Cobalt-catalyzed cross addition of silylacetylenes to internal alkynes

Article abstract of DOI:10.1021/jo400064b

A CoCl₂·6H₂O/Zn reagent using 2-(2,6-diisopropylphenyl)iminomethylpyridine (dipimp), 1,2- bis(diphenylphosphino)ethane (dppe), or 1,2-bis(diphenylphosphino)benzene (dppPh) as a ligand effectively catalyzed the cross-addition reaction

Full text of DOI:10.1021/jo400064b

Article
pubs.acs.org/joc
Cobalt-Catalyzed Cross Addition of Silylacetylenes to Internal
Alkynes
Tetsuya Sakurada, Yu-ki Sugiyama, and Sentaro Okamoto*
Department of Material and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
S
* Supporting Information
ABSTRACT: A CoCl2·6H2O/Zn reagent using 2-(2,6-
diisopropylphenyl)iminomethylpyridine (dipimp), 1,2-bis-
(diphenylphosphino)ethane (dppe), or 1,2-bis-
(diphenylphosphino)benzene (dppPh) as a ligand effectively
catalyzed the cross-addition reaction of silylacetylene to
internal alkynes. The reaction of some unsymmetrical internal
alkynes, such as 3-arylpropargyl alcohols, proceeded in a highly
regioselective manner in the presence of dppe or dppPh but gave a nearly 1:1 mixture of regioisomers in the presence of dipimp.
The results of reactions using 1-deuterated 2-silylacetylene revealed that the reaction involves a direct oxidative addition of the
silylacetylenic C−H bond to cobalt.
INTRODUCTION
■
Hydroalkynylation of alkynes is a straightforward, atom-
economical way to access conjugated enynes, which are versatile
as the main structures or substructures of organic materials and
biologically important molecules and as synthetic intermediates.1
Although the catalytic homodimerization of terminal alkynes
has been extensively studied,2 the cross addition of terminal
alkynes to internal alkynes is rather difficult because of
competitive homodimerization and oligomerization. Successful
cross addition of terminal alkynes to activated internal alkynes
bearing an electron-withdrawing group has been developed
using palladium,3 rhodium,4 iridium,5 ruthenium,6 titanium,7
and uranium8 catalysts. Alkyne addition to unactivated internal
alkynes is not easy and has been relatively less studied.
However, some successful examples using this process with
terminal silylacetylenes have recently been reported using
nickel,9 ruthenium,10 rhodium,11 iridium,12 and palladium13
catalysts. Most of these reactions, other than a few
exceptions,9b,c,10 used silylacetylenes with a bulky silyl group,
such as SiMe2(t-Bu), Si(i-Pr)3, and SiPh3, for minimizing the
competitive dimerization of the silylacetylene.
In the course of our study on cyclotrimerization reactions of
alkynes,14 we initially found that a 2-(2,6-diisopropylphenyl)-
iminomethylpyridine (dipimp)/CoCl2·6H2O/Zn catalyst (for
the structure of dipimp, see Figure 1) converted a mixture of
trimethylsilylacetylene (1a) and diphenylacetylene (2a) to the
hydroalkynylated product 3aa (99% E) in good yield without
any cyclotrimerization products (Scheme 1, R = Me, R′ = Ph).
Herein we report the development of the first cobalt-catalyzed
cross addition of terminal alkynes to internal alkynes,15−17 in
which 1a is used as the substrate.
Figure 1. Structures of ligands.
reacting 1a or triisopropylacetylene (1b) with 2a or 3-phenyl-2-
propyn-1-ol (2b) in N-methylpyrrolidone (NMP) as bench-
mark reactions (Scheme 1 and Table 1).
The substrates were recovered when ligands were not used
(Table 1, run 1). When dipimp was used as a ligand, (3:1)−
(1:3) mixtures of 1a and 2a yielded hydroalkynylated product
3aa in good yield (>84%) with >97% E geometric purity (runs
2−5). The use of a slight excess of silylacetylene (1.3 equiv)
was thereafter treated as a standard condition. Similarly, 1b
smoothly reacted with 2a to afford 86% yield of 3ba but with
somewhat reduced geometric purity (run 6).
In the reaction of diarylacetylene 2a, other ligands, such
as 2,2′-bipyridyl, triphenylphosphine, xantphos, 1,2-bis-
(diphenylphosphino)ethane (dppe), and 1,3-bis-
(diphenylphosphino)propane (dppp), resulted in comparably
low yields (runs 7−11). Reactions with a diphosphine ligand
often involved geometric isomerization (runs 8, 9, and 11).
For the reaction of 1 and unsymmetrical internal alkyne 2b,
which may result in the formation of the two regioisomers 3 and
4, reactions using various ligands were explored (runs 12−19).
Catalysis with dipimp, dppe, or dppPh proceeded smoothly and
RESULTS AND DISCUSSION
■
On the basis of the initial results mentioned above, we first
optimized the catalyst system and reaction conditions by
Received: January 12, 2013
© XXXX American Chemical Society
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Scheme 1. Addition of Silylacetylenes to Diphenylacetylene
and 3-Phenylprop-2-yn-1-ol
Scheme 2. Substrates for Cobalt-Catalyzed
Hydroalkynylation
Table 1. Optimization of the Addition of Silylacetylenes to
a
Diphenylacetylene and 3-Phenylprop-2-yn-1-ol
c
3:4 (% E or
total
d
b
c
run
ligand
substrate
1:2
Z of 3)
yield, %
1
1a + 2a
1a + 2a
1a + 2a
1a + 2a
1a + 2a
1b + 2a
1b + 2a
1b + 2a
1b + 2a
1b + 2a
1a, 2a
1.3:1
1:3
trace
e
2
dipimp
dipimp
dipimp
dipimp
dipimp
dipyr
(99 E)
84
e
3
1:1.3
1.3:1
3:1
(97 E)
90
4
(>99 E)
99
89
86
58
50
9
5
(98 E)
6
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
1.3:1
(92 E)
provided the corresponding hydroalkynylated products in good
yields and with high geometric purity. The reactions of aliphatic
alkynes 2e and 2h did not proceed as silylacetylene 1a was mainly
consumed through dimerization18a (runs 4 and 8). However, the
product 3af was quantitatively obtained from 1,4-dimethoxybut-2-
yne (2f), presumably because of the coordinating effect and/or
electronic (−I) effect of its propargylic oxygen(s) (run 5). The
catalyst loading could be reduced to 2 mol % without any loss of
activity (run 6). As shown in runs 9−11, unsymmetrical internal
alkynes 2, bearing an aromatic or silyl substituent, smoothly reacted
with 1a to afford a nearly 1:1 mixture of regioisomers 3 and 4.
