Synthesis of γ-functionalized allyltrichlorosilanes and their application in the asymmetric allylation of aldehydes

Article abstract of DOI:10.1016/j.tetasy.2010.03.026

Isomerically pure trans-and cis-γ-bromoallyltrichlorosilanes 4 and 5 have been synthesized and shown to react with aromatic aldehydes 1 in the presence of Lewis-basic catalysts (e.g., DMF) to produce the corresponding anti-and syn-allylbromohydrins 8 and 9, respectively, as single diastereoisomers. With BINAPO 25 as a chiral catalyst, promising enantioselectivity (≤50% ee) has been attained.

Full text of DOI:10.1016/j.tetasy.2010.03.026

Tetrahedron: Asymmetry 21 (2010) 1173–1175
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Synthesis of
c-functionalized allyltrichlorosilanes and their application in
the asymmetric allylation of aldehydes
a
a,
*
Andrei V. Malkov a,b, , Claire MacDonald , Pavel Kocovsky
*
ˇ
´
a WestChem, Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK
b Department of Chemistry, Loughborough University, Leics LE11 3TU, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Isomerically pure trans- and cis-c-bromoallyltrichlorosilanes 4 and 5 have been synthesized and shown
Received 2 March 2010
Accepted 26 March 2010
Available online 3 May 2010
to react with aromatic aldehydes 1 in the presence of Lewis-basic catalysts (e.g., DMF) to produce the cor-
responding anti- and syn-allylbromohydrins 8 and 9, respectively, as single diastereoisomers. With
BINAPO 25 as a chiral catalyst, promising enantioselectivity (650% ee) has been attained.
Ó 2010 Elsevier Ltd. All rights reserved.
Dedicated to Professor Henri Kagan on the
occasion of his 80th birthday
Brønsted
base
1. Introduction
ArCHO
OH
X2
1
O
Ar
*
The addition of allymetal reagents to aldehydes constitutes a
popular method for the stereocontrolled construction of a car-
bon–carbon bond with the concomitant formation of up to two ste-
reogenic centers. The more recent variant, namely the asymmetric
allylation of aldehydes with allyltrichlorosilanes, catalyzed by chi-
ral Lewis bases (Scheme 1), has now evolved into an efficient and
practical method for the synthesis of enantiomerically enriched
homoallylic alcohols,1 thereby providing a viable alternative to
the protocols that employ stoichiometric chiral allylboranes. In
general, the reaction displays an excellent diastereocontrol in the
case of trans- and cis-crotylsilanes, which is believed to originate
from a cyclic transition state.1 However, the synthetic applications
are currently limited to simple alkyl homologues of allyltrichloro-
silanes.1,2 We reasoned that the use of isomerically pure allylsi-
Ar
*
X1
X1
* *
X1 = Hal
SiCl3
Lewis
base
X2
X
2 = H
10
2, X1 = Cl, X2 = H
3, X1 = H, X2 = Cl
4, X1 = Br, X2 = H
5, X1 = H, X2 = Br
6, X1 = Cl, X2 = H
7, X1 = H, X2 = Cl
8, X1 = Br, X2 = H
9, X1 = H, X2 = Br
Scheme 1.
per(I)-catalyzed hydrosilylation of the commercially available
1,3-dichloropropene (E/Z ꢀ1.3:1) with trichlorosilane in the pres-
ence of an equimolar amount of triethylamine.5 The latter 2/3 mix-
ture was then reacted with benzaldehyde (rt, 2 h) in DMF, which
acted both as a solvent and as a Lewis-basic catalyst, to afford a
1:1 mixture of the diastereoisomeric chlorohydrins 6 and 7 in
62% yield.
With this promising result in hand, we set out to synthesize
pure isomers E-2/Z-3 or E-4/Z-5. Since the initial attempts of the
separation of the cis- and trans-isomers of commercial 1,3-dichlo-
ropropene by fraction distillation were unsuccessful, an alternative
route had to be developed; we focused on the vinyl bromides E-4/
Z-5.
