Modular synthesis of planar-chiral para-substituted paracyclophanes by double Suzuki coupling

Article abstract of DOI:10.1002/ejoc.201201043

The first synthesis of enantiomerically pure 4,7-paracyclophane ditriflate starting from a quinone was reported. Using this molecule, mono-or dicoupling with boronic acids delivered enantiomerically pure arylparacyclophanes.

Full text of DOI:10.1002/ejoc.201201043

Job/Unit: O21043
/KAP1
Date: 19-09-12 17:44:00
Pages: 5
SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201043
Modular Synthesis of Planar-Chiral para-Substituted Paracyclophanes by
Double Suzuki Coupling
Murat Cakici[a,b] and Stefan Bräse*[a,c]
Keywords: Cyclophanes / Cross-coupling / Chirality / Boron / Hydrocarbons
The first synthesis of enantiomerically pure 4,7-paracyclo-
phane ditriflate starting from a quinone was reported. Using
this molecule, mono- or dicoupling with boronic acids deliv-
ered enantiomerically pure arylparacyclophanes.
Introduction
[2.2]Paracyclophanes belong to a unique class of strained
hydrocarbons with an increasing number of applications[1]
in disciplines such as material science[2] or catalysis.[3]
Therefore, the synthesis of chiral paracyclophanes is still a
field of interest. There are a variety of methods for the syn-
thesis of chiral, non-racemic ortho- (i.e., 4,5-) or monosub-
stituted paracyclophanes.[4] For an application in material
science, we needed a modular synthesis of para-substituted
paracyclophanes (i.e., 4,7-).[5–8] However, there are only a
few known examples of 4,7-diarylcyclophanes that give ac-
cess to racemic material.[9,10] The methods used were mostly
bottom-up syntheses from the two layers.
Figure 1. para-Substituted paracyclophanes 1-X. X = Hal, OH,
OTf (this work).
a hydroquinone, the reduction product of a known quin-
one.[14] An asymmetric synthesis of this quinone is re-
ported.[15]
The synthesis of non-racemic quinone 3 was therefore
envisaged. Our synthesis started with the preparation of
enantiopure hydroxyparacyclophane (SP)-2-OH, which is
accessible on gram scale in three steps from parent paracy-
clophane 2-H by using our optimized protocol (Scheme 1,
steps 1–3).[4a] Conversion to quinone (SP)-3 proceeded
smoothly in 67% yield by addition of a diazonium salt at
the para position and subsequent reduction of the azo com-
pound followed by oxidation with iron(III) sulfate accord-
ing to a literature procedure.[16] The absolute configuration
of quinone (SP)-3 can be traced back to the configuration
of phenol (SP)-2-OH and/or aldehyde (SP)-2-CHO.
Results and Discussion
Retrosynthetically, para-substituted paracyclophanes can
be obtained from difunctionalized paracyclophanes 1-X
(Figure 1), which in turn can undergo cross-coupling reac-
tions. To our surprise, this was not investigated before, al-
though a number of 4,7-dihydroxylated paracyclophanes (X
= OH) are known.[11]
Although 4,7-dihaloparacyclophanes 1-Hal (Hal = Br)
are known,[12] an asymmetric synthesis might be tedious.
Therefore, we turned our attention to (so far unknown)
para-ditriflate 1-OTf.[13] The latter might be accessible from
[a] Soft Matter Synthesis Lab, Institute for Biological Interfaces I,
Karlsruhe Institute of Technology,
76131 Karlsruhe, Germany
[b] Faculty of Sciences, Department of Chemistry,
Ataturk University,
25240 Erzurum, Turkey
[c] Karlsruhe Institute of Technology, Campus South,
Institute of Organic Chemistry,
Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
Fax: +49-721-6088581
E-mail: braese@kit.edu
Homepage: http://www.ioc.kit.edu/braese
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201043.
Scheme 1. Asymmetric synthesis of quinone 3.
