Novel synthesis of pentacarbonylbenzopyranylidenetungsten(0) complexes and their Diels - Alder reaction with electron-rich alkenes [11]

Full text of DOI:10.1021/ja001481p

10226
J. Am. Chem. Soc. 2000, 122, 10226-10227
Scheme 1
Novel Synthesis of
Pentacarbonylbenzopyranylidenetungsten(0)
Complexes and Their Diels-Alder Reaction with
Electron-Rich Alkenes
Nobuharu Iwasawa,* Masahide Shido,†
Katsuya Maeyama,† and Hiroyuki Kusama
Department of Chemistry, Tokyo Institute of Technology
O-okayama, Meguro-ku, Tokyo, 152-8551
Department of Chemistry, Graduate School of Science
The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033
a 5 M amounts of W(CO)5‚THF were employed.
ReceiVed April 28, 2000
Scheme 2
It is known that terminal alkynes and carbonyl complexes of
low valent group 6 metals such as M(CO)5‚L (M ) Cr, Mo, W;
L ) THF, Et3N, etc.) are in equilibrium with their vinylidene
complexes.1 Although these complexes are expected to have
unique characteristics, their use in synthetic reactions has been
quite limited.2,3 We previously reported the highly efficient
W(CO)5‚THF-catalyzed electrocyclization of aromatic enynes via
vinylidene intermediates.4,5 This led us to expect that novel
benzopyranylidene complexes6,7 could be obtained from o-
ethynylphenyl ketones by the dienone-electrocyclization8,9 of the
corresponding vinylidene intermediates. In this paper we describe
the realization of this approach, and its application to the synthesis
of substituted naphthalenes through a Diels-Alder reaction with
electron-rich alkenes.
When o-ethynylbenzophenone 1a (R ) Ph) was treated with
1.5 M amounts of preformed W(CO)5‚THF at room temperature,
a dark blue color gradually appeared. After the mixture had been
stirred for 3 days, the crude product was purified by silica gel
chromatography to give a dark-blue, air-stable, crystalline solid.
The presence of a typical carbene carbon in the 13C NMR spectra
(δ ) 230.9 ppm) and the elemental analysis both supported the
conclusion that the expected benzopyranylidene complex 2a had
actually been produced by this procedure (75% yield). The yield
of 2a was further improved to 82% by carrying out the reaction
using 3 M amounts of W(CO)5‚THF at room temperature for 1
day.
* Address correspondence to this author at the Tokyo Institute of Technol-
ogy. Phone: 81-3-5734-2746. FAX: 81-3-5734-2746. E-mail: niwasawa@
chem.titech.ac.jp.
† The University of Tokyo.
(1) (a) Landon, S. J.; Shulman, P. M.; Geoffroy, G. L. J. Am. Chem. Soc.
1985, 107, 6739. (b) Szymanska-Buzar, T.; Downs, A. J.; Greene, T. M.;
Marshall, A. S. J. Organomet. Chem. 1995, 495, 163.
Various substituted benzopyranylidene complexes were ob-
tained by this procedure as summarized in Scheme 1. Not only
the benzophenone derivative 1a but also alkyl phenyl ketone
derivatives 1b-d gave the corresponding complexes in good
yield. Furthermore, a propenyl derivative 1e gave the complex
2e in moderate yield. However, when the o-ethynylbenzaldehyde
1f (R ) H) was employed, only unidentified polymerized products
were obtained. Direct observation of the reaction mixture by 1H
NMR revealed that some of the desired complex 2f was formed
at first, but that it decomposed completely within 1 day.
We next examined the reactivity of these complexes. When
2a was treated with Et2NH,10 the complex was consumed
instantaneously at 0 °C, and o-methylbenzophenone 6 was isolated
in 63% yield. Formation of 6 can be explained as follows: First,
Et2NH adds in a 1,6-manner to give aminal 3a (R ) Et). Then
elimination of W(CO)6 occurs to give enamine 4, which is
hydrolyzed to give 6, via iminium salt 5 (Scheme 2). Unexpect-
edly, when the same reaction was carried out using i-Pr2NH, imine
7 (R ) i-Pr) and 3-methyl-1-phenylnaphthalene 8 were isolated
in 9% and 14% yield, respectively, along with o-methylben-
(2) For the synthetic use of vinylidene intermediates containing group 6
metals, see: McDonald, F. E. Chem. Eur. J. 1999, 5, 3103 and references
therein.
