Regioselective haloaromatization of 1,2-bis(ethynyl)benzene via halogen acids and PtCl2. Platinum-catalyzed 6-π electrocyclization of 1,2-bis(1′-haloethenyl)benzene intermediates

Article abstract of DOI:10.1021/jo0518295

Treatment of 1,2-bis(ethynyl)benzene (1) with aqueous HX (X = Br, I) in hot 3-pentanone (100-105 °C, 2 h) afforded 1,2-bis(1′-haloethenyl)benzene species 2-Br and 2-I in 98% and 95% yields, respectively. The hydrochlorination of endiyne 1 failed to proceed at elevated temperature but was implemented efficiently by PtCl₂ (5 mol %) in hot 3-pentanone (100 °C, 2 h) to give 1,2-bis(1′-chloroetheny)benzene 2-Cl in 80% yield. In the presence of PtCl₂ (5 mol %), these halides 2-Cl, 2-Br, and 2-I were subsequently converted to 1-halonaphthalenes 3-Cl, 3-Br, and 3-I in the mother solution via sequential 6-π electrocyclization and dehalogenation reactions. PtCl₂ (5 mol %) also effected direct haloaromatization of endiyne 1 with HX (X = Cl, Br, I) and gave 1-halonaphthalenes 3-Cl, 3-Br, and 3-I in 64-71% yields. This investigation reports the scope and the regioselectivity of haloaromatization of various enediynes catalyzed by PtCl₂.

Full text of DOI:10.1021/jo0518295

Regioselective Haloaromatization of 1,2-Bis(ethynyl)benzene via
Halogen Acids and PtCl2. Platinum-Catalyzed 6-π
Electrocyclization of 1,2-Bis(1′-haloethenyl)benzene Intermediates
Ching-Yu Lo,† Manyam Praveen Kumar,† Hsu-Kai Chang,† Shie-Fu Lush,‡ and
Rai-Shung Liu*,†
Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan 30043, ROC, and
Department of Medical Technology, Yuanpei Institute of Science and Technology,
Hsinchu, Taiwan 30043, ROC
rsliu@mx.nthu.edu.tw
Received August 30, 2005
Treatment of 1,2-bis(ethynyl)benzene (1) with aqueous HX (X ) Br, I) in hot 3-pentanone (100-
105 °C, 2 h) afforded 1,2-bis(1′-haloethenyl)benzene species 2-Br and 2-I in 98% and 95% yields,
respectively. The hydrochlorination of endiyne 1 failed to proceed at elevated temperature but was
implemented efficiently by PtCl2 (5 mol %) in hot 3-pentanone (100 °C, 2 h) to give 1,2-bis(1′-
chloroethenyl)benzene 2-Cl in 80% yield. In the presence of PtCl2 (5 mol %), these halides 2-Cl,
2-Br, and 2-I were subsequently converted to 1-halonaphthalenes 3-Cl, 3-Br, and 3-I in the mother
solution via sequential 6-π electrocyclization and dehalogenation reactions. PtCl2 (5 mol %) also
effected direct haloaromatization of endiyne 1 with HX (X ) Cl, Br, I) and gave 1-halonaphthalenes
3-Cl, 3-Br, and 3-I in 64-71% yields. This investigation reports the scope and the regioselectivity
of haloaromatization of various enediynes catalyzed by PtCl2.
Introduction
can be introduced onto aromatic products via suitable
nucleophiles.6,7 A widespread application of this method
suffers from its limited scope: the reaction requires either
anionic nucleophiles, such as sodium methoxide and
thiophenoxide, or strained enediynes. The ruthenium-
catalyzed cyclization of enediynes with nucleophiles that
we recently reported proceeded highly regioselectively8
and was compatible with a wide range of nucleophiles
including water, alcohols, aniline, pyrrole, acetylacetone,
Bergman aromatization of enediynes1 has attracted
considerable attention because of its perspective applica-
tions in materials and medicinal chemistry.2,3 Although
cyclization of unfunctionalized enediynes has been at-
tempted with various approaches, including diradical
pathways,1b,4 electrophilic additions,5a-c and radical
cations,5d these reactions only gave benzene or fulvene
products of special types. In contrast, the aromatization
of enediynes via nucleophilic addition (anionic Bergman
cyclization) is more useful because organic functionality
(4) For cyclization of enediynes via diradical pathways, see ref 1b
and: (a) Jones, L. H.; Hartwig, C. W.; Wentworth, P., Jr.; Simeonov,
A.; Wentworth, A. D.; Py, S.; Ashley, J. A.; Lerner, R. A.; Janda, K. D.
