PtBr2-catalyzed consecutive enyne metathesis-aromatization of 1-(1-methoxy-but-3-enyl)-2-(1-alkynyl)benzenes: Dual role of the Pt catalyst

Article abstract of DOI:10.1021/jo048273q

(Chemical Equation Presented) 1,7-Enynes 1, connected through an aromatic ring and bearing a leaving methoxy group at the 4-position, underwent the PtBr₂-catalyzed enyne metathesis followed by aromatization in one pot to afford vinyl naphthalen

Full text of DOI:10.1021/jo048273q

PtBr -Catalyzed Consecutive Enyne Metathesis-Aromatization of
2
1
-(1-Methoxy-but-3-enyl)-2-(1-alkynyl)benzenes: Dual Role of the
Pt Catalyst
Gan B. Bajracharya, Itaru Nakamura, and Yoshinori Yamamoto*
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
yoshi@yamamoto1.chem.tohoku.ac.jp
Received September 30, 2004
1
4
,7-Enynes 1, connected through an aromatic ring and bearing a leaving methoxy group at the
2
-position, underwent the PtBr -catalyzed enyne metathesis followed by aromatization in one pot
to afford vinyl naphthalenes 3 in good to acceptable yields. The cyclobutene intermediate 11a and
another intermediate 2a were isolated, indicating that PtBr acts as a dual role catalyst: (1) as a
2
transition metal catalyst, it induces the enyne metathesis to produce 11a starting from 1a, and (2)
as a Lewis acid catalyst, it facilitates elimination of MeOH from 2a to give the aromatized product
3a.
Introduction
SCHEME 1. Cycloisomerization of 1,n-Enynes
In the past decade, the metal-mediated cycloisomer-
ization of 1,n-enynes has emerged as an extremely
attractive and unique tool for the one-pot synthesis of
1
various types of cyclic compounds (Scheme 1). A wide
range of transition metal complexes have been used for
SCHEME 2. Mechanisms in Enyne Metathesis
this transformation.2
-8
On the basis of the mechanisms involved in the enyne
metathesis, the cycloisomerization can be classified into
three categories. First is the metal carbene-mediated
enyne metathesis and second is the cycloisomerization
through metallacyclic intermediates (Scheme 2). The
metal carbene reacts first with the yne unit and the
resulting carbene complex further reacts with the ene
(
1) For reviews on enyne metathesis, see: (a) Trost, B. M. Acc. Chem.
Res. 1990, 23, 34-42. (b) Trost, B. M. Angew. Chem., Int. Ed. Engl.
995, 34, 259-281. (c) For textbook, see: Tsuji, J. Palladium Reagents
1
and Catalysts: Innovations in Organic Synthesis; John Wiley & Sons:
Chichester, UK, 1995; pp 476-488. (d) Ojima, I.; Tzamarioudaki, M.;
Li, Z. Y.; Donovan, R. J. Chem. Rev. 1996, 96, 635-662. (e) Trost, B.
M.; Krische, M. J. Synlett 1998, 1-16. (f) Mori, M. Top. Organomet.
Chem. 1998, 1, 133-154. (g) Trost, B. M. Chem. Eur. J. 1998, 4, 2405-
2
412. (h) Fletcher, A. J.; Christie, S. D. R. J. Chem. Soc., Perkin Trans.
unit to give the intermediate A, which subsequently
produces the 1,3-dienes together with the metal carbene
catalyst (pathway a). The oxidative metallacycloaddition
to enynes forms the metallacycle B, which undergoes
subsequent â-hydride elimination to give a mixture of the
1
2000, 1657-1668. (i) Trost, B. M.; Toste, F. D.; Pinkerton, A. B.
Chem. Rev. 2001, 101, 2067-2096. (j) Aubert, C.; Buisine, O.; Malacria,
M. Chem. Rev. 2002, 102, 813-834. (k) S e´ meril, D.; Bruneau, C.;
Dixneuf, P. H. Adv. Synth. Catal. 2002, 344, 585-595. (l) Poulsen, C.
S.; Madsen, R. Synthesis 2003, 1-18. (m) Lloyd-Jones, G. C. Org.
Biomol. Chem. 2003, 1, 215-236. (n) Schmidt, B. Angew. Chem., Int.
Ed. 2003, 42, 4996-4999. (o) Fairlamb, I. J. S. Angew. Chem., Int.
Ed. 2004, 43, 1048-1052. (p) Diver, S. T.; Giessert, A. J. Chem. Rev.
