One-step construction of five successive rings by rhodium-catalyzed intermolecular double [2+2+2] cycloaddition: Enantioenriched [9]helicene-like molecules

Full text of DOI:10.1002/anie.200901962

Communications
DOI: 10.1002/anie.200901962
Helical Structures
One-Step Construction of Five Successive Rings by Rhodium-
Catalyzed Intermolecular Double [2+2+2] Cycloaddition:
Enantioenriched [9]Helicene-Like Molecules**
Ken Tanaka,* Naohiro Fukawa, Takeshi Suda, and Keiichi Noguchi
Helicenes and helicene-like molecules have long attracted
much attention because of their potential applications to
optical or electronic functional materials.[1] Therefore, flexible
as well as convenient methods for their syntheses are highly
desired for structural alterations and supply of sufficient
quantities.[2] The most frequently employed method for the
synthesis of helicenes is the oxidative photocyclization of
stilbene-type precursors, however this procedure cannot be
Scheme 1. Rhodium-catalyzed intramolecular [2+2+2] cycloaddition
conducted on a large scale because of the highly dilute
reaction conditions.[3] Several non-photochemical methods
for the synthesis of [6]- and [7]helicenes and helicene-like
molecules have been developed to date.[4] As such, transition
metal mediated intramolecular [2+2+2] cycloadditions of
triynes are useful methods for the synthesis of [6]- and
[7]helicene-like molecules through the formation of three
successive rings.[5,6] Starꢀ and co-workers pioneered this
strategy by using cobalt- or nickel-mediated or catalyzed
[2+2+2] cycloadditions.[5] Following this pioneering work, we
recently reported a cationic rhodium(I)/chiral bisphosphine
complex, which catalyzed the intramolecular [2+2+2] cyclo-
additions of 2-naphthol-linked triynes, leading to enantioen-
riched [7]helicene-like molecules (Scheme 1).[6,7] Notably,
non-photochemical methods that can furnish higher ordered
(ꢀ [8]) helicenes and helicene-like molecules are rare.[8]
To access sterically more demanding [9]helicene-like
molecules[9] starting from commercially available 2-naphthol,
we designed an intermolecular double [2+2+2] cycloaddition
leading to enantioenriched [7]helicene-like molecules.
between a 2-naphthol-linked tetrayne and a dialkynylke-
tone,[10] which forms five successive rings (Scheme 2). This
method would furnish various [9]helicene-like molecules,
containing a densely substituted fluorenone core, by changing
substituents of each cycloaddition partner.
The 2-naphthol-linked tetraynes 4 were readily prepared
by a three-step sequence starting from a known terminal
alkyne 1[7] as shown in Scheme 3. A copper-mediated
Scheme 2. Rhodium-catalyzed intermolecular double [2+2+2] cyclo-
addition leading to enantioenriched [9]helicene-like molecules.
[*] Prof. Dr. K. Tanaka, N. Fukawa, T. Suda
Department of Applied Chemistry, Graduate School of Engineering
Tokyo University of Agriculture and Technology
Koganei, Tokyo 184-8588 (Japan)
Fax: (+81) 42-388-7037
E-mail: tanaka-k@cc.tuat.ac.jp
Prof. Dr. K. Noguchi
Instrumentation Analysis Center
Tokyo University of Agriculture and Technology
Koganei, Tokyo 184-8588 (Japan)
[**] This work was supported partly by a Grant-in-Aid for Scientific
Research (Nos. 20675002 and 20·8746) from MEXT (Japan). We
thank Prof. Dr. Kenji Ogino (TUAT) for the measurement of
fluorescence spectra, Dr. Katsuya Maeyama (TUAT) for the
measurement of UV spectra, and Takuya Osaka (TUAT) for his
preliminary experiment. We also thank Takasago International
Corporation for the gift of segphos [(4,4‘-bi-1,3-benzodioxole)-5,5‘-
diylbis(diphenylphosphine)] and H8-binap (2,2’-bis(diphenylphos-
phanyl)-1,1’-binaphthyl), and Umicore for generous support in
supplying the rhodium complex.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901962.
