Iridium-catalyzed direct tetraborylation of perylene bisimides

Article abstract of DOI:10.1021/ol2004534

Treatment of perylene bisimides (PBIs) with bis(pinacolato)diboron in the presence of an iridium catalyst provides tetraborylated PBIs at 2,5,8,11-positions in good yields with perfect regioselectivity. The planar structure of the perylene core has been c

Full text of DOI:10.1021/ol2004534

ORGANIC
LETTERS
2011
Vol. 13, No. 10
2532–2535
Iridium-Catalyzed Direct Tetraborylation
of Perylene Bisimides
Takuro Teraoka, 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 February 18, 2011
ABSTRACT
Treatment of perylene bisimides (PBIs) with bis(pinacolato)diboron in the presence of an iridium catalyst provides tetraborylated PBIs at 2,5,8,11-
positions in good yields with perfect regioselectivity. The planar structure of the perylene core has been confirmed by X-ray diffraction analysis.
Oxidation of tetraborylated PBI with hydroxylamine hydrochloride affords tetrahydroxy PBI in excellent yield, which exhibits a substantially blue-
shifted absorption spectrum due to an intramolecular hydrogenbonding interaction between carbonyl and hydroxy groups.
Perylene tetracarboxylic acid bisimide (PBI) is an im-
portant class of dyes for widespread use. This molecule has
received much attention for wide areas of applications
toward organic material devices such as organic solar cells,
organic light-emitting diodes, single molecule spectro-
scopy, field effect transistors, and biomedical sensors.1
The synthesis of novel PBI derivatives often utilizes mod-
ification of the perylene core by halogenation at the bay
area, namely, 1,6,7,12-positions. This is due to the high
reactivity of the bay area toward electrophilic substitu-
tions. On the other hand, functionalization at 2,5,8,11-
positions of PBIs has been unavailable until our recent
reports on direct alkylation and arylation of PBIs at
2,5,8,11-positions through Ru-catalyzed CꢀH bond
cleavage.2,3 We have demonstrated that the substitutions
at 2,5,8,11-positions maintain planar geometry of the
perylene π-plane, while the substitutions at the bay area
often induced significant distortion. We have also demon-
strated that alkylation of PBIs at 2,5,8,11-positions sub-
stantially enhances solid state emission and arylation at
these positions has a significant impact on their optical and
electrochemical properties in solution. However, introduc-
tion of heteroatoms and other functionalities at these
positions of PBIs has not been achieved so far.
Direct CꢀH borylation has proven to be a powerful tool
in organic synthesis.4 Recently, Miyaura, Ishiyama, and
co-workers have reported iridium-catalyzed ortho-boryla-
tion of benzoate esters.5 Encouraged by this pioneering
€
(1) (a) Herrmann, A.; Mullen, K. Chem. Lett. 2006, 35, 978. (b)
€
Wurthner, F. Chem. Commun. 2004, 1564. (c) Wasielewski, M. R. J. Org.
Chem. 2006, 71, 5051. (d) Langhals, H. Helv. Chim. Acta 2005, 88, 1309.
(e) Langhals, H. Heterocycles 1995, 40, 477. (f) Langhals, H. Molecular
Devices. Chiral, Bichromophoric Silicones: Ordering Principles in Com-
plex Molecules. In Silicon Based Polymers; Ganachaud, F., Boileau, S.,
Boury, B., Eds.; Springer: New York, 2008; pp 51ꢀ63.
(4) For reviews, see: (a) Mkhalid, I. A. I.; Barnard, J. H.; Marder,
T. B.; Murphy, J. M.; Hartwig, J. F. Chem. Rev. 2009, 110, 890. (b)
Ishiyama, T.; Miyaura, N. J. Organomet. Chem. 2003, 680, 3. (c)
Ishiyama, T.; Miyaura, N. Chem. Rec. 2004, 3, 271. (d) Ishiyama, T. J.
Synth. Org. Chem. Jpn. 2005, 63, 440. (e) Ishiyama, T.; Miyaura, N. In
Boronic Acids; Hall, D., Ed.; Wiley-VCH: Weinheim, 2005; pp 101. (f)
Ishiyama, T.; Takagi, J.; Nobuta, Y.; Miyaura, N. Org. Synth. 2005, 82,
126.
(5) (a) Ishiyama, T.; Isou, H.; Kikuchi, T.; Miyaura, N. Chem.
Commun. 2010, 46, 159. (b) Miyaura, N. Bull. Chem. Soc. Jpn. 2008,
81, 1535. (c) Kawamorita, S.; Ohmiya, H.; Hara, K.; Fukuoka, A.;
Sawamura, M. J. Am. Chem. Soc. 2009, 131, 5058.
(2) For leading references on direct CꢀH transformations, see: (a)
Handbook of CꢀH Transformations; Dyker, G., Ed.; Wiley-VCH: Weinheim,
2005. (b) Kakiuchi, F.; Chatani, N. Adv. Synth. Catal. 2003, 345, 1077. (c)
Godula, K.; Sames, D. Science 2006, 312, 67. (d) Activation of Unreactive
Bonds and Organic Synthesis; Murai, S., Ed.; Springer: Berlin, 1999. (d)
Alberico, D.; Scott, M. E.; Lautens, M. Chem. Rev. 2007, 107, 174.
(3) (a) Nakazono, S.; Imazaki, Y.; Yoo, H.; Yang, J.; Sasamori, T.;
Tokitoh, N.; Cedric, T.; Kageyama, H.; Kim, D.; Shinokubo, H.; Osuka,
A. Chem.;Eur. J. 2009, 15, 7530. (b) Nakazono, S.; Easwaramoorthi, S.;
Kim, D.; Shinokubo, H.; Osuka, A. Org. Lett. 2009, 11, 5426.
r
10.1021/ol2004534
Published on Web 04/20/2011
2011 American Chemical Society