1
,3- and 1,4-dienes (pathway b). The third category
involves the π-complexation of an electrophilic transition
2
004, 104, 1317-1382. (q) Nakamura, I.; Yamamoto, Y. Chem. Rev.
004, 104, 2127-2198.
2
metal onto the alkyne moieties that leads to the forma-
10.1021/jo048273q CCC: $30.25 © 2005 American Chemical Society
892
J. Org. Chem. 2005, 70, 892-897
Published on Web 01/05/2005

PtBr
2
-Catalyzed Enyne Metathesis-Aromatization
1
9
tion of the polarized η -alkyne complex C bearing a
positive charge at the â-position (pathway c). Delocal-
ization of a positive charge produces the isomers D, E,
F, and G. Finally, the intermediate H is afforded through
bond rearrangement processes, which produces the 1,3-
diene along with other products.
produces conjugated dienes. Murai et al. used a wisely
designed starting material and succeeded in trapping the
carbene intermediate in the Pt-catalyzed cyclization of
enynes.5b Recently, Echavarren and co-workers reported
that 1,6-enynes react with alcohols or water in the
2
to give new carbo- or heterocycles with
alkoxy or hydroxy functional groups via formation of the
presence of PtCl
2
f,i
3a,5a,b
and F u¨ rstner3b,d-f proposed the
Trost, Murai,
formation of the cyclopropylmetal carbene intermediate
1
0
cyclopropyl platinum-carbene intermediate.
(
(
F in Scheme 2) in the Pd(II)-, Rh(II)-, Ru(II)-, and Pt-
Generally, the cycloisomerization of 1,6-enynes affords
the products with a newly generated five-membered ring.
Obviously, one may expect the formation of a six-
membered ring by cyclization of 1,7-enynes. However,
formation of a six-membered ring from 1,7-enynes in the
metathesis reaction is not easy due to the poorer ability
II)-catalyzed skeletal rearrangements of enynes that
(2) For Pd-catalyzed cycloisomerization of enynes, see: (a) Trost,
B. M.; Tanoury, G. J. J. Am. Chem. Soc. 1988, 110, 1636-1638. (b)
Trost, B. M.; Lautens, M.; Chan, C.; Jebaratnam, D. J.; Mueller, T. J.
Am. Chem. Soc. 1991, 113, 636-644. (c) Trost, B. M.; Trost, M. K.
Tetrahedron Lett. 1991, 32, 3647-3650. (d) Trost, B. M.; Pfrengle, W.;
Urabe, H.; Dumas, J. J. Am. Chem. Soc. 1992, 114, 1923-1924. (e)
Trost, B. M.; Chang, V. K. Synthesis 1993, 824-832. (f) Trost, B. M.;
Hashmi, A. S. K. Angew. Chem., Int. Ed. Engl. 1993, 32, 1085-1087.
1
j
of 1,7-enynes to function as a bidentate ligand. Trost
partially solved this problem by introducing a free
carboxylic acid substituent at the alkene terminus that
2g
coordinated with the metal in the catalytic cycle. Murai
et al. reported a few examples of cyclorearrangement of
(
(
(
g) Trost, B. M.; Gelling, O. J. Tetrahedron Lett. 1993, 34, 8233-8236.
h) Trost, B. M.; Shi, Y. J. Am. Chem. Soc. 1993, 115, 5, 9421-9438.
i) Trost, B. M.; Hashmi, A. S. K. J. Am. Chem. Soc. 1994, 116, 2183-
1
,7-enynes catalyzed by metal halides (e.g., PtCl
2
and
2
184. (j) Trost, B. M.; Tanoury, G. J.; Lautens, M.; Chan, C.;
GaCl etc.) leading to the formation of six-membered
3
3
MacPherson, D. T. J. Am. Chem. Soc. 1994, 116, 4255-4267. (k) Trost,
B. M.; Romero, D. L.; Rise, F. J. Am. Chem. Soc. 1994, 116, 4268-
4
a,h
rings. However, compare to the 1,6-enynes, 1,7-enynes
are rarely used in the enyne cycloisomerization reaction.
Furthermore, reports on formation of an aromatic ring
in the enyne cyclization are very rare.1 Recently, P e´ rez-
Castells et al. apparently reported the formation of a
naphthalene derivative as the side product by means of
the intramolecular Pauson-Khand reaction of aromatic
enynes promoted by molecular sieves in the presence of
278. (l) Goeke, A.; Sawamura, M.; Kuwano, R.; Ito, Y. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 662-663. (m) Yamada, H.; Aoyagi, S.;
Kibayashi, C. Tetrahedron Lett. 1996, 37, 8787-8790. (n) Trost, B.
