Liquid-phase synthesis of ethyl (Z)-2-(thiocyanatomethyl)alk-2-enoates from PEG-bound α-phenylselenopropionate

Article abstract of DOI:10.3184/030823407X234554

Aldolisation of novel PEG-bound α-phenylselenopropionate with aldehydes, followed by oxidation-elimination with 30% hydrogen peroxide and then treatment with potassium thiocyanate formed PEG-bound 2-(thiocyanatomethyl)alk- 2-enoates. Subsequent cleavage f

Full text of DOI:10.3184/030823407X234554

442 RESEARCH PAPER
AUGUST, 442–444
JOURNAL OF CHEMICAL RESEARCH 2007
Liquid-phase synthesis of ethyl (Zꢀ)-2-(thiocyanatomethyl)alk-2-enoates
from PEG-bound a-phenylselenopropionate
Qiao-Sheng Hua,b, La-Mei Yub, Wu-Kang Suna, Shou-Ri Shenga,b* and Xiao-Ling Liua
aCollege of Chemistry and Life Science, Gannan Normal University, Ganzhou, 341000, P. R. China
bCollege of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, 330027, P. R. China
Aldolisation of novel PEG-bound a-phenylselenopropionate with aldehydes, followed by oxidation–elimination with
30% hydrogen peroxide and then treatment with potassium thiocyanate formed PEG-bound 2-(thiocyanatomethyl)alk-
2-enoates. Subsequent cleavage from the PEG efficiently afforded ethyl (Z)-2-(thiocyanatomethyl)alk-2-enoates in
good yields and high purities.
Keywords: liquid-phase organic synthesis, PEG-supported a-selenopropionate, ethyl (Z)-2-(thiocyanatomethyl)alk-2-enoates
Recently, much attention has been focused on liquid-phase
organic synthesis (LPOS) because it combines the advantages
of the classical homogeneous solution methodology (high
reactivity and simple analytical procedures) with those
of solid-phase organic synthesis (SPOS) (easy isolation
and purification of the final products and high stability of
the system polymer-supported molecule).1 Polyethylene
glycol (PEG), as a inexpensive polymer, is an ideal support
for liquid-phase combinatorial synthesis since it can be
functionalised with different spacers or reactive groups and
are also commercially available, inexpensive, non-toxic,
highly resistant to drastic physical and chemical conditions,
and soluble in a wide variety of solvents such as CH2Cl2,
THF, CH3OH and H2O at room temperature, and can be
precipitated from a solution by addition of diethyl ether,
tert-butyl methyl ether, propan-2-ol or hexane.2 Furthermore,
the PEG-bound intermediate products can be adequately
characterised by using routine analytical techniques like TLC,
into organic substrates under extremely mild conditions.8
For example, the phenylseleno group is readily converted to
a leaving group giving access to carbon–carbon double bond
via oxidation followed by β-elimination. Recently, several
research groups9 and our group10 have developed selenium-
based approaches for solid-phase synthesis with a combined
advantage of decrease volatility and simplification of product
work-up. However, to our knowledge, organic selenium
compounds attached to soluble polymer supports have not
been reported. As part of an ongoing research program focused
on the use of organoselenium reagents in organic synthesis,
we report here a new method for the synthesis of ethyl
(Z)-2-(thiocyanatomethyl)alk-2-enoates based on PEG-
bound a-phenylselenopropionate (Scheme 1).
As shown in Scheme 1, esterification of commercially
available difunctional PEG (MW = 4,000) with a-phenylseleno-
propanoic acid in the presence of dicyclohexylcarbodiimide
(DCC) and 4-dimethylaminopyridine (DMAP) in anhydrous
CH2Cl2 at room temperature for 24 h readily gave rise to the
corresponding PEG bound a-phenylselenopropionate ester
2. The conversion of terminal hydroxyl groups on PEG was
determined by 1H NMR analysis to be quantitative. The PEG-
bound ester 2 showed complete disappearance of the hydroxyl
OH stretching vibration and the appearance of a strong C=O
stretching vibration at 1728 cm-1. The PEG-bound ester 2
can be stored at room temperature for long time without
diminution of capacity or the liberation of disagreeable odors.