Scheme 3. Syntheses of tetraynes 4a and 4b. DMAP=4-dimethyl-
aminopyridine, DCC=dicyclohexylcarbodiimide.
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Table 2: Rhodium-catalyzed double [2+2+2] cycloadditions of tetrayne
4b with diynes 5a–d.
homocoupling of 1 and subsequent treatment with aqueous
HCl furnished the 2-naphthol-substituted 1,3-diyne 3. Ether-
ification of 3 with 1-bromo-2-butyne gave the ether-linked
tetrayne 4a. Ester-linked tetrayne 4b was also prepared by
esterification of 3 with 2-butynoic acid.
Upon obtaining tetraynes 4a and 4b, we first investigated
an intermolecular double [2+2+2] cycloaddition of the ether-
linked tetrayne 4a with dialkynylketone 5a in the presence of
cationic rhodium(I) complexes and various bis(phosphine)
ligands (Figure 1). The study revealed that the use of
Entry 5 (R)
Ligand
6
Yield [%][a] (ee [%])
1
2
5a (Me)
5b (nBu)
5c (Ph)
(R)-binap
(P)-(+)-6ba
42 (26)
58 (32)
61 (31)
32 (60)
(R)-H8-binap (ꢁ)-6bb
3
(R)-segphos (ꢁ)-6bc
4[b]
5d (CH2OMe) (R)-segphos (+)-6bd
[a] Yield of isolated product. [b] Catalyst: 40 mol%. Solvent: (CH2Cl)2.
Temperature: 408C.
Figure 1. Structures of axially chiral biaryl bisphosphine ligands.
ceeded by employing the cationic rhodium(I)/(R)-H8-binap
or (R)-segphos complex (20 mol%) as the catalyst. Although
a high loading of the cationic rhodium(I)/(R)-segphos com-
plex was required, the reaction of 4b and methoxymethyl-
substituted diethynylketone 5d proceeded to yield the
corresponding [9]helicene-like molecule 6bd with the highest
ee value (Table 2, entry 4).
As expected, fluorenone-containing [9]helicene-like mol-
ecules 6, possessing large overlapping rings, did not undergo
racemization at room temperature in CH2Cl2. In contrast,
fluorenone-containing [7]helicene-like molecule 8, which was
prepared from phenol-linked tetrayne 7 and dialkynylketone
5a, underwent gradual racemization under the same condi-
tions (Scheme 4).
10 mol% of a [Rh(cod)2]BF4/(R)-segphos (cod = cycloocta-
1,5-diene) complex at room temperature furnished the
expected fluorenone-containing [9]helicene-like molecule
6aa as a yellow solid in 56% yield upon isolation and
Table 1: Rhodium-catalyzed double [2+2+2] cycloadditions of tetrayne
4a with diynes 5a–d.
Entry 5 (R)
Ligand
6
Yield [%][a] (ee [%])
1[b]
2
5a (Me)
5b (nBu)
5c (Ph)
(R)-segphos (P)-(+)-6aa
(R)-segphos (+)-6ab
(R)-H8-binap (ꢁ)-6ac
56 (47)
40 (39)
53 (18)
26 (26)
56 (10)
3
4[c]
5[c]
5d (CH2OMe) (R)-segphos (+)-6ad
5d (CH2OMe) (R)-H8-binap (ꢁ)-6ad
[a] Yield of isolated product. [b] 5a: 2 equiv. [c] Catalyst: 20 mol%.
47% ee (Table 1, entry 1). In addition to 5a, n-butyl- (Table 1,
entry 2), phenyl- (Table 1, entry 3), and methoxymethyl
substituted diethynylketones (Table 1, entries 4 and 5) par-
ticipated in this reaction at room temperature by employing a
cationic rhodium(I)/(R)-segphos or (R)-H8-binap complex
(10–20 mol%) as the catalyst.