lanes functionalized in the
c-position 2–5 would broaden the
scope of this reaction and produce halohydrins 6–9 (Scheme 1),
which could further undergo cyclization to afford the correspond-
ing vinyl epoxides in a stereoselective fashion: thus, a ring closure
in the case of anti-halohydrins 6 or 8 should result in the formation
of trans-epoxides 10.3 It is noteworthy that in an analogous allyla-
tion of aldehydes employing
c-chloroallyltin reagents, the corre-
sponding chlorohydrins could not even be isolated due to their
spontaneous cyclization to afford vinylepoxides 10 directly.4
The synthesis of the trans-isomer E-4 is shown in Scheme 2. Fol-
lowing the method of Just et al.6 the addition of hydrobromic acid
to propiolic acid 11 afforded trans-b-bromoacrylic acid 12 (75%),
which was reduced with lithium aluminum hydride to give E-3-
bromo-propenol 13 in 62% yield as a single stereoisomer. The sub-
sequent conversion of the latter allylic alcohol 13 into chloride 14
is worthy of note: several methods, including the use of N-chloro
succinimide with triphenyl phosphine, thionyl chloride, oxalyl
chloride in DMF, and phosphorus trichloride in pyridine proved
2. Results and discussion
In order to explore the feasibility of this approach, a mixture of
3-chloroallylsilanes 2 and 3 (E/Z ꢀ1.3:1) was prepared by a cop-
* Corresponding authors. Tel.: +44 1509222580 (A.V.M.).
E-mail addresses: a.malkov@lboro.ac.uk (A.V. Malkov), pavelk@chem.gla.ac.uk
ˇ
´
(P. Kocovsky).
0957-4166/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2010.03.026

1174
A. V. Malkov et al. / Tetrahedron: Asymmetry 21 (2010) 1173–1175
Table 1
aq. HBr
100 o
Allylation of aldehydes in DMFa
C
CO2H
CO2H
11
Br
O
(75%)
12
DMF
OH
OH
Ar
1a-e
H
LiAlH4, Et2O
(62%)
OH
Br
0
oC
Ar
Ar
CO(CCl3)2,
24 h
0 oC
X1 X2
PPh3, 0 o
C
X1
SiCl3
Br
Br
Cl
19a-e
8a-d
9a-e
X2
(71%)
13
(E)-
14
(>99:1)
4/5
HSiCl3, Et3N
CuCl, Et2O
Entry
Aldehyde, Ar
Silane
X1, X2
Productb, yieldc (%)
Br
SiCl3
1
2
3
4
5
6
7
8
9
1a, Ph
4
4
4
4
5
5
5
5
5
Br, H
Br, H
Br, H
Br, H
H, Br
H, Br
H, Br
H, Br
H, Br
8a, 38
8b, 36d
8c, 41
8d, 25
9a, 48
9b, 38
9c, 48
9d, 31
9e, 32
-4
(E)
1b, 2-Naphthyl
1c, 4-CF3C6H4
1d, 4-ClC6H4
1a, Ph
1b, 2-Naphthyl
1c, 4-CF3C6H4
1d, 4-ClC6H4
1e, 4-MeOC6H4
Scheme 2.
unsuccessful, with the yields of the desired product not exceeding
22%. Finally, after further experimentation, we found that the pro-
cedure employing hexachloroacetone as the source of the chloride,
produced the required allylchloride 14 in 71% yield. The reaction
proceeded under mild conditions with retention of the regio- and
stereo-integrity. Furthermore, purification of the product can be
accomplished by its distillation directly from the reaction mixture.
Silane E-4 was then obtained by a CuCl-catalyzed hydrosilyation of
14 with trichlorosilane in the presence of an equimolar amount of
triethylamine.5 After removing the solid residues by filtration un-
der an inert atmosphere, an aliquot sample was analyzed by 1H
NMR spectroscopy, which confirmed a quantitative conversion.
To avoid decomposition, the remaining material was used in the
next step without further purification.
For the synthesis of Z-5 (Scheme 3), ethyl propiolate 15 was
readily converted into Z-bromoacrylate 16 (92%) by reaction with
lithium bromide in acetic acid. Here, the nucleophilic addition of
the halide anion to the electron-deficient carbon–carbon triple
bond, stereodirected by coordination of the lithium cation to the
carbonyl group, was proposed to account for the high stereoselec-
tivity observed for this reaction.7 Ester 16 was then reduced with
lithium aluminum hydride at 0 °C to provide the cis-alcohol 17 in
77% yield. The last two steps toward the target Z-5 were carried
out in the same way as described for the synthesis of E-4.
a
The reactions were carried out on a 2.0 mmol scale at 0 °C for 24 h, using a 1:1
aldehyde to silane ratio at 0.4 M concentration.
b
Variable quantities (up to 30%) of the isomeric products 19a–e were also
formed. The products were isolated as almost pure diastereoisomers (P95:5) as
shown by 1H NMR spectroscopy. The relative configuration was established by 1H
NMR spectroscopy in analogy with other crotyl derivatives.9,10
The yields are shown for the two-step sequence (14/18?4/5?8/9) since the
silanes 4 and 5 were used without isolation.
c
d
The product was difficult to purify; the diastereoisomeric ratio was P9:1.
