Titanium(IV)/tridentate BINOL derivative as catalyst for meso-aziridine ring-opening reactions: High enantioselectivity, strong positive non-linear effect and structural characterization

Article abstract of DOI:10.1002/adsc.200800402

A titanium-based chiral Lewis acid was found to be effective for the ring-opening reactions of meso-aziridines with aniline nucleophiles. The products were generally isolated in high yields and with high to excellent enantioselectivity. The catalytic syst

Full text of DOI:10.1002/adsc.200800402

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DOI: 10.1002/adsc.200800402
Titanium(IV)/Tridentate BINOL Derivative as Catalyst for meso-
Aziridine Ring-Opening Reactions: High Enantioselectivity,
Strong Positive Non-Linear Effect and Structural Characterization
Rongmin Yu,a Yasuhiro Yamashita,a and Shu¯ Kobayashia,*
a
Department of Chemistry, School of Science and Graduate School of Pharmaceutical Sciences, The University of Tokyo,
The HFRE Division, ERATO, JST, Hongo, Bunkyo-ku, Tokyo 113-0030, Japan
Fax : (+81)3-5684-0634; e-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
Received: June 28, 2008; Revised: October 15, 2008; Published online: December 19, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800402.
Abstract: A titanium-based chiral Lewis acid was
found to be effective for the ring-opening reactions
of meso-aziridines with aniline nucleophiles. The
products were generally isolated in high yields and
with high to excellent enantioselectivity. The cata-
lytic system was studied by X-ray single crystal
analysis. In experiments on non-linear effects a
strong non-linear effect of the catalyst system was
observed.
allowing many useful molecules to be synthesized.
Recently, enantioenriched ring-opened products have
been obtained when reactive nucleophiles such as
TMSCN, MeMgBr, and TMSN3 were employed,[2]
however, use of weaker nucleophiles like anilines has
been less successful. In our previous report, we de-
tailed asymmetric reactions of meso-azirdines with
anilines promoted by the Nb(V)/tetradentate BINOL
ligand (L1) that provided vicinal diamines in good to
high yields, but the best ee was 84% [Scheme 1,
Keywords: meso-aziridines; desymmetrization; non-
linear effect; titanium; vicinal diamines; X-ray
structure
M(OR)n =NbACTHNGUTERNNUG
(OMe)5].[3] Because of the importance
of the reaction, we searched for more effective and
selective catalysts. Herein, we report a Ti(IV)/triden-
tate BINOL derivative L2 [Scheme 1, M(OR)n =
TiACTHNUGTRNE(UNG O-i-Pr)4] catalyst that leads to excellent enantiose-
lectivity in the reaction.
In combination with various titanium sources,
The synthesis and transformation of aziridines has at- BINOL and its derivatives form highly effective
tracted much attention especially in the last decade.[1] asymmetric catalysts that have been used successfully
Due to the strain of the three-membered ring, aziri- in a range of enantioselective syntheses.[4] In most sys-
dines are reactive towards a variety of nucleophiles, tems, however, the substrates are limited to alde-
Scheme 1. Asymmetric aziridine ring opening reaction with M(OR)n/BINOL-derived complex.
Adv. Synth. Catal. 2009, 351, 147 – 152
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147

Rongmin Yu et al.
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hydes, ketones, and epoxides. The fact that Ti(IV)/ atic. We scrutinized the reaction conditions and ascer-
BINOL catalysts activate nitrogen-containing sub- tained that the presence of water was indispensable to
strates effectively to perform enantioselective reac- form an effective catalyst in the system.[6]
tions is unusual,[5] probably due to the strong affinity
We then investigated in detail the optimization of
between Ti(IV) and the substrates. Thus, it was very the the reaction conditions. When the ratio of
surprising when we found that the combination of H2O:Ti(O-i-Pr)4 was 2: 1, the reaction gave the best
Ti(O-i-Pr)4 and BINOL-derived tridentate ligand L2 result (Table 1, entry 4). The addition of anhydrous
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
smoothly catalyzed the reaction of aziridine 1a and MgSO4 was still required to improve the reaction
aniline 2a with promising results (Table 1, entry 1). To (entry 5). Comparing solvents revealed that toluene
the best of our knowledge, this is the first example of was the best medium for this reaction, and, surprising-
a Ti complex promoting a ring-opening reaction of an ly, the addition order was critical. The best results
aziridine.
were obtained when water was added to a mixture of
the catalyst, aziridine 1a and MgSO4. Investigation of
starting material stoichiometries revealed that an
excess aziridine leads to higher selectivity and slow
addition of aniline was conducive to high enantiose-
lectivity. Overall, under the optimized conditions the
ring-opened product was obtained in high yield and
excellent enantioselectivity (93% yield and 95% ee).
