Chiral phosphine oxide BINAPO as a catalyst for enantioselective allylation of aldehydes with allyltrichlorosilanes

Article abstract of DOI:10.1016/j.tetlet.2004.10.168

The effectiveness of chiral phosphine oxide BINAPO as a catalyst for the enantioselective addition of allyltrichlorosilanes to aldehydes was demonstrated, wherein the combination of diisopropylethylamine and tetrabutylammonium iodide as additives is cruci

Full text of DOI:10.1016/j.tetlet.2004.10.168

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 157–159
Chiral phosphine oxide BINAPO as a catalyst for enantioselective
allylation of aldehydes with allyltrichlorosilanes
Makoto Nakajima,a,* Shunsuke Kotani,a,b Tadao Ishizukaa and Shunichi Hashimotob
aFaculty of Medical and Pharmaceutical Sciences, Kumamoto University, Kumamoto 862-0973, Japan
bGraduate School of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan
Received 6 October 2004; revised 25 October 2004; accepted 27 October 2004
Abstract—The effectiveness of chiral phosphine oxide BINAPO as a catalyst for the enantioselective addition of allyltrichlorosilanes
to aldehydes was demonstrated, wherein the combination of diisopropylethylamine and tetrabutylammonium iodide as additives is
crucial for accelerating the catalytic cycle.
Ó 2004 Elsevier Ltd. All rights reserved.
The asymmetric allylation of carbonyl compounds to
generate two successive stereogenic centers has been
the subject of investigation in recent years.1 While high
enantioselectivities have been achieved with allylmetals
in the presence of chiral Lewis acids as catalysts,2 these
processes preferentially afford syn homoallylic alcohols
from both stereoisomers of allylmetals via acyclic transi-
tion states. On the other hand, Lewis base-catalyzed
allylations with allyltrichlorosilanes are presumed to
proceed via chair-like transition states that involve
hypervalent silicates,3,4 in which the addition of (E)-
and (Z)-silane provide the anti- and syn-product,
respectively, with high diastereoselectivity. Asymmetric
versions of the Lewis base-catalyzed allylations using
chiral phosphoramide,5 formamide,6 and pyridine N-
oxide7 derivatives as catalysts were reported to afford
the homoallylic alcohols with good enantioselectivities.8
Although phosphine oxides possess a notable electron-
pair donor property and form complexes with various
metals,9 less attention has been paid to chiral phosphine
oxides in the field of asymmetric catalysis.10 Quite re-
cently, Kobayashi and co-workers reported an allylation
of acylhydrazones with allyltrichlorosilane promoted by
achiral phosphine oxides, that required stoichiometric
amounts of phosphine oxides as promoters.11 Herein
we describe the first enantioselective allylation of alde-
hydes with allyltrichlorosilanes promoted by substoichio-
metric amounts of chiral phosphine oxide.
We initially investigated the allylation of benzaldehyde
with allyltrichlorosilane at rt in dichloromethane using
10mol% of BINAPO,10c a precursor of BINAP which
is the most common chiral phosphine ligand. The reac-
tion did proceed, however, the catalytic activity was so
low that only 32% of homoallyl alcohol was obtained
after 24h. (Table 1, entry 1). We previously reported
that the addition of diisopropylethylamine significantly
enhanced N-oxide-promoted allylation with allyltrichlo-
rosilane.7a In fact the addition of the amine increased
the reactivity of BINAPO, but the yield of the alcohol
was still modest (entry 2). After considerable screening,
we found the addition of tetrabutylammonium iodide12
to the above system dramatically increased the reactivity
(entry 4).13 Since the addition of tetrabutylammonium
iodide alone does not give comparable result (entry 3),
the combination of diisopropylethylamine and tetrabut-
ylammonium iodide was proved to be essential for the
reactivity, although the details are unclear.
With appropriate additives in hand, we then examined
allylations of benzaldehyde with trichlorosilanes of var-
ious substituent patterns. As shown in Table 2, c-allyl-
ated syn-homoallylic alcohol was obtained from (Z)-
crotyltrichlorosilane (entry 1) while the corresponding
anti-alcohol was produced from (E)-crotyltrichlorosi-
lane (entry 2). These results suggest that these allylations
Keywords: Allylation; Catalyst; Enantioselectivity; Phosphine oxide.
*
Corresponding author. Tel.: +81 96 371 4680; fax: +81 96 362 7692;
e-mail: nakajima@gpo.kumamoto-u.ac.jp
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.10.168

