3) and phenanthro[3,4-b]thiophene (4) were proto-
the basis of changes in C NMR chemical shift (¦¤ C) are
discussed.
Bull. Chem. Soc. Jpn. 2014, 87, 1235–1244 | doi:10.1246/bcsj.20140182
© 2014 The Chemical Society of Japan | 1235

2
5
O
1
1
O
4
11
3
5
10
3
FSO3H
H
9
4
SO2ClF
2
4
H
5
1
6
1
8
5
5
1
7
6
3
2
5
2
2
4
1
O
3
S
3
S
O
3
FSO3H
4
4
SO2ClF
2
1
H
H
7
5
2
8
6
1
H
H
O
2
5
3
S
S
3
FSO H
3
4
SO2ClF
1
4
3
9
2
H
Figure 2. Structures of dibenzo[b,d]furan (5), benzo[b]-
naphtho[1,2-d]furan (6), benzo[b]naphtho[2,3-d]furan (7),
benzo[b]naphtho[2,1-d]furan (8), and dinaphtho[2,1-b:
1¤,2¤-d]furan (9).
2
H
1
S3
FSO H
3
S
SO2ClF
4
[
Pd(PPh ) Cl ],
4
O
3 2 2
LiCl, DBU
DMF
−
6
47%
+
7
8%
BF4
R
S
OTf
RI/AgBF4
2
ClCH2CH2Cl
HO OH
+
2
2
Me (R = Me)
+
Et (R = Et)
HY Zeolite
1,2-dichlorobenzene
9
74%
Figure 1. Protonation of benzo[a]fluorene (1), benzo[b]-
naphtho[2,1-d]thiophene (2), phenanthro[4,3-b]thiophene
(
3), and phenanthro[3,4-b]thiophene (4) in FSO3H/
+
+
SO2ClF, and synthesis of sulfonium salts 2Me and 2Et .
Scheme 1. Synthesis of benzo[b]naphtho[1,2-d]furan (6),
benzo[b]naphtho[2,3-d]furan (7), and dinaphtho[2,1-b:
1¤,2¤-d]furan (9).
Results and Discussion
Benzofurans 6,20 7,20,21 and 922 were prepared according to
modified procedures described in the literature. Harvey and co-
workers reported that the cyclization of 2-(2-naphthoxy)phenyl
trifluoromethanesulfonate20 with a palladium catalyst afforded
exclusively 7. In our hands the reaction products obtained by a
similar method were found to be a mixture of 6 and 7 at a ratio
benzo[b,d]furan with dibromoethane afforded 4-bromodibenzo-
[b,d]furan in a 64% yield by a similar method to that de-
scribed in the literature. A Grignard reagent derived from 4-
bromobenzo[b,d]furan with magnesium reacted with succinic
anhydride to give 10 in 32% (Scheme 2).
2
3
Butanoic acid 10 was reduced to 4-(4-dibenzo[b,d]furanyl)-
1
24
of 9:1 by H NMR analysis (Scheme 1). Both compounds were
butanoic acid (11) with hydrazine and KOH by a Wolff­
isolated by SiO column chromatography and subsequent gel
Kishner reduction in a 57% yield (Scheme 2). Cyclization
in P O ­H PO afforded 4-oxo-1,2,3,4-tetrahydrobenzo[b]-
naphtho[2,1-d]furan (12) in a 77% yield. Reduction with
2
permeation chromatography in 47% and 8% yields, respec-
tively. Compound 9 was prepared by dehydration of 1,1¤-bi-2-
naphthol with HY zeolite in 1,2-dichlorobenzene in 74% yield
2
5
3
4
2
4
LiAlH followed by dehydration with p-toluenesulfonic acid
4
2
2
(Scheme 1).
in CH Cl gave 1,2-dihydrobenzo[b]naphtho[2,1-d]furan (14),
2
2
Benzofuran 8 could be prepared by alternative synthetic
whose dehydroaromatization with 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone (DDQ) in benzene gave 8 in 88% yield
(Scheme 2).
routes from commercially available 5 as shown in Scheme 2.
Compound 5 was selectively lithiated at the C-4 position with
2
5
2
3
n-butyllithium. The lithiated compound reacted with succinic
anhydride in solution to give 4-(4-dibenzo[b,d]furanyl)-4-
oxobutanoic acid (10) in a 17% yield. This compound could be
also synthesized by Grignard reaction. Reaction of the lithiated
Protonation of 5­9 in Superacidic Solutions. The benzo-
furan derivatives 5­9 were allowed to react in superacidic
media, and their solutions were analyzed by NMR at low
1
13
temperatures using H, C, DIFNOE, H/H COSY, HMQC,
1236 | Bull. Chem. Soc. Jpn. 2014, 87, 1235–1244 | doi:10.1246/bcsj.20140182
© 2014 The Chemical Society of Japan

O
O
O
O
OH
1) BuLi/hexane/THF
O
H NNH
OH
2
2
H PO
3
4
5
KOH
P O
5
2
) succinic anhydride
2
1
0
(HOCH CH ) O
2 2 2
11
3
) HCl aq.
57%
O
1
7%
O
O
LiAlH4
THF
p-TsOH
DDQ
Benzene
8
O
OH CH Cl
2
2
8
8%
1
2 77%
1
3 89%
14 91%
Br
O
1
) BuLi/hexane/THF
1
) Mg/Et O
2
5
10
2
) BrCH CH Br
2) succinic anhydride
) HCl aq.
2
2
3
32%
64%
Scheme 2. Synthesis of benzo[b]naphtho[2,1-d]furan (8).