Next, an intermolecular double [2+2+2] cycloaddition of
ester-linked tetrayne 4b with 5a was also investigated,
revealing that the use of 20 mol% of a [Rh(cod)2]BF4/(R)-
binap complex at room temperature furnished the expected
fluorenone-containing [9]helicene-like molecule 6ba as a red
solid in 42% yield and 26% ee (Table 2, entry 1). The
reactions of 4b with n-butyl- (Table 2, entry 2) and phenyl-
substituted diethynylketones (Table 2, entry 3) also pro-
Scheme 4. Synthesis and racemization of fluorenone-containing
[7]helicene-like molecule 8.
The enantiopure crystal of (P)-(+)-6aa was readily
prepared by recrystallization (Figure 2).[11] The pseudohelical
axes of [9]helicene-like molecules are nearly parallel to the a-
axis (ca. 16.58). The molecules that are related by the
crystallographic 21-axes along the c-axis are packed almost
parallel to the bc-plane. Adjacent molecular layers along the
a-axis are related by the other 21-axes along the b-axis.
Therefore, the helically chiral molecules are stacked in a
partially overlapped fashion along the a-axis.
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Figure 3. Packing structure of enantiopure (P)-(+)-6ba. A) c-projection
view, B) b-projection view.
Table 3: Photophysical data of helicene-like molecule 6 and 8.[a]
25[b]
Figure 2. Packing structure of enantiopure (P)-(+)-6aa. A) a-projection
view, B) c-projection view.
Entry
Compound
½aꢂD
UV Absorption
Emission
lmax [nm]
lmax [nm][c]
1
2
3
4
5
6
7
8
9
(+)-6aa
(+)-6ab
(ꢁ)-6ac
(+)-6ad
(+)-6ba
(ꢁ)-6bb
(ꢁ)-6bc
(+)-6bd
(+)-8
1250
1499
1151
1585
2050
1752
1605
2033
1245
356
357
377
379
313
376
341
381
349
556
554
420, 444
561
580
The enantiopure acetone solvate crystals of (P)-(+)-6ba
was also readily prepared by recrystallization (Figure 3).[11]
The pseudohelical axes of 6ba are almost parallel to the c-axis
(ca. 3.98). The molecules that are related by crystallographic
two-fold axes along the b-axis form a columnar structure. In
this column, the fluorenone parts of the 6ba molecules are
stacked along the c-axis. In addition, intermolecular stacking
between naphthalene rings was observed with an average ring
separation of 3.415(2) ꢁ.
578
423, 572
472, 575
559
[a] Measured in CHCl3. [b] Values are calculated as 100% ee. [c] Only the
longest absorption maximum wavelengths are given.
Photophysical properties of helicene-like molecules 6 and
8 were briefly examined as shown in Table 3. As expected,
highly conjugated ester-linked [9]helicene-like molecules
6ba–d (Table 3, entries 5–8), containing two a-pyrone rings,
exhibit larger optical rotation values than ether-linked
[9]helicene-like molecules 6aa–d (Table 3, entries 1–4). Inter-
estingly, ether-linked [7]- and [9]helicene-like molecules 8 and
6aa exhibit very close optical rotation values (Table 3,
entries 1 and 9). In general, when the compounds possess
two a-pyrone rings (Table 3, entries 5–8), bathochromic shifts
in emission were observed when compared to 6aa–d (Table 3,
entries 1–4).
In conclusion, we have achieved the flexible and conven-
ient syntheses of enantioenriched fluorenone-containing
[9]helicene-like molecules by rhodium-catalyzed intermolec-
ular double [2+2+2] cycloadditions. Their unique crystal
structures and photophysical properties have also been
determined. Future studies will focus on expanding the
substrate scope including [11]helicene-like molecules and
developing new materials by additional functionalization of
these new helically chiral molecules.
Received: April 13, 2009
Published online: June 18, 2009
Keywords: alkynes · asymmetric catalysis · fluorenones ·
.
helicenes · rhodium
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