Palladium-catalyzed phosphinylthiolation of terminal alkynes

Article abstract of DOI:10.1016/j.tetlet.2012.02.035

Phosphinylthiolation of terminal alkynes with Ph₂P(O)SBu or a related compound proceeds in the presence of a palladium-PEt₃ catalyst system. Activity and stereoselectivity are highly dependent on the nature of solvent, ethylbenzene, and n-hexanol (or t-amyl alcohol) being E- and Z-selective, respectively.

Full text of DOI:10.1016/j.tetlet.2012.02.035

Tetrahedron Letters 53 (2012) 2078–2081
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Tetrahedron Letters
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Palladium-catalyzed phosphinylthiolation of terminal alkynes
⇑
Nobutaka Hoshi, Taigo Kashiwabara, Masato Tanaka
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Phosphinylthiolation of terminal alkynes with Ph2P(O)SBu or a related compound proceeds in the pres-
ence of a palladium–PEt3 catalyst system. Activity and stereoselectivity are highly dependent on the nat-
ure of solvent, ethylbenzene, and n-hexanol (or t-amyl alcohol) being E- and Z-selective, respectively.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 20 January 2012
Revised 6 February 2012
Accepted 7 February 2012
Available online 16 February 2012
Keywords:
Palladium
Phosphinylthiolation
Alkyne
Homogeneous catalysis
The addition reaction of inter-element bonds is a research area
of vital growth these days. However, as far as P–S and related
bonds are concerned, only a limited number of publications have
appeared. One of the protocols is based on the radical process as
reported for P(O)–SR (R = alkyl) species by Malacria and co-work-
ers1 and for Ph2P–SR (R = aryl, alkyl) by Oshima and co-workers.2
As another protocol, one of us has reported the palladium-
catalyzed addition of (RO)2P(O)SPh and (RO)2P(O)SePh.3 Attempted
reactions starting with Ph2P(O)SPh aiming at expanding the scope
of the protocol did not furnish the desired adducts in appreciable
yields.4 Further attempts, however, have uncovered that the use
of alkylthio congeners in the place of Ph2P(O)SPh does work to give
the desired adducts in acceptable yields and that the stereochem-
istry of the products is highly dependent on the solvents, which
will be reported in this Letter.
from the mechanistic viewpoint (vide infra) that 1-octyn-1-yldi-
phenylphosphine oxide (4aA; 7%) and 2-butylthio-1-octene (5aA;
13%) were also found in the reaction mixture. The structures of
(E)-3aA and other products obtained in other reactions were char-
acterized spectroscopically and confirmed unambiguously by X-
ray diffraction in two cases, (E)-3aA and (Z)-3aH.5
Choice of the solvent has proved to be another important factor
that affects the reactivity, the stereochemistry in particular. Under
the same conditions using PEt3, the yield ratio of the Z/E products
observed with various solvents increased as follows; ethylbenzene
(dielectric constant
= 13.1) 17/57 < n-octane (
chlorobenzene ( = 5.6) 25/5 < butyronitrile (
= 37.6) 33/trace < n-hexanol ( = 13.3) 83/trace. Thus, the reac-
e
= 2.4) 8%/70% < methyl isobutyl ketone
= 2.0) 16/44 < THF ( = 7.58) 26/45 <
= 20.7) 22/1 < DMF
(e
e
e
e
e
(e
e
tion is Z-selective in n-hexanol, unlike the E-selectivity observed
in ethylbenzene (Scheme 1). It is also interesting to note that other
alkanols, sterically more demanding and less nucleophilic ones in
particular, are more advantageous over n-hexanol in the high
yielding formation of Z-isomers, as exemplified by the reaction of
5-hexynenitrile (Table 2).6 However, we are unable to find any
correlation between the selectivity and the dielectric constant of
the solvent.
