Boron trifluoride-diethyl etherate complex mediated allylation of Baylis-Hillman adducts: A facile synthesis of 1,5-dienes

Article abstract of DOI:10.1055/s-2008-1067049

Morita-Baylis-Hillman adducts undergo smooth allylic nucleophilic substitution (S_N2′) with allyltrimethylsilane in the presence of BF₃·OEt₂ under mild reaction conditions to afford 1,5-diene derivatives in good yields with high selectivity.

Full text of DOI:10.1055/s-2008-1067049

PAPER
1943
Boron Trifluoride–Diethyl Etherate Complex Mediated Allylation of
Baylis–Hillman Adducts: A Facile Synthesis of 1,5-Dienes
Synthesis
h
of 1,5-Diene
i
s
llu S. Yadav,* Basi V. Subba Reddy, Satadru S. Mandal, Ashutosh Pratap Singh, Ashok K. Basak
Division of Organic Chemistry I, Indian Institute of Chemical Technology, Hyderabad 500007, India
Fax +91(40)27160512; E-mail: yadavpub@iict.res.in
Received 27 February 2008
us.11 However, Baylis–Hillman acetates derived from
acrylonitrile failed to undergo allylation under these con-
ditions. Furthermore, there have been no reports on the
direct allylation of Baylis–Hillman adducts with allyl-
trimethylsilane using the Lewis acid catalyst.
Abstract: Morita–Baylis–Hillman adducts undergo smooth allylic
nucleophilic substitution (SN2¢) with allyltrimethylsilane in the
presence of BF3·OEt2 under mild reaction conditions to afford 1,5-
diene derivatives in good yields with high selectivity.
Key words: Morita–Baylis–Hillman adducts, boron reagents, ally-
lation, 1,5-dienes
In continuation of our research on Baylis–Hillman ad-
ducts,12 we report herein a direct, one-pot allyaltion of
Baylis–Hillman adducts with allyltrimethylsilane using
BF3·OEt2 as a novel promoter. First, we examined the re-
action of methyl 2-[hydroxyl(phenyl)methyl] acrylate
(1a) with allyltrimethylsilane (2) in the presence of 1.2
equivalents of BF3·OEt2 in dichloromethane. The reaction
proceeded smoothly at 0 °C and the desired product,
methyl 2-benzylidenehex-5-enoate (3a) was obtained in
84% yield (Scheme 1).
The coupling of activated vinylic systems with aldehydes
in the presence of 1,4-diazabicyclo[2,2,2]octane (DAB-
CO), known as the Morita–Baylis–Hillman reaction, is
widely used for the direct synthesis of a-hydroxyalkyl or
arylvinyl systems.1 These adducts are well-known carbon
electrophiles capable of reacting with various nucleo-
philes and their ability to undergo nucleophilic substitu-
tion reactions contributes largely to their synthetic value.2
The versatility of the functionality in such Baylis–Hill-
man adducts makes them valuable synthetic intermediates
for the synthesis of a variety of heterocycles3,4 such as
quinolines, pyrimidones, isoxazolines, pyrazolones, pyr-
rolidines, indolizines, azetidinones, diazacyclophanes,
and chromanones, as well as biologically active natural
products including a-alkylidene-b-lactams, a-methylene-
g-butyrolactones, mikanecic acids, frontalin, trimethop-
rim, sarkomycin, ilmofosine nuciferol, and many oth-
ers.2,5,6
The product 3a was obtained as a mixture of E- and Z-iso-
mers in 9:1 ratio favoring the E-isomer. The ratio of E-
and Z-isomers was determined on the basis of integration
ratios of isomeric olefinic proton and allylic methylene
1
protons in the H NMR spectrum of the product. The E-
stereochemistry of the product was assigned on the basis
of the chemical shift values of vinyl and allylic protons in
the 1H NMR spectrum of the product and also by the com-
