Copper(II)-catalyzed allylation of propargylic and allylic alcohols by allylsilanes: a facile synthesis of 1,5-enynes

Article abstract of DOI:10.1016/j.tetlet.2007.11.143

Propargylic alcohols undergo smooth deoxygenative allylation with allylsilanes in the presence of a solution of 10 mol % of copper(II) tetrafluoroborate in acetonitrile to afford the corresponding 1,5-enynes in good to high yields under mild and neutral c

Full text of DOI:10.1016/j.tetlet.2007.11.143

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Tetrahedron Letters 49 (2008) 614–618
Copper(II)-catalyzed allylation of propargylic and allylic alcohols
by allylsilanes: a facile synthesis of 1,5-enynes
J. S. Yadav *, B. V. Subba Reddy, T. Srinivasa Rao, K. V. Raghavendra Rao
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 3 October 2007; revised 16 November 2007; accepted 23 November 2007
Abstract
Propargylic alcohols undergo smooth deoxygenative allylation with allylsilanes in the presence of a solution of 10 mol % of copper(II)
tetrafluoroborate in acetonitrile to afford the corresponding 1,5-enynes in good to high yields under mild and neutral conditions. Scan-
dium triflate is also found to catalyze efficiently the nucleophilic substitution of propargylic alcohols with allylsilanes.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Allylation; Propargylic alcohols; Allylic alcohols; 1,5-Enynes; Cu(II) reagents
The Lewis acid catalyzed coupling of propargyl alcohols
and halides with allylsilanes is an important transforma-
tion, as it provides a direct approach to synthetically valu-
able 1,5-enynes.1–4 The Nicholas reaction has been widely
used as a powerful tool for the substitution of propargylic
alcohols.5 However, this method generally requires a stoi-
chiometric amount of Co2(CO)8 and several steps are nece-
ssary to obtain the propargylic product from propargylic
alcohols through cationic propargylic complexes.6–9 Sub-
sequently, several transition metal catalyzed propargylic
substitutions have been reported using ruthenium, rhe-
nium(V), and gold(III) catalysts.10–15 Recently, boron(III)
and bismuth(III) reagents have also been used to accom-
plish this transformation.16–19 In most cases, either a high
reaction temperature or an additive is required to enhance
the leaving ability of the hydroxyl group. Therefore, the
direct catalytic substitution of alcohols with allylsilanes
using a simple and efficient catalyst is highly desirable.
In this Letter, we report a direct and facile method for
the nucleophilic substitution of propargylic alcohols with
allylsilanes. Initially, we attempted the allylation of 1,3-
diphenyl-2-propyn-1-ol (1) with allyltrimethylsilane (2) in
the presence of 10 mol % of Cu(BF4)2. The reaction pro-
ceeded smoothly at room temperature in acetonitrile to
give 1,5-enyne 3a in 85% yield (Scheme 1).
This interesting catalytic activity of Cu(BF4)2 in acetoni-
trile provided the incentive for further study of reac-
tions with different propargylic alcohols. A wide range of
OH
10 mol% Cu(BF4)2
Si
+
Ph
Ph
CH3CN, r.t.
2
3a
1
Scheme 1. Preparation of 3a.
*
Corresponding author. Tel.: +91 40 2193535; fax: +91 40 2160512.
E-mail address: yadavpub@iict.res.in (J. S. Yadav).
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.11.143

J. S. Yadav et al. / Tetrahedron Letters 49 (2008) 614–618
615
Table 1
Cu(BF4)2-catalyzed allylation of propargyl alcohols with allylsilanes
Entry
Propargyl alcohol
Allylsilane
Producta
Time (h)
4.0
Yieldb (%)
85
OH
SiMe3
a
Ph
Ph
Ph
Ph
Ph
OH
b
c
4.0
4.5
3.5
4.0
86
78
86
85
SiMe3
SiMe3
Ph
Ph
Ph
Ph
OH
Ph
OH
OH
SiMe3
SiMe3
SiMe3
d
e
OH
MeO
MeO
MeO
f
3.5
3.5
88
86
Ph
Ph
Ph
Ph
OMe
OMe
OH
MeO
g
SiMe3
OMe
OH
OMe
SiMe3
SiMe3
h
i
5.5
3.0
3.0
78
85
84
Me
OH
Me
S
S
Ph
Ph
Ph
Ph
OH
j
SiMe3
SiMe3
S
S
OH
k
l
6.0
4.0
86
84
Cl
Cl
OH
SiMe3
SiMe3
SiMe3
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
OH
OH
m
n
4.0
4.5
88
85
Ph
(continued on next page)