With the foregoing results of trial experiments in hand, we ran a
series of reactions of 1a with various alkynes (Table 3). Aliphatic
alkynes, inclusive of those substituted by polar functional groups,
afford the desired products in acceptable yields of either (Z)- or
(E)-adducts depending on the solvent. However, t-butylacetylene
(2C) and 3-phenyl-1-propyne (2F), which produce (Z)-adducts in
a rather high yield in an alcoholic solvent, mainly afford the same
stereoisomer in very low yields in ethylbenzene, partially because
of intrinsic low reactivity. Moreover, the reaction of 2F also
Since the first several attempts using a catalyst system compris-
ing of CpPd(p-allyl) (5 mol %) and PPh3 (10 mol %) did not work
successfully, a series of trial experiments to search for better
phosphine ligands were run at 130 °C for 6 h using diphenyl(butyl-
thio)phosphine oxide (1a, 1.0 mmol) and 1-octyne (2A, 1.0 mmol)
in ethylbenzene (2.0 mL). Under the conditions, strongly
electron-donating and sterically less demanding phosphines have
proven to be better performing (Table 1). Among the ligands exam-
ined, PEt3 is the ligand of choice, which affords (E)-3aA in a 70%
NMR yield along with the (Z)-isomer (8%). Since the related reac-
tion of (RO)2P(O)SPh with terminal alkynes results in cis-addition
giving (Z)-isomers as the major products, the predominant forma-
tion of (E)-3aA is entirely unexpected. It is also interesting to note
⇑
Corresponding author.
E-mail address: m.tanaka@res.titech.ac.jp (M. Tanaka).
formed
(E)-1-phenyl-2-butylthio-3-diphenylphosphinyl-1-pro-
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.02.035

N. Hoshi et al. / Tetrahedron Letters 53 (2012) 2078–2081
2079
Table 1
Table 2
Ligand effects on the reaction of 1a with 2Aa
Effect of alcohol solvent in the reaction of 5-hexynenitrilea
Ph2P(O)SBu
CpPd(π-allyl)
(CH2)3-CN
SBu
CpPd(π-allyl)
C6H13
SBu
(E)-3aA
Ph2(O)P
C6H13
SBu
(Z)-3aA
2PEt3
1a
Ligand
+
(CH2)3-CN
Ph2P(O)SBu
+
+
130 °C, 18 h
Ph2(O)P
3aB
Ph2(O)P
Ethylbenzene
130 °C, 6 h
1a
n-C6H13
2B
2A
Solvent
Yield of (Z)-3aBb (%)
Yield of (E)-3aBb (%)
Entry
Ligand
Conversion of 1ab (%)
Yieldc (%)
(Z)-3aA (E)-3aA
OH
40
45
Trace
3
OH
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
PMe3
PEt3
PPr3
PBu3
90
98
51
58
42
93
49
4
66
28
51
6
64
88
51
43
57
38
63
11
8
28
34
0
31
70
9
15
0
64
8
OH
OH
P(t-Bu)3
67
74
11
4
Ph-SMAPd
11
1
51
8
P(CH2O)3CCH3
P(OCH2)3CEt
PCy3
ꢀ0
16
8
0
16
3
OH
a
PPh3
Reaction conditions: Ph2P(O)SBu (1a) (1.0 mmol), 5-hexynenitrile (2B)
(1.5 mmol), CpPd(p-allyl) (0.05 mmol), PEt3 (0.10 mmol), solvent (2.0 mL), 130 °C
P(p-An)3
P(OEt)3
dmpe
dppe
dppp
12
6
5
19
Trace
9
for 18 h.
Determined by 1H NMR spectroscopy using 1,1,2,2-tetrachloroethane as an
b
Trace
0
7
Trace
Trace
7
13
Trace
15
6
8
2
internal standard.
dppb
dcpee
dppf
Xantphos
Besides diphenyl(butylthio)phosphine oxide (1a), diphenyl(t-
butylthio)phosphine oxide (1b) also reacts smoothly with 2A to
preferentially form (E)- and (Z)-3bA in ethylbenzene and n-hexa-
nol, respectively, although the solvent dependence of the stereo-
chemistry is not as distinct as that observed with 1a (Scheme 2).
Although we assume that the oxidative addition of the P–S bond
triggers the catalysis,3 attempted isolation of intermediates to pro-
pose the mechanism has been unsuccessful.9,10 However, following
observation merits consideration of the origin of the solvent-
dependent stereoselectivity. Figure 1a illustrates the time course
of the reaction in n-hexanol, which displays a monotonous increase
of the yield of (Z)-3aA. On the other hand, Figure 1b illustrating the
reaction in ethylbenzene indicates that (Z)-3aA is formed only in
the beginning stage. In addition, 4aA is also formed as another ma-
jor product in the beginning stage. However, the quantities of (Z)-
3aA and 4aA start decreasing slightly and instead (E)-3aA shows a
somewhat sigmoidal increase to eventually be the major product.