As expected, bromohydrin 8a was readily cyclized upon treat-
ment with NaH in CH2Cl2 (0 °C, 2 h) to afford the pure trans-epox-
ide 10a (Ar@Ph) in 82% yield.
In some cases, the allylation of aldehydes with silanes 4/5 was
accompanied by the formation of variable quantities of the iso-
meric linear products 19, presumably formed by the allylic rear-
rangement of halohydrins 8/9, which in turn was catalyzed by
traces of residual CuCl left in the reaction mixture from the previ-
ous step. The isomers can be separated by chromatography but for
obtaining the epoxides this may not be necessary, since the corre-
sponding iodo-analogues of 19 have been shown to undergo a fac-
ile cyclization into vinylepoxides 10 anyway (upon treatment with
NaH).8
To develop an enantioselective variant of this process, several
chiral Lewis bases were examined as potential catalysts (Fig. 1).
First, we used our pyridine-type N-oxide catalysts METHOX 202,9
and QUINOX 21,10 which have previously been shown by us to ex-
hibit high catalytic activity and enantioselectivity in the addition of
allyltrichlorosilane and its non-functionalized homologues to a
variety of aromatic aldehydes.9–11 However, in the case of bromo-
allyltrichlorosilanes 4 and 5, both N-oxides 20 and 21 proved
LiBr, AcOH
MeCN, 80 oC
CO2Et
15
Br
CO2Et
16
LiAlH4, Et2O
0 o
(92%)
(77%)
C
CO(CCl3)2,
PPh3, 0 o
C
Br
OH
Br
Cl
18
17
(85%)
(Z)-
(> 99:1)
O
MeO
Ph2P
HSiCl3, Et3N
CuCl, Et2O
N
O
O
OMe
O
OMe
Br
SiCl3
N
-5
(Z )
O
O
Me
Ph2P
Scheme 3.
Me
O
O
20
21
22
With the geometrically pure allylsilanes 4 and 5 in hand, we first
investigated a racemic variant of the allylation reaction, employing
DMF as a Lewis-basic activator.1 The results are summarized in
Table 1; since silanes 4 and 5 were prepared in situ, without isola-
tion, the yields are given over the two steps. The reaction proved
highly diastereoselective: thus, silane E-4 produced exclusively
the anti-isomers 8a–d, whereas Z-5 afforded syn-9a–e contami-
nated with only trace amounts of the opposite diastereoisomers
(P95:5), as revealed by the 1H NMR spectra of the crude products.
O
O
O
PPh2
P
P
P
P
PPh2
O
23
24
Figure 1. Chiral catalysts.
25

A. V. Malkov et al. / Tetrahedron: Asymmetry 21 (2010) 1173–1175
1175
Hall, D. G. Angew. Chem., Int. Ed. 2003, 42, 4732–4739; (e) Oestreich, M.;
Rendler, S. Synthesis 2005, 1727–1747; (f) Orito, Y.; Nakajima, M. Synthesis
2006, 1391–1401; (g) Benaglia, M.; Guizzetti, S.; Pignataro, L. Coord. Chem. Rev.
unreactive (Table 2, entries 1 and 2), suggesting that the nucleo-
philicity of the -carbon in silanes 4 and 5 is significantly reduced
(compared to allyltrichlorosilane) due to the electron-withdrawing
effect of the bromine. Therefore, we turned our attention to phos-
phine oxides as potentially more reactive catalysts.
c
ˇ
´
2007, 252, 492–512; (h) Malkov, A. V.; Kocovsky, P. Eur. J. Org. Chem. 2007, 29–
ˇ
´
36; (i) Kocovsky, P.; Malkov, A. V. Chiral Lewis Bases as Catalysts. In
Enantioselective Organocatalysis; Dalko, P. I., Ed.; Wiley-VCH: Weinheim,
2007; p 255; (j) Denmark, S. E.; Beutner, G. L. Angew. Chem., Int. Ed. 2008, 47,
1560–1638.