J. Am. Chem. Soc. 2001, 123, 3607. (b) Grissom, J. W.; Gunawardena,
G. U. Tetrahedron Lett. 1995, 36, 4951. (c) Schottelius, M.; Chen, P.
J. Am. Chem. Soc. 1996, 118, 4896. (d) Dai, W.-M. Curr. Med. Chem.
2003, 10, 2265.
(5) (a) Whitlock, H. W., Jr.; Sandvick, P. E.; Overman, L. E.;
Reichardt, P. B. J. Org. Chem. Soc. 1969, 34, 879. (b) Whitlock, H. W.,
Jr.; Sandvick, P. E. J. Am. Chem. Soc. 1966, 88, 4525. (c) Schreiner,
P. R.; Prall, M.; Lutz, V. Angew. Chem., Int. Ed. 2003, 42, 5757. (d)
Schmittel, M.; Kiau, S. Liebigs Ann/Recueil 1997, 1391.
(6) For anionic Bergman cyclization, see: (a) Wu, M.-J.; Lin, C.-F.;
Lu, E.-D. J. Org. Chem. 2002, 67, 5907. (b) Wu, M.-J.; Lee, C.-Y.; Lin,
C.-F. Angew. Chem., Int. Ed. 2002, 41, 4077. (c) Magnus, P.; Eisenbeis,
S. A.; Rose, W. C.; Zein, N.; Solmon, W. J. Am. Chem. Soc. 1993, 115,
12627. (d) Sugiyama, H.; Yamashita, K.; Nishi, M.; Saito, I. Tetrahe-
dron Lett. 1992, 33, 515.
† National Tsing-Hua University.
‡ Yuanpei Institute of Science and Technology.
(1) (a) Bergman, R. G. Acc. Chem. Res. 1973, 6, 25. (b) Lockhart, T.
P.; Comita, P. B.; Bergman, R. G. J. Am. Chem. Soc. 1981, 103, 4082.
(2) Reviews: (a) Enediyne Antibiotics as Antitumor Agents; Borders,
D. B., Doyle, T. W., Eds.; Marcel Dekker: New York, 1995. (b) Xi, Z.;
Goldberg, I. H. In Comprehensive Natural Product Chemistry; Barton,
D. H. R., Nakanishi, K., Eds.; Pergamon: Oxford, 1999; Vol. 7, p 553.
(3) Recent reviews: (a) Basak, A.; Mandal, S.; Bag, S. S. Chem. Rev.
2003, 103, 4077. (b) Rawat, D. S.; Zaleski, J. M. Synlett 2004, 393. (c)
Bowles, D. M.; Palmer, G. J.; Landis, C. A.; Scott, J. L.; Anthony J. E.
Tetrahedron 2001, 57, 3753. (d) Grissom, J. W.; Gunawardena, G. U.;
Klingberg, D.; Huang, D. Tetrahedron 1996, 52, 6453. (e) Wang, K. K.
Chem. Rev. 1996, 96, 207.