M.; Li, Y. J. Am. Chem. Soc. 1996, 118, 6625-6633. (o) Oh, C. H.; Rhim,
C. Y.; Hwang, S. K. Chem. Lett. 1997, 777-778. (p) Yamada, H.;
Aoyagi, S.; Kibayshi, C. Tetrahedron Lett. 1997, 38, 3027-3030. (q)
Widenhoefer, R. A.; Stengone, C. N. J. Org. Chem. 1999, 64, 8681-
1,12
8
6
692. (r) Trost, B. M.; Krische, M. J. J. Am. Chem. Soc. 1999, 121,
131-6141. (s) Trost, B. M.; Haffner, C. D.; Jebaratnam, D. J.; Krische,
M. J.; Thomas, A. P. J. Am. Chem. Soc. 1999, 121, 6183-6192. (t)
Harada, K.; Tonoi, Y.; Kato, H.; Fukuyama, Y. Tetrahedron Lett. 2002,
4
3, 3829-3832. (u) Fairlamb, I. J. S.; Pike, A. C.; Ribrioux, S. P. C. P.
(6) For Ti-catalyzed cyclization of enynes, see: (a) Berk, S. C.;
Grossman, R. B.; Buchwald, S. L. J. Am. Chem. Soc. 1993, 115, 4912-
4913. (b) Urabe, H.; Hata, T.; Sato, F. Tetrahedron Lett. 1995, 36,
4261-4264. (c) Suzuki, K.; Urabe, H.; Sato, F. J. Am. Chem. Soc. 1996,
118, 8729-8730. (d) Urabe, H.; Suzuki, K.; Sato, F. J. Am. Chem. Soc.
1997, 119, 10014-10027. (e) Sturla, S. J.; Kablaoui, N. M.; Buchwald,
S. L. J. Am. Chem. Soc. 1999, 121, 1976-1977.
(7) For Zr-mediated cyclization of enynes, see: (a) Davis, J. M.;
Whitby, R. J.; Jaxa-Chamiec, A. Synlett 1994, 110-112. (b) Miura, K.;
Funatsu, M.; Saito, H.; Ito, H.; Hosomi, A. Tetrahedron Lett. 1996,
37, 9059-9062. (c) Negishi, E.-i.; Ma, S.; Sugihara, T.; Noda, Y. J. Org.
Chem. 1997, 62, 1922-1923.
(8) For Ni/Cr-, Ni-, and Ni/Zn-mediated cyclization of enynes, see:
(a) Ni/Cr: Trost, B. M.; Tour, J. M. J. Am. Chem. Soc. 1987, 109, 5268-
5270. (b) Ni: Tamao, K.; Kobayashi, K.; Ito, Y. J. Am. Chem. Soc. 1988,
110, 1286-1288. (c) Ni/Zn: Savchenko, A. V.; Montgomery, J. J. Org.
Chem. 1996, 61, 1562-1563.
(9) For recent formulations of the reaction mechanism, see: (a) Oi,
S.; Tsukamoto, I.; Miyano, S.; Inoue, Y. Organometallics 2001, 20,
3704-3709. (b) Mainetti, E.; Mouri e´ s, V.; Fensterbank, L.; Malacria,
M.; Marco-Contelles, J. Angew. Chem., Int. Ed. 2002, 41, 2132-2135.
(10) (a) M e´ ndez, M.; Mu n˜ oz, M. P.; Echavarren, A. M. J. Am. Chem.
Soc. 2000, 122, 11549-11550 and references therein. (b) M e´ ndez, M.;
Mu n˜ oz, M. P.; Nevado, C.; C a´ rdenas, D. J.; Echavarren, A. M. J. Am.
Chem. Soc. 2001, 123, 10511-10520. (c) Nevado, C.; C a´ rdenas, D. J.;
Echavarren, A. M. Chem. Eur. J. 2003, 9, 2627-2635. (d) Nevado, C.;
Charruault, L.; Michelet, V.; Nieto-Oberhuber, C.; Mu n˜ oz, M. P.;
M e´ ndez, M.; Rager, M.-N.; Gen eˆ t, J.-P.; Echavarren, A. M. Eur. J. Org.
Chem. 2003, 706-713. See also: (e) Fern a´ ndez-Rivas, C.; M e´ ndez, M.;
Nieto-Oberhuber, C.; Echavarren, A. M. J. Org. Chem. 2002, 67, 5197-
5201. (f) Echavarren, A. M.; M e´ ndez, M.; Mu n˜ oz, M. P.; Nevado, C.;
Mart ´ı n-Matute, B.; Nieto-Oberhuber, C.; C a´ rdenas, D. J. Pure Appl.