Next, reactions using dppe or dppPh as the ligand were studied,
and the results are shown in Table 3 (Scheme 2). Similar to the
case for the dipimp/cobalt catalyst, dppe/CoCl2·6H2O/Zn did not
catalyze the reaction of aliphatic internal alkynes 2e and 2h (runs 1
and 2),18b but 2-butyn-1,4-diol derivatives 2f, 2l, and 2g were
smoothly converted to 3af, 3al, and 3ag, respectively, again
presumably owing to their coordinating effect and/or the electronic
(−I) effect of the oxygen atom (runs 3−7). Reducing the catalyst
loading to 2 mol % was possible without any loss of activity
(run 4). Aromatic substitution in the internal alkyne promoted the
reaction, affording the corresponding adduct(s) in moderate to good
yields (runs 8−16). In clear contrast to the reaction with dipimp, the
reactions of 3-aryl-2-propyn-1-ols 2j and 2k with dppe or dppPh
produced the highly predominant regioisomer 3 (runs 8−10). The
results of the reactions with substrates bearing an electron-donating
or -withdrawing group at the para position of the benzene ring
indicate that the electronic nature of the alkynes may affect the
regioselectivity. The reactions of phenylbutylacetylene (2m) and
secondary propargylic alcohol 2n were slow and gave a mixture of 3
and 4, respectively, in low yields with moderate regioselectivity
(runs 11−14). The use of a 1:3 mixture of 1a and 2n as substrates
did not improve the yield (run 13). The use of dppPh as a ligand
instead of dppe promoted selective formation of 3an but still in
low yield (run 14). However, in addition to phenyl-substituted 2,
7
(96 E)
8
dppe
(14 E)
9
dppp
(75 E)
10
11
12
13
14
15
16
17
18
19
2 PPh3
xantphos
dipimp
dipyr
(>99 E)
6
(2 E)
67
99
26
15
90
9
1a + 2b
1a + 2b
1a + 2b
1a + 2b
1a + 2b
1a + 2b
1a + 2b
1b + 2b
52:48 (>99 Z)
62:38 (>99 Z)
>99:1 (>99 Z)
96:4 (>99 Z)
75:15 (>99 Z)
>99:1 (>99 Z)
>99:1 (>99 Z)
dppm
dppe
dppp
dppPh
xantphos
2 PPh3
95
6
trace
a
Conditions: 1 (0.50−0.65 mmol), 2 (0.50−3 mmol), CoCl2·6H2O
(5 mol %), ligand (6 mol %), zinc powder (10 mol %), NMP (2.0 mL),
b
c
50 °C, 24 h. See Figure 1 for the ligand structures. Determined by
d
1H NMR analysis. Unless otherwise stated, the total yield of 3 and 4
1
is based on 2; determined by H NMR analysis using an internal
standard. Yield based on 1a.
e
quantitatively provided geometrically pure 3ab and 4ab (runs 12,
15, and 17). The reactions with dppe and 1,2-bis-
(diphenylphosphino)benzene (dppPh) showed high regioselec-
tivity, giving (Z)-2-benzylidene-4-(trimethylsilyl)but-3-yn-1-ol
(3ab) (runs 15 and 17); however, a nearly 1:1 mixture of 3ab
and 4ab was obtained from the reaction with dipimp (run 12).
With these results in hand, we selected the dipimp/
CoCl2·6H2O/Zn, dppe/CoCl2·6H2O/Zn, and dppPh/
CoCl2·6H2O/Zn catalysts for further exploration of reactions
with other representative substrates 2, as illustrated in Scheme 2.
The results of the reactions of various internal acetylenes 2
using dipimp as a ligand (Scheme 2) are summarized in Table 2.
The reactions of 2a (run 1) and diarylacetylenes with electron-
donating and -withdrawing groups (runs 2 and 3, respectively)
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a
Table 2. dipimp/Cobalt-Catalyzed Hydroalkynylation
a
Conditions: 1a (0.65 mmol), 2 (0.50 mmol), CoCl2·6H2O (5 mol %), ligand (6 mol %), zinc powder (10 mol %), NMP (2.0 mL), 50 °C, 24 h.
Determined by H NMR analysis. Total yield of 3 and 4. 2 mol % of CoCl2·6H2O and 2.4 mol % of dipimp were used.
b
c
d
1
heteroaryl- and alkenyl-substituted propargylic alcohols 2o, 2p, and
2q were exclusively converted to the corresponding 3 in good
yields (runs 15−17).
The results depicted in Scheme 3 demonstrate the
application of the method to double cross-addition reactions.
The reaction of 3,3′-(1,4-phenylene)bis(prop-2-yn-1-ol) (2r)
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a
Table 3. (dppe or dppPh)/Cobalt-Catalyzed Hydroalkynylation
a
Conditions: 1a (0.65 mmol), 2 (0.50 mmol), CoCl2·6H2O (5 mol %), ligand (6 mol %), zinc powder (10 mol %), NMP (2.0 mL), 50 °C, 24 h.
b
c
d
e
Determined by 1H NMR analysis. Total yield of 3 and 4. 2 mol % of CoCl2·6H2O and 2.4 mol % of dipimp were used. dppPh was employed as a
f
g
ligand instead of dppe. A 1:3 mixture of 1a and 2 was employed. Yields were based on 1a.
with silylacetylene gave the corresponding bis-enyne compound
3ar in 68% yield.
complete deuterium incorporation, but 3af was not deuterated
at all. The reaction was, therefore, found to be an evident
hydroalkynylation reaction, in which the olefinic hydrogen in the
product comes from silylacetylene, without any scrambling of the
hydrogen atoms.19 Therefore, we suggest that the reaction does
not involve a deprotonation/metalation of silylacetylenes 1 and
that the C−H cleavage of 1 and formation of the new C−H and
C−C bonds must occur on a single metal center of the catalyst.
To investigate the reaction with terminal acetylene substrates
other than silylacetylenes (Figure 2), we next performed the
reaction of 1-hexyne (1c) or 2-methylbut-3-yn-2-ol (1d) with
Next, reactions with deuterated silylacetylenes using anhydrous
CoCl2 were investigated, and the results are illustrated in Scheme 4.