work, we decided to explore borylation of PBIs at 2,5,8,11-
positions under iridium-catalysis.
catalyst system effected exclusive borylation on the
2,5,8,11-positions of the perylene core: no borylation oc-
curred on the phenyl groups of the imide substituents of 1c
to furnish tetraborylated PBI 2c (entry 5). In sharp contrast,
borylation with L1 took place only on the phenyl groups but
afforded a mixture of several products due to formation of
regioisomers.
Table 1. aIr-Catalyzed Direct Borylation of PBI 1
Single crystals of 2a suitable for X-ray diffraction ana-
lysis were obtained by vapor diffusion of acetone into a
chloroform solution of 2a. The solid-state structure of 2a
was unambiguously determined to elucidate the highly
planar structure of the perylene core, of which mean plane
6
˚
deviation is calculated to be only 0.015 A (Figure 1). This
characteristic is in sharp contrast to the functionalizaed
PBIs at the bay areas, of which perylene backbone are
considerably twisted. The boryl groups are tilted to the
perylene core by 86.0 or 97.3° and the CꢀB bond lengths
between the perylene core and theboryl groups range from
˚
1.569 to 1.585 A. These structural features imply weak
electronic communication between the boryl substituents
and the perylene moiety. The crystal packingof2adoes not
show any πꢀπ interaction because of steric hindrance of
bulky 2,2,3,3-tetramethyldioxaborolanyl substituents
(Supporting Information, SI).
yield (%)
entry
ligand
R
2
3
4
1
2
3
4
5
L1
L2
L3
L3
L3
2-ethylpropyl (1a)
2-ethylpropyl (1a)
2-ethylpropyl (1a)
2-hexylheptyl (1b)
2-phenethyl (1c)
13
37
12
59
78
64
a Reaction conditions: PBI 1 (0.25 mmol), bis(pinacolato)diboron,
(2.0 mmol), [Ir(OMe)cod]2 (0.0075 mmol), 1,4-dioxiane (5 mL), ligand-
(0.003 mmol), 110 °C, 48ꢀ96 h.
Table 1 illustrates Ir-catalyzed direct borylation of PBI. In
the presence of 3.0 mol % of [Ir(OMe)cod]2 and di-tert-
butylbipyridyl (L1) as a ligand, a mixture of bis(N-ethyl-
propyl)PBI 1a and bis(pinacolato)diboron was heated in
refluxing 1,4-dioxane. Purification of the reaction mixture
by silica gel chromatography afforded only monoborylated
PBI 3a in 13% yield (entry 1). Miyaura et al. have reported
that tris(3,5-bis(trifluoromethyl)phenyl)phosphine (L2) is
effective for borylation of benzoates. In fact, the use of L2
significantly improved the yield of 3a to 37% and also
provided diborylated PBI 4a in 12% yield but without
formation of tetraborylated PBI 2a (entry 2). Eventually,
we found that the use of tris(pentafluorophenyl)phosphine
(L3) is quite effective to furnish 2a and 2b in 59 and 78%
yields, respectively (entries 3 and 4). Interestingly, this
Figure 1. X-ray crystal structure of (a) top view and (b) side view
of tetraborylted PBI 2a. The thermal ellipsoids were scaled to the
50% probability level. Hydrogen atoms, solvents, and boryl
substituents in the side view were omitted for clarity.
SuzukiꢀMiyaura cross-coupling of 2b was then exam-
ined. After several trials, the use of SPhos as a phosphine
ligand was found to be optimal to afford heteroarylated
(6) Crystal data for 2a: C67H92B4N2O15, Mw = 1208.67, orthorhom-
˚
˚
bic, space group P212121(No. 19), a = 11.010(5) A, b = 18.968(8) A, c =
33.023(15) A, V = 6896(5) A , Z = 4, Dcalc = 1.164 g/cm3, T = 153(2)
K, R = 0.0918 (I > 2.0σ(I)), Rw = 0.3186 (all data), GOF = 0.934 (I >
2.0σ(I)).
3
˚
˚
Org. Lett., Vol. 13, No. 10, 2011
2533

