Synthesis of oxygen-substituted hexa-peri-hexabenzocoronenes through ir-catalyzed direct borylation

Article abstract of DOI:10.1021/ol300743f

Direct C-H borylation of hexa-peri-hexabenzocoronenes (HBCs) has been achieved under iridium catalysis, which allows efficient synthesis of hydroxy-substituted HBCs by oxidation of the boryl groups. Further oxidation of dihydroxy HBC with phenyliodine bis(trifluoroacetate) (PIFA) afforded tetraoxo-substituted HBC without any regioisomers, which can be considered as a π-extended quinone.

Full text of DOI:10.1021/ol300743f

ORGANIC
LETTERS
2012
Vol. 14, No. 10
2472–2475
Synthesis of Oxygen-Substituted
Hexa-peri-hexabenzocoronenes
through Ir-Catalyzed Direct Borylation
Ryuichi Yamaguchi, Satoru Hiroto, and Hiroshi Shinokubo*
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University,
Chikusa-ku, Nagoya 464-8603, Japan
hshino@apchem.nagoya-u.ac.jp
Received March 22, 2012
ABSTRACT
Direct CÀH borylation of hexa-peri-hexabenzocoronenes (HBCs) has been achieved under iridium catalysis, which allows efficient synthesis of
hydroxy-substituted HBCs by oxidation of the boryl groups. Further oxidation of dihydroxy HBC with phenyliodine bis(trifluoroacetate) (PIFA)
afforded tetraoxo-substituted HBC without any regioisomers, which can be considered as a π-extended quinone.
Polycyclic aromatic hydrocarbons (PAHs) such as
pyrenes, triphenylenes, and hexa-peri-hexabenzocoronenes
(HBCs) have attracted much attention as fragments of
graphene.1 The structure and functionality at the edge
of graphenes significantly alter the physical properties
of graphenes.2 Consequently, effective functionalization
methods of PAHs are required to prepare model com-
pounds for the edge structures for functionalized graphenes.
Among various PAHs, HBCs have been extensively
investigated as the building block for liquid crystals and
motifs in supramolecules because of their disk-shaped
structures, producing a discotic liquid crystalline meso-
phase efficiently.3 However, introduction of various func-
tional groups at the peripheral positions of the HBC core
has been limited. Introduction of functionalities after the
synthesis of the HBC core is generally difficult due to low
solubility.4 Furthermore, the presence of reactive substi-
tuents often induces undesired reactions during Scholl
oxidation of the hexaphenylbenzene precursors with
FeCl3. In particular, the synthesis of alkoxy-substituted
HBCs is difficult because of severe side reactions.5 In
addition, hydroxy-substituted HBCs have never been
synthesized, although they would serve asprecursors for π-
extended quinone derivatives and novel self-assembling
units with hydrogen bondings between hydroxy groups.
Recently, transition metal catalyzed CÀH functionali-
zation has been under considerable development. Among
the various methods, CÀH direct borylation is a conve-
nient protocol for the introduction of boryl groups to
functional π-systems.6,7 The resulting borylated products
are useful substrates for further transformations such as
SuzukiÀMiyaura coupling and oxidation reactions.
Initially, we attempted to employ tetra-tert-butyl HBC
as a substrate for CÀH functionalization.8 However, poor
solubility of this compound in common organic solvents
hampered the conduction of further reactions. To improve
(1) (a) Haley, M. M., Tykwinski, R. R., Eds. Carbon-Rich Compound;
Wiley-VCH: Weinheim, 2006. (b) Harvey, R. G. Polycyclic Aromatic
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M€ullen, K. Pure. Appl. Chem. 2009, 81, 2203.
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(5) (a) Weiss, K.; Beernink, G.; Dotz, F.; Birkner, A.; Mullen, K.;
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X.; Bodwell, G. J.; Wagner, M.; Enkelmann, V.; Mullen, K. Org. Lett.
2007, 9, 2485. (c) Wadumethrige, S. H.; Rathore, R. Org. Lett. 2008, 10,
5139. (d) Feng, X.; Pisula, W.; Takase, M.; Dou, X.; Enkelmann, V.;
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(2) Dutta, S.; Pati, S. K. J. Mater. Chem. 2010, 20, 8207.
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(3) (a) Wu, J.; Pisula, W.; Mullen, K. Chem. Rev. 2007, 107, 718. (b)
Wagner, M.; Ding, N.; Mullen, K. Chem. Mater. 2008, 20, 2872.
Hill, J. P.; Jin, W.; Kosaka, A.; Fukushima, T.; Ichihara, H.; Shimomura,
T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004, 304, 1481.
(4) Wasserfallen, D.; Kastler, M.; Pisula, W.; Hofer, W. A.; Fogel,
(6) (a) Mkhalid, I. A. I.; Barnard, J. H.; Marder, T. B.; Murphy,
J. M.; Hartwig, J. F. Chem. Rev. 2010, 110, 890. (b) Miyaura, N.
Bull. Chem. Soc. Jpn. 2008, 81, 1535. (c) Ishiyama, T.; Miyaura, N.
J. Organomet. Chem. 2003, 680, 3.
€
Y.; Wang, Z.; Mullen, K. J. Am. Chem. Soc. 2006, 128, 1334.
r
10.1021/ol300743f
Published on Web 05/02/2012
2012 American Chemical Society

