Synthesis of multiply functionalized benzenes via ruthenium-catalyzed cycloaddition of diiododiynes

Article abstract of DOI:10.1016/j.tet.2007.09.086

Highly functionalized benzenes were precisely synthesized via multi-step processes consisting of ruthenium-catalyzed [2+2+2] cycloaddition of diiododiynes with an ethynylboronate or terminal alkynes, and subsequent chemo- and regio-selective palladium-cat

Full text of DOI:10.1016/j.tet.2007.09.086

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Tetrahedron 64 (2008) 847e855
www.elsevier.com/locate/tet
Synthesis of multiply functionalized benzenes via ruthenium-catalyzed
cycloaddition of diiododiynes
*
Yoshihiko Yamamoto , Kozo Hattori
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Received 27 June 2007; accepted 17 September 2007
Available online 24 October 2007
Abstract
Highly functionalized benzenes were precisely synthesized via multi-step processes consisting of ruthenium-catalyzed [2þ2þ2] cyclo-
addition of diiododiynes with an ethynylboronate or terminal alkynes, and subsequent chemo- and regio-selective palladium-catalyzed CeC
bond-forming reactions of the resulting cycloadducts. The sequential cycloaddition/coupling process was applied to the synthesis of oligo-
( p-phenylene ethynylene)s.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
cyclooctadiene).6 We further succeeded in applying the
Cp*RuCl-catalyzed cyclotrimerization to alkynylboronates to
obtain cycloadducts without loss of the boronate moiety, pro-
viding powerful synthetic routes to highly functionalized ben-
zenes in combination with various transition-metal-catalyzed
transformations of the resultant arylboronates.7 Then, we also
realized for the first time the Cp*RuCl-catalyzed cycloaddition
of diiodo-1,6-diynes with acetylene to furnish p-diiodobenz-
enes, which were further subjected to two-fold palladium-
catalyzed couplings such as Sonogashira, MizorokieHeck,
and SuzukieMiyaura reactions (Scheme 1).8 Although this
sequential process made the assembly of highly conjugated
molecules straightforward, the obtained products were limited
to symmetrical systems.8a
Despite its synthetic performance assembling highly
substituted benzenes in a single operation, transition-metal-
catalyzed [2þ2þ2] cyclotrimerization of alkynes has been
considered to be less useful in synthetic organic chemistry.1,2
This is because intermolecular reactions of several different
alkynes usually lead to a mixture of cycloadducts as a result
of low chemo- and regio-selectivities. Thus, considerable
efforts have mainly focused on intramolecular variants with
diynes and triynes, providing a reliable route to polycyclic
benzene derivatives.3
Numerous transition-metal elements have been found to
promote alkyne cyclotrimerizations, and most attention has
particularly focused on group 9 and 10 transition elements
such as Co, Rh, Ir, Ni, and Pd.1,3 With respect to group 8
triads, however, some stoichiometric and catalytic cyclotrime-
rizations with limited scope have been reported.4 Recently,
catalytic intramolecular cyclotrimerizations of triynes and
1,6-diynes were accomplished by Blechert, Witulski,
and their co-workers with first generation Grubbs catalyst,5
and by us with Cp*RuCl(cod) (1: Cp*¼h5-C5Me5, cod¼1,5-
X
X
5 mol %
[Cp*RuCl(cod)] 1
1 atm acetylene
DCE, rt, 0.5 h
I
I
I
I
styrene
or
cat. Pd
phenylacetylene
or
phenylboronic acid
X
R = CH=CHPh,
C≡CPh, Ph
R
R
* Corresponding author. Tel./fax: þ81 3 5734 3339.
E-mail address: omyy@apc.titech.ac.jp (Y. Yamamoto).
Scheme 1.
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.09.086

848
Y. Yamamoto, K. Hattori / Tetrahedron 64 (2008) 847e855
Table 1
Cycloaddition of diiododiynes 2aef with ethynylboronate 3a
X
X
Ru
Run
Product
Yield (%)
I
I
I
I
I
I
+
R
PdLn
Figure 1.
PdLn
R
O
1
4a, 81 (87)b
O
B
O
One can envision that the stepwise derivatization of the
I
I
two CeI bonds of the diiodobenzenes may lead to a diverse
array of unsymmetrical conjugated molecules. Toward this
aim, an unsymmetrical cycloadduct derived from a diiodo-
diyne and a terminal alkyne is an ideal scaffold, where one
of the two CeI bonds would be strictly discriminated from
the other with the difference in their steric environments im-
posed by the substituent R (Fig. 1). In addition, if the R group
is a boronate, three different catalytic couplings would be
sequentially executed on the single aromatic nucleus. In this
paper, we wish to report our study along these lines on the
synthesis of highly functionalized benzenes from
diiododiynes.
TsN
2
3
4
5
4b, 85
4c, 66
4d, 54
O
B
O
I
MeO2C
MeO2C
O
B
O
I
I
I
NC
NC
O
B
O
I
2. Results and discussion
O
O
4e, 73
4f, 82
O
B
O
2.1. Cp*RuCl-catalyzed cycloaddition of diiododiynes
with ethynylboronate
I
I
I
With the goal of developing the efficient synthetic route
to highly substituted unsymmetrical benzenes, we first
investigated the cycloaddition of the diiododiynes with
2-ethynyl-5,5-dimethyl-1,3,2-dioxaborinane (3), yielding
unsymmetrical p-diiodobenzenes possessing a CeB bond,
which would be a useful synthetic handle for further chemo-
selective functionalization. Diiododiynes 2aef, which were
used in our previous study,8a reacted with 3 equiv of 3 in
the presence of 10 mol % 1 at room temperature for 12 h
(Scheme 2). Purification by silica gel chromatography gave
the desired cycloadducts 4aef bearing a boronate moiety as
well as two CeI bonds in the yields summarized in Table
1. The ruthenium-catalyzed cycloaddition tolerated func-
tional groups such as an ether, a sulfonamide, an ester, and
a nitrile (runs 1e4). As opposed to conventional aromatic io-
dination under acidic conditions,9 an acid-sensitive acetal can
be used as a protecting group (run 5). Polyaromatic product
4f was also synthesized in a good yield with an exact substi-
tution pattern (run 6).
6
O
B
O
a
A solution of 2 (0.3 mmol) in DCE was added to a DCE solution of
10 mol % 1 and ethynylboronate 3 (0.9 mmol) by a syringe over 15 min,
and the solution was stirred for 12 h at room temperature.
b
Yield from 5 mmol of 2a.
