The Synthesis of New C2-Symmetric Chiral 1,4-Diamino Motif and Application in Catalytic Asymmetric Alkynylation of meso-Epoxides

Article abstract of DOI:10.1055/s-2004-815401

The efficient asymmetric preparation of the C₂-symmetric chiral 1,4-diamine, (1R,2S,4R,5S)-1,4-diamino-2,5-dimethylcyclohexane (5), and its salen ligands is described. With the Mitsunobu reaction as the key step, the overall yield of the four-s

Full text of DOI:10.1055/s-2004-815401

LETTER
465
The Synthesis of New C2-Symmetric Chiral 1,4-Diamino Motif and
Application in Catalytic Asymmetric Alkynylation of meso-Epoxides
The Synthesis of
h
New C2-Symemetric Chi
n
ral 1,4-Dia
m
g
ino Motif jian Zhu,*a,b Minghua Yang,a Jiangtao Sun,a Yuhua Zhu,a Yi Pana
a
School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093,
P. R. China
b
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, P. R. China
Fax +86(25)3317761; E-mail: cjzhu@netra.nju.edu.cn
Received 29 October 2003
reduction in up to 96% yield.6 Following the established
procedure,7 exposure of 2 to excess enantiomerically pure
monoisopinocampheylborane [from (1R)-(+)-a-pinene]
followed by oxidation with basic hydrogen peroxide, gave
Abstract: The efficient asymmetric preparation of the C2-symmet-
ric chiral 1,4-diamine, (1R,2S,4R,5S)-1,4-diamino-2,5-dimethylcy-
clohexane (5), and its salen ligands is described. With the
Mitsunobu reaction as the key step, the overall yield of the four-step
synthesis of 5 is 45%. The enantioselective alkynylation of meso- the enantiomerically pure diol 3, (1S,2S,4S,5S)-1,4-dihy-
epoxides catalyzed by chiral gallium complexes is also achieved.
droxy-2,5-dimethylcyclohexane, in 66% yield after sepa-
ration by silica gel chromatography and recrystallization.8
The absolute configuration was determined by compari-
son of the optical rotation with its enantiomer which was
formed from (1S)-(–)-a-pinene.7b
Key words: 1,4-diamine, salen ligand, alkynylation, gallium com-
plex, asymmetric synthesis
The development of novel chiral ligands is crucial to the
advancement of asymmetric catalysis.1 Diamine ligands
are one of the most common types of nitrogen-containing
ligands which are becoming applicable for catalytic asym-
metric synthesis, and enantiomerically pure diamines are
constituents of many natural compounds and commonly
used in various areas, particularly in pharmaceutical and
medicinal chemistry.2 It has been reported that chiral 1,4-
diamine have served as key intermediate in the synthesis
of potent HIV protease inhibitors.3 Although numerous
studies describing the enantioselective synthesis and the
applications in catalytic asymmetric synthesis of 1,2-di-
amines have been performed, there are only a few exam-
ples involving 1,4-diamine have been reported in
literature.4 This prompted us towards the development of
new chiral 1,4-diamine ligands, which can be applicable
to miscellaneous asymmetric reactions. We envisaged
that the ligands derived from 1,4-diamine with cyclohex-
ane backbone could provide a better face or site discrimi-
nation in asymmetric reactions. Very recently, Berkessel
reported that the chiral 1,4-diamine (endo,endo-2,5-di-
amino-norborane) based salen ligands promoted efficient
and highly enantioselective catalytic Nozaki–Hiyama–
Kishi reactions.5 Here we wish to report the synthesis of a
new C2-symmetric chiral ligand, (1R,2S,4R,5S)-1,4-di-
amino-2,5-dimethylcyclohexane, and the application of
its salen ligands to the asymmetric alkynylation reaction
of meso epoxides catalyzed by gallium complexes.
Scheme 1 Synthesis of 1,4-diamine 5. Reagents and conditions: (a)
Li/NH3 (l)–EtOH, –40 °C, 96%; (b) (i) IpcBH2, Et2O, –25 °C; (ii)
H2O2, NaOH, 66%; (c) (PhO)2P(O)N3, Ph3P, EtO2CN=NCCO2Et,
THF, 85%; (d) (i) LiAlH4, THF, 0 °C; (ii) NaF, H2O; (iii) HCl (g),
THF, 83%.