parison of the spectral data with authentic samples.11 In
the 13C NMR spectra of trisubstituted olefins, the allylic
carbon cis- to the aryl group appears upfield while the
same carbon trans to aryl group appears downfield.12
The stereoselective addition of allylmetal reagents to al-
dehydes, imines and epoxides is one of the most important
carbon–carbon bond-forming reactions in organic synthe-
sis.7 Similarly, Lewis acid catalyzed carbon–carbon bond-
forming reactions are of great significance in organic syn-
thesis because of their high reactivity, selectivity, and
mild reaction conditions.8 Despite a large number of
methods reported for the allylation of aldehydes with al-
lyltrimethylsilane,9,10 only the zinc-mediated allylation of
Baylis–Hillman acetates has recently been reported by
These results encouraged us to examine the reactivity of
various substituted Baylis–Hillman adducts. The scope of
the BF3·OEt2 promoted allylic substitution was investigat-
ed with respect to various Baylis–Hillman adducts and the
results are presented in Table 1. Interestingly, adducts de-
rived from both aliphatic and aromatic aldehydes partici-
pated well in this reaction. In all cases, the reactions were
clean and stereoselective affording the trisubstituted ole-
fins in good yields. In the case of crotonaldehyde, the cor-
OH
CO2Me
BF3⋅OEt2
CO2Me
Si
+
CH2Cl2, 0 °C
1a
2
3a (E-isomer)
Scheme 1
SYNTHESIS 2008, No. 12, pp 1943–1947
x
x.
x
x
.
2
0
0
8
Advanced online publication: 29.04.2008
DOI: 10.1055/s-2008-1067049; Art ID: Z05408SS
© Georg Thieme Verlag Stuttgart · New York

1944
J. S. Yadav et al.
PAPER
Table 1 BF3·OEt2-Mediated Allylation of Baylis–Hillman Adducts with Allyltrimethylsilane
Entry
Substrate 1
Product 3/4a
Ratio E/Z
Time (min)
20
Yield (%)b
84
OH
CO2Me
CO2Me
OH
a
90:10
CO2Me
CO2Me
b
85:5
20
82
MeO
MeO
OH
CO2Me
c
d
e
95:5
25
30
20
78
76
79
CO2Me
OH
CO2Me
70:30
75:25
CO2Me
CO2Et
HO
CO2Et
O
O
OH
CO2Me
CO2Me
f
70:30
15
76
OPh
OPh
OH
CO2Et
CO2Et
g
h
i
70:30
75:25
10:90
20:80
15:85
25
20
25
30
20
82
83
79
78
80
Me
Me
Cl
OH
OH
CO2Me
CO2Me
Cl
CN
CN
Me
Me
OH
CN
CN
j
CN
OH
MeO
MeO
k
CN
OMe
OMe
OH
CN
l
15:85
10:90
15
20
81
78
CN
MeO
S
MeO
OH
m
CN
S
CN
a All products were characterized by 1H NMR and IR spectroscopy, and mass spectrometry.
b Yield refers to pure products after column chromatography.
Synthesis 2008, No. 12, 1943–1947 © Thieme Stuttgart · New York

PAPER
Synthesis of 1,5-Dienes
1945
OH
CN
BF3⋅OEt2
+
Si
CN
CH2Cl2, 0 °C
Me
Me
1i
2
4i (Z-isomer)
Scheme 2
manner. The resulting mixture was stirred at 0 °C for the appropri-
ate time (see Table 1) as required for completion. After complete
conversion as indicated by TLC, the mixture was quenched with sat.
aq NaHCO3 (10 mL) and extracted with CH2Cl2 (2 × 10 mL). The
combined organic layers were washed with brine (10 mL), dried
(Na2SO4), and evaporated under reduced pressure. The crude prod-
uct was purified by silica gel column chromatography (100–200
mesh) using a gradient mixture of hexane–EtOAc (9:1) to afford the
pure allyl derivatives 3 and 4 (Table 1).
responding product, methyl (2E,4E)-2-(but-3-enyl)hexa-
2,4-dienoate was obtained as a major isomer (Table 1, en-
try d). Next, we examined the reactivity of Baylis–Hill-
man adducts derived from acrylonitrile. Thus, treatment
of 2-[hydroxyl(p-tolyl)methyl]acrylonitrile (1i) with al-
lyltrimethylsilane in presence of BF3·OEt2 gave (Z)-2-(4-
methylbenzylidene)pent-4-enenitrile (4i) in 79% yield
(Table 1, entry i, Scheme 2).