Chem. 2004, 76, 453-463.
(11) For recent advances in the transition metal-catalyzed formation
of aromatic rings, see: (a) Saito, S.; Yamamoto, Y. Chem. Rev. 2000,
100, 2901-2915 and references therein. (b) W: Miura, T.; Iwasawa,
N. J. Am. Chem. Soc. 2002, 124, 518-519. (c) Shen, H.-C.; Pal, S.;
Lian, J.-J.; Liu, R.-S. J. Am. Chem. Soc. 2003, 125, 15762-15763. See
also: (d) Imamura, K.-i.; Yoshikawa, E.; Gevorgyan, V.; Yamamoto,
Y. Tetrahedron Lett. 1999, 40, 4081-4084.
Tetrahedron Lett. 2002, 43, 5327-5331.
3) For Pt-mediated cyclization of enynes, see: (a) Chatani, N.;
Furukawa, N.; Sakurai, H.; Murai, S. Organometallics 1996, 15, 901-
03. (b) F u¨ rstner, A.; Szillat, H.; Gabor, B.; Mynott, R. J. Am. Chem.
(
9
Soc. 1998, 120, 8305-8314. (c) Trost, B. M.; Doherty, G. A. J. Am.
Chem. Soc. 2000, 122, 3801-3810. (d) F u¨ rstner, A.; Szillat, H.; Stelzer,
F. J. Am. Chem. Soc. 2000, 122, 6785-6786. (e) F u¨ rstner, A.; Stelzer,
F.; Szillat, H. J. Am. Chem. Soc. 2001, 123, 11863-11869. (f) Mamane,
V.; Gress, T.; Krause, H.; F u¨ rstner, A. J. Am. Chem. Soc. 2004, 126,
8
654-8655. (g) Bernard, M.; Cariou, K.; Mainetti, E.; Mouri e´ s. V.;
Dhimane, A.-L.; Fensterbank, L.; Malacria, M. J. Am. Chem. Soc. 2004,
26, 8656-8657. (h) For GaCl -mediated cyclization of enynes, see:
Chatani, N.; Inoue, H.; Kotsuma, T.; Murai, S. J. Am. Chem. Soc. 2002,
24, 10294-10295.
1
3
1
(4) For Rh-mediated cyclization of enynes, see: (a) Wender, P. A.;
Sperandio, D. J. Org. Chem. 1998, 63, 4164-4165. (b) Gilbertson, S.
R.; Hoge, G. S.; Genov, D. G. J. Org. Chem. 1998, 63, 10077-10080.
(
c) Cao, P.; Wang, B.; Zhang, X. J. Am. Chem. Soc. 2000, 122, 6490-
6
4
8
2
491. (d) Cao, P.; Zhang, X. Angew. Chem., Int. Ed. 2000, 39, 4104-
106. (e) Lei, A.; He, M.; Zhang, X. J. Am. Chem. Soc. 2002, 124, 8198-
199. (f) Lei, A.; He, M.; Wu, S.; Zhang, X. Angew. Chem., Int. Ed.
002, 41, 3457-3460. (g) Lei, A.; Waldkirch, J. P.; He, M.; Zhang, X.
Angew. Chem., Int. Ed. 2002, 41, 4526-4529. (h) Tong, X.; Zhang, Z.;
Zhang, X. J. Am. Chem. Soc. 2003, 125, 6370-6371. (i) Lei, A.; He,
M.; Zhang, X. J. Am. Chem. Soc. 2003, 125, 11472-11473. (j) Tong,
X.; Li, D.; Zhang, Z.; Zhang, X. J. Am. Chem. Soc. 2004, 126, 7601-
7
6
607. (k) Mikami, K.; Kataoka, S.; Yusa, Y.; Aikawa, K. Org. Lett. 2004,
, 3699-3701.
(5) For Ru-mediated cyclization of enynes, see: (a) Chatani, N.;
Morimoto, T.; Muto, T.; Murai, S. J. Am. Chem. Soc. 1994, 116, 6049-
6
050. (b) Chatani, N.; Kataoka, K.; Murai, S.; Furukawa, N.; Seki, Y.
