Practical approach for catalytic asymmetric allylation of aldehydes with a chiral bidentate titanium(IV) complex

Article abstract of DOI:10.1016/S0040-4039(01)00025-9

A new, chiral bidentate Ti(IV) complex of type 1 was successfully designed and can be utilized for simultaneous coordination to aldehyde carbonyls, thereby allowing the precise enantioface discrimination of such carbonyls for a new catalytic, practical en

Full text of DOI:10.1016/S0040-4039(01)00025-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1935–1939
Practical approach for catalytic asymmetric allylation of
aldehydes with a chiral bidentate titanium(IV) complex
a,b
a,b,
Satoshi Kii and Keiji Maruoka
*
a
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
b
Department of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810l, Japan
Received 9 November 2000; accepted 28 December 2000
Abstract—A new, chiral bidentate Ti(IV) complex of type 1 was successfully designed and can be utilized for simultaneous
coordination to aldehyde carbonyls, thereby allowing the precise enantioface discrimination of such carbonyls for a new catalytic,
practical enantioselective allylation of aldehydes with allyltributyltin. © 2001 Elsevier Science Ltd. All rights reserved.
As one of the fundamental yet powerful CꢀC bond-
forming reactions, enantioselective allylation of alde-
hydes attracts considerable attention in asymmetric
of cinnamaldehyde, previously reported catalytic pro-
cesses generally exhibited less satisfactory enantioselec-
4–7
tivity (ꢀ90% ee).
This is mainly because previous
1
synthesis. Given the versatile chemistry of the resulting
strategies utilize a single coordination complex A
between an aldehyde and chiral monodentate Lewis
acids which inevitably causes free rotation at the MꢀO
bond, and the anti-coordination A of a Lewis acid
(ML*) toward an aldehyde would also decrease the
enantioselection compared to the unfavorable syn-
coordination B. In order to overcome these intrinsic
problems by the approaches currently known, we
are interested in the possiblity of forming double
coordination complex C with chiral bidentate Lewis
acids, thereby allowing more precise enantioface dis-
crimination of aldehyde carbonyl. Here we wish to
disclose a new catalytic, practical enantioselective
2
homoallylic alcohols, this asymmetric transformation
constitutes a valuable process. Despite considerable
recent progress in this area using a stoichiometric
3
amount of chiral Lewis acids, only a few types of
reagents have been reported for catalytic versions, and
continuing improvements in the efficiency of these
Lewis acids have been made within the field of asym-
4
–7
metric catalysis.
In this respect, a most practical
catalytic allylation should utilize an inexpensive and
readily available chiral ligand, and provide uniformly
high and predictable enantioselectivity across a wide
range of aldehyde substrates. For instance, in the case
Scheme 1.
Keywords: allylation; binaphthol; asymmetric reactions; titanium and compounds.
*
Corresponding author. Tel./fax: +00 81 75 753 4041; e-mail: maruoka@kuchem.kyoto-u.ac.jp
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00025-9

1
936
S. Kii, K. Maruoka / Tetrahedron Letters 42 (2001) 1935–1939
i
allylation of aldehydes with allyltributyltin using a
newly designed, chiral bidentate Ti(IV) complex of type
monodentate Ti(IV) complex 2 derived from Ti(OPr ) ,
4
(S)-binaphthol, and 4-chloro-2-(tritylamino)benzo-
phenone as monodentate ligand (each 5 mol%). In
addition, a chiral, in situ generated Ti((S)-binaph-
1, as shown in Scheme 1.
i
i
thoxy)(OPr ) (6) (5 or 10 mol%) from Ti(OPr ) and
2
4
(
S)-binaphthol retarded the allylation to a great extent,
giving homoallylic alcohol 4 (R=Ph) in only 3% yield
71% ee) or 5% yield (72% ee).
