Catalytic asymmetric allylations of achiral and chiral aldehydes via BINOL-Zr complex

Article abstract of DOI:10.1016/S0040-4039(02)00139-9

The complex generated from BINOL, Zr(O^tBu)₄, and 4 ? MS in toluene-pivalonitrile is very effective for catalytic asymmetric allylation of aldehydes using allyltributyltin. The reactions of achiral aldehyde under these conditions are completed within 3 h using 10-20 mol% of the complex at -20°C. The ees of homoallylic alcohols can be enhanced up to 98% via the tandem asymmetric allylation-Oppenauer oxidations. The scope and limitations of these conditions for β-alkoxy aldehydes were extensively examined.

Full text of DOI:10.1016/S0040-4039(02)00139-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1765–1769
Catalytic asymmetric allylations of achiral and chiral aldehydes
via BINOL–Zr complex
Michio Kurosu* and Miguel Lorca
Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, FL 32306, USA
Received 25 October 2001; revised 15 January 2002; accepted 17 January 2002
t
,
Abstract—The complex generated from BINOL, Zr(O Bu)4, and 4 A MS in toluene–pivalonitrile is very effective for catalytic
asymmetric allylation of aldehydes using allyltributyltin. The reactions of achiral aldehyde under these conditions are completed
within 3 h using 10–20 mol% of the complex at −20°C. The ees of homoallylic alcohols can be enhanced up to 98% via the tandem
asymmetric allylation–Oppenauer oxidations. The scope and limitations of these conditions for b-alkoxy aldehydes were
extensively examined. © 2002 Elsevier Science Ltd. All rights reserved.
The additions of the acetate unit to aldehydes play a
significant role in the syntheses of most natural prod-
ucts of polyketide origin.1 In a number of total synthe-
ses of such molecules asymmetric allylations of
aldehydes are often the start in constructing 1,3-polyol
units, and the asymmetric allylation via chiral
allyldialkylboranes2 and allyldialkoxylboranes3 has
been widely utilized.4 Since the stereochemical course is
strongly controlled by a closed chair-like transition
state, these methods have often suffered from lower
selectivity when chiral aldehydes (mismatched sub-
strates) were employed.5 In order to overcome mis-
matched systems, it is especially important to induce
the desired stereochemistry through an open-chained
(acyclic) transition state. Although impressive advances
have been achieved in Lewis acid and Lewis base-cata-
lyzed enantioselective allylation of aromatic and reac-
tive aldehydes,6 few procedures can be applied to
various types of aldehydes. Among the catalytic asym-
metric allylation reactions developed so far, catalysts
temperatures, (3) no generalization has been possible
for catalytic symmetric allylation reactions of chiral
aldehydes. We now wish to report a very efficient
catalytic asymmetric allylation of aldehydes via BINOL
and Zr(OtBu)4,10 and its application to syntheses of
1,3-polyols.
In our studies on the structure elucidation of the
BINOL-group IVB transition metals complexes by
Fourier transform ion cyclotron resonance (FT-ICR)
mass spectroscopy,11 a 1:1 mixture of BINOL and
i
,
Ti(O Pr)4 in CH2Cl2 in the presence of 4 A molecular
sieves (MS) formed a complex which contains four Ti
atoms (calculated mass=1819.0445). This complex,12
however, exhibited very poor catalytic activity in the
allylation of hydrocinnamaldehyde using allytributyltin
(less than 5% conversion after 12 h at −20°C, the
isolated product was 86% ee). On the other hand, the
same reaction using 10 mol% of a 1:1 mixture of
(S)-BINOL and Zr(OtBu)4 in toluene13 at −20°C for 2.5
h afforded (R)-1-phenyl-hex-5-ene-3-ol in 75% yield
with 88% ee. After extensive optimization of the reac-
tion conditions using the BINOL-Zr(OtBu)4 complex,
several critical findings emerged: (1) the reaction pro-
ceeded even at −78 to −60°C and was completed with
150 mol% of the catalyst, (2) the reactions at 0°C
caused reduction of the aldehydes, (3) the addition of 4
prepared from either BINOL/Ti(OiPr)4,7 or BINOL/
TiCl2(OiPr)2
(BINOL=1,1%-binaphtalen-2,2%-diol)
8
using allyltributyltin are applicable to a certain type of
chiral aldehydes. In many model studies described in
the literature these reactions provide homoallylic alco-
hols in excellent enantiomeric excess (ee) but need to be
further developed, because (1) in our hands, reported
yields and ees were not reproducible,9 (2) the catalysts
require 20 mol% (with respect to BINOL), (3) the
reactions require very long reaction times at controlled
,
A MS was very important for minimizing the self-con-
densation of aldehydes, (4) the addition of 10% of
pivalonitrile14 to toluene was effective in increasing the
,
ees (up to 93%), (5) in the presence of 4 A MS and
pivalonitrile addition of an excess of Zr(OtBu)4 did not
result in a decrease of ees, (6) an excess (1.5–2.0 equiv.)
