Angewandte
Chemie
Table 2: Screening of chiral ligands for enantioselective reaction.
the tetraphenylene skeleton, and we finally determined the
structure of cycloadduct 3ab by X-ray diffraction analysis (the
ORTEP diagram is depicted in the Supporting Information).
The carbon- and oxygen-tethered triynes 1bb and 1cb were
also transformed into the corresponding tetraphenylenes 3bb
and 3cb in high yield using the same catalyst (Table 1,
entries 4 and 5).
We considered a plausible mechanism for the above-
mentioned reaction, which includes consecutive inter- and
intramolecular cycloadditions (Scheme 2). Oxidative cou-
Entry
Ligand
T [8C]
t [h]
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
(R)-MeO-biphep
(R)-Cl-MeO-biphep
(S)-binap
RT
RT
RT
RT–60
RT–60
reflux
reflux
reflux
reflux
reflux
2
2
1
5
5
6
3
6
6
88
80
45
84
83
97
57
65
65
65
14
15
À15
(R)-Cy-binap
79
(+)-Cy-segphos
(S,S)-Me-duphos
(S,S)-Et-duphos
(R,R)-QuinoxP*
(S,S)-QuinoxP*
(R,R)-QuinoxP*
57
À48
À76
88
9
À87
10[a]
24
87
[a] 2 mol% chiral catalyst was used. MeO-biphep=6,6’-dimethoxy-2,2’-
bis(diphenylphosphino)-1,1’-biphenyl, Cl-MeO-biphep=5,5’-dichloro-
6,6’-dimethoxy-2,2’-bis(diphenylphosphino)-1,1’-biphenyl, Cy-segphos=
5,5’-Bis(dicyclohexylphosphino)-4,4’-bi-1,3-benzodioxole, Me-duphos=
1,2-bis(2,5-dimethylphospholano)benzene,
Et-duphos=1,2-bis(2,5-
diethylphospholano)benzene, QuinoxP*=2,3-bis(tert-butyl-methylphos-
phino)quinoxaline.
By using either the Rh–Cy-binap or Rh–QuinoxP*
catalyst, we investigated the reaction of several triynes
(Table 3). For the phenyl-substituted triyne 1ac, the reaction
using the Cy-binap induced far better results than did the
QuinoxP* catalyst, with ee values of more than 90% (Table 3,
entries 1 and 2). For carbon-tethered triynes 1bb and 1bc
having methyl and phenyl groups, respectively, on their
alkyne termini, the reaction with the Cy-binap catalyst
yielded the corresponding tetraphenylenes 3bb and 3bc
with excellent ee values (Table 3, entries 3–5). The methoxy
group on the benzene ring was tolerable, and the tetrame-
thoxy tetraphenylene 3bd was prepared from the dimethoxy
triyne 1bd (Table 3, entry 6). Oxygen-tethered triynes 1cb
and 1cc were also good substrates but the ee values of the
products were lower than those of their carbon-tethered
counterparts (Table 3, entries 7–9). However, when a 4-
bromophenyl group was introduced, almost perfect enantio-
selectivity was achieved by using the Cy-binap catalyst
(Table 3, entry 10). These results indicate that Cy-binap is a
generally good chiral ligand, and QuinoxP* is well suited for
use with methyl-substituted triynes.
Scheme 2. Plausible mechanism of dimerization of triyne 1.
pling of the 1,6-diyne moiety of the first triyne gave metal-
lacyclopentadiene A. Chemo- and regioselective intermolec-
ular coupling with terminal alkyne moiety of the second
triyne gave primary cycloadduct B. Oxidative coupling of 1,6-
diyne moiety of the second triyne and intramolecular
coupling with the remaining terminal alkyne moiety of the
first triyne gave tetraphenylene 3.[13] Thus, the present
protocol provides a new and concise approach to substituted
tetraphenylenes. We additionally investigated enantioselec-
tive variants of the present reaction.
Using nitrogen-tethered triyne 1ab as a model substrate,
we examined several chiral phosphine ligands (Table 2).
Chiral biphep derivatives definitely induced enantioselectiv-
ity but with poor ee values (Table 2, entries 1 and 2). Binap
itself was ineffective (Table 2, entry 3) but Cy-binap was the
ligand of choice after thorough screening (Table 2, entry 4).
Among ligands other than binap derivatives, duphos ligands
proved effective, and good ee values were achieved by using
Et-duphos under reflux conditions in 1,2-dichloroethane
(Table 2, entries 6 and 7). Finally, QuinoxP*[14] induced the
best enantioselectivity and both enantiomers were obtained
using (R,R)- and (S,S)-ligands, respectively (Table 2, entries 8
and 9).[15,16] Using 2 mol% of the catalyst achieved the same
results, but a longer reaction time was required (Table 2,
entry 10).
In summary, we realized the dimerization of triynes,
possessing phenylene-bridged 1,5-diyne moiety, by consec-
utive inter- and intramolecular cycloadditions. It provides a
new way to access substituted tetraphenylene derivatives. By
using chiral rhodium catalysts, the first catalytic and highly
enantioselective synthesis of chiral tetraphenylenes was
achieved. Evaluations of chemical and physical properties
of the chiral tetraphenylenes and their use as a new family of
functional molecules are in progress.
Experimental Section
Typical experimental procedure (Table 3): [Rh(cod)2]BF4 (2.0 mg,
0.005 mmol) and Cy-binap or QuinoxP* (0.005 mmol) were placed in
a Schlenk tube, which was then evacuated and backfilled with argon
Angew. Chem. Int. Ed. 2009, 48, 8066 –8069
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Shibata, Takanori
