Rapid access to dibenzohelicenes and their functionalized derivatives

Article abstract of DOI:10.1002/anie.201301739

Spiraling up: Easy access to dibenzo[5]-, dibenzo[6]-, and dibenzo[7]helicenes (see picture) as well as their functionalized derivatives includes Sonogashira and Suzuki-Miyaura couplings, desilylation, and [2+2+2] alkyne cycloisomerization. The simplicity

Full text of DOI:10.1002/anie.201301739

Angewandte
Chemie
DOI: 10.1002/anie.201301739
Helical Structures
Rapid Access to Dibenzohelicenes and their Functionalized
Derivatives**
ˇ ˇ
ˇ
ˇ
ˇ
Andrej Jancarꢀk, Jirꢀ Rybꢁcek, Kevin Cocq, Jana Vacek Chocholousovꢁ, Jaroslav Vacek,
ˇ
´
Radek Pohl, Lucie Bednꢁrovꢁ, Pavel Fiedler, Ivana Cꢀsarovꢁ, Irena G. Starꢁ,* and Ivo Stary*
Dedicated to E. Peter Kꢂndig
Visions of applying unique helically chiral p-electron systems
in diverse areas of chemistry and physics have fueled the
intensifying research aimed at the development of a practical
and general methodology for the preparation of functional-
ized helicenes and their heteroanalogues.[1] Despite the fact
that significant progress has recently been achieved in this
regard,[1a–e] there is plenty of opportunity to substantially
shorten, simplify, and generalize the synthesis of helicenes. If
such an endeavor meets with success, then an on-demand
preparation of tailormade helicenes would ultimately not be
a limiting step for their wider exploitation.
dibenzohelicenes 1–3 represent, in fact, a formal fusion of
picene (i.e., [5]phenacene) with the corresponding parent
carbohelicenes. Recently, Tanaka et al. developed the syn-
thesis of complementary 1,1’-bitriphenylenes by a rhodium-
catalyzed double [2+2+2] cycloaddition of biaryl-linked
tetraynes with 1,4-diynes,[4a] thus providing other dibenzo
analogues of parent helicenes. Durola et al. synthesized tert-
butylated dibenzo[5]helicene and its triply helical analogue
by a Scholl cyclization featuring an intriguing regioselectivi-
ty,[4b] and Siegel and Ernst et al. reported the new dibenzo[5]-
helicene 1 as prepared from difluoroquinquephenyl by an
Herein, we report on the development of a simple and
versatile synthesis of a series of dibenzo[5]-, dibenzo[6]-, and
dibenzo[7]helicenes (1–3; Figure 1)[2] and their functionalized
derivatives, including heteroanalogues. Construction of
dibenzohelicenes relies on the pivotal Sonogashira and
Suzuki–Miyaura couplings to assemble the key aromatic
triynes, with a subsequent [2+2+2] alkyne cycloisomeriza-
tion[3] to form the helical backbones. The structures of the
intramolecular Friedel–Crafts-type arylation through silylium
[4c]
ꢀ
ion promoted C F bond activation.
There are good reasons for paying attention to dibenzo-
helicenes: 1) The presence of two extra annulated benzene
rings simplifies the synthesis and makes it more efficient (see
below), 2) the methodology allows the preparation of fully
aromatic, helically chiral systems, which are known to exhibit
remarkable chiroptical properties,[5] and 3) the lateral exten-
sion of the aromatic helix by additional benzene ring(s) can be
advantageous for efficient chirality transfer in enantioselec-
tive reactions mediated by helicene-based catalysts.[6] Extend-
ing the aromatic scaffold raises a practical question about its
solubility, which could be substantially lowered owing to an
extensive intermolecular p–p stacking. However, the twist of
dibenzohelicenes (i.e., nonplanarity) should minimize such an
interaction (see below).