FSO H-SbF
O
O
O
3
5
H
H
H
H
(
1:1)
CF SO H
3 3
5
+
6
SO ClF
SO ClF
2
2
H
–70 °C
–70 °C
aH+
5bH+
5
6H+
H
7
.77 (0.20)
7.86 (0.29)
H
(
1
26.4)
82.6 120.7 (9.1)
6
.86 (–0.84)
(10.4)
1.3) H
1
H
(2.3)
7.14 (–0.63)
H
O
(–2.0)
152.5
(
1.7) 131.3 158.5
(
O
(
0.30)7.75
8.73 (1.28)
1
66.4
144.1 (31.3)
13.3
(
0.19)
H
H
(
9.8)
(45.9)
173.0
42.9
7.69
H
H
37.2
(
5.7) 132.8
H
2.67
1
34.0
(
–4.2)
1
43.4
1
18.4
+
(
5.9) 128.6
H
0.34) 7.67
(17.4)
126.8
120.9
(
–5.8)
H
155.1
(37.7)
125.7
145.3 (14.7)
1
23.9
H
4
.45
1
67.0
9.18 H
(46.4)
(
H
3.3)
(3.5) H 127.0
7.12 (4.9) H
7.11 (–0.92)
(
8
.05
0.10)
H
(
1.23)
(
–3.5)
(
128.9 (–0.4)
(
–0.36)
7.26
aH+
H
7
.48
5
(–1.14)
1
29.0
5.4)
136.0 (11.5)
129.1 H
(
–1.14)
(
Figure 3. Protonation of dibenzo[b,d]furan (5) in FSO3H­
SbF5/SO2ClF, and experimental NMR data and charge
delocalization modes for 5aH in FSO3H­SbF5 (4:1)/
SO2ClF at ¹70 °C. Relative NMR chemical shifts, ¦¤ H
and ¦¤ C, to those of 5 in parentheses. Dark circles are
6
H+
H
7.62 (0.07)
(1.8)
+
7.41 (–0.31)
1
Figure 4. Protonation of benzo[b]naphtho[1,2-d]furan (6),
and experimental NMR data and charge delocalization
modes for 6H in CF3SO3H/SO2ClF at ¹70 °C. Relative
NMR chemical shifts, ¦¤ H and ¦¤ C, to those of 6
in parentheses. Dark circles are roughly proportional to
¦¤ C: Threshold: 10 ppm.
1
3
1
3
roughly proportional to ¦¤ C: Threshold: 10 ppm.
+
1
13
and HMBC methods.26 Quenching of the cationic solutions
gave intact materials. The NMR results are summarized in
Figures 3­7. The charge delocalization modes are represented
as dark circles on the basis of the changes in ¤ C upon
protonation. The NMR chemical shifts give insight into
electronic molecular structures.
1
3
13
structures by B3LYP/6-311+G(d,p)// B3LYP/6-31G(d), and
the charge delocalization mode based of the changes in ¤ C.
The reliability of the NMR chemical shifts by the DFT­GIAO
1
3
2
7,30
DFT Calculations. The structures for benzofurans 5­9 and
method have been reported in the literature.
Compared with
their protonation carbocations were optimized by the DFT
the experimental data, our DFT calculations slightly over-
estimate the NMR chemical shifts and delocalization of the
positive charge, but the overall trends are consistent with the
experimental results.
method at the B3LYP/6-31G(d) level2
7,28
and their energies
are summarized in Tables S1­S5 (Supporting Information).
The π-systems of 5­8 and their protonation cations are planar in
their optimized structures, whereas the structures for 9 and its
protonation cations are twisted. Figure S1 (Supporting Infor-
mation) shows the GIAO-NMR29 values for the most stable
Isotropic magnetic tensors (· ) for oxygen in 5­9 and their
iso
1
7
carbocations and their change (¦¤ O) relative to those of the
corresponding neutral compounds were computed, and the
Bull. Chem. Soc. Jpn. 2014, 87, 1235–1244 | doi:10.1246/bcsj.20140182
© 2014 The Chemical Society of Japan | 1237

H
H
FSO
4:1)
SO ClF
–70 °C
3
H-SbF
5
O
O
(
FSO H
3
8
7
+
2
SO ClF
2
H
7aH+
H+
–
70 °C
8
H
O
7
.57 (–0.07)
(
7.3)
H
(
–0.42) 8.03
(28.8)
H128.2 130.5 (4.0)
(
–0.27) 7.44
H
7.79 (0.23)
H
139.4 (13.3)
7
bH+
H
H
(2.4)
58.4
(–1.4)
120.0
O
1
180.8
(
1.6) 113.4
(
0.14) 7.61
5
.19
129.6 (1.2)
H
H
(
0.37) 7.94
(–6.0)
119.1
(
40.1)
O 194.9
35.7
H
(12.0)
(
1.6)
8
9.1)
42.1
.09 (0.12)
151.0
(17.9)
H
H 39.6
H
H
(6.4) 132.6
131.2
+
7.52 (–0.46)
(
2.4) 114.0
0.35) 7.86
159.2
(
H
(5.8) 128.7
H
0.15) 7.54 (2.7)
a
(
131.1 (3.3)
8.22 (0.70)
1
165.4
H
123.0
(
–3.6)
(3.4)
(46.9)H
H
7
(
(
4.0) 132.3
6.4) 129.1
120.3
H(15.1)
40.9
1
28.8
(1.2)
131.4
8.47
(0.48)
3.86
.71
(–0.29)
+
1
(
1
68.3
49.2)
a
8H+
H 123.4
1
31.2 (6.9)
138.7 H
7.97 (0.50)
(
H
H
(
0.43) 7.80(2.1)
8
.25
9
.78
1.37)
aH+
H
Figure 6. Protonation of benzo[b]naphtho[2,1-d]furan (8),
and experimental NMR data and charge delocalization
modes for 8H in FSO3H­SbF5 (4:1)/SO2ClF at ¹70 °C.
Relative NMR chemical shifts, ¦¤ H and ¦¤ C, to those
of 8 in parentheses. Dark circles are roughly proportional
(
10.4)
(
0.18)
(
8
.39 (0.36)
+
7
1
13
(
0.31) 7.88
9.16 (1.24)
H(45.5)
(
1
9.3)
66.9
(
1.3)
13
H
to ¦¤ C: Threshold: 10 ppm.
O
8.33 (0.39)
(
3.8) 115.4
0.69) 8.20
156.1
1
52.4
b
H
(
+
138.4 (10.6)
.92 (0.40)
H
44.4
7
O
1
27.7
H
H
1
H
(15.6)
(
16.1)
146.3
FSO H
(
3.8)
35.7
H
b
3
(
16.1)
b
165.3
(39.9)
9
SO ClF
H 127.3
139.9
2
H
–
70 °C
(
6.0)H
(
0.39) 7.76
131.0 H
2.7)
8
(
.30
0.23)
5.28
8
.15H
(
8.15
H+
9
(0.12)
(0.68)
7
bH+ b: 132.8, 128.7, nd
8
H
.37 (0.53)
(
20.1)
(
–0.17) 7.67H
(1.7)
O
Figure 5. Protonation of benzo[b]naphtho[2,3-d]furan (7),
and experimental NMR data and charge delocalization
174.5
1
56.1
(0.1) 112.9
144.0 (31.2)
(
0.68) 8.64
+
+
modes for 7aH and 7bH in FSO3H/SO2ClF at ¹70 °C.