of allyltributyltin was required to facilitate the catalytic
* Corresponding author. Tel.: (850) 644-9624; fax: (850) 644-8281;
e-mail: kurosu@chem.fsu.edu
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00139-9

1766
M. Kurosu, M. Lorca / Tetrahedron Letters 43 (2002) 1765–1769
pivalonitrile. Because (S)- and (R)-BINOLs are now
available very cheaply17 and because BINOLs can be
recovered relatively easily from the reaction mixtures by
chromatography, the catalytic asymmetric allylation
reaction described here can be utilized for syntheses of
relatively large amounts of optically active homoallyl
alcohols; we have carried out allylation of the alde-
hydes on a 5–10 g scale (entries 3 and 5 in Table 1).
Thus, these procedures are much more practical than
others previously described.
cycle, and (7) under these conditions the ees of products
were not affected by the amount of catalyst used; the
reactions with 10, 20, 100% of the catalyst gave the
same values of ees. We have applied the optimized
conditions to over 20 different aldehydes and the repre-
sentative results are summarized in Table 1.15
In most cases the reactions were remarkably rapid;
allylations of the achiral aldehydes were completed
within 3 h and the isolated products exhibited 90–93%
ees. Allylation of 3-benzyloxy-propionate afforded the
desired homoallylic alcohol in greater than 80% yield
with 93% ee (entry 7). In addition, the allylation of the
alkynal gave rise to the propargyl alcohol with satisfac-
tory ee (entry 8); asymmetric allylations that are con-
trolled via a cyclic transition state usually result in poor
asymmetric induction. Although modified BINOLs–
Zr(OtBu)4 complexes have been applied to catalytic
asymmetric Mannich-type reactions,10b Strecker-type
reactions10e and aldol reactions10f with excellent enan-
tioselectivities, unmodified BINOL–Zr complexes do
not exhibit a useful level of asymmetric induction with-
out the addition of complex additives.6j,10c,16 However,
we were able to attain a useful catalytic asymmetric
allylstannylation of aldehydes via unmodified BINOL
in combination with a solvent system such as toluene–
We recognized that the ees of homoallylic alcohols
could be enhanced by subsequent Oppenauer oxida-
tions. Using the complex generated from a 1.2:1 mix-
ture of Zr(OtBu)4 and BINOL a tandem catalytic
asymmetric allylation–Oppenauer oxidation (AA–
Oppenauer reaction) took place. The tandem AA–
Oppenauer reactions were especially useful for
relatively reactive aldehydes such as aromatic aldehydes
and hydrocinnamaldehyde (Scheme 1 and Table 2).18
The catalytic allylation reactions using 1.1 equiv. of
allytributyltin yielded the desired homoallyl alcohol in
60–70% yields. The intermediate, a homoallylic tin
alkoxide, reacts with a chiral BINOL–Zr complex and
the unreacted aldehyde 1 was utilized as a hydride
Table 1. Catalytic asymmetric allylation of achiral aldehydes via BINOL-Zr(OtBu)4
Entry
Aldehyde (R=)
Catalysts (mol%)
Time (h)
Yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
Ph
10
10
10
10
10
10
20c
10
10
10
1.5
2.5
2.5
2
90
85
86
80–90
80–90
75
88
89
85–90a
93a
PhCH2CH2
C7H15
93b
TIPSOCH2(CH2)3
BnOCH2(CH2)3
TBSOCH2CH2
BnOCH2CH2
CH3(CH2)4CC
(E)-PhCHꢀCH
MeO2C(CH2)4
90–93b
90–93b
92b
2
2.5
2.5
1
2.5
2
93b
90b
85
86
93b
10
92b
a % ee determined by the HPLC analysis on a Chiralcel OD column.