(
Other selected examples are listed in Table 1. Several
characteristic features of the present allylation follow:
The design of chiral bidentate Ti(IV) catalysts origi-
nates from our recently developed bidentate Lewis acid
(
1) The chiral bidentate Ti(IV) catalyst 1 (R%=Tr;
8
,9
X=Cl) exhibits uniformly high asymmetric induction
as well as high chemical yield. As a general tendency,
the i-PrOH-free catalyst 1 (R=Tr; X=Cl) is more
reactive than the in situ generated 1, and the asymmet-
ric allylation with the i-PrOH-free catalyst 1 (3–5
mol%; R=Tr; X=Cl) proceeds at lower or similar
reaction temperature with shorter reaction time. In the
case of low enantioselectivity observed in entries 17 and
chemistry, and one of the most characteristic features
in this study is the rational control of the desired
two-Ti distance for carbonyl trapping by dative interac-
tions rather than ionic interactions as utilized previ-
8
ously. Accordingly, the requisite chiral bidentate
catalyst 1 (R%=Tr; X=Cl) is prepared by mixing of
2
,2%-bis(tritylamino)-4,4%-dichlorobenzophenone
5
(5
i
mol%; R%=Tr; X=Cl) with Ti(OPr ) (10 mol%) in
4
2
3, the i-PrOH-free catalyst 1 (3 mol%; R=Tr; X=Cl)
CH Cl at room temperature for 1 h and subsequent
2
2
was pretreated with 4 equiv. of i-PrOH (12 mol%) to
furnish the catalyst C, which should be similar to the in
situ generated 1, and indeed exhibited similar reactivity
and enantioselectivity (entries 18 and 24). (2) The
simultaneous coordination and double activation ability
of the bidentate Ti(IV) catalyst 1 (R%=Tr; X=Cl)
toward aldehyde carbonyls strongly accelerated the rate
of allylation compared to previously reported BINOL–
treatment with (S)-binaphthol (10 mol%) at room tem-
perature for 15 h [Method I in Scheme 2]. Reaction of
benzaldehyde with allyltributyltin (1 equiv.) under the
influence of in situ generated chiral bidentate catalyst 1
(
5 mol%; R=Tr; X=Cl) in CH Cl at 0°C for 2 h gave
2 2
rise to 1-phenyl-3-buten-1-ol 4 (R=Ph) in 95% yield
with 99% ee.
1
0–12
The absolute configuration of the
homoallylic alcohol was determined to be S by correla-
i
5
Ti(OPr ) complexes. (3) The enantioselectivity of the
4
5b
tion to the authentic sample. The amount of catalyst
can be reduced to 3 mol% with similar chemical yield
present allylation is largely insensitive to the reaction
temperature (i.e. 98–99% ee at −15–20°C in the case of
benzaldehyde). (4) This asymmetric procedure is opera-
1
and enantioselectivity (96% yield, 98% ee) except for
the need of longer reaction time, as shown in Table 1
tionally quite simple. Thus, commercially available
(
entry 1).
i
Ti(OPr ) and chiral binaphthol are sequentially added
4
to 2,2%-bis(tritylamino)-4,4%-dichlorobenzophenone (5)
The chiral bidentate catalyst 1 (R=Tr; X=Cl) can be
(
R%=Tr; X=Cl) in CH Cl at room temperature to
2 2
i
13
also prepared from Ti((S)-binaphthoxy)(OPr )2 (6)
and 5 (R=Tr; X=Cl) in CH Cl [Method II in Scheme
generate the requisite catalyst 1 (R%=Tr; X=Cl), which
is then treated with aldehyde 3 and allyltributyltin at
2
2
2
]. This i-PrOH-free catalyst 1 (R=Tr; X=Cl) is more
0
°C for several hours. (5) The present asymmetric
reactive than the in situ generated chiral catalyst 1, and
the asymmetric allylation with the i-PrOH-free catalyst
allylation is highly chemoselective in the presence of
other carbonyl moieties such as ketone and ester func-
tionalities, leaving these groups intact (entry 25).