ˇ
´
2. Malkov, A. V.; Kabeshov, M. A.; Barłóg, M.; Kocovsky, P. Chem. Eur. J. 2009, 15,
1570–1573.
Table 2
Asymmetric allylation of aldehydes 1 with 4 and 5a
3. For asymmetric synthesis of vinylepoxides, see: (a) Hu, S.; Jayaraman, S.;
Oehlschlager, A. C. J. Org. Chem. 1996, 61, 7513–7520; (b) Solladie-Cavallo, A.;
Bouerat, L.; Roje, M. Tetrahedron Lett. 2000, 41, 7309–7312; (c) Bandini, M.;
Cozzi, P.; Melchiorre, P.; Morganti, S.; Umani-Ronchi, A. Org. Lett. 2001, 3,
1153–1155; (d) Zanardi, J.; Lamazure, D.; Miniere, S.; Reboul, V.; Metzner, P. J.
Org. Chem. 2002, 67, 9083–9086; (e) Aggarwal, V. K.; Alonso, E.; Bae, I.; Hynd,
G.; Lydon, K. M.; Palmer, M. J.; Patel, M.; Porcelloni, M.; Richardson, J.; Stenson,
R. A.; Studley, J. R.; Vasse, J.-L.; Winn, C. J. J. Am. Chem. Soc. 2003, 125, 10926–
10940; (f) Aggarwal, V. K.; Bae, I.; Lee, H.-Y.; Richardson, J.; Williams, D. T.
Angew. Chem., Int. Ed. 2003, 42, 3274–3278.
4. Shibata, I.; Fukuoka, S.; Baba, A. Chem. Lett. 1998, 533–534.
5. (a) Furuya, N.; Sukawa, T. J. Organomet. Chem. 1975, 96, C1–C3; (b) Nishiyama,
H.; Narimatsu, S.; Itoh, K. Tetrahedron Lett. 1981, 22, 5289–5292.
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7. (a) Ma, S.; Lu, X. J. Chem. Soc., Chem. Commun. 1990, 1643–1644; (b) Ma, S.; Lu,
X. Org. Synth. 1999, Coll. Vol. 9, 415–417.
Entry
Catalyst
(mol %)
Aldehyde,
Ar
Silane,
X1, X2
T
(°C)
Product,
Eec
(%)
yieldb (%)
1
2
3
4
5
6
8
9
20 (10)
21 (10)
22 (10)
23 (20)
24 (10)
24 (10)
25 (10)
25 (10)
1a, Ph
1c, 4-CF3C6H4
1a, Ph
1a, Ph
1a, Ph
1a, Ph
1a, Ph
1c, 4-CF3C6H4
4, H, Br
5, Br, H
5, Br, H
4, H, Br
4, H, Br
5, Br, H
5, Br, H
5, Br, H
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ10
8a, 0
9c, 0
n/a
n/a
18
9a, 17
8a, 23
8a, 10
9a, 43
9a, 34
9c, 22
29
15
25d
50e
43
a
For the Scheme, see Table 1. The reactions were carried out in MeCN on a
0.5 mmol scale with a 1:1.1 aldehyde/silane ratio in the presence of the catalyst
overnight.
8. Kazmaier, U.; Lucas, S.; Klein, M. J. Org. Chem. 2006, 71, 2429–2433.
b
ˇ
´
9. Malkov, A. V.; Bell, M.; Castelluzzo, F.; Kocovsky, P. Org. Lett. 2005, 7, 3219–
The absolute configuration of the products was inferred from the known
3222.
absolute configuration of the (S,S)-(ꢁ)-bromohydrin 9a (as shown in the Schemes)
and the corresponding chlorohydrin18 and extrapolated to the rest of the series. This
configuration corresponds to that of the laevorotatory product obtained from the
reaction of PhCHO with Z-crotyltrichlorosilane catalyzed by (S)-25.17 The crude
products were almost pure diastereoisomers (P95:5) as shown by 1H NMR spec-
troscopy. The relative configuration was established by 1H NMR spectroscopy in
analogy with other crotyl-derived products;9,10 8a and 9a are known compounds.18
ˇ
´
10. (a) Malkov, A. V.; Dufková, L.; Farrugia, L.; Kocovsky, P. Angew, Chem., Int. Ed.