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M. Cakici, S. Bräse
SHORT COMMUNICATION
After some experimentation, we were able to convert
quinone (SP)-3 via hydroquinone (SP)-1-OH into triflate
(SP)-1-OTf in the presence of triflate anhydride and a base
(Scheme 2). It was crucial to exclude air under these condi-
tions, as re-oxidation of novel hydroquinone (SP)-1-OH
proceeds fast with oxygen. However, triflate (SP)-1-OTf is
a stable molecule. The enantiomeric excess (determined by
HPLC on chiral column) was identical to that of starting
quinone (SP)-3.
Conclusions
We have demonstrated that a chiral, non-racemic ditri-
flate is a suitable starting material for aryl-substituted para-
cyclophanes. This triflate should also be a suitable precur-
sor for other cross-coupling and/or polymerization reac-
tions.
Experimental Section
[2.2]Paracyclophane-4,7-quinone (3): Racemic and optically active 3
were synthesized from rac- and (S)-4-hydroxy[2.2]paracyclo-
phane[4a] by the procedure reported in the literature.[16] M.p.
1
Ͼ200 °C (dec.); Rf (CH2Cl2) = 0.37. H NMR (300 MHz, CDCl3):
δ = 6.85 (d, J = 7.9 Hz, 2 H), 6.73 (d, J = 7.9 Hz, 2 H), 5.82 (s, 2
H), 3.28–3.20 (m, 2 H), 3.17–3.01 (m, 4 H), 2.36–2.26 (m, 2
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 187.4, 149.2, 139.0,
134.8, 133.5, 131.5, 33.5, 28.9 ppm. IR (KBr): ν = 2930, 2858, 1649,
˜
1597, 1413, 1351, 1233, 1115, 1093 cm–1. [α]20 = +224.0 (c = 0.185,
benzene) {ref.[15b] [α]20 = +211(c = 0.185, benzene)}. HPLC (Chi-
ralcel AS-H column, n-hexane/iPrOH = 90:10, flow rate = 1 mL
min–1, λ = 254 nm): tR = 17.4 (S), 18.6 min (R); Ͼ99%ee.
Scheme 2. Synthesis of non-racemic triflate (SP)-1-OTf.
(SP)-[2.2]Paracyclophane-4,7-bistriflate [(SP)-1-OTf]: Hydrazine hy-
drate (0.051 mL, 1.050 mmol) was added to a solution of (SP)-[2.2]-
paracyclophane-4,7-quinone [(SP)-3; 100 mg, 0.420 mmol] in an-
hydrous EtOH (15 mL). The reaction mixture was heated at reflux
for 2 h, the solvent and the excess amount of hydrazine hydrate
were distilled off in vacuo, and the flask was filled with argon. The
crude [2.2]paracyclophane-4,7-hydroquinone was dissolved in dry
dichloromethane (10 mL) under an atmosphere of nitrogen at room
temperature followed by the addition of dry pyridine (0.204 mL,
2.520 mmol). Trifluoromethanesulfonic anhydride (triflic anhy-
dride; 0.282 mL, 1.680 mmol) in dry dichloromethane (2 mL) was
then added dropwise. The color of the solution changed from yel-
low to orange and white fumes evolved. The reaction mixture was
allowed to stir at room temperature for 2 h. Water (10 mL) was
then added, and the reaction mixture was extracted with dichloro-
methane (3ϫ15 mL). The combined organic phase was washed
with brine and dried with MgSO4. The residue after rotary evapo-
ration was purified by column chromatography (silica gel; hexane/
ethyl acetate, 90:10). Unconsumed quinone was re-isolated (30 mg,
0.126 mmol, 30%). Triflate (SP)-1-OTf (85 mg, 0.169 mmol, 57%
with respect to consumed quinone) was obtained and recrystallized
(hexane) to give colorless crystals. M.p. 99–100 °C; Rf (hexane/
EtOAc = 90:10) = 0.79. 1H NMR (400 MHz, CDCl3): δ = 6.93 (dd,
J = 8.0, 1.6 Hz, 2 H), 6.50 (dd, J = 8.0, 1.6 Hz, 2 H), 6.32 (s, 2 H),
3.42–3.36 (m, 2 H), 3.24–3.13 (m, 4 H), 2.83–2.75 (m, 2 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 147.4, 139.1, 135.5, 133.1, 130.6,
Gratifyingly, double Suzuki coupling of racemic or
enantiopure ditriflate 1-OTf with aryl boronic acids 4 pro-
ceeded smoothly to yield diarylparacyclophanes 5 in good
yields (Table 1, entries 3 & 4). The enantiomeric excess was
over 99%ee for 5b (determined by HPLC). Under different
conditions (fewer equivalents of the boronic acid, lower
temperature, different base), monoarylated paracyclo-
phanes 6 were isolated in good yields (Table 1, entries 1 &
2).