H
H
H
(
1
2.8)
22.3
(37.0)
+156.5
4.68
38.2
1
13
Relative NMR chemical shifts, ¦¤ H and ¦¤ C, to those
of 7 in parentheses. Dark circles are roughly proportional
to ¦¤ C: Threshold: 10 ppm. a and b denote interchange-
able assignments and nd means not detected.
(24.2) 152.6
(
0.5) 131.8
145.1 (13.8)
.01 (–0.06)
1
3
128.3 127.1
(
0.09) 8.16
8
(
–0.4)
(–1.6)
H
(
0.7)
126.4
(
4.3)
H
(
3.3) 132.9
1
30.0
130.1 (0.5)
H
H
8
.96 9.07
data are summarized in Table 1. The GIAO calculations of
(5.6) 130.1
135.9 (11.4)
(
–0.20) (–0.09)
1
7
H
(0.28)
7
.87
132.9
(6.6)
H
¤
O have been reported and successfully applied to many
129.1
H
7
1
4,31
H
8.00 (0.41)
(
2.8)
systems.
.98 (0.23)
7
.85 (0.10)
Protonation of Dibenzo[b,d]furan (5). Reaction of 5 in
FSO H/SO ClF at ¹70 °C gave a clear pale yellow solution,
9H+
3
2
whose NMR spectra were consistent with the parent compound
itself. It is indeed surprising that compound 5 resists pro-
tonation in FSO H. The use of more acidic FSO H­SbF (4:1)/
Figure 7. Protonation of dinaphtho[2,1-b:1¤,2¤-d]furan (9)
in superacidic solutions, and experimental NMR data and
3
3
5
+
charge delocalization modes for 9H in FSO3H/SO2ClF at
SO ClF produced a yellow solution, whose NMR spectra show
1
13
2
¹
70 °C. Relative NMR chemical shifts, ¦¤ H and ¦¤ C,
signals of neutral 5 and two protonation carbocations at
to those of 9 in parentheses. Dark circles are roughly
4
.8:1.0:0.2 ratio. The methylene protons of the two carboca-
13
proportional to ¦¤ C: Threshold: 10 ppm.
tions were observed at ¤ 4.45 and 4.31. In the more acidic
FSO H­SbF (1:1)/SO ClF the ratio became 2.7:4.0:1.0 at
3
5
2
¹
70 °C (¤ 4.54 and 4.14). The increased acidity increased the
with protonation at the C(2) position (Figure 3). The signal for
H(1) in 5aH was observed as a relatively broad singlet in the
most deshielded region (¤ 9.18) and was correlated with the
+
amount of the carbocations, but 35% of 5 still remained. The
major cation was definitely determined to be a cation (5aH )
+
1238 | Bull. Chem. Soc. Jpn. 2014, 87, 1235–1244 | doi:10.1246/bcsj.20140182
© 2014 The Chemical Society of Japan

Table 1. Isotropic Magnetic Tensors for Oxygen and Their
Change between Cations and Parent Compounds (¦¤ O)
by GIAO­B3LYP/6-311+G(d,p)// B3LYP/6-31G(d)
Protonation of Benzo[b]naphtho[1,2-d]furan (6).
The
17
reaction of 6 in FSO H/SO ClF afforded a dark green sus-
3
2
pension at ¹70 °C, whose NMR observation shows relatively
+
small broad signals for a protonation cation 6H probably due
1
7
Isotropic magnetic tensor
for oxygen/ppm
¦¤ O
Compound
to the coproduction of radical cations. Changing the superacid
to CF SO H with lesser oxidative power gave a clear dark
a)
/ppm
3
3
5
99.0
20.6
90.0
91.3
91.7
107.5
6.6
79.6
95.6
35.5
84.3
84.5
(0)
78.4
9.0
(0)
¹0.4
(0)
100.9
27.9
(0)
60.1
(0)
green solution, whose NMR signals were less broad, thus
allowing the determination of the structure of the generated
+
5
5
6
6
7
7
7
8
8
9
9
aH
bH
+
+
cation as 6H with protonation at C(5) by 2D NMR techniques
+
(Figure 4). The protonation position is pseudo-meta to its
oxygen atom, indicating the influence of oxygen might be
H
+
+
+
limited. Signals for H(9) and H(3) in 6H were observed at
aH
bH
the most deshielded region, 7.69 and 7.62 ppm. The most
deshielded carbons were observed at 166.4 ppm for C(7a), at
155.1 ppm for C(11b), and at 152.5 ppm for C(6a). Compared
+
H
+
with the NMR data of 6, most of the protons of 6H were
+
H
¹0.2
shielded, and especially signals for H(1) and H(11) at a fjord
region were significantly shielded by 1.14 ppm. The positive
a) Isotropic magnetic tensor for oxygen relative to that of
corresponding parent compound.
1
3
charge based on ¦¤ C is delocalized into limited carbons, five
carbons in the benzofuran ring and another carbon.
+
The experimentally observed cation 6H was calculated
methylene protons for H(2) at ¤ 4.54 by COSY. Irradiation
of the signal of H(1) in a DIFNOE spectrum resulted in the
to be the most stable one among the possible protonation
carbocations for 6 by the DFT (Table S2 in the Supporting
Information). The second most stable carbocation is a cation
1
enhancement of a doublet signal assigned as H(9). Other H
1
3
+
¹1
and C NMR signals for 5aH could be assigned by 2D NMR
techniques.
with protonation at the C(9) position and is 3 kcal mol higher
1
in energy. Computed H NMR data (Figure S1) were over-
Compared with the NMR data of 5, all the CH protons in
aH were deshielded by 0.2­1.3 ppm and the signals for H(1)
estimated in comparison with the experimental values, and all
+
+
5
the CH protons are predicted to be deshielded in 6H . The
1
7
+
and H(3) are the most deshielded. Signals for C(1), C(3), and
C(4a) were significantly deshielded, and other CH carbons
were deshielded by 1­9 ppm except C(9a), which was shielded
by 6 ppm. Increase in the temperature to ¹50 °C did not change
the ratio of the two cations.
¦¤ O for 6H has a negligible value (Table 1).
Protonation of Benzo[b]naphtho[2,3-d]furan (7).