Using dipimp as the ligand, the deuterated silylacetylene d-1b
(>98% D) reacted with 2f to afford 3bf (87% isolated yield) with
complete deuterium incorporation (>98% D) at the olefinic
position. Similar results were obtained using dppe as the ligand in-
stead of dipimp. The reaction of a 1:1 mixture of d-1b (0.5 mmol)
and 1a (0.5 mmol) with 1.3 mmol of 2f produced 3bf and 3af in
99% and 98% yields, respectively. The resulting 3bf showed
D
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Scheme 3. Double Addition Reaction
Scheme 5. Proposed Reaction Mechanism
Scheme 4. Results of the Reactions with Deuterated
Substrates
Scheme 6. Reactions of 1a with a (dipimp or dppe)/
CoCl2·6H2O/Zn Catalyst
alkynylmetalation of II with the internal alkyne. The results in
the tables indicate that the reactivity order of internal alkynes is
[ArCCAr, ArCCCH2OR, ROCH2CCCH2OR] >
[ArCC(alkyl), Me3SiCCCH2OR] ≫ [(alkyl)CC(alkyl),
(alkyl)CCCH2OR] for both dipimp/cobalt and dppe/cobalt
catalysts and that electrondeficient, less hindered alkynes are
more reactive. In the reactions of propargyl alcohols or ethers,
coordination by the propargylic oxygen did not affect either the
reactivity or the regioselectivity, and the CH2OR moiety acts as
an electron-withdrawing group (−I effect). The difference in
steric bulk between the two substituents of the internal alkynes
may determine the regioselectivity, and the selectivity was higher
in the reactions with dppe or dppPh than in those with dipimp,
because dppe and dppPh are more sterically demanding than
dipimp.
Although the reactions in the desired catalytic cycle, I → II →
III (or III′) → I, compete with the formation of a silylacetylene
dimer(s)2 via the common intermediate II, the reaction rate for
the formation of 3 is reasonably faster than that for dimerization
of the silylacetylene, although the dimer of the silylacetylene was
obtained as a byproduct in each reaction. Interestingly, the
coproduced dimer from the dipimp/cobalt system was 2,4-
bis(trimethylsilyl)but-1-en-3-yne (7), while the dimer obtained
Figure 2. Alkynes other than silylacetylenes.
2f in the presence of (dipimip or dppe)/CoCl2·6H2O/Zn in
NMP. None of these reactions afforded the corresponding
adduct 3. The reaction of 1c yielded homocyclotrimerization
product(s) 6c, while no reaction proceeded with alkyne 1d.
On the basis of these observations, we postulate two possible
mechanisms (Scheme 5) for the reaction. As described for other
metal cataysts,9−13 cobalt(I) species I, generated by reduction of
LnCoCl2 (L = ligand) with Zn powder, can undergo an oxidative
addition reaction with silylacetylenes to generate the alkynylco-
balt hydride complexes II, followed by a hydrometalation
reaction with internal alkynes, giving III, the reductive
elimination of which produces products 3 and regenerates I.
Alternatively, 3 can be produced by the reductive elimination of
alkenyl hydride complex III′, which would be generated via
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standard) in NMP (1.0 mL) was added, and the resulting mixture was
stirred at 50 °C for 24 h. After addition of saturated aqueous NH4Cl, the
mixture was extracted with Et2O. The combined organic layers were
washed with brine, dried over MgSO4, filtered through a pad of Celite,
and concentrated in vacuo to give a crude mixture, which was analyzed
by H NMR. The residue was purified by column chromatography on
silica gel to afford the hydroalkynylated product 3ak (148 mg, 98% yield
with >99% of Z geometric purity) as a colorless oil.
from the dppe/cobalt system was (E)-1,4- bis(trimethylsilyl)but-
1-en-3-yne (8). As shown in Scheme 6, treatment of 1a with
dipimp/CoCl2·6H2O/Zn or dppe/CoCl2·6H2O/Zn selectively
produces 720 and 8,21 respectively.
1
CONCLUSION
■
In summary, we found that a CoCl2·6H2O/Zn reagent catalyzes
addition of trimethylsilylacetylene or triisopropylsilylacetylene to
internal alkynes in the presence of a dipimp or 1,2-diphosphine
(dppe or dppPh) ligand to afford the corresponding but-1-en-3-
yne derivatives with high geometric purity. Some unsymmetrical
internal alkynes, such as 3-aryl- and 3-alkenylpropargyl alcohols,
were converted to the adduct with high regioselectivity in the
presence of dppe or dppPh. To the best of our knowledge, this
reaction is the first demonstration of the cobalt-catalyzed
hydroalkynylation of internal alkynes.
Ethyl (Z)-4-(2-(hydroxymethyl)-4-(trimethylsilyl)but-1-en-3-yn-1-
1
yl)benzoate (3ak): H NMR (CDCl3, 600 MHz) δ 8.03 (d, 2H, J =
8.4 Hz), 7.33 (d, 2H, J = 8.4 Hz), 7.04 (s, 1H), 4.38 (q, 2H, J =
7.2 Hz), 4.34 (d, 2H, J = 6.0 Hz), 1.99 (t, 1H, J = 6.0 Hz), 1.40 (t, 3H,
J = 7.2 Hz), 0.25 (s, 9H); 13C NMR (CDCl3, 150 MHz) δ 166.2,
139.8, 137.6, 129.9, 129.7, 129.0, 126.0, 104.7, 98.0, 61.2, 60.8, 14.4,
0.005; IR (neat) 3444, 2959, 2933, 2140, 1714, 1621, 1337, 1276
cm−1; HR-MS m/z calcd for C17H22NaO3Si [M + Na]+ 325.1236,
found 325.1226.
Ethyl (E)-4-(5-hydroxy-1-(trimethylsilyl)pent-3-en-1-yn-3-yl)-
benzoate (4ak): analytical sample obtained as a colorless oil from
the crude mixture by column chromatography (run 11, Table 2);
1H NMR (CDCl3, 600 MHz) δ 8.04 (d, J = 7.8 Hz, 2H), 7.41 (d, J =
7.8 Hz, 2H), 6.42 (t, 1H, J = 6.6 Hz), 4.39 (q, 2H, J = 7.2 Hz), 4.31−
4.26 (m, 2H), 1.47 (t, 1H, J = 6.0 Hz), 1.40 (t, 3H, J = 7.2 Hz), 0.20
(s, 9H); 13C NMR (CDCl3, 150 MHz) δ 166.2, 140.8, 139.0, 130.0,
129.4, 128.6, 125.3, 105.0, 94.9, 61.1, 59.8, 14.3, −0.17; IR (neat)
3352, 2956, 2930, 2136, 1509, 1298, 1187 cm−1; HR-MS m/z calcd for
C17H22NaO3Si [M + Na]+ 325.1236, found 325.1242.