These observations appear to suggest that (Z)-3aA isomerizes to
(E)-3aA in ethylbenzene.
a
Reaction conditions: Ph2P(O)SBu (1a) (1.0 mmol), 1-octyne (2A) (1.0 mmol),
CpPd(p-allyl) (0.05 mmol), ligand (P/Pd = 2.0), ethylbenzene (2.0 mL), 130 °C for
6 h.
b
Determined by GC. Conversion of 2A was not determined due to peak over-
lapping with the solvent.
Determined by 1H NMR spectroscopy using 1,1,2,2-tetrachloroethane as an
c
internal standard.
d
4-Phenyl-1-phospha-4-silabicyclo[2.2.2]octane.
1,2-Bis(dicyclohexylphosphino)ethane.
e
C6H13
SBu
Ph2(O)P
(E)-3aA
(Z)-3aA
1a
Ph2P(O)SBu
+
n
n-C6H13 2A
C6H13
SBu
Ph2(O)P
To substantiate the possibility of the isomerization, an ethyl-
benzene (5.0 mL) solution of (Z)-3aA (0.5 mmol) was heated in
Scheme 1. Solvent dependence on the stereoselectivity.
the presence of CpPd(p-allyl) (0.025 mmol) and PEt3 (0.05 mmol)
at 130 °C for 6 h (Scheme 3). Analysis of the resulting mixture re-
vealed that (E)-3aA was indeed formed in a 23% yield,11 although
the isomerization was not sufficiently rapid to rationalize that
the isomerization is a major provenance of (E)-3aA.12
pene [(E)-3aF0], an isomer of 3aF, in a 9% yield (both entries 11 and
12).7 On the other hand, the reaction of phenylacetylene (2H) is
somewhat slow as compared with aliphatic alkynes and less selec-
tive, ending up with the yield of the desired adduct lower than 50%.
Detailed analysis of entries 15 and 16 has revealed that the low
yield is due to the more extensive formation of byproducts such
as 1-(butylthio)styrene (13% and 21% in entries 15 and 16, respec-
tively), 1,2-di(butylthio)styrene (8% and 4%), and oligomers of 2H
(11% and 15%).8 In agreement with aliphatic alkynes being more
reactive and more selective, p-anisylacetylene (2J) substituted
with an electron-releasing methoxy group is better performing
than the parent phenylacetylene, although the reaction of p-fluor-
ophenylacetylene (2K) cannot be understood in the same line. The
stereochemical preference is solvent-dependent as found with ali-
phatic alkynes. However, the dependence is not as distinct and,
subjected to the substituent bound to the phenyl ring, both n-hex-
anol and ethylbenzene afford the same stereoisomer as a major
product, as exemplified by the reaction of p-methoxyphenylacety-
lene and p-fluorophenylacetylene.
Another interesting observation that merits further mechanistic
consideration is that 1-octyn-1-yldiphenylphosphine oxide (4aA)
and 2-butylthio-1-octene (5aA) were formed as byproducts in
the reaction of 2A run in ethylbenzene (vide supra). The formation
of 4aA may have come from the metathetical reaction of 1a and 2A,
which is envisioned to generate butane-1-thiol as a coproduct.