In the cases of Baylis–Hillman adducts derived from acry-
lonitrile, the corresponding products were obtained with
high Z-selectivity. The substrate derived from cinnamal-
dehyde also gave the respective (2Z,4E)-2-allyl-5-phenyl-
penta-2,4-dienenitrile in 78% yield (entry j, Table 1).
Adducts derived from heteroaromatic aldehydes were
also equally effective for this transformation (entries e and
m, Table 1). The reaction conditions were compatible
with various functionalities such as olefins, halides, aryl
methyl ethers, and esters (Table 1). The products were
characterized by 1H NMR, IR, and mass spectroscopy. In
the absence of BF3·OEt2, no nucleophilic substitution oc-
curred even after long reaction times (9–13 h) under re-
fluxing conditions. Various Lewis acids such as InCl3,
InBr3, FeCl3, and CeCl3·7H2O were screened and none
gave the desired product. Furthermore, metal triflates
such as Sc(OTf)3, Yb(OTf)3, and In(OTf)3 also failed to
give the product under similar conditions. As solvent,
dichloromethane gave the best results.
3a
Light yellow oil.
IR (neat): 3071, 2943, 2848, 1692, 1596, 1513, 1431, 1301, 1258,
1118, 1022, 917, 819 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.23–2.34 (m, 2 H), 2.55–2.65 (m,
2 H), 3.80 (s, 3 H), 4.90–5.09 (m, 2 H), 5.74–5.89 (m, 1 H), 7.14–
7.33 (m, 5 H), 7.64 (s, 1 H).
ESI-MS: m/z = 239 (M + Na).
3b
Light yellow oil.
IR (neat): 3074, 2950, 2841, 1707, 1605, 1510, 1437, 1303, 1253,
1128, 1032, 912, 828 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.23–2.34 (m, 2 H), 2.59–2.67 (m,
2 H), 3.78 (s, 3 H), 3.81 (s, 3 H), 4.93–5.09 (m, 2 H), 5.75–5.92 (m,
1 H), 6.87 (d, J = 8.8 Hz, 2 H), 7.30 (d, J = 8.8 Hz, 2 H), 7.58 (s, 1
H).
13C NMR (75 MHz, CDCl3): d = 26.8, 32.0, 51.8, 55.2, 114.0, 114.8,
128.0, 129.6, 133.7, 137.8, 159.8, 169.0.
ESI-MS: m/z = 246 (M+).
In conclusion, we have described an efficient protocol for
the direct allylation of Baylis–Hillman adducts with allyl-
trimethylsilane via SN2¢ type allylic substitution using
BF3·OEt2. The method offers several advantages such as
operational simplicity, mild reaction conditions, cleaner
reaction profiles, simple workup procedure, and the use of
inexpensive and readily available reagents, which makes
it a useful and attractive strategy for the preparation of
1,5-dienes in a single step.
3c
Light brown liquid.
IR (neat): 2925, 2854, 1716, 1638, 1458, 1260, 1089, 1021, 913,
805 cm–1.
1H NMR (300 MHz, CDCl3): d = 0.91 (t, J = 6.7 Hz, 3 H), 1.24–
1.50 (m, 6 H), 2.09–2.22 (m, 3 H), 3.73 (s, 3 H), 4.92–5.04 (m, 2 H),
5.68–5.90 (m, 1 H), 6.73 (t, J = 7.5 Hz, 1 H).
13C NMR (75 MHz, CDCl3): d = 13.8, 22.4, 28.4, 28.5, 29.0, 31.5,
32.3, 51.4, 114.7, 131.3, 138.0, 143.4, 168.2.