EXPERIMENTAL SECTION
General Considerations. NMR spectra were recorded with use of
a solution in CDCl3 at 600, 500, and 270 MHz for 1H and 150 and 125
MHz for 13C. Chemical shifts are reported in parts per million (ppm, δ)
■
1
relative to Me4Si (δ 0.00) or residual CHCl3 (δ 7.26 for H NMR)
and CDCl3 (δ 77.0 for 13C NMR). IR spectra were recorded on an
FT-IR spectrometer. High-resolution mass spectra (HR-MS) were
measured on a TOF-MS equipped with an ESI ionization unit. All
reactions sensitive to oxygen and/or moisture were performed under
an argon atmosphere. All of the other commercially obtained
chemicals, unless otherwise indicated, were used as received. 2-(2,6-
Diisopropylphenyliminomethyl)pyridine (dipimp) was prepared from
pyridine-2-carboxaldehyde and 2,6-diisopropylaniline by the reported
procedure.22 Compounds 1a, 1b, 2a, 2b, 2e, 2f, 2h, 2l, 2m, and 2n are
available commercially. Compounds 2c and 2d and were synthesized
according to the procedure reported in the literature.23 Compounds
2g, 2i, 2j, and 2k were prepared by conventional reaction procedures.
1H NMR spectra of 720 and 821 obtained were in good agreement with
(E)-(3,4-Diphenylbut-3-en-1-yn-1-yl)triisopropylsilane (3ba):.9b
177 mg of 3ba obtained as a colorless oil in 98% isolated yield (run 4,
1
Table 1); H NMR (CDCl3, 600 MHz) δ 7.40−7.05 (m, 11H), 1.11
(br s, 21H); 13C NMR (CDCl3, 150 MHz) δ 137.5, 136.8, 136.1,
129.3, 129.1, 128.3, 128.1, 127.7, 127.6, 124.5, 109.7, 91.0, 18.7, 11.4;
IR (neat) 2952, 2929, 2864, 2140, 1714, 1592, 1487 1288 cm−1; HR-
MS m/z calcd for C25H33Si [M + H]+ 361.2352, found 361.2344.
(Z)-2-Benzylidene-4-(trimethylsilyl)but-3-yn-1-ol (3ab): 107 mg of
3ab obtained as a colorless oil in 93% isolated yield (run 17, Table 1);
1H NMR (CDCl3, 600 MHz) δ 7.36 (dd, 2H, J = 7.2 Hz, 7.2 Hz), 7.29
(dd, 1H, J = 7.2 Hz, 7.2 Hz), 7.26 (d, 2H, J = 7.2 Hz), 7.04 (s, 1H),
4.36 (d, 2H, J = 6.6 Hz), 1.97 (t, 1H, J = 6.6 Hz), 0.24 (s, 9H); 13C
NMR (CDCl3, 150 MHz) δ 138.7, 135.4, 129.0, 128.4, 128.1, 124.0,
105.0, 96.5, 60.7, −0.05; IR (neat) 3370, 3024, 2957, 2139, 1644,
1487, 1249 cm−1; HR-MS m/z calcd for C14H19OSi [M + H]+
231.1205, found 231.1213.
those reported.
Typical Procedure for Hydroalkynylation of Silylacetylenes
1 to Internal Alkynes 2 Catalyzed by a dipimp/CoCl2·6H2O/Zn
Reagent (Table 1, Run 3). To zinc powder (3.3 mg, 0.05 mmol) was
added a solution of CoCl2·6H2O (6.0 mg 0.025 mmol) and dipimp
(8.0 mg, 0.03 mmol) in NMP (1.0 mL). The resulting mixture was
stirred at ambient temperature for 1 h. A solution of trimethylsily-
lacetylene (1a; 90 μL, 0.65 mmol), diphenylacetylene (2a; 116 mg.
0.65 mmol), and hexamethylbenzene (40.6 mg, 0.25 mmol, used as an
internal standard) in NMP (1.0 mL) was added, and the resulting
mixture was stirred at 50 °C for 24 h. After addition of saturated
aqueous NH4Cl, the mixture was extracted with Et2O. The combined
organic layers were washed with brine, dried over MgSO4, filtered
through a pad of Celite, and concentrated in vacuo to give a crude
mixture, which was analyzed by 1H NMR. The residue was purified by
column chromatography on silica gel to afford the hydroalkynylated
product 3aa (124 mg, 90% yield with 97% of E geometric purity) as a
colorless oil.
(E)-3-Phenyl-5-(trimethylsilyl)pent-2-en-4-yn-1-ol (4ab): analytical
sample obtained as a colorless oil from the crude mixture by column
1
chromatography (run 12, Table 1); H NMR (CDCl3, 600 MHz) δ
7.40−7.29 (m, 5H), 6.36 (t, 1H, J = 6.8 Hz), 4.32−4.28 (m, 2H),
1.44−1.38 (br, 1H), 0.20 (s, 9H); 13C NMR (CDCl3, 150 MHz) δ
137.9, 136.3, 128.7, 128.2, 128.1, 126.0, 105.7, 94.2, 60.0, −0.11; IR
(neat) 3352, 2958, 2897, 2141, 1614, 1493, 1249 cm−1; HR-MS m/z
calcd for C14H18NaOSi [M + Na]+ 253.1025, found 253.1016.
(E)-(3,4-Bis(4-methoxyphenyl)but-3-en-1-yn-1-yl)trimethylsilane
(3ac):.24 135 mg of 3ac obtained as a colorless oil in 80% isolated yield
1
(run 2, Table 2); H NMR (CDCl3, 600 MHz) δ 7.32−7.28 (m, 2H),
7.04−7.00 (m, 2H), 6.96 (s, 1H), 6.84−6.81 (m, 2H), 6.71−6.67
(m, 2H), 3.81 (s, 3H), 3.76 (s, 3H), 0.21 (s, 9H); 13C NMR (CDCl3,
150 MHz) δ 159.1, 136.2, 130.7, 130.3, 129.9, 128.8, 121.6, 113.8, 113.5,
108.2, 93.3, 55.2 (2C), 0.05; IR (neat) 2960, 2928, 2128, 1505, 1303,
1258 cm−1; HR-MS m/z calcd for C21H25O2Si [M + H]+ 337.1624,
found 337.1631.