Nowadays, the intra/intermolecular [2+2+2] cycloisome-
rization of alkynes has been established as a standard method
for the synthesis of helical (hetero)aromatics as reported by
Figure 1. Dibenzo[5]-, dibenzo[6]-, and dibenzo[7]helicene (1–3).
our group,[7] Vollhardt et al.,[8] Teply et al.,[9] Tanaka et al.,[4a]
´
ˇ ˇ
ˇ
ˇ
[*] A. Jancarꢀk, Dr. J. Rybꢁcek, K. Cocq, Dr. J. Vacek Chocholousovꢁ,
Dr. J. Vacek, Dr. R. Pohl, Dr. L. Bednꢁrovꢁ, Dr. P. Fiedler,
Shibata et al.,[10] Carbery et al.,[11] and Diederich et al.[12] By
utilizing this methodology, we previously carried out the
direct cycloisomerization of aromatic Z,Z dienetriynes into
helicenes (4!5; Scheme 1).[7d] As this method of helicene
synthesis has limits in the chemical stability (and accessibility)
of the starting Z,Z dienetriynes, we propose herein their
stabilized analogues as exemplified by 4 where the (Z)-1,2-
vinylene fragments are embedded in the ortho-phenylene
moieties. Provided cycloisomerization is feasible, the forma-
tion of 1 from 4 should be highly exergonic as the calculated
value of DG at 258C in THF is ꢀ124.7 kcalmolꢀ1 (comparable
to DG = ꢀ135.1 kcalmolꢀ1 for 4!5).[13]
´
Dr. I. G. Starꢁ, Dr. I. Stary
Institute of Organic Chemistry and Biochemistry
Academy of Sciences of the Czech Republic
Flemingovo nꢁm. 2, 166 10 Prague 6 (Czech Republic)
E-mail: stara@uochb.cas.cz
stary@uochb.cas.cz
ˇ
Dr. I. Cꢀsarovꢁ
Department of Inorganic Chemistry, Faculty of Science
Charles University in Prague, Prague 2 (Czech Republic)
[**] This work was supported by the Czech Science Foundation (P207/
10/2207), the Ministry of Education, Youth and Sports of the Czech
Republic (MSM0021620857), and the Institute of Organic Chemis-
try and Biochemistry, Academy of Sciences of the Czech Republic
(RVO: 61388963). Reinhard Kaiser is acknowledged for his
contribution to the synthesis development.
Indeed, the incorporation of the ortho-phenylene tethers
into the aromatic triynes like 4 substantially simplified their
synthesis whether they were symmetrical or not. Based purely
on the powerful coupling methodologies (except for the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301739.
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Communications
the case of the synthesis of symmetrical triynes, trimethyl-
silylacetylene in the first Sonogashira coupling was replaced
by gaseous acetylene.[16]
Thus having a representative series of triynes 4, 11–18 and
27–31 including functionalized ones, which were stable (in
contrast to the Z,Z dienetriynes mentioned above) and
prepared basically within three to five synthetic operations,
we embarked on studying their intramolecular [2+2+2]
cycloisomerization to dibenzohelicenes (Table 1 and
Table 2). Gratifyingly, the test model triyne 4 underwent
a smooth and clean cyclization reaction under Ni0/PPh3
catalysis, thus affording a fully aromatic corresponding
dibenzo[5]helicene 1 in nearly quantitative yield (Table 1,
entry 1). Encouraged by this result, we screened a collection
of related triynes (11–18) and found that they gave rise to new
functionalized dibenzo[5]helicenes (19–26) in good to high
yields (Table 1, entries 2–9). To our delight, the nickel(0)-
catalyzed cyclization exhibited a broad tolerance to substitu-
ents and functional groups such as Ph (entry 2), CH3
(entries 3, 4, 6, and 7), CH3O (entries 4 and 5), Cl (entry 6),
NO2 (entry 7), and the incorporated pyridine unit(s) placed at
the termini of the helicene scaffold (entries 8 and 9). Note
that the presence of the CH3 group in position 1 of 20, 21, 23,
and 24 significantly increases the racemization barrier of the
dibenzo[5]helicene scaffold.
Scheme 1. [2+2+2] Alkyne cycloisomerization in the formation of
a helicene scaffold; a) [Ni(cod)2], PPh3, 64%.[7d] cod=cyclo-1,5-octa-
diene.
desilylation step(s); see below), we developed complemen-
tary step-economic, high-yielding, synthetic protocols, which
we have generally used in the preparation of the desired
aromatic triynes as illustrated in Scheme 2. To obtain unsym-
metrical triynes such as the prototypal 11, the diaryl acetylene
8 was prepared from the aryliodides 6 and 7 and trimethyl-
However, the versatility of any methodology for the
helicene synthesis is judged by the fact of whether it is suitable
for the preparation of higher homologues of [5]helicene. We
therefore attempted the preparation of dibenzo[6]- and
dibenzo[7]helicenes (Table 2). The triynes 27–31 were assem-
bled from dihalogenated benzene/naphthalene building
blocks using the straightforward Sonogashira and Suzuki–
Miyaura coupling methodology (for details, see the Support-
ing Information).
