Dioxygen-triggered transannular dearomatization of benzo[5]helicene diols: Highly efficient synthesis of chiral π-extended diones

Article abstract of DOI:10.1002/anie.201400486

Oxidative dearomatization is a powerful strategy in organic synthesis. However, reports on using dioxygen as the oxidant, as it is environmentally friendly, readily available (air), and easy to handle, are rather limited. Helicene diols can undergo transannular dearomatization triggered by dioxygen to give diones in quantitative yields within several minutes. By virtue of this, the chirality is successfully transferred from helicity to central chirality to form distorted π-extended diones having two all-carbon quaternary stereocenters. The optical resolution was achieved by column chromatography, and the structures and the absolute configurations of the chiral diones were determined by X-ray analysis. A bigger slice of π: In the title reaction, the helicity was successfully transferred into two quaternary all-carbon stereocenters of the π-extended diones. The optical resolution was easily achieved by column chromatography, and the absolute configurations were determined by X-ray analysis.

Full text of DOI:10.1002/anie.201400486

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Angewandte
Communications
DOI: 10.1002/anie.201400486
Synthetic Methods
Dioxygen-Triggered Transannular Dearomatization of
Benzo[5]helicene Diols: Highly Efficient Synthesis of Chiral
p-Extended Diones**
Yun Shen, Hai-Yan Lu, and Chuan-Feng Chen*
Abstract: Oxidative dearomatization is a powerful strategy in
organic synthesis. However, reports on using dioxygen as the
oxidant, as it is environmentally friendly, readily available
(air), and easy to handle, are rather limited. Helicene diols can
undergo transannular dearomatization triggered by dioxygen
to give diones in quantitative yields within several minutes. By
virtue of this, the chirality is successfully transferred from
helicity to central chirality to form distorted p-extended diones
having two all-carbon quaternary stereocenters. The optical
resolution was achieved by column chromatography, and the
structures and the absolute configurations of the chiral diones
were determined by X-ray analysis.
conjugated skeletons and intriguing helical chirality.[5]
Recently, optically pure helicenes having diverse functional
groups are employed in several different fields, such as high-
optical-rotation complexes,[6] organoelectronics,[7] asymmetric
gold catalysis,[8] redox-triggered chiroptical switches,[9] and
proteinlike tetrameric aabb-heteroaggregates.[10] Although
the stereoselective synthesis of helicenes has been realized by
virtue of organometallic catalysis[11] and Diels–Alder addi-
tions[12] during the last decade, most of the helicenes
employed in optically active forms, as in asymmetric syn-
thesis[13] and self-assembly,[14] were resolved by semi-prepa-
rative or preparative high performance liquid chromatogra-
phy (HPLC), which is an expensive and impractical strategy.
It has been a significant challenge to find a convenient and
efficient strategy to obtain optically pure helicenes.
Herein, we report a practical method for the synthesis of
helicenes and their optical resolution. Moreover, the helicene
diols could undergo highly efficient transannular dearomati-
zation reactions triggered by dioxygen (Figure 1). To the best
of our knowledge, this work represents the first example of
the transannular dearomatization reactions using dioxygen as
the oxidant.
T
he physical and chemical properties of aromatic materials
have been hot topics not only in academia but also in industry.
Dearomatization—the use of aromaticity as a functional
group—is considered as a powerful transformation.[1] Prac-
tical applications have been realized not only in the con-
struction of carbon–carbon and carbon–heteroatom
bonds,[1c,d] but also in materials science for memory and
switches.[2] For oxidative dearomatization, phenols are the
most frequently used substrates in the preparation of complex
compounds.[3] Recently, Koyama, Hiroto, and Shinokubo[4]
provided an exceptionally facile method to construct p-
extended [2,2]-metacyclophanes utilizing intermolecular oxi-
dative dearomatization. However, most of the oxidants used
in oxidative dearomatization are hypervalent iodine reagents
and transition-metal reagents, which are usually expensive,
highly reactive, and require harsh reaction conditions.[1c,d] The
reports on utilizing dioxygen, as a benign and clean oxidant, in
oxidative dearomatization, especially for the large p-conju-
gated systems, are rather limited.