The
ionization of 7 in FSO3H/SO2ClF gave a dark orange solution,
whose NMR spectra at ¹70 °C showed the formation of a
+
+
mixture of 7aH and 7bH with protonation at pseudo-ortho
and pseudo-meta positions to the oxygen in a ratio of 1.0:0.21
(Figure 5). Singlet signals were observed at ¤ 9.78 for H(11)
+
The protonation position for the mainly formed cation 5aH
is pseudo-para to the oxygen atom, probably due to its
electron-donating effect. The charge delocalization map on the
+
and 5.19 for H(6) for 7aH and at ¤ 9.16 for H(6) and 5.28
13
+
+
+
+
basis of change in ¤ C from the neutral to 5aH indicates that
positive charge is delocalized into three carbons in a protonated
benzene ring.
for H(11) for 7bH . The H(11) protons for 7aH and 7bH are
+
more deshielded than those of the H(6) protons for 7bH and
7aH , respectively.
+
The protonation site of the minor cation could not be
determined experimentally because of relatively weak signals.
The DFT calculations (see below) suggested that the carboca-
Increase in temperature to ¹50 °C did not change the ratio
1
3
of the two cations. Based on the ¦¤ C, positive charge is
+
delocalized into four carbons in the naphthalene ring for 7aH
+
+
tions protonated at C(2) and C(3) (5aH and 5bH shown in
and five carbons in the naphthalene ring and another carbon for
+
+
+
+
Figure 3) are the most stable. Since 5aH was observed as the
7bH . The charge is more delocalized in 7bH than in 7aH ,
+
+
major cation, the minor cation is likely 5bH .
presumably because the oxygen in 7aH stabilizes the cations
more effectively than that in 7bH .
Cations 7aH and 7bH were calculated to be the most
stable cations by the DFT, and 7bH is somewhat more stable
(1.3 kcal mol ) than 7aH (Table S3). Computed positive
charges for 7aH and 7bH are dominantly delocalized into
their naphthalene rings (Figure S1). The ¦¤ O for 7aH is
considerably large, and is larger than that of the isomeric 7bH
+
+
The DFT calculations show that carbocations 5aH and
+
+
+
5
bH , generated by the protonation at C(2) and C(3), are the
¹1
+
most stable ones, whose energies are similar within 1 kcal mol
¹1
+
(Table S1 in the Supporting Information). These cations were
+
+
the experimentally observed ones. An onium ion with pro-
tonation at the oxygen atom is much less stable. The positive
1
7
+
+
+
+
charges in both 5aH and 5bH were delocalized into the
protonated benzene ring and two carbons in the other benzene
(Table 1). These data suggest that positive charge is delocalized
+
+
rings. Computed charge in 5aH shown in Figure S1 is more
into the oxygen atom in 7aH .
+
largely delocalized in its protonated benzene ring, and 5bH
Protonation of Benzo[b]naphtho[2,1-d]furan (8). Com-
pound 8 ionized neither in FSO H/SO ClF nor in neat FSO H.
1
7
has apparently alternant delocalization mode. The ¦¤ O for
3
2
3
+
+
5
aH was considerably larger than that of the isomeric 5bH
The reaction in FSO H­SbF (4:1)/SO ClF gave a dark orange
3 5 2
(
Table 1), suggesting that positive charge is delocalized into
solution, whose NMR spectra at ¹70 °C indicated the forma-
tion of 8H with protonation at the pseudo-para position to the
+
+
the oxygen atom in 5aH .
Bull. Chem. Soc. Jpn. 2014, 87, 1235–1244 | doi:10.1246/bcsj.20140182
© 2014 The Chemical Society of Japan | 1239

–
8.3
–12.9
–
16.0
O
–9.4
(18.8)
–
27.7
–
10.1
–20.6
(7.6)
(
7.0)
(2.4)
–28.2
–
15.3
O
–25.2
(0.5)
–28.2
(18.1)
–28.2
O
O
H
H
H
5aH+
5bH+
–28.4
5
6
H
–
10.6
–
4.5
–0.7
(28.6)
H
–24.9
–
19.7 (5.4)
–11.6
–
9.8
(
20.7)
(
8.5)
–27.2
–29.3
–27.6
O
(1.8)
O
H
H
–
28.1
H
(
–0.4)
O
(
3.2)
–
25.7
7
H+
(2.7)
6
7
aH+
–
15.2
–
0.5
–5.4
(10.1)
O
(
–3.6)
–26.6
(1.4)
(
28.8)
-25.6
(2.0)
–
15.5
O
–
20.7
–24.6
–
28.0
O
(
6.5)
(3.5)
-
27.6
H
bH+
H
–
25.5
–4.8
H
7
H
8
H+
(20.7)
8
–
17.2
O
–9.5 / –9.3
(7.7) / (7.9)
–
25.3 / –23.7
–
23.6 / -25.1
O
H
H
–11.1 / –12.1
(12.5) / (13.0)
–2.7 / –1.8
(22.6) / (21.9)
–25.0 / –27.6
(1.9) / (1.7)
–26.4 / –24.4
(3.1) / (2.4)
–
26.9 / –29.3
–29.5 / –26.8
above / below
9H+
9
Figure 8. NICS(1)zz values for 5­9, and their protonation carbocations by GIAO­B3LYP/6-311+G(d,p)// B3LYP/6-31G(d)
Relative NICS values to those of the corresponding parent hydrocarbon in parentheses.).
(
+
oxygen (Figure 6). H­H COSY showed a correlation cross
peak between H(5) at ¤ 3.86 and H(6) at ¤ 8.47. Compared with
The most deshielded protons for 9H were observed at 9.07
and 8.96 ppm. Compared with the NMR data for 9, signals for
H(1), H(4), H(8), and H(13) in 9H were shielded, and other
+
8
, the peak for H(6) was remarkably deshielded by 0.48 ppm,
whereas H(1), H(7), and H(10) were shielded by 0.42, 0.29,
and 0.27 ppm. The most deshielded carbons were observed at
CH protons were deshielded. Peaks for C(6), C(7a), C(9), and
C(13c) were deshielded by 31.2, 20.1, 24.2, and 37.0 ppm.
1
3
1
¦
80.8 ppm for C(11a) and at 165.4 ppm for C(6). According to
¤ C, the positive charge delocalization is limited to the
According to the changes in ¤ C, the positive charge delo-
calization is limited to a biphenyl structure. The delocalization
13
+
carbons in the protonated benzene ring and one carbon in the
adjacent benzene ring.
mode is similar to that of 6H .