As illustrated, reaction of the PEG-bound ester 2 with TiCl4
Followed by i-Pr2NEt and NMP in CH2Cl2 at 0°C generate the
titanium enolate (following Crimmins’ procedure) 11 to which
are added different aldehydes furnished the corresponding
PEG-bound intermediate 3 in excellent yields. Without further
isolation of 3, we carried out the next step of oxidation–
elimination reaction directly with 30% hydrogen peroxide at
1
IR and H NMR.3 Thiocyanates, as useful substrates, can be
easily converted into various sulfur containing molecules4
such as thiols, thiophenoils, and disulfides which serve
as important synthetic intermediates in agricultural and
pharmaceutical chemistry.5 Although some synthetic methods
have been developed for preparation thiocyanates,3b,6 in view
of their significance in organic synthesis, it is still necessary to
develop a more efficient alternative method with operational
simplicity.
The Baylis–Hillman reaction is well known as a powerful
carbon–carbon bond-forming methods in organic synthesis.7
Nucleophilic displacement of Baylis–Hillman acetates is
one of the most straightforward reactions for stereoselective
synthesis of trisubstituted alkenes in organic chemistry.
On the other hand, organoselenium reagents are commonly
used as a powerful tool for introducing new functional groups
TiCl4, i-Pr2NEt
NMP, CH2Cl2
O
O
OH
DCC, DMAP, CH2Cl2
SePh
CH3
OH
O
O
R
PhSeCHMeCOOH
RCHO
H3C SePh
dihydroxy-PEG
MW = 4000
3
1
2
O
H
O
H
O
OH
H2O2
rt
EtONa
KSCN
DMF
O
R
EtO
R
O
R
EtOH, rt
SCN
SCN
Scheme 1
* Correspondent. E-mail: shengsr@jxnu.edu.cn
PAPER: 07/4681

JOURNAL OF CHEMICAL RESEARCH 2007 443
Preparation of PEG-bound a-phenylselenopropionate 2
0°C and then at room temperature to obtain the corresponding
PEG-bound Baylis–Hillman products 4, which exhibited a
broad hydroxyl absorption in 3300–3400 cm-1 region, an
intense carbonyl absorption at 1709–1722 cm-1, C=C olefin
absorption at 1625–1630 cm-1 in its FT–IR spectra and was
found to have lost all its selenium by elemental analysis,
indicating the oxidation–elimination was complete.
To a stirred solution of PEG (5.0 g, 2.5 mmol) in CH2Cl2 (20 ml)
were added a-phenylselenopropanoic acid (1.72 g, 7.5 mmol), DCC
(2.06 g, 10 mmol) and DMAP (0.305 g, 2.5 mmol). After the mixture
was stirred at room temperature for 24 h, the precipitate was removed
by filtration and the polymer was precipitated by addition of diethyl
ether (200 ml) to the filtrate. For completion of the precipitation, the
suspension was left at 0°C for another 30 min. The precipitate was
collected and washed with diethyl ether (3 × 30 ml of each), and dried
in vacuo to afford 5.15 g (98%) of the PEG bound ester 2 as a pale
yellow solid: 1H NMR (CDCl3) δ: 7.61–7.59 (m, 2H), 7.31–7.29 (m,
3H), 4.18 (t, J = 4.98 Hz, 2H, PEG-OCH2CH2OCO), 4.08 (q, J = 6.8
Hz, 1H), 3.46–3.81 (m, PEG CH2), 1.54 (d, J = 6.8 Hz, 3H); IR (KBr)
ν: 2886, 1728, 1644, 1455, 1350, 1252, 1146, 950, 849 cm-1.
Then the reaction conditions of PEG-bound B-H adducts
with potassium thiocyanate were firstly optimised.
