Oxidative skeletal rearrangement of 1,1′-binaphthalene-2,2′- diamines (BINAMs) via C-C bond cleavage and nitrogen migration: A versatile synthesis of U-shaped azaacenes

Article abstract of DOI:10.1039/c4cc04911j

An oxidative skeletal rearrangement of 1,1′-binaphthalene-2,2′- diamines (BINAMs) that involves the cleavage of a strong C-C single bond of the binaphthalene unit and the nitrogen migration has been discovered. The unprecedented rearrangement enables access to a series of U-shaped azaacenes otherwise difficult to prepare in a selective manner by classical methods. Moreover, physicochemical properties of the unique azaacenes have been comprehensively investigated. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04911j

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Oxidative skeletal rearrangement of 1,10-
binaphthalene-2,20-diamines (BINAMs) via C–C
bond cleavage and nitrogen migration: a versatile
synthesis of U-shaped azaacenes†
Cite this: Chem. Commun., 2014,
50, 10291
Received 27th June 2014,
Accepted 13th July 2014
Youhei Takeda,*ab Masato Okazakib and Satoshi Minakata*b
DOI: 10.1039/c4cc04911j
www.rsc.org/chemcomm
An oxidative skeletal rearrangement of 1,10-binaphthalene-2,20- of a Pd-catalyzed double amination of activated ortho-dichloroarenes
diamines (BINAMs) that involves the cleavage of a strong C–C single with ortho-arylenediamines and a successive oxidation leading to
bond of the binaphthalene unit and the nitrogen migration has been linear azaacenes in an efficient manner, has been reported.5
discovered. The unprecedented rearrangement enables access to a On the other hand, Hiroto and Shinokubo have reported a
series of U-shaped azaacenes otherwise difficult to prepare in a DDQ-promoted oxidative annulation of aromatic amines to selec-
selective manner by classical methods. Moreover, physicochemical tively give kinked azaacenes and aza[7]helicenes depending on the
properties of the unique azaacenes have been comprehensively conditions applied.6 Herein we present a new strategy to synthesize
investigated.
azaacenes based on the discovery of an unprecedented oxidative
skeletal rearrangement of 1,10-binaphthalene-2,20-diamines
Azaacenes, in which parts of CQCH fragments of acenes are (BINAMs) induced by an iodine-containing oxidant, which
formally replaced with isoelectronic imine (CQN) moieties, exclusively provides a series of dibenzo[a, j]phenazine-cored
have been emerging as promising candidates for electron- U-shaped azaacenes (Scheme 1a). As a relevant work, there is
transporting (n-type) materials in organic electronics.1 The a single report that describes selective preparation of dibenzo[a, j]-
introduction of electronegative nitrogen atoms into p-conjugated phenazine through an oxidative cross-annulation of a- and
carbon frameworks allows increasing electron affinities (EAs) to b-aminonaphthalenes mediated by a strong base (t-BuOK)
lower the barriers for electron injection from metal electrodes.2 under an O2 atmosphere (Scheme 1b).7 However, this known
Furthermore, owing to the decrease in the number of C–Hꢀꢀꢀp method intrinsically requires two regioisomers of amino-
contacts, azaacenes show a high propensity for adopting densely naphthalenes and suffers from the concomitant production
stacked packing-structures in the solid states, which favor a of a constitutional [a,h]-isomer, thereby lacking the versatility
carrier-transporting process.3 However, due to limited synthetic as a synthetic approach to U-shaped azaacenes.
accessibility of azaacenes, the exploration of azaacene-based
During our studies on the development of oxidative trans-
materials has fatally lagged behind those of acenes. Therefore, formations of various amines utilizing iodine-containing oxidants,8
the development of novel synthetic approaches to azaacenes we serendipitously found out that the treatment of BINAM (1a) with
is fundamentally important from the viewpoints of not only tert-butyl hypoiodite (t-BuOI)9 produced dibenzo[a,j]phenazine (2a)
synthetic chemistry but also materials science. Traditionally,
syntheses of azaacenes have exclusively relied on condensation/
oxidation protocols starting from ortho-arylenediamines and
ortho-quinone derivatives.4 In this context, recently, a new
synthetic protocol toward the preparation of azaacenes consisting
a Frontier Research Base for Global Young Researchers, Graduate School of
Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan
b Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan.
