Barbier coupling in water: SnCl2-mediated and Co(acac) 2-catalyzed allylation of carbonyls

Article abstract of DOI:10.1016/j.tetlet.2005.07.060

Co(acac)₂·2H₂O efficiently catalyzes SnCl ₂-mediated Barbier coupling in water between carbonyls, including aromatic, aliphatic and α,β-unsaturated aldehydes, ketones, sugars and allyl bromide to afford the corresponding h

Full text of DOI:10.1016/j.tetlet.2005.07.060

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 6247–6251
Barbier coupling in water: SnCl -mediated and Co(acac) -catalyzed
2
2
allylation of carbonyls
*
Mihir K. Chaudhuri, Sanjay K. Dehury and Sahid Hussain
Department of Chemistry, Indian Institute of Technology, Guwahati 781039, India
Received 11 May 2005; revised 5 July 2005; accepted 14 July 2005
Abstract—Co(acac) Æ2H O efficiently catalyzes SnCl -mediated Barbier coupling in water between carbonyls, including aromatic,
2
2
2
aliphatic and a,b-unsaturated aldehydes, ketones, sugars and allyl bromide to afford the corresponding homoallylic alcohols in high
yields. The catalyst was reused for several cycles with consistent activity.
ꢀ
2005 Elsevier Ltd. All rights reserved.
1
The Barbier reaction is a century old C–C bond form-
ing reaction involving a coupling reaction between a car-
bonyl function and an organic halide mediated by a
metal to afford the corresponding alcohol. Its synthetic
utility was overtaken mainly by Grignard and related
organometallic reactions, largely because the classical
Barbier reactions suffered from the problems of interfer-
ence from a host of side reactions especially reductions
and couplings (cf. pinacol coupling of carbonyls and
to the products can be incomplete presumably because
of passivity of the catalyst caused by oxo or hydroxo
metal coating in the course of the reaction. The use of
4
a,b
hydrobromic acid
saturated aqueous NH Cl/THF to activate the metal,
or ultrasonication together with
3
4
improves the yields. Our experience in catalytic organic
1
9
transformations in conjunction with information in
2
–9,11–18
recent publications
on metal mediated C–C bond
forming reactions, suggested to us that a combination of
2
Wurtz coupling of halides) in organic solvents. Signifi-
a water-soluble reductive salt (e.g., SnCl ) as a mediator
2
cantly, if it is carried out in an aqueous medium, the
desired product is obtained, almost quantitatively, if
not exclusively. This realization gained momentum
and an appropriately water soluble, redox active, reus-
able metal complex as the catalyst would perhaps serve
as a general activator for the metal. The bimetallic nat-
ure of the reagent and the significant effect of water to
facilitate the reaction by in situ generation of reactive
hydrated/hydroxyl organotin species are important. A
suitable catalyst would be cost effective, reusable over
several cycles of reactions and have appreciable shelf
life, some solubility in water without decomposition or
hydrolysis and easily accessible variable oxidation
states. An appropriate catalyst is essential since in water,
3
4
through the elegant contributions of Luche, Nakami,
5
2,6
7
8
Benezra, Li and Chan, Whitesides, Roy and Liu
9
and Guo. An important incentive in this development
is that the solvent water satisfies the demands of ꢀGreen
1
0
Chemistry.ꢁ The developments in Barbier reactions in
water are: (i) a large number of reagents and reaction
protocols have been developed for the synthesis of
3
–9
homoallylic alcohols,
(ii) many metals viz. alumin-
1
1
6a,12
6g
13
14
ium, indium,
antimony, bismuth, lead, man-
SnCl alone cannot mediate the desired reaction.
2
6
e
6f
zinc,15 tin,
2,16
cadmium,17
ganese,
magnesium,
1
8
9a
20
Cu(II)8a,9a or
gallium and copper have been reported to be effec-
tive. Being heterogeneous in nature, metal-mediated
reactions often pose operational problems (e.g., stir-
ring), in addition, the conversion of starting materials
Four recent reports using Pd(II),
9
b
Ti(III) as efficient catalysts are of direct relevance to
the present work. However, Pd and Ti based catalysts
are expensive, the former also needing biphasic condi-
tions, while TiCl has inherent stability problems and re-
3
quires careful handling. The Cu(II) catalyzed reactions
were conducted under nitrogen since the active catalyst
Keywords: Cobalt(II) acetylacetonate; Allylation; Water; Carbonyl;
Cu(I) is prone to easy oxidation. TiCl is not efficient
Ketones; Sugars.
3
9
b
*
Corresponding author. Tel./fax: +91 361 2690762; e-mail: mkc@
iitg.ernet.in
for ketones
and Cu(II) is poor in allylation of
aldehydes, RCHO, with R being cinnamyl, n-hexyl or
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.07.060