158
M. Nakajima et al. / Tetrahedron Letters 46 (2005) 157–159
Table 1. Effect of additives in BINAPO-catalyzed allylation
tern on allylsilane. trans-Crotylsilane gave a selectivity
similar to that of the parent allylsilane (entry 2), while
cis-crotylsilane (entry 1) and prenylsilane (entry 3) dra-
matically decreased the enantioselectivity. It is note-
worthy that the reaction of b-substituted silanes (entries
4–6), which gave lower selectivities with other Lewis
bases,7a afforded the adducts in good enantio-
selectivities.
Entry
Additive
(equiv)
Time
(h)
Yield
(%)a
Ee
(%)b
1
2
3
4
None
iPr2NEt(5)
24
24
12
4
32
79
54
92
36
37
46
43
Table 3 summarizes the results obtained in the reaction
of a variety of aldehydes with methallyltrichlorosilane.
Allylation of aromatic aldehydes gave results similar
to those with benzaldehyde (entries 1–7). Although the
electronic factor on benzene ring does not significantly
affect the enantioselectivity (entries 1, 2, 6, and 7), a,b-
unsaturated aldehydes, and aliphatic aldehydes were
unsuitable substrates in terms of both chemical yield
and enantioselectivity (entries 10, 11). The best enantio-
Bu4N+IÀ(1.2)
iPr2NEt(5) + Bu4N+IÀ(1.2)
a Isolated yield.
b Determined by HPLC (Daicel Chiralcel OD).
mediated by BINAPO proceed via cyclic chair-like tran-
sition structures that involve hypervalent silicates. The
enantioselectivity strongly depended on substituent pat-
Table 2. BINAPO-catalyzed allylation with trichlorosilanes
Entry
R1
R2
R3
Time (h)
Yield (%)a
Ee (%)b(confgn)c
[a]D (c, solvent)
1d
2f
3
Me
H
H
H
4
2
4
1
1
3
92e
87g
63
4 (1S,2R)
46 (1R,2R)
4 (R)
À2.3 (0.56, CHCl3)
+44.7 (0.75, CHCl3)
+2.3 (0.95, CHCl3)
+40.0 (0.58, C6H6)
À16.7 (1.72, CHCl3)
+40.2 (1.17, CHCl3)
Me
Me
H
H
Me
H
H
4
Me
Ph
73
80
66 (R)
59 (R)
64
5
H
H
6
H
–(CH2)4–
81
a Isolated yield.
b Determined by HPLC (Daicel Chiralcel OD-H or Chiralpak AD-H).
c Configuration assignment by comparison to the literature values of optical rotations, see Supplementary Data.
d E:Z = 1:99.
e syn:anti = 99:1.
f E:Z = 77:23.
g syn:anti = 23:77.
Table 3. BINAPO-catalyzed methallylation of aldehydes
Entry
R4
Time (h)
Yield (%)a
Ee (%)b (confgn)c
[a]D (c, solvent)
1
4-ClC6H4
4-MeOC6H4
2-Furyl
2
1
77
75
53
57
75
65
61
67
70
67
59
65 (R)
55 (R)
63 (R)
53
+34.7 (0.55, Et2O)
+42.4 (0.45, C6H6)
+31.7 (0.49, CHCl3)
+59.8 (0.54, CHCl3)
+56.7 (0.51, CHCl3)
+26.4 (0.89, CHCl3)
+27.0 (1.15, CHCl3)
+43.8 (0.97, CHCl3)
+46.2 (1.19, CHCl3)
+13.5 (0.39, C6H6)
À5.7 (1.0, CHCl3)
2
3
1
4
1-Naphthyl
2-Naphthyl
3,5-(CF3)2C6H3
3,4,5-(MeO)3C6H2
3,5-Me2C6H3
3,5-Me2C6H3
PhCH@CH
PhCH2CH2
4
5
4
62
56
6
4
7
4
57
71
8
2
9d
10
11
72
1
79
32 (R)
29 (S)
24
a Isolated yield.
b Determine by HPLC (Daicel Chiralcel OD-H, OJ-H or Chiralpak AD-H).
c Configuration assignment by comparison to literature values of optical rotations, see Supplementary data.
d The reaction was conducted at À23°C.