E-mail: takeda@chem.eng.osaka-u.ac.jp, minakata@chem.eng.osaka-u.ac.jp
† Electronic supplementary information (ESI) available: Synthetic procedures,
spectroscopic data, NMR spectra, and physicochemical properties. CCDC
1004407. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c4cc04911j
Scheme 1 Construction of dibenzophenazine-based U-shaped azaacenes.
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Table 2 Scope and limitation of the rearrangement of 1a
in 28% yield along with the production of reasonably expected
compounds 3a10 (15%) and 4a11 (7%) [eqn (1)].
(1)
The structure of 2a was unambiguously confirmed by the
X-ray crystallographic analysis of its yellow single crystal (ESI†).
Most importantly, the discovered transformation leading to 2a
formally involves the cleavage of the strong C(Ar)–C(Ar) bond
[a in eqn (1)] of the binaphthyl unit (e.g., the bond dissociation
energy (BDE) of Ph–Ph is as large as 118 kcal molꢁ1 12
without
)
the aid of transition metal complexes13 and also involves the
migration of a nitrogen atom [b in eqn (1)] to the adjacent
carbon. Encouraged by this finding, we envisioned that the
utilization of this unique rearrangement reaction would offer a
versatile route to U-shaped azaacenes. To establish a novel
approach to synthesize U-shaped azaacenes, reaction parameters
were scrutinized to identify the conditions to selectively provide
2a (ESI†). Table 1 summarizes the effect of halogen-containing
oxidants. To our delight, the use of N-iodolactams as oxidants
like NIS, NIPy, and DIH in t-BuOH selectively gave 2a in
moderate yields along with trace amounts of 4a without afford-
ing 3a. The use of 8 mol equivalents of DIH gave 2a in 77%
yield, although the role of excess of DIH is not clear at present.
In sharp contrast, when N-chloro-containing oxidant (t-BuOCl)
was used, diaza[5]helicene 3a was exclusively formed in a high
yield (89%). Other types of oxidants like I2, NBS, DDQ, PhI(OAc)2,
and MnO2 failed to furnish 2a.
a
Reaction conditions: 1 (0.2 mmol) and DIH (1.6 mmol) in t-BuOH
b
(20 mL) at rt for the indicated time. MeOH was used as a solvent.
Having identified the optimal conditions for the rearrange-
ment reaction, the scope and limitation of BINAM substrates
were surveyed (Table 2). The treatment of BINAMs bearing
two electron-donating substituents (Me and MeO) at the
3,30-position with DIH successfully provided azaacenes 2b
and 2c in 66% and 96% yields, respectively. The reaction using
3,30-dibrominated BINAM 1d gave desired product 2d in a low
yield, probably because of the poor solubility of the diamine in
t-BuOH. Notably, the rearrangement of 3,30-diphenyl diamine
1e also efficiently proceeded to afford 2e in a high yield, indicating
that steric hindrance around the amino moieties does not affect
the reaction efficiency. In sharp contrast, diamine bearing two
ester groups (CO2Me) at the 3,30-position did not undergo the
rearrangement at all. This significant lack of reactivity might be
ascribed to the low nucleophilicity of amino moieties, inhibiting
the formation of N–I bonds (vide infra). Diamines bearing the 7,70-
and 6,60-disubstituent also underwent rearrangement to selectively
provide 2f and 2g in moderate yields. Notably, the rearrangement
of bianthracene diamine 1h gave highly conjugated heptacyclic
azaacene 2h, which should be quite difficult to synthesize by
conventional organic reactions.
Table 1 Effect of halogen-containing oxidantsa,b
Regarding mechanistic aspects, importantly, the existence of
at least one naphthalene unit is indispensable for the unique
rearrangement (Scheme 2): when biaryldiamines bearing one (5) or
no (8) naphthyl unit were subjected to the reaction conditions, the
ratios of rearranged products/benzocinnoline products were found
to be positively correlated with the number (n) of naphthalene
units. These results might indicate the involvement of dearoma-
tization processes in the reaction pathway (vide infra), which would
be more favorable with naphthalene units than benzene units.14
On the other hand, a cross-over experiment applying an equimolar
mixture of 1a and 1c resulted in no production of cross-products at
a
Reaction conditions: 1a (0.2 mmol) and oxidant (0.8 mmol) in t-BuOH
b
(20 mL) at rt for 3 h. Yields were determined by the integration of
1H NMR charts of the crude products. Isolated yield.
c
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Scheme 2 Influence of a naphthalene unit.