10.1021/jo0518295 CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/12/2005
10482
J. Org. Chem. 2005, 70, 10482-10487

Regioselective Haloaromatization of 1,2-Bis(ethynyl)benzene
with literature reports.11 Thermal 6-π-electrocyclization
of 1,2-divinylbenzenes to 1,2-dihydronaphthalenes nor-
mally proceeds at elevated temperatures (250-300 °C).11
We were pleased to find that PtCl2 (10 mol %) species
effected the aromatization of 1,2-divinylbenzenes 2-Br
and 2-I and gave 1-halonaphthalene 3-Br and 3-I in 46-
56% yields (entries 7 and 8), whereas unreacted 2-Br and
2-I were recovered in 23% yields. In these two cases,
species 2-Br and 2-I were generated in situ from endiyne
1 and HX (X ) Br, I) according to the operations in
entries 1 and 2, and PtCl2 catalyst was subsequently
added to the same solution. Naphthalenes 3-Cl, 3-Br, and
3-I were produced more efficiently (64-71%) from simul-
taneous treatment of of diyne 1, aqueous HX (2.0 equiv),
and PtCl2 catalyst (10 mol %) in hot 3-pentanone, and
the reaction period is considerably shorter (entries 9-11).
In the iodoaromatization of species 1, 1,2-divinylbenzene
2-I was confirmed to be the reaction intermediate and
isolated in 26% yield in a short period (100 °C, 1 h).
Notably, PtCl2 catalyst (10 mol %) failed to catalyze
haloaromatization of isolated and pure 2-Cl, 2-Br, and
2-I alone in hot 3-pentanone even at a prolonged period
(100 °C, 24 h). Based on these observations, we conclude
that the PtCl2 complex serves as precursors for genera-
tion of unknown active platinum species. Such an active
platinum catalyst has dual roles in catalytic activities:
acceleration of the hydrohalogenation of diynes to gener-
ate 1,2-bis(1′-halovinyl)benzene intermediates and the
subsequent catalytic 6-π-electrocyclization of these 1,2-
bis(1′-halovinyl)benzene species.
SCHEME 1
and dimethyl malonate (Scheme 1. eq 1). The mechanism
of this aromatization has been elucidated to involve
ruthenium-π-alkyne intermediates.8 Unfortunately, this
catalytic cyclization failed to work with hydrohalogena-
tion (NuH ) HCl, HBr, HI) to give desired 1-halonaph-
thalenes, which are important building blocks in many
synthetic applications. In this study, we report the first
realization of this catalytic process using PtCl2 catalyst,
and the mechanism proceeds via a distinct pathway from
those shown in Scheme 1 (eq 1). Prior to this study,
haloaromatization of enediyne was shown to give fulvene
products via a postulated vinyl cation intermediate
(Scheme 1, eq 2).5a-5c
The preceding platinum-catalyzed cyclization of 1,2-
bis(ethynyl)benzene 1 with HX (entries 10-12, Scheme
2) comprises consecutive hydrohalogenation and aroma-
tization reactions. According to the same approach,
Scheme 3 shows the cyclization efficiency of 1,2-bis-
(ethynylbenzene) (1) with HI-catalyzed by various metal
chloride salts (10 mol %) including RuCl3, PdCl2, PdCl2-
(PhCN)2, IrCl3, RhCl3, and AuCl3, and only RuCl3 showed
low activity for such a cyclization to give 2-iodonaphalene
3-I in 24% yield in addition to the 1,2-divinylbenzene
intermediate 2-I (53%, entry 1). The remaining metal
catalysts gave only hydroiodonation product 2-I (70-78%
yields) exclusively (entries 2-6).
We prepared various 1,2-bis(ethynyl)benzenes 5-7 to
study the generality of this cyclization. The catalytic
reactions were performed via treatment of species 5-7
with aqueous HX (2.0 equiv) and PtCl2 (10 mol %) in hot
3-pentanone (100 °C, 2-10 h). As shown in Table 1,
diynes 1 and 2 bearing an electron-rich benzene are very
suitable for this haloaromatization, and their 1-chloro-
and 1-bromonaphthalene products 8-Cl, 8-Br, 9-Cl, and
9-Br were obtained with yields as high as 81-92%.
