Bis(((S)-binaphthoxy)(isopropoxy)titanium) oxide as a μ-oxo-type chiral Lewis acid: Application to catalytic asymmetric allylation of aldehydes

Article abstract of DOI:10.1021/ja020338o

A new, chiral bis-Ti(IV) oxide of type 1 was successfully designed and can be utilized for strong activation of aldehyde carbonyls, thereby allowing a new catalytic enantioselective allylation of aldehydes with allyltributyltin. The chiral bis-Ti(IV) cata

Full text of DOI:10.1021/ja020338o

Published on Web 01/29/2003
Bis(((S)-binaphthoxy)(isopropoxy)titanium) Oxide as a µ-Oxo-Type Chiral
Lewis Acid: Application to Catalytic Asymmetric Allylation of Aldehydes
Hideo Hanawa, Takuya Hashimoto, and Keiji Maruoka*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo, Kyoto 606-8502, Japan
Received March 4, 2002 ; E-mail: maruoka@kuchem.kyoto-u.ac.jp
Catalytic asymmetric synthesis in the efficient production of
enantiomerically rich compounds is of considerable current interest
in organic synthesis, and hence a number of chiral metal catalysts
possessing various chiral auxiliaries have been devised to a useful
Scheme 1
1
level in recent years. The development of efficient enantioselective
methods by the utilization of new chiral metal catalysts increasingly
requires the rational design of new reagents and processes. In this
regard, we recently communicated that the Al-O-Al unit in MAO
(methylalumoxane) and bis(dimethylaluminum) oxide as effective
Lewis acids is crucially important for the strong activation of
2
carbonyl and epoxide oxygens. This observation prompted us to
design a new, chiral Lewis acid possessing the key M-O-M unit
Scheme 2
(M: metal) including a certain chiral auxiliary. Accordingly, we
prepared a new, binaphthyl-modified bis-Ti(IV) oxide of type 1
for the enantioselective activation of carbonyl moieties (Scheme
1). The reactivity and the chiral efficiency of bis-Ti(IV) oxide
(S,S)-1 as a chiral Lewis acid was examined by doing the catalytic
asymmetric allylation of aldehydes with allyltributyltin in com-
parison with (S)-binaphthoxytitanium diisopropoxide (2) as a
corresponding mono-Ti(IV) Lewis acid.3
The requisite bis-Ti(IV) oxide (S,S)-1 was synthesized by two
different routes [A] and [B] starting from triisopropoxytitanium
Table 1. Asymmetric Allylation of Aldehydes by Chiral Bis-Ti(IV)
Oxide 1a
i
4
chloride, (Pr O)
3
TiCl as shown in Scheme 2. Positive ESI-MS
Ti catalyst
(mol %)
%eec
+
% yieldb
(config)d
clearly showed an m/z peak at 943 (M‚2THF + H ), indicating
the formation of (S,S)-1‚2THF coordination complex by comparison
with the theoretical molecular ion peak. Reaction of hydrocin-
entry
aldehyde
CH CHO
time (h)
1
2
3
4
5
6
7
8
9
PhCH
2
2
1 (10)
1 (10)e
1 (5)
1 (10)
2 (20)
1 (5)
1 (10)
2 (20)
1 (10)
1 (10)
1 (10)e
1 (5)
4
84
82
77 [95]
85
14
99 (R)
98 (R)
16
12 [7]
12
f
98 [98] (R)
namaldehyde 3 (R ) CH
under the influence of in situ generated chiral bis-Ti(IV) oxide
S,S)-1 (10 mol %) [prepared by method A] in CH Cl at 0 °C for
h afforded 1-phenyl-5-hexen-3-ol 4 (R ) CH CH Ph) in 84%
2 2
CH Ph) with allyltributyltin (1.1 equiv)
g
CH
3
(CH
2
)
6
CHO
99 (R)
81 (R)
g
12
(
2
2
24 [12]
28 [18]
28
15
7
f
f
86 [92]
99 [98] (R)
g
h
4
2
2
(CH
3
)
2
CHCHO
71 [91] >99 [99] (S)
h
5
7
70
90
81
[94]
85
85 (S)
95 (S)
96 (S)
96 (S)
[97] (S)
98 (S)
97 (S)
yield with 99% ee. Notably, both the reaction rate and the
PhCHdCHCHO
PhCHO
enantioselectivity of the allylation are much lowered (e.g., 10%
yield and 72% ee for hydrocinnamaldehyde) under similar reaction
conditions with a chiral mono-Ti(IV) catalyst 2 (20 mol %).6
10
11
12
24
[9]
f
1
1
3
4
p-bromobenzaldehyde 1 (10)
furfural 1 (10)
15
18
Alternatively, chiral bis-Ti(IV) oxide (S,S)-1, prepared by method
96
7
,8
B, exhibited similar reactivity and enantioselectivity (81% yield
with 98% ee) compared to the catalyst by method A.5
a
Unless otherwise noted, the reaction of aldehyde and allyltributyltin
1.1 equiv) was carried out in the presence of chiral bis-Ti(IV) oxide 1 (by
(
Other selected examples are listed in Table 1. Several charac-
teristic features of the present allylation follow: (1) The chiral bis-
Ti(IV) oxide (S,S)-1 exhibits uniformly high asymmetric induction
as well as high chemical yield. (2) The use of Ti-O-Ti unit in
the chiral bis-Ti(IV) catalyst (S,S)-1 toward aldehyde carbonyls
strongly accelerates the rate of allylation compared to the corre-
sponding mono-Ti(IV) catalyst 2 (entries 5 and 8). (3) The