Figure 1. Dioxygen-triggered transannular dearomatization reactions.
Helicenes, as a member of polycyclic aromatic hydro-
carbons formed by ortho-annulated aromatic or heteroaro-
matic rings, have attracted increasing interest because of their
Our work focuses on exploring efficient synthetic strat-
egies for functionalized helicenes and their applications in
asymmetric synthesis. Previously, our group reported an
efficient method to construct multifunctionalized benzo[5]-
helicenes.[15] Unfortunately, they were all optically unstable at
room temperature and could not be resolved. Thus, we
rationally designed an optically stable diol (1) as shown in
Figure 1: 1) the two methyl groups at C1 and C16 could
effectively hinder racemization; 2) the two hydroxy groups at
C2 and C15 could be used for optical resolution and as
binding sites; 3) the two Br atoms at C3 and C14 could be
modified after helicene formation to form chiral spaces.
Given the above-mentioned features, we first tried to
synthesize the 1,16-disubstituted helicene diols (Scheme 1).
Through the reductive coupling of 2a and 2b,[16] the dienes 3a
and 3b, respectively, were obtained.[17] The subsequent Diels–
Alder reactions proceeded smoothly, thus giving the adducts
[*] Y. Shen, Prof. C.-F. Chen
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Molecular Recognition and Function
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (China)
E-mail: cchen@iccas.ac.cn
Y. Shen, Prof. H.-Y. Lu
University of Chinese Academy of Sciences
Beijing 100049 (China)
[**] We thank the National Basic Research Program (2011CB932501),
and the National Natural Science Foundation of China (21272264,
21332008) for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400486.
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Table 1: The Suzuki–Miyaura cross-coupling reactions of 6 and the
dearomatization reactions of diaryl-functionalized helicenes 10a–e.
Entry Ar
Product Yield [%][a] Product Yield [%][b]
1
2
3
4
5
phenyl
3-thienyl
4-fluorophenyl
4-chlorophenyl
4-nitrophenyl
10a
10b
10c
10d
10e
96
90
93
85
75
11a
11b
11c
11d
11e
99
99
99
99
99
[a] Yield of product isolated after step a. [b] Yield of product isolated after
steps b and c.
According to the crystal structure, our proposed structure 8
was not correct. And the transannular dearomatization
reaction took place to give the dione 9 (Figure 1) with two
all-carbon quaternary stereocenters.
Scheme 1. Preparation of the dehydrobenzo[5]helicene 5 and benzo[5]-
helicene diol 1: a) Zn (2.0 equiv), TMSCl (4.0 equiv), conc. HCl, THF,
ꢀ788C to RT, 3 h; b) 2-carboxybenzene diazonium chloride (2.0 equiv),
DCE, 2-methyloxirane, reflux, 2 h; c) DDQ (10.0 equiv), xylene, reflux,
12 h; d) BBr3, CH2Cl2, 0–58C, 30 min. DCE=1,2-dichloroethane,
DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone, THF=tetrahydro-
furan, TMS=trimethylsilyl.
By comparison of the crystal structures of 5, 6, and 11a,
the helical framework was found to be more stable, from
a thermodynamic point of view, after the dearomatization
reactions. For 6, the torsional angles for C1-C16d-C16c-C16b,
C16d-C16c-C16b-C16a, and C16c-C16b-C16a-C16 were
28.758, 28.198, and 25.328, respectively (sum of 82.268). For
11a, they were 10.658, 8.788, and 2.978, respectively (sum of
22.48). By comparison, 5, which had a total torsion angle of
22.48 (1.238, 14.328, and 6.858 for each torsion) caused by the
repulsion of the two methoxy groups, it was fair to draw the
conclusion that the transannular dearomatization process
greatly released the torsional strain of the helical structure.
Moreover, for 11a, along the single bond between C1 and
C16, the two methyl groups are in an anti conformation
4a and 4b in excellent yields. After the oxidation of the
adducts by DDQ, 5 and 6 were obtained in satisfactory yields
(for the crystal structures of 5 and 6, see the Supporting
Information). Finally, the desired helicene diol 1 was pre-
pared quantitatively by demethylation.