The amount of catalyst could be reduced to 1 mol%
without significantly affecting the enantioselectivity.
Having established the optimal catalyst and reac-
tion conditions, we examined the scope of the reac-
tion with a variety of differently substituted anilines.
As shown in Table 2, excellent enantioselectivity was
obtained in the aziridine ring-opening reaction with
several anilines. Both electron-withdrawing and elec-
tron-donating substituents were suitable. Generally,
meta-substituted anilines provided better selectivity,
and ortho-substituted anilines gave the desired 1,2-di-
amines in lower yields and selectivity.
Table 1. Asymmetric ring-opening reaction of meso-aziridine
1a by chiral Ti(IV)/L2 complex.[a]
Entry
H2O:Ti
(O-i-Pr)4
Solvent
Yield [%]
ee [%]
1[b]
2
–
–
–
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
Et2O
toluene
toluene
toluene
toluene
toluene
70
73
35
91
97
97
61
51
quant.
quant.
88
93
79
64
77
56
84
89
75
33
51
82
90
90
95
92
3[c]
4[b]
5
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
6[d]
7[b]
8[b]
9[e]
10[f]
11[g]
12[g,h]
13[g,i]
[a]
Reaction between 1a (0.2M) and 2a (1.0 equiv.) was con-
ducted in the presence of Ti(O-i-Pr)4/L2 (10 mol%) and
AHCTUNGTRENNUNG
MgSO4 in organic solvent at 08C for 40 h.
No MgSO4.
Glassware was flame-dried.
Water was added immediately followed by addition of 1a
[b]
[c]
[d]
Table 2. Substrate scope of asymmetric ring-opening reac-
tion of meso-aziridine 1a by chiral Ti(IV)/L2 complex.
and MgSO4.
[e]
[f]
[g]
[h]
[i]
2a: 1.5 equiv.
2a: 0.5 equiv.
À108C.
Entry
Cat. X
[mol%]
Amine
Yield
[%]
ee
[%]
G
1[b]
2[b]
3
2
aniline (2a)
p-toluidine (2b)
m-toluidine (2c)
o-toluidine (2d)
p-chloroaniline (2e)
m-chloroanilne (2f)
o-chloroaniline (2g)
p-fluoroaniniline (2h)
m-fluoroaniline (2i)
p-bromoaniline (2j)
m-bromoaniline (2k)
p-iodoaniline(2l)
m-iodoaniline (2m)
p-anisidine(2n)
88
63
81
42
90
84
49
62
82
62
74
69
54
51
51
92
90
93
73
91
93
82
85
91
89
88
89
78
73
79
2a: 1.2 equiv, slow addition over 22.5 h.
Catalyst loading was 1.0 mol%, and substrate concentra-
tion was 0.1M.
10
10
5
10
5
10
2.5
5
5
5
5
5
4
5[b]
6[b]
7[b]
8
The Ti(IV)/L2 complex was prepared from the re-
action of TiACHTUNGTRENNUNG(O-i-Pr)4 and one equivalent of L2 at
608C for 3 h. Owing to the potential decomposition
of titanium phenoxide in water, we reasoned that the
removal of trace water from the reaction system
9
10
11
should be helpful. After trials of several drying re- 12
13
agents, we were pleased to find that the selectivity of
the reaction was improved considerably when anhy-
drous MgSO4 was added (Table 1, entry 2). However,
when the reaction was performed under rigorously
anhydrous conditions, it was found that the yield and
the selectivity decreased (Table 1, entry 3). In addi-
tion, the reproducibility of the reaction was problem-
14[b]
15
10
5
m-anisidine (2o)
[a]
Reaction between 1a (0.2M) and 2 (1.0 equiv.) was con-
ducted in the presence of Ti(O-i-Pr)4/L2 (X mol%), H2O
AHCTUNGTRENNUNG
(2X mol%) and MgSO4 in toluene at À108C for 40 h.
[b]
2 (1.2 equiv.) was added over 22.5 h.
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Titanium(IV)/Tridentate BINOL Derivative as Catalyst
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Other aziridines were also tested in the Ti(IV)/L2 While high yield and enantioselectivity were also ob-
catalyst system, and the results are shown in Table 3. tained using aziridine 1a, it was surprising to find that
It was found that the Ti(IV)/L2 catalyst worked well especially the enantioselectivity decreased using simi-
with some other aziridines. Whilst aziridine 1b lar aziridines 1e and 1f. It is noted that aziridine 1g
showed lower reactivity and enantioselectivity without the o-alkoxy substituent gave the correspond-
(entry 2), the reaction at higher temperature im- ing 1,2-diamine in 94% yield with 72% ee.
proved the yield with still relatively good ee (entry 3).