enantioselectivity of the present allylation is not sensitive to the
reaction temperature (entries 1 vs 2 and 10 vs 11). (4) The amount
of chiral bis-Ti(IV) oxide (S,S)-1 can be reduced to 5 mol % without
affecting the enantioselectivity (entries 3, 6, and 12).
b
method A) or chiral mono-Ti(IV) 2 in CH2Cl2 at 0 °C. Isolated yield.
c
d
Determined by HPLC analysis using Chiralcel OD and OJ. Determined
by comparison of the sign of optical rotation with reported values. See ref
3
p. At -15 °C. Use of 2 equiv of allyltributyltin. g Determined by GC
e
f
analysis using a chiral column (Chrompack CP-CHIRASIL-DEX CB).
Determined by chiral GC analysis after conversion to its benzoate.
h
1, (S,R)-1, and minor (R,R)-1. Selected results are summerized in
Table 2, which clearly shows a positive nonlinear effect in
2 2
correlating the enantiopurity of allylation product 4 (R ) CH CH -
Ph) with the ee of (S)-binaphthol.3g In marked contrast, however,
starting from optically pure (S)- and (R)-binaphthol, optically pure
bis-Ti(IV) oxides, (S,S)-1 and (R,R)-1 were prepared independently,
and asymmetric allylation by mixing of such (S,S)-1 and (R,R)-1
We also carried out the asymmetric allylation by using nonra-
cemic bis-Ti(IV) oxide 1 (by method C) with partially resolved
(S)-binaphthol as chiral auxiliary, which includes a mixture of (S,S)-
2 2
(by method D) gave rise to product 4 (R ) CH CH Ph) in rather
1708
9
J. AM. CHEM. SOC. 2003, 125, 1708-1709
10.1021/ja020338o CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
Table 2. Chiral Amplification in Asymmetric Allylation Catalyzed
by Nonracemic Bis-Ti(IV) Oxide 1
discussions. H.H. thanks the Japan Society for the Promotion of
Science for Young Scientists for Research Fellowships.
a
method Cc
method Dc
ee of1b
% eed
% eed
Supporting Information Available: Representative experimental
procedures (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
%
% yield
% yield
0
38
50
63
69
84
0
64
87
96
99
70
74
75
75
84
3
23
52
75
99
2
5
7
5
0
5
References
100
(
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b
%
(
) in CH2Cl2 (0.033 M) at 0 °C for 4 h. In the nonracemic binaphthol,
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d
by HPLC analysis using Chiralcel OD.
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2
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1
exists as a monomeric species rather than dimeric species in
9
solution; (3) bis-Ti(IV) oxide 1 is coordinatively stable, and no
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(
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10
1
. First, chiral bis-Ti(IV) oxide 1 might not exist as an equilibrium
11
mixture of ((S)-binaphthoxy)TidO (5) and (S)-binaphthoxytita-
nium diisopropoxide (2). It should be noted that (S)-binaphthoxy-
titanium µ-oxo complex 6 showed only low reactivity and selectivity
under similar allylation condition. In addition, Keck’s mono-Ti-
6
12
i
(
4
IV) reagent derived from Ti(OPr ) and (S)-binaphthol (2 equiv)
(
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the other titanium intramolecularly, thereby enhancing the otherwise
weak Lewis acidity of the original Ti(IV) center for the carbonyl
activation as shown in 7. Alternatively, a carbonyl oxygen
coordinates simultaneously to two Ti centers as an intermediate or
as a dynamic species, thereby allowing the strong activation of such
an aldehyde carbonyl as depicted in 8. Although the elucidation of
the most plausible reaction mechanism involving chiral bis-Ti(IV)
catalyst 1 must await further research, the present asymmetric
approach provides a very useful way for obtaining high reactivity
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the design of chiral Lewis acid catalysts.
2
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i
(
(
(
3
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(
8) The formation of the Ti-O-Ti unit by these methods was confirmed by
2
reaction of cis-[TiCl
2
(η -guaiacolato)
2
] (2 equiv) with Ag
2
O (1 equiv) in
2
CH Cl to furnish [Ti
2
2
2
(µ-O)Cl
2
(η -guaiacolato) ], which is spectroscopi-
4
cally identical to that by the reported procedure (Sobota, P.; Przybylak,
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(10) For details, see Supporting Information.
(
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Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research (No. 13853003) from the Ministry of
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