Design and synthesis of planar chiral heterocyclic carbene precursors derived from [2.2]paracyclophane

Article abstract of DOI:10.1021/jo800468x

(Chemical Equation Presented) A unique family of planar chiral symmetrical N-heterocyclic carbene precursors with restricted flexibility derived from [2.2]paracyclopane were obtained by a new synthetic route. The resolution of 4-amino-13-bromo[2.2]paracyclophane was achieved with relatively high efficiency. Starting from (45_p,13R_p)-4-amino-13-bromo[2.2] paracyclophane, the planar chiral pseudogem-disubstituted [2.2]paracyclophanyl dihydroimidazoliums were prepared in a four-step sequence with good yields. The resulting dihydroimidazolium salts were fully characterized with a series of methods including single-crystal X-ray diffraction technique.

Full text of DOI:10.1021/jo800468x

for these complexes in catalysis reaction. NHCs offer good
potential for coordination with metal ions as a neutral two-
electron-donating (s-donating) ligand with negligible π back-
bonding. As a result, dihydroimidazolium salts were revealed
to give corresponding carbene as very effective ligands in both
ruthenium- and palladium-catalyzed transformations. In addition,
the complexes of NHCs with transition metals have demon-
strated catalytic activity for a series of reactions such as
hydrosilylation,6 Heck,7 Suzuki-Miyaura,8 Kumada,9 Sono-
gashiracouplings,10,11 olefincyclopropanation,12 arylamination,13,14
and olefin metathesis.15
Design and Synthesis of Planar Chiral
Heterocyclic Carbene Precursors Derived from
[2.2]Paracyclophane
Wenzeng Duan,†,‡ Yudao Ma,*,† Houqi Xia,† Xueying Liu,†
Qingshuang Ma,† and Junshan Sun‡
Department of Chemistry, Shandong UniVersity, Shanda
South Road No. 27, Jinan 250100, People’s Republic of
China, Department of Chemistry, Taishan UniVersity, Taian
271021, People’s Republic of China
It is well-known that the increase in the bulkiness of chiral
groups may lead to increased enantiocontrol in asymmetric
reactions. Many strategies therefore have been developed during
the past years for introducing different chirality elements into
NHC ligands, which include the alkyl side chains containing
stereogenic center, a chiral backbone in the heterocycle, chiral
biaryl units, and the combination with ferrocene-based planar
chirality.16 In particular, the catalytic activity revealed for
imidazole-NHCs attracted great research interests in chiral NHCs
for asymmetric catalysis.17 The chemistry of the parent imida-
zole system is well established; the preparation of tailor-made
NHC precursors bearing bulky [2.2]paracyclophane substituent,
however, is still a challenge for organic chemists.
ydma@sdu.edu.cn
ReceiVed February 28, 2008
A unique family of planar chiral symmetrical N-heterocyclic
carbene precursors with restricted flexibility derived from
[2.2]paracyclopane were obtained by a new synthetic route.
The resolution of 4-amino-13-bromo[2.2]paracyclophane was
achieved with relatively high efficiency. Starting from
(4Sp,13Rp)-4-amino-13-bromo[2.2]paracyclophane, the planar
chiral pseudogem-disubstituted [2.2]paracyclophanyl dihy-
droimidazoliums were prepared in a four-step sequence with
good yields. The resulting dihydroimidazolium salts were
fully characterized with a series of methods including single-
crystal X-ray diffraction technique.
Planar chiral [2.2]paracyclophane-based ligand possesses a
rigid [2.2]paracyclophanyl unit, and such a versatile backbone
structure opens the possibility of designing different types of
chiral ligands.18 The thus far reported [2.2]paracyclophane-based
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* Corresponding author. Phone: 0086-531-88361869. Fax: 0086-531-
88565211.
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A. H. J. Am. Chem. Soc. 2000, 122, 8168–8179. (c) Bujok, R.; Bieniek, M.;
Masnyk, M.; Michrowska, A.; Sarosiek, A.; Stepowska, H.; Arlt, D.; Grela, K.
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† Shandong University.
‡ Taishan University.
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10.1021/jo800468x CCC: $40.75 2008 American Chemical Society
Published on Web 04/29/2008