Melting points were recorded on a Büchi R-535 apparatus and are
uncorrected. IR spectra were recorded on a Perkin-Elmer FT-IR
240-c spectrophotometer using KBr discs. 1H NMR and 13C spectra
were recorded on Gemini-200 spectrometer (200 MHz) in CDCl3
using TMS as the internal standard. Mass spectra were recorded on
a Finnigan MAT 1020 mass spectrometer operating at 70 eV. Col-
umn chromatography was performed using Merck silica gel (60–
120 mesh).
ESI-MS: m/z = 233 (M + Na).
3d
Light yellow liquid.
IR (neat): 2926, 2859, 1706, 1634, 1490, 1354, 1244, 1097, 1080,
937, 725 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.16 (s, 3 H), 2.12–2.25 (m, 4 H),
3.73 (s, 3 H), 5.05–5.19 (m, 3 H), 5.76–5.94 (m, 2 H), 7.34 (d,
J = 6.2 Hz, 2 H).
13C NMR (75 MHz, CDCl3): d = 19.1, 26.6, 34.2, 51.8, 115.2, 127.7,
129.1, 138.2, 138.6, 140.6, 168.2.
BF3·OEt2-Mediated Allylation of Baylis–Hillman Adducts with
Allyltrimethylsilane; General Procedure
To a stirred solution of the respective Baylis–Hillman adduct 1 (1
mmol) and allyl trimethylsilane (2; 1.5 mmol) in anhyd CH2Cl2 (5
mL) at 0 °C was added slowly BF3·OEt2 (1.2 mmol) in a dropwise
ESI-MS: m/z = 180 (M+).
Synthesis 2008, No. 12, 1943–1947 © Thieme Stuttgart · New York

1946
3e
J. S. Yadav et al.
PAPER
13C NMR (75 MHz, CDCl3): d = 21.4, 32.3, 35.6, 109.3, 116.3,
Light brown liquid.
117.5, 128.5, 129.4, 130.9, 136.1, 140.3, 143.8.
IR (neat): 3076, 2948, 2851, 1709, 1634, 1551, 1434, 1367, 1261,
1128, 1086, 915, 746 cm–1.
ESI-MS: m/z = 197 (M+).
4k
1H NMR (300 MHz, CDCl3): d = 1.34 (t, J = 6.8 Hz, 3 H), 2.20–
2.30 (m, 2 H), 2.76–2.82 (m, 2 H), 4.24 (q, J = 6.8 Hz, 2 H), 4.92–
5.08 (m, 2 H), 5.79–5.93 (m, 1 H), 6.46 (dd, J = 1.5, 3.0 Hz, 1 H),
6.57 (d, J = 3.7 Hz, 1 H), 7.36 (s, 1 H), 7.50 (d, J = 1.5 Hz, 1 H).
Light yellow oil.
IR (neat): 3430, 3067, 3029, 2924, 2854, 2211, 1726, 1639, 1448,
1216, 1074, 972, 755 cm–1.
13C NMR (75 MHz, CDCl3): d = 15.6, 26.8, 32.9, 60.6, 111.1, 109.5,
1H NMR (300 MHz, CDCl3): d = 2.30–2.45 (m, 4 H), 4.95–5.17 (m,
2 H), 5.60–5.86 (m, 1 H), 6.75 (d, J = 11.3 Hz, 1 H), 7.10 (d,
J = 15.8 Hz, 1 H), 7.23–7.37 (m, 4 H), 7.45 (dd, J = 1.5, 8.3 Hz, 2
H).
13C NMR (75 MHz, CDCl3): d = 32.2, 33.7, 112.5, 116.3, 116.5,
124.5, 127.2, 128.8, 129.1, 136.0, 139.0, 140.3, 144.4.
114.7, 116.9, 125.7, 138.0, 143.8, 155.4, 166.1.
ESI-MS: m/z = 243 (M + Na).
HRMS: m/z calcd for C15H15O + Na: 243.1021; found: 243.1024.
3f
Light yellow oil.
ESI-MS: m/z = 209 (M+).
