A facile stereoselective synthesis of (Z)-2-ethoxycarbonyl-substituted 1,3-enynesfrom (E)-α-stannyl-α,β-unsaturated esters and alkynyl bromides

Article abstract of DOI:10.3184/030823409X401772

Palladium-catalysed hydrostannylation of alkynyl esters in benzene at room temperature gives regio- and stereoselectively (E)-α-stannyl-α, β-unsaturated esters in good yields. (E)-α-Stannyl-α,β;- unsaturated ethyl esters are difunctional group reagents wh

Full text of DOI:10.3184/030823409X401772

JOURNAL OF CHEMICAL RESEARCH 2009
MARCH, 137-139
RESEARCH PAPER 137
A facile stereoselective synthesis of (Z)-2-ethoxycarbonyl-substituted
1,3-enynes from (E)-a-stannyl-a,f3-unsaturated
Ruchun Dai, Zhiwen Xi and Mingzhong Cai*
esters and alkynyl bromides
Department of Chemistry, Jiangxi Normal University, Nanchang 330022, P. R. China
Palladium-catalysed
hydrostannylation
of alkynyl esters in benzene at room temperature
gives regio- and
stereoselectively
(E)-a-stannyl-a,13-unsaturated esters in good yields. (E)-a-Stannyl-a,13-unsaturated ethyl esters
are difunctional group reagents which undergo Stille coupling reactions with alkynyl bromides in the presence of
Pd(PPh3)4 and Cui co-catalyst to afford stereoselectively (Z}-2-ethoxycarbonyl-substituted 1,3-enynes in good yields.
Keywords: alkynyl ester, hydrostannylation, (E)-a-stannyl-a,13-unsaturated ester, (Z)-2-ethoxycarbonyl-substituted 1,3-enyne,
stereoselective synthesis
Enyne systems have attracted much attention from synthetic
organic chemists as enynes show interesting chemical
and biological reactivities.I-3 Conjugated enynes are also
important synthetic intermediates since the conjugated
enyne moiety can be readily converted in a stereospecific
manner into the corresponding diene system.4-6 Recently,
Takahashi et aU described the formation of highly substituted
enynes using a coupling reaction between alkenylzirconium
compounds and alkynyl halides. Gimeno and coworkers8
reported the stereoselective synthesis of chiral terminal (E)-
1,3-enynes derived from optically active aldehydes. The
synthesis of 1,3-enynes containing metal or heteroatom
functional groups has also attracted considerable interest
in organic syntheses because many useful functional group
transformations can be achieved by introduction and removal
of metal or heteroatom functions. The stereoselective
synthesis of 1,3-enynylsilanes,9,101,3-enynylsulfides, 11,121,3-
enynyltellurides,13,14 1,3-enynylselenides, 15-18 1,3-enynyl-
stannanes,19,20 1,3-enynylsulfones 21,22and fluor023 or CFr
substituted 1,3-enynes24 have already been described in
the literature. 1,3-Enynyl esters are important synthetic
intermediates since the ester group both activates the
adjacent multiple carbon-carbon bonds and provides
However, the cross-coupling reaction of vinylstannanes
with haloalkynes has rarely been reported.35-37Palladium-
catalysed hydrostannylation of phenylthioalkynes,38 alkynyl
selenides,39 and alkynyl sulfoxides4o has been reported to
be highly regio- and stereoselective, providing a direct route
for the stereoselective synthesis of l,l-difunctional group
reagents containing heteroatoms and tin. Kazmaier et al.41
reported that Mo(CO)lNC-t-Bu)3
was an efficient catalyst for
regioselective hydrostannylation of alkynes. Rossi et al.42,43
reported palladium-catalysed hydrostannylation of alkynyl
esters in THF. However, the hydrostannylation of ethyl 3-
phenylpropiolate with BU3SnHgave the addition products with
90% a-regioselectivity in 71% yield. In order to prepare highly
regioselectively (E)-a-stannyl-a,13-unsaturated esters, we
investigated palladium-catalysed hydrostannylation of alkynyl
esters with BU3SnHin benzene at room temperature and found
that benzene was better solvent than THF or toluene and that
(E)-a-stannyl-a,13-unsaturated esters 1 were obtained with high
regio- and stereoselectivity in high yields (Scheme 1).