(E)-(3,4-Diphenylbut-3-en-1-yn-1-yl)trimethylsilane (3aa):9b 1H
NMR (CDCl3, 600 MHz) δ 7.38−7.33 (m, 2H), 7.30−7.26 (m, 3H),
7.16−7.12 (m, 3H), 7.08 (s, 1H), 7.07−7.02 (m, 2H), 0.22 (s, 9H); 13C
NMR (CDCl3, 150 MHz) δ 137.3, 136.0, 129.3, 129.1, 128.4, 128.1,
127.8, 127.7, 124.2, 107.6, 94.4, −0.01; IR (KBr) 3019, 2959, 2132,
1594, 1488, 1250 cm−1; HR-MS m/z calcd for C19H21Si [M + H]+
277.1413, found 277.1404.
Typical Procedure for Hydroalkynylation of Silylacetylenes
1 to Internal Alkynes 2 Catalyzed by a dppe/CoCl2·6H2O/Zn
Reagent (Table 3, Run 10). To zinc powder (3.3 mg, 0.05 mmol)
was added a solution of CoCl2·6H2O (6.0 mg, 0.025 mmol) and dppe
(12.0 mg, 0.03 mmol) in NMP (1.0 mL). The resulting mixture was
stirred at ambient temperature for 1 h. A solution of trimethylsilylace-
tylene (1a; 90 μL, 0.65 mmol), internal alkyne 2k (102 mg, 0.50 mmol),
and hexamethylbenzene (40.6 mg, 0.25 mmol, used as an internal
Diethyl (E)-4,4′-(4-(trimethylsilyl)but-1-en-3-yne-1,2-diyl)-
dibenzoate (3ad): 162 mg of 3ad obtained as a colorless oil in 77%
isolated yield (run 3, Table 2); 1H NMR (CDCl3, 600 MHz) δ 7.98−
7.93 (m, 2H), 7.84−7.80 (m, 2H), 7.42−7.37 (m, 2H), 7.15 (s, 1H),
7.06 (d, 2H, J = 10 Hz), 4.38 (q, 2H, J = 7.2 Hz), 4.33 (q, 2H, J =
7.2 Hz), 1.39 (t, 3H, J = 7.2 Hz), 1.36 (t, 3H, J = 7.2 Hz), 0.23 (s, 9H);
13C NMR (CDCl3, 150 MHz) δ 166.2, 166.1, 141.6, 140.0, 137.1,
129.8, 129.4, 129.2, 129.1, 125.5, 106.3, 96.7, 61.04, 60.98, 14.30,
14.27, −0.15; IR (neat) 2956, 2929, 2864, 2140, 1730, 1487, 1340,
F
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1252 cm−1; HR-MS m/z calcd for C25H28NaO4Si [M + Na]+
443.1655, found 443.1663.
J = 6.6 Hz), 4.21 (s, 2H), 2.35−2.25 (br, 1H), 2.09−1.99 (br, 1H),
0.20 (s, 9H); 13C NMR (CDCl3, 150 MHz) δ 138.4, 125.5, 104.2,
95.3, 60.6, 58.5, −0.13; IR (neat) 3176, 2972, 2937, 2156, 1660, 1404,
1243 cm−1; HR-MS m/z calcd for C9H16NaO2Si [M + Na]+ 207.0817,
found 207.0811.
(Z)-(5-Methoxy-3-(methoxymethyl)pent-3-en-1-yn-1-yl)-
trimethylsilane (3af):9b 104 mg of 3af obtained as a colorless oil in 98%
1
isolated yield (run 5, Table 2); H NMR (CDCl3, 600 MHz) δ 6.21
(t, 1H, J = 6.6 Hz), 4.09 (d, 2H, J = 6.6 Hz), 4.00 (s, 2H), 3.36 (s, 3H),
3.33 (s, 3H), 0.19 (s, 9H); 13C NMR (CDCl3, 150 MHz) δ 138.8, 122.7,
105.0, 93.8, 70.0, 68.4, 58.2, 58.1, −0.08; IR (neat) 2979, 2928, 2168,
1402, 1266, 1128 cm−1; HR-MS m/z calcd for C11H21O2Si [M + H]+
213.1311, found 213.1313.
(Z)-3-(((Triisopropylsilyl)oxy)methyl)-5-(trimethylsilyl)pent-2-en-
4-yn-1-ol (3ag): 44 mg of a mixture of 3ag and 4ag obtained as a
colorless oil in 26% isolated yield (run 7, Table 2); analytical sample
obtained from the crude mixture by column chromatography; 1H
NMR (CDCl3, 600 MHz) δ 6.25 (t, 1H, J = 6.6 Hz), 4.33 (s, 2H), 4.28
(dd, 2H, J = 6.6 Hz, J = 6.6 Hz), 2.41−2.55 (br, 1H), 1.09 (d, 21H,
J = 6.6 Hz), 0.17 (s, 9H); 13C NMR (CDCl3, 150 MHz) δ 139.2,
125.2, 105.3, 94.2, 62.3, 59.2, 17.9, 11.9, −0.16; IR (neat) 3312, 2956,
2934, 2866, 2144, 1733, 1248, 1095 cm−1; HR-MS m/z calcd for
C18H36NaO2Si2 [M + Na]+ 363.2152, found 363.2151.
(Z)-(3-Benzylidenehept-1-yn-1-yl)trimethylsilane (3am): 45 mg of
a mixture of 3am and 4am obtained as a colorless oil in 35% yield (run 11,
Table 3). 1H NMR selected peaks for 3am (CDCl3, 600 MHz): δ 7.37−
7.21 (m, 5H), 6.90 (s, 1H), 2.39−2.34 (m, 2H), 1.65−1.57 (m, 2H),
1.42−1.33 (m, 4H), 0.90 (t, 3H, J = 7.2 Hz), 0.22 (s, 9H). Selected peaks
1
of H NMR for (E)-trimethyl(3-phenyloct-3-en-1-yn-1-yl)silane (4am)
(CDCl3, 600 MHz): δ δ 7.37−7.21 (m, 5H), 6.25 (t, 1H, J = 7.8 Hz),
2.24−2.17 (m, 2H), 1.65−1.57 (m, 2H), 1.33−1.26 (m, 4H), 0.85 (t, 3H,
J = 7.2 Hz), 0.19 (s, 9H). 13C NMR of 3am and 4am (CDCl3, 150 MHz,
measured using a mixture of isomers): δ 141.6, 137.3, 136.7, 136.6, 128.79,
128.75, 128.2, 128.0, 127.3, 127.2, 125.6, 123.4, 107.8, 107.1, 93.8, 91.4,
31.6, 31.0, 30.6, 29.3, 22.4, 22.3, 13.9, 13.8, 0.05. IR (neat, measured using
a mixture of isomers): 2957, 2929, 2860, 1614, 1505, 1249 cm−1. HR-MS:
m/z calcd for C17H25Si [M + H]+ 257.1726, found 257.1721 (measured
using a mixture of isomers).
