Ruthenium-Catalyzed Aromatization of Aromatic Enynes via the 1,2-Migration of Halo and Aryl Groups: A New Process Involving Electrocyclization and Skeletal Rearrangement

Full text of DOI:10.1021/ja0379159

Published on Web 12/02/2003
Ruthenium-Catalyzed Aromatization of Aromatic Enynes via the 1,2-Migration
of Halo and Aryl Groups: A New Process Involving Electrocyclization and
Skeletal Rearrangement
Hung-Chin Shen, Sitaram Pal, Jian-Jou Lian, and Rai-Shung Liu*
Department of Chemistry, National Tsing-Hua UniVersity, Hsinchu, Taiwan, R.O.C
Received August 14, 2003; E-mail: rsliu@mx.nthu.edu.tw
Scheme 1
Electrocyclization of aromatic enynes can be achieved by a metal-
catalyzed reaction via the generation of a metal vinylidene
intermediate (Scheme 1, eq 1).1,2 Merlic and Pauly first reported1
the aromatization of dienyl alkynes using a RuCl2(p-cymenes)PPh3
catalyst. Iwasawa and co-workers2a achieved similar results with
W(CO)5(THF) catalyst. This tungsten catalyst also effected the
electrocyclization of o-(iodoethynyl)styrenes to give iodo-substituted
naphthalene,2b and the mechanism is thought to involve tungsten-
iodinated vinylidene intermediates. On the basis of this principle,
the metal-catalyzed aromatization of o-(ethynyl)phenyl ketones3 and
aldehyde4 has recently been developed to give useful oxygen-
containing compounds. Surprisingly, none of these catalytic reac-
tions is accomplished by cationic metal complexes, which may lead
to a novel skeletal rearrangement in electrocyclization via the
generation of reactive carbocation intermediates.5 In this report,
we describe the cleavage of carbon-carbon and carbon-halide
bonds in the aromatization of o-(ethynyl)styrenes using a cationic
ruthenium complex. Notably, the regiochemistries for the 1,2-shifts
of aryl and halo substituents are completely different (Scheme 1,
eq 2).
Table 1. Aromatization of Various O-(Ethynyl)styrenes
We first examined the catalytic transformations of various
o-(ethynyl)styrenes with TpRu(PPh3)(CH3CN)2PF6 (10 mol %)
catalyst6 to study the structural effects of the substrates. The results
are summarized in Table 1. As shown in entries 1-3, this catalyst
effected the aromatization of monosubstituted, 1′,2′- or 1′,1′-
disubstituted, and 1′,1′,2′-trisubstituted styrene derivatives 1-4 to
naphthalene derivatives 6-9 (>88% yields) without migration of
the styryl substituent. This method is also applicable to the
cyclization of 2′-iodovinyl derivative 5 (E/Z )1.1),7 but with a
remarkable 1,2-shift of the iodo-substituent to the terminal alkyne
carbon, based on the results with a deuterated sample d-5a. A
similar phenomenon is observed for the 1,2-bromo shift based on
the cyclization of a deuterated sample d-5b, but the yield of
3-bromonaphthalene d-10b is low (30%).
a 10 mol % catalyst, [substrate] ) 0.05 M in toluene, 110 °C, 6-8 h.
b Yields were reported after separation from the silica column. c A mixture
of Z/E isomers was present for 5a, d-5a, and 5b.
We also observed a 1,2-aryl shift for the aromatization of various
2-(2′-aryl-vinyl)ethynylbenzenes even though the parent compound
d-2 failed to show such a phenomenon (Table 1, entry 2). As shown
in Table 3 (entries 1,2), the 2′-aryl group preferably underwent a
1,2-shift to the 1′-vinyl carbon rather than the terminal alkyne
carbon. The NMR spectral data of naphthalene derivatives 32A
and 32B are identical to those of authentic samples prepared from
independent routes.9 This shift is favored by electron-donating
groups mainly on the migrating aryl R group and less on the para-
phenyl substituent X according to the results in entries 1-7.
We prepared deuterated samples d-14b and d-31 to better
understand the mechanism of migration. As shown in Scheme 2,
the 1′-vinyl proton of compound 14b remains unshifted, whereas
the alkynyl proton undergoes 1,2-migration to the internal alkynyl
carbon (entries 1-2). In contrast, the 1′-vinyl proton of compound
31 undergoes 1,2-migration to its 2′-carbon (entry 3), whereas the
alkynyl proton migrates to the internal carbon (entry 4). We finally
prepared a 13C-containing sample 31 which has 10 atom %
enrichment at the dCHPh carbon. Notably, the 13C NMR spectrum
of the resulting product 38A shows the enrichment at δ 140.0 that
is assigned to be the quarternary CPh carbon according to 13C-1H
HMBC spectra.