Table 1. Suzuki coupling of racemic and enantiopure ditriflate 1-
OTf with aryl boronic acids 4.
130.3, 118.8 (q, J = 320.8 Hz, 1 C), 34.0, 31.3 ppm. IR (ATR): ν =
˜
2931, 2857, 1489, 1412, 1202, 1135, 1106, 1094, 1033 cm–1. HRMS:
calcd. for C18H14F6O6S2 504.0136; found 504.0138. [α]20 = +17.5
(c = 1, DCM). HPLC (Chiralcel OD-H column, n-hexane/iPrOH
= 90:10, flow rate = 1 mLmin–1, λ = 220 nm): tR = 6.7 (R), 8.8 min
(S); Ͼ99%ee.
Entry
R
H
Base Temp. Product
[°C]
Conv.
[%]
Yield
[%]
52 (81)[c]
71 (89)[c]
77
1[a]
2[a]
3[b]
4[b]
K2CO3
80
80
rac-6a
rac-6b
rac-5a
64
80
quant.
CO2Me K2CO3
H
K3PO4 100
CO2Me K3PO4 100
(SP)-5b[d] quant.
88
General Procedure for the Suzuki Coupling: [2.2]Paracyclophane-
4,7-bistriflate [rac- or (SP)-1-OTf; 150 mg 0.300 mmol), powdered
K3PO4 (191 mg, 0.900 mmol), and boronic acid 4 (1.20 mmol) were
added to a 10-mL, two-necked flask that had been purged with
argon. A solution of Pd(PPh3)4 (21 mg, 0.018 mmol) in dry dioxane
(6 mL) was added, and the reaction mixture was stirred under an
atmosphere of argon at 100 °C for 2 d. The reaction mixture was
cooled and filtered through a short pad of Celite. The filtrate was
[a] Reaction conditions: rac-1-OTf (1.0 equiv.), boronic acid
(3.0 equiv.), Pd(PPh3)4 (5.0 mol-%), K2CO3 (3.0 equiv.), 24 h, under
argon atmosphere. [b] Reaction conditions: rac- or (SP)-1-OTf
(1.0 equiv.), boronic acid (4.0 equiv.), Pd(PPh3)4 (6.0 mol-%),
K3PO4 (3.0 equiv.), 48 h, under argon atmosphere. [c] Calculated
with respect to consumed 1-OTf. [d] Ͼ99%ee (determined by
HPLC).
2
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Modular Synthesis of Planar-Chiral para-Substituted Paracyclophanes
concentrated under reduced pressure to remove the solvents. The
crude product was purified by column chromatography [silica gel;
hexane/EtOAc = 98:02 (for 5a), hexane/EtOAc = 90:10 (for 5b)] to
give diaryl paracyclophanes 5.