+
The observed cation 9H was computed to be the most stable
+
The DFT calculations show cation 8H is the most stable
carbocation by the DFT, and the second possible protonation
¹
1
among the possible protonation carbocations derived from
carbocation is less stable by 9 kcal mol (Table S5). Computed
+
8
(Table S4). The positive charge is estimated to be delo-
positive charge for 9H (Figure S1) appears to be extensively
1
7
calized into the protonated benzene ring and two other carbons
delocalized into carbons on all the benzene rings. The ¦¤ O for
17
+
+
(
Figure S1). The ¦¤ O for 8H is considerably large, sug-
9H has a little value (Table 1).
gesting that positive charge delocalization of the oxygen in
Aromaticity of Protonation Carbocations Calculated by
GIAO­NICS Method. Changes in aromaticity by protonation
+
8
H
(Table 1).
Protonation of Dinaphtho[2,1-b:1¤,2¤-d]furan (9). The
protonation of 9 in FSO H/SO ClF gave a clear dark red
of 5­9 were investigated by NICS calculations. NICS(1) is
zz
3
2
an indicator of aromaticity, and its values for the optimized
structures of 5­9 and their protonation carbocations are sum-
marized in Figure 8. The level of the calculations is considered
to be adequate in order to discuss the overall aspects of
3
2
+
solution, whose NMR shows the formation of a cation (9H )
protonated at C(5) (Figure 7). The compound ionized more
easily than dibenzofuran 5, as shown by successful protonation
using a less acidic superacid. The protonation position is
pseudo-meta to the oxygen atom, which means that 9 could
have 1-arylnaphthalene character.
3
3
aromaticity according to the results reported in the literature.
The values for neutral 5­9 were calculated to be between ¹12
and ¹17 for furan rings and between ¹24 and ¹30 for benzene
1240 | Bull. Chem. Soc. Jpn. 2014, 87, 1235–1244 | doi:10.1246/bcsj.20140182
© 2014 The Chemical Society of Japan

rings. These large negative values suggest that all the rings
have substantial aromaticity. The NICS(1)zz values for all the
rings in protonation carbocations are negative and all the π-
rings remain aromatic. Changes in NICS(1)zz between the
hexane­CH Cl (1:1) afforded a mixture of 6 and 7 (9:1) as a
2 2
partially solidified colorless oil (92 mg) and 2-(2-naphthoxy)-
phenol as colorless crystals (30 mg). Further purification by Gel
permeation chromatography (GPC) of the mixture of 6 and 7
neutrals and protonation cations, ¦NICS(1) , are positive
using CHCl as eluent gave 7 as colorless crystals (9 mg, yield
zz
3
21
+
+
except limited rings in 7aH and 7bH , whose change is
relatively small. The increase in NICS(1)zz may be attributed to
a decrease in the ring currents caused by positive charge
delocalization. Compared with the charge delocalization maps
8%) and 6 as a colorless oil (54 mg, yield 47%).
1
6: H NMR (400 MHz, CDCl3): ¤ 8.62 (d, J = 8.0 Hz, 1H),
8.40 (dd, J = 6.8, 2.4 Hz, 1H), 8.03 (d, J = 8.0 Hz, 1H), 7.93
(d, J = 9.2 Hz, 1H), 7.77 (d, J = 9.2 Hz, 1H), 7.75­7.69 (m,
1
3
13
on the basis of ¦¤ C, generally the NICS(1) values became
2H), 7.55 (t, J = 7.6 Hz, 1H), 7.53­7.45 (m, 2H); C NMR
zz
less negative for π-rings with delocalized positive charge.
(100 MHz, CDCl ): ¤ 156.0 (C), 154.5 (C), 130.6 (C), 129.3
3
(
(
CH), 129.2 (C), 128.7 (CH), 127.3 (CH), 126.0 (CH), 125.1
C), 124.5 (CH), 123.6 (CH), 123.3 (CH), 122.1 (CH), 117.4
Summary
In summary, compound 5 is protonated mainly at C(2) in
FSO3H­SbF5 (4:1) and (1:1)/SO2ClF, and 6, 8, and 9 are
protonated exclusively at C(5) in CF SO H and FSO H/
(C), 112.8 (CH), 112.0 (CH).
7: H NMR (300 MHz, CDCl3): ¤ 8.41 (s, 1H), 8.07 (d, J =
1
7.5 Hz, 1H), 8.03 (d, J = 8.1 Hz, 1H), 7.97 (d, J = 7.5 Hz, 1H),
7.92 (s, 1H), 7.59­7.45 (m, 4H), 7.37 (t, J = 6.9 Hz, 1H).
4-(4-Dibenzo[b,d]furanyl)-4-oxobutanoic Acid (10) from
Dibenzo[b,d]furan (5). n-Butyllithium (2.55 M, 0.750 mL,
1.91 mmol) in hexane was added dropwise to a stirred solution
of 5 (303 mg, 1.80 mmol) in dry THF (1.5 mL) cooled with an
ice­water bath under argon atmosphere. The resulting mixture
was stirred for 1 h and added dropwise to a solution of succinic
anhydride (480 mg, 4.79 mmol) in dry THF (10 mL) cooled
with an ice­water bath under argon atmosphere. The mixture
was stirred at 0 °C for 4 h, and then at room temperature for
another 18 h. The reaction mixture was quenched with water
and concentrated by evaporation. The residue was diluted with
ether and extracted with 10% sodium carbonate. The aqueous
layer was washed with ether and acidified with 10% HCl.
Precipitated crystals were collected by filtration to give pure 10
as colorless crystals (80 mg, 17% yield): mp 164.0­168.5 °C;
IR (KBr): 3067, 2921, 2766, 2677, 2581, 1707, 1679, 1594,
3
3
3
SO ClF, whereas 7 is protonated at C(6) and C(11) to form
2
two species. The positive charge delocalization in the hetero-
PAH carbocations is generally rather limited and the cations
protonated at the pseudo-ortho and pseudo-para positions
relative to oxygen atoms have more limited delocalization path
than those protonated at the pseudo-meta positions. All the
π-rings for 5­9 and their protonation cations are aromatic.
¦
NICS(1) is more positive at the π-rings with the delocalized
zz
17
positive charge. On the basis of ¦¤ O, the positive charges are
delocalized into oxygen atoms in 5aH , 7aH , and 8H .