Elucidation of the nature and structure of catalysts
When aziridines 1c and 1d were employed, good or their precursors provides important information
yields and moderate to good enantioselectivities were for the rational design of new catalysts. Ti/BINOL
obtained (entries 4 and 5).
complexes are infamous in that they exhibit various
Furthermore, the effect of N-substituents was ex- coordination numbers and geometries, undergo rapid
amined in the Ti(IV)/L3 catalyst system (Table 4). and facile change in coordination environment, and
have a distinct tendency to form multipart oligomers
or supramolecular assemblies.[7] Such structural com-
Table 3. Asymmetric ring-opening reactions of meso-aziri-
dines by chiral Ti(IV)/L2 complex.[a]
plexity often frustrates efforts towards structural ra-
tionalization of catalytic behavior. Our efforts to
grow crystals suitable for single crystal diffraction
studies of enantiopure Ti(IV)/L2 complex have been
unsuccessful to date. After several disappointing
trials, we changed our strategy and grew crystals from
racemic mixtures. Finally, crystals of a binuclear tita-
Entry
Aziridine
Yield [%]
ee [%]
1
1a
93
30
89
95
71
60
2
3[b]
1b
1c
1d
nium compound, Ti
and a pentatitanium compound, Ti
(O-i-Pr)2 (rac-5),[9] were obtained. Their structures are
2ACHTUNGTREN(GNUN R-L2)ACHTUNTRGEG(NNUN S-L2)ACUTHNGTRENNUNG
C
CHTUNGTRENNUNG
4[c]
85
74
68
61
AHCTUNGTRENNUNG
shown in Figure 1 and Figure 2 (see also Scheme 2).
Yellow crystals of compound RS-4 were isolated
from a mixture of an equal amount of enantiopure
Ti(IV)/L2 complexes in ether. As shown in Figure 1,
the molecule possesses a crystallographic inversion
center. The titanium atoms are five-coordinate with a
highly distorted trigonal bipyrimidal geometry. The
tridentate ligands adopt a double bidenate chelating
form with the central aryloxo as a bridge to link the
5[c]
[a]
Reaction between aziridine 1 (0.2m) and 2a (1.0 equiv)
was conducted in the presence of TiACTHNUTRGEN(UNG O-iPr)4/L2 (10
mol%) and MgSO4 in toluene at À108C for 40 h.[b]
508C.[c] 08C.
Table 4. Asymmetric ring-opening reactions of meso-aziri-
dines by chiral Ti(IV)/L3 complex.[a]
Entry
1
Aziridine
Yield [%]
94
ee [%]
1a
91
2
3
1e
68
72
1f
1g
quant.
94
76
72
4
[a]
Reaction between aziridine 1 (0.2M) and 2a (1.0 equiv.)
was conducted in the presence of Ti(O-i-Pr)4/L3 (10
mol%) and MgSO4 in toluene at À108C for 40 h.
Figure 1. ORTEP drawing of RS-4 (40% ellipsoids). Hydro-
gen and selected carbon atoms have been omitted for clari-
ty.
G
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Rongmin Yu et al.
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Figure 2. ORTEP drawing of rac-5 (40% ellipsoids). Hydrogen and selected carbon atoms have been omitted for clarity.
observed in the crytal structure (one unit is depicted
in Figure 2 for clarity).
To obtain further information of the catalysts, we
carried out catalysis with low ee ligands/complexes to
ascertain if any non-linear effects (NLE) were operat-
ing. While the catalytic systems of titanium and
Scheme 2. Preparation of the chiral Ti(IV) catalysts.
BINOL frequently exhibit strong NLE in several re-
actions,[10] our results shown in Table 5 and Figure 3
titanium atoms. The dihedral angle of the binaphthyl are interestingly unique if a little complicated. First,
is 64.01(9)8.