Investigations of the crude products
1 by IH NMR
spectroscopy (400 MHz) showed regioisomeric purities of
more than 98%. One olefinic proton signal of compounds
la and Ib splits characteristically into a triplet at Ii = 6.04
a
useful functional group for further transformation.
with coupling constant J
=
6.8-7.2 Hz, which indicated that
l-Alkoxycarbonyl-substituted 1,3-enynes can be conveniently
prepared by palladium-catalysed cross-coupling reactions
of terminal alkynes with 13-halo-a,13-unsaturated esters.25-27
However, the synthesis of 2-alkoxycarbonyl-substituted 1,3-
enynes is limited.28We report here that (Z)-2-alkoxycarbonyl-
substituted 1,3-enynes can be synthesised stereoselectively
by palladium-catalysed hydrostannylation of alkynyl esters,
followed by a cross-coupling reaction with alkynyl bromides
in the presence of Pd(PPh3)4and Cul co-catalyst.
Stille has reported29 the use of organotin reagents to
obtain enynes by cross-coupling reaction of vinyl triflates
or vinyl iodides with alkynylstannanes in the presence of
palladium catalysts.29-31Interest in the method is due to a
high tolerance for functional groups such as allylic ether,
vinylic thioethers, esters, ketones or trimethylsilyl ether.32-34
the hydrostannylation to the alkynyl esters had taken place
with strong preference for the addition of the tin atom at the
carbon adjacent to the ester group. The stereochemistry of
the addition was readily apparent from the IH NMR spectra
of compounds 1 which showed a (3JSn-H) coupling constant
of 64 Hz, fully in accord with an E geometry and overall cis
addition of tin hydride.44
(E)-a-Stannyl-a,13-unsaturated esters 1 are difunctional
group reagents in which two synthetically versatile groups
are linked to the same olefinic carbon atom and can be
considered both as vinylstannanes and as a,13-unsaturated
esters. Palladium-catalysed
cross-coupling reaction of
vinylstannanes with alkynyl bromides has already been
reported.16,45With a convenient route to the (E)-a-stannyl-
a,13-unsaturated esters 1 we decided to establish the feasibility
R.>=<C0Et
2
Benzene, r.t., 4 h
H
SnBu3
1
la: R = n-C4H9, Isolated yield: 91%
Ib: R = n-C6H13, Isolated yield: 89%
Ie: R = Ph, Isolated yield: 87%
Scheme 1
* Correspondent.E-mail:caimzhong@163.com

138 JOURNAL OF CHEMICAL RESEARCH 2009
R~C02Et
----R]
B,
Pd(PPh3)4/CuI_
_
+
DMF, r.t., 24 h
H
~
2
3
]
R
Scheme 2
Table
1
Synthesis of (Z)-2-ethoxyca rbonyl-substituted
of using 1 in cross-coupling
reactions with alkynyl bromides
1,3-e nynes 3a-h
2. Gratifyingly, when the cross-coupling
reactions of 1 with
a variety of alkynyl bromides 2 were conducted in DMF at
room temperature using Pd(PPh3)4 and CuI as co-catalyst
Entry
R
R1
Product
Yield/Yo"
1
2
3
4
5
6
7
8
n-C4H
g
n-C4H
Ph
Ph
n-C6H13
g
3a
3b
3c
3d
3e
3f
86
81
73
78
75
79
80
82
(Scheme 2), fairly rapid reactions occurred affording stereo-
selectively the desired coupling products 3 in good yields. The
experimental results are summarised in Table I. However, we
found that when the cross-coupling reactions of (E)-a-stannyl-
a,13-unsaturated esters 1 with alkynyl iodides were performed
under the same conditions, only trace coupling products were
obtained.