Although the iodination reaction has a shorter period (2
h, entries 3 and 6), 1-iodonaphthalenes 8-I and 9-I were
obtained in lower yields (63-64%). This catalytic reaction
is less efficient with 4,5-difluoro-1,2-bis(ethynyl)benzene
7, and the its cyclized products 10-Cl, Br, and I were
obtained only in 32-45% yields.
Results and Discussion
Heating a mixture of enediyne 1 (1.0 equiv) and HI
(1.0-2.0 equiv) with TpRuPPh3(CH3CN)2PF6 (10 mol %)
catalyst in hot 3-pentanone (100-105 °C, 2 h) led to
polymerization and 1,2-bis(1′-iodoethenyl)benzene 2-I in
12% yield.8 We found that treatment of enediyne 1 with
aqueous HI (2 equiv) in hot 3-pentanone alone (2 h)
afforded vinyl iodide species 2-I in 95% yield (Scheme 2,
entry 1). The yield of product 2-I remained at 91% for a
prolonged period (24 h). A similar hydrobromination
occurs for enediyne 1 with HBr under similar conditions
(entry 2). In contrast, the hydrochlorination reaction was
unattainable through heating 1 with HCl (2 equiv) alone
in hot 3-pentanone (entry 3); it was achieved efficiently
with PtCl2 catalyst9 (3 mol %, entry 4) in a short period
(100 °C, 2 h) to give 2-Cl in 80% yield. Heating pure 1,2-
divinylbenzenes 2-Cl, 2-Br, and 2-I in hot 3-pentanone
(100 °C, 24 h) or 1,4-xylene (150 °C, 36 h) did not lead to
6-π-electrocyclization reaction10 (entries 5-6), consistent
(7) For the cyclization of strained enediynes with anionic nucleo-
philes, the enediynes actually underwent traditional Bergman or
Saito-Myers diradical processes after attack of anionic nucleophiles.
For examples, see: (a) Nicolaou, K. C.; Dai, W. M. Angew. Chem., Int.
Ed. Engl. 1991, 30, 1387 and references therein. (b) Hirama, M.;
Fujiwara, K.; Shigematsu, K.; Fukazara, Y. J. Am. Chem. Soc. 1989,
111, 4120. (c) Wender, P. A.; Hartmata, M.; Jefferey, D.; Mukai, C.;
Suffert, J. Tetrahedron Lett. 1988, 29, 909. (d) Bekele, T.; Brunette,
S. R.; Lipton, M. A. J. Org. Chem. 2003, 68, 8741.
(8) Odedra, A.; Wu, C.-J.; Pratap, T. P.; Huang, C.-W.; Ran, Y.-F.;
Liu, R.-S. J. Am. Chem. Soc. 2005, 127, 3406.
(9) For metal-catalyzed hydrohalogenation of alkynes to alkenyl
halides, see, for example: (a) Comprehensive Organic Synthesis; Trost,
B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 4, Part 1.7, p
269. (b) Kamiya, N.; Chikami, Y.; Ishii, Y. Synlett 1990, 675. (c) Kim,
G.; Jung, S.-d.; Lee, E.-j.; Kim, N. J. Org. Chem. 2003, 68, 5395.
(10) (a) Maier, G. Angew. Chem., Int. Ed. Engl. 1967, 6, 402. (b)
Tessier, P. L.; Nguyen, N.; Clay, M. D.; Fallis, A. G. Org. Lett. 2005,
7, 767. (c) Rautenstrauch, V. J. Org. Chem. 1984, 49, 950.
(11) Lamberts, J. J. M.; Laarhoven, W. H. J. Org. Chem. 1984, 49,
100.
J. Org. Chem, Vol. 70, No. 25, 2005 10483

Lo et al.