2.2. Differentiation of two CeI bonds with Sonogashira
coupling and consecutive functionalization of
diiodophenylboronate by three different catalytic CeC
bond formations
Having obtained the cycloadducts with three reactive CeX
bonds (X¼I or B(OR)2), we turned our attention to selective
transformations of these bonds by taking advantage of palla-
dium-catalyzed cross coupling techniques. At the outset, we
attempted the differentiation of the two CeI bonds in different
steric environments. Toward this end, Sonogashira coupling is
the method of choice,10 because it enables the formation of
a Csp2eCsp bond without affecting the boronate moiety as re-
ported in the recent literature.11 In the presence of 5 mol %
PdCl2(PPh3)2 and 10 mol % CuI, boronate 4a reacted with
O
O
I
I
+
X
B
2
3 (3 equiv)
I
I
i
1.5 equiv of phenylacetylene in Pr2NH at room temperature
10 mol% 1
DCE, rt, 12 h
X
O
(Table 2, run 1). As a result, 4a was completely consumed
within 2 h, but the expected mono coupling product 6a was ac-
companied by the undesired double coupling product 7a. Their
isolated yields were 43% and 33%, respectively. To improve
B
O
4
Scheme 2.

Y. Yamamoto, K. Hattori / Tetrahedron 64 (2008) 847e855
849
Table 2
Sonogashira coupling of diiodobenzenes 4a, 5aed with phenylacetylene
O
O
O
5 mol%
PdCl2(PPh3)2
+
+
Ph
1.5 equiv
Ph
Ph
I
I
I
Ph
10 mol% CuI
conditions
R
R
R
4a, 5a-d
6
7
Run
1
Diiodide, R
Solvent
iPr2NH
Additive (1.5
equiv)
T (ꢀC)
t (h)
Product, yields (%)
6
7
d
rt
2
6a, 43
7a, 33
O
4a, B
O
5a, nBu
5b, Ph
5c, OnOct
5d, CO2Me
2
3
4
5
6
tBuOMe
tBuOMe
tBuOMe
tBuOMe
tBuOMe
iPr2NH
iPr2NH
iPr2NH
iPr2NH
iPr2NH
0
0
0
0
0
12
4
6a, 64
6b, 90
6c, 91
6d, 94
6e, 90
7a, 17
7b, <5
7c, <5
7d, <5
7e, <5
8
4
6
the selectivity, the reaction was repeated at 0 ꢀC otherwise un-
der the same conditions. Although the formation of 7a was
completely suppressed, the reaction failed to go to completion
within 24 h, and 6a turned out to be inseparable from remain-
ing starting material 4a by silica gel flash chromatography.
This result forced us to search for a more efficient Sonoga-
shira-coupling protocol. Thus, with a decreased amount of
iPr2NH (1.5 equiv), the same coupling reaction was carried
O
5 mol% Pd(OAc)2
10 mol% PPh3
4a
I
I
1 atm CO
p-benzoquinone
MeOH, rt, 1h
CO2Me
5d 71%
O
Table 2
run 6
Ph
I
t
out in less coordinating solvent BuOMe at 0 ꢀC (run 2). As
CO Me
6e 90%
2
expected, the reaction reached completion within 12 h, and
6a was obtained in an increased yield of 64%. Since further
attempts failed to improve the yield of 6a, the impact of the
substituent R on the selectivity was next investigated (runs
3e5). To our surprise, the Sonogashira coupling of n-butyl-
and phenyl-substituted 5a and 5b completed within 4 and
8 h, respectively, under the optimal conditions. In addition,
the desired mono coupling products 6b and 6c were almost ex-
clusively obtained in more than 90% isolated yields. With
these encouraging results in hand, the electronic influence of
the substituent R was further examined (runs 5 and 6). Elec-
tron-rich ether 5c was prepared in 88% yield from boronate
4a by the treatment with hydrogen peroxide under basic con-
ditions and subsequent etherification of the resultant phenol
(Scheme 3), while electron-deficient benzoate 5d was obtained
in 71% yield via Pd(II)-catalyzed methoxycarbonylation of 4a
with our own protocol (Scheme 4).7d,e The Sonogashira coup-
ling of 5c and 5d uneventfully proceeded under the same
2.5 mol% Pd2(dba)3
MeO
11 mol% SPhos
K3PO4, toluene
100 °C, 1 h
B(OH)2
OMe
O
Ph
CO2Me
8 86%
Scheme 4.
conditions to selectively afford the corresponding diarylacetyl-
enes 6d and 6e in 94% and 90% yields, respectively. These
results clearly suggested that lower selectivity for 4a has no
relation to the electronic nature of the boronate moiety. It is
tentatively proposed that the palladium species released after
the first coupling might be directed to the second coupling
by the coordination with the oxygen atom on the boronate
goup.12
O
H2O2, aq. NaOH
According to the above notion, we concluded that the boro-
nate group should be converted to other groups at the very early
stage of the consecutive functionalization of the diiodophenyl-
boronates. Along this line, we carried out first the palladium-
catalyzed methoxycarbonylation of 4a to obtain benzoate 5d,
which then successfully underwent highly selective Sonoga-
shira coupling with phenylacetylene to give 6e in 90% yield.
Finally, SuzukieMiyaura coupling of 6e and p-methoxyphe-
nylboronic acid was executed in the presence of 2.5 mol %
Pd2(dba)3, 11 mol % SPhos (2-dicyclohexylphosphino-20,60-
4a
I
I
THF, rt, 6 h
OH
O
CH3(CH2)7I
3 equiv
I
I
K2CO3
Acetone reflux, 3 h
O(CH2)7CH3
5c 88%
Scheme 3.

850
Y. Yamamoto, K. Hattori / Tetrahedron 64 (2008) 847e855
dimethoxybiphenyl),13 and K3PO4 in toluene at 100 ꢀC for 1 h
to afford highly substituted biphenyl 8 in 86% yield
(Scheme 4).
homo coupling became conspicuous as bulkier 9 was used in
place of phenylacetylene under exactly the same conditions,
but a similar trend has been observed in earlier studies.15
This type of homo coupling is considered to catalytically pro-
ceed in the presence of CuI and adventitious molecular oxy-
gen.16 To suppress homo coupling, copper-free protocols
have been developed.17 Thus, in the absence of CuI, 9 and
1.1 equiv 5a was treated with 2.5 mol % [(h3-C3H5)PdCl]2,
10 mol % PPh3, and 2 equiv of DABCO in DMF at room
temperature for 20 h (a modified Soheili’s protocol).17e This
copper-free reaction, however, failed to go to completion, and
gave 10 and 11 in 62% and 30% yields, respectively, with the
34% recovery of 5a. According to the report of Ho and
co-workers,18 we also attempted to inhibit oxidative coupling
by carrying out Sonogashira coupling under a reductive atmo-
sphere of H2þN2, but no improvement was observed. Finally,
slow addition of 9 into the reaction mixture had no favorable
effect.