1
(5 mol%; R=Tr; X=Cl) proceeded at −20°C for 2 h
to furnish homoallylic alcohol 4 (R=Ph) in 85% yield
14
with 98–99% ee (entry 4). It should be noted that both
the reaction rate and the enantioselectivity of the allyla-
tion are much lower (e.g. 9 and 90% ee for benzalde-
hyde) under similar reaction conditions with a chiral
Among several possible reaction pathways, the high
reactivity and selectivity of the bidentate Ti(IV) com-
plex 1 (R%=Tr; X=Cl) is ascribed to the simultaneous
coordination of two Ti to a carbonyl oxygen, thereby
Scheme 2.

S. Kii, K. Maruoka / Tetrahedron Letters 42 (2001) 1935–1939
1937
Table 1. Asymmetric allylation of aldehydes with allyltributyltin catalyzed by chiral bidentate Ti(IV) complex 1 (R%=Tr;
a
X=Cl)
Entry
Aldehyde
PhCHO
Catalystb (mol%)
Condition (°C, h)
% Yieldc
% Eed (config)e
1
2
3
4
5
6
7
8
9
[A] (3)
[A] (5)
[B] (3)
[B] (5)
[A] (5)
[B] (3)
[B] (5)
[A] (5)
[B] (5)
[A] (3)
[A] (5)
[B] (3)
[A] (10)
[B] (5)
[A] (3)
[A] (5)
[B] (3)
[C] (3)
[A] (5)
[B] (3)
[A] (3)
[A] (5)
[B] (3)
[C] (3)
0, 5
0, 2
96
95
74
85
82
75
88
54
77
89
83
71
79
88
87
97
98
90
92
99
85
82
88
84
98 (S)
99 (S)
97 (S)
98 (S)
97 (R)
98 (R)
−15, 15
−20, 2
0, 12
0, 2
0, 0.5
0, 24
0, 1
f
CH (CH ) CHO
3
2 6
f
f
99 (R)
98 (R)
g
(CH ) CHCHO
3
2
g
\99 (R)
h
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
PhCH CH CHO
0, 4
0, 2
97 (S)
98 (S)
2
2
h
h
−10, 12
0, 10
0, 5
0, 12
0, 12
0, 3
0, 12
0, 10
0, 6
0, 5
0, 6
0, 2
0, 4
98 (S)
PhCHꢁCHCHO
97 (S)
97 (S)
i
p-Bromobenzaldehyde
97 (S)
i
98 (S)
94 (S)
i
i
97 (S)
2-Naphthaldehyde
Furfural
98
97
97
98
95
97
2
5
[A] (10)
0, 20
96
97
a
Unless otherwise noted, the reaction of aldehyde and Bu SnCH CHꢁCH (1 equiv.) was carried out in the presence of chiral bidentate Ti(IV)
3
2
2
catalyst 1 (3–5 mol%; R%=Tr; X=Cl) in CH Cl under the given reaction conditions.
2
2
b
Catalyst [A] refers to the in situ generated 1 (R%=Tr; X=Cl) by Method I. Catalyst [B]: i-PrOH-free catalyst 1 by Method II. Catalyst [C]:
i-PrOH-free catalyst 1+4i-PrOH.
Isolated yield.
c
d
e
f
Determined by HPLC analysis using Chiralcel OD and OJ.
Determined by comparison of the sign of optical rotation with reported values. See Refs. 3o and 5b.
Determined by GC analysis using a chiral column (GL SCIENCE CP-CHIRASIL-DEX CB).
Determined by GC analysis (chiral GL SCIENCE CP-CHIRASIL-DEX CB column) after conversion to its benzoate.
Correlated to (S)-4 (R=CHꢁCHPh) (entry 13) by the catalytic hydrogenation of 4 (R=CH CH Ph) with Pd/C and H to 1-phenyl-3-hexanol.
g
h
i
2
2
2
Correlated to (S)-4 (R=Ph) (entry 1) by debromination of 4 (R=p-bromophenyl) with BuLi in ether.