ligands include diphosphanes,19 oxazoline-phosphanes,20 imi-
dazoliums,21 oxazoline-alcohols,22 and imine ligands.23 Re-
cently, a series of planar chiral pseudo-ortho-disubstituted
[2.2]paracyclophanyl dihydroimidazoliums have been prepared
by this group and their applications as rhodium or ruthenium
complexes in highly enantioselective transformations have also
been demonstrated.24 Very lately, efforts have been paid to
extend the previous studies toward the synthesis of planar chiral
pseudogem-disubstituted [2.2]paracyclophanyl dihydroimida-
zoliums. These novel ligands are expected to give higher
stereoselectivity in asymmetric reactions due to their increased
steric hindrance. As part of our continuous study for employing
chiral [2.2]paracyclophanyl dihydroimidazoliums in asymmetric
reactions, herein we describe the synthesis of novel planar chiral
pseudogem-disubstituted [2.2]paracyclophanyl dihydroimida-
zoliums.
SCHEME 1
(()-4-Amino-13-bromo[2.2] paracyclophane 4 was prepared
first by Cram in 1975.25 However, to the best of our knowledge,
the resolved enantiomers of this compound have never been
reported. As a consequence, our synthetic strategy was based
on constructing the (4Sp,13Rp)-4-amino-13-bromo[2.2]paracyclo-
phane 6 core by utilization of readily available building blocks
(Scheme 1). Staring with the commercial available [2.2]para-
cyclophane 1, 4-nitro[2.2]paracyclophane 2 was obtained from
the nitration of 1 with fuming nitric acid and acetic acid.26
Bromination of 2 under standard condition led to the isolation
of the desired compound 3 in good chemoselectivity.27 Accord-
ingtoapreviouslydescribedprocedure,25 4-nitro-13-bromo[2.2]para-
cyclophane 3 was reduced to afford 4 in excellent yield. The
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initial trial to resolve (()-4 by modifying an authentic method
using fractional crystallization of the diastereoisomeric salts of
(1R)-(-)-10-camphorsulfonic acid in various solvents failed to
give a satisfactory result.28 We therefore turned our attention
to resolve (()-4 by means of a chiral auxiliary reagent, (1R)-
(-)-10-camphorsulfonyl chloride ((-)-CSC). In the presence
of triethylamine, (()-4 was converted to a mixture of diaster-
eomeric amides 5, which were successfully separated by
chromatography on silica gel. The lower polarity portion (-)-5
(47% yield, >99% de determined by 1H NMR) was hydrolyzed
with 48% HBr and propionic acid, affording (-)-6 in 45% yield.
The single-crystal X-ray diffraction analysis result of (-)-5
confirms the absolute stereochemistry of (-)-6, whose config-
uration was assigned to (4Sp,13Rp)-4-amino-13-bromo[2.2]para-
cyclophane (Figure S1, Supporting Information).
As reported previously by this group, Suzuki coupling with
arylboron compounds under palladium-NHC catalysis gave the
4-amino-12-aryl[2.2]paracyclophanes.24a Rozenberg and co-
workers have also investigated the synthesis of pseudogem-
substituted aryl[2.2]paracyclophanes using the Suzuki cross-
coupling reaction.29 A palladium-catalyzed Suzuki cross-
coupling reaction is therefore expected to be suitable for forming
the hindered amino[2.2]paracyclophane derivatives (7a-d).
Repeated experimental results led to the optimal reaction
condition including the reaction temperature of 80 °C and time
of 96 h for the reaction between Pd-DPPF and KF in dioxane.
Moreover, it has been found that addition of 5% Pd-DPPF in
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J. Org. Chem. Vol. 73, No. 11, 2008 4331