2.3. Synthesis of oligo( p-phenylene ethynylene)s via
iterative Sonogashira coupling
Having secured the site-selective transformation of the un-
symmetrical diiodobenzenes, we then attempted the synthesis
of oligo( p-phenylene ethynylene)s by means of iterative Sono-
gashira coupling. As highlighted in a recent review of Klok
and co-workers, a repetitive coupling strategy is quite advan-
tageous for the synthesis of uniform conjugated oligomers
that have attracted considerable interest as molecular electron-
ics.14 As the starting point, 6b was subjected to the Sonoga-
shira coupling with trimethylsilylacetylene (Scheme 5). The
reaction, however, led to the incomplete conversion of 6b
within 24 h under the conditions optimized for the synthesis
of 6. Thus, we employed the microwave (MW) irradiation
conditions that proved to be effective in the recent study by
Wang and co-workers.11 To our delight, 6b was totally con-
sumed within 15 min upon treatment with 5 mol %
Although the oxidative homo coupling of arylalkynes of-
ten hampered the Sonogashira coupling route to oligo(phenyl-
ene ethynylene)s, the formed 1,3-diyne derivatives are also
valuable rod-like conjugate molecules, having potential ap-
plications as functional materials.19 Therefore, the catalytic
protocol providing an easy access to this structural motif
is highly important. As shown in Scheme 5, high-yielding
and exclusive formation of 11 was achieved, when 9 was
simply treated with 5 mol % PdCl2(PPh3)2, 10 mol % CuI,
i
PdCl2(PPh3)2/CuI and 15 equiv of Pr2NH under MW irradia-
tion in DMF at 120 ꢀC, and desilylation with K2CO3 in MeOH
of the crude coupling product gave the desired terminal alkyne
9 in 96% yield. The next coupling of 9 with diiodide 5a was
carried out at room temperature to furnish 10 in 69% yield
along with a noticeable amount of homo dimer 11 (17%). It
is worthy to note that the Cu/Pd ratio plays a critical role in
the selective formation of 10 over 11. In fact, our Sonoga-
shira-coupling protocol for the synthesis of 6 (Cu/Pd¼2:1) re-
sulted in the exclusive formation of 11 in 73% yield, while
36% of 9 was recovered intact along with 10 and 11 (46%
and 9% yields, respectively) upon treatment with a decreased
amount of CuI (Cu/Pd¼1:5) for 24 h. These results indicate
that the copper salt is required for the efficient conversion,
while its increased loading facilitates the undesired Glaser-
type homo coupling. We have been unable to ascertain why
i
t
and 1.5 equiv of Pr2NH in BuOMe at room temperature
for 1 h.
Further elongation of 10 was uneventfully accomplished by
repeating the Sonogashira coupling with trimethylsilylacetyl-
ene followed by desilylation, furnishing 12 in 82% yield
(Scheme 6). Unfortunately, the subsequent coupling of 12
with 5a led to an intractable mixture of the starting materials
and the expected cross/homo coupling products. The Sonoga-
shira coupling of 10 with phenylacetylene was executed under
the MW irradiation conditions to afford unsymmetrical
oligo( p-phenylene ethynylene) 13 in 86% yield (Scheme 6).
O
1) 3 mol% PdCl2(PPh3)2 / CuI
i
15 equiv Pr2NH
DMF, MW 120 °C, 15 min
6a
+
+
SiMe3
1.5 equiv
2) K2CO3, MeOH, rt, 1 h
n
9 96%
Bu
O
O
5 mol% PdCl2(PPh3)2
2.5 mol% CuI
5a
11 17%
+
9
I
i
1.5 equiv Pr2NH
BuOMe, rt, 2 h
1.1 equiv
t
n
n
10 69%
Bu
Bu
O
O
5 mol% PdCl2 (PPh3)2
10 mol% CuI
9
i
1.5 equiv Pr2NH
t
BuOMe, rt, 1 h
n
n
Bu
11 83%
Bu
Scheme 5.

Y. Yamamoto, K. Hattori / Tetrahedron 64 (2008) 847e855
851
O
O
1) 3 mol% PdCl2(PPh3)2 / CuI
i
15 equiv Pr2NH
DMF, MW 120 °C, 15 min
2) K2CO3, MeOH, rt, 1 h
+
+
10
SiMe3
1.5 equiv
n
n
Bu
Bu
12 82%
O
O
3 mol% PdCl2(PPh3)2
3 mol% CuI
10
Ph
i
15 equiv Pr2NH
1.5 equiv
DMF, MW 120 °C, 15 min
n
n
Bu
Bu
13 86%
Scheme 6.
3. Conclusion
4.2. Synthesis of Cp*RuCl(cod) (1)
20
We have succeeded in the selective cycloaddition of diiodo-
diynes with 2-ethynyl-5,5-dimethyl-1,3,2-dioxaborinane under
the ruthenium catalysis to obtain diiodophenylboronates in
54e87% yields, and further precise transformations of the
cycloadducts into highly functionalized aromatic molecules.
We found that a mild Sonogashira-coupling protocol is highly
effective for the differentiation of the two CeI bonds of the
unsymmetrical cycloadducts, providing that the boronate
moiety is converted into other groups prior to the site-selective
Sonogashira coupling. Finally, we synthesized several oligo-
( p-phenylene ethynylene)s by means of the iterative Sonoga-
shira-coupling strategy.
In a 100 mL flask, [Cp*RuCl2]n (1.35 g, 2.20 mmol) and
1,5-cyclooctadiene (15 mL, 122.5 mmol) were heated in de-
gassed refluxing EtOH for 1 h under Ar atmosphere. After
cooling to room temperature, the solvent was roughly removed
by a rotary evaporator, and the residue was subjected to short
column chromatography on silica gel (ca. 10 g) eluted with
dichloromethane to remove excess 1,5-cyclooctadiene and
insoluble materials. The orange band was collected and the
concentrated rapidly to ca. one third volume by a rotary eva-
porator. To the concentrated solution was added small volume
of ether and the solution was rapidly cooled in dry ice/EtOH
bath to give rise to precipitates. The precipitates were col-
lected by filtration and washed with ether. Finally, the desired
complex 1 was obtained as golden micro plates (1.09 g, 65%).