+
+
+
Experimental
NMR spectra for the neutral compounds were recorded on
a 300 or 400 MHz spectrometer at room temperature. NMR
spectra for cations were measured with a 400 MHz instrument
1
13
at low temperatures ( H: 400 MHz, C: 100 MHz). IR data
were collected by using an FT-IR instrument. Electron ioni-
zation mass spectra (EI-MS) were measured with a GC-MS
instrument.
¹
1
1
1415, 1260, 1180, 750 cm ; H NMR (300 MHz, CDCl3):
¤ 8.16 (dd, J = 7.5, 1.3 Hz, 1H), 8.10 (dd, J = 7.8, 1.4 Hz, 1H),
7.99 (d, J = 7.7 Hz, 1H), 7.68 (d, J = 8.5 Hz, 1H), 7.53 (td,
J = 7.8, 1.2 Hz, 1H), 7.47­7.38 (m, 2H), 3.73 (t, J = 6.4 Hz,
SO ClF was prepared according to a modified procedure of
2
3
4
Prakash et al. 2-(2-Naphthoxy)phenyl trifluoromethanesulfo-
nate was prepared by a procedure similar to that described in
1
3
2H), 2.92 (t, J = 6.5 Hz, 2H); C NMR (75 MHz, CDCl3):
¤ 196.1 (CO), 177.3 (COOH), 156.1 (C), 154.7 (C), 127.9
(CH), 127.7 (CH), 125.9 (C), 125.7 (CH), 123.5 (CH), 123.2
(C), 122.9 (CH), 121.7 (C), 120.7 (CH), 112.0 (CH), 37.8
(CH ), 27.9 (CH ); MS (EI, 70 eV): m/z (rel intensity) 268
2
0
the literature. Dry reagents were prepared by standard meth-
ods. Dibenzo[b,d]furan (5) and other reagents were commer-
cially available and were used as received. NMR spectra of the
cations in CF SO H, FSO H, and FSO H­SbF (4:1)/SO ClF
3
3
3
3
5
2
2
2
+
+
+
were recorded with CD Cl as an internal reference. NMR
(M , 32), 196 (M ¹ C H O , 15), 195 (M ¹ C H O , 100),
3 4 2 3 5 2
167 (M ¹ C H O , 7); HRMS (EI, 70 eV): m/z calcd for
4 5 3
2
2
+
spectra of the cationic solution in FSO H­SbF (1:1)/SO ClF
3
5
2
were measured with an acetone-d capillary as an external
C H O : 268.0735, found: 268.0738.
16 12 4
6
1
23
standard, whose H chemical shifts were corrected by adding
4-Bromodibenzo[b,d]furan.
A solution of n-BuLi in
0
.96 ppm to the observed values.9
hexane (1.49 M, 0.860 mL, 1.28 mmol) was added dropwise
to a solution of 5 (200 mg, 1.19 mmol) in dry THF (1.0 mL)
cooled by an ice­water bath under Ar atmosphere. The solution
was stirred at room temperature for 5.5 h. The solution was
cooled with an ice­water bath, and 1,2-dibromoethane (0.180
mL, 2.09 mmol) in dry THF (0.30 mL) was added dropwise.
The mixture was stirred at room temperature for 2 h, and two
drops of water were added. Most of the solvent was evaporated,
and the residue was diluted with CH2Cl2. The solution was
washed with H O and 5% K CO and dried (MgSO ). Removal
Benzo[b]naphtho[1,2-d]furan (6) and Benzo[b]naph-
tho[2,3-d]furan (7). Lithium chloride was dried by heating
at 130 °C for 30 min. A mixture of 2-(2-naphthoxy)phenyl tri-
fluoromethanesulfonate (196 mg, 0.531 mmol), [Pd(PPh ) Cl ]
3
2
2
(39 mg, 0.055 mmol), lithium chloride (191 mg, 4.5 mmol),
and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 0.10 mL, 0.67
mmol) in dry DMF (2.0 mL) was stirred and heated at 145 °C
for 18 h under argon atmosphere. The cooled mixture was
diluted with ether (20 mL) and filtered. The filtrate was washed
with water and the organic layer was dried (Na SO ). The
2
2
3
4
of the solvent gave a pale yellow oil (285 mg), whose NMR
spectra showed the formation of 5, 4-bromodibenzo[b,d]furan,
and 4,6-dibromodibenzo[b,d]furan (0.5:1:0.08, 64% for 4-
2
4
removal of the solvent gave pale yellow crystals (143 mg),
whose purification by column chromatography (SiO ) with
2
Bull. Chem. Soc. Jpn. 2014, 87, 1235–1244 | doi:10.1246/bcsj.20140182
© 2014 The Chemical Society of Japan | 1241

bromodibenzo[b,d]furan). SiO column chromatography using
hexane as eluent gave a mixture of 5 and 4-bromodibenzo-
evaporated to give a pale brown oil, whose purification by SiO
2
2
4
2
column chromatography using CH Cl as eluent afforded 12
2
2
[
b,d]furan as colorless crystals (0.062:1.0, 157 mg) and a mix-
ture of 4-bromodibenzo[b,d]furan and 4,6-dibromodibenzo-
b,d]furan (1.0:0.44, 50 mg) as colorless crystals. Since 4-
as pale yellow crystals (67 mg, 77% yield): IR (KBr): 3058,
2953, 1674, 1629, 1285, 1264, 748 cm ; H NMR (300 MHz,
¹
1 1
[
CDCl ): ¤ 8.07 (d, J = 8.3 Hz, 1H), 7.99 (d, J = 7.7 Hz, 1H),
3
bromodibenzo[b,d]furan could not be isolated, the mixtures
were used for further experiments.
4
7.86 (d, J = 8.3 Hz, 1H), 7.62 (d, J = 8.1 Hz, 1H), 7.52 (td,
J = 7.8, 1.4 Hz, 1H), 7.38 (td, J = 7.5, 0.9 Hz, 1H), 3.30 (t,
J = 6.2 Hz, 2H), 2.76 (t, J = 6.6 Hz, 2H), 2.32­2.24 (m, 2H);
-Bromodibenzo[b,d]furan: 1H NMR (300 MHz, CDCl3):
1
3
¤
7.93 (d, J = 7.6 Hz, 1H), 7.88 (d, J = 7.6 Hz, 1H), 7.66 (d,
C NMR (75 MHz, CDCl ): ¤ 197.7 (C=O), 157.5 (C), 153.3
3
J = 8.3 Hz, 1H), 7.61 (d, J = 7.7 Hz, 1H), 7.50 (td, J = 7.8,
(C), 131.2 (C), 128.9 (C), 128.3 (CH), 127.9 (C), 123.9 (C),
123.2 (CH), 121.8 (CH), 121.5 (CH), 118.2 (CH), 111.9 (CH),
39.0 (CH ), 23.0 (CH ), 22.8 (CH ).