For the sake of pursuing intramolecular [2+2+2] cyclo-
isomerization and thus forming the helicene backbone, the
triynes 27–31 were treated with a catalytic amount of the in
situ generated Ni0/PPh3 complex. To our delight, the triynes
27–29 and 31 provided the corresponding dibenzo[6]-, azadi-
benzo[6]-, or dibenzo[7]helicenes in good to high yields
regardless of whether the tethered alkyne units were terminal
or substituted by p-tolyl groups (Table 2, entries 1–3 and 5).
However, to our disappointment, we failed to cyclize the
triyne 30 into the dibenzo[7]helicene 3 because the starting
material polymerized (Table 2, entry 4, reaction condi-
tions A). We were therefore obliged to screen other reaction
conditions. Indeed, the use of PCy3 instead of PPh3 in the
nickel catalysis suppressed polymerization and promoted
cyclization to deliver the desired dibenzo[7]helicene 3 in good
yield (Table 2, entry 4, reaction conditions B). Additionally,
the conversion of 30 into 3 could also be enforced by using the
[CpCo(CO)(fum)] catalyst[17] under microwave irradiation
(Table 2, entry 4, reaction conditions C). The single-crystal
analyses of racemic 2 and 25, which formed racemic
compounds in the solid state, were performed to confirm
the structures (for details, see the Supporting Information).
Analytical samples of the racemic dibenzohelicenes 2, 3,
and 20 were resolved to optically pure enantiomers by HPLC
on a Chiralpak IA column (n-heptane/chloroform 70:30; for
Scheme 2. The general synthetic route to the unsymmetrical aromatic
ꢁ
triynes such as 11: a) TMS-C CH (1.0 equiv), [Pd(PPh3)2Cl2] (6 mol%),
CuI (10 mol%), diisopropylamine (6.0 equiv), benzene, room temper-
ature, 2.5 h, then 7 (1.0 equiv), DBU (excess), water (40 mol%), 458C,
18 h, 75%; b) 9 (2.5 equiv), [Pd(PPh3)2Cl2] (8 mol%), K2CO3
(2.0 equiv), toluene/ethanol/water (4:4:1), 908C, 2 h, 75%; c) nBu4NF
(2.2 equiv), THF, room temperature, 15 min, 97%. DBU=1,8-
diazabicyclo[5.4.0] undec-7-ene, THF=tetrahydrofuran, TIPS=triiso-
propylsilyl.
silylacetylene by a chemoselective Sonogashira coupling/in
situ desilylation/chemoselective Sonogashira coupling
sequence as a one-pot operation (for details, see the
Supporting Information).[14] Alternatively, such a multistep
protocol was modified if a product of the first Sonogashira
coupling (i.e., trimethylsilylethynyl aromatics) was commer-
cially available or the stepwise approach to diarylacetylene
resulted in its higher overall yield. The ortho-phenylene-
tethered alkyne units were attached by the treatment of 8 with
the arylboronic acid pinacol ester 9 (or with the correspond-
ing arylboronic acid)[15] under the Suzuki–Miyaura coupling
conditions to receive the protected triyne 10, which upon
smooth desilylation provided the key aromatic triyne 11. In
2
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Table 1: [2+2+2] Cycloisomerization of the aromatic triynes 4 and 11–18 into
the corresponding dibenzo[5]helicenes.
Table 1: (Continued)
Entry Triyne
Cond.[a]
Helicene[b]
Yield [%][c,d]
Entry
Triyne
Cond.[a]
Helicene[b]
Yield [%][c,d]
85
(60%, 4)
8
17
18
A
25
26
96
(67%, 4)
1
4
A
1
80
(31%, 4)
9
A
88
(58%, 5)
2
12
A
19
[a] Reaction conditions A: [Ni(cod)2] (20 mol%), PPh3 (40 mol%), THF, room
temperature, 10–40 min. [b] Racemates; only P enantiomers are shown for
clarity. [c] Yield of isolated product. [d] The overall yield of the synthetic
sequence starting from commercial chemicals and the number of steps (one-
pot operations) are given within parentheses.