Analytical data of 1 was reported in the original report by
Suzuki and Moro-oka.20
4. Experimental
4.1. General
4.3. Representative procedure for cycloaddition of
diiododiynes 2 with ethynylboronate 3
Flash chromatography was performed with a silica gel col-
umn (Cica silica gel 60N) eluted with mixed solvents [hexane/
AcOEt]. TLC analyses were performed with Merck TLC plate
silica gel 60 F254. 1H and 13C NMR spectra were measured on
a Gemini 2000 NMR spectrometers as CDCl3 solutions at
25 ꢀC. 1H NMR chemical shifts are reported in terms of chem-
ical shift (d, ppm) relative to the singlet at 7.26 ppm for chlo-
roform. Splitting patterns are designated as follows: s, singlet;
d, doublet; t, triplet; q, quartet; quint, quinted; sext, sextet; sept,
septet; m, multiplet. Coupling constants are reported in hertz.
13C NMR spectra were fully decoupled and are reported in
terms of chemical shift (d, ppm) relative to the triplet at
77.0 ppm for CDCl3. Mass spectra were recorded on a JEOL
JMS-T100CS mass spectrometer as MeOH solutions. Elemen-
tal analyses were performed with a PerkineElmer 2400II
CHNS/O elemental analyzer. Melting points were obtained
by a Yamato Melting Point Apparatus MP-12 and are uncor-
rected. Microwave irradiation experiments were carried out
with a single-mode microwave reactor (CEM Discover Lab-
Mate). Closed reaction vessels were used, and the temperature
was monitored by an on-line IR detector. 1,2-Dichloroethane,
tBuOMe, iPr2NH, and DMF were distilled from CaH2, and de-
gassed before use. 2-Ethynyl-5,5-dimethyl-1,3,2-dioxaborinane
(3) and diiododiynes 2aef, and diiodobenzenes 5a,b, 14 were
prepared according to previously reported procedures.7e,8a
To a solution of Cp*RuCl(cod) (1) (11.4 mg, 0.030 mmol)
and ethynylboronate 3 (124.5 mg, 0.902 mmol) in dry degassed
1,2-dichloroethane (1.5 mL) was added a solution of diiodo-
diyne 2a (104.2 mg, 0.301 mmol) in dry degassed 1,2-dichloro-
ethane (2 mL) over 20 min via a syringe at room temperature
under Ar atmosphere. The solution was stirred at room temper-
ature under Ar atmosphere for 12 h, and then, the solvent was
removed under reduced pressure. The residue was purified by
silica gel flash column chromatography (hexane/AcOEt 20:1e
10:1) to give 4a (118.5 mg, 81%) as a colorless solid: mp
1
151.0e151.5 ꢀC; H NMR (300 MHz, CDCl3): d 1.07 (s, 6H),
3.79 (s, 4H), 5.14 (s, 4H), 7.73 (s, 1H); 13C NMR (75 MHz,
CDCl3): d 22.0, 31.7, 72.4, 79.4, 80.4, 87.1, 92.5, 142.9,
145.0, 145.1; MS (ESI): calcd for C13H15BI2O3 (483.9), found
m/z 522.9 [MþK]þ; EA calcd (%) for C13H15BI2O3 (483.88): C
32.27, H 3.12; found: C 32.22, H 3.00.
4.3.1. Compound 4b
Mp 196.5 ꢀC decomp. (eluent, hexane/AcOEt 10:1e3:1);
1H NMR (300 MHz, CDCl3): d 1.04 (s, 6H), 2.42 (s, 3H),
3.76 (s, 4H), 4.65 (s, 4H), 7.34 (d, J¼8.4 Hz, 2H), 7.68 (s,
1H), 7.79 (d, J¼8.4 Hz, 2H); 13C NMR (75 MHz, CDCl3):

852
Y. Yamamoto, K. Hattori / Tetrahedron 64 (2008) 847e855
d 21.4, 21.8, 31.6, 59.8, 60.8, 72.4, 89.3, 94.9, 127.6, 130.1,
133.7, 142.3, 142.4, 143.4, 144.1; MS (ESI): calcd for
C17H16BI2NO4S as a dimethyl boronate (597.0), found m/z
619.9 [MþNa]þ; EA calcd (%) for C20H22BI2NO4S
(637.08): C 37.71, H 3.48, N 2.20; found: C 37.98, H 3.23,
N, 2.18.
and extracted with AcOEt (20 mLꢁ2). The combined organic
layer was washed with brine (50 mL), dried with MgSO4, and
concentrated in vacuo. The crude product was then treated
with n-octyliodide (2.161 g, 9.00 mmol) and K2CO3
(2.490 g, 18.0 mmol) in dry acetone at ambient temperature.
After refluxing for 3 h, the reaction mixture was washed
with satd NH4Cl (50 mL), and the aqueous layer was extracted
with AcOEt (20 mLꢁ3). The organic layer was combined,
dried with MgSO4, and concentrated in vacuo. The residue
was purified by silica gel flash column chromatography (hex-
ane/AcOEt 50:1e10:1) to give 5c (1.314 g, 88%) as colorless
crystals: mp 56.8e58.5 ꢀC; 1H NMR (300 MHz, CDCl3):
d 0.89 (t, J¼7.2 Hz, 3H), 1.21e1.55 (m, 10H), 1.83 (quint,
J¼7.2 Hz, 2H), 3.99 (t, J¼6.3 Hz, 2H), 5.09 (d, J¼1.5 Hz,
2H), 5.11 (d, J¼1.5 Hz, 2H), 6.95 (s, 1H); 13C NMR
(75 MHz, CDCl3): d 14.0, 22.5, 25.9, 28.9, 29.1 (2C), 31.7,
70.0, 78.7, 79.7, 79.8, 86.6, 119.7, 136.5, 146.0, 157.7; MS
(ESI): calcd for C16H22I2O2 (500.0), found m/z 523.0
[MþNa]þ; EA calcd (%) for C16H22I2O2 (500.15): C 38.42,
H 4.43; found: C 38.38, H 4.22.
4.3.2. Compound 4c
1
Mp 164.9e165.8 ꢀC (eluent, hexane/AcOEt 20:1e5:1); H
NMR (300 MHz, CDCl3): d 1.05 (s, 6H), 3.69 (s, 2H), 3.72 (s,
2H), 3.76 (s, 6H), 3.77 (s, 4H), 7.64 (s, 1H); 13C NMR
(75 MHz, CDCl3): d 21.8, 31.6, 47.0, 48.2, 53.1, 56.3, 72.4,
92.7, 98.2, 142.9, 145.4, 145.7, 171.6; MS (ESI): calcd for
C15H17BI2O6 as a dimethyl boronate (557.9), found m/z
580.9 [MþNa]þ; EA calcd (%) for C18H21BI2O6 (597.98): C
36.15, H 3.54; found: C 36.31, H 3.61.