With 1 in hand, we tried to achieve optical resolution and
determine the racemization barrier. Therefore, (1S)-(+)-10-
camphorsulfonyl chloride (7) was used to obtain the diaste-
reomers. Interestingly, the reaction was completed within
several minutes, and only a minute amount of the sulfonate
was obtained. According to the characterization results, the
quinone-like helical dione 8 (Figure 1) was supposed to be
produced. By screening the influence of various factors, we
found that this process was greatly accelerated under an
oxygen atmosphere rather than under inert conditions, and
light had negligible effects on the reaction (for details, see the
Supporting Information). Thus, we suggested that such
a transformation was achieved through a dioxygen-triggered
radical process.[3]
To investigate whether this transformation was universal,
five different aryl groups were introduced to the scaffold
(Scheme 2). After demethylation of 10a–e and subsequent
stirring in a CH2Cl2 solution open to the air, similar trans-
formations took place (Table 1). Fortunately, we obtained
a single crystal of 11a (for details, see the Supporting
Information) by slow evaporation of the acetone solution.
ꢀ
ꢀ
(173.128), while the C1 C2 and C16 C15 bonds are in
a gauche conformation (65.898). With the distance between
C1 and C16 decreasing from 3.077 ꢀ in 6 to 1.550 ꢀ in 11a,
the distance between the two oxygen atoms was shortened
from 4.685 ꢀ to 2.694 ꢀ, thus making the molecule a potential
bidentate ligand.
To obtain optically active diones the helicenes needed to
be first resolved. Since all the helicene diols were susceptible
to oxygen in the solution, we incorporated the chiral auxiliary
before the oxidation (Scheme 3). Given the solubility, we
chose the diphenyl-substituted Diels–Alder adduct as a pre-
cursor, which was readily synthesized from 4b in 95% yield.
By demethylation and reaction with 7, a mixture of diaste-
reomers of sulfonates was obtained. After the oxidation of the
sulfonates and the subsequent optical resolution by column
chromatography, (ꢀ)-(S,S,M)-13 and (+)-(S,S,P)-14 could be
À
Á
25
easily obtained, and their optical rotations ½aꢁD were found
to be ꢀ14608 and + 16608 (c = 1.0 mgmLꢀ1, CH2Cl2), respec-
tively.
[5]Helicenes usually undergo racemization even at room
temperature. Therefore, we examined the racemization
barrier of the 1,16-dimethylbenzo[5]helicene (DMB[5]H).
Because the sulfonates were transformed into diones after
hydrolysis, it was difficult for us to determine this barrier of
the optically pure helicene diols directly. Because (ꢀ)-
(S,S,M)-13 and (+)-(S,S,P)-14 had two O atoms, which are
smaller than a methyl group, at C2 and C15, it was reasonable
that the two chiral auxiliaries had a very small effect on the
racemization barrier.[18]
Scheme 2. The Suzuki–Miyaura cross-coupling reactions of 6 and the
dearomatization reactions of 10a–e: a) arylboronic acid (3.0 equiv),
[Pd(PPh3)4] (10 mol%), K2CO3 (10.0 equiv), toluene/EtOH/H2O (v/v/v,
2:2:1), reflux, 12 h; b) BBr3 (10.0 equiv), CH2Cl2, 0–58C, 20 min;
c) CH2Cl2, stirring under air, RT, 15 min.
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Figure 2. ECD spectra (c, 0.1 mm, CH2Cl2, 258C) of (ꢀ)-(S,S,M)-13,
(+)-(S,S,P)-14, (+)-(S,S)-11a, and (ꢀ)-(R,R)-11a.
Scheme 3. Synthesis and optical resolution of the diones: a) phenyl-
boronic acid (3.0 equiv), [Pd(PPh3)4] (10 mol%), K2CO3 (10.0 equiv),
toluene/EtOH/H2O (v/v/v, 2:2:1), reflux, 12 h; b) BBr3 (10.0 equiv),
CH2Cl2, 0–58C, 20 min; c) 7 (3.0 equiv), Et3N, DMAP (5 mol%),
CH2Cl2, RT, 5 h; d) DDQ (10.0 equiv), xylene, reflux, 12 h; e) optical
resolution by column chromatography (for details, see the Supporting
Information); f) KOH (10.0 equiv), CH2Cl2/MeOH (v/v, 1:1), 608C in
open air, 12 h. The de values were determined by 1H NMR spectros-
copy. The ee values were determined by HPLC using a chiral stationary
phase. DMAP=4-(N,N-dimethylamino)pyridine.