b % ee determined by 1H NMR spectroscopic analysis of its Mosher ester.
c Allylation reaction using 10 mol% of catalyst gave the product ranging from 70 to 75% yields.
Scheme 1. Tandem catalytic asymmetric allylation–Oppenauer oxidation.

M. Kurosu, M. Lorca / Tetrahedron Letters 43 (2002) 1765–1769
1767
Table 2.
Entry
Aldehyde (R=)
Catalysts (mol%)
Time (h)
Yield (%)
eea (%)
1
2
3
4
Ph
4-BrPh
(E)-PhCHꢀCH
PhCH2CH2
10
10
10
10
24
24
24
24
50
45
58
50
98
98
96
97
a % ee determined by the HPLC analysis on a Chiralcel OD column.
aldehydes were used for the asymmetric allylations
using both (S)- and (R)-BINOLs. As summarized in
Table 3, the allylation reactions using BINOL–Zr com-
plex were significantly influenced by protecting groups
at the b-position. The reaction of 5a (R1=Me) pro-
ceeded in favor of the anti-product regardless of the
chirality of BINOL (entries 1 and 2). The TBS pro-
tected substrate 5e did not afford a useful level of
asymmetric induction (entries 9 and 10). Allylation
reactions of the Bn and MPM protected substrates, 5c
and 5d, using (R)-BINOL as a chiral source afforded
anti:syn selectivity of 4.2:1 and 4.0:1, respectively. How-
ever, reactions using (S)-BINOL gave poor selectivity
of anti:syn=1:1.2 and 1:1.4, respectively (entries 5–8).
The MOM protecting group for the b-hydroxyl turned
out to be superior to others in the BINOL–Zr-catalyzed
allylation reactions (entries 3 and 4). syn-3,5-Isopropyl-
idene ketal aldehyde 6a was a good substrate for syn-
thesizing both 4,6-syn- and 4,6-anti-1,3-diol systems
using the (S)- and (R)-BINOL–Zr complexes. Although
in the allylation of anti-3,5-isopropylidene ketal alde-
hyde 6b, the anti-induction using (S)-BINOL showed a
reasonable selectivity of 5:1, the syn-induction using
(R)-BINOL resulted in poor diastereoselectivity (entries
13 and 14).21
acceptor to produce ketone 4 via transition model I.
Undesired homoallyl tinalkoxides were oxidized faster
than desired tinalkoxides. The terminal double bond of
4 was isomerized during work-up to afford enone 3.