J. Am. Chem. Soc. 1998, 120, 9104-9105. (c) Nishida, M.; Adachi, N.;
Onozuka, K.; Matsumura, H.; Mori, M. J. Org. Chem. 1998, 63, 9158-
9
9
2
1
159. (d) Trosy, B. M.; Toste, F. D. J. Am. Chem. Soc. 1999, 121, 9728-
729. (e) Paih, J. L.; Rodr ı´ guez, D. C.; D e´ rien, S.; Dixneuf, P. H. Synlett
000, 95-97. (f) Trost, B. M.; Toste, F. D. J. Am. Chem. Soc. 2000,
22, 714-715. (g) S e´ meril, D.; Cl e´ ran, M.; Bruneau. C.; Dixneuf, P.
H. Adv. Synth. Catal. 2001, 343, 184-187. (h) Ackermann, L.; Bruneau,
C.; Dixneuf, P. H. Synlett 2001, 397-399. (i) S e´ meril, D.; Cl e´ ran, M.;
Perez, A. J.; Bruneau. C.; Dixneuf, P. H. J. Mol. Catal. A: Chem. 2002,
1
5
(12) For the transition metal-catalyzed cyclization of heterocycles
through a Friedal-Crafts-type reaction and/or formation of cyclopro-
pylmetal-carbenes, see: (a) Dankwardt, J. W. Tetrahedron Lett. 2001,
42, 5809-5812. (b) F u¨ rstner, A.; Mamane, V. J. Org. Chem. 2002, 67,
6264-6267. (c) Mart ´ı n-Matute, B.; Nevado, C.; C a´ rdenas, D. J.;
Echavarren, A. M. J. Am. Chem. Soc. 2003, 125, 5757-5766.
90, 9-25. (j) Trost, B. M.; Toste, F. D. J. Am. Chem. Soc. 2002, 124,
025-5036. (k) Mori, M.; Saito, N.; Tanaka, D.; Takimoto, M.; Sato,
Y. J. Am. Chem. Soc. 2003, 125, 5606-5607. (l) For Ir-mediated
cyclization of enynes, see: Chatani, N.; Inoue, H.; Morimoto, T.; Muto,
T.; Murai, S. J. Org. Chem. 2001, 66, 4433-4436.
J. Org. Chem, Vol. 70, No. 3, 2005 893

Bajracharya et al.
SCHEME 3. Preparation of Starting Materials
2 8
a stoichiometric amount of Co (CO)
.13 The same group
TABLE 1. Optimization of the Reaction Conditionsa
subsequently utilized the aromatic enynes in the Grubbs
catalyst-mediated ring-closing metathesis producing 1,3-
dienes, which underwent a subsequent Diels-Alder
process to construct natural product frameworks.14 To the
best of our knowledge, a naphthalene core has not been
prepared by the transition metal-mediated enyne me-
tathesis-aromatization employing 1,7-enynes including
an aromatic ring in the tether part.
additive
temp, yield,
b
entry
catalyst
PdBr
solvent
(40 mol %)
°C
%
We report that a six-membered ring can readily be
obtained in good to acceptable yields in the enyne
1
2
3
4
5
6
7
8
9
2
MeCN
100 42 (38)
100 34
100 44
100 41
100 47
100 49
120 53
120 56
120 46
120 43
PdCl
PtCl
PtCl (PhCN)
2
MeCN
2
MeCN
metathesis of 1,7-enynes 1 by using PtBr
2
catalyst (eq
MeCN
2
2
PtCl
2
(CH
2
3
CN)
2
MeCN
1
). Here, PtBr exhibits a dual role: (1) as a transition
2
PtBr
PtBr
PtBr
PtBr
PtBr
PtBr
PtBr
PtBr
PtBr
PtBr
MeCN
2
2
2
2
2
2
2
2
2
MeCN
1,4-dioxane
hexane
toluene
10
11
12
13
14
15
120
7
1,4-dioxane p-benzoquinone 120 51
1,4-dioxane â-pinene
1,4-dioxane
120 49
c
120 79
d
1,4-dioxane
120 81 (75)
a
The reaction of 1a (0.3 mmol) was carried out in the presence
metal, PtBr
the six-membered 1,3-dienes 2, and (2) as a Lewis acidic
catalyst, PtBr
2
catalyzes the enyne metathesis to produce
of 10 mol % of catalyst under Ar for 15 h unless otherwise noted.
b
1
Yield calculated from H NMR integration with CH2Br2 as an
c
2
assists elimination of MeOH from 2 to
internal standard; the isolated yield is in parentheses. 2 mol %
d
of catalyst was used. 1 mol % of catalyst was used.
afford the vinylnaphthalene derivatives 3.
