Zn/[bmim]PF6-mediated Markovnikov allylation of unactivated terminal alkynes

Article abstract of DOI:10.1016/j.tetlet.2005.09.147

A simple and highly regioselective method has been developed for the allylation of unactivated terminal alkynes with allyl bromide using Zn/[bmim]PF₆ as an inexpensive, readily available and recyclable reagent system. High conversions and enhan

Full text of DOI:10.1016/j.tetlet.2005.09.147

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8411–8413
Zn/[bmim]PF -mediated Markovnikov allylation of
6
I
unactivated terminal alkynes
*
J. S. Yadav, B. V. S. Reddy, P. Murali Krishna Reddy and Manoj K. Gupta
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 19April 2005; revised 12 September 2005; accepted 23 September 2005
Available online 13 October 2005
Abstract—A simple and highly regioselective method has been developed for the allylation of unactivated terminal alkynes with allyl
bromide using Zn/[bmim]PF as an inexpensive, readily available and recyclable reagent system. High conversions and enhanced
6
selectivity together with the environmentally benign nature of the Zn/[bmim]PF
alternative to established methods.
6
reagent system makes this method an attractive
ꢀ
2005 Elsevier Ltd. All rights reserved.
Metal mediated processes play a significant role in many
C–C bond forming reactions. In continuation of our
unique properties such as good solvating ability, wide
liquid range, tunable polarity, high thermal stability,
immiscibility with a number of organic solvents, negligi-
1
work on the use of metals such as zinc, indium and mag-
2
5
nesium for various transformations, we describe a Zn/
ble vapour pressure and ease of recyclability (Fig. 1).
[
bmim]PF reagent system for the allylation of unacti-
6
vated terminal alkynes with allyl bromide. Only a few
methods have been developed for the allylation of termi-
nal alkynes with allyl halides using group III metals such
as indium and gallium. The disadvantage of previous
procedures is the formation of anti-Markovnikov by-
products. Recently, Lee et al. reported the Markovnikov
allylation of unactivated alkynes with allyl bromide
using gallium metal. However, this method is also limi-
ted due to the difficulty in handling and the high cost of
gallium.
Due to stabilization of charged intermediates by ionic
liquids, they can promote unprecedented selectivities
and increase reaction rates over conventional solvents.
Because of their advantages, ionic liquids can make a
contribution to green chemistry. This is the first report
of the allylation of unactivated terminal alkynes with
allyl bromide using Zn/[bmim]PF6.
6
3
4
Treatment of terminal alkynes 1 with allyl bromide 2 in
the presence of Zn/[bmim]PF afforded the products 3 in
6
70–91% yields (Scheme 1).
The use of solvents like water, supercritical fluids and
ionic liquids, has recently received a great deal of atten-
tion in different areas of organic chemistry. Particularly,
ionic liquids are being used as Ôgreen solventsÕ with
The scope and generality of this process is illustrated
with respect to various alkynes and the results are pre-
sented in Table 1.
In all cases, the reaction proceeded smoothly at room
temperature with high selectivity. The reaction did not
+
+
N
N
PF -
6
N
N
BF4-
[
Bmim]PF6
[Bmim]BF4
Zn/[bmim]PF6
Br
R
+
R
Figure 1. Chemical structures of some ionic liquids (ILs).
r. t.
1
2
3
Keywords: Terminal alkynes; Zinc; Allyl bromide; Ionic liquids.
q
IICT Communication No. 051001.
R = aryl, alkyl
*
Corresponding author. Tel.: +91 40 27193434; fax: +91 40
7160512; e-mail: yadavpub@iict.res.in
2
Scheme 1.
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.09.147

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412
J. S. Yadav et al. / Tetrahedron Letters 46 (2005) 8411–8413
-mediated allylation of terminal alkynes with allyl bromide
Table 1. Zn/[bmim]PF
6
a
b
Entry
a
Terminal alkynes 1
Allyl bromide 2
Product 3
Time (h)
3.0
Yield (%)
88
Br
O
O
b
c
Br
Br
Br
Br
2.5
2.5
3.0
3.0
85
89
90
82
O
O
Me
Me
Ph
Cl
O
O
O
d
e
PhO
O
Cl
O
O
O
f
Br
Br
3.0
3.0
80
91
O
g
Cl
Cl
Me
Me
O
O
h
i
Br
Br
Br
2.5
3.5
4.0
90
85
82
O
O
OTBDMS
OTBDMS
j
Me
Me
O
k
Br
4.5
70
O
O
O
O
O
l
Br
Br
3.5
4.0
85
82
m
BnO
BnO
n
Br
4.5
70
a
1
All products were characterized by H NMR, IR spectroscopy and mass spectrometry.
Yield refers to the isolated pure products after column chromatography.
b
take place in the absence of either zinc or the ionic
liquid, even after long reaction times.
References and notes
1
. (a) Normant, J. F.; Alexakis, A. Synthesis 1981, 841–870;
b) Oppolzer, W. Angew. Chem., Int. Ed. Engl. 1989, 28, 38–
2; (c) Knochel, P. In Comprehensive Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford,
1991; Vol. 4, p 865; (d) Yamamoto, Y.; Asao, N. Chem.
Rev. 1993, 93, 2207–2293; (e) Knochel, P. In Comprehensive
Organometallic Chemistry II; Able, E. W., Stone, F. G. A.,
Wilkinson, G., Eds.; Pergamon Press: Oxford, 1995; Vol.
(
5
In summary, we have described a simple, convenient and
efficient protocol for the allylation of unactivated termi-
nal alkynes with allyl bromide using the Zn/[bmim]PF ,
6
7
recyclable reagent system.
Acknowledgements
1
1, p 159.
2
. (a) Yadav, J. S.; Srinivas, D.; Reddy, G. S.; Bindu, K. H.
Tetrahedron Lett. 1997, 38, 8745–8748; (b) Yadav, J. S.;
Reddy, B. V. S.; Reddy, G. S. K. K. Tetrahedron Lett. 2000,
B.V.S., M.K.G. thank CSIR, New Delhi, and P.M.R.
thanks UGC, New Delhi, for the award of fellowships.