SCHEME 2a
a 2.1 equiv of HX (X ) Cl, 37 wt %; Br, 48 wt %; I, 52 wt %). b 3-Pentanone, [substrate] ) 0.45 M. c Yields shown are after separation
from a silica column. d 2-Cl,Br,I were generated in situ from 1 and HX according to entries 1, 2, and 4. e Pure 2-Cl,Br,I were used after
purification.,
SCHEME 3a
TABLE 1. Hydrohalogenation of
1,2-Bis(ethynyl)benzenes
entries
substrates
HX time (h) products (yields, %)
1
2
3
4
5
6
7
8
9
R,R ) -OCH2O- (5) HCl
6
4
8-Cl (86)
8-Br (81)
8-I (63)
9-Cl (92)
9-Br (88)
9-I (64)
10-Cl (45)
10-Br (48)
10-I (32)
5
HBr
HI
5
2
R ) OMe (6)
HCl
HBr
HI
6
6
6
4
2
R ) F (7)
HCl
HBr
HI
10
8
7
7
4
a 2.1 equiv of HX (X ) Cl, 37 wt %; Br, 48 wt %; I, 52 wt %).
b 3-pentanone, [substrate] ) 0.45 M. c Yields shown are after
separation from a silica column.
a 10 mol % catalyst, [substrate] ) 0.25 M, 3-pentanone, 100 °C,
24 h. b Yields are given after separation from a silica column.
1,2-bis(ethynyl)benzene 12 with HX (X ) Cl, Br, I)
produced only one regioisomer and gave cyclized products
16-Cl(A), 16-Br(A), and 16-I(A) in 65-83% yields. In
such a cyclization, the halide adds selectively to the C(1′)-
ethynyl carbon para to the oxygen atom. The structure
assignment of 16-Br(A) and 16-I(A) is made on the basis
We extended this haloaromatization to various 1,2-bis-
(ethynyl)benzenes 11-14 bearing two unequivalent ter-
minal alkynes. The regioselectivity of the reaction is
varied as noted in Table 1. For 4-methyl-1,2-bis(ethynyl)-
benzene 11, only the bromo derivative 15-Br showed mild
regioselectivity (A/B ) 76/24). The structure of 15-Br(A)
was confirmed by 1H-NOE spectra.12 In contrast, the
PtCl2 (10 mol %)-catalyzed aromatization of 4-methoxy-
(12) The 1H-NOE map of compounds 15-Br(A), 16-Br(A), 16-I(A),
18-Cl(A), 18-Br(A), and 18-I(B) is provided in the Supporting
Information.
10484 J. Org. Chem., Vol. 70, No. 25, 2005

Regioselective Haloaromatization of 1,2-Bis(ethynyl)benzene
SCHEME 4a
SCHEME 5
SCHEME 6
a 2.1 equiv of HX (X ) Cl, 37 wt %; Br, 48 wt %; I, 52 wt %).
b 3-Pentanone (100 °C), [substrate] ) 0.45 M. c Yields shown are
after separation from a silica column.
of its proton NOE effect.12 A high regioselectivity is also
maintained for the cyclization of 2,3-bis(ethynyl)benzo-
[b]furan 13 and its thiophene analogue 14, but the
structures of their resulting products 17-Cl, Br, I and
18-Cl, Br, I (74-85% yields) depend on the type of
halogen substituents: the chloro- and bromo groups of
17-Cl(A), 17-Br(A), 18-Cl(A), and 18-Br(A) are located
at the C(1)-carbon, whereas the iodo groups of 17-I(B)
and 18-I(B) are placed at the C(4)-carbon. The 1H NMR
patterns of 17-I(B) and 18-I(B) were very distinct from
those of 17-Cl(A), 17-Br(A), 18-Cl(A), and 18-Br(A).
Structural assignment of these dibenzofuran and diben-
zothiophene products is made on the basis of the proton
NOE effect of the representative derivatives 18-Cl(A),
18-Br(A), and 18-I(B).13
Scheme 4 shows the haloaromatization of acyclic
enediyne 19 in the presence or absence of PtCl2 catalyst
(entries 1-4). In contrast with preceding 1,2-bis(ethynyl)-
benzenes, haloaromatization of this enediyne proceeded
smoothly in hot 3-pentanone in the absence of PtCl2
catalyst, and the 1,2-divinyl species 20-I(A) was evidently
the reaction intermediate because it was isolated in 33%
yield at a short reaction period (1 h, entry 3). The 6-π
electrocyclization of intermediates 20-Cl, -Br, and -I(A)
avoids the dearomatization process, and it can proceed
with 100 °C. In the presence of PtCl2 (5 mol %), the same
cyclized products 20-Cl(B) and 20-Br(B) were obtained
after brief duration of reaction (2-3 h, entries 5 and 6).