We prepared various 2-(2′-iodoethenyl)ethynylbenzenes to ex-
amine the generality of this cycloisomerization. The halide species
11a-17 contain a mixture of Z and E (E/Z ) 1.5-1.0) isomers.7
Most of the samples were prepared in deuterated form to confirm
the 1,2-shift. Entries 1-4 show the suitability of this reaction with
t
a change in the para-phenyl substituents (R ) OMe, Bu, Me, F),
with the methoxy group being the most effective. The relative
positions of the deuterium, iodo, and methoxy groups of product
18a were confirmed by 1H NOE spectra.8 Such an iodo shift works
well not only for meta-methoxy, 3,4-(methylenedioxy), and 3,4-
dimethoxyphenyl groups 12, 13, 14b (entries 5, 6, and 8), but also
for dienyl alkyne derivatives 15 and 16 (entries 9, 10). A low yield
(40%) of naphthalene 21a was obtained for the bromo shift (entry
7). For 1-ethyl-2-iodovinyl species 17, two naphthalenes 24a and
24b were obtained in equal proportions, resulting from nonmigration
and 1,2-migrations pathways, respectively.
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J. AM. CHEM. SOC. 2003, 125, 15762-15763
10.1021/ja0379159 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Aromatization of 2-(2′-Iodovinyl)ethynylbenzene
Scheme 3
a 1,2-shift of a carbon-carbon bond to generate intermediate F.
Notably, a loss of deuterium content of naphthalene 38A (75%)
was observed as compared to its starting compound d-31 (95%).
This information suggests that species F undergoes a 1,2-hydride
shift to give the more stable diphenyl methyl cation G. Dissociation
of a proton from species G, followed by cleavage of the ruthenium-
naphthyl bond, produces active ruthenium species and naphthalene
product.
a 10 mol % catalyst, [substrate] ) 0.05 M in toluene, 110 °C, 6-8 h.
b Yields were reported after separation from the silica column.
Table 3. 1,2-Aryl Shift for 2-(2′-Arylvinyl)ethynylbenzene
In summary, we have reported unusual 1,2-iodo and aryl shifts
in the electrocyclization of o-(ethynyl)styrenes.12 Isotopic labeling
experiments were performed to elucidate the reaction mechanism,
and the results indicate that the 1,2-aryl shift arises from 5-endo-
dig electrocyclization of a ruthenium-vinylidene species, whereas
the 1,2-iodo shift follows a 6-endo-dig pathway.
substrates
products (yields)a,b
(1) X ) Y ) H, R ) Me (25)
32A (35%), 32B (35%)
33A (60%), 33B (15%)
34A (54%), 34B (18%)
35A (64%), 35B (6%)
36A (18%), 36B (56%)
37A (61%), 37B (15%)
38A (73%), 38B (trace)
Acknowledgment. The authors wish to thank the National
Science Council, Taiwan, for support of this work.
Supporting Information Available: Experimental procedures,
synthetic schemes and spectral data of compounds 1-38, and 13C NMR
and 13C-1H HMBC spectra of 13C-enriched sample 38A (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
(2) X ) Y ) H, R ) OMe (26)
(3) X ) OMe, Y ) H, R ) Me (27)
(4) X ) OMe, Y ) H, R ) OMe (28)
(5) X ) Y ) O-CH2-O, R ) H (29)
(6) X ) Y ) O-CH2-O, R ) Me (30)
(7) X ) Y ) O-CH2-O, R ) OMe (31)
a 8 mol % catalyst, [substrate] ) 0.05 M in toluene, 110 °C, 6-8 h.
References
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Scheme 2
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A plausible mechanism (Scheme 3) involves an equilibrium3,10
between ruthenium-alkyne complexes A and ruthenium-vi-
nylidene species B. Electrocyclization of species B via a 6-endo-
dig pathway (a) gives ruthenium naphthylidene species C which
subsequently undergoes a 1,2-iodo shift to give ruthenium-η2-
naphthalene D, ultimately producing the expected product and active
ruthenium species. The mechanism in the transformation of species
C to D is analogous to the classical conversion of a methyl
substituted carbene to a metal-olefin species.11 On the basis of 2H-
and 13C-labeling results, we propose that a 1,2-aryl shift arises from
the 5-endo-dig electrocyclization (pathway b) of species B to give
ruthenium fluorenyl species E which bears a benzyl cation to induce
(11) (a) Roger, C.; Bodner, G. S.; Hatton, W. G.; Gladysz, J. A. Organome-
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(12) Our results indicate that the 1,2-iodo and phenyl shifts are not applicable
to o-(ethynyl)styrenes bearing internal alkynes.
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