For the Oppenauer oxidation with the BINOL–Zr com-
t
plex use of BuOMe was most effective among the
solvents investigated.19
Although asymmetric allylations via acyclic transition
states might be useful for chiral aldehydes, few such
applications were found in the literature.20 The major
drawback in these reactions of aldehydes possessing a
b-chiral center using reported methods is that the reac-
tions require an extremely long reaction time (6 days at
−20°C). In addition, the diastereoselectivities of allyl-
ated products seem to be influenced significantly by the
protecting groups used. On the basis of data provided
by FT-ICR mass spectroscopy, the rather large catalyst
(calculated mass=1775.2592) generated from BINOL-
Zr(OtBu)4 in toluene–pivalonitrile might interact with
the functional groups at the b-position. To investigate
how the BINOL–Zr catalyst ignores the b-chiral center
of aldehydes, we first synthesized a variety of b-hydroxy
aldehydes possessing different protecting groups. These
Table 3. Asymmetric allylation of chiral aldehydes via BINOL-Zr(OtBu)4
a
Entry
Aldehyde
BINOL
Selectivity (anti:syn)b
Yield (%)
1
2
3
4
5
6
7
8
R1=Me (5a)
R1=Me (5a)
R1=MOM (5b)
R1=MOM (5b)
R1=Bn (5c)
R1=Bn (5c)
R1=MPM (5d)
R1=MPM (5d)
R1=TBS (5e)
R1=TBS (5e)
3,5-syn (6a)
S
R
S
R
S
R
S
R
S
R
S
R
S
R
7a:8a=2.3:1
7a:8a=8.5:1
7b:8b=1:8.0
7b:8b=8.5:1
7c:8c=1:1.2
7c:8c=4.2:1
7d:8d=1:1.4
7d:8d=4.0:1
7e:8e=1:1.5
7e:8e=1.5:1
9a:10a=1:6
9a:10a=10:1
9b:10b=1:5.0
9b:10b=1.5:1
90
90
90
85
80
78
88
89
85
85
90
90
87
85
9
10
11
12
13
14
3,5-syn (6a)
3,5-anti (6b)
3,5-anti (6b)
a All reactions were carried out in the range of 0.1–0.2 mmol scale using 100 mol% of the BINOL–Zr(OtBu)4 complex at −20°C for 3 h. Allylations
with 20 mol% of the catalyst require 12 h to complete the reactions.
b Diastereoselectivity of the products was established by 1H NMR.

1768
M. Kurosu, M. Lorca / Tetrahedron Letters 43 (2002) 1765–1769
Acknowledgements
The tendency in the diastereoselectivities observed in
the allylation reactions of a series of chiral aldehydes
using BINOL–Zr(OtBu)4 clearly indicates that it is
difficult to attain high 1,3-syn selectivity except in the
case of entries 3 and 11 in Table 3. Because of the
higher oxophilicity of the BINOL–Zr complex the reac-
tion rate was far greater than with the BINOL–Ti
complexes.9 This catalyst causes a significant interac-
tion with the coordinative oxygen atoms at the b-posi-
tion of aldehydes to increase the chelation controlled
allylation products.22
This work was supported by The Florida State Univer-
sity. Dr. Alan Marshall and Mr. Robert Bossio are
gratefully acknowledged for high-resolution mass spec-
tral measurements.
References
1. (a) Heachcock, C. H. In Asymmetric Synthesis; Morison,
J. D., Ed.; Academic Press: New York, 1984; Vol. 3, p.
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Stereochem. 1982, 13, 1; (c) Mukaiyama, T. Org. React.
(N.Y.) 1982, 28, 203.
2. Brown, H. C.; Randad, R. S.; Bhat, K. S.; Zaidlewiez,
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3. Roush, W. R.; Hoong, L. K.; Palmer, M. A. J.; Park,
J.-C. J. Org. Chem. 1990, 55, 4109.
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1992, 92, 807; (c) Duthaler, R. O.; Hafner, A. Chem. Rev.
1992, 92, 807.
In conclusion, an expeditious catalytic allylation of
aldehydes with BINOL–Zr(OtBu)4 is reported. The
reaction proceeds within a few hours at −20°C and the
ees of the homoallylic alcohols are greater than 90%.
The ees of the products can be enhanced via tandem
catalytic asymmetric allylation–Oppenauer reactions.
These tandem reactions are especially useful for reactive
aldehydes. The asymmetric allylation with BINOL–
Zr(OtBu)4 may be useful for a variety of protected
chiral b-hydroxy aldehydes. However, the choice of
protecting groups is crucial for inducing useful levels of
diastereoselectivity.
Representative procedure for the synthesis of (R)-undec-
1-en-4-ol (entry 3 in Table 1). Zr(OtBu)4 was purchased
from either Aldrich or Strem and stored as a 0.55 M
toluene solution. This was stored in a desiccator over
KOH pellets. To a stirred mixture of (S)-BINOL (892
5. Roush, W. R.; Hoong, L. K.; Palmer, M. A.; Staub, J.
A.; Palkowitz, A. D. J. Org. Chem. 1990, 55, 4117.