Fig. 1 A tentative pathway of the rearrangement.
all (Scheme 3), and a control experiment using b-naphthylamine
under the optimal conditions led to the production of the
[a,h]-isomer without the formation of 2a [eqn (2)], suggesting
the intramolecularity of the rearrangement.
(2)
Although a precise mechanism should await further investiga-
tion, in conjunction with the accumulated knowledge of the reacti-
vities of aromatic amines toward electrophilic iodinating oxidants,
a tentative reaction pathway is illustrated in Fig. 1. The reaction
would start with exchange of N–H hydrogen(s) with iodine, leading
to mono or di N-iodoamine A.8 The dearomatization process to form
azirine B could explain the indispensability of the naphthyl unit for
the rearrangement.15 Upon the formation of B, the highly strained
intermediate would be attacked by an intramolecular amino nucleo-
phile (Ar–NH2), and the developing anion would be trapped by the
extra iodinating reagent to form C.16 Nitrene D could be generated
from C driven by the release of its strain energy, and the subsequent
insertion of nitrene to the C(Ar)–C(Ar) bond would give phenazine
2a. Alternatively, the path from C to 2a could be interpreted as a
variant of Stieglitz rearrangement.17 Carbazole 4a might be produced
through a [3,3]-sigmatropic rearrangement of the diimine that would
be formed by the protonation of 1a with generated HI, and followed
by the elimination of ammonia.11
Scheme 4 Pd-catalyzed functionalization of 2g.
In light of bromo functionality of 2d and 2g, these compounds
should serve as useful building blocks for functionalized azaacenes.
Synthetic versatility of 2g was clearly demonstrated by the prepara-
tion of 2i–2l via Pd catalysis (Scheme 4). With slight modifications
of the Nolan’s original conditions,18 NHC/Pd-catalyzed cross-
coupling of 2g with an arylboronic acid and cyclic amines gave
the corresponding coupled products 2i–2k in good yields.
Furthermore, TIPS-acetylene was efficiently cross-coupled with
2g to afford conjugation-extended azaacene 2l in 76% yield.
Basic physicochemical properties of U-shaped azaacenes 2
were investigated. UV/Vis and fluorescence (FL) spectra as well
as a cyclic voltammogram for 2a are shown in Fig. 2 as a
representative example (for other compounds, see the ESI†).
The mirror-image type UV and FL spectra with fine vibrational
structures and a small Stokes shift (9 nm) reflects the rigid
structure of 2a like typical (aza-)acenes (Fig. 2a). Likewise,
diluted CH2Cl2 solutions of other azaacenes 2 emit fluorescence
ranging from blue (lem = 425 nm) to yellow (lem = 561 nm)
depending on the substituents on the conjugated core. Especially,
the introduction of sterically demanding and strongly electron-
donating groups like piperidino and morpholino functionalities
(i.e., 2j and 2k) resulted in significant Stokes shifts and greatly
enhanced quantum yields (2a: lem = 425 nm, Ff 0.14; 2j: lem
=
561 nm, Ff 0.47; 2k: lem = 543 nm, Ff 0.42), probably ascribed to
intramolecular charge-transfer (ICT) emission. Notably, most of
the azaacenes 2 showed one pair of reversible redox waves
at the potentials ranging from ꢁ1.76 to ꢁ1.98 V against the
Fc/Fc+ redox couple, as shown in Fig. 2b, indicating the good
Scheme 3 Cross-over experiment.
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In conclusion, we have discovered an unprecedented oxidative
skeletal rearrangement of BINAMs and established an approach
to constructing novel types of azaacenes that are otherwise
difficult to synthesize by conventional methodologies. Investi-
gation into the creation of functional conjugated materials based
on these unique azaacenes are ongoing in our laboratory.
We deeply thank Professor Toshiyuki Moriuchi of Osaka
University for his assistance in single-crystal X-ray diffraction
experiments. This research was partly supported by the research
Grants from the Izumi Science and Technology foundation and
from the Murata Science Foundation, the Sasakawa Scientific
Research Grant from the Japan Science Society, and the Research
Encouragement Grants from The Asahi Glass Foundation (to Y.T.).
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