For the conversion of the diol 3 to the diamine 5, we ini-
tially attempted to introduce the azide functionality by
displacement of the tosylate or triflate of 3 with sodium or
lithium azide.9 Unexpectedly, both the tosylate and triflate
of 3 are unreactive to those reagents. After other several
unsuccessful attempts had been tried, we based our strat-
egy for the synthesis of the diamine on the Mitsunobu re-
action,10 which proved to be the best key step in terms of
the yields and simplicity. Substitution of the diol 3 with
diphenylphosphoryl azide under Mitsunobu condition in
the presence of triphenylphosphine and diethyl azodicar-
As outlined in Scheme 1, the asymmetric synthesis of 5 boxylate in tetrahydrofuran gave the diazide 4 in 85%
started with the conversion of 1,4-dimethylbenzene (1) yield.11 Compound 4 was then reduced with LiAlH4 in tet-
into 1,4-dimethylcyclohexadiene (2) by a modified Birch rahydrofuran at 0 °C to give 1,4-diamino-2,5-dimethylcy-
clohexane,12 which was then treated with dry hydrogen
chloride provided dihydrochloride salt of 5 (83% yield
from 4) for easy purification and higher stability.13 The
absolute configuration of 1,4-diamino-2,5-dimethylcyclo-
SYNLETT 2004, No. 3, pp 0465–0468
Advanced online publication: 12.01.2004
DOI: 10.1055/s-2004-815401; Art ID: U22403ST
© Georg Thieme Verlag Stuttgart · New York
1
8.
0
2.
2
0
0
4

466
C. Zhu et al.
LETTER
hexane was determined as (1R,2S,4R,5S), since the provided by equivalent BF3·OEt2 system using tridentate
stereospecific conversion of the stereocenters during oxygen-containing chiral ligand (22% ee).16 Comparing
Mitsunobu reaction. It could be expected that this new entries 1 and 2, 5 and 6, it can be found that the steric bulk-
chiral motif could be derivatized in a broad range to serve iness of t-Bu substituents in the salicylidene component
as variable stereo directing reagents or ligands in mis- induced a positive effect in the enantioselectivity of the
cellaneous asymmetric synthesis.
reaction. By the way, the addition of 4Å molecular sieves
did not improve the reactivity or selectivity for this reac-
tion. To our best knowledge, this is the first example of
asymmetric alkynylation reaction of meso-epoxides cata-
lyzed by chiral gallium complexes (Scheme 3).
The desymmetrization of meso epoxides via the enantio-
selective addition of nucleophiles is an efficient strategy
for asymmetric synthesis since it simultaneously estab-
lishes two contiguous stereogenic centers.14 This type of
reaction has been successfully accomplished with various
nucleophiles.15 The oxirane ring cleavage by metal acetyl-
ides forms trans-b-hydroxyacetylenes, which could be
one of the very useful synthons for the synthesis of a vari-
ety of optically active compounds. Tomioka and cowork-
ers have reported the enantioselective reaction of
cyclohexene oxide with phenylethynyl lithium using
stoichiometric amount of BF3·OEt2 in very poor enantio-
selectivity.16 Given that trimethylgallium, a remarkably
effective catalyst for the ring opening reaction of oxiranes
with alkylnyl lithium,17 we considered the asymmetric
C
CPh
HO
O
GaMe3 + chiral ligand
(10 mol%)
*
*
PhC CLi
+
Toluene
n
n
Scheme 3 Enantioselective phenylethynylation of meso-epoxides.
Table 1 Results of Enantioselective Ring Opening of meso-Ep-
oxides with Phenylethynyl Lithium Catalyzed by Chiral Gallium
Complexes
alkynylation reaction of meso-epoxide catalyzed by chiral Entry Epoxide
gallium complexes with 5 based salen ligands.
Ligand Temp Time Yield Ee
(°C)
(h)
(%)a
(%)b
Following the procedure established in the literature,18 the
chiral salen ligands 6 and 7 were readily prepared by the
reaction of dihydrochloride salt of 5 with salicylaldehyde
or its derivative 3,5-di-tert-butyl-2-hydroxybenzaldehyde
1
6
7
8
9
6
7
9
7
0
48
54
34
O
2
0
0
0
0
0
0
48
48
48
60
60
60
60
50
57
46
42
45
34
55
30
47
27
45
38
41
O
in ethanol in the yield of 86% and 89%, respectively
(Scheme 2).19
3
O
4
O
N
N
CHO
OH
5
6
7
8
O
O
O
+ R
5
HO
R
R
OH
R
R
R
6 R = H
7 R = t-Bu
Reflux 72
O
Scheme 2 Synthesis of salen ligands 6 and 7. Reagents and condi-
tions: K2CO3, EtOH, 80 °C.
a Isolated yields.
b Enantioselectivity excess values were determined by HPLC analysis
using a DAICEL CHIRAL OD-H column.