when non-racemic L2 was used for the preparation of
Compound Ti5ACHTNUGTNERN(UNG m3-O)2ACHUTNGTREGN(NNU m-O)(L2)4ACUTHNGTRNNEN(UG O-i-Pr)2 (rac-5) non-racemic titanium catalysts and RS-4, a significant
were obtained from an equimolar mixture of R-4/H2O
and S-4/H2O. This compound contains a new arrange-
ment of five Ti(IV) atoms which are linked by two m3-
O atoms and a m2-O atom. The molecule has a crystal-
lographic C2 symmetry axis that runs through the m2-
O atom (O2) and Ti3 metal (Figure 2). The central ti-
tanium atom (Ti3) is six-coordinate with adopts a dis-
torted octahedral geometry. The other Ti atoms are
five-coordinate with distorted trigonal bipyramidal ar-
rangements. BINOL is capable of an impressive struc-
ture flexibility allowing it to accommodate the range
of required chelating or bridging coordination geome-
tries without significant strain, this is also true for the
tridenate ligand L2. In compound 5, one unique tri-
dentate ligand adopts a double bidenate chelating
form as that in compound RS-4, and its dihedral angle
is 59.3(2)8. However, another independent tridenate
ligand has a different coordinative mode where it
binds to all three independent Ti atoms. The outer
BINOL-oxo links two Ti atoms., and the dihedral
angle of B is 51.5(2)8. It should be reiterated that 5 is
racemic, and equal amounts of both enantiomer are
Table 5. NLE results in asymmetric ring-opening reaction of
aziridines by Ti(IV)/L.[a]
Method A[b]
Method B[b]
Method C[b]
ee [%] ee [%] of ee [%] of ee [%] of ee [%] ee [%] of
of R-L2 product R-4/H2O product of R-4 product
23
42
70
100
84
88
89
90
25
50
75
100
26
45
63
76
25
50
75
100
42
69
84
88
[a]
Reaction between 1a (0.2M) and 2a (1.0 equiv.) was con-
ducted in the presence of non-racemic catalyst (5 mol%),
H2O (10 mol%) and MgSO4 in toluene at À108C for
40 h.
[b]
In Method A, non-racemic L2 was used for the prepara-
tion of the catalyst. In Method B, the catalyst was pre-
pared from mixing R-4/H2Oand S-4/H2O, which were
prepared and treated with H2O separately in advance. In
Method C, the catalyst was prepared from mixing R-4
and S-4, which were prepared separately in advance.
H2O was added after mixing.
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Titanium(IV)/Tridentate BINOL Derivative as Catalyst
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from the studies of NLE indicate the complexity of
this catalytic system. Studies on the substrate scope
and the application of this new Ti/L2 catalyst in other
reactions are in progress.
Experimental Section
Typical Procedure for Asymmetric Ring Opening
Reaction of Aziridines
TiACTHNUTRGNEU(GN O-i-Pr)4 (7.1 mg, 0.025 mmol) and L2 (10.8 mg,
0.025 mmol) in toluene (1.25 mL) was stirred at 608C for
3 h. After cooling to room temperature, the resulting yellow
solution was added to a 10-mL flask in which aziridine 1a
(0.25 mmol) and MgSO4 (50 mg) were placed. The yellow
suspension was stirred at 08C for 30 min, and then 3.0M so-
lution of water in 2-propanol (16.5 mL) was added. The mix-
ture turned to dark red, and was stirred at the temperature
for another 30 min. The mixture was cooled to À108C, and
2a (0.30 mmol) in toluene (0.25 mL) was added over a
period of 22.5 h using a syringe pump. After the addition of
2a, the reaction mixture was stirred for further 17.5 h, and
then quenched with saturated NaHCO3 (10 mL). The mix-
ture was extracted with CH2Cl2 (3ꢁ10 mL). The organic ex-
tracts were combined, and dried with anhydrous Na2SO4.
After filtration and concentration, the crude product was
purified by preparative thin-layer chromatography (ben-
Figure 3. NLE profiles of chiral Ti/L catalyst prepared by
Method A (diamond), Method B (triangle), and Method C
(square).
positive NLE was observed (Method A). This result
might be explained by the nature of homochiral com-
plexes. Indeed, RS-4 is not soluble in common sol-
vents, like CH2Cl2, CH3OH, toluene, and diethyl
ether. It is very stable, and no changes were observed
when water was added to its suspension in toluene. In
contrast, it was assumed that the homochiral com-
plexes reacted readily with water in toluene to form
catalytically active species. Thus, the preferential for-
mation of RS-4, the difference of the reactivity to
water and solubility in toluene between homochiral
compounds and RS-4 were considered to be the rea-
sons for the strong NLE. Next, R-4/H2O and S-4/H2O
were used for the preparation of non-racemic titani-
um catalysts, and NLE experiments were performed
(Method B). Interestingly, the ees of the products
were almost equal to the ees of the catalysts. It was
assumed that rac-5 was formed preferentially, and
that the catalyst was stable in solution, no ligand
scrambling was observed under the reaction condi-
tions. In addition, it was confirmed that rac-5 was
active for the aziridine ring-opening reaction. Finally,
NLE experiments were conducted using non-racemic
titanium catalysts prepared from R-4 and R-5
(Method C). In this case, a slight positive NLE was
observed. It is most likely that homogeneous com-
plexes R-4 and S-4 are not stable in solutions and that
ligand scrambling might occur. It was confirmed that
RS-4 could be obtained quantitatively from their race-
mic mixture.