Although butane-1-thiol is not found in the reaction mixture, the
formation of 5aA is rationalized by assuming that butane-1-thiol
generated during the catalysis has added across the triple bond
of 1-octyne.13 This assumption is supported by the reaction of
2A-d1 with 1a forming 5aA (20% NMR yield),14 in which the D con-
tent at the methylene carbon estimated by 1H NMR spectroscopy
was higher than 90% both at syn and anti positions (Scheme 4). A
similar observation has been reported by Ogawa and co-workers
in the mechanistic study on the palladium-catalyzed hydropho-
sphination of alkynes with tetraphenyldiphosphine.15

2080
N. Hoshi et al. / Tetrahedron Letters 53 (2012) 2078–2081
Table 3
Phosphinylthiolation of terminal alkynes with Ph2P(O)SBua
CpPd(π-allyl)
2PEt3
R
Ph2P(O)SBu
1a
+
R
130 °C, 12 h
SBu
Ph2(O)P
2X
3aX
Entry
1e
Alkyne 2, R=
Solventb
Conversion of 1ac (%)
Yield of (Z)-3aXd (%)
Yield of (E)-3aXd (%)
2A, n-hexyl
2A, n-hexyl
2B, 3-cyanopropyl
2B, 3-cyanopropyl
2C, t-Bu
t-AA
EB
t-AA
EB
H
EB
t-AA
EB
t-AA
EB
t-AA
EB
H
EB
H
EB
H
EB
H
EB
H
98
98
93
95
93
38
97
79
99
51
96
38
85
43
81
65
81
65
99
89
86
81
89 (81)
8
74 (68)
13
61 (46)
3
73 (70)
ꢀ0
e
2
70 (60)
f
3
4
4f
64 (41)
f
5
6
ꢀ0
f
2C, t-Bu
ꢀ0
7
8
9
2D, 3-(carbomethoxy)propyl
2D, 3-(carbomethoxy)propyl
2E, 3-hydroxylpropyl
2E, 3-hydroxylpropyl
2F, benzyl
3
63 (61)
0
17 (16)
76 (71)
9
10
11f
12f
13
14
15
16
17
18
19
20
21
22
71 (70)g
ꢀ0
2F, benzyl
7g
4
2G, 1-cyclohexenyl
2G, 1-cyclohexenyl
2H, phenyl
2H, phenyl
2I, p-tolyl
38
3
47 (36)
trace
47
22
36 (20)
8
37 (34)
8
37 (35)
46
51
1
2I, p-tolyl
ꢀ0
2J, p-methoxyphenyl
2J, p-methoxyphenyl
2K, p-fluorophenyl
2K, p-fluorophenyl
31
18
25 (21)
27
EB
12
a
b
c
d
e
f
Reaction conditions: 1a (1.0 mmol), 2X (1.5 mmol), CpPd(
t-AA = t-amyl alcohol, EB = ethylbenzene, H = n-hexanol.
Determined by GC.
p-allyl) (0.05 mmol), PEt3 (0.10 mmol), solvent (2.0 mL), 130 °C for 12 h.
Determined by 1H NMR spectroscopy. The figures in parentheses are isolated yields.
Quantity of 2A = 1.0 mmol, reaction time = 6 h.
Reaction time = 18 h.
A double bond isomer was also formed in a 9% yield. See the text.
g
On the basis of these considerations, addition of butane-1-thiol
across the triple bond of 4aA appears to be another candidate route
to (E)-3aA. Indeed, when a toluene-d8 solution of 4aA (0.07 mmol),
Ph2P(O)S(t-Bu)
CpPd(π-allyl)
+ 2PEt3
130 °C, 12 h
C6H13
C6H13
Ph2(O)P
1b
+
n-C6H13
2A
+
Ph2(O)P
S(t-Bu)
S(t-Bu)
(E)-3bA
butane-1-thiol (0.07 mmol), CpPd(p-allyl) (0.01 mmol), and PEt3
(Z)-3bA
in ethylbenzene 21%
in n-hexanol 43% (39%)
(0.02 mmol) was heated at 110 °C for 6 h, (E)-3aA was formed in
a 64% yield along with only a trace of (Z)-3aA (Scheme 5).16,17 An-
other reaction without using the palladium complex under other-
wise identical conditions afforded a mixture of (Z)-3aA (24%) and
(E)-3aA (45%).
40% (32%)
23%
Scheme 2. Phosphinylthiolation with diphenyl(t-butylthio)phosphine oxide (fig-
ures in parentheses are isolated yields).
100
100
80
(Z)-3aA
80
60
60
1a
(E)-3aA
40
40
1a
4aA
(Z)-3aA
20
20
4aA
(E)-3aA
0
0
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Reaction time (h)
Reaction time (h)
Figure 1. Time course of the reaction of 1a with 2A run (a) in n-hexanol (left) and (b) in ethylbenzene (right). Conditions are basically the same as those for Table 1.

N. Hoshi et al. / Tetrahedron Letters 53 (2012) 2078–2081
2081
CpPd(π-allyl) (5 mol %)
PEt3 (10 mol %)
ethylbenzene, 130 °C, 6 h
References and notes
C6H13
SBu
C6H13
SBu
Ph2(O)P
1. (a) Carta, P.; Puljic, N.; Robert, C.; Dhimane, A.-L.; Fensterbank, L.; Lacôte, E.;
Malacria, M. Org. Lett. 2007, 9, 1061; See also: (b) Sato, A.; Yorimitsu, H.;
Oshima, K. Tetrahedron 2009, 65, 1553.