Results and Discussion
1
). Similarly, PdCl
CN) afforded the product 3a in comparable yields
entries 2-5). The yield of the product was slightly
improved by using PtBr as a catalyst (entry 6). The use
of other catalysts such as Pd(PPh , Pd (dba) ‚CHCl , Pt-
(PPh , NiBr (PPh , RhCl(PPh , etc. did not afford 3a
at all. We also evaluated some Lewis acids, for example,
Cu(OTf) , AgOTf, AuBr , GaCl , etc., in 1,4-dioxane, but
2 2 2 2 2 3
, PtCl , PtCl (PhCN) , and PtCl (CH -
2
The starting materials 1 were prepared by the allyla-
tion of o-alkynylbenzaldehydes 5 (Scheme 3). The precur-
sors 5 were prepared from the corresponding 2-bromoben-
zaldehyde derivatives 4 with a Sonogashira cross-
(
2
3
)
)
3 3
4
2
3
3
1
5
)
3 4
2
3
)
2
coupling. The direct allylation of the o-alkynylbenz-
aldehydes 5 was carried out by reaction with allyltri-
methylsilane in the presence of scandium triflate at
ambient temperature to produce the corresponding aro-
2
3
3
the reaction did not proceed. The yield of the product was
improved by increasing the temperature up to 120 °C
1
6
17
matic enynes 1. Alternatively, the acetalization of 5,
followed by allylation also produced the desired 1,7-
enyne.
(
entry 7). Among the solvents examined, 1,4-dioxane
provided the best results (compare entry 8 with entries
and 10). Under neat conditions, the product yield was
9
First, we used 1a as a model substrate for optimization
of the reaction conditions (eq 2). The results are sum-
marized in Table 1. We found that in the presence of 10
mol % of PdBr in MeCN at 100 °C, the enyne 1a
2
underwent cyclization-aromatization to afford 1-(1-me-
thylenebutyl)naphthalene 3a in 38% isolated yield (entry
drastically reduced (entry 11). The use of additional
additives that were thought to enhance the reaction rate
18
by π-coordination, such as p-benzoquinone and â-pinene,
did not improve the product yield (entries 12 and 13).
When the loading of PtBr was reduced to 2 mol % in 1,4-
dioxane (0.15 M), a high yield of the product was obtained
entry 14). A better yield was obtained by further
2
(
(
13) Blanco-Urgoiti, J.; Casarrubios, L.; Dominguez, G.; P e´ rez-
Castells, J. Tetrahedron Lett. 2001, 42, 3315-3317 and references
lowering the loading of catalyst to 1 mol % (entry 15).
Next, we examined the scope of this reaction. The
results are summarized in Table 2. It is noteworthy to
therein.
(
14) . (a) Rosillo, M.; Casarrubios, L.; Dom ı´ nguez, G.; P e´ rez-Castells,
J. Tetrahedron Lett. 2001, 42, 7029-7031. (b) Rosillo, M.; Dom ´ı nguez,
G.; Casarrubios, L.; Amador, U.; P e´ rez-Castells, J. J. Org. Chem. 2004,
2
mention that only 1 mol % of PtBr is sufficient to
promote this reaction; however, longer reaction times
6
9, 2084-2093.
(
15) For a review, see: Brandsma, L.; Vasilevsky, S. F.; Verkruijsse,
H. D. Application of Transition Metal Catalyst in Organic Synthesis;
Springer-Verlag: Berlin, Germany, 1999; Chapter 10.
(18) Recently, we found that the use of p-benzoquinone or â-pinene
as an additional additive enhanced the reaction rate in the Pt-catalyzed
intramolecular carboalkoxylation of alkynes, see: Nakamura, I.;
Bajracharya, G. B.; Wu, H.; Oishi, K.; Mizushima, Y.; Gridnev, I. D.;
Yamamoto, Y. J. Am. Chem. Soc. 2004, 126, 15423-15430.
(
16) Yadav, J. S.; Subba Reddy, B. V.; Srihari, P. Synlett 2001, 673-
6
75.
(17) Luche, J.-L.; Gemal, A. L. J. Chem. Soc., Chem. Commun. 1978,
9
76-977.
894 J. Org. Chem., Vol. 70, No. 3, 2005

PtBr
2
-Catalyzed Enyne Metathesis-Aromatization
TABLE 2. Synthesis of Vinylnaphthalenesa
π-bond of 1 coordinates to the electron-deficient PtBr
2
as
shown in 6, and subsequently the intermediate 7 is
generated. Cyclization of 7 would occur in exo-fashion to
produce 8. A subsequent rearrangement leads to forma-
tion of the platinum-carbene complex 9 as shown in
route a. Alternatively, a rearrangement takes place to
form the more stable tertiary carbocationic species 10 as
depicted in route b. The skeletal rearrangement of both
b
entry
1
1
R
time, h
yield of 3, %
9
and 10 leads to the formation of the cyclobutene
c
1b
1b
1c
1c
1d
1e
1f
1g
1h
1i
n-Bu
n-Bu
79
117
21
45
93
52
35
d
c
intermediate 11. Formation of the cyclobutene intermedi-
ate 11 may be explained also via formation of the
platinacyclopentene intermediate 12 by oxidative coup-
ling followed by the reductive elimination of PtLn.