4
When dichloromethane, tetrahydrofuran, and acetonitrile
were used as solvent, the reaction proceeded slowly
and gave a poor yield; fairly good yield of PEG-bound
2-(thiocyanatomethyl)alk-2-enoates 5 showing characteristic
absorption at 2120–2250 cm-1 (CN) in FT–IR spectra was
obtained in anhydrous N,N-dimethylformamide (DMF) while
the best result was found in DMF at 80°C for 2–3 h with
addition of ammonium hydrogen carbonate to the reaction
mixture. After precipitation by addition of diethyl ether to the
mixture and separated by simple filtration, the PEG-bound
products 5 were cleaved with 0.1 moll-1 EtONa/EtOH solution
at room temperature for approximately 8 h to afford target
compounds 6 in good yields and purities (Table 1).
Preparation of ethyl (Z)-2-(thiocyanatomethyl)alk-2-enoates (6a–6i);
general procedure
To a stirred solution of PEG-bound ester 2 (1.0 mmol) in CH2Cl2
(10 ml) at 0°C, was added TiCl4 (0.12 ml, 1.05 mmol) and the mixture
was stirred for 20 min. i-Pr2NEt (0.19 ml, 1.1 mmol) was added
and the mixture was stirred for 40 min before adding NMP (0.1 ml).
The mixture was stirred for 10 min and the appropriate aldehyde
(1.2 mmol) was added. After 30 min, NH4Cl was added and the
reaction mixture was warmed to 0°C and 30% aqueous hydrogen
peroxide (1.0 ml, 11.6 mmol) were added. After 30 min, the ice bath
was removed and the mixture was stirred at room temperature for
1–2 h. After completion of the reaction, the reaction mixture
was cooled and the diethyl ether (100 ml) was added to allow the
precipitation of PEG-bound B-H adducts 4, which were collected
by filtration and washed with diethyl ether (3 × 30 ml of each).
To a solution of PEG-bound B-H adducts 4 (1.0 mmol) in anhydrous
DMF (10 ml) was added KSCN (194 mg, 2.0 mmol) and NH4HCO3
(79 mg, 1.0 mmol) and the reaction mixture was stirred at 80°C
for 2–3 h. Then the PEG-bound intermediate 5 was precipitated by
addition of diethyl ether (100 ml) to the mixture. For completion
of the precipitation, the suspension was left at 0°C for another
30 min. The white precipitate was collected, washed several times
with diethyl ether (2 × 10 ml) and dried in vacuo. The PEG-bound
intermediate 5 was treated with 0.1 mol/l EtONa in EtOH (10 ml),
the obtained mixture was stirred at room temperature for 8 h, then
Et2O (60 ml) was added to the mixture and the organic phase was
separated. The organic extracts were dried over anhydrous Na2SO4
and concentrated to afford crude products 6a–6i, which were further
purified by passing the crude product through a pad of silica gel (ethyl
acetate–hexane as the eluent, 2:8), if necessary.
As seen from Table 1, the present method was effective
for substrates possessing either aryl (entries 1–7) or alkyl
substituents (entries 8–9). Besides good yields, the present
process also exhibited high stereoselectivity. The exclusive
(Z)-stereoselectivity of target products 6 was assigned by
1
comparing the chemical shifts in H NMR with reported
relevant values of trisubstituted alkenes12 and no E isomers
were observed from the spectra. In some cases, a trace amount
of the PEG residue might contaminate the final products 6,
which could be easily purified by passing the crude product
through a pad of silica gel (ethyl acetate–hexane as the
eluent, 2:8).
In summary, a novel and efficient procedure for the liquid-
phase synthesis of ethyl (Z)-2-(thiocyanatomethyl)alk-
2-enoates in good yields and purities using PEG-bound
a-phenylselenopropionate reagent with advantages of
decrease volatility and simplification of product work-up has
been developed.