6248
M. K. Chaudhuri et al. / Tetrahedron Letters 46 (2005) 6247–6251
2
-furyl.9a Moreover, it is not clear if carbohydrate allyl-
O
OH
SnCl , Co(acac) .2H O (cat.)
ation would be possible with these catalysts.
Br
2
2
2
+
H
R
R
water, RT
(
81-97%)
We now disclose our results on the Barbier allylation of
a variety of substrates using SnCl /Co(acac) Æ2H O as
2
2
2
Scheme 1.
2
1
the reagent system. Incidentally, we have reported a
2
2
very easy access to metal acetylacetonates including
Co(acac) Æ2H O. The chosen catalyst has easily accessi-
ble variable oxidation states, the required solubility in
water with no apparent hydrolysis and is capable of act-
ing as a Lewis acid (Scheme 1).
2
2
2 2
Table 1. SnCl -mediated and Co(acac) -catalyzed carbonyl allylation
a
Entry
Substrate
Product
Time (h)
6
Yield%
OH
CHO
1
97
OH
CHO
2
3
6
6
93
91
O
O
OH
CHO
Cl
Cl
OH
Cl
CHO
Cl
4
5
6
6
94
93
OH
Cl
CHO
Cl
CHO
Cl
Cl
OH
6
7
6
6
96
94
NO2
NO2
OH
CHO
O N
2
O N
2
OH
OH
CHO
CHO
8
9
6
6
6
92
95
91
MeO
MeO
H N
2
H N
2
OH
CHO
1
0
NH
2
NH2
OH
CHO
CHO
1
1
1
2
6
8
90
92
CHO
OH
CHO
OH
13
8
85
CHO

M. K. Chaudhuri et al. / Tetrahedron Letters 46 (2005) 6247–6251
6249
Table 1 (continued)
a
Entry
Substrate
Product
Time (h)
8
Yield%
OH
14
n-C
7
H15CHO
90
n-C H
7
15
19
OH
1
1
5
6
n-C
9
H19CHO
8
88
85
n-C H
9
O
OH
18
OH
O
1
1
7
8
18
18
81
89
OH
OH
O
HO
OH OH
1
2
9
0
D-Arabinose
L-Mannose
18
18
81
83
CH OH
2
OH OH
OH OH OH
OH OH
CH OH
2
a
Isolated yield.
No organic co-solvent or assistance of ultrasonic irradi-
ation is required thereby providing a clear advantage
over the reactions catalyzed by zero valent metals. The
experimental results are summarized in Table 1. The
identity of the products was confirmed by comparing
In view of a facile Co(II)–Co(III) redox cycling, it is pos-
sible that an allyl halide radical is first formed by the
transfer of an electron from cobalt(II), which in turn
.
interacts with SnCl to give an
SnX radical. This
2
2
would then react with cobalt(III) to produce cobalt(II)
and a neutral allylated derivative, which would finally
add to the carbonyl group yielding the homoallylic alco-
hol. This reaction might also be facilitated by simulta-
neous Lewis acid (cf. Co(II)) catalyzed activation of
the carbonyl group. We refrain from making any further
comment on the mechanism at the moment in the ab-
sence of further results. A notable feature of the Co(acac)2Æ
1
the H NMR (300 and 400 MHz) and FTIR spectral
data with reported results.7c,9,23
Following our typical reaction procedure,21 aldehydes
and ketones (entries 1–15 and 16–18), aliphatic and aro-
matic carbonyls (entries 12–20 and 1–11) and nitro
substituted substrates (entries 6 and 7) were allylated
efficiently. For the substrates containing an active
hydrogen, the reagent works well without any need for
protection (entries 14–17). Similarly, for a,b-unsatu-
rated aldehydes (entries 12 and 13) only 1,2-addition
products were obtained. No pinacol coupling was ob-
served in such reactions. All these results point to a large
scope for the SnCl /Co(acac) Æ2H O system.
2
H O catalyst is its reusability. The results presented in
2
2
2
2
It is significant that the present protocol successfully
effects allylation of unprotected sugars in water. Thus,
the SnCl /Co(acac) Æ2H O combination brought about
2
2
2
smooth reactions between D-arabinose or L-mannose
and allyl bromide affording the corresponding homo-
allylic alcohols in 81% and 83% yields.
In order to further broaden the scope, a few reactions
were conducted with allyl chloride. Thus, aromatic car-
bonyl substrates (Table 1, entries 1–3) were reacted with
allyl chloride under conditions similar to those used for
allyl bromide. The corresponding homoallylic products
were obtained in comparable yields.
Figure 1. Recycling of the catalyst for the allylation of benzaldehyde.