SCHEME 2
SCHEME 3
three portions is more effective than adding it in one portion.
As shown in Scheme 1, the highest isolated product yield was
reached with phenylboronic acid at 97%. 3-Methoxyphenylbo-
ronic acid, at 70%, was shown to be inferior. 1-Naphthalenebo-
ronic acid and 2-methoxyphenylboronic acid displayed moderate
reactivity.
Our first approach to NHC precursors relied on the utilization
of N,N′-disubstituted ethylenediamine derivatives as a key
intermediate since their well-established ring closure with
trialkyl orthoformates was expected to give the desired dihy-
droimidazolium salts. The preparation of the diamines (9a-d)
followed a two-step condensation-reduction sequence. Treat-
ment of the pseudogem-substituted amino[2.2]paracyclophanes
(7a-d) with aqueous gloxal gave corresponding diimines
(8a-d), which were reduced to the diamines (9a-d) with
NaBH4/20% H2SO4 in a yield range of 55-66%. The open-
chain intermediates (9a-d) were subjected to ring-closure with
triethyl orthoformate in the presence of a catalytic amount of
formic acid. The desired dicyclophane dihydroimidazolium
tetrafluoroborates (10a-d) were obtained in good yield
(72%-91%) by using ammonium tetrafluoroborate.30 It is worth
noting that ammonium chloride and hydrochloride are less
efficient since the analogous dihydroimidazolium chlorides could
not be purified by chromatography and recrystallization.2b,31
Both mass and 1H NMR spectroscopic results on the synthetic
compounds 10a-d, and in particular the X-ray diffraction
analysis result on 10c (Figure S2, Supporting Information),
proved the expected nature of these products. Additionally,
(-)-6 reacted with excess aqueous glyoxal, providing glyoxal
diimine 11. This compound, however, could not be reduced by
NaBH4/20% H2SO4 due to its insolubility (Scheme 2).
of a catalytic amount of formic acid gave the expected
dihydroimidazolium 14 in the yield of 81% (Scheme 3). This
newly synthesized compound was characterized by mass and
1H NMR spectroscopy.
In summary, novel planar chiral dihydroimidazolium salts
have been designed and synthesized by using a simple route
starting with commercially available [2.2]paracyclophane. The
large-scale synthesis of these new carbene precursors is in
progress in our laboratory and their application in the asym-
metric synthesis reaction is under way.
Experimental Section
General Procedure for the Synthesis of N,N′-Bis[(4Sp,13Rp)-
(-)-13-(1-naphthyl)-4-[2.2]paracyclophanyl]-4,5-dihydroimida-
zolium Tetrafluoroborate. The mixture of (4Sp,13Rp)-4-amino-
13-bromo[2.2]paracyclophane(6,501.3mg,1.66mmol),1-naphthalene-
boronic acid (428.3 mg, 2.49 mmol), KF (289.7 mg, 4.98 mmol),
and Pd-DPPF (13.6 mg, 1.66 × 10-2 mmol) in 1,4-dioxane (5 mL)
was stirred at 80-90 °C for 24 h under a slight positive pressure
of nitrogen. At 24, 48, and 72 h, 1-naphthaleneboronic acid (95.2
mg, 0.55 mmol), KF (72.4 mg, 1.25 mmol), and Pd-DPPF (13.6
mg, 1.66 × 10-2 mmol) were added to the flask, respectively. After
completion of the reaction as indicated by TLC, the mixture was
cooled to room temperature, water (5 mL) was added, and the
solution was filtered. The solution was then extracted by dichlo-
romethane (3 × 10 mL), and the solvent was removed by rotary
evaporation. The crude material was purified by chromatography
on silica gel (eluent: hexanes/ethyl acetate ) 20:1), and pure
(4Sp,13Rp)-4-amino-13-(1-naphthyl)[2.2]paracyclophane (7c) was
The second approach to NHC precursor 14 was based on the
reported route by employing the acylation of amine.31c-f Oxalyl
chloride and (-)-6 were used to generate oxalamide 12, which
then underwent a reduction with BH3 ·S(CH3), yielding diamine
13 in good yield. Treatment of the diamine 13 with triethyl
orthoformate and ammonium tetrafluoroborate in the presence
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2001, 66, 7729–7738. (c) Seiders, T. J.; Ward, D. W.; Grubbs, R. H. Org. Lett.
2001, 3, 3225–3227. (d) Scarborough, C. C.; Grady, M. J. W.; Guzei, I. A.;
Gandhi, B. A.; Bunel, E. E.; Stahl, S. S. Angew. Chem., Int. Ed. 2005, 44, 5269–
5272. (e) Paczal, A.; Be´nyei, A. C.; Kotschy, A. J. Org. Chem. 2006, 71, 5969–
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obtained as a white solid (490 mg, 84%). Mp 234-236 °C; Rf 0.64
1
(hexanes/ethyl acetate ) 5:1); [R]20 +61.5 (c 0.31, CHCl3); H
D
NMR (300 MHz, CDCl3) δ 7.97-8.10 (m, 2H), 7.84-7.87 (d, 2H),
7.55-7.60 (t, 1H), 7.34-7.46 (m, 2H), 6.97 (s, 1H), 6.63 (s, 2H),
6.48-6.50 (d, 1H), 6.24-6.27 (dd, 1H), 5.77 (s, 1H), 3.06-3.18
(m, 6H), 2.90-3.00 (m, 2H), 2.17-2.84 (m, 2H); 13C NMR (75
MHz, CDCl3) δ 146.1, 140.8, 139.4, 139.1, 138.4, 137.4, 135.3,
134.4, 133.5, 133.3, 131.8, 131.7, 128.2, 127.7, 127.3, 126.4, 126.0,
125.5, 124.9, 124.0, 122.8, 119.9, 35.3, 35.1, 33.3, 30.9. Anal. Calcd
for C26H23N: C, 89.36; H, 6.63; N, 4.01. Found: C, 89.03; H, 6.62;
N, 3.93.
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4332 J. Org. Chem. Vol. 73, No. 11, 2008