2003, 42, 3674–3677; (b) Malkov, A. V.; Ramírez-López, P.; Biedermannová, L.;
ˇ
´
Rulíšek, L.; Dufková, L.; Kotora, M.; Zhu, F.; Kocovsky, P. J. Am. Chem. Soc. 2008,
130, 5341–5348.
11. For other chiral N-oxide catalysts, see Refs. 1h–j and the following: (a) Malkov,
ˇ
´
A. V.; Orsini, M.; Pernazza, D.; Muir, K. W.; Langer, V.; Meghani, P.; Kocovsky, P.
Org. Lett. 2002, 4, 1047–1049; (b) Malkov, A. V.; Bell, M.; Vassieu, M.; Bugatti,
c
Established by chiral HPLC.
ˇ
´
V.; Kocovsky, P. J. Mol. Catal. A: Chem. 2003, 196, 179–186; (c) Malkov, A. V.;
½ ꢃ ¼ ꢁ8:1 (c 0.5, CH2Cl2); lit (Ref. 18) gives ½aꢃD ¼ ꢁ34:7 (c 2.1, CHCl3) for the
a 2D0
d
Bell, M.; Orsini, M.; Pernazza, D.; Massa, A.; Herrmann, P.; Meghani, P.;
highly enantiomerically enriched (1S,2S)-9a.
ˇ
´
Kocovsky, P. J. Org. Chem. 2003, 68, 9659–9668; (d) Malkov, A. V.; Westwater,
½ ꢃ ¼ ꢁ19:8 (c 0.5, CH2Cl2).
a 2D0
e
ˇ
ˇ
M.-M.; Kadlcíková, A.; Gutnov, A.; Hodacová, J.; Rankovic, Z.; Kotora, M.;
ˇ
´
Kocovsky, P. Tetrahedron 2008, 64, 11335–11348; (e) Malkov, A. V.; Gordon, M.
ˇ
ˇ
´
R.; Stoncius, S.; Hussain, J.; Kocovsky, P. Org. Lett. 2009, 11, 5390–5393; (f)
ˇ
ˇ
Hrdina, R.; Valterová, I.; Hodacová, J.; Císarová, I.; Kotora, M. Adv. Synth. Catal.
Indeed, the allylation of benzaldehyde with Z-5 in the presence
of the phosphine dioxide 22 (10 mol %),12 derived from Kagan’s
DIOP,13 produced syn-9a, though in low yield (17%) and with only
18% ee (entry 3). Monoxide14 23 and dioxide15 24 exhibited similar
levels of reactivity and enantioselectivity (entries 4–6). The most
promising results were obtained with (S)-BINAPO16,17 25
(10 mol %), which catalyzed the formation of syn-9a in 50% ee (en-
try 7) and syn-9c in 43% ee (entry 8).18 High diastereoselectivities
(P95:5) were again observed.19
ˇ
ˇ
2007, 349, 822–826; (g) Hrdina, R.; Kadlcíková, A.; Valterová, I.; Hodacová, J.
Tetrahedron: Asymmetry 2007, 17, 3185–3191; (h) Hrdina, R.; Boyd, T.;
Valterová, I.; Hodacová, J.; Kotora, M. Synlett 2008, 3141–3144; (i) Hrdina, R.;
Dracinsky, M.; Valterová, I.; Hodacová, J.; Císarová, I.; Kotora, M. Adv. Synth.
Catal. 2008, 350, 1449–1456; (j) Kadlcíková, A.; Hrdina, R.; Valterová, I.; Kotora,
M. Adv. Synth. Catal. 2009, 351, 1279–1283; (k) Hrdina, R.; Opekar, F.; Roithová,
J.; Kotora, M. Chem. Commun. 2009, 2314–2316; For chiral sulfoxide-type
catalysts, see: (l) Massa, A.; Malkov, A. V.; Kocovsky, P.; Scettri, A. Tetrahedron
Lett. 2003, 44, 7179–7181.
ˇ
ˇ
´
ˇ
ˇ
ˇ
ˇ
´
12. Hashmi, A. S. K.; Naumann, F.; Probst, R.; Bats, J. W. Angew. Chem., Int. Ed. 1997,
36, 104–106.