Ethyl (Z)-3-Phenyl-2-(thiocyanatomethyl)acrylate (6a): Colourless
oil (Lit.6d colourless oil); 1H NMR: δ = 7.96 (s, 1H), 7.50–7.46
(m, 2H), 7.39–7.36 (m, 3H), 4.25 (q, J = 6.8 Hz, 2H), 4.06 (s, 2H),
1.38 (t, J = 6.8 Hz, 3H); IR (film): ν = 2985, 2116, 1708, 1630, 1495,
1369, 1245, 1065, 1028, 910 cm-1.
Experimental
All reactions were conducted under a nitrogen atmosphere. NMP and
DMF were refluxed with calcium hydride and distilled under reduced
pressure prior to use. Dichloromethane was dried by distillation
from phosphorous pentoxide prior to use. a-Phenylselenopropanoic
acid13 was prepared according to the literature procedures. PEG
4000 and other reagents were purchased from commercial sources.
Melting points were determined on X4 melting point apparatus and
are uncorrected. 1H NMR spectra were recorded on a Bruker Avance
(400 MHz) spectrometer. FT–IR spectra were taken on a Perkin-
Elmer SP One FT–IR spectrophotometer. Mass spectra (EI, 70eV)
were recorded on a HP5989B mass spectrometer. Microanalyses
were performed with a PE 2400 elemental analyser. HPLC analysis
was carried out on an Agilent 1100 (column, ODS 5 μm 250 × 4
mm, gradient elution with CH3CN/H2O, UV absorption detector at
254 nm).
Ethyl
(Z)-3-(4-Methoxyphenyl)-2-(thiocyanatomethyl)acrylate
1
(6b): Colourless oil; H NMR: δ = 7.78 (d, J = 8.8 Hz, 2H), 7.70
(s, 1H), 7.03 (d, J = 8.8 Hz, 2H), 4.25 (q, J = 6.8 Hz, 2H), 4.04 (s, 2H),
3.84 (s, 3H), 1.41 (t, J = 6.8 Hz, 3H); 13C NMR (CDCl3, 75 MHz):
δ = 166.6, 140.8, 130.3, 129.3, 124.6, 116.2, 114.5, 113.8, 60.1, 55.2,
45.5, 14.1; IR (film): ν = 2986, 2124, 1710, 1630, 1498, 1375, 1242,
1068, 1030, 910, 824 cm-1; EIMS: m/z (%) = 277 (M+); Anal. Calcd
for C14H15NO3S: C, 60.63; H, 5.45; N, 5.05. Found: C, 60.70; H,
5.52; N, 5.12.
Ethyl (Z)-3-(4-Fluorophenyl)-2-(thiocyanatomethyl)acrylate (6c):
Colourless oil (Lit.6d colourless oil); 1H NMR: δ = 7.74 (s, 1H),
7.53-7.40 (m, 2H), 7.04 (t, J = 8.1 Hz, 2H), 4.27 (q, J = 7.1 Hz, 2H),
3.66 (s, 2H), 1.38 (t, J = 7.1 Hz, 3H); IR (film): ν = 2985, 2120, 1712,
1632, 1501, 1370, 1244, 1065, 1033, 914, 822, 765 cm-1.