The mixture of 7c (258 mg, 0.738 mmol) and 40% glyoxal (42
mg, 0.886 mmol) in THF (1 mL) was stirred at room temperature
over 5 h. After the reaction was completed as indicated by TLC,
the solution was concentrated and the residue was purified by silica
gel chromatography with dichloromethane, affording diimine 8c.
To a solution of 8c (263 mg, 0.365 mmol) in THF (4 mL) at 0
°C was added NaBH4 (192 mg, 5.074 mmol) in 40-mg portions.
H2SO4 (20%, 0.73 mL) in THF (1 mL) was slowly added
dropwisely for 15 min. After being stirred at 0 °C for another 15
min, the suspension was poured into a 2% NaOH (36 mL) solution.
The solution was extracted with dichloromethane (3 × 30 mL).
The solvent was removed on a rotary evaporator and the crude
product was subjected to chromatography on silica gel with hexanes/
dichloromethane (1:1) to give the desired glyoxal diamine 9c as a
solvent was removed on a rotary evaporator. The crude product
was subjected to chromatography on silica gel (eluent: dichlo-
romethane/ethanol ) 20:1), and the target N,N′-bis[(4Sp,13Rp)-(-)-
13-(1-naphthyl)-4-[2.2]paracyclophanyl]-4,5-dihydroimidazolium tet-
rafluoroborate was obtained as a yellow solid (95 mg, 72%). Mp
245 °C dec; Rf 0.84 (dichloromethane/ethanol ) 10:1), [R]20
D
-129.6 (c 0.25, CHCl3); 1H NMR (300 MHz, CDCl3) δ 8.05-8.07
(m, 1H), 7.74-7.76 (d, 2H), 7.66-7.69 (d, 2H), 7.60-7.62 (d, 2H),
7.25-7.44 (m, 7H), 6.72-6.87 (m, 10H), 6.53-6.55 (d, 3H), 4.72
(m, 2H), 4.17 (m, 2H), 3.36-3.39 (m, 2H), 3.11-3.30 (m, 10H),
2.84-2.92 (m, 4H); 13C NMR (75 MHz, CDCl3) δ 162.2, 151.7,
143.2, 140.6, 137.8, 137.3, 137.1, 134.4, 133.7, 133.3, 133.2, 133.1,
132.6, 130.6, 128.9, 128.5, 127.8, 127.7, 126.5, 125.9, 125.6, 124.9,
123.2, 50.5, 35.1, 35.0, 34.9, 32.7; LRMS [ESI (methanol)] ms
(+) m/z (%) 635.5 (100), 639.5 (10); ms (-) m/z (%) 679.7 (43),
725.8 (100), 771.5 (16); mass calcd for C55H47N2BF4 822.8. Anal.
Calcd for C55H47N2BF4 ·0.5H2O: C, 79.42; H, 5.82; N, 3.37. Found:
C, 79.46; H, 5.74; N, 3.37.
white solid (148 mg, 56%). Mp 175-177 °C; Rf 0.58 (hexanes/
1
ethyl acetate ) 5:1); [R]20 -39.1 (c 0.1, CHCl3); H NMR (300
D
MHz, CDCl3) δ 7.90-7.97 (m, 6H), 7.77-7.79 (d, 2H), 7.63-7.68
(t, 2H), 7.45-7.50 (t, 2H), 7.35-7.40 (t, 2H), 6.79 (s, 2H),
6.49-6.58 (m, 4H), 6.30-6.33 (d, 2H), 6.06-6.09 (d, 2H), 5.16
(s, 2H), 3.73 (s, 2H), 3.05-3.08 (d, 2H), 2.76-2.98 (m, 12H),
2.47-2.52 (m, 2H), 2.25-2.35 (m, 4H); 13C NMR (75 MHz,
CDCl3) δ 146.8, 141.8, 139.5, 138.9, 138.1, 136.8, 134.8, 134.1,
133.6, 132.8, 131.7, 131.4, 128.2, 127.5, 126.9, 126.4, 126.0, 125.5,
124.6, 124.1, 120.9, 114.6, 43.8, 43.4, 35.3, 34.9, 32.9, 31.7. Anal.
Calcd for C54H48N2: C, 89.46; H, 6.67; N, 3.86. Found: C, 89.23;
H, 6.72; N, 3.91.
Acknowledgment. Financial support from the National
Natural Science Foundation of China (Grant Nos. 20441004
and 20671059) and the Department of Science and Technology
of Shandong Province is gratefully acknowledged.
Supporting Information Available: Experimental proce-
dures, characterization data for selected compounds, and crystal-
lographic data (CIF files) for (-)-5 and 10c. This material is
available free of charge via the Internet at http://pubs.acs.org.
A mixture of the diamine 9c (116 mg, 0.16 mmol), triethyl
orthoformate (2 mL), one drop of formic acid, and ammonium
tetrafluoroborate (16.8 mg, 0.16 mmol) was stirred at 125 °C for
10 h. After completion of the reaction as indicated by TLC, the
JO800468X
J. Org. Chem. Vol. 73, No. 11, 2008 4333