1
.0 Hz, 1H), 7.37 (t, J = 7.5 Hz, 1H), 7.23 (t, J = 7.9 Hz, 1H);
1
3
C NMR (75 MHz, CDCl ): ¤ 156.0 (C), 153.2 (C), 130.0
3
2
2
2
(
(
CH), 127.8 (CH), 125.7 (C), 124.1 (C), 123.9 (CH), 123.2
CH), 121.0 (CH), 119.6 (CH), 112.1 (CH), 104.4 (C).
4-Hydroxy-1,2,3,4-tetrahydrobenzo[b]naphtho[2,1-d]-
furan (13). A solution of 12 (55 mg, 0.23 mmol) in THF (0.6
mL) was added dropwise to a suspension of LiAlH4 (9.6 mg,
0.24 mmol) in THF (0.2 mL) under nitrogen atmosphere. The
mixture was stirred at room temperature for 3 h and refluxed
for 1 h. The cold reaction mixture was quenched by dropwise
addition of water and filtered with the help of ether. The solvent
of the filtrate was evaporated to give colorless crystals, whose
purification by SiO column chromatography using CH Cl as
4
-(4-Dibenzo[b,d]furanyl)-4-oxobutanoic Acid (10) from
4
-Bromodibenzo[b,d]furan. A Grignard reagent was pre-
pared from magnesium (34 mg, 1.40 mmol) and a mixture of 5,
-bromodibenzo[b,d]furan, and 4,6-dibromodibenzo[b,d]furan
0.21:1.0:0.13, 178 mg) in dry ether (1.5 mL). The reagent was
added dropwise into a solution of succinic anhydride (171 mg,
.71 mmol) in dry ether (5.0 mL)­dry THF (0.5 mL) cooled
4
(
1
2
2
2
2
4
with an ice­water bath, and stirred for 3 h. The mixture was
diluted with water, and most of the organic solvent was evapo-
rated. The residue was diluted with sat. NaHCO aq. (5.0 mL)
eluent afforded 13 as colorless crystals (50 mg, 89% yield):
¹
1
IR (KBr): 3366, 3060, 2937, 1582, 1456, 1193, 742 cm
;
1
H NMR (400 MHz, CDCl ): ¤ 7.92 (d, J = 7.7 Hz, 1H), 7.78
3
3
and washed with ether. The aqueous layer was acidified with
(d, J = 7.9 Hz, 1H), 7.57 (d, J = 8.3 Hz, 1H), 7.46­7.41 (m,
2H), 7.32 (t, J = 7.7 Hz, 1H), 4.94 (s, 1H), 3.21­3.12 (m, 1H),
3.01­2.91 (m, 1H), 2.17­1.87 (m, 4H), 1.80 (s, 1H, OH);
concd HCl and extracted with CHCl . The organic layer was
3
dried (MgSO ). Removal of the solvent afforded 10 as colorless
4
1
3
crystals (48 mg, 32%).
C NMR (100 MHz, CDCl ): ¤ 156.4 (C), 154.1 (C), 137.9
3
4
-(4-Dibenzo[b,d]furanyl)butanoic Acid (11). A solution
of 10 (196 mg, 0.732 mmol) and KOH (85% purity, 491 mg,
.43 mmol) in diethylene glycol (6.0 mL) was stirred and
heated to 100 °C. NH NH ¢H O (0.440 mL, 9.04 mmol) was
(C), 126.8 (CH), 124.4 (C), 123.3 (CH), 122.7 (C), 122.6 (CH),
121.7 (C), 120.7 (CH), 118.1 (CH), 111.6 (CH), 68.0 (CH),
32.1 (CH2), 22.9 (CH2), 17.8 (CH2).
7
1,2-Dihydrobenzo[b]naphtho[2,1-d]furan (14). A solu-
2
2
2
added dropwise over 5 min. The resulting solution was heated
at 120 °C for 2 h and at 180 °C for 1 h. The cooled mixture was
poured into water. The solution was acidified with concd HCl,
tion of 13 (38 mg, 0.16 mmol) and p-TsOH¢H O (8.0 mg,
2
0.042 mmol) in dichloromethane (1.0 mL) was heated under
reflux for 30 min. The cooled solution was washed with sat.
NaHCO and dried (MgSO ). The removal of the solvent by
and extracted with CHCl . The organic layer was washed with
3
3
4
water and dried (MgSO ). Removal of the solvent gave an
evaporation gave pale yellow crystals, whose purification by
4
orange oil (230 mg), which was dissolved in sat. NaHCO3
SiO column chromatography using hexane as eluent gave 14
2
(
30 mL). After filtration, the filtrate was acidified with concd
as colorless crystals (32 mg, 91% yield); mp 63.0­64.2 °C; IR
¹1
HCl and precipitated pale yellow crystals were collected by
filtration and dried under vacuum to give pure 1124 as pale
yellow crystals (106 mg, 57% yield): mp 109.0­111.0 °C; IR
(KBr): 3027, 2930, 1455, 1416, 1341, 1198, 835, 747 cm ;
1
H NMR (400 MHz, CDCl ): ¤ 7.89 (d, J = 7.8 Hz, 1H), 7.70
3
(d, J = 7.9 Hz, 1H), 7.54 (d, J = 8.5 Hz, 1H), 7.41 (td, J = 7.9,
1.3 Hz, 1H), 7.30 (t, J = 7.7 Hz, 1H), 7.03 (d, J = 7.7 Hz, 1H),
6.60 (d, J = 9.7 Hz, 1H), 6.11­6.05 (m, 1H), 3.15 (t, J = 8.5
(KBr): 3050, 2940, 2925, 2867, 1691, 1452, 1423, 1184, 754
¹
1 1
cm ; H NMR (300 MHz, CDCl ): ¤ 7.93 (d, J = 7.6 Hz, 1H),
7
3
1
3
.81 (m, 1H), 7.57 (d, J = 8.1 Hz, 1H), 7.44 (td, J = 7.7, 0.9
Hz, 2H), 2.48­2.40 (m, 2H); C NMR (100 MHz, CDCl ): ¤
3
Hz, 1H), 7.35­7.26 (m, 3H), 3.06 (t, J = 7.5 Hz, 2H), 2.46
(
156.5 (C), 153.7 (C), 133.3 (C), 128.4 (CH), 127.8 (CH), 126.7
(CH), 124.7 (C), 123.2 (C), 122.6 (CH), 121.3 (CH), 120.4
t, J = 7.4 Hz, 2H), 2.21­2.11 (m, 2H); 1 C NMR (75 MHz,
3
CDCl ): ¤ 179.1 (COOH), 156.0 (C), 154.7 (C), 127.3 (CH),
(CH), 118.4 (C), 117.9 (CH), 111.5 (CH), 22.3 (CH ), 20.1
3
2
+
1
27.0 (CH), 125.1 (C), 124.5 (C), 123.9 (C), 122.8 (CH), 122.6
(CH ); MS (EI, 70 eV): m/z (rel intensity) 220 (M , 100), 219
2
(
(
(
CH), 120.7 (CH), 118.6 (CH), 111.7 (CH), 33.3 (CH ), 29.1
(60), 218 (38), 205 (30), 189 (26), 191 (22); HRMS (EI,
2
CH ), 24.7 (CH ); MS (EI, 70 eV): m/z (rel intensity) 254
70 eV): m/z calcd for C H O: 220.0888, found: 220.0884.