Scheme 1. SuzukiꢀMiyaura Coupling of 2b
Scheme 2. Introduction of Hydroxy and Methoxy Groups
PBIs in good yields (Scheme 1). Although arylation of
PBIs at 2,5,8,11-positions can be achieved by Ru-catalyzed
direct arylation with arylboronic esters,3b introduction of
heteroaryl groups was difficult. The borylationꢀcross-
coupling sequence would be useful for the synthesis of this
type of heteroarylated PBIs.
We also attempted the oxidation of tetraborylated PBI
2b to tetrahydroxy PBI 6, since the effect of heteroatoms at
the 2,5,8,11-positions has not been investigated. Oxidation
of arylboronic acid pinacol esters often employs Oxone7 or
basic hydrogen peroxide,8 but these protocols afforded
only a trace amount of the desired tetrahydroxy PBI 6. After
several trials, we found that the use of hydroxylamine hydro-
chloride as an oxidant in the presence of NaOH successfully
provided 6 in 82% yield (Scheme 2).9 Conversion of 6 to 7 by
methylation of OH group was carried out with an excess
amount of iodomethane in the presence of Ag2CO3 as bases.
In the 1H NMR spectrum of 6, the hydroxyl protons were
observed in a substantially downfield region at δ = 13.2
ppm, suggesting intramolecular hydrogen bonding interac-
tion between carbonyl and hydroxy moieties.
in 6 was confirmed by the IR spectrum, in which imide
carbonyl stretching bands shifted down to lower wave-
numbers than that of 7 (SI). Theoretical calculations at the
B3LYP/6-31G(d) level revealed considerable stabilization
of HOMO in the case of 6, while destabilization was
observed for 7 (Figure 3). Lowering of the HOMO energy
was also confirmed by the cyclic voltammetry (SI). The
first oxidation potentials (Eox1) of 1b, 6, and 7 are 1.23,
1.56, and 1.10V (vs Fc/Fcþ), respectively. Accordingly, the
potential differences between the first oxidation potentials
and the first reduction potentials are 2.33, 2.68, and 2.28 V,
respectively. This trend in electrochemical HOMOꢀ
LUMO gaps are well matched with the optical HOMOꢀ
LUMO gaps as well as theoretical calculations.
Interestingly, tetraborylated PBI 2b was found to
be emissive even in the solid state. A powder sample of
2b emits at 609 nm with a relatively high quantum yield
of 0.18, which was determined by a calibrated inte-
grating sphere system. The enhancement of photolu-
minescence efficiency in the solid state is probably due
to the bulky pinacolboryl substituents, which reduce
intermolecular interaction in the excited state of PBI.
In summary, we have accomplished regioselective
4-fold direct borylation of PBI at the 2,5,8,11-posi-
tions of the perylene core under Ir-catalysis. The boron
substituents can be oxidized to hydroxy groups with
hydroxylamine. Interestingly, introduction of the
Figure 2 shows UV/vis absorption spectra of 1b, 2b, 6,
and 7 in dichloromethane. Notably, introduction of OH
groups induced a significant blue-shift of the lowest energy
band from 525 to 512 nm, despite that the electron-
donating substituents often cause a red-shift by lifting
the HOMO energy. This spectral change can be explained
by intramolecular hydrogen bonding between carbonyl
and hydroxy groups in tetrahydroxy PBI 6. The hydrogen
bonding interaction enhances the electron-withdrawing
nature of imide carbonyl groups and stabilizes the HOMO
level to increase the HOMOꢀLUMO gap of 6. The role of
hydrogen bonding can be suggested from the red-shifted
feature in the absorption spectrum of methoxy derivative
7. The existence of the intramolecular hydrogen bondings
(8) (a) Simon, J.; Salzbrunn, S.; Prakash, G. K. S.; Petasis, N. A.;
Olah, G. A. J. Org. Chem. 2000, 66, 633. (b) Ely, R. J.; Morken, J. P. J.
Am. Chem. Soc. 2010, 132, 2534.
(9) Kianmehr, E.; Yahyaee, M.; Tabatabai, K. Tetrahedron Lett.
2007, 48, 2713.
(7) (a) Webb, K. S.; Levy, D. Tetrahedron Lett. 1995, 36, 5117. (b)
Maleczka, R. E.; Shi, F.; Holmes, D.; Smith, M. R., III J. Am. Chem.
Soc. 2003, 125, 7792.
2534
Org. Lett., Vol. 13, No. 10, 2011