86
3
4
5
13
11
14
A
A
A
20
21
22
(24%, 5)
details, see the Supporting Information). We could see that
the formal annulation of two benzene rings to the backbone
of the parent 1-methyl[5]-, [6]-, and [7]helicene significantly
diminished the specific optical rotatory power of 2, 3, and 20,
respectively (Table 3, entries 3 versus 4 and 5 versus 6), but
the barriers to racemization were affected only slightly
(Table 3, entries 1 versus 2, 3 versus 4, and 5 versus 6). To
assign the helicity of the individual enantiomers of the
representative dibenzohelicenes 2, 3, 20, and 32 we correlated
their experimental ECD spectra with those of parent [5]-, [6]-
and [7]helicenes or, as given for (ꢀ)-(M)-20, with the
calculated ECD spectrum (for details, see the Supporting
Information). Given the results, the rule of thumb for helicene
chemistry, that is, fully aromatic systems are of P helicity (or
M helicity) if they are dextrorotatory (or laevorotatory), was
obeyed in the case of dibenzohelicenes.
91
(50%, 4)
98
(54%, 4)
From a practical and economic point of view, an asym-
metric synthesis of nonracemic dibenzohelicenes by enantio-
selective [2+2+2] alkyne cycloisomerization would be the
most advantageous. Thus, based on our previous experi-
ence,[24] we focused on the challenging enantioselective
nickel(0) catalysis employing axially chiral monophosphines
(Table 4). Indeed, the cyclization of the the model triyne 27
provided the dibenzo[6]helicene 2 with moderate ee values if
BOP,[25] or N- and O-PINAP[26] ligands were used (Table 4,
entries 1–3). The best ligand we identified was QUINAP,[27]
which allowed achievement of up to 80% ee when conducting
the cyclization at ꢀ208C (Table 4, entries 4 and 5). A further
decrease in temperature resulted, however, in unreactive 27
(Table 4, entry 6). To make the transition state of cyclization
more sterically congested and, perhaps, to achieve higher
enantiocontrol, we attached p-tolyl groups to the pendant
alkyne units as in the triyne 28. Then, by using QUINAP, we
obtained 32 in 85% ee when performing the cyclization at
room temperature and 87% ee when lowering the temper-
84
(55%, 4)
6
7
15
16
A
A
23
24
63
(53%, 4)
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Table 2: [2+2+2] Cycloisomerization of aromatic the triynes 27–31 into the
corresponding dibenzo[6]- and dibenzo[7]helicenes.
Table 3: Optical rotations and barriers to racemization of representative
dibenzohelicenes (2, 3, and 20) compared to the properties of the parent
helicenes.
Entry
Triyne
Cond.[a]
Helicene[b]
Yield [%][c,d]
25[a]
¼
Entry Dibenzohelicene or helicene
½aꢂD
DDG [kcalmolꢀ1
]
1
2
3
4
5
6
1-methyldibenzo[5]helicene (20)
1,3,6-trimethyl[5]helicene
dibenzo[6]helicene (2)
[6]helicene
+529[b]
37.5[c]
38.7[d]
35.8[f]
36.2[h]
40.9[c]
42.5[k]
+1470[e]
+3750[g]
+1773[i]
+5900[j]
92
(53%, 5)
1
27
A
2
dibenzo[7]helicene (3)
[7]helicene
[a] Optical rotations of (+)-enantiomers given; measured at l=589 nm
in CHCl3 (unless stated otherwise). [b] Measured in CH2Cl2, 98% ee.
[c] Determined in hexadecane at 2308C. [d] Determined in
(nBuOCH2CH2)2O at 2008C (Ref. [19]). [e] 95% ee. [f] Determined in
hexadecane at 2008C. [g] Ref. [20]. [h] Determined in naphthalene at
1888C.[21] [i]>99% ee. [j] Determined at l=579 nm.[22] [k] Determined in
naphthalene at 2398C.[23]
90
(43%, 6)
2
3
4
28
29
30
A
32
33
3
Table 4: Enantioselective [2+2+2] cycloisomerization of the aromatic
triynes 27 and 28 into the dibenzo[6]helicenes 2 and 32, respectively.