n-C4H
g
Ph
Ph
Ph
Ph
CH30CH2
n-C4H
g
n-C6
H13
Ph
3g
3h
n-C4H
g
n-C
6
H13
In summary,
a
convenient
synthetic method for (Z)-2-
"Isolated
yield based on the (E)-a-stannyl-a,l3-unsaturated
esters 1 used.
ethoxycarbonyl-substituted
the palladium-catalysed
I ,3-enynes has been developed by
hydrostannylation
of alkynyl esters,
Ie: Oil. IR (film):
1183,1034,788,695;
V
(cm-') 3059,2958,2923,
'H NMR (400 MHz, CDCI3): 1) 7.32-7.29 (m,
1700, 1596, 1463,
followed by a cross-coupling
in the presence of Pd(PPh3)4 and CuI. The present method
reaction with alkynyl bromides
5H), 6.70 (s, 3JSn_H= 64 Hz, 1H), 4.17 (q, J = 7.2 Hz, 2H), 1.58-
1.52 (m, 6H), 1.37-1.32 (m, 6H), 1.22 (t, J = 7.2 Hz, 3H), 1.07
(t, J = 8.0 Hz, 6H), 0.91 (t, J = 7.2 Hz, 9H); l3C NMR (100 MHz,
CDCI3): 1) 173.2, 142.1, 139.8, 137.0, 128.3, 128.1, 128.0,60.3,28.9,
27.3, 14.2, 13.7, 10.6; MS (E1): m/z 466 (W , 1.5),409 (100), 407
(87), 179 (54), 177 (46); Anal. Calcd for C23H3802Sn: C, 59.37; H,
8.23. Found: C, 59.57; H, 8.35%.
has the advantages
of readily available starting materials,
straightforward
and simple procedures, mild reaction
conditions and good yields.
Experimental
'H NMR spectra were recorded on a Bruker AC-P400 (400 MHz)
spectrometer with TMS as an internal standard using CDCI3 as the
solvent. l3C NMR (100 MHz) spectra were recorded on a Broker AC-
P400 (400 MHz) spectrometer using CDCI3 as the solvent. IR spectra
were determined on an FTS-185 instrument as neat films. Mass spectra
were obtained on a Finnigan 8239 mass spectrometer. Microanalyses
were measured using a Yanaco MT-3 CHN microelemental analyser.
All reactions were carried out in pre-dried glassware (150°C, 4 h) and
cooled under a stream of dry Ar. Benzene was distilled from sodium
prior to use. DMF was dried by distillation over calcium hydride.
General procedure for the synthesis of (Z)-2-ethoxycarbonyl-
substituted 1.3-enynes 3a-h
(E)-a-Stannyl-a,13-unsaturated
ester I (1.0 mmol) and alkynyl
bromide 2 (1.1 mmol) were dissolved in DMF (10 mL) under Ar at
room temperature. Pd(PPh3)4 (0.05 mmol) and CuI (0.75 mmol) were
then added. The mixture was stirred for 20-24 h at room temperature
and monitored by TLC (Si02) for the disappearance of the starting
(E)-a-stannyl-a,13-unsaturated ester 1. The reaction mixture was
diluted with diethyl ether (30 mL), filtered and then treated with 20%
aqueous KF (10 mL) for 30 min before being dried and concentrated.
The residue was purified by column chromatography on silica gel
(eluent: light petroleum ether/EtOAc, 19:I).