616
J. S. Yadav et al. / Tetrahedron Letters 49 (2008) 614–618
Table 1 (continued)
Entry
Propargyl alcohol
Allylsilane
Producta
Time (h)
2.0
Yieldb (%)
88
OH
SiMe3
o
Ph
Ph
Me
Ph
Ph
Me
Ph
OH
Ph
SiMe3
p
3.0
86
a
All products were characterized by NMR, IR, and mass spectrometry.
Yield refers to pure products after chromatography.
b
OH
10 mol% Cu(BF4)2
CH3CN, r.t.
Si
+
Ph
Ph
3m
Scheme 2. Preparation of 3m.
OH
10 mol% Cu(BF4)2
CH3CN, r.t.
Si
+
3p
Scheme 3. Preparation of 3p.
propargylic alcohols underwent smooth allylation with
allylsilanes to afford the corresponding 1,5-enynes in high
yields (Table 1, entries b–k). In all cases, the reactions pro-
ceeded smoothly at room temperature under the influence
of 10 mol % of Cu(BF4)2. In addition, doubly activated
entries b, e, g, j, and l). It should be noted that the allyla-
tion of all substrates led exclusively to the formation of
propargylic products and no traces of allenic side products
were detected. However, in the absence of catalyst, the
reaction did not proceed even after a long reaction time.
The scope and generality of this process is illustrated in
Table 1.20
(E)-1,5-diphenyl-1-penten-4-yn-3-ol
underwent
facile
nucleophilic substitution with allylsilanes to furnish the
respective 3-allylpent-1-en-4-yne derivatives (Scheme 2,
Table 1, entries l, m, and n).
Likewise, doubly activated allylic alcohols reacted rap-
idly with allyltrimethylsilane to furnish 1,5-dienes in excel-
lent yields (Table 1, entries o–p, Scheme 3).
The method is compatible with halides, aryl alkyl ethers,
alkynes, and alkenes present in the molecule. Similarly, 2-
methylallylsilane also reacted well with propargylic alco-
hols to provide methyl substituted 1,5-enynes (Table 1,
Furthermore, alkyl substituted propargylic alcohols
failed to undergo allylation under the present reaction con-
ditions. As solvent, acetonitrile gave the best results. All
1
products were characterized by H, 13C NMR, IR, and
mass spectrometry. No additives or activators were
required for activation of the –OH group. The effects of
various copper(II) salts such as Cu(BF4)2, Cu(OTf)2, Cu(a-
cac)2, and Cu(ClO4)2 were screened for this conversion. Of
these catalysts, Cu(BF4)2 was found to be most effective in
Table 2
Sc(OTf)3-catalyzed allylation of propargyl alcohols with allylsilanes
Entry
Propargyl alcohol
Allylsilane
Producta
Time (h)
1.0
Yieldb (%)
84
OH
SiMe3
a
Ph
Ph
Ph
Ph
Ph
OH
b
1.0
80
SiMe3
Ph
Ph
Ph

