Unexpected formation of poly-functionalized fulvenes by the reaction of a tetraaryl[5]cumulene with iodine

Article abstract of DOI:10.1039/d1ob01270c

Longer cumulenes have come to draw considerable attention due to their unique properties and reactivities, leading to various hydrocarbons. In this manuscript, we describe the reaction of tetrakis(p-methoxyphenyl)[5]cumulene with iodine to afford poly-fun

Full text of DOI:10.1039/d1ob01270c

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Unexpected formation of poly-functionalized
fulvenes by the reaction of a tetraaryl[5]cumulene
Cite this: DOI: 10.1039/d1ob01270c
with iodine†
Received 1st July 2021,
b
Keita Hoshi,a Masashi Yasuda,a Takumi Nakamura,b Yasushi Yoshida,
Accepted 14th August 2021
Shoko Ueta,a Keiji Minagawa,a Yasuhiko Kawamura,a Yasushi Imada a and
DOI: 10.1039/d1ob01270c
a,c
Fumitoshi Yagishita
*
rsc.li/obc
Longer cumulenes have come to draw considerable attention due tively rare due to their instabilities. It has been known that [5]
to their unique properties and reactivities, leading to various cumulenes, shown in Fig. 1, undergo thermal cycloaddition,
hydrocarbons. In this manuscript, we describe the reaction of tet- leading to unique hydrocarbons such as radialenes. For
rakis(p-methoxyphenyl)[5]cumulene with iodine to afford poly- example, the thermal dimerization of [5]cumulenes occurs at
functionalized fulvenes via unexpected migration of a terminal aryl the central γ-bond to afford the symmetrical [4]radialene,8
ring under ambient conditions. The obtained iodinated fulvenes whereas the Ni-catalyzed dimerization occurs at the β-bond.9
were utilized in Suzuki–Miyaura cross-coupling reactions affording In addition, some 2 + 2 cycloaddition reactions of [5]cumu-
lenes with alkenes,10,11 alkynes,11,12 or fullerene C60 have
been reported and afforded various 2 + 2 cycloadducts. On the
other hand, other transformations of [5]cumulenes are rare
except for the formation of five-membered metallacycloalk-
ynes.13 A few examples such as the oxidation reaction using
epoxidizing agents14 and the trimerization reaction involving a
radical formation process15 or a 4 + 2 cycloaddition process16
have been reported so far, which encouraged us to explore the
novel reactivity of [5]cumulenes. Herein, we describe the reac-
tion of tetrakis(p-methoxyphenyl)[5]cumulene (1) with iodine
to afford iodinated tetraarylfulvene 2 via unexpected migration
of a terminal aryl ring of [5]cumulene under ambient con-
12
penta- and fully-arylated fulvenes successfully.
Fulvenes and their related derivatives have been recognized as
an important class of hydrocarbons due to their utilities in the
fields of medicinal chemistry,1 materials science,2 and organo-
metallics.3 Recently, poly-arylated fulvenes have become the
candidates for organic electronic devices.4 To construct such
scaffolds, some successful examples have been developed. For
example, Iyoda and co-workers reported the nickel-catalyzed
oligomerization of 1,1-dibromoethylenes to afford hexaarylated
fulvenes in the presence of diarylacetylenes.5 Heck and co-
workers also reported the palladium-catalyzed reaction of
β-bromo styrene with diphenylacetylenes, leading to pentaphe-
nylfulvene.6 Very recently, palladium-catalyzed vinylic geminal
double C–H activation and alkyne annulation have been devel-
oped to provide hexaarylated fulvenes.7
[n]Cumulenes (n is the number of cumulated double
bonds) have come to draw considerable attention because they
show unique properties and reactivities. In comparison with
the research studies on the reaction of shorter cumulenes Fig. 1 The structure of [5]cumulene.
including allenes, those on longer cumulenes (n ≥ 5) are rela-
aDepartment of Applied Chemistry, Tokushima University, 2-1 Minamijosanjima,
Tokushima 770-8506, Japan. E-mail: yagishitaf@tokushima-u.ac.jp
bDepartment of Applied Chemistry and Biotechnology, Graduate School of
Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
cDepartment of Post-LED Photonics Research, Institute of Post-LED Photonics, 2-1
Minamijosanjima, Tokushima 770-8506, Japan
†Electronic supplementary information (ESI) available: Additional figures, crys-
tallographic data in CIF, and NMR spectra. CCDC 2089131, 2089132, 2089134,
2089167 and 2089168. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/d1ob01270c
Scheme 1 Construction of the fulvene scaffold by the reaction of [5]
cumulene with iodine.