Scheme 1. Synthesis of Mesityl-Substituted Hexa-peri-hexa-
benzocoronene 3a
Scheme 2. Ir-Catalyzed Direct Borylation and Oxidation of 3a
the solubility, more bulky substituents were required at the
HBC periphery. The introduction of mesityl groups was
achieved by palladium-catalyzed coupling of tetrabromo-
hexaphenylbenzene 1a with mesitylmagnesium bromide in
95% yield.9 The Scholl reaction of 2a with FeCl3 in a
CH2Cl2/EtOH mixed solvent provided tetramesityl HBC
3a in 96% yield (Scheme 1), which was highly soluble in
organic solvents such as dichloromethane and toluene.
With a soluble HBC in hand, we then investigated
the iridium-catalyzed direct borylation of 3a (Scheme 2).
The reaction was performed in the presence of 4.0 equiv
of bis(pinacolate)diboron and 6 mol % of [Ir(OMe)cod]2/
dtbpy. No reaction proceeded in cyclohexane and 1,4-
dioxane, which are often employed as standard solvents
for direct borylation. After several trials, we found that
a mixed solvent of mesitylene/tert-butyl methyl ether
(2/1, v/v) was effective in providing fully borylated product
4a in 88% yield under refluxing conditions.10 The boryl groups
were easily converted into hydroxy groups by oxidation
with hydrogen peroxide to furnish 5a in 85% yield. The
structure of 5a was unambiguously confirmed by X-ray
(7) Examples of CÀH direct borylation of large π-conjugated com-
pounds: (a) Coventry, D. N.; Basanov, A. S.; Goeta, A. E.; Howard,
J. A. K.; Marder, T. B.; Perutz, R. N. Chem. Commun. 2005, 2172. (b)
Hata, H.; Shinokubo, H.; Osuka, A. J. Am. Chem. Soc. 2005, 127, 8264.
(c) Kimoto, T.; Tanaka, K.; Sakai, Y.; Ohno, A.; Yoza, K.; Kobayashi,
K. Org. Lett. 2009, 11, 3658. (d) Ozawa, R.; Yoza, K.; Kobayashi, K.
Chem. Lett. 2011, 40, 941. (e) Teraoka, T.; Hiroto, S.; Shinokubo, H.
Org. Lett. 2011, 13, 2532. (f) Battagliarin, G.; Li, C.; Enkelmann, V.;
Figure 1. X-ray crystal structure of 5a. (a) Top view, (b) side
view, and (c) packing structure in the crystal. The thermal
ellipsoids are scaled to 50% probability level. Mesityl groups
are omitted for clarity.
€
Mullen, K. Org. Lett. 2011, 13, 3012. (g) Crawford, A. G.; Liu, Z.;
Mkhalid, I. A. I.; Thibault, M.-H.; Schwarz, N.; Alcaraz, G.; Steffen, A.;
Collings, J. C.; Batsanov, A. S.; Howard, J. A. K.; Marder, T. B.
Chem.;Eur. J. 2012, 18, 5022.
(8) (a) Sadhukhan, S. K.; Viala, C.; Gourdon, A. Synthesis 2003, 10,
€
1521. (b) Herwig, P. T.; Enkelmann, V.; Schmelz, O.; Mullen, K.
Chem.;Eur. J. 2000, 6, 1834.
(9) (a) Wolf, C.; Xu, H. J. Org. Chem. 2008, 73, 162. (b) Li, G. Y.
Angew. Chem., Int. Ed. 2001, 40, 1513.
(10) Harrisson, P.; Morris, J.; Steel, P. G.; Marder, T. B. Synlett 2009,
147.
diffraction analysis (Figure 1). The hydroxy groups were
incorporated at the least hindered positions. The π-plane
of the HBC core was slightly distorted, and the mean
Org. Lett., Vol. 14, No. 10, 2012
2473