J. S. Yadav et al. / Tetrahedron Letters 46 (2005) 8411–8413
8413
4
1, 2695–2697; (c) Kumar, H. M. S.; Anjaneyulu, S.;
the resulting product was directly charged onto a small silica
gel column and eluted with a mixture of ethyl acetate–
n-hexane (1:9) to afford pure products. The remaining
Reddy, B. V. S.; Yadav, J. S. Synlett 1999, 551–552; (d)
Yadav, J. S.; Reddy, B. V. S.; Reddy, M. M. Tetrahedron
Lett. 2000, 41, 2663–2665; (e) Yadav, J. S.; Reddy, B. V. S.;
Reddy, G. S. K. K. New J. Chem. 2000, 24, 571–573; (f)
Kumar, H. M. S.; Reddy, B. V. S.; Anjaneyulu, S.; Yadav,
J. S. Tetrahedron Lett. 1999, 40, 8305–8306.
. Ranu, B. C.; Majee, A. Chem. Commun. 1997, 1225–1226.
. Lee, P. H.; Heo, Y.; Seomoon, D.; Kim, S.; Lee, K. Chem.
Commun. 2005, 1874–1876.
6
[bmim]PF ionic liquid was washed with saturated brine
solution, (4 · 5 mL) and dried under vacuum (ꢀ5 h) for the
next run. Spectral data for products: compound 3a: Liquid,
IR (KBr): mmax: 2981, 1635, 1494, 1445, 1017, 909, 759,
À1
1
3
4
702 cm . H NMR (200 MHz, CDCl ): d 7.46–7.23 (m,
3
5H), 6.08–5.84 (m, 1H), 5.43 (s, 1H), 5.24–5.04 (m, 3H), 3.25
+
(d, J = 6.3 Hz, 2H). EI-MS: m/z (%): 144 (M , 15), 128 (20),
5
6
7
. (a) Yadav, J. S.; Reddy, B. V. S.; Eeshwaraiah, B.; Gupta,
M. K. Tetrahedron Lett. 2004, 45, 5873–5876; (b) Welton,
T. Chem. Rev. 1999, 99, 2071–2083; (c) Wasserscheid, P.;
Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772–3789.
. (a) Sheldon, R. Chem. Commun. 2001, 2399–2407; (b)
Yadav, J. S.; Reddy, B. V. S.; Baishya, G.; Harshavardhan,
S. J.; Chary, C. J.; Gupta, M. K. Tetrahedron Lett. 2005,
115 (80), 103 (10), 91 (5), 77 (8), 63 (7), 39 (5). HRMS calcd
for C11
H12: 144.0939. Found: 144.0932. Compound 3b:
Liquid, IR (KBr): mmax: 2928, 2984, 1696, 1354, 1296, 1218,
À1
1
1094, 968, 771, 670 cm . H NMR (200 MHz, CDCl ): d
3
7.38–7.21 (m, 2H), 7.02–6.90 (m, 3H), 6.42–6.38 (m, 1H),
5.14–4.99 (m, 4H), 4.40 (s, 2H), 2.90 (d, J = 6.4 Hz, 2H). EI-
+
MS: m/z (%): 174 (M , 100), 134 (30), 94 (20), 79 (20), 39
4
6, 3569–3572, and references cited therein.
. General procedure: A mixture of terminal alkyne (1 mmol)
and allyl bromide (1.2 mmol) in Zn[bmim]PF (2 mL) was
(20). HRMS calcd for C H O: 174.1044. Found: 174.1040.
1
2
14
Compound 3k: Liquid, IR (KBr): mmax 2925, 2875, 1683,
À1
1
6
1442, 1376, 1251, 996, 913 cm . H NMR (300 MHz,
stirred at 27 ꢁC for the appropriate time (Table 1). After
completion of the reaction, as indicated by TLC, the
reaction mixture was extracted with diethyl ether
CDCl ): d 5.90–5.64 (m, 2H), 5.11–4.84 (m, 3H), 4.60–4.56
3
(m, 1H), 4.16–3.76 (m, 3H), 3.50–3.34 (m, 2H), 2.90–2.70
+
(overlap, 3H), 2.01–1.48 (m, 4H). EI-MS: m/z (%): 182 (M ,
(
3 · 10 mL). The combined organic extracts were washed
5), 146 (5), 132 (5), 120 (20), 86 (100), 41 (45). HRMS calcd
for C H O (Na): 205.1204. Found: 205.1200.
with brine solution, dried, then concentrated in vacuo and
1
1
18
2