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ditions (Scheme 1). The iodinated fulvene was easily converted
to fully arylated fulvene under the Suzuki–Miyaura coupling
conditions.
We selected tetrakis(p-methoxyphenyl)[5]cumulene (1),
which is a known compound,17 as a substrate because the
iodocyclization of tetraarylcumulene tends to occur from the
electron-rich aromatic ring to the cationic iodonium inter-
mediate according to our previous report.18 When 5 mL of
MeNO2 solution containing 0.15 mmol of 1 and 3 eq. of iodine
was stirred for 30 min at room temperature, 2,3-diiodo-4,5,6,6-
tetrakis(p-methoxyphenyl)fulvene (2) was isolated in 31% yield
(Scheme 1 and entry 1 in Table 1). The structure was deter-
mined by NMR spectroscopy and HRMS. In addition, ca. 23%
yield of a relatively unstable byproduct was also obtained. This
1
byproduct shows a similar H NMR spectrum to that of 2 with
the exception of one additional singlet peak observed at
6.70 ppm. On the basis of NMR and HRMS spectroscopy, we
presumed that 4-iodo-2,3,6,6-tetrakis(p-methoxyphenyl)fulvene
(3) was also formed as the major byproduct.
Fig. 2 ORTEP drawings of 2 and 3. The values are torsion angles of aryl
rings for the fulvene plane.
Fortunately, single crystals of 2 and 3 suitable for X-ray crys-
tallographic analysis were successfully prepared by recrystalli-
zation from chloroform solution of each compound.
Therefore, we conducted X-ray analysis to evaluate their
detailed structure in the crystalline lattice and found that ful-
venes 2 and 3 crystallized in the monoclinic space group P21/n
Next, we conducted the optimization of reaction conditions
for the construction of fulvenes using [5]cumulene 1. The
results are presented in Table 1. Interestingly, a longer reaction
time resulted in a lower yield of 2, whereas the yield of 3 was
increased (entry 2). The reaction of 1 at a lower temperature
resulted in low conversion and fulvene 2 was not obtained
after 30 min (entry 3). When the reaction was examined using
alternative solvents, fulvene 2 was obtained in moderate-to-
good yields except MeOH (entries 4–7). Notably, benzene was
the most effective solvent for this reaction and fulvene 2 was
obtained in 56% yield (entry 7). A longer reaction time slightly
decreased the yield of 2 in benzene (entry 8).
Next, we investigated the reaction mechanism for the con-
struction of the fulvene scaffold from tetraaryl[5]cumulene.
Because a longer reaction time tends to afford mono-iodinated
fulvene 3 with a higher yield than that of di-iodinated fulvene
2, we consider that fulvene 2 is an intermediate to form 3. To
verify this hypothesis, we conducted control experiments
(Scheme 2). When the benzene solution containing 2 was
stirred for 30 min in the presence of 3 eq. of iodine, no reac-
ˉ
and the triclinic space group P1, respectively (Fig. 2). As
expected, the structure of mono-iodinated fulvene 3 was
unequivocally established by X-ray analysis. The X-ray analysis
also revealed the twisted conformation through C–C bonds
between the substituted aryl rings and fulvene core. In both
fulvenes, the torsion angles of the substituted aryl rings at the
4-, 5-, and 6-positions (A, B, and C rings in Fig. 2) for fulvene
cores show similar values, 53.4° for the A ring, around 60° for
the B ring, and around 34° for the C ring. On the other hand,
the D ring of 2 shows a more twisted conformation in compari-
son with that of 3.
Table 1 Reaction of tetrakis(p-methoxyphenyl)[5]cumulene (1) with
iodine
Entry
Time
Solvent
Temp.
Yield of 2 a (%)
1
2
3
4
5
6
7
8
30 min
60 min
30 min
30 min
30 min
30 min
30 min
10 h
MeNO2
MeNO2
MeNO2
MeCN
rt
rt
31 (23)b
4 (38)b
0 (0)b
−25 °C
rt
rt
rt
rt
rt
16 (28)b
0 (0)b
MeOH
Toluene
Benzene
Benzene
32 (18)b
56 (27)b
50 (33)b
a Isolated yields. b The values in parentheses are the yields of 3.
Scheme 2 Control experiments to elucidate the reaction mechanism.