6
250
M. K. Chaudhuri et al. / Tetrahedron Letters 46 (2005) 6247–6251
2
-mediated allylation of benzaldehyde using various
J. Org. Chem. 1999, 64, 3230–3236; (g) Li, C. J.; Chan,
Table 2. SnCl
metal acetylacetonate catalysts
T. H. Tetrahedron Lett. 2000, 41, 5009–5012, and refer-
ences cited therein.
. (a) Schmid, W.; Whitesides, G. M. J. Am. Chem. Soc.
Entry
Catalyst
Time (h)
Yield (%)
7
1
2
3
4
Co(acac)
2
Æ2H
2
O
6
30
30
18
97
15
10
95
1
991, 113, 6674–6675; (b) Gordon, D. M.; Whitesides, G.
VO(acac)
2
M. J. Org. Chem. 1993, 58, 7937–7938; (c) Kim, E.;
Gordon, D. M.; Schmid, W.; Whitesides, G. M. J. Org.
Chem. 1993, 58, 5500–5507; (d) Gao, J.; Harter, R.;
Gordon, D. M.; Whitesides, G. M. J. Org. Chem. 1994, 59,
Fe(acac)
Cr(acac)
3
3
3
714–3715.
8
. (a) Kundu, A.; Prabhakar, S.; Vairamani, M.; Roy, S.
Organometallics 1997, 16, 4796–4799; (b) Kundu, A.;
Prabhakar, S.; Vairamani, M.; Roy, S. Organometallics
Figure 1 show that six cycles of the reaction could be
conducted with high throughput and one time loading
of the catalyst.
1
999, 18, 2782–2785; (c) Kundu, A.; Roy, S. Organomet-
allics 2000, 19, 105–108; (d) Sinha, P.; Kundu, A.; Roy, S.;
Prabhakar, S.; Vairamani, M.; Sankar, A. R.; Kunwar, A.
C. Organometallics 2001, 20, 157–162; (e) Sinha, P.; Roy,
S. Chem. Commun. 2001, 1798–1799; (f) Sinha, P.; Roy, S.
Organometallics 2004, 23, 67–71.
It may be relevant to mention that allylations of benzal-
dehyde using different metal acetylacetonate catalysts,
for example, VO(acac) , Fe(acac) , Cr(acac) and Co-
2
3
3
(
acac) Æ2H O (Table 2) did not proceed with equal alac-
2 2
9. (a) Tan, X. H.; Shen, B.; Liu, L.; Guo, Q. X. Tetrahedron
Lett. 2002, 43, 9373–9376; (b) Tan, X. H.; Shen, B.; Deng,
W.; Zhao, H.; Liu, L.; Guo, Q. X. Org. Lett. 2003, 5,
1833–1835.
10. Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and
Practice; Oxford University Press: New York, 1997.
1. Araki, S.; Jin, S. J.; Idou, Y.; Butsugan, Y. Bull. Chem.
Soc. Jpn. 1992, 65, 1736–1738.
2. (a) Cintas, P. Synlett 1995, 1087–1096; (b) Chan, Y. H.;
Yang, Y. J. Am. Chem. Soc. 1999, 121, 3228–3229; (c)
Loh, T. P.; Zhou, J. R.; Yin, Z. Org. Lett. 1999, 1, 1855–
rity. For VO(acac) and Fe(acac) , the yields were very
2
3
low (10–15%) even after 30 h of reaction. Cr(acac)3
and Co(acac) Æ2H O were far more effective in catalyz-
2
2
ing the allylations although the former took relatively
longer. Thus, Co(acac) Æ2H O was the catalyst of choice.
2
2
1
In conclusion, Co(acac) Æ2H O appears to be an effec-
2
2
1
tive, reusable catalyst for SnCl mediated carbonyl ally-
2
lation of a wide range of aldehydes, ketones and
carbohydrates. The main advantages of the catalyst
include ease of preparation, stability and high activity
and solubility in water.
1
857; (d) Hilt, G.; Smolko, K. I.; Waloch, C. Tetrahedron
Lett. 2002, 43, 1437–1439.
13. Wada, M.; Ohki, H.; Akiba, K. Y. Bull. Chem. Soc. Jpn.
990, 63, 1738–1747.
1
1
4. Zhou, J. Y.; Jia, Y.; Sun, G. F.; Wu, S. H. Synth.
Commun. 1997, 27, 1899–1906.
Acknowledgements
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S.K.D. is an IITG research fellow. S.H. thanks the
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India, for providing a research fellowship.
1
1
528; (b) Wu, S. H.; Huang, B. Z.; Gao, X. Synth.
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3
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697–5698; (b) Talaga, P.; Schaeffer, M.; Benezra, C.;
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21. Typical procedure for allylation of carbonyls: To 15 mL of
water, were added Co(acac) Æ2H O (0.1 mmol, 0.029 g),
2
2
SnCl Æ2H O (1.2 mmol, 0.271 g), allyl bromide (1.2 mmol,
2
2
(
e) Li, C. J.; Meng, Y.; Yi, X. H.; Ma, J. H.; Chan, T. H. J.
0.144 g) and benzaldehyde (1 mmol, 0.106 g), and the
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reaction mixture was stirred at room temperature for the