M. Nakajima et al. / Tetrahedron Letters 46 (2005) 157–159
159
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selectivity (79% ee) was observed in the reaction of 3,5-
dimethylbenzaldehyde at À23°C (entry 9).
In summary, we have demonstrated the effectiveness of
chiral phosphine oxide BINAPO as a catalyst for the
enantioselective addition of allyltrichlorosilanes to alde-
hydes, wherein a combination of diisopropylethylamine
and tetrabutylammonium iodide as additives is crucial
for accelerating the catalytic cycle. The present reaction
provides the first example that utilizes chiral phosphine
oxide as a catalyst in the enantioselective reaction. Stu-
dies on the mechanism as well as the design of chiral
phosphine oxides to further enhance enantioselectivity
are currently in progress.
6. Iseki, K.; Mizuno, S.; Kuroki, Y.; Kobayashi, Y. Tetra-
hedron Lett. 1998, 39, 2767–2770.
7. (a) Nakajima, M.; Saito, M.; Shiro, M.; Hashimoto, S. J.
Am. Chem. Soc. 1998, 120, 6418–6419; (b) Malkov, A. V.;
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Acknowledgments
9. Grushin, V. V. Chem. Rev. 2004, 104, 1629–1662.
10. Mono(phosphine oxide)–Pd complex: (a) Dai, W.-M.;
Yeung, K. K. Y.; Leung, W. H.; Haynes, R. K. Tetrahe-
dron: Asymmetry 2003, 14, 2821–2826, Bis(phosphine
oxide)–Fe complex; (b) Matsukawa, S.; Sugama, H.;
Imamoto, T. Tetrahedron Lett. 2000, 41, 6461–6465,
Bis(phosphine oxide)–Sm complex; (c) Mikami, K.;
Yamaoka, M. Tetrahedron Lett. 1998, 39, 4501–4504,
For Hybrid phosphine oxide ligand, see Ref. 9.
This work was partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Sci-
ence, Sports, and Culture of Japan and SUNBOR
GRANT.
Supplementary data
11. Ogawa, C.; Konishi, H.; Sugiura, M.; Kobayashi, S. Org.
Biomol. Chem. 2004, 2, 446–448.
12. Short, J. D.; Attenoux, S.; Berrisford, D. J. Tetrahedron
Lett. 1997, 38, 2351–2354.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2004.10.168.
13. Typical procedure: A solution of benzaldehyde (50mg,
0.47mmol) in CH2Cl2 (0.5mL), diisopropylethylamine
(0.41mL, 2.4mmol), and allyltrichlorosilane (0.10mL,
0.69mmol) were successively added to a solution of (S)-
BINAPO (32mg, 0.028mmol) and tetrabutylammonium
iodide (207mg, 0.56mmol) in CH2Cl2 (0.5mL) at room
temperature (20–25°C). After stirring for 4h, the reaction
was quenched by 10% NaOH (1mL). The mixture was
extracted with AcOEt (30mL) and the organic layer was
successively washed with 10% HCl (10mL), satd NaHCO3
(10mL), and brine. Drying over Na2SO4 and evaporating
the solvent furnished the crude product, which was
purified by column chromatography (SiO2 7g, hexane/
AcOEt = 12/1) to give the alcohol (63.5mg, 92%, [a]D
+21.8 (c 0.96, C6H6)) as a colorless oil. Ee was determined
to be 43% by chiral HPLC (Daicel Chiralcel OD, 1mL/
min, hexane/2-propanol = 40:1).
References and notes
1. For reviews on asymmetric allylation of carbonyl com-
pounds, see: (a) Hoffmann, R. W. Angew. Chem., Int. Ed.
Engl. 1987, 26, 489–594; (b) Yamamoto, Y.; Asao, N.
Chem. Rev. 1993, 93, 2207–2293; (c) Marshall, J. A. Chem.
Rev. 1996, 96, 31–47.
2. For a review on chiral Lewis acid-catalyzed allylation of
aldehydes, see: Cozzi, P. G.; Tagliavini, E.; Umani-
Ronchi, A. Gazz. Chim. Ital. 1997, 127, 247–254.
3. (a) Kobayashi, S.; Nishio, K. Tetrahedron Lett. 1993, 34,
3453–3456; (b) Kobayashi, S.; Nishio, K. J. Org. Chem.
1994, 59, 6620–6628.
4. For recent reviews on hypervalent silicates, see: (a)
Sakurai, H. Synlett 1989, 1–8; (b) Chuit, C.; Corriu, R.