Although this pathway is not completely ruled out, we
prefer the mechanism proceeding via the zwitterionic
vinylmetal complex 7 because Pt(0) [or Pd(0)] completely
halted the enyne metathesis reaction. Under reflux
reaction conditions, rearrangement of the cyclobutene
intermediate 11 takes place to generate the 1,3-diene 2.
2
3
4
n-Hex
n-Hex
60
60
d
c
5
6
7
8
cyclopropyl
cyclohexyl
Ph
p-CF3-C6H4
p-Me-C6H4
p-MeO-C6H4
50
c
168
45
52
76
69
51
43
83
c
9
0
92
e
1
88
a
The reaction of 1 (0.5 mmol) was carried out in the presence
of 2 mol % of PtBr2 in 1,4-dioxane at 120 °C under Ar unless
otherwise noted. Isolated yield. Significant amounts of starting
material were recovered. d 1 mol % of PtBr2 was used. 5 mol %
of PtBr2 was used.
b
c
2
At this stage, PtBr acts as a Lewis acid and induces the
elimination of MeOH from 2, leading to the aromatized
final product 3.
e
The intermediates 11 and 2 were isolated successfully
in support of the proposed mechanism. The substrate 1a,
were required when we employed substrates 1b and 1c
(
compare entries 1 and 2 and entries 3 and 4). Hence,
2
upon treatment with 4 mol % of PtBr with MeCN as a
use of 2 mol % of PtBr
standard conditions for further studies. In the presence
of 2 mol % of PtBr in 1,4-dioxane at 120 °C, the
2
in 1,4-dioxane was chosen as the
solvent at lower temperature (60 °C), afforded the cy-
clobutene derivative 11a in a moderate yield under
20
2
nonoptimized reaction conditions (Scheme 5). The reac-
cyclization-aromatization of 1b and 1c afforded the
corresponding vinylnaphthalenes 3b and 3c, respectively,
in moderate yields (entries 1 and 3). Longer reaction
times were observed with cyclopropyl and cyclohexyl
substituents on the alkynyl terminus (entries 5 and 6).
The reaction of 1f proceeded smoothly producing 3f in
tion of 11a in the presence of a catalytic amount of PtBr
at elevated temperature (120 °C) in CH CN afforded the
final product 3a. In the absence of the catalyst, when 11a
was heated at 120 °C in MeCN, the thermodynamically
stable 1,3-diene 2a was produced. Treatment of 2a with
PtBr (2 mol %) in CH CN at 120 °C gave the final
2 3
product 3a.
2
3
7
6% yield (entry 7). Trifluoromethyl, methyl, and meth-
oxy substituents at the para position of the aromatic ring,
as in 1g, 1h, and 1i, were tolerated under the reaction
conditions and produced the corresponding products in
good to acceptable yields (entries 8-10).
The reaction of 1j bearing an electron-withdrawing
substituent on the para position of the aromatic ring
proceeded in a similar manner to afford 3j in 45% yield
Conclusion
The 1,7-enynes, connected through an aromatic ring
and bearing a methoxy group at the 4-position, under-
went Pt-catalyzed enyne metathesis followed by aroma-
tization to afford vinylnaphthalene derivatives in one pot.
Although formation of a six-membered ring through
enyne metathesis is not easy, this procedure allows a
quick access to a new six-membered ring. The presence
of a methoxy substituent in the tether further extends
the scope of this reaction to generate an aromatic ring.
(
eq 4). The reaction of 1k bearing an electron-donating
group on the aromatic ring produced only a trace amount
of the product 3k (eq 5).19
2
The dual role of PtBr as a transition metal catalyst and
a Lewis acid was exemplified in this reaction. The
cyclobutene intermediate in the reaction was isolated and
fully characterized, which supported that pathway c
(
Scheme 2) was involved in the metathesis. Further
research and improvement of the reaction conditions are
under investigation in our laboratory.