General procedure for the synthesis of (E)-a-stannyl-a.13-unsaturated
esters la-e
3a: Oil. IR (film):
V
(cm-') 2958, 2931, 2872, 2220,1728,1447,
CAUTION: Appropriate care was taken in handling benzene due
to its carcinogenicity
1374, 1211, 1174, 1024; 'H NMR (400 MHz, CDCI3): 1) 6.48 (t,
J = 7.6 Hz, 1H), 4.23 (q, J = 7.2 Hz, 2H), 2.59-2.53 (m, 2H), 2.33
(t, J = 6.8 Hz, 2H), 1.55-1.28 (m, 11H), 0.96-0.85 (m, 6H); l3C
NMR (100 MHz, CDCI3): 1) 165.2, 152.8, 116.6, 89.8, 77.8, 60.9,
31.1,30.7,29.6,22.4,21.9,19.0,14.1,13.9,13.6;
(M+, 15),207 (41),167 (50),115 (100); Anal. Calcd forC,sH2402: C,
76.23; H, 10.24. Found: C, 76.05; H, 10.32%.
A 25 mL, two-necked, round-bottom flask equipped with a magnetic
stir bar and argon was charged sequentially with alkynyl ester
(I mmol), benzene (4 mL), Pd(PPh3)4 (0.02 mmol) and BU3SnH
(1.05 mmol). If toluene in used instead of benzene the reactions
occurred with low regioselectivity and low yields. The mixture was
stirred at room temperature for 4 h. After removal of the solvent
under reduced pressure, the residue was diluted with light petroleum
ether (20 mL) and filtered to remove the palladium catalyst.
The resulting solution was concentrated under reduced pressure
and the residue was purified by flash chromatography on silica gel
(eluent: light petroleum ether/EtOAc, 19:I).
MS (E1): m/z 236
3h: Oil. IR (film):
V
(cm-') 2958, 2928, 2856, 2216, 1722, 1597,
1490, 1443, 1371, 1210, 1138, 756, 690; 'H NMR (400 MHz,
CDCI3): 1) 7.47-7.44 (m, 2H), 7.33-7.30 (m, 3H), 6.68 (t,J= 7.6 Hz,
1H), 4.27 (q, J = 7.2 Hz, 2H), 2.68-2.63 (m, 2H), 1.49-1.34 (m,
7H), 0.93 (t, J = 7.2 Hz, 3H); l3C NMR (100 MHz, CDCI3): 1) 164.8,
154.6,131.6,128.4,128.3,123.2,116.4,88.8,86.8,61.1,31.1,29.8,
22.5,14.2,13.9; MS (E1): m/z 256 (W , 5.1),129 (30), 115 (41),105
(100); Anal. Calcd for C17H2002: C, 79.65; H, 7.86. Found: C, 79.38;
H,7.64%.
la: Oil. IR (film):
V
(cm-') 2958,2929, 1709, 16m, 1464, 1182,
1038; 'HNMR(400MHz, CDCI3): 1) 6.04 (t,J= 6.8 Hz, 3JSn_H=64Hz,
1H), 4.14 (q, J = 7.2 Hz, 2H), 2.44-2.40 (m, 2H), 1.58-1.26 (m,
19H), 0.95-0.84 (m, 18H); l3C NMR (100 MHz, CDCI3): 1) 171.3,
3e: Oil. IR (film):
1215,1094,755,690;
V
(cm-') 3062,2957,2226,
1726, 1595, 1490,
'H NMR (400 MHz, CDCI3): 1) 7.52-7.49 (m,
153.6,135.6,59.9,31.8,31.4,29.9,27.3,22.3,14.4,
13.9, 13.7, 10.3;
2H), 7.39-7.33 (m, 8H), 7.27 (s, 1H), 4.27 (q, J= 7.2 Hz, 2H), 1.25
(t, J = 7.2 Hz, 3H); l3C NMR (100 MHz, CDCI3): 1) 165.8, 143.2,
134.7, 131.7, 129.2, 128.8, 128.6, 128.4, 128.3, 122.8, 116.9, 91.7,
87.0,61.7, 13.9; MS (E1): m/z 276 (W , 89), 231 (54),203 (100), 105
(90); Anal. Calcd for C'9H'602: C, 82.58; H, 5.84. Found: C, 82.32;
H,5.61%.