68
(39%, 5)
A
0
75
(34%, 5)
80
A
B
C
Entry Tri- Ligand
yne
Reaction
conditions
Product ee [%][a]
Yield
[%][b]
(36%, 5)
1
2
3
4
5
6
7
8
27 (S)-BOP[c]
RT, 10 min
(ꢀ)-2
(+)-2
(+)-2
(+)-2
31
33
>90
>90
>90
>90
>90
–
27 (R)-N-PINAP[c] RT, 10 min
27 (R)-O-PINAP[c] RT, 10 min
27 (R)-QUINAP[d] RT, 10 min
40
72 (95)[e]
27 (R)-QUINAP[c] ꢀ208C, 16 h (+)-2
80
–
27 (R)-QUINAP[c] ꢀ408C, 24 h
28 (R)-QUINAP[c] RT, 5 h
28 (R)-QUINAP[c] 08C, 16 h
–
(+)-32 85 (>99)[e]
(+)-32 87
80
30
80
(30%, 6)
5
31
A
34
[a] Determined by HPLC on a Chiralpak IA column (250ꢂ4.6 mm, 5 mm,
n-heptane–chloroform 85:15, 1.0 mLminꢀ1). [b] Yields determined by
1H NMR spectroscopy using an internal standard (p-methoxyacetophe-
none). [c] Ni: 20 mol%, L*: 40 mol%). [d] Ni: 3 mol%, L*: 6 mol%.
[e] After single crystallization from THF/2-propanol (ca. 60% yield).
[a] Reaction conditions A: [Ni(cod)2] (20 mol%), PPh3 (40 mol%), THF, room
temperature, 10 min. Reaction conditions B: [Ni(cod)2] (20 mol%), PCy3
(40 mol%), THF, room temperature, 10 min. Reaction conditions C:
[CpCo(CO)(fum)] (1.0 equiv), 1-butyl-2,3-dimethylimidazolium tetrafluorobo-
rate (25 mL/1 mL), THF, microwave reactor, 1808C, 10 min. [b] Racemates.
Only P enantiomers are shown for clarity. [c] Yield of isolated product. [d] The
overall yield of the synthetic sequence starting from commercial chemicals and
the number of steps (one-pot operations) are given within parentheses.
Cp=cyclopentadienyl, fum=dimethyl fumarate, Tol=p-tolyl.
ature to 08C, thus identifying the reactivity limit of 28
(Table 4, entries 7 and 8). Finally, a single crystallization of
the enantioenriched samples of 2 and 32 from THF/2-
propanol allowed further increase in their ee values to 95%
and 99%, respectively, with approximately 60% yield from
the crystallization (Table 4, entries 4 and 7).
their functionalized derivatives. These helically chiral aro-
matics can usually be synthesized within four to six operations
in overall yields ranging from 24% to 67% by employing
a short sequence of reliable processes such as Sonogashira
coupling, Suzuki–Miyaura coupling, desilylation, and
[2+2+2] alkyne cycloisomerization. With a few exceptions,[4]
there are only scattered examples of dibenzohelicenes
In summary, we have developed a general approach to
dibenzo[5]-, dibenzo[6]-, and dibenzo[7]helicenes as well as
4
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Chemie
described in the literature[2] and this study on their prepara-
tion and properties is so far the most comprehensive.
Dibenzohelicenes have an advantage over the parent heli-
cenes because of the simplicity of their non-photochemical
preparation and, therefore, they have the potential to mimic
or even substitute parent helicenes in envisaged applications.
Moreover, we have demonstrated one of the highest degrees
of enantiocontrol (see Ref. [4a], [10], and [28]) in the
asymmetric synthesis of helicenes by transition-metal-cata-
lyzed [2+2+2] alkyne cycloisomerization, and developed
a straightforward approach to the optically pure paradigmatic
dibenzo[6]helicene.
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Received: February 28, 2013
Revised: June 13, 2013
Published online: && &&, &&&&
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Angew. Chem. Int. Ed. 2013, 52, 1 – 7
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Angewandte
Chemie
Communications
Helical Structures
ˇ ˇ
ˇ
A. Jancarꢁk, J. Rybꢂcek, K. Cocq,
ˇ
J. Vacek Chocholousovꢂ, J. Vacek, R. Pohl,
ˇ
L. Bednꢂrovꢂ, P. Fiedler, I. Cꢁsarovꢂ,
´
I. G. Starꢂ,* I. Stary*
&&&& — &&&&
Rapid Access to Dibenzohelicenes and
their Functionalized Derivatives
Spiraling up: Easy access to dibenzo[5]-,
dibenzo[6]-, and dibenzo[7]helicenes (see
picture) as well as their functionalized
derivatives includes Sonogashira and
Suzuki–Miyaura couplings, desilylation,
and [2+2+2] alkyne cycloisomerization.
The simplicity of this non-photochemical
approach combined with the potential for
helicity control favors dibenzohelicenes
over the parent helicenes for practical
applications.
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!