M. K. Chaudhuri et al. / Tetrahedron Letters 46 (2005) 6247–6251
6251
time given in Table 1. The reaction was monitored by TLC
and on completion of the reaction, the product was
extracted with ethyl acetate (3 · 30 mL). The combined
organic layers were washed with water several times until
5.20–5.14 (m, 2H), 4.75 (t, J = 6.6 Hz, 1H), 2.50–2.46 (m,
2H), 2.20–1.85 (br, 1H).
22. Chaudhuri, M. K.; Dehury, S. K.; Bora, U.; Dhar, S. S.;
Choudary, B. M.; Kantam, M. L. U.S. Pat. Appl. Publ.
2004, 8 pp. US 2004127690 A1 20040701, CAN 141:63885
AN 2004:534033.
23. (a) Yu, C. M.; Kim, J. M.; Shin, M. S.; Cho, D.
Tetrahedron Lett. 2003, 44, 5487–5490; (b) Kadota, I.;
Matsukawa, Y.; Yamamoto, Y. J. Chem. Soc., Chem.
Commun. 1993, 1638–1641, and references cited therein.
2 4
the aqueous layer became colourless, then dried (Na SO ),
concentrated and purified by column (silica) chromatog-
raphy (hexane/ethyl acetate, 90/10, v/v) to afford the pure
À1
compound. Yield: 0.144 g, 97%. IR (cm ) 3400, 3070,
1
1
641, 1600, 1496, 1446, 1049. H NMR (300 MHz,
CDCl ): d (ppm) 7.40–7.25 (m, 5H), 5.90–5.75 (m, 1H),
3