(Z)-4-((Triisopropylsilyl)oxy)-2-((trimethylsilyl)ethynyl)but-2-en-1-
ol (4ag): mixture of 3ag and 4ag obtained as a colorless oil in 26%
isolated yield (run 7, Table 2); analytical sample obtained from the crude
mixture by column chromatography; 1H NMR (CDCl3, 600 MHz)
δ 6.15 (t, 1H, J = 6.0 Hz), 4.38 (d, 2H, J = 6.0 Hz), 4.17 (d, 2H, J =
6.6 Hz), 2.36 (t, 1H, J = 6.6 Hz), 1.06 (d, 21H, J = 6.6 Hz), 0.20 (s, 9H,
Si(CH3)3); 13C NMR (CDCl3, 150 MHz) δ 138.9, 124.1, 104.2, 94.8,
60.9, 59.9, 17.9, 11.9, −0.07; IR (neat) 3466, 2952, 2933, 2857, 1714,
1260 1010 cm−1; HR-MS m/z calcd for C18H36NaO2Si2 [M + Na]+
363.2152, found 363.2148.
(Z)-4-(Trimethylsilyl)-2-((trimethylsilyl)methylene)but-3-yn-1-ol
(3ai): 45 mg of a mixture of 3ai and 4ai obtained as a colorless oil in 40%
isolated yield (run 9, Table 2); analytical sample obtained from the crude
mixture by column chromatography; 1H NMR (CDCl3, 600 MHz) δ 6.25
(s, 1H), 4.17 (d, 2H, J = 6.6 Hz), 1.77 (t, 1H, J = 6.6 Hz), 0.20 (s, 9H),
0.15 (s, 9H); 13C NMR (CDCl3, 150 MHz) δ 141.3, 138.0, 104.9, 95.4,
64.0, −0.10, −0.13; IR (neat) 3368, 2957, 2897, 2868, 2136, 1574, 1306
cm−1; HR-MS m/z calcd for C11H22NaOSi2 [M + Na]+: 249.1107, found
249.1101.
(Z)-3,5-Bis(trimethylsilyl)pent-2-en-4-yn-1-ol (4ai): mixture of 3ai
and 4ai obtained as a colorless oil in 40% isolated yield (run 9, Table 2);
analytical sample obtained from the crude mixture by column
chromatography; 1H NMR (CDCl3, 600 MHz) δ 6.76 (t, 1H, J =
6.6 Hz), 4.29−4.23 (m, 2H), 1.39−1.31 (br, 1H), 0.23 (s, 9H), 0.18
(s, 9H); 13C NMR (CDCl3, 150 MHz) δ 151.6, 127.3, 107.6, 97.2, 61.9,
−0.03, −0.21; IR (neat) 3307, 2958, 2899, 2873, 2121, 1514, 1249 cm−1;
HR-MS m/z calcd for C11H23OSi2 [M + H]+ 227.1287, found 227.1290.
(Z)-2-(4-Methoxybenzylidene)-4-(trimethylsilyl)but-3-yn-1-ol
(3aj): 90 mg of a mixture of 3aj and 4aj obtained as a colorless oil in 69%
isolated yield (run 8, Table 3); analytical sample was obtained from the
crude mixture by column chromatography; 1H NMR (CDCl3, 600 MHz)
δ 7.22 (d, J = 9.0 Hz, 2H), 6.98 (s, 1H), 6.89 (d, J = 9.0 Hz, 2H), 4.36
(d, 2H, J = 6.0 Hz), 3.82 (s, 3H), 1.97 (t, 1H, J = 6.0 Hz), 0.23 (s, 9H);
13C NMR (CDCl3, 150 MHz) δ 159.5, 138.5, 130.6, 128.1, 122.0, 113.9,
(Z)-3-Benzylidene-5-(trimethylsilyl)pent-4-yn-2-ol (3an): 40 mg of
a mixture of 3an and 4an obtained as a colorless oil in 33% isolated
yield (run 12, Table 3); analytical sample obtained from the crude
1
mixture by column chromatography; H NMR (CDCl3, 600 MHz)
δ 7.35 (dd, 2H, J = 7.2 Hz, 7.2 Hz), 7.31−7.24 (m, 3H), 6.97 (s, 1H),
4.80−4.73 (m, 1H), 1.84 (d, 1H, J = 7.8 Hz), 1.45 (d, 3H, J = 6.0 Hz),
0.25 (s, 9H); 13C NMR (CDCl3, 150 MHz) δ 137.2, 135.5, 128.8,
128.7, 128.4, 127.9, 103.2, 97.4, 64.4, 22.7, −0.001; IR (neat) 3361,
2959, 2928, 2139, 1517, 1384, 1250 cm−1; HR-MS m/z calcd for
C15H20NaOSi [M + Na]+: 267.1181, found 267.1185.
(E)-4-Phenyl-6-(trimethylsilyl)hex-3-en-5-yn-2-ol (4an): mixture of
3an and 4an obtained as a colorless oil in 33% isolated yield (run 11,
Table 3); analytical sample was obtained from the crude mixture by
1
column chromatography; H NMR (CDCl3, 600 MHz) δ 7.39−7.26
(m, 5H), 6.17 (d, 1H, J = 9.6 Hz), 4.54−4.47 (m, 1H), 1.67−1.56 (br, 1H),
1.32 (d, 3H, J = 6.6 Hz), 0.19 (s, 9H); 13C NMR (CDCl3, 150 MHz)
δ 142.1, 136.6, 128.7, 128.3, 128.0, 125.0, 105.9, 94.2, 64.7, 23.3, −0.11; IR
(neat) 3271, 2962, 2927, 2143, 1714, 1505, 1250 cm−1; HR-MS m/z calcd
for C15H20NaOSi [M + Na]+ 267.1181, found 267.1175.