Treatment of the ligands with equal equivalents of trime-
thyl gallium gave the catalysts, which were not isolated,
and directly used. Toluene was chosen as the solvent ac-
cording to the literature.16 Asymmetric ring openings of
cyclohexene oxide, cyclopentene oxide and 1,4-dihy-
dronaphthalene oxide with phenylethynyl lithium were
examined in the presence of chiral gallium catalysts with
various kinds of ligands.20 As summarized in Table 1, the
expected b-phenylethynyl hydrins have been obtained in
yields varying from 34% to 60%, with the enantioselectiv-
ity varying from 27% to 55% depending on the nature of
the catalyst used. The reaction of cyclohexene oxide with
phenylethynyl lithium using ligand 7 afforded the product
in 55% ee (entry 1), which is much higher than that
N
N
HO
R
R
OH
R
R
8 R = H
9 R = t-Bu
Figure 1
Synlett 2004, No. 3, 465–468 © Thieme Stuttgart · New York

LETTER
The Synthesis of New C2-Symmetric Chiral 1,4-Diamino Motif
*
467
N
N
GaMe2
O
HO
A
_
*
CH3
OH
N
N
Li+
Ga
O
O
O
Ph
CH3
B
_
_
*
*
H3C
H3C
N
N
N
N
Li+
Li+
Ga
Ga
O
O
O
O
CH3
CH3
O
O
-C
C
Ph
C
D
Ph
C
C- Li+
Scheme 4 Possible mechanism for the ring opening of meso epoxide with phenylethynyl lithium catalyzed by chiral Ga-complex.
For comparison of the ligand effect, salen ligands 8 and 9 Mitsunobu reaction, was efficiently accomplished. We
(Figure 1), which were derivatized from (1R,2R)-1,2-di- also achieved the first enantioselective alkynylation of
aminocyclohexane, were also examined in this reaction meso epoxides catalyzed by chiral gallium complexes. Al-
(entries 3, 4 and 7), and provided lower selectivity than though the enantioselectivity of the present reaction is
that of corresponding 1,4-analogues. This result prelimi- modest, an obvious improvement of our new ligand than
narily demonstrates that 1,4-diamino motifs are more ef- that of 1,2-diaminocyclohenxane was observed. Further
ficient on the enantioselectivity in the alkynylation studied on the application of 1,4-diamino-2,5-dimethyl-
reaction than that of 1,2-diamino analogues.
cyclohexane on other asymmetric reaction are now in
progress in our research group.
A proposal mechanism for the catalytic asymmetric ep-
oxide opening is shown in Scheme 4. The salen ligand
reacted with trimethylgallium gave a monometallic open
chain compound A as a precatalyst.21,22 Ring closure ate
complex B, which was the de facto catalytic species in the
catalytic cycle, was most possibly formed as A was treat-
ed with phenylethynyl lithium.23 The gallium metal ap-
peared to function as a Lewis acid, activating epoxide and
also controlling the orientation of epoxide due to coordi-
nation of an axial lone pair, allowing for the cleavage of
C–O bond by backside attack. The catalyst derived from
1,4-diamino ligand with the cyclohexene backbone as a
‘chiral wall’, can provide a better site discrimination
around the center metal than that of 1,2-diamino counter-
part, in which the chiral centers are just in the plane with
the center metal.
Acknowledgment
We gratefully acknowledge the National Natural Science Foundati-
on of China, the 863 High Technology Program, Science Foundati-
on of Jiangsu Province and the Key Laboratory of Fine
Petrochemical Technology of Jiangsu Province for their financial
support.
References
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In summary, the synthesis of new chiral C2-symmetric
1,4-diamino-2,5-dimethylcyclohexane, based on the
Synlett 2004, No. 3, 465–468 © Thieme Stuttgart · New York

468
C. Zhu et al.
LETTER
2 H), 2.00–2.08 (m, 4 H), 1.81–1.85 (m, 2 H), 1.06 (d,
(4) (a) Tanyeli, C.; Ozcubukcu, S. Tetrahedron: Asymmetry
2003, 14, 1167. (b) Jiang, B.; Feng, Y.; Hang, J. F.
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J = 6.6 Hz, 6 H) ppm. IR (KBr): nC=N = 1627.6 cm–1. MS
(EI): m/z (%) = 350.2 (100) [M+], 335.1 (3), 320.1 (2), 229.1
(43), 188.1 (57), 122.1 (78). Anal. Calcd for C22H26N2O2: C,
75.40; H, 7.48; N, 7.99. Found: C, 75.21; H, 7.44; N, 7.83.