˚
plane deviation was calculated to be 0.045 A. The distance
between two oxygen atoms on the adjacent molecules was
˚
2.68 A, indicating the existence of hydrogen bonding. It is
noteworthy that the hydrogen bondings construct a 1D
chain network of 5a in the crystal.
To investigate the generality of the present protocol,
the direct borylation of trisubstituted HBC 3b was exam-
ined (Scheme 3).11 Borylation of 3b with 6.0 equiv of
Scheme 3. Ir-Catalyzed Direct Borylation and Oxidation of 3b
Figure 2. X-ray crystal structure of 5b. (a) Top view, (b) side
view, and (c) packing structure in the crystal. The thermal
ellipsoids are scaled to 50% probability level. Mesityl groups
are omitted for clarity.
Scheme 4. PIFA Oxidation of 5a
bis(pinacolate)diboron in tert-butyl methyl ether afforded
triborylated product 4b in 73% yield. Treatment of 4b
with H2O2/NaOH aq furnished trihydroxy HBC 5b in
69% yield. The structure was also confirmed by X-ray
diffraction analysis, which reveals that HBC 5b also forms
an assembly through hydrogen bondings in the crystal
(Figure 2). The structure of 5b was also slightly distorted,
probably as a result of the crystal packing force.
We then attempted further oxidation of 5a to synthesize
dioxo-substituted HBC as a π-extended quinone on the
basis of the HBC skeleton. However, the quinone product
was not stable enough to be isolated.12 Instead, oxidation
of 5a with 4.0 equiv of phenyliodine bis(trifluoroacetate)
(PIFA) in DMF afforded tetraoxo product 6 in 84% yield
without any regioisomers (Scheme 4).13 The presence of
tetraoxo substituents was confirmed by high-resolution
mass spectrum. Tetraone 6 was reduced with NaBH4 into
tetrahydroxy HBC 7 in 79% yield. The structure of 7 was
assigned by the NOESY spectrum and finally confirmed
by preliminary X-ray diffraction analysis of acetylated
derivative 8 (Figure S29). In tetraacetate 8, the acetoxy
groups were selectively introduced in a syn manner and
the π-plane was distorted on one side due to the steric
repulsion of four acetoxy groups.14
Figure 3 shows UV/vis absorption spectra of 3a, 5a, 6,
and 7 in CH2Cl2. HBCs 3a, 5a, and 7 exhibited typical
absorption spectra for aromatic hydrocarbons.15 Hydroxy
groups induce a slight bathochromic shift of absorption
spectra due to their electron-donating nature. The broad-
ening of the spectral shape of 7 is probably due to lower
symmetry. On the other hand, tetraone 6 exhibited quite a
broad absorption spectrum reaching the near-infrared
region. The significantly decreased extinction coefficient
and the blue shift of the main band around 300 nm can
€
(11) Feng, X.; Wu, J.; Enkelmann, V.; Mullen, K. Org. Lett. 2006, 8,
1145.
(12) The instability of the possible dioxo-substituted HBC is prob-
ably due to its biradical character.
(14) We now speculate that the regioselectivity results from distor-
tion of the HBC core after the introduction of the oxo-substituents.
€
(15) Kastler, M.; Schmidt, J.; Pisula, W.; Sebastiani, D.; Mullen, K.
J. Am. Chem. Soc. 2006, 128, 9526.
(13) Wu, A.; Duan, Y.; Xu, D.; Penning, T. M.; Harvey, R. G.
Tetrahedron 2010, 66, 2111.
2474
Org. Lett., Vol. 14, No. 10, 2012

and reduction potentials is substantially small (1.06 V).
The high reduction potential of 6 suggests a prospective
feature of 6 as an electron-accepting molecule.
In summary, we have demonstrated that iridium-cata-
lyzed direct borylation of tetraaryl- and triaryl-substituted
HBCs proceeded efficiently to afford diborylated and
triborylated HBCs in good yields. The borylated HBCs
have been successfully converted into hydroxy HBCs,
which are difficult to prepare by Scholl oxidation. In addi-
tion, π-extended quinone 6 has been synthesized through
oxidation of dihydroxy HBC 5a. The present synthetic
method would be useful for modification of HBCs, which
allows introduction of various functionalities after con-
struction of the HBC skeleton. Further derivatization of
the borylated HBCs and properties of the HBC derivatives
are currently under investigation.
Figure 3. UV/vis absorption spectra 3a, 5a, 6, and 7 in CH2Cl2.
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research (Nos. 22750036 and 23655033)
from MEXT, Japan and the Global COE Program in
Chemistry of Nagoya University. H.S. acknowledges the
Daiko Foundation for financial support.
be accounted for by the breaking of the π-conjugation of
HBC by the carbonyl groups. The observed large bath-
ochromic shift can be accounted for by the intramolecular
charge-transfer interaction between the HBC core and
the carbonyl moieties on the basis of DFT calculations
(Figure S31). The HOMO and LUMO energies of 6 were
estimated by electrochemical analysis by cyclic voltamme-
try (FigureS26). Reversible reduction and oxidation waves
were observed at À0.64 and 0.42 V (vs ferrocene/ferroce-
nium cation), respectively. The gap between the oxidation
Supporting Information Available. General procedures,
spectral data for compounds, and CIF files for the X-ray
analysis of 5a, 5b, and 8. This material is available free of
charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 10, 2012
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