(3) For a review on vinylidene complexes, see: (a) Bruneau, C.; Dixneuf,
P. H. Acc. Chem. Res. 1999, 32, 311. (b) Bruce, M. I. Chem. ReV. 1991, 91,
197. (c) Bruce, M. I.; Swincer, G. A. AdV. Organomet. Chem. 1983, 22, 59.
(4) Maeyama, K.; Iwasawa, N. J. Org. Chem. 1999, 64, 1344. See also:
Maeyama, K.; Iwasawa, N. J. Am. Chem. Soc. 1998, 120, 1928.
(5) For the first example of the transition-metal catalyzed electrocyclization
of dienynes, see: Merlic, C. A.; Pauly, M. E. J. Am. Chem. Soc. 1996, 118,
11319.
(6) Synthesis of an isomeric benzo[e]pyranylidene complex was reported
using the aldol-type reaction of a methyl carbene complex with salicylaldehyde;
however, synthesis of benzo[d]pyranylidene complexes of group 6 metals has
not been reported before. Aumann, R.; Heinen, H. Chem. Ber. 1987, 120,
537. See also: Licandro, E.; Maiorana, S.; Papagni, A.; Zanotti, G. A.; Cariati,
F.; Bruni, S.; Moret, M.; Chiesi V. A. Inorg. Chim. Acta 1994, 220, 233.
(7) Several methods have been reported for the synthesis of pyranylidene
complexes. All of them employ carbene complexes as precursors. Aumann,
R.; Meyer, A. G.; Fro¨hlich, R. J. Am. Chem. Soc. 1996, 118, 10853. References
are cited therein.
(8) For a general review, see: Okamura, W. H.; de Lera, A. R. In
ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Perga-
mon: New York, 1991; Vol. 5, pp 699-750.
(9) Professor Berke and co-workers have mentioned in one of their papers
the possibility of the formation of a benzopyranylidene complex by electro-
cyclization of vinylidene intermediate which is produced by the dimerization
of propiolate on reaction with chromium carbonyl complex. However, they
have proposed this possibility based only on the observation that the color of
the solution of this vinylidene complex changed depending on the polarity of
the solvent, and they neither detected nor isolated the pyranylidene complex
at all. See: Berke, H.; Ha¨rter, P.; Huttner, G.; Zsolnai, L. Z. Naturforsh. 1981,
36b, 929.
(10) Aumann et al. have reported that aminolysis of the 2H-pyran-2-
ylidenetungsten complex affords amino-1-tungsta-1,3,5-hexatrienes having
different structures depending on the reaction temperature, and the type of
amine involved. See: Aumann, R.; Roths, K.; Jasper, B.; Fro¨hlich, R.
Organometallics 1996, 15, 1257. See also: Yu, Z.; Aumann, R.; Fro¨hlich,
R.; Roths, K.; Hecht, J. J. Organomet. Chem. 1997, 541, 187.
10.1021/ja001481p CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/29/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 41, 2000 10227
Table 2. Synthesis of Various Naphthalene Derivatives
Table 1. Reaction of Benzopyranylidene Complexes with
Electron-Rich Olefins
R
R1, R2, X
yield (%)
Ph
Ph
Ph
i-Pr
i-Pr
t-Bu
t-Bu
R1 ) R2 ) H, X ) O-n-Bu
84a (9a)
58b (9b)
64 (9c)
66a (9f)
54 (9h)
65 (9g)
62 (9i)
R1 ) H, R2 ) X ) OEt
R1 ) R2 ) -(CH2)3-, X ) morpholine
R1 ) R2 ) H, X ) O-n-Bu
R1 ) H, R2 ) X ) OEt
R1 ) R2 ) H, X ) O-n-Bu
R1 ) H, R2 ) X ) OEt
reaction
time
yield
(%)
R
R1, R2, X
a A catalytic amount of p-toluenesulfonic acid monohydrate was
added. b This reaction was carried out at 0 °C.