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tion took place (condition A). Also, the reaction using sodium
iodide as an iodide anion source gave the same result (con-
dition B). These results indicated that molecular iodine and
the iodide anion did not contribute to form fulvene 3 from
fulvene 2 in this reaction. On the other hand, the addition of
hydroiodic acid dramatically affected the reaction, and fulvene
4 was obtained in 85% yield (condition C). It is known that the
protonation of fulvene takes place at the 2- and 3-positions,
and the 2-position is more favorable than the 3-position for
protonation.19 Thus, we considered that fulvenes 3 and 4 were
formed via protonation followed by deiodination via 2 in the
presence of a Brønsted acid. We also evaluated whether a
reversible reaction between fulvenes 2 and 3 occurs or not.
However, fulvene 2 was not obtained from 3 under the opti-
mized conditions (condition D).
Scheme 4 Suzuki–Miyaura cross-coupling reaction of 2 with p-meth-
oxyphenylboronic acid. Hydrogen atoms are omitted for clarity in the
ORTEP drawing of 5. Two methoxy groups are disordered.
Finally, we demonstrate the use of the obtained fulvenes as
a coupling partner for the transition metal catalyzed cross-
coupling reaction. The reaction of 2 with p-methoxyphenyl-
boronic acid gave hexaarylfulvene 5 in 81% yield under the
Suzuki–Miyaura cross coupling conditions (Scheme 4). In
addition, mono-iodinated fulvene 3 was also converted to pen-
taarylfulvene 6 in 52% yield by the Suzuki–Miyaura coupling
reaction using 4-methoxyphenylboronic acid (Scheme S1†).
Fortunately, a single crystal of 5 was obtained by recrystalliza-
tion from the dichloromethane solution. X-ray analysis of 5
revealed that each aryl ring tends to have a twisted confor-
mation to the fulvene core and shows a “propeller-like” struc-
ture. Fully arylated fulvene 5 shows a broad absorption region,
which reaches around 550 nm, and the absorption peak was
observed at 415 nm (Fig. S3†). Although the procedure
reported by Iyoda and co-workers provides hexaarylfulvenes
with the same aryl substituents in one step,5 our results
enable access to not only fully functionalized fulvenes but also
penta-functionalized fulvenes with the same or different aryl
substituents via common cross coupling reactions.
On the basis of these results, we proposed the reaction
mechanism as shown in Scheme 3. Initially, the iodonium ion
intermediate A is formed by the reaction of [5]cumulene 1 with
iodine at the β-bond, which is the shortest double bond of [5]
cumulene and is slightly longer than a typical triple bond.18,20
Then, the nucleophilic attack of the iodide ion on A affords bis
(iodide) adduct B.21 After the second formation of iodonium
ion intermediate C at the β′-bond, cyclization followed by
migration of the aryl group occurs leading to intermediate D.
As a result, fulvene 2 is formed by the elimination of the
iodine atom. In addition, a longer reaction time gives fulvene
3 by the reaction of 2 with the Brønsted acid HI, which is
formed by the reaction of iodine with water contained in the
solvent or by the decomposition of iodine.22 To elucidate this
hypothesis, we conducted a control experiment using D2O.
After a deuterated benzene solution containing cumulene 1,
iodine and D2O was stirred at room temperature, we measured
In conclusion, we have found a novel type of reaction of tet-
raaryl[5]cumulene with iodine, leading to poly-functionalized
fulvenes under mild reaction conditions. The structures of the
fulvenes were equivocally established by X-ray analysis. In
addition, the obtained fulvene was used as a precursor for a
cross-coupling reaction and was successfully converted to fully-
arylated fulvene under the Suzuki–Miyaura coupling conditions.
1
the H NMR spectrum of the reaction mixture without purifi-
cation. As a result, the singlet peak at 7.0 ppm assignable to
the vinyl proton of 3 disappeared (Fig. S2†).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors would like to thank Dr H. Sato (Rigaku Corp.) for
supporting the X-ray analysis. We also thank the Research
Clusters program of Tokushima University (no. 1802001,
1802003, and 2003006) for partial financial support. This work
was supported by JSPS KAKENHI, grant number 20H02767,
and the project on the Promotion of Regional Industries and
Universities from the Cabinet Office, Japan, by the Creation
and Application of Next-Generation Photonics by Tokushima
Prefecture from the Tokushima Prefectural Government,
Japan.
Scheme 3 Plausible reaction mechanism.
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