25
(½aꢁD ¼ + 2808 c = 1.0 mgmLꢀ1, CH2Cl2) and (ꢀ)-(R,R)-11a
25
(½aꢁD ¼ꢀ2708 c = 1.0 mgmLꢀ1, CH2Cl2) were obtained quan-
titatively. For the diones, a small negative Cotton effect (or
positive Cotton effect) at the maximum absorption peak was
found for (+)-(S,S)-11a [or (ꢀ)-(R,R)-11a] (Figure 2). To
make sure of the absolute configuration of the diones, we
prepared (+)-(S,S)-9 using a similar procedure and optical
resolution based on 4b. Fortunately, the single crystal of
(+)-(S,S)-9[21] was obtained, in which the Br atoms helped
ascertain the absolute configuration of the diones (see the
Supporting Information).
To investigate the racemization barrier of the DMB[5]H
core, we first performed variable-temperature 1H NMR
experiments (for details, see the Supporting Information).
According to the results, (ꢀ)-(S,S,M)-13 did not undergo
racemization, even at 383 K. Afterwards, we examined the
sample at higher temperatures, but, unfortunately, the
sulfonates decomposed (> 1508C). However, when keeping
the sample under 1208C for 12 hours, no racemization was
observed. So it is fair to conclude that 1,16-dimethylbenzo[5]-
helicenes do not undergo racemization at temperatures below
1208C. This temperature is sufficiently high that the majority
of common reactions could be performed without loss of
optical purity. Recently, Suemune, Usui, and co-workers[19]
found that the racemization barrier of 1-methyl[5]helicene
(DG° 38.6 kcalmolꢀ1, 473 K) was even higher than that of
[6]helicene (DG° 36.9 kcalmolꢀ1, 469 K). Our helicenes had
two methyl groups at the most sterically hindered positions, so
it was reasonable to say that 1,16-dimethylbenzo[5]helicenes
had better thermal stability than [6]helicene.
In conclusion, we have discovered a highly efficient
dioxygen-triggered transannular dearomatization reaction of
1,16-dimethylbenzo[5]helicene diols, and it resulted in chiral
p-extended diones, having two quaternary carbon centers, in
quantitative yields. Given the satisfactory thermal stability of
the scaffold, helicene sulfonates could be easily resolved by
column chromatography. Absolute configurations were
undoubtedly assigned as (+)-(P)-helicene diols [or (ꢀ)-(M)-
helicene diols] and gave (ꢀ)-(R,R)-diones [or (+)-(S,S)-
diones], as determined with the help of X-ray analysis. The
results presented herein will provide new opportunities for
the construction of chiral p-extended systems and chiral
bidentate ligands. The application of these p-extended diones
in asymmetric synthesis are under way in our laboratory.
Received: January 16, 2014
Revised: February 26, 2014
Published online: April 1, 2014
To assign the absolute configuration of the helicenes, we
compared the electronic circular dichroism (ECD) spectra
with those of other [5]helicenes.[20] Because the two chiral
auxiliaries had no absorption from l = 330 to 500 nm, the
ECD spectra (Figure 2) in this region displayed the signals of
helicity. For the reported [5]helicenes, the negative Cotton
effect (or positive Cotton effect) was always observed at the
maximum absorption peak for P helicenes (or M helicenes).
Moreover, as a general principle, laevorotatory (or dextro-
rotatory) carbohelicenes had M helicity (or P helicity). By
virtue of these points above, the absolute configurations of
the diastereomers were unambiguously assigned: (ꢀ)-13
possessed M helicity, while (+)-14 possessed P helicity.[5e]
After the reactions of hydrolysis and dearomatization
(Scheme 3), the optically active diones (+)-(S,S)-11a
Keywords: chiral resolution · dearomatization · helicenes ·
oxidation · synthetic methods
.
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