4.3.3. Compound 4d
1
Mp 195.4e196.5 ꢀC (eluent, hexane/AcOEt 10:1e5:1); H
NMR (300 MHz, CDCl3): d 1.07 (s, 6H), 3.80 (s, 4H), 3.85 (s,
2H), 3.88 (s, 2H), 7.78 (s, 1H); 13C NMR (75 MHz, CDCl3):
d 21.7, 29.8, 31.6, 50.5, 51.7, 72.4, 92.2, 97.9, 115.7, 142.0,
144.3; MS (ESI): calcd for C13H11BI2N2O2 as a dimethyl boro-
nate (491.9), found m/z 514.8 [MþNa]þ; EA calcd (%) for
C16H15BI2N2O2 (531.92): C 36.13, H 2.84, N 5.27; found: C
36.34, H 2.73, N, 5.07.
4.5. Synthesis of diiodobenzene 5d from 4a
To the degassed solution of 4a (483.9 mg, 1.00 mmol) in
dry MeOH (20 mL) were added Pd(OAc)2 (24.5 mg,
0.0506 mmol), PPh3 (26.2 mg, 0.0999 mmol), and p-benzo-
quinone (108.2 mg, 1.00 mmol), and the reaction mixture
was stirred at room temperature under CO atmosphere for
1 h. The solution was diluted with AcOEt (20 mL), and
washed with satd Na2CO3 (50 mL). The aqueous layer was ex-
tracted with AcOEt (20 mLꢁ2). The combined organic layer
was washed with brine (50 mL), and died with MgSO4. The
solvent was removed in vacuo, and the crude material was pu-
rified by silica gel flash column chromatography (hexane/
AcOEt 50:1e10:1) to give 5d (303.2 mg, 71%) as a colorless
solid: mp 121.1e122.0 ꢀC; 1H NMR (300 MHz, CDCl3):
d 3.93 (s, 3H), 5.18e5.22 (m, 4H), 8.05 (s, 1H); 13C NMR
(75 MHz, CDCl3): d 52.6, 79.3, 80.8, 86.4, 87.5, 135.0,
139.2, 147.1, 147.4, 165.3; MS (ESI): calcd for C10H8I2O3
(429.9), found m/z 452.9 [MþNa]þ; EA calcd (%) for
C10H8I2O3 (429.98): C 27.93, H 1.88; found: C 28.04, H 1.73.
4.3.4. Compound 4e
Mp 154.5e156.0 ꢀC (eluent, hexane/AcOEt 20:1e10:1);
1H NMR (300 MHz, CDCl3): d 1.05 (s, 6H), 1.47 (s, 6H),
2.97 (s, 2H), 3.00 (s, 2H), 3.73 (s, 4H), 3.77 (s, 4H), 7.62 (s,
1H); 13C NMR (75 MHz, CDCl3): d 21.8, 23.2, 24.1, 31.6,
38.3, 47.0, 47.9, 68.6, 72.4, 94.2, 98.0, 99.6, 142.5, 147.0,
147.3; MS (ESI): calcd for C16H21BI2O4 as a dimethyl boro-
nate (542.0), found m/z 564.9 [MþNa]þ; EA calcd (%) for
C19H25BI2O4 (582.02): C 39.21, H 4.33; found: C 39.14, H
4.18.
4.3.5. Compound 4f
1
Mp 214 ꢀC decomp. (eluent, hexane/AcOEt 10:1e5:1); H
NMR (300 MHz, CDCl3): d 1.10 (s, 6H), 3.56 (s, 2H), 3.58 (s,
2H), 3.83 (s, 4H), 7.24e7.39 (m, 6H), 7.71 (s, 1H), 7.75 (d,
J¼6 Hz, 2H); 13C NMR (75 MHz, CDCl3): d 21.9, 31.7,
52.1, 53.2, 53.5, 72.5, 93.5, 98.9, 119.9, 122.5, 127.7, 127.9,
139.7, 142.7, 148.5, 148.7, 151.9; MS (ESI): calcd for
C23H19BI2O2 as a dimethyl boronate (592.0), found m/z
614.9 [MþNa]þ; EA calcd (%) for C26H23BI2O2 (632.08): C
49.40, H 3.67; found: C 49.11, H 3.96.
4.6. Representative procedure for Sonogashira coupling
of cycloadducts 4a, 5aed with phenylacetylene
A solution of PdCl2(PPh3)2 (10.6 mg, 0.0151 mmol), CuI
(5.76 mg, 0.0302 mmol), 4a (145.2 mg, 0.300 mmol), phenyl-
i
acetylene (46.0 mg, 0.450 mmol), and Pr2NH (0.063 mL,
t
0.45 mmol) in degassed BuOMe (5.0 mL) was stirred at
4.4. Synthesis of diiodobenzene 5c from 4a
0 ꢀC for 12 h. The reaction mixture was diluted with AcOEt
(20 mL), and washed with satd NH4Cl (10 mL). The aqueous
layer was extracted with AcOEt (20 mLꢁ2). The combined
organic layer was then washed with brine (20 mL) and dried
with MgSO4. After removing the solvent in vacuo, the residue
was purified by flash column chromatography on silica gel
eluted with hexane/AcOEt 20:1e10:1) to give 6a (87.9 mg,
To a solution of 4a (1.452 g, 3.00 mmol) in THF (20 mL)
was added a mixture of 1 M aq NaOH and 30% hydrogen per-
oxide (2:1 v/v, 30 mL) at ambient temperature, and the reac-
tion mixture was stirred for 6 h. The reaction mixture was
diluted with AcOEt (20 mL) and washed with satd NH4Cl
(50 mL). The aqueous layer was acidified with 1 M HCl,
1
64%) as a pale-yellow solid: mp 122.5e123.5 ꢀC; H NMR

Y. Yamamoto, K. Hattori / Tetrahedron 64 (2008) 847e855
853
(300 MHz, CDCl3): d 1.08 (s, 6H), 3.81 (s, 4H), 5.07 (m, 2H),
5.39 (m, 2H), 7.32e7.37 (m, 3H), 7.48e7.51 (m, 2H), 7.56 (s,
1H); 13C NMR (75 MHz, CDCl3): d 21.7, 31.4, 72.2, 75.3,
79.4, 85.8, 92.7, 93.9, 116.0, 122.7, 128.3, 128.5, 131.5,
137.3, 142.6, 144.4; MS (ESI): calcd for C18H16BIO3 as a di-
methyl boronate (418.0), found m/z 441.0 [MþNa]þ; EA calcd
(%) for C21H20BIO3 (458.10): C 55.06, H 4.40; found: C
54.91, H 4.20.