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rac-5a: Recrystallization (hexane) gave colorless crystals. Yield
77%; m.p. 157–159 °C; Rf (hexane/EtOAc = 98:02) = 0.57. 1H
NMR (400 MHz, CDCl3): δ = 7.57 (d, J = 7.5 Hz, 4 H), 7.50 (t, J
= 7.6 Hz, 4 H), 7.38 (t, J = 7.3 Hz, 2 H), 6.77 (d, J = 7.8 Hz, 2 H),
6.70 (s, 2 H), 6.67 (d, J = 7.8 Hz, 2 H), 3.59 (ddd, J = 12.2, 10.0,
2.5 Hz, 2 H), 3.00–2.84 (m, 4 H), 2.65 (ddd, J = 12.2, 10.0, 5.4 Hz,
2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 141.0, 141.0, 139.7,
137.2, 135.2, 132.1, 129.9, 129.9, 128.7, 127.0, 34.7, 34.3 ppm. IR
(ATR): ν = 2968, 2925, 2853, 1598, 1502, 1472, 1444, 1210, 1074,
˜
1023 cm–1. HRMS: calcd. for C28H24 360.1878; found 360.1876.
(SP)-5b: Recrystallization (hexane) gave colorless crystals. Yield
88%; m.p. 219–220 °C; Rf (hexane/EtOAc = 90:10) = 0.39. 1H
NMR (400 MHz, CDCl3): δ = 8.17 (d, J = 8.1 Hz, 4 H), 7.62 (d,
J = 8.1 Hz, 4 H), 6.73 (s, 2 H), 6.70 (d, J = 7.9 Hz, 2 H), 6.66 (d,
J = 7.9 Hz, 2 H), 3.98 (s, 6 H), 3.57–3.52 (m, 2 H), 3.05–2.83 (m,
4 H), 2.64–2.58 (m, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
167.0, 145.1, 140.5, 139.4, 137.5, 135.2, 131.9, 129.9, 129.7, 129.5,
[3]
[4]
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128.7, 52.1, 34.5, 34.1 ppm. IR (ATR): ν = 2945, 1705, 1602, 1431,
˜
1276, 1262, 1180, 1101, 1014 cm–1. HRMS: calcd. for C32H28O4
476.1988; found 476.1989. [α]20 = +403.0 (c = 1, DCM). HPLC
(Chiralcel OD-H column, n-hexane/iPrOH = 90:10, flow rate =
1 mLmin–1, λ = 365 nm): tR = 14.5 (S), 25.5 min (R); Ͼ99%ee.
rac-6a: Recrystallization (hexane) gave colorless crystals. Yield
[5]
[6]
[7]
1
81%; m.p. 90–92 °C; Rf (hexane/EtOAc = 98:02) = 0.51. H NMR
(400 MHz, CDCl3): δ = 7.51–7.39 (m, 5 H), 7.02 (dd, J = 7.9,
1.9 Hz, 1 H), 6.68 (dd, J = 7.9, 1.9 Hz, 1 H), 6.65 (s, 1 H), 6.59–
6.54 (m, 2 H), 6.27 (s, 1 H), 3.48–3.36 (m, 2 H), 3.21 (dd, J = 8.4,
6.0 Hz, 2 H), 2.93–2.67 (m, 4 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 147.2, 142.6, 139.8, 139.4, 139.3, 139.0, 135.0, 132.6,
132.3, 131.9, 130.2, 130.0, 129.5, 129.4, 128.7, 127.5, 120.3, 34.3,
33.9, 33.5, 31.3 ppm. IR (KBr): ν = 2932, 2857, 1597, 1477, 1420,
˜
1247, 1211, 1141, 1071, 1029 cm–1. C23H19F3O3S (432.46): calcd. C
63.88, H 4.43, S 7.41; found C 63.56, H 4.58, S 7.49.