2. Wada, T.; Kondoh, A.; Yorimitsu, H.; Oshima, K. Org. Lett. 2008, 10, 1155.
3. (a) Han, L.-B.; Tanaka, M. Chem. Lett. 1999, 28, 863; (b) Han, L.-B.; Choi, N.;
Tanaka, M. J. Am. Chem. Soc. 1996, 118, 7000.
Ph2(O)P
(Z)-3aA
conversion 38%
(E)-3aA
23% (NMR)
Scheme 3. Isomerization of (Z)-3aA to (E)-3aA.
4. Tanaka, M. unpublished result.
5. CCDC 863310–863311 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
6. Since the reactions run in n-hexanol under the standard conditions formed n-
hexyl diphenylphosphinate as a byproduct (9–15%), the better performance of
t-amyl alcohol can be due partially to its low nucleophilicity, which depresses
the reactivity in the transesterification (butylthio and alkoxy exchange).
7. Another reaction of 2F in n-hexanol, the formation of (E)-3aF0 was more
extensive (36% 1H NMR yield), relative to the formation of (Z)-3aF (26% 1H
NMR yield). Since phenylallene was totally unreactive towards 1a under the
conditions, the formation of (E)-3aF0 is not due to isomerization of 2F to
phenylallene prior to addition of 1a.
+
n-C6H13
P(O)SBu
Ph2
D
2A-d1 (1.0 mmol)
1a (1.0 mmol)
D content: 99%
CpPd(π-allyl) (5 mol %)
Ha
Hb
n-C6H13
SBu
5aA-d2
20% (NMR yield)
D content:
Ha = 95%
PEt3 (P/Pd = 2)
130 °C, 6 h
ethylbenzene
H
b = 91%
8. To improve the yield, screening of phosphine ligands and solvent effect study
were performed for phenylacetylene. However we were unable to find a better
ligand than triethylphosphine and a better solvent than n-hexanol and t-amyl
alcohol.
Scheme 4. Formation of 2-butylthio-1-octene-d2 (5aA-d2) in the reaction of 1-
octyne-d1 (2A-d1) with diphenyl(butylthio)phosphine oxide (1a).
9. One of the species generated in the reaction of 1a with CpPd(
(0.05 mmol) and PMe3 (0.05 mmol) in benzene-d6 run at 110 °C for 3 h
appeared dimeric such as [{Ph2P(O)}Pd(
-SBu)]2, as judged on the basis of 31P
p-allyl)
C6H13
P(O)Ph2 CpPd(π-allyl) (14 mol %)
PEt3 (P/Pd = 2)
C6H13
SBu
l
Ph2(O)P
4aA
+
NMR, 67.1 ppm (d, J = 37.1 Hz, P@O), ꢁ3.7 ppm (d, J = 37.1 Hz, PMe), while the
same reaction run in MeOH-d4 at 60 °C for 3 h appeared to generate, among
others, a monomeric species, Ph2P(O)Pd{S(t-Bu)}(PMe3)2 which displayed 31P
NMR signals at 74.0 ppm (t, J = 122.4 Hz) and ꢁ29.3 ppm (d, J = 122.4 Hz),
indicative of a trans-configuration. However, depending on the conditions of
these and similar reactions using 1b, polymeric palladium species also
appeared to be formed on the basis of ESI-MS analysis, and we have been
unable to further characterize these species due to the complexity of the
mixture. For the generation of polymeric and oligomeric palladium sulfide
toluene-d8, 110 °C, 6 h
BuSH
(E)-3aA 64% (NMR)
Scheme 5. Palladium-catalyzed facile addition of butane-1-thiol with 1-octyn-1-
yldiphenylphosphine oxide (4aA).
species, see Ref.10
.
P-S
1
R
2X
10. (a) Ogawa, A. Top. Organomet. Chem. doi:10.1007/3418_2011_19.; (b) Stash, A.
I.; Levashova, V. V.; Lebedev, S. A.; Hoskov, Y. G.; Mal’kov, A. A.; Romm, I. P.