Notably, we obtained 1,4-diiodobenzene 20-I(C) (73%)
exclusively from the platinum-catalyzed cyclization of
enediyne 19 with HI, whereas the expected iodobenzene
20-I(B) was obtained in only 3% yield.
mol %) and H2O in various proportions. The resulting
1-bromonaphthalene d3-3-Br contains a 0.75 deuterium
content equally at the C2-C4-carbons (X ) 0.75 D); such
a deuterium distribution remained unchanged with
increasing proportion of external H2O present in hot
3-pentanone.
Scheme 6 shows a plausible mechanism to account for
the platinum-catalyzed 6-π-electrocyclization of 1,2-bis-
(1-iodovinyl)benzene, which is the key step in the halo-
aromatization of 1,2-bis(ethynyl)benzenes. This proposed
mechanism rationalize the regioselective halogenation of
1,2-bis(ethynyl)benzene 12 bearing a methoxy group.
PtCl2 serves as a precursor for generation of unknown
active platinum species. We propose that this active Pt2+
species coordinates with the C(2)-vinyl group as the
methoxy activates the C(1) olefin group14 to facilitate an
intramolecular cyclization. This mechanism resembles
that of the Pd(II)-catalyzed Cope rearrangement of acyclic
1,5-diene.15 This cyclization is expected to give species
II bearing a platinum cyclohexyl moiety, and a subse-
quent loss of proton leads to formation of aromatic species
III. To account for the deuterium-labeling experiment,
we propose a loss of iodide anion of intermediate III to
We performed deuterium-labeling experiments to char-
acterize the reaction mechanism. As shown in Scheme
5, the bis(1-bromovinyl)benzene species d4-2-Br was first
generated by treatment of diyne d2-1 with Me3SiBr and
D2O (2.0 equiv), and the deuterium content of species d4-2
is estimated to be 0.80 D. In acidic medium, the proton
exchange between 3-pentanone and water at 100 °C may
account for such an incomplete deuterium content (X )
0.80 D) of species d4-2. This species was subsequently
converted to 1-bromonaphthalene d3-3-Br with PtCl2 (5
(13) The alcohol derivative of species 18-I(A) was prepared according
to the synthetic scheme shown below. The 1H-NOE map of this alcohol
is provided in the Supporting Information.
(14) (a) Bastian, G.; Royer, R.; Cavier, R. Eur. J. Med. Chem. 1983,
18, 365. (b) Kaluza, F.; Perold, G. Chem. Ber. 1955, 88, 597.
(15) Overman, L. E.; Knoll, F. M. J. Am. Chem. Soc. 1980, 102, 866.
J. Org. Chem, Vol. 70, No. 25, 2005 10485

Lo et al.
TABLE 2. Regioselectivity in the Hydrohalogenation of
4-Substituted 1,2-Bis(ethynyl)benzenes
nism for the platinum-catalyzed 6-π electrocyclization of
1,2-bis(1′-haloethenyl)benzene intermediates.
Experimental Section
General Procedures. Unless otherwise noted, all reactions
were carried out under a nitrogen atmosphere in oven-dried
glassware using standard syringe, cannula, and septa ap-
paratus. Benzene, diethyl ether, tetrahydrofuran, and hexane
were dried with sodium benzophenone and distilled before use.