allowing the double activation of the aldehyde car-
bonyls. Such activation is also observed by C NMR
around the single bond can be determined. The free
13
trans-4-methoxy-3-buten-2-one exists in both s-cis and
and IR spectroscopy using 2,6-dimethyl-g-pyrone as the
s-trans conformations, since irradiation of CH CꢁO
3
13
carbonyl substrate: C NMR (CD Cl at 20°C) l
protons resulted in moderate NOE to both a and b
methoxyvinyl protons (3.1 and 5.6%, respectively). On
complexation with monodentate Ti(IV) Lewis acid 2,
2
2
1
65.32 (free pyrone b carbons); l 166.26 (pyrone b
carbons of the 1:1 2,6-dimethyl-g-pyrone/monodentate
2
complex); l 167.75 (pyrone b carbons of the 1:1
irradiation of CH CꢁO protons again resulted in simi-
3
2
,6-dimethyl-g-pyrone/bidentate (R%=Tr; X=Cl)
1
lar NOE to both a and b methoxyvinyl protons (1.7
and 3.4%, respectively). This result implies that both
s-cis and s-trans conformers, (D) and (E), exist in a
similar ratio to the free enone in solution. A similar
tendency is also observed with a complex derived from
chelation complex); l 166.40 (pyrone b carbons of the
2
:1 2,6-dimethyl-g-pyrone/bidentate 1 (R%=Tr; X=Cl)
−1
chelation complex); IR (CH Cl ) 1669 cm
(free
2
2
−
1
pyrone CꢁO); 1656 cm (pyrone CꢁO of the 1:1 2,6-
−
1
5c
dimethyl-g-pyrone/monodentate 2 complex); 1652 cm
pyrone CꢁO of the 1:1 2,6-dimethyl-g-pyrone/bidentate
(R%=Tr; X=Cl) chelation complex).
trans-4-methoxy-3-buten-2-one and Keck’s reagent
(
1
(2.7 and 5.3% for the a and b protons, respectively). In
marked contrast, however, in the bidentate 1 (R%=Tr;
X=Cl)/enone complex, irradiation of the methyl pro-
ton of enone resulted in moderate NOE to the only b
proton of the trans-methoxyvinyl group, implying the
existence of the s-trans conformer (F) predominantly
because of the double coordination.
The double coordination phenomenon of 1 (R%=Tr;
X=Cl) is supported by the difference NOE measure-
ment with trans-4-methoxy-3-buten-2-one at low tem-
perature (−20°C), where the enone conformation

1
938
S. Kii, K. Maruoka / Tetrahedron Letters 42 (2001) 1935–1939
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1
This work was supported by a Grant-in-Aid for Scien-
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Control of Stereochemistry) from the Ministry of Edu-
cation, Science, Sports and Culture. We thank Profes-
sor K. Mikami for helpful informations and
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t
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2
t
X=H) derived from 5 (R%=Pr and SiBu Ph ; X=H),
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2
i
4
2
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h). Among various spacers examined for designing of
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be the most effective.
5
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1
1. Introduction of chloro substituents (X=Cl) in ligand 5 is
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9
3
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2
2
homoallylic alcohol 4 (R=Ph) in 85% yield (99% ee)
accompanied by some cleavage of the tritylamino moiety
in ligand 5 (R%=Tr; X=H).
1
Chem. Soc., Chem. Commun. 1998, 443. (e) Saied, O.;
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1
2. Attempted addition of 2 more equiv. of (S)-binaphthol to
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ical yield (87%) in the asymmetric allylation of benzalde-
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1
0. Our initial attempt to use chiral bidentate Ti(IV) catalysts
of type 1 (5 mol%) from the parent 2,2%-diaminoben-
1
zophenone 5 (R%=X=H) as a spacer by combining
i
Ti(OPr ) and chiral (S)-binaphthol turned out to be less
4
satisfactory (11% yield and 91% ee at 0°C for 6 h) in the
asymmetric allylation of benzaldehyde with allyl-
tributyltin. Introduction of alkyl or silyl moieties on the
amino groups in 5 in order to make two Ti closely
aligned by their steric repulsion resulted in enhancement
of reactivity of the asymmetric allylation. Indeed, biden-
(
c) Mikami, K. In Encyclopedia of Reagents for Organic
Synthesis; Paquette, L. A., Ed.; Wiley: Chichester, Vol. 1,
p. 407.
14. Mikami, K.; Matsukawa, S.; Volk, T.; Terada, M.
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.
.