For 7: yellow solid, mp >210 °C. [a]D25 +17.5 (c 2.0,
CH2Cl2). 1H NMR (300 MHz, CDCl3): d = 13.78 (s, 2 H),
8.42 (s, 2 H), 7.42 (d, J = 2.1 Hz, 2 H), 7.14 (d, J = 2.1 Hz,
2 H), 3.43–3.46 (m, 2 H), 2.01–2.07 (m, 4 H), 1.82–1.88 (m,
2 H), 1.49 (s, 18 H), 1.36 (s, 18 H), 1.06 (d, J = 5.9 Hz, 6 H)
ppm. IR (KBr): nC=N = 1628.8 cm–1. MS (EI): m/z (%) =
574.2 (100) [M+], 559.3 (17), 544.3 (2), 341.3 (29), 272.1
(32), 69.0 (100). Anal. Calcd for C38H58N2O2: C, 79.39; H,
10.17; N, 4.87. Found: C, 79.30; H, 10.23; N, 4.72.
Organometallics 2000, 19, 3824.
(8) (1S,2S,4S,5S)-(+)-1,4-dihydroxy-2,5-
Dimethylcyclohexane(3): white solid, mp 120–121 °C.
[a]D25 +32.7 (c 1.0, CH2Cl2). 1H NMR (300 MHz, CDCl3):
d = 3.51–3.57 (m, 2 H), 1.87–1.93 (m, 2 H), 1.68–1.78 (m, 4
H), 1.53–1.59 (m, 2 H), 1.01 (d, J = 6.9 Hz, 6 H) ppm. IR
(KBr): nOH = 3313.8 cm–1. MS (EI): m/z (%) = 144.1 (4)
[M+], 126.0 (4), 111.1 (10), 72 (100). Anal. Calcd for
C8H16O2: C, 66.63; H, 11.18. Found: C, 66.34; H, 11.22.
(9) (a) Reddy, D. R.; Thornton, E. R. J. Chem. Soc., Chem.
Commun. 1992, 172. (b) Schmidt, R. R.; Zimmermann, P.
Tetrahedron Lett. 1986, 27, 481.
(10) (a) Lal, B.; Pramanik, B. N.; Manhas, M. S.; Bose, A. K.
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K.; Mackin, G. H.; Overman, L. E.; Shaka, A. J. J. Org.
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(11) (1R,2S,4R,5S)-(–)-1,4-diazido-2,5-dimethylcyclohexane(4):
yellow oil. [a]D25 –44.6 (c 0.2, THF). 1H NMR (300 MHz,
CDCl3): d = 3.59–3.64 (m, 2 H), 1.95 (br. s, 2 H), 1.83 (ddd,
J = 13.5, 7.5, 4.2 Hz, 2 H), 1.64 (ddd, J = 13.5, 7.5, 4.2 Hz,
2 H), 1.11 (d, J = 7.0 Hz, 6 H) ppm. IR (KBr): n (-N3) =
2097.9 cm–1. MS (EI): m/z (%) = 194.0 (1) [M+], 166.1 (1),
141.0 (1), 109.1 9 (7), 81.0 (20), 42 (100). Anal. Calcd for
C8H14N6: C, 49.47; H, 7.27; N, 43.27. Found: C, 49.32; H,
7.23, N 43.41.
(20) A Typical Procedure for the Ring Opening of meso
Epoxides by Phenylethynyl Lithium: GaMe3 (0.2 mL, 0.1
mmol, 0.5 M in toluene) was added dropwise to the toluene
solution of ligand 7 (57 mg, 0.10 mmol) under argon
atmosphere. The mixture was stirred for 2 h at r.t. to form the
salen–Ga complex. The toluene solution of phenylethynyl
lithium, which was prepared from the reaction of n-BuLi
(1.6 mmol in 1 mL hexane) and phenylacetylene (153 mg,
1.5 mmol) at –78 °C before use, was transferred to above
solution of salen-Ga complex at 0 °C and stirred for another
1 h. Then the cyclohexene oxide (101 mL, 1.0 mmol) was
added slowly to the mixture using a syringe at 0 °C. The
resulting mixture was stirred for 48 h at the same
temperature before being quenched with a sat. NH4Cl
solution and extracted with EtOAc. The organic phase was
dried over Na2SO4, and removed the solvent. After being
separated by preparative silica gel TLC, 2-(2-phenylethynl)-
1-cyclohexanol (120 mg) was obtained as yellow oil in the
yield of 60% with 55% ee. [a]D25 –57.2 (c 0.7, CHCl3). The
ee value was determined by HPLC analysis (DAICEL
CHIRAL OD-H column, hexane/i-PrOH = 30:1, 0.5 mL/
min, 254 nm): tR = 22.35 min, 23.77 min. 1H NMR16 (300
MHz, CDCl3): d = 7.39–7.48 (m, 2 H), 7.30–7.39 (m, 3 H),
3.53–3.60 (m, 1 H), 2.46–2.48 (m, 1 H), 1.24–2.11 (m, 9 H)
ppm. IR(neat): n = 3429.4, 2229.8 cm–1.