The diyne substrates 1, 5-7, and 11-14 were prepared
according to the procedures described in the literature.8
Standard Procedure for Preparation of 1,2-Bis(1′-
bromoethenyl)benzene (2-Br). To a 3-pentanone solution
(1.0 mL) of 1,2-bis(1′-ethenyl)benzene 1 (100 mg, 0.79 mmol)
was added aqueous HBr (48 wt %, 0.19 mL), and the mixture
was heated at 100 °C for 1.5 h. The solution was concentrated,
extracted with ether, and washed with water. The ether
extract was dried over MgSO4, and eluted through a silica
column to give product 2-Br as a yellow oil (223 mg, 0.78 mmol,
98%).
Synthesis of 1,2-Bis(1′-chloroethenyl)benzene (2-Cl).
To a 3-pentanone solution (1.0 mL) of 1,2-bis(1′-ethenyl)-
benzene (100 mg, 0.79 mmol) (1) were added aqueous HCl (37
wt %, 0.14 mL) and PtCl2 (10.5 mg, 0.04 mmol), and the
mixture was heated at 100 °C for 2 h. The solution was
concentrated, extracted with diethyl ether, and washed with
water. The ether extract was dried over MgSO4 and eluted
through a silica column to give 1,2-bis(1′-chloroethenyl)-
benzene (126 mg, 0.63 mmol, 80%) (2-Cl) as a yellow oil.
Catalytic Transformation of 1,2-Bis(1′-ethenyl)ben-
zene (1) to 1-Iodonaphthalene (3-I). To a 3-pentanone
solution (1.0 mL) of 1,2-bis(1′-ethenyl)benzene (100 mg, 0.79
mmol) (1) were added aqueous HI (52 wt %, 0.41 mL) and PtCl2
(21 mg, 0.079 mmol), and the mixture was heated at 100 °C
for 4 h. The solution was concentrated, extracted with diethyl
ether, and washed with water. The ether extract was dried
over MgSO4 and eluted through a silica column to give
1-iodonaphthalene 3-I (129 mg, 0.50 mmol).
a 2.1 equiv of HX (X ) Cl, 37 wt %; Br, 48 wt %; I, 52 wt %).
b PtCl2 (10 mol %), 3-pentanone, 100 °C, [substrate] ) 0.40 M.
c Yields shown are after separation from a silica column.
form platinum carbene intermediate IV, which ultimately
gives the observed 1-iodonaphthalene product 16-I(A)
(Table 2, entry 6) through a 1,2-hydrogen shift.16 To
account for the doubly iodinated product 20-I(C) given
from enediyne 19, we propose that the corresponding
intermediate III′ undergoes â-hydrogen elimination to
give the desired product and (Pt-H)+1 species, which
finally regenerates Pt2+ and H2 in the presence of proton.
The halide-dependent regioselectivity for dibenzofurans
17-Cl(A), Br(A), and 17-I(B) and dibenzothiophene 18-
Cl(A), Br(A), and 18-I(B) is somewhat surprising,
particularly for the formation of 17-I(B) and 18-I(B) in
which the iodo group was located at the C4-carbon.
Species 17-I(B) and 18-I(B) represent two exceptions to
the proposed mechanism in Scheme 6, and their forma-
tion mechanism was unclear at this stage.
In summary, we have reported platinum-catalyzed
hydrohalogenation of 1,2-bis(ethynyl)benzenes that gives
1-halonaphthalene products efficiently. This cyclization
proceeds via platinum-catalyzed 6-π electrocyclization of
1,2-bis(1′-haloethenyl)benzene intermediates, and such
a mechanism is distinct from that for our previous
ruthenium-catalyzed aromatization of enediynes with
nucleophiles.8 PtCl2 serves as a precursor for generation
of unknown active platinum species. On the basis of
deuterium-labeling experiments, we propose a mecha-
Spectral Data for 1,2-Bis(1′-chloroethenyl)benzene (2-
1
Cl). H NMR (400 MHz, CDCl3): δ 5.44 (s, 2H), 5.55 (s, 2H),
7.31-7.33 (m, 2H), 7.39-7.41 (m, 2H). 13C NMR (100 MHz,
CDCl3): δ 122.0, 127.4, 128.6, 129.6, 138.5. HRMS: calcd for
C10H8Cl2 198.0003, found 198.0001.