(s, 1H); 13C NMR (75 MHz, CDCl3): d 52.5, 75.4, 79.9,
84.7, 87.3, 95.1, 117.0, 122.3, 128.6, 129.1, 131.8, 133.3,
133.9, 144.4, 146.9, 166.2; MS (ESI): calcd for C18H13IO3
(404.0), found m/z 427.0 [MþNa]þ; EA calcd (%) for
C18H13IO3 (404.20): C 53.49, H 3.24; found: C 53.59, H 3.07.
4.7. SuzukieMiyaura coupling of 6e with p-methoxy-
phenylboronic acid
Further elution (hexane/AcOEt 10:1e5:1) gave 7a
1
(22.4 mg, 17%) as a orange solid: mp 120 ꢀC decomp.; H
A solution of Pd2(dba)3$CHCl3 (7.78 mg, 0.00752 mmol),
SPhos (13.5 mg, 0.0329 mmol), 6e (121.3 mg, 0.300 mmol),
p-methoxyphenylboronic acid (68.5 mg, 0.451 mmol), and
K3PO4 (254.5 mg, 1.20 mmol) in degassed toluene (5 mL)
was stirred at 100 ꢀC for 1 h. The reaction mixture was diluted
with AcOEt (20 mL), and washed with satd NH4Cl (50 mL).
The aqueous layer was extracted with AcOEt (20 mLꢁ2).
The combined organic layer was then washed with brine
(50 mL) and dried with MgSO4. After removing the solvent
in vacuo, the residue was purified by flash column chromato-
graphy on silica gel eluted with hexane/AcOEt 20:1e5:1 to
give 8 (99.0 mg, 86%) as a colorless solid: mp 157.5e
NMR (300 MHz, CDCl3): d 1.08 (s, 6H), 3.85 (s, 4H), 5.31
(s, 4H), 7.34e7.37 (m, 6H), 7.49e7.52 (m, 4H), 7.86 (s,
1H); 13C NMR (75 MHz, CDCl3): d 21.8, 31.8, 72.5, 74.6,
74.8, 86.7, 87.8, 94.2, 96.4, 115.9, 120.6, 123.0, 123.6,
127.7, 128.4, 128.5, 128.6, 131.7, 135.3, 137.1, 142.2,
143.0; MS (ESI): calcd for C26H21BO3 as a dimethyl boronate
(392.2), found m/z 415.2 [MþNa]þ; EA calcd (%) for
C29H25BO3 (432.32): C 80.57, H 5.83; found: C 81.08, H 5.17.
4.6.1. Compound 6b
1
Mp 64.5e65.0 ꢀC (eluent, hexane/AcOEt 70:1e50:1); H
1
NMR (300 MHz, CDCl3): d 0.98 (t, J¼7.2 Hz, 3H), 1.44
(sext, J¼7.2 Hz, 2H), 1.59 (quint, J¼7.2 Hz, 2H), 2.72 (t,
J¼7.2 Hz, 2H), 5.06 (s, 2H), 5.38 (s, 2H), 7.22 (s, 1H),
7.34e7.37 (m, 3H), 7.49e7.53 (m, 2H); 13C NMR (75 MHz,
CDCl3): d 13.8, 22.3, 32.3, 38.1, 75.5, 79.4, 85.8, 93.5, 93.7,
116.6, 122.7, 128.5, 128.7, 130.7, 131.7, 139.2, 144.7,
144.9; MS (ESI): calcd for C20H19IO (402.1), found m/z
425.0 [MþNa]þ; EA calcd (%) for C20H19IO (402.27): C
59.71, H 4.76; found: C 59.93, H 4.29.
158.1 ꢀC; H NMR (300 MHz, CDCl3): d 3.66 (s, 3H), 3.85
(s, 3H), 4.99 (m, 2H), 5.31 (m, 2H), 6.94 (d, J¼9 Hz, 2H),
7.14 (d, J¼9 Hz, 2H), 7.35e7.39 (m, 3H), 7.50e7.55 (m,
2H), 7.95 (s, 1H); 13C NMR (75 MHz, CDCl3): d 52.0, 55.1,
74.2, 74.3, 85.5, 93.9, 113.8, 115.8, 122.7, 128.5, 128.9,
129.0, 130.7, 130.8, 131.8, 132.7, 136.3, 140.1, 144.7,
159.2, 167.9; MS (ESI): calcd for C25H20O4 (384.1), found
m/z 407.1 [MþNa]þ; EA calcd (%) for C25H20O4 (384.42):
C 78.11, H 5.24; found: C 78.26, H 4.92.
4.6.2. Compound 6c
4.8. Representative procedure for Sonogashira coupling
with microwave irradiation
Mp 120.0e125.5 ꢀC decomp. (eluent, hexane/AcOEt
70:1e50:1); 1H NMR (300 MHz, CDCl3): d 5.13 (s, 2H),
5.46 (s, 2H), 7.33e7.50 (m, 11H); 13C NMR (75 MHz,
CDCl3): d 75.5, 79.6, 85.5, 91.6, 94.3, 116.7, 122.6, 128.0,
128.2, 128.5, 128.9, 129.4, 131.6, 131.7, 140.4, 142.8,
145.3, 146.1; MS (ESI): calcd for C22H15IO (422.0), found
m/z 445.0 [MþNa]þ; EA calcd (%) for C22H15IO (422.26):
C 62.58, H 3.58; found: C 62.47, H 3.57.