6. (a) Furuta, K.; Miwa, Y.; Iwanaga, K.; Yamamoto, H. J.
Am. Chem. Soc. 1988, 54, 1481; (b) Furuta, K.; Mouri,
M.; Yamamoto, H. Synlett 1991, 561; (c) Marshall, J. A.;
Tang, Y. Synlett 1992, 653; (d) Ishihara, K.; Mouri, M.;
Gao, Q.; Maruyama, T.; Furuta, K.; Yamamoto, H. J.
Am. Chem. Soc. 1993, 115, 11490; (e) Denmark, S. E.;
Coe, D. M.; Pratt, N. E.; Griedel, B. D. J. Org. Chem.
1994, 59, 6161; (f) Kobayashi, S.; Nishio, K. J. Am.
Chem. Soc. 1995, 117, 6392; (g) Iseki, K.; Mizuno, S.;
Kuroki, Y.; Kobayashi, Y. Tetrahedron 1999, 55, 977; (h)
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Am. Chem. Soc. 1998, 120, 641; (i) Chataigner, I.;
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Tetrahedron Lett. 2000, 41, 5543 and references cited
therein.
7. (a) Keck, G. E.; Tarbet, K. H.; Geraci, L. S. J. Am.
Chem. Soc. 1993, 115, 8268; (b) Keck, G. E.; Geraci, L.
S. Tetrahedron Lett. 1993, 34, 7827; (c) Yu, C.-M.; Choi,
H.-S.; Yoon, S.-K.; Jung, W.-H. Synlett 1997, 889.
8. Costa, A. L.; Piazza, M. G.; Tagliavini, E.; Trombini, C.;
Umani-Ronchi, A. J. Am. Chem. Soc. 1993, 115, 7001.
9. Modified Keck’s protocols (BINOL-Ti(OR)4=2:1) are
effective in allylations of aldehydes with b-substituted
allylstannanes, see: Weigand, S.; Bru¨ckner, R. Chem. Eur.
J. 1996, 2, 1077.
,
mg, 3.11 mmol), and 4 A MS (1.5 g) in dry toluene (15
mL) and dry pivalonitrile (1.5 mL) was added
Zr(OtBu)4 (5.6 mL, 3.11 mmol). The reaction mixture
was stirred for 30–60 min at rt. At −50 to −78°C
allyltributyltin (14.4 mL, 46.7 mmol) and octanal (4.9
mL, 31.1 mmol) were added. After 2 h at −20°C,
saturated NaHCO3 solution (20 mL) was added and the
reaction mixture was stirred for an additional 30 min.
An Et2O/water partition was conducted;23 the organic
phase was washed with brine, dried over Na2SO4 and
concentrated in vacuo to afford the crude homoallyl
alcohol. This was purified by silica gel chromatography
(hexanes:EtOAc:CH2Cl2, 20:1:2 to 10:1:2) to afford
(R)-1-heptybut-3-en-1-ol (4.65 g, 27.4 mmol, 88%), [h]D
+6.9° (c 1.0, CHCl3 at 27°C).
Representative procedure for the synthesis of (R)-1-
phenyl-5-hexen-3-ol (entry 4 in Table 2) via the tandem
asymmetric allylation–Oppenauer oxidation. To
a
stirred mixture of (S)-BINOL (100 mg, 0.35 mmol),
,
and 4 A MS (200 mg) in dry toluene (1.8 mL) and dry
pivalonitrile (0.18 mL) was added Zr(OtBu)4 (0.84 mL,
0.46 mmol). The reaction mixture was stirred for 30–60
min at rt. At −50 to −78°C allyltributyltin (1.19 mL,
3.84 mmol) and hydrocinnamaldehyde (0.46 mL, 3.5
10. Applications of the BINOL–Zr complexes in catalytic
asymmetric reactions, see: (a) Bedeschi, P.; Casolari, S.;
Costa, A. L.; Tagliavini, E.; Umani-Ronchi, A. Tetra-
hedron Lett. 1995, 36, 7897; (b) Ishitani, H.; Ueno, M.;
Kobayashi, S. J. Am. Chem. Soc. 1997, 119, 7153; (c)
Casolari, S.; Cozzi, P. G.; Orioli, P.; Tagliavini, E.;
Umani-Ronchi, A. Chem. Commun. 1997, 2123; (d)
Kobayashi, S.; Ishitani, H.; Ueno, M. J. Am. Chem. Soc.