Experimental Section
Materials. Starting materials 1 were prepared from the
coupling of corresponding 2-bromobenzaldehyde derivatives
1
5
A plausible reaction mechanism is outlined in Scheme
. First, in a manner similar to that of the ordinary
transition metal complexes, the electron-rich alkynyl
with alkynes by using a Sonogashira reaction followed by
scandium triflate-catalyzed allylation with allyltimethylsi-
4
1
6
lane. 2-Bromobenzaldehyde, alkynes, and allyltrimethylsi-
(
19) The starting material 1k was decomposed under the reaction
(20) Structure of 11a was fully characterized by NMR experiments
[see the Supporting Information].
conditions.
J. Org. Chem, Vol. 70, No. 3, 2005 895

Bajracharya et al.
SCHEME 4. A Plausible Reaction Mechanism
SCHEME 5
lane were purchased and used as received. 2-Bromo-5-
Representative Procedure for the Preparation of 1-(1-
2
1
(
trifluoromethyl)benzaldehyde and 2-bromo-5-methoxyben-
Methylenebutyl)naphthalene (3a). To a mixture of PtBr
2
2
2
zaldehyde were prepared according to the reported proce-
dures. Cyclopropylacetylene was prepared according to the
(1.1 mg, 0.003 mmol, 1 mol %) and 1-(1-methoxy-but-3-enyl)-
2-(pent-1-ynyl)benzene (1a) (68.5 mg, 0.3 mmol) was added
1,4-dioxane (2.0 mL, 0.15 M) under an argon atmosphere in a
Wheaton microreactor. The mixture was stirred for 20 min at
room temperature then heated at 120 °C for 15 h. The product
was filtered through a short column of SiO with hexane/ethyl
2
acetate (4/1) as an eluent, and the resulting filtrate was
2
3
literature. All the catalysts used were commercially avail-
able.
(
21) (a) Sˇ indel a´ rˇ , K.; Dlaba cˇ , A.; Mety sˇ ov a´ , J.; Kak a´ cˇ , B.; Holubek,
J.; Sv a´ tek, E.; Sediv y´ , Z.; Protiva, M. Collect. Czech. Chem. Commun.
975, 40, 1940-1959. (b) Perchonock, C. D.; Uzinskas, I.; McCarthy,
1
M. E.; Erhard, K. F.; Gleason, J. G.; Wasserman, M. A.; Muccitelli, R.
M.; DeVan, J. F.; Tucker, S. S.; Vickery, L. M.; Kirchner. T.; Weichman,
B. M.; Mong, S.; Scott, M. O.; Chi-Rosso, G.; Wu, H.-L.; Croole, S. T.;
Newton, J. F. J. Med. Chem. 1986, 29, 1442-1452.
(22) Gies, A.-E.; Pfeffer, M. J. Org. Chem. 1999, 64, 3650-3654.
(23) Corley, E. G.; Thompson, A. S.; Huntington, M. In Organic
Synthesis; Hart, D. J., Ed.; 1999; Vol. 77, pp 131-235.
896 J. Org. Chem., Vol. 70, No. 3, 2005

2
PtBr -Catalyzed Enyne Metathesis-Aromatization
concentrated. The residue was purified by column chromatog-
raphy (silica gel, hexane/ethyl acetate; 100/1) to afford 1-(1-
903, 801, 778 cm-1. Anal. Calcd for C15
H, 8.22. Found: C, 92.05; H, 8.13. HRMS (EI) calcd for C H
16
H16 (196.29): C, 91.78;
1
5
1
methylenebutyl)naphthalene (3a) in 75% yield (44.0 mg). H
+
(
M ) 196.1252, found 196.1248.
NMR (400 MHz, CDCl
3
) δ 0.92 (t, J ) 7.4 Hz, 3H), 1.48-1.39
(
m, 2H), 2.49 (t, J ) 7.4 Hz, 2H), 5.07 (d, J ) 2.0 Hz, 1H),
5
.39-5.38 (m, 1H), 7.27 (dd, J ) 1.2, 6.8 Hz, 1H), 7.48-7.40
Supporting Information Available: Characterization
data and spectra of all products. This material is available free
of charge via the Internet at http://pubs.acs.org.
(
m, 3H), 7.76-7.74 (m, 1H), 7.86-7.82 (m, 1H), 8.06-8.02 (m,
1
3
1
1
1
3
H). C NMR (100 MHz, CDCl ) δ 13.9, 21.3, 40.8, 115.1,
24.9, 125.1, 125.5, 125.6, 125.8, 127.0, 128.1, 131.2, 133.6,
41.4, 148.8. IR (neat) 3059-2870, 1636, 1590, 1506, 1463,
JO048273Q
J. Org. Chem, Vol. 70, No. 3, 2005 897