A solution of PdCl2(PPh3)2 (42.1 mg, 0.0600 mmol), CuI
(11.3 mg, 0.0593 mmol), 6b (766.0 mg, 1.90 mmol), trime-
i
thylsilylacetylene (295 mg, 3.00 mmol), and Pr2NH (4.0 mL,
29.0 mmol) in degassed DMF (1.0 mL) was heated at
120 ꢀC for 15 min with a microwave reactor. The reaction
mixture was diluted with AcOEt (20 mL), and washed with
satd NH4Cl (50 mL). The aqueous layer was extracted with
AcOEt (20 mLꢁ2). The combined organic layer was then
washed with brine (50 mL) and dried with MgSO4. After re-
moving the solvent in vacuo, the residue was then treated
with K2CO3 (1.11 g, 8.00 mmol) in MeOH (20 mL) at room
temperature for 1 h. The reaction mixture was diluted with
AcOEt (20 mL) and washed with 1 N NaOH (50 mL). The
aqueous layer was extracted with AcOEt (20 mLꢁ2). The
combined organic layer was then washed with satd NH4Cl
(50 mL), and dried with MgSO4. After removing the solvent
in vacuo, the residue was purified by flash column chromato-
graphy on silica gel eluted with hexane/AcOEt 70:1e20:1 to
give 9 (546.9 mg, 96%) as a pale-yellow solid: mp 72.0e
4.6.3. Compound 6d
1
Mp 51.7e54.5 ꢀC (eluent, hexane/AcOEt 70:1); H NMR
(300 MHz, CDCl3): d 0.89 (t, J¼7.2 Hz, 3H), 1.21e1.55 (m,
10H), 1.85 (quint, J¼7.2 Hz, 2H), 4.04 (t, J¼6.3 Hz, 2H),
5.04 (s, 2H), 5.33 (s, 2H), 6.78 (s, 1H), 7.34e7.37 (m, 3H),
7.49e7.53 (m, 2H); 13C NMR (75 MHz, CDCl3): d 14.0,
22.5, 25.9, 29.0, 29.1, 31.7, 69.8, 75.0, 78.7, 80.2, 86.0,
93.6, 113.2, 117.0, 122.7, 128.5, 128.8, 131.7, 134.1, 145.9,
157.2; MS (ESI): calcd for C24H27IO2 (474.1), found m/z
497.1 [MþNa]þ; EA calcd (%) for C24H27IO2 (474.37): C
60.77, H 5.74; found: C 60.84, H 5.28.
1
4.6.4. Compound 6e
72.5 ꢀC; H NMR (300 MHz, CDCl3): d 0.95 (t, J¼7.2 Hz,
Mp 119.5e121.5 ꢀC (eluent, hexane/AcOEt 70:1e10:1);
1H NMR (300 MHz, CDCl3): d 3.95 (s, 3H), 5.12 (m, 2H),
5.42 (m, 2H), 7.34e7.39 (m, 3H), 7.49e7.53 (m, 2H), 7.86
3H), 1.39 (sext, J¼7.2 Hz, 2H), 1.64 (quint, J¼7.2 Hz, 2H),
2.79 (t, J¼7.2 Hz, 2H), 3.47 (s, 1H), 5.21 (s, 2H), 5.24 (s,
2H), 7.26 (s, 1H), 7.34e7.37 (m, 3H), 7.49e7.53 (m, 2H);

854
Y. Yamamoto, K. Hattori / Tetrahedron 64 (2008) 847e855
13C NMR (75 MHz, CDCl3): d 13.8, 22.3, 32.8, 33.4, 74.3,
74.6, 79.4, 85.5, 86.4, 94.2, 115.1, 116.8, 122.8, 128.5,
128.8, 130.7, 131.7, 138.9, 143.1, 145.1; MS (ESI): calcd
for C22H20O (300.2), found m/z 323.2 [MþNa]þ; EA calcd
(%) for C22H20O (300.39): C 87.96, H 6.71; found: C 88.00,
H 6.27.
2H), 5.36 (s, 2H), 7.17 (s, 1H), 7.29 (s, 1H), 7.34e7.38 (m,
3H), 7.49e7.54 (m, 2H); 13C NMR (75 MHz, CDCl3):
d 13.8, 13.9, 22.3, 22.5, 32.4, 32.8, 33.7, 39.2, 74.4, 74.6,
75.4, 79.5, 86.6, 89.6, 93.9, 94.3, 94.5, 115.6, 116.3, 116.8,
122.8, 128.5, 128.8, 130.5, 130.9, 131.7, 139.0, 139.1,
142.3, 144.3, 145.0, 145.2; MS (ESI): calcd for C34H33IO2
(600.2), found m/z 623.2 [MþNa]þ; EA calcd (%) for
C34H33IO2 (600.53): C 68.00, H 5.54; found: C 68.20, H 5.23.
Further elution (hexane/AcOEt 10:1) gave 11 (5.05 mg,
17%) as a yellow solid: mp 166.5e168.0 ꢀC decomp.; 1H
NMR (300 MHz, CDCl3): d 0.98 (t, J¼7.2 Hz, 3H), 1.42
(sext, J¼7.2 Hz, 2H), 1.67 (quint, J¼7.2 Hz, 2H), 2.82 (t,
J¼7.2 Hz, 2H), 5.25 (s, 4H), 7.28 (s, 2H), 7.34e7.38 (m,
6H), 7.49e7.54 (m, 4H); 13C NMR (75 MHz, CDCl3):
d 13.8, 22.3, 32.9, 33.6, 74.3, 74.5, 79.9, 81.4, 86.5, 95.0,
114.6, 117.5, 122.7, 128.6, 128.9, 130.9, 131.8, 139.2,
143.7, 146.3; MS (ESI): calcd for C44H38O2 (598.3), found
m/z 621.3 [MþNa]þ; EA calcd (%) for C44H38O2 (598.77):
C 88.26, H 6.40; found: C 88.52, H 5.96.
4.8.1. Compound 12
Mp 128e131 ꢀC decomp. (eluent, hexane/AcOEt 50:1e
10:1); 1H NMR (300 MHz, CDCl3): d 0.96 (t, J¼7.2 Hz,
3H), 0.97 (t, J¼7.2 Hz, 3H), 1.41 (sext, J¼7.2 Hz, 2H), 1.43
(sext, J¼7.2 Hz, 2H), 1.59e1.74 (m, 4H), 2.81 (t, J¼7.2 Hz,
2H), 2.83 (t, J¼7.2 Hz, 2H), 3.49 (s, 1H), 5.22 (s, 4H), 5.26
(s, 2H), 5.27 (s, 2H), 7.22 (s, 1H), 7.30 (s, 1H), 7.34e7.38
(m, 3H), 7.49e7.54 (m, 2H); 13C NMR (75 MHz, CDCl3):
d 13.9, 14.0, 22.5, 22.6, 32.8, 32.9, 33.5, 33.8, 74.3, 74.4,
74.5, 74.6, 79.3, 85.8, 86.5, 90.0, 94.4, 94.9, 115.4, 115.5,
116.4, 116.7, 122.7, 128.4, 128.7, 130.5, 130.8, 131.6,
138.6, 138.9, 142.1, 143.1, 144.2, 145.2; MS (ESI): calcd
for C36H34O2 (498.3), found m/z 521.3 [MþNa]þ; EA calcd
(%) for C36H34O2 (498.65): C 86.71, H 6.87; found: C
86.54, H 7.43.