1998, 120, 431; (e) Ishitani, H.; Komiyama, S.;
t
mmol) were added. After 3 h at −20°C, dry BuOMe (3
mL) was added and the reaction mixture was stirred at
0°C for 24 h. The crude mixture was purified by silica
gel chromatography (hexanes:EtOAc:CH2Cl2, 20:1:2 to
10:1:2) to afford (R)-1-phenyl-5-hexen-3-ol (308 mg,
1.75 mmol, 50%).

M. Kurosu, M. Lorca / Tetrahedron Letters 43 (2002) 1765–1769
1769
Hasegawa, Y.; Kobayashi, S. J. Am. Chem. Soc. 2000,
122, 762; (f) Ishitani, H.; Yamashita, Y.; Shimizu, H.;
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S. D. H.; Marshall, A. J. Am. Soc. Mass Spectrom. 1997,
8, 970.
12. The structures of the BINOL–Ti complexes are unknown,
but they appear to be m-oxo species, see: (a) Corey, E. J.;
Letavic, M. A.; Noe, M. C.; Sarshar, S. Tetrahedron Lett.
1994, 35, 7553; (b) Terada, M.; Matsumoto, Y.; Naka-
mura, Y.; Mikami, K. Inorg. Chim. Acta 1999, 296, 267.
13. The same reaction in CH2Cl2, Et2O, and tBuOMe
resulted in poor conversion yields with lower ees (70–
80%).
14. The addition of 30–40% of pivalonitrile in 0.2–0.3 M
toluene solution of reaction mixtures resulted in slower
conversion, but the ees of products were retained. The
same reactions in other nitrile solvents resulted in lower
conversion yields and ees (50% yield (80% ee) in acetoni-
trile, 50–60% yield (82% ee) in propionitrile).
reductions of carbonyls, see: Lorca, M.; Kuhn, D.;
Kurosu, M. Tetrahedron Lett. 2001, 42, 6243.
17. (S)-BINOL is now available from Kankyo Kagaku Cen-
ter Co., Ltd. at approximately US $0.35 per gram.
18. For Mg, and Zr metal-catalyzed Oppenauer oxidations
using aldehydes, see: (a) Byrne, B.; Karras, M. Tetra-
hedron Lett. 1987, 28, 769; (b) Krohn, K.; Knauer, B.;
Ku¨pke, J.; Seebach, D.; Beck, A. K.; Haykawa, M.
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J. Org. Chem. 1996, 61, 1467.
19. The detailed reaction mechanism of these reactions is
currently under investigation.
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118, 9422; (b) Roush, W. R.; Champoux, J.; Peterson, B.
C. Tetrahedron Lett. 1996, 37, 8989.
21. Stereochemistries of the skipped-triols were confirmed by
using Kishi’s universal NMR databases after 9 and 10
had been converted to the saturated triols, see:
Kobayashi, Y.; Tan, C.-H.; Kishi, Y. Angew. Chem., Int.
Ed. 2000, 39, 4279.
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25, 1883.
15. The absolute chemistries of the homoallyl alcohols were
determined by comparison of the sign of optical rotations
with reported values.
23. A mixture of powdered 2:1, CsF:CsOH and silica gel was
effective to remove both tin and Ti species, see: Edelson,
B. J.; Stoltz, B. M.; Corey, E. J. Tetrahedron Lett. 1999,
40, 6729. We are preparing a full article including a
detailed work-up procedure to be published in J. Org.
Chem.
16. Catalytic asymmetric allylations using the BINOL–
Zr(OiPr)4·iPrOH was reported, see: Ref. 10a. In our
hands, allylation of octanal under reported conditions
afforded the homoallyl alcohol in 30–50% yield with
80–85% ee together with octanol (ꢀ40%). For a useful
application of BINOL–Zr(OiPr)4·iPrOH in selective