Figure 3. Calculated HOMO and LUMO levels of 1, 6, and 7 at
the B3LYP/6ꢀ31G(d) level. Calculated absorption wavelengths
by the TD-DFT method are shown in the parentheses.
HOMO level. This feature is specific to hydroxylation
at the 2,5,8,11-positions of PBI, which enables intra-
molecular hydrogen bonding with imide carbonyl
groups. In addition, borylated PBIs would serve as a
nice building block for molecular assemblies on the
basis of PBI though transition metal-catalyzed trans-
formations. Further direct functionalization of PBIs is
currently underway in our group.
Acknowledgment. This work was supported by Grant-
in-Aids for Scientific Research (Nos. 21685011 and
21108510 “pi-Space”) from MEXT, Japan and the Global
COE program in Chemistry of Nagoya University. H.S.
acknowledges Asahi Glass Foundation for financial
support.
Figure 2. (a) UV/vis absorption spectra of 1b (black), 2b, 6, and 7
measured in CH2Cl2. (b) Emission spectra of 1b, 2b, 6, and 7
measured in CH2Cl2. Excitation wavelength is at 450 nm.
Supporting Information Available. General procedures,
spectral data for compounds, and CIF file for the X-ray
analysis of 2a. This material is available free of charge via
the Internet at http://pubs.acs.org.
hydroxy group induced a substantial blue-shift in the
UV/vis absorption spectrum due to an intramolecular
hydrogen bonding interaction, which stabilizes the
Org. Lett., Vol. 13, No. 10, 2011
2535