4.10. Homo coupling of 9
A solution of PdCl2(PPh3)2 (7.25 mg, 0.0103 mmol), CuI
(4.20 mg, 0.0221 mmol), 9 (60.1 mg, 0.200 mmol), and
iPr2NH (0.042 mL, 0.30 mmol) in degassed tBuOMe
(3.0 mL) was stirred at room temperature for 1 h. The reaction
mixture was diluted with AcOEt (10 mL), and washed with
satd NH4Cl (10 mL). The aqueous layer was extracted with
AcOEt (10 mLꢁ2). The combined organic layer was then
washed with brine (20 mL) and dried with MgSO4. After re-
moving the solvent in vacuo, the residue was purified by re-
crystallization from AcOEt to give 11 (49.7 mg, 83%) as
a yellow solid.
4.8.2. Compound 13
Mp 145e148 ꢀC decomp. (eluent, hexane/AcOEt 10:1e
5:1); H NMR (300 MHz, CDCl3): d 0.98 (t, J¼7.2 Hz, 6H),
1
1.43e1.51 (m, 4H), 1.65e1.76 (m, 4H), 2.85 (t, J¼7.2 Hz,
2H), 2.87 (t, J¼7.2 Hz, 2H), 5.27 (s, 8H), 7.25 (s, 1H), 7.30
(s, 1H), 7.35e7.40 (m, 6H), 7.49e7.54 (m, 4H); 13C NMR
(75 MHz, CDCl3): d 13.8, 13.9, 22.5, 32.8, 32.9, 33.7, 74.4,
74.5, 74.6, 74.7, 85.2, 86.6, 90.0, 94.4, 95.2, 98.3, 115.8,
115.9, 116.7, 122.8, 123.0, 128.5, 128.6, 128.8, 130.7,
130.9, 131.6, 131.8, 138.7, 139.0, 142.3, 142.4, 144.3,
144.6; MS (ESI): calcd for C42H38O2 (574.3), found m/z
597.3 [MþNa]þ; EA calcd (%) for C42H38O2 (574.75): C
87.77, H 6.66; found: C 87.76, H 6.21.
Acknowledgements
This research was partially supported by the MEXT, Grant-
in-Aid for Young Scientists (A) (17685008), and Tokyo Tech
Award for Challenging Research, 2006. We thank Profs. T.
Ikariya and S. Kuwata for use of the ESI mass spectrometer,
and Profs. K. Tomooka and K. Igawa for use of the NMR spec-
trometer. We also thank Profs. H. Suzuki, T. Takao, and M.
Oishi for elemental analyses.
4.9. Sonogashira coupling of 9 with 5a
A solution of PdCl2(PPh3)2 (3.52 mg, 0.00501 mmol), CuI
(0.48 mg, 0.0025 mmol), 9 (30.0 mg, 0.0999 mmol), 5a
i
(47.1 mg, 0.110 mmol), and Pr2NH (0.021 mL, 0.15 mmol)
in degassed tBuOMe (2.0 mL) was stirred at room temperature
for 2 h. The reaction mixture was diluted with AcOEt (10 mL),
and washed with satd NH4Cl (20 mL). The aqueous layer was
extracted with AcOEt (20 mLꢁ2). The combined organic
layer was then washed with brine (20 mL) and dried with
MgSO4. After removing the solvent in vacuo, the residue
was purified by flash column chromatography on silica gel
eluted with hexane/AcOEt 100:1e70:1) to recover 5a
(9.2 mg, 20%).
References and notes
1. For reviews of cyclotrimerization, see: (a) Shore, N. E. Comprehensive
Organic Synthesis; Trost, B. M., Fleming, I., Paquette, L. A., Eds.; Perga-
mon: Oxford, 1991; Vol. 5, pp 1129e1162; (b) Grotjahn, D. B. Compre-
hensive Organometallic Chemistry II; Hegedus, L. S., Abel, E. W., Stone,
F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, 1995; Vol. 12, pp 741e
770.
Further elution (hexane/AcOEt 50:1e20:1) gave 10
1
(41.3 mg, 69%) as a colorless solid: mp 143.5e145.5 ꢀC; H
2. Carruthers, W.; Coldham, I. Modern Methods of Organic Synthesis, 4th
ed.; Cambridge University Press: Cambridge, 2004; p 89.
NMR (300 MHz, CDCl3): d 0.97 (t, J¼7.2 Hz, 3H), 0.98 (t,
J¼7.2 Hz, 3H), 1.42 (sext, J¼7.2 Hz, 2H), 1.44 (sext,
J¼7.2 Hz, 2H), 1.56e1.74 (m, 4H), 2.73 (t, J¼7.2 Hz, 2H),
2.82 (t, J¼7.2 Hz, 2H), 5.07 (s, 2H), 5.25 (s, 2H), 5.27 (s,
3. For reviews of intramolecular cyclotrimerization, see: (a) Saito, S.;
Yamamoto, Y. Chem. Rev. 2000, 100, 2901e2915; (b) Yamamoto, Y.
Curr. Org. Chem. 2005, 9, 503e519; (c) Kotha, S.; Brahmachary, E.;
Lahiri, K. Eur. J. Org. Chem. 2005, 4741e4767.

Y. Yamamoto, K. Hattori / Tetrahedron 64 (2008) 847e855
4. (a) Lindner, E.; Jansen, R.-M.; Mayer, H. A.; Hiller, W.; Fawzi, R.
855
8. (a) Yamamoto, Y.; Hattori, K.; Nishiyama, H. J. Am. Chem. Soc. 2006,
128, 8336e8340; Also, see: (b) Yamamoto, Y.; Hashimoto, T.; Hattori,
K.; Kikuchi, M.; Nishiyama, H. Org. Lett. 2006, 8, 3565e3568.
9. Smith, M. B.; March, J. March’s Advanced Organic Chemistry, 5th ed.;
Wiley: New York, NY, 2001; Chapter 11, p 704 and references cited
therein.
Organometallics 1989, 8, 2355e2360; (b) Lindner, E.; Kuhbauch, H.
¨
J. Organomet. Chem. 1991, 403, C9eC14; (c) Ren, C. Y.; Cheng,
W. C.; Chan, W. C.; Yeung, C. H.; Lau, C. P. J. Mol. Catal. 1990, 59,
´
L1eL8; (d) Bohanna, C.; Esteruelas, M. A.; Herrero, J.; Lopez, A. M.;
Oro, L. A. J. Organomet. Chem. 1995, 498, 199e206; For recent exam-
ples, see: (e) Breschi, C.; Piparo, L.; Pertici, P.; Caporusso, A. M.; Vitulli,
G. J. Organomet. Chem. 2000, 607, 57e63; (f) Ruba, E.; Schmid, R.;
10. (a) Sonogashira, K.; Tohda, Y.; Hagihara, H. Tetrahedron Lett. 1975, 16,
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