(Z)-2-(Thiophen-2-ylmethylene)-4-(trimethylsilyl)but-3-yn-1-ol
(3ao): 73 mg of 3ao obtained as a colorless oil in 62% isolated yield
1
(run 15, Table 3); H NMR (CDCl3, 600 MHz) δ 7.38 (dd, J = 0.6,
4.8 Hz, 1H), 7.08 (br s, 1H), 7.07 (br d, J = 3.6 Hz, 1H), 7.05 (dd, J =
4.2, 5.4 Hz, 1H), 4.48 (dd, J = 1.2, 6.6 Hz, 2H), 1.97 (br t, J = 6.3 H,
1H), 0.23 (s, 9H); 13C NMR (CDCl3, 150 MHz) δ 138.4, 130.1,
129.7, 128.0, 127.6, 121.5, 104.7, 97.9, 60.9, −0.04; IR (neat) 3347,
2953, 2893, 2131, 1591, 1415, 1319, 1247, 1027, 873, 842, 757, 704
cm−1; HR-MS m/z calcd for C12H16NaOSSi [M + Na]+ 259.0589,
found 259.0585.
(Z)-2-(Pyridin-2-ylmethylene)-4-(trimethylsilyl)but-3-yn-1-ol
(3ap): 73 mg of 3ap obtained as a colorless oil in 63% isolated yield (run
16, Table 3); 1H NMR (CDCl3, 600 MHz) δ 8.58 (br d, J = 4.8 Hz, 1H),
7.72 (dt, J = 1.8, 7.8 Hz, 1H), 7.27 (d, J = 7.8 Hz, 1H), 7.20 (ddd, J = 1.2,
4.8, 7.8 Hz, 1H), 6.93 (s, 1H), 4.36 (s, 2H), 0.22 (s, 9H); 13C NMR
(CDCl3, 150 MHz) δ 154.1, 148.9, 137.4, 136.1, 131.0, 125.4, 122.2,
107.0, 98.0, 62.7, −0.13; IR (neat) 3561, 2956, 2896, 2339, 1606, 1587,
1582, 1471, 1427, 1252, 1144 cm−1; HR-MS m/z calcd for
C13H17NNaOSi [M + Na]+ 254.0977, found 254.0980.
(2Z,4E)-5-Phenyl-2-((trimethylsilyl)ethynyl)penta-2,4-dien-1-ol
(3aq): 64 mg of 3aq obtained as a colorless oil in 50% isolated yield
(run 17, Table 3); 1H NMR (CDCl3, 600 MHz) δ 7.43 (d, J = 7.8 Hz,
2H), 7.33 (t, J = 7.5 Hz, 2H), 7.27 (t, J = 7.2 Hz, 1H), 7.06 (dd, J = 12,
15.6 Hz, 1H), 6.69 (d, J = 12 Hz, 1H), 6.66 (d, J = 16 Hz, 1H), 4.38
(d, J = 6 Hz, 2H), 1.81 (t, J = 6.3 Hz, 1H), 0.23 (s, 9H); 13C NMR
(CDCl3, 150 MHz) δ 138.2, 136.6, 136.4, 128.7, 128.4, 126.9, 123.0,
122.5, 105.5, 97.9, 60.3, −0.02; IR (neat) 3408, 3032, 2958, 2896,
2129, 1612, 1448, 1249, 1158, 1141, 1018, 992, 969, 875, 754, 690 cm−1;
HR-MS m/z calcd for C16H20NaOSi [M + Na]+ 279.1181, found
279.1176.
105.4, 95.9, 60.9, 55.2, 0.009; IR (neat) 3378, 2960, 2928, 2142, 1604,
1339, 1219 cm−1; HR-MS m/z calcd for C15H21O2Si [M + H]+ 261.1311,
found 261.1302.
(E)-3-(4-Methoxyphenyl)-5-(trimethylsilyl)pent-2-en-4-yn-1-ol
(4aj): mixture of 3aj and 4aj obtained as a colorless oil in 61% isolated
yield (run 10, Table 2); analytical sample obtained from the crude mixture
by column chromatography; 1H NMR (CDCl3, 600 MHz) δ 7.28 (d, J =
9.0 Hz, 2H), 6.90 (d, J = 9.0 Hz, 2H), 6.29 (t, 1H, J = 6.6 Hz), 4.34−4.28
(m, 2H), 3.83 (s, 3H), 1.40 (t, 1H, J = 6.0 Hz), 0.20 (s, 9H); 13C NMR
(CDCl3, 150 MHz) δ 159.4, 136.7, 130.0, 128.7, 125.5, 113.6, 106.0, 93.8,
106.0, 93.8, 60.0, 55.3, −0.08; IR (neat) 3422, 2959, 2923, 2140, 1505,
1278, 1179 cm−1; HR-MS m/z calcd for C15H20NaO2Si [M + Na]+
283.1130, found 283.1136.
(Z)-2-((Trimethylsilyl)ethynyl)but-2-ene-1,4-diol (3al): 90 mg of
3al obtained as a colorless oil in 98% isolated yield (run 5, Table 3);
1H NMR (CDCl3, 600 MHz) δ 6.22 (t, 1H, J = 6.6 Hz), 4.28 (d, 2H,
G
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(2Z,2′Z)-2,2′-(1,4-Phenylenebis(methanylylidene))bis(4-(trimethyl-
silyl)but-3-yn-1-ol) (3ar): 130 mg of 3ar obtained as a colorless oil in
68% yield (Scheme 3); H NMR (CDCl3, 600 MHz) δ 7.26 (s, 4H),
(6) (a) Yi, C. S.; Liu, N. Organometallics 1998, 17, 3158.
(b) Nishiyama, T.; Washitake, Y.; Uemura, S. Adv. Synth. Catal.
2007, 349, 2563.
1
7.01 (s, 2H), 4.36 (d, J = 6 Hz, 4H), 1.97 (t, J = 6.3 Hz, 2H), 0.24 (s,
18H); 13C NMR (CDCl3, 150 MHz) δ 138.0, 135.2, 129.1, 124.6,
104.9, 97.3, 60.8, −0.04; IR (neat) 3434, 2957, 2897, 2864, 2143,
1407, 1249, 1167, 1046, 970, 885, 759, 727 cm−1; HR-MS m/z calcd
for C12H16NaOSSi [M + Na]+ 259.0589, found 259.0585.