IR (neat): 3075, 2939, 2842, 1701, 1607, 1518, 1437, 1300, 1259,
1109, 1018, 915, 812 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.13–2.32 (m, 2 H), 2.42–2.64 (m,
2 H), 3.78 (s, 3 H), 4.84–5.05 (m, 2 H), 5.55–5.82 (m, 1 H), 6.81–
7.08 (m, 6 H), 7.20–7.38 (m, 3 H), 7.58 (s, 1 H).
4l
Light yellow liquid.
IR (neat): 3448, 2927, 2842, 2205, 1604, 1511, 1257, 1179, 1030,
913, 829 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.33–2.50 (m, 4 H), 3.82 (s, 3 H),
5.00–5.15 (m, 2 H), 5.71–5.87 (m, 1 H), 6.79 (s, 1 H), 6.88 (d,
J = 7.5 Hz, 2 H), 7.66 (d, J = 7.5 Hz, 2 H).
13C NMR (75 MHz, CDCl3): d = 32.3, 35.4, 55.2, 107.5, 114.0,
116.2, 119.0, 126.3, 130.1, 136.1, 143.2, 160.7.
ESI-MS. m/z = 331 (M + Na).
3g
Light brown oil.
IR (neat): 3069, 2947, 2839, 1696, 1603, 1515, 1428, 1298, 1256,
1110, 1029, 907, 809 cm–1.
ESI-MS: m/z = 236 (M + Na).
1H NMR (300 MHz, CDCl3): d = 1.22 (s, 3 H), 2.30 (s, 3 H), 2.42–
2.54 (m, 2 H), 2.55–2.65 (m, 2 H), 4.89–5.08 (m, 2 H), 5.73–5.90
(m, 1 H), 7.15 (d, J = 7.5 Hz, 2 H), 7.24 (d, J = 7.5 Hz, 2 H), 7.60
(s, 1 H).
HRMS: m/z calcd for C14H15NO + Na: 236.1051; found: 236.1058.
4m
Light brown liquid.
ESI-MS: m/z = 267 (M + Na).
IR (neat): 3442, 2936, 2830, 2195, 1624, 1513, 1246, 1159, 1027,
908, 819 cm–1.
3h
1H NMR (300 MHz, CDCl3): d = 2.36–2.48 (m, 4 H), 4.94–5.17 (m,
2 H), 5.64–5.91 (m, 1 H), 6.98 (s, 1 H), 7.06 (dd, J = 3.6, 5.1 Hz, 1
H), 7.41 (d, J = 5.1 Hz, 1 H), 7.47 (d, J = 3.6 Hz, 1 H).
13C NMR (75 MHz, CDCl3): d = 30.4, 32.8, 112.0, 113.8, 114.7,
128.0, 128.1, 128.4, 134.8, 134.9, 142.6;
Light yellow oil.
IR (neat): 3078, 2923, 2828, 1699, 1586, 1523, 1437, 1311, 1268,
1098, 1012, 913, 811 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.13–2.29 (m, 2 H), 2.37–2.59 (m,
2 H), 3.74 (s, 3 H), 4.84–5.06 (m, 2 H), 5.45–5.85 (m, 1 H), 7.06 (d,
J = 8.0 Hz, 2 H), 7.25 (d, J = 8.0 Hz, 2 H), 7.52 (s, 1 H).
ESI-MS: m/z = 212 (M + Na).
HRMS: m/z calcd for C11H11NS + Na: 212.0509; found: 212.0517.
ESI-MS: m/z = 273 (M + Na).
4i
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Light brown oil.
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760 cm–1.
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5.02–5.19 (m, 2 H), 5.73–5.91 (m, 1 H), 6.81 (d, J = 9.02 Hz, 1 H),
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4j
Light brown liquid.
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759 cm–1.
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5.04–5.18 (m, 2 H), 5.75–5.90 (m, 1 H), 6.87 (s, 1 H), 7.21 (d,
J = 7.5 Hz, 2 H), 7.62 (d, J = 7.5 Hz, 2 H).
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Synthesis 2008, No. 12, 1943–1947 © Thieme Stuttgart · New York