Spectral Data for 1,2-Bis(1′-bromoethenyl)benzene (2-
Br). 1H NMR (400 MHz, CDCl3): δ 5.90∼5.93 (m, 4H), 7.31-
7.33 (m, 2H), 7.39-7.41 (m, 2H). 13C NMR (100 MHz, CDCl3):
δ 122.0, 127.4, 128.6, 129.6, 138.5. HRMS: calcd for C10H8Br2
285.8993, found 285.8995.
Spectral Data for 1,2-Bis(1′-iodoethenyl)benzene (2-
1
I). H NMR (400 MHz, CDCl3): δ 6.22 (dd, J ) 4.4, 1.2 Hz,
4H), 7.23-7.25 (m, 4H). 13C NMR (100 MHz, CDCl3): δ 102.4,
128.2, 129.5, 131.3, 140.8. HRMS: calcd for C10H8I2 381.8715,
found 381.8718.
Spectral Data for 1-Chloronaphthalene (3-Cl). 1H NMR
(600 MHz, CDCl3): δ 7.37 (t, J ) 7.6 Hz, 1H), 7.48-7.60 (m,
3H), 7.75 (d, J ) 8.2 Hz, 1H), 7.85 (d, J ) 8.2 Hz, 1H), 8.26 (d,
J ) 8.2 Hz, 1H) 13C NMR (150 MHz, CDCl3): δ 124.4, 125.7,
126.1, 126.6, 127.0, 127.1, 128.2, 130.8, 131.9, 134.5. HRMS:
calcd for C10H7Cl 162.0236, found 162.0235.
Spectral Data for 1-Bromonaphthalene (3-Br). 1H NMR
(500 MHz, CDCl3): δ 7.30 (t, J ) 8.0 Hz, 1H), 7.52 (t, J ) 8.0,
1H), 7.58 (t, J ) 8.0 Hz, 1H), 7.76-7.83 (m, 3 H), 8.23 (d, J )
8.0 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 122.8, 126.1, 126.6,
127.0, 127.3, 127.9, 128.2, 129.8, 131.9, 134.6. HRMS: calcd
for C10H7Br 205.9731, found 205.9731.
(16) (a) Roger, C.; Bodner, G. S.; Hatton, W. G.; Gladysz, J. A.
Organometallics 1991, 10, 3266. (b) Kusama, H.; Takaya, J.; Iwasawa,
N. J. Am. Chem. Soc. 2002, 124, 11592. (c) Bly, R. S.; Silverman, G.
S.; Bly, R. K. J. Am. Chem. Soc. 1988, 110, 7730.
Spectral Data for 1-Iodonaphthalene (3-I). 1H NMR
(600 MHz, CDCl3): δ 7.17 (t, J ) 7.8 Hz, 1H), 7.50 (t, J ) 7.8
Hz, 1H), 7.57 (t, J ) 7.8 Hz, 1H), 7.76 (d, J ) 8.2 Hz, 1H),
10486 J. Org. Chem., Vol. 70, No. 25, 2005

Regioselective Haloaromatization of 1,2-Bis(ethynyl)benzene
7.82 (d, J ) 8.2 Hz, 1H), 8.07∼8.09 (m, 2H). 13C NMR (150
MHz, CDCl3): δ 99.5, 126.6,126.8, 127.6, 128.5, 128.9, 132.0,
134.0, 134.2, 137.3. HRMS: calcd for C10H7I 253.9592, found
253.9595.
Supporting Information Available: NMR spectra and
spectral data of compounds 1, 2-(Cl-I) , 3(Cl-I), 5-7, 8(Cl-
I), 9(Cl-I), 10(Cl-I), 11-14, 15(Cl-I), 16(Cl-I), 17(Cl-I),
18(Cl-I), 19, and 20-I. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the National Science
Council, Taiwan, for supporting this work.
JO0518295
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