(7) Akita, M.; Yasuda, H.; Nakamura, A. Bull. Chem. Soc. Jpn. 1984,
57, 480.
(8) Wang, J.; Kapon, M.; Berthet, J. C.; Ephritikhine, M.; Eisen, M. S.
Inorg. Chim. Acta 2002, 334, 183.
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Chem. Commun. 1988, 804. (b) Matsuyama, N.; Tsurugi, H.; Satoh, T.;
Miura, M. Adv. Synth. Catal. 2008, 350, 2274. (c) Matsuyama, N.;
Hirano, K.; Satoh, T.; Miura, M. J. Org. Chem. 2009, 74, 3576.
(d) Ogata, K.; Murayama, H.; Sugasawa, J.; Suzuki, N.; Fukuzawa, S. J.
Am. Chem. Soc. 2009, 131, 3176. (e) Ogata, K.; Atsumi, Y.; Fukuzawa,
S. Org. Lett. 2011, 13, 122. For addition of terminal acetylenes to 1,3-
dienes, methylidenecyclopropanes, styrenes, norbornenes, and their
derivatives, see: (f) Shirakura, M.; Suginome, M. J. Am. Chem. Soc.
2008, 130, 5410. (g) Shirakura, M.; Suginome, M. Org. Lett. 2009, 11,
523. (h) Shirakura, M.; Suginome, M. J. Am. Chem. Soc. 2009, 131,
(Z)-Triisopropyl(5-methoxy-3-(methoxymethyl)pent-3-en-1-yn-1-yl)-
silane (d-3bf):13 146 mg of d-3bf obtained as a colorless oil in 98%
1
yield (Scheme 4); H NMR (CDCl3, 600 MHz) δ 4.09 (s, 2H), 4.02
(s, 2H), 3.36 (s, 3H), 3.35 (s, 3H, CH2OCH3), 1.08 (d, 21H); 13C
NMR (CDCl3, 150 MHz) δ 137.8 (t, J = 24 Hz), 123.0, 107.0, 90.3,
70.2, 68.3, 58.3, 58.0, 18.6, 11.2; IR (neat) 2931, 2864, 2144, 1714,
1260, 1192 cm−1; HR-MS m/z calcd for C17H31DNaO2Si [M + Na]+
320.2132, found 320.2132.
ASSOCIATED CONTENT
* Supporting Information
Figures giving spectroscopic data for the compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
5060. (i) Villarino, L.; Lopez, F.; Gastedo, L.; Mascarenas, J. L. Chem.
́
̃
Eur. J. 2009, 15, 13308. (j) Shirakawa, M.; Suginome, M. Angew.
Chem., Int. Ed. 2010, 49, 3827. (k) Fan, B.-M.; Yang, Q.; Hu, J.; Fan,
C.; Li, S.; Yu, L.; Huang, C.; Tsang, W. W.; Kwong, F. Y. Angew.
Chem., Int. Ed. 2012, 51, 7821. (l) Hu, J.; Yang, Q.; Xu, J.; Huang, C.;
Fan, B.; Wang, J.; Lin, C.; Bian, Z.; Chan, A. S. C. Org. Biomol. Chem.
2013, 11, 814.
AUTHOR INFORMATION
Corresponding Author
*S.O.: e-mail, okamos10@kanagawa-u.ac.jp; tel, +81-45-481-5661;
fax, +81-45-413-9770.
■
(10) (a) Yi, C. S.; Liu, N. Organometallics 1998, 17, 3158.
(b) Katayama, H.; Yari, H.; Tanaka, M.; Ozawa, F. Chem. Commun.
2005, 4336. See also ref 6a.
(11) (a) Katagiri, T.; Tsurugi, H.; Funayama, A.; Satoh, T.; Miura, M.
Chem. Lett. 2007, 36, 830. (b) Nishimura, T.; Guo, X.-X.; Ohnishi, K.;
Hayashi, T. Adv. Synth. Catal. 2007, 349, 2669. See also ref 9c.
(12) Ogata, K.; Oka, O.; Toyota, A.; Suzuki, N.; Fukuzawa, S. Synlett
2008, 2663.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Scientific Frontier Research Project of the
Ministry of Education, Culture, Sports, Science and Technol-
ogy (MEXT) of Japan for financial support. We also thank the
Japan Society for the Promotion of Science (JSPS) for a grant
to Y.S.
(13) Tsukada, N.; Ninomiya, S.; Aoyama, Y.; Inoue, Y. Org. Lett.
2007, 9, 2919.
(14) (a) Saino, N.; Amemiya, F.; Tanabe, E.; Kase, K.; Okamoto, S.
Org. Lett. 2006, 8, 1439. (b) Goswami, A.; Ito, T.; Okamoto, S. Adv.
Synth. Catal. 2007, 349, 2368. (c) Saino, N.; Kawaji, T.; Ito, T.;
Matsushita, Y.; Okamoto, S. Tetrahedron Lett. 2010, 51, 1313.
(d) Watanabe, J.; Sugiyama, Y.; Nomura, A.; Azumatei, S.; Goswami,
A.; Saino, N.; Okamoto, S. Macromolecules 2010, 43, 2213.
(e) Sugiyama, Y.; Kato, R.; Sakurada, T.; Okamoto, S. J. Am. Chem.
Soc. 2011, 133, 9712.
(15) Reviews for alkyne cyclotrimerization: (a) Grotjahn, D. B. In
Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F. G. A.,
Wilkinson, G., Hegedus, L. S., Eds.; Pergamon: Oxford, U.K., 1995;
Vol. 12, p 741. (b) Schore, N. E. In Comprehensive Organic Synthesis;
Trost, B. M., Fleming, I., Paquette, L. A., Eds.; Pergamon: Oxford,
U.K., 1991; Vol. 5, p 1129. (c) Saito, S.; Yamamoto, Y. Chem. Rev.
2000, 100, 2901. (d) Malacria, M.; Aubert, C.; Renaud, J. L. In Science
of Synthesis: Houben-Weyl Methods of Molecular Transformations;
Lautens, M., Trost, B. M., Eds.; Georg Thieme Verlag: Stuttgart,
Germany, 2001; Vol. 1, pp 439−530. (e) Yamamoto, Y. Curr. Org.
Chem. 2005, 9, 503. (f) Kotha, S.; Brahmachary, E.; Lahiri, K. Eur. J.
Org. Chem. 2005, 4741. (g) Gandon, V.; Aubert, C.; Malacria, M. Curr.
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