Ph
Ph
Ph
R1 ) R2 ) H, X ) O-n-Bu
10 h
15 min
95a (9a)
74b (9b)
R1 ) H, R2 ) X ) OEt
R1 ) R2 ) -(CH2)3-, X ) morpholine instantaneous 79 (9c)
n-Pr R1 ) R2 ) H, X ) O-n-Bu
1 h
79 (9d)
h giving 1-phenylnaphthalene 9a in 95% yield (eq 1). In Table 1
is summarized the preparation of naphthalenes variously substi-
tuted at the 1-, 2-, and 3-positions by the reaction of various
benzopyranylidene complexes 2 and typical electron-rich alkenes.
All the reactions proceeded smoothly without the necessity of
employing a large excess of the dienophiles,12 and in particular,
the reaction with an enamine proceeded instantaneously to give
a 1,2,3-trisubstituted naphthalene in good yield.
This naphthalene synthesis could be carried out without
isolating the intermediate complexes. Thus, on treatment of the
o-ethynylphenyl ketone derivatives 1 with W(CO)5‚THF for 1
day followed by addition of a dienophile, we could obtain various
substituted naphthalene derivatives in good overall yield without
the necessity of isolating the intermediate complexes (eq 2, Table
2).
In summary, we have developed a novel method for the
synthesis of benzopyranylidene complexes from o-ethynylphenyl
ketones by electrocyclization of the vinylidene intermediates, and
have applied this method to the synthesis of substituted naph-
thalenes by the Diels-Alder reaction with electron-rich alkenes.
We are currently studying this naphthalene synthesis with a
catalytic amount of W(CO)6.
n-Pr R1 ) R2 ) -(CH2)3-, X ) morpholine instantaneous 73 (9e)
i-Pr R1 ) R2 ) H, X ) O-n-Bu
t-Bu R1 ) R2 ) H, X ) O-n-Bu
2.5 h
overnight
81 (9f)
88 (9g)
a A catalytic amount of p-toluenesulfonic acid monohydrate was
added. b A catalytic amount of TMSCl was added.
zophenone 6 in 48% yield. Formation of 7 and 8 suggests that
elimination of propene from iminium salt 5 occurs and that the
propene formed reacts with the complex 2a to give the Diels-
Alder product, which undergoes elimination of W(CO)6 followed
by air-oxidation to give naphthalene 8 (see eq 1).11
From this result the reactivity of 2a as a diene in the Diels-
Alder reaction is expected to be high, and so we next tried to
develop a method for the synthesis of substituted naphthalene
derivatives by the reaction of various dienophiles with the
benzopyranylidene complexes 2.12 When 2a (RdPh) was treated
with 4 equiv of butyl vinyl ether (R1 ) R2 ) H, X ) O-n-Bu) at
room temperature in THF, the reaction was complete within 10
(11) When we examined the reaction of 1-pentene with the benzopyra-
nylidene complex 2a in THF, the corresponding Diels-Alder product,
1-phenyl-3-propylnaphthalene, was obtained in 19% yield. It is also possible
that an enamine, generated during the reaction by abstraction of an isopropyl
methine proton by ortho quinodimethane intermediate 4, is the reactive species,
although we have not detected the expected amine ({(o-methylphenyl)methyl}-
isopropylamine) in the crude product.
(12) Wulff et al. have reported that electron-rich olefins react with
pyranylidene complexes to give dihydrobenzene derivatives. However, their
reactivity is not very high, the dienophiles are usually employed as solvent,
and it takes a long time for completion of the reaction. Wang, S. L. B.; Wulff,
W. D. J. Am. Chem. Soc. 1990, 112, 4550. Aumann et al. have also reported
the same type of reactions using a stoichimetric amount of enamines as
dienophiles. Aumann, R.; Meyer, A. G.; Fro¨hlich, R. J. Am. Chem. Soc. 1996,
118, 10853.
Acknowledgment. This research was partly supported by the Toray
Science Foundation and a Grant-in-Aid for Scientific Research from the
Ministry of Education, Science, Sports and Culture of Japan.
Supporting Information Available: Preparative methods and spectral
and analytical data of compounds 1, 2, and 6-9 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA001481P