13. Dang, T. P.; Kagan, H. B. J. Chem. Soc., Chem. Commun. 1971, 481–482.
14. Cote, A.; Desrosiers, J.-N.; Boezio, A. A.; Charette, A. B. Org. Synth. 2006, 83, 1–4.
15. Cote, A.; Boezio, A. A.; Charette, A. B. Angew. Chem., Int. Ed. 2004, 43, 6525–
6528.
3. Conclusion
16. (a) Takaya, H.; Mashima, K.; Koyano, K.; Yagi, M.; Kumobayashi, H.; Taketomi,
T.; Akutagawa, S.; Noyori, R. J. Org. Chem. 1986, 51, 629–635; (b) Takaya, H.;
Akutagawa, S.; Noyori, R. Org. Synth. 1989, 67, 20–32; (c) Gladiali, S.;
Pulacchini, S.; Fabbri, D.; Manassero, M.; Sansoni, M. Tetrahedron: Asymmetry
1998, 9, 391–395; (d) Berthod, M.; Saluzzo, C.; Mignani, G.; Lemaire, M.
Tetrahedron: Asymmetry 2004, 15, 639–645; (e) Nakajima, M.; Sugiura, M.;
Kotani, S.; Tando, T.; Shimoda, Y. Tetrahedron: Asymmetry 2009, 20, 1369–1370.
17. The related allylation of aldehydes with E- and Z-crotyl-trichlorosilane,
catalyzed by BINAPO, exhibited 46% and 4% ee, respectively: Kotani, S.;
Hashimoto, S.; Nakajima, M. Tetrahedron 2007, 63, 3122–3132.
18. The (1R,2R)-(+)- and (1S,2S)-(ꢁ)-bormohydrin 9a was previously obtained via
In conclusion, we have synthesized isomerically pure trans- and
cis-c-bromoallyltrichlorosilanes 4 and 5, which in the Lewis base-
catalyzed addition to aromatic aldehydes 1 produced the corre-
sponding allylbromohydrins 8 and 9, respectively, as highly pure
diastereoisomers (P95:5). In the enantioselective variant, promis-
ing enantioselectivity (50% ee) was obtained with BINAPO 25 as a
catalyst. It is noteworthy that all these reactions proceed solely via
the
c-attack of the allylic moiety; the formation of the products
corresponding to the a-attack was not observed within the limits
the BF3-catalyzed stoichiometric addition of the enantiomeric [Z-c-
of the 1H NMR spectroscopy.20
bromoallyl]B(Ipc)2 to PhCHO in 74% ee (at ꢁ78 °C) and 86% ee (at ꢁ90 °C);
the syn:anti ratio was 90:10 and 94:6, respectively.3a
19. Silane 5 (0.55 mmol) was added to a solution of 25 (0.05 mmol), i-PrEt2N
(2.5 mmol), and PhCH@O (0.50 mmol) in MeCN (2 mL) under argon at ꢁ20 °C
and the mixture was stirred at ꢁ20 °C overnight. The reaction was quenched
with aq. NaHCO3 (1 mL) and the mixture was extracted with ether (3 ꢂ 10 mL).
The extract was washed with brine (2 ꢂ 10 mL), dried (Na2SO4), and
evaporated. Purification by flash chromatography on silica gel (15ꢂ1 cm)
with petroleum ether/AcOEt (6:1) afforded (ꢁ)-9a (34%). HPLC (Chiracel IB,
Acknowledgments
We acknowledge the EPSRC for Grant No. GR/T27051/01 and
Loughborough University for an additional support.
flow
rate:
0.5 mL/min,
hexane/i-PrOH
99:1;
tminor = 40.30 min,
References
tmajor = 42.84 min) showed 50% ee.
20. Formation of small amounts of the products of
a
-attack (610%) was observed
-haloallylboranes.18
1. For leading reviews, see: (a) Denmark, S. E.; Stavenger, R. A. Acc. Chem. Res.
2000, 33, 432–440; (b) Denmark, S. E.; Fu, J. Chem. Commun. 2003, 167–170; (c)
Denmark, S. E.; Fu, J. Chem. Rev. 2003, 103, 2763–2793; (d) Kennedy, J. W. J.;
for the stoichiometric allylation of aldehydes with
c