Russ. J. Coord. Chem. 2009, 35, 136; (c) Stash, A. I.; Perepelkova, T. I.; Noskov, Yu.
G.; Buslaeva, T. M.; Romm, I. P. Russ. J. Coord. Chem. 2001, 27, 585; (d) Higgins, J.
D.; Suggs, W. Inorg. Chim. Acta 1998, 145, 247; (e) Jain, V. K.; Jain, L. Coord.
Chem. Rev. 2010, 254, 2848.
11. The conversion of (Z)-3aA was 38%, suggesting that other unknown products
were also formed during the isomerization.
12. Although diethyl 2-(p-tolylthio)ethenylphosphonate, a related compound, is
known to undergo a thermal Z-to-E isomerization, (Z)-3aA did not isomerize to
(E)-3aA by simple heating with or without PEt3. See: Acheson, R. M.; Ansell, P.
J.; Murry, J. R. J. Chem. Res., Synop. 1986, 378.
[Pd]
P-[Pd]-S
+ SH
P
R
4aX
-[Pd]
(Z)-3aX
R
2X
P = P(O)Ph2
S = SBu
(E)-3aX
Scheme 6. Possible pathways leading to (E)-3aA and (Z)-3aA.
13. Palladium-catalyzed addition of thiols across C–C triple bonds is
a well
To summarize, the possible pathways leading to (Z)-3aA and
(E)-3aA can be proposed as illustrated in Scheme 6. In alcoholic
solvents, the catalysis is carried presumably by the straightforward
shuttle between [Pd] and [Ph2P(O)-Pd-SBu] species, which is in
good agreement with the stereoselective formation of (Z)-3aA. In
ethylbenzene, on the other hand, (Z)-3aA formed isomerizes to
(E)-3aA. Moreover, the [Ph2P(O)-Pd-SBu] species somehow reacts
with terminal alkynes to generate 4aX and butane-1-thiol and
these two intermediates react together forming mainly (E)-3aA,
along with (Z)-3aA to a lesser extent.18 The reason for the lack of
isomerization in n-hexanol is uncertain at this moment.
established process. See Ref.10a
.
14. In this reaction, 4aA and (E)-3aA-d1 were formed in 9% and 60% yields,
respectively. The product distribution in this reaction was basically the same as
that found in a reaction using 2A under otherwise the same conditions, where
the yield of 5aA was 19%.
15. (a) Nagata, S.; Kawaguchi, S.; Matsumoto, M.; Kamiya, I.; Nomoto, A.; Sonoda,
M.; Ogawa, A. Tetrahedron Lett. 2007, 48, 6637; See also: (b) Arisawa, M.;
Onoda, M.; Hori, C.; Yamaguchi, M. Tetrahedron Lett. 2006, 47, 5211.
16. Details will be reported separately. The same reaction of 4aA and butane-1-
thiol run without CpPd(p-allyl) (0.01 mmol) and PEt3 or in the presence of
AIBN did not afford (E)-3aA at all.
17. Similar reactions of selenols in the presence of the Wilkinson’s catalyst have
been reported very recently; see: Kawaguchi, S.-i.; Kotani, M.; Atobe, S.;
Nomoto, A.; Sonoda, M.; Ogawa, A. Organometallics 2011, 30, 6766; Palladium-
catalyzed anti-hydrothilation of alkynylphosphines was reported; see: Kondoh,
A.; Yorimitsu, H.; Oshima, K. Org. Lett. 2007, 9, 1383.
Acknowledgments
18. Although less likely, a radical mechanism cannot be rigorously excluded as far
as the reaction in ethylbenzene is concerned. When a reaction similar to entry
2, Table 1 was run in the presence of TEMPO (50 mol %) resulted in a 70%
conversion of 1a, giving (E)-3aA (5% yield), (Z)-3aA (13%), and 4aA (42%). On
the other hand, another reaction using AIBN (10 mol %) instead of the
palladium complex and PEt3 run at 80 °C for 6 h in benzene did not proceed
at all. We presume that the low yield of isomeric 3aA observed when TEMPO
was added is due to the deterioration of the catalyst.
We thank Professor Masaya Sawamura for providing Ph-SMAP.
This work was supported by a Grant-in-Aid for Scientific Research
on Priority Areas (No. 18065008) from MEXT, Japan.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.02.035.