2
2
16 12
+
M , 59), 195 (20), 194 (100), 182 (15), 181 (63); HRMS (EI,
0 eV) m/z calcd for C H O : 254.0943, found: 254.0947.
Benzo[b]naphtho[2,1-d]furan (8).
A solution of 14
7
(42 mg, 0.19 mmol) and DDQ (2,3-dichloro-5,6-dicyano-1,4-
benzoquinone, 50 mg, 0.22 mmol) in benzene (2.0 mL) was
stirred under reflux for 3 h. The cooled mixture was diluted
with CH Cl , washed with 5% NaOH and water, and dried
16
14
3
4
-Oxo-1,2,3,4-tetrahydrobenzo[b]naphtho[2,1-d]furan
12). A mixture of 11 (93 mg, 0.36 mmol), 85% H3PO4 (6.50
mL, 95.0 mmol), and P O (1.40 g, 9.84 mmol) was heated with
(
2
5
2
2
a 150 °C oil bath for 3 h. The cooled mixture was poured into
(MgSO ). The solvent was evaporated to give pale yellow
4
ice­water and extracted with CHCl . The organic layer was
crystals, whose purification by SiO column chromatography
using hexane as eluent gave 8 as colorless crystals (37 mg,
3
2
2
1
washed with sat. NaHCO and dried (MgSO ). The solvent was
3
4
1242 | Bull. Chem. Soc. Jpn. 2014, 87, 1235–1244 | doi:10.1246/bcsj.20140182
© 2014 The Chemical Society of Japan

1
8
1
8
1
1
8% yield): H NMR (400 MHz, CDCl ): ¤ 8.45 (d, J = 8.4 Hz,
the same level of theory. Nuclear independent chemical shifts
3
3
2
H), 8.01­7.97 (m, 3H), 7.77 (d, J = 8.4 Hz, 1H), 7.71 (d, J =
.0 Hz, 1H), 7.64 (td, J = 7.1, 1.3 Hz, 1H), 7.56 (td, J = 7.0,
.4 Hz, 1H), 7.47 (td, J = 7.8, 1.5 Hz, 1H), 7.39 (td, J = 7.7,
(NICS) were calculated by the GIAO method by B3LYP/
6-311+G(d,p)// B3LYP/6-31G(d). The “out-of-plane” compo-
nents of the NICS tensors were calculated at 1 ¡ above the ring
centers by rotating the planes of the rings orthogonal to z axis,
1
3
.2 Hz, 1H); C NMR (100 MHz, CDCl ): ¤ 156.0 (C), 152.0
3
(
(
(
C), 133.1 (C), 128.4 (CH), 126.5 (CH), 126.2 (CH), 126.1
CH), 125.1 (C), 123.3 (CH), 122.9 (CH), 121.4 (C), 120.9
CH), 120.3 (CH), 119.2 (C), 118.5 (CH), 111.8 (CH).
which were denoted as NICS(1) . The ring centers were
zz
defined as the simple average of Cartesian coordinates for all
the carbons in the ring. The planes of the rings were calculated
by the least-square method using the coordinates of all the
carbons in the rings.
Dinaphtho[2,1-b:1¤,2¤-d]furan (9).
Compound 9 was
prepared according to a modified procedure described in the
2
2
literature. HY Zeolite (HS-320) purchased from Wako
Chemicals (363 mg) was dried under vacuum with a heat gun
for 20 min and added to a solution of 1,1¤-bi-2-naphthol
This work was supported in part by a Grant-in-Aid for
Grant-in-Aid for Scientific Research (C) (Nos. 21550043 and
24550155) and a Grant-in-Aid for Scientific Research on
Innovative Areas “Reaction Integration” (No. 21106013),
Japan.
(145 mg, 0.505 mmol) in dry o-dichlorobenzene (4.0 mL). The
resulting suspension was heated under reflux for 6 days. The
mixture was cooled to room temperature and filtered. Evapo-
ration of the filtrate afforded dark-green crystals (146 mg),
Supporting Information
which were purified by column chromatography (SiO ) with
2
hexane as eluent to give 9 as pale yellow crystals (101 mg,
Figures S1 (GIAO­NMR data), and S2­S28 (selected NMR
spectra). Tables S1­S5 (energy data) and S6­S17 (Cartesian
coordinates for the computed structures). This material is
available free of charge on J-STAGE.
1
yield 74%): H NMR (400 MHz, CDCl ): ¤ 9.17 (d, J = 8.6 Hz,
3
2
H), 8.07 (d, J = 8.1 Hz, 2H), 7.96 (d, J = 8.8 Hz, 2H), 7.84 (d,
J = 8.8 Hz, 2H), 7.75 (td, J = 7.7, 1.3 Hz, 2H), 7.59 (t, J =
1
3
7
.4 Hz, 2H); C NMR (100 MHz, CDCl3): ¤ 154.4 (C), 131.3
(
(
C), 129.6 (CH), 128.7 (C), 128.4 (CH), 126.3 (CH), 125.7
CH), 124.5 (CH), 119.5 (C), 112.8 (CH).
References
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13
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1244 | Bull. Chem. Soc. Jpn. 2014, 87, 1235–1244 | doi:10.1246/bcsj.20140182
© 2014 The Chemical Society of Japan