MS (E1): m/z 446 (W , 2.3), 389 (69), 387 (48), 205 (50),105 (100),
73 (75); Anal. Calcd for C2,H4202Sn: C, 56.64; H, 9.50. Found: C,
56.34; H, 9.25%.
Ih: Oil. IR (film):
V
(cm-') 2957, 2927, 1709, 1603, 1464, 1377,
1180; 'H NMR (400 MHz, CDCI3): 1) 6.04 (t, J= 7.2 Hz, 3JSn_H= 64
Hz, 1H), 4.14 (q, J= 7.2 Hz, 2H), 2.45-2.39 (m, 2H), 1.53-1.26 (m,
23H), 0.96-0.84 (m, 18H); l3C NMR (100 MHz, CDCI3): 1) 171.3,
153.7, 135.5, 59.9, 32.1, 31.7, 29.2, 29.0, 28.9, 27.3, 22.6, 14.4,
14.1,13.7,10.3; MS (E1): m/z 417 (W-Bu, 100),371 (21),291 (19),
235 (28), 179 (38); Anal. Calcd for C23H4602Sn: C, 58.36; H, 9.79.
Found: C, 58.08; H, 9.62%.
3d: Oil. IR (film):
1447,1373,1211,1174,1024,753,694;
V
(cm-') 3025, 2930, 2218, 1728, 1598, 1493,
'HNMR(400MHz,CDCl3):
1) 7.31-7.29 (m, 5H), 7.10 (s, 1H), 4.21 (q, J= 7.2 Hz, 2H), 2.39 (t,
J = 7.2 Hz, 2H), 1.45-1.25 (m, 8H), 1.20 (t, J = 7.2 Hz, 3H), 0.92
(t, J = 7.2 Hz, 3H); l3C NMR (100 MHz, CDCI3): 1) 166.3, 141.8,

JOURNAL OF CHEMICAL RESEARCH 2009 139
2
3
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(46), 143 (82), 129 (97); Anal. Calcd for C19H2402:C, 80.24; H, 8.51.
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4
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7
3e: Oil. IR (film):
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(em-I) 3062, 2932, 2217, 1727, 1599, 1493,
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V
(em-I) 3060, 3025, 2930, 2858, 2218, 1729,
1598, 1492, 1447, 1373, 1211, 1174,753,694; IH NMR (400 MHz,
CDC13): 87.31-7.26 (m, 5H), 7.10 (s, lH), 4.21 (q, J= 7.2 Hz, 2H),
2.39 (t, J= 7.2 Hz, 2H), 1.38-1.22 (m, 4H), 1.20 (t, J= 7.2 Hz, 3H),
0.92 (t, J= 7.2 Hz, 3H); l3CNMR(lOO MHz, CDC13):8 166.2, 141.8,
135.0, 128.7, 128.5, 128.2, 117.4,93.3,78.1,61.5,30.6,22.0,
13.8, 13.6; MS (EI): m/z 256 (M+, 81), 211 (l00), 141 (83); Anal.
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V
(em-I) 2928, 2857, 2215, 1722, 1597, 1490,
1443, 1372, 1210, 1137, 1027, 756, 690; IH NMR (400 MHz,
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= 7.2 Hz, 2H), 2.71-2.65 (m, 2H), 1.53-1.28 (m,
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V
(em-I) 2957, 2930, 2858, 2212, 1722, 1466,
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We thank the National Natural Science Foundation of China
(Project No. 20462002) and the Natural Science Foundation
of Jiangxi Province in China (0420015) for financial support.
Received 10 September 2008 ,-accepted 13 December 2008
Paper 08/0163 doi: 10.3184/030823409X401772
Published online: 3 April 2009
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