Table 1 The yields and purities of ethyl (Z)-2-thiocyanatomethylalk-2-enoates 6
Entry
R
Product 6
Yield/%a
Purity/%b
1
2
3
4
5
6
7
8
9
C6H5
4-CH3OC6H4
4-FC6H4
2-BrC6H4
4-NCC6H4
4-NO2C6H4
2-furyl
(E)-CH3CH=CH
CH3CH2CH2
6a
6b
6c
6d
6e
6f
6g
6h
6i
85
86
86
83
80
80
82
85
87
94
94
95
92
90
91
93
95
96
aIsolated yield based on loading of original HO-PEG-OH. bDetermined by HPLC analysis of the crude products before purification
PAPER: 07/4681

444 JOURNAL OF CHEMICAL RESEARCH 2007
Ethyl
(Z)-3-(2-Bromophenyl)-2-(thiocyanatomethyl)acrylate
References
1
(6d): White solid; m.p. 80–82°C; H NMR: δ = 7.73 (s, 1H), 7.65
(d, J = 7.9 Hz, 1H), 7.40–7.24 (m, 3H), 4.27 (q, J = 7.0 Hz, 2H),
3.54 (s, 2H), 1.36 (t, J = 7.0 Hz, 3H); 13C NMR (CDCl3, 75 MHz):
δ = 167.5, 139.8, 134.7, 133.3, 131.5, 130.2, 127.8, 123.8, 123.4,
114.5, 60.1, 45.6, 14.2; IR (KBr): ν = 2978, 2230, 1706, 1628, 1475,
1240, 1106, 1025, 788 cm-1; EIMS: m/z (%) = 326 (M+); Anal. Calcd
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Ethyl (Z)-3-(4-Cyanophenyl)-2-(thiocyanatomethyl)acrylate (6e):
Light yellow solid; m.p. 128–130°C (Lit.6d m.p. 129–131°C);
1H NMR: δ = 7.72 (s, 1H), 7.51 (d, J = 8.5 Hz, 2H), 7.04 (d, J = 8.5 Hz,
2H), 4.28 (q, J = 7.2 Hz, 2H), 3.54 (s, 2H), 1.36 (t, J = 7.2 Hz,
3H); 13C NMR (CDCl3, 75 MHz): δ = 167.2, 139.5, 139.1, 132.1,
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Brown oil (Lit.6d brown oil); 1H NMR: δ = 8.15 (d, J = 8.4 Hz, 2H),
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1632, 1599, 1521, 1347, 1235, 1158, 1109, 851 cm-1.
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yellow solid; m.p. 161–163°C (Lit.6d m.p. 159–162°C); 1H NMR:
δ = 7.50 (d, J = 7.4 Hz, 2H), 6.72 (d, J = 3.1 Hz, 1H), 6.38
(d,J=2.3Hz,1H),4.27(q,J=7.0Hz,2H),3.57(s,2H),1.33(t,J=7.0Hz,
3H); IR (KBr): ν = 2926, 2125, 1705, 1631, 1595, 1368, 1240, 1105,
765 cm-1.
Ethyl (2Z, 4E)-2-(Thiocyanatomethyl)hexa-2,4-dienoate (6h):
White solid; m.p. 87–89°C (Lit.6d m.p. 83–87°C); 1H NMR:
δ = 7.25–7.18 (m, 1H), 6.50–6.35 (m, 1H), 6.12–5.99 (m, 1H),
4.26 (q, J = 7.0 Hz, 2H), 3.24 (s, 2H), 1.80 (t, J = 6.5 Hz, 3H), 1.35
(t, J = 7.0 Hz, 3H); IR (KBr): ν = 2962, 2120, 1715, 1630, 1615,
1370, 1238, 1158, 725, 620 cm-1.
Ethyl (Z)-2-(Thiocyanatomethyl)hexa-2-enoate (6i): Colourless oil;
1H NMR: δ = 6.75 (t, J = 7.5 Hz, 1H), 4.20 (q, J = 6.8 Hz, 2H), 3.13
(s, 2H), 2.30-2.12 (m, 2H), 1.24–1.17 (m, 5H), 0.97 (t, J = 7.0 Hz,
3H); 13C NMR (CDCl3, 75 MHz): δ = 168.2, 148.2, 129.8, 116.2,
60.3, 49.5, 27.2, 19.2, 14.3, 13.2; IR (film): ν = 2965, 2120, 1716,
1617, 1459, 1368, 1235, 1148, 724, 621 cm-1; EIMS: m/z (%) =
213 (M+); Anal. Calcd for C10H15NO2S: C, 56.31; H, 7.09; N, 6.57.
Found: C, 56.37; H, 7.15; N, 6.62.
5
6
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We gratefully acknowledge financial support from the National
Natural Science Foundation of China (No. 20562005) and
NSF of Jiangxi Province (No. 0620021).
Received 2 June 2007; accepted 20 July 2007
Paper 07/4681 doi: 10.3184/030823407X234554
13 N. Petragnani and H.M.C. Rerraz, Synthesis, 1978, 476.
PAPER: 07/4681