[8]
rac-6b: Recrystallization (hexane) gave colorless crystals. Yield
89%; m.p. 122–125 °C; Rf (hexane/EtOAc = 90:10) = 0.53. 1H
NMR (400 MHz, CDCl3): δ = 8.16 (d, J = 8.1 Hz, 2 H), 7.53 (d,
J = 8.1 Hz, 2 H), 7.02 (dd, J = 8.0, 1.6 Hz, 1 H), 6.67 (s, 1 H), 6.63
(dd, J = 8.0, 1.6 Hz, 1 H), 6.59–6.54 (m, 2 H), 6.29 (s, 1 H), 3.97
(s, 3 H), 3.48–3.42 (m, 1 H), 3.39–3.30 (m, 1 H), 3.23–3.20 (m, 2 H),
2.95–2.78 (m, 3 H), 2.68–2.60 (m, 1 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 167.0, 147.8, 144.1, 141.7, 140.2, 139.4, 135.3, 132.8,
132.6, 132.4, 130.5, 130.3, 130.2, 129.8, 129.7, 129.5, 120.5, 52.4,
[9]
[10]
[11]
34.6, 34.1, 33.7, 31.6 ppm. IR (KBr): ν = 2949, 2858, 1722, 1609,
˜
1421, 1281, 1247, 1213, 1141, 1070, 1019 cm–1. C25H21F3O5S
(490.49): calcd. C 61.22, H 4.32, S 6.54; found C 60.95, H 4.41, S
6.62.
Supporting Information (see footnote on the first page of this arti-
cle): All preparation procedures as well as 1H NMR and 13C NMR
spectra for all compounds and HPLC chromatograms.
[12]
[13]
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Monotriflate 2-OTf is known: a) B. Ortner, H. Hubner, P.
Gmeiner, Tetrahedron: Asymmetry 2001, 12, 3205–3208; b) M.
Kreis, S. Bräse, Adv. Synth. Catal. 2005, 347, 313–319; c) M.
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7394; for the nonaflate: d) M. Kreis, M. Nieger, S. Bräse, J.
Organomet. Chem. 2006, 691, 2171–2181.
Note Added in Proof (September 19, 2012): The stereochemistry of
one product has been proven by a molecular structure determined
by X-ray diffraction.[17]
Acknowledgments
M. C. acknowledges the Ataturk University for the financial sup-
port.
[14]
I. V. Fedyanin, K. A. Lyssenko, N. V. Vorontsova, V. I. Rozen-
berg, M. Y. Antipin, Mendeleev Commun. 2003, 15–16.
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M. Cakici, S. Bräse
SHORT COMMUNICATION
[15] a) N. V. Vorontsova, D. Antonov, L. Rozenberg, E. V. Voron-
tosv, Z. A. Starikova, Y. N. Bubnov, Modern Trends in Organo-
metallic Catalytic Chemistry, Mark VolЈpin, Memorial Inter-
national Symposium, Moscow, Russia, 2003, May 16–23, p. 95;
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Starikova, Eur. J. Org. Chem. 2003, 761–770.
Antonov, Z. A. Starikova, Y. N. Bubnov, Russ. Chem. Bull. Int.
Ed. 2002, 51, 1483–1490.
M. Nieger, M. Cakici, S. Bräse, unpublished results.
Received: August 2, 2012
[17]
Published Online: ᭿
[16] a) D. J. Cram, A. C. Day, J. Org. Chem. 1966, 31, 1227–1232;
b) N. V. Vorontsova, V. I. Rozenberg, E. V. Vorontsov, D. Y.
4
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Modular Synthesis of Planar-Chiral para-Substituted Paracyclophanes
Cyclophanes
The reaction of a chiral, non-racemic para-
cyclophane ditriflate under Suzuki con-
ditions gave – depending on the reaction
conditions – the mono- or diarylated (pic-
tured) enantiopure paracyclophane.
M. Cakici, S. Bräse* ........................ 1–5
Modular Synthesis of Planar-Chiral para-
Substituted Paracyclophanes by Double
Suzuki Coupling
Keywords: Cyclophanes / Cross-coupling /
Chirality / Boron / Hydrocarbons
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