(12) Kobayashi, Y.; Shiozaki, M.; Ando, O. J. Org. Chem. 1995,
60, 2570.
(13) Dihydrochloride of (1R,2S,4R,5S)-(+)-1,4-diamino-2,5-
25
dimethylcyclohexane(5): white solid, mp >200 °C. [a]D
+3.3 (c 0.5, H2O). 1H NMR (300 MHz, D2O): d = 3.40–3.45
(m, 2 H), 2.13–2.18 (m, 2 H), 1.75–1.88 (m, 4 H), 1.00 (d,
J = 7.3 Hz, 6 H) ppm. IR (KBr): n = 3456.1, 3022.3, 2895.0,
1652.9, 1558.8, 1465.2, 1454.8, 1399.5 cm–1. MS (EI): m/z
(%) = 142.1(2) [M+– 2HCl], 125.1 (7), 110.1 (6), 83.1 (6),
72.0 (100). Anal. Calcd for C8H18N2Cl2: C, 45.08; H, 8.51;
N, 13.14. Found: C, 44.93; H, 9.30; N, 13.25.
(21) Hill, M. S.; Wei, P.; Atwood, D. A. Polyhedron 1998, 17,
811.
(22) Identification of the pre-catalysts. Analytical data of the
complex formed from ligand 6 and GaMe3 (1:1 molar ration
at r.t. in toluene): 1H NMR (300 MHz, CDCl3): d = 13.43 (s,
1 H), 8.36 (s, 1 H), 8.11 (s, 1 H), 7.30–7.39 (m, 4 H), 6.73–
7.01 (m, 4 H), 3.90–3.94 (m, 1 H), 3.38–3.44 (m, 1 H), 2.37–
2.39 (m, 1 H), 2.20–2.24 (m, 2 H), 1.93–2.20 (m, 2 H), 1.59–
1.63 (m, 1 H), 1.00–1.02 (d, J = 7.3 Hz, 6 H), –0.21 (s, 3 H),
–0.31 (s, 3 H) ppm. MS (EI): m/z (%) = 434.9 (6) (M+ – 15),
417.0 (100), 333.0 (2), 222.9 (5), 188.9 (8), 122.0 (7), 109.0
(3), 68.9 (12). Anal. Calcd for C24H31N2O2Ga: C, 64.17; H,
6.95; N, 6.24. Found: C, 64.35; H, 7.10; N, 6.38. Analytical
data of the complex formed from ligand 7 and GaMe3 (1:1
molar ration at r.t. in toluene): 1H NMR (300 MHz, CDCl3):
d = 13.55 (s, 1 H), 8.11 (s, 1 H), 8.08 (s, 1 H), 7.42–7.43 (d,
J = 2.3 Hz, 1 H), 7.13–7.14 (d, J = 2.3 Hz, 1 H), 7.03–7.04
(d, J = 2.5 Hz, 1 H), 6.90–6.91 (d, J = 2.5 Hz, 1 H), 3.50–
3.51 (m, 1 H), 3.37–3.39 (m, 1 H), 1.51–2.42 (m, 6 H), 1.48
(s, 9 H), 1.48 (s, 9 H) 1.33 (s, 9 H), 1.31 (s, 9 H), 0.99–1.02
(d, J = 6.3 Hz, 6 H), –0.21 (s, 3 H), –0.36 (s, 3 H). MS (EI):
m/z (%) = 657.4 (14) (M+ – 15), 642.4 (100), 321.6 (5), 313.8
(10), 68.9 (9). Anal. Calcd for C40H63N2O2Ga: C, 71.32; H,
9.43; N, 4.16. Found: C, 71.54; H, 9.52; N, 3.98.
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NMR (300 MHz, CDCl3): d = 13.57 (s, 2 H), 8.40 (s, 2 H),
7.28–7.36 (m, 4 H), 6.88–7.02 (m, 4 H), 3.50 (d, J = 3.0 Hz,
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Synlett 2004, No. 3, 465–468 © Thieme Stuttgart · New York