AHCTUNGTREGzNNNU ene:hexane:ethyl acetate=19:10:1) to afford the desired
product. The optical purity was determined by HPLC analy-
sis using a chiral column.
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In summary, we report the first example of a Ti/
BINOL derivative catalyst in the highly enantioselec-
tive ring-opening reactions of meso-aziridines. The
isolation and structural characterization of Ti
(S-L2)(O-i-Pr)2 (RS-4) and Ti5A(m3-O)2A(m-O)(L2)
Pr)2A(rac-5) sheds a light on the possible formation
and the structure of the actual catalyst. The results
(R-L2)-
G
R
G
N
CHTUNGTRENNUNG
CHTUNGTRENNUNG
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Rongmin Yu et al.
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[8] Yellow crystals of Ti2ACHTUNGRTEN(NNUG R-L2)ACHTUNRTGENN(GUN S-L2)CATHUNGTREN(NNGU O-i-Pr)2 (RS-4)
were isolated from a mixture of an equal amount of
enantiopure R-4 and S-4 in ether. Crystal data for RS-
4: triclinic, space group P-1, a=11.418(1) ꢃ, b=
12.178(1) ꢃ,
c=12.996(1) ꢃ,
a=75.57(3)8,
b=
84.14(3)8, g=63.74(3)8, V=1569.3(2) ꢃ3, Z=1, R value
[I> 2ø] R1=0.0331. CCDC 685274 contains the sup-
plementary crystallographic data RS-4. These data can
be obtained free of charge from The Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[9] Compound Ti5CAHTNUGTRENN(NGU m3-O)2AHCNUTRTGEG(NNUN m-O)(L2)4ACHTNUGERTN(NUGN O-i-Pr)2 (rac-5) was
obtained from a racemic mixture of enantiopure RR-4/
H2O and SS-4/H2O in diethyl ether. Crystal data for
rac-5: monoclinic, space group C2/c, a=31.936(6) ꢃ,
b=23.275(5) ꢃ, c=17.572(4) ꢃ, b=106.96(3)8, V=
[5] a) A. Tillack, V. Khedkar, H. Jiao, M. Beller, Eur. J.
Org. Chem. 2005, 5001; b) G. Sundararajan, N. Praba-
garan, B. Varghese, Org. Lett. 2001, 3, 1973.
[6] a) G. Pescitelli, L. D. Bari, P. Salvadori, J. Organomet.
Chem. 2006, 691, 2311; b) Y. Yamashita, H. Ishitani, H.
Shimizu, S. Kobayashi, J. Am. Chem. Soc. 2002, 124,
3292; c) L. J. P. Martyn, S. Pandiaraju, A. K. Yudin, J.
Organomet. Chem. 2000, 603, 98; d) M. Terada, Y. Mat-
sumoto, Y. Nakamura, K. Mikami, Chem. Commun.
1997, 281; e) G. H. Posner, H. Dai, D. S. Bull, J.-K. Lee,
F. Eydoux, Y. Ishihara, W. Welsh, N. Pryor, S. Petr Jr. ,
J. Org. Chem. 1996, 61, 671.
[7] a) Q.-S. Guo, H.-B. Song, J.-S. Li, Z. Anorg. Allg.
Chem. 2006, 632, 721; b) B. Liu, F.-Y. Jiang, H.-B. Song,
J.-S. Li, Tetrahedron: Asymmetry 2006, 17, 2149; c) H.-
Z. Tang, P. D. Boyle, B. M. Novak, J. Am. Chem. Soc.
12494(4) ꢃ3, Z=4,
R value [I> 2ø] R1=0.0692.
CCDC 685275 contains the supplementary crystallo-
graphic data for rac-5. These data can be obtained free
of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[10] a) H. Bao, J. Zhou, Z. Wang, Y. Guo, T. You, K. Ding,
J. Am. Chem. Soc. 2008, 130, 10116; b) K. Mikami, M.
Terada, T. Korenaga, Y. Matsumoto, M. Ueki, R. Ange-
laud, Angew. Chem. 2000, 112, 3676; Angew. Chem. Int.
Ed. 2000, 39, 3532; c) C. Girard, H. B. Kagan, Angew.
Chem. 1998, 110, 3088; Angew. Chem. Int. Ed. 1998, 37,
2922, and references cited therein.
152
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Adv. Synth. Catal. 2009, 351, 147 – 152