J. S. Yadav et al. / Tetrahedron Letters 49 (2008) 614–618
617
Table 2 (continued)
Entry
Propargyl alcohol
Allylsilane
Producta
Time (h)
1.5
Yieldb (%)
86
OH
SiMe3
c
Ph
Ph
OH
OH
SiMe3
SiMe3
SiMe3
d
e
0.5
0.5
88
86
OH
MeO
MeO
MeO
MeO
f
0.5
0.5
1.5
92
90
84
Ph
Ph
Ph
Ph
OMe
OH
OMe
OMe
g
h
SiMe3
SiMe3
SiMe3
OMe
OH
Me
S
Me
S
OH
OH
i
j
0.25
0.25
96
95
Ph
Ph
Ph
Ph
SiMe3
SiMe3
S
S
OH
k
l
1.2
0.6
0.5
78
80
82
Cl
Cl
OH
SiMe3
SiMe3
Ph
Ph
Ph
Ph
Ph
Ph
OH
m
Ph
Ph
OH
OH
SiMe3
SiMe3
n
o
0.8
1.4
80
86
Ph
Ph
Ph
Ph
Ph
Me
Me
OH
Ph
SiMe3
p
2.0
84
Ph
Ph
a
All products were characterized by NMR, IR, and mass spectrometry.
Yield refers to pure products after chromatography.
b

618
J. S. Yadav et al. / Tetrahedron Letters 49 (2008) 614–618
11. Luzung, M. R.; Toste, F. D. J. Am. Chem. Soc. 2003, 125, 15760–
15761.
12. Sherry, B. D.; Radosevich, A. T.; Toste, F. D. J. Am. Chem. Soc.
2003, 125, 6076–6077.
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terms of conversion. A solution of 10 mol % of LiBF4 in
acetonitrile was not so effective for this reaction, albeit,
acetonitrile can form the complex CH3CN+–BF3 which
acts as the catalytic species. The use of Cu(BF4)2 makes this
procedure very mild, simple, and convenient. Alternatively,
5 mol % of scandium triflate was also found to be an
equally effective catalyst for this conversion and compara-
tive results are presented in Table 2.
15. Kuninobu, Y.; Ishii, E.; Takai, K. Angew. Chem., Int. Ed. 2007, 46,
3296–3299.
In summary, a solution of 10 mol % of Cu(II) tetrafluo-
roborate in acetonitrile was shown to be a highly efficient
and convenient catalytic medium for the preparation of
1,5-enynes via the allylation of propargylic and allylic alco-
hols with allylsilanes. In addition to its simplicity and mild
reaction conditions, this method provides good yields of
1,5-enynes with high selectivity, which makes it a useful
and attractive process.
16. Schwier, T.; Rubin, M.; Gevorgyan, V. Org. Lett. 2004, 6, 1999–2001.
17. Zhan, Z. P.; Yang, W. Z.; Yang, R. F.; Yu, J. L.; Li, J. P.; Liu, H. J.
Chem. Commun. 2006, 3352–3354.
18. Zhan, Z. P.; Yu, J. L.; Liu, H. J.; Cui, Y. Y.; Yang, R. F.; Yang, W.
Z.; Li, J. P. J. Org. Chem. 2006, 71, 8298–8301.
19. Kabalka, G. W.; Yao, M. L.; Borella, S. J. Am. Chem. Soc. 2006, 128,
11320–11321.
20. General procedure: To a stirred solution of the propargylic alcohol
(1 mmol) and Cu(BF4)2 (10 mol %) or 5 mol % Sc(OTf)3 in aceto-
nitrile (10 mL), allyltrimethylsilane (1.5 mmol) was added slowly
dropwise at 0 °C and the resulting mixture allowed to stir at room
Acknowledgment
temperature for the appropriate time (Tables
1 and 2). After
completion of the reaction, as indicated by TLC, the reaction mixture
was quenched with water and extracted with dichloromethane
(2 Â 10 mL). The combined organic extracts were dried over anhy-
drous Na2SO4. Removal of the solvent followed by purification on
silica gel using ethyl acetate/n-hexane (1:9) as eluent afforded pure 1,5-
enyne. Spectral data for selected products: Entry d (Table 1): IR
(KBr): m 3055, 2957, 2930, 2865, 1637, 1601, 1506, 1436, 1368, 1323,
T.S.R. and K.V.R. thank CSIR, New Delhi, for the
awards of fellowships.
References and notes
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