Tin- and germanium-substituted enol ethers from fischer carbene complexes and group 14 vinyl derivatives

Article abstract of DOI:10.1021/om9703999

Trialkyltin- or trialkylgermanium-substituted enol ethers 5 and 2-alkoxy-1,3-butadienes 7 have been prepared by the reaction of Fischer carbene complexes 1-3 and 6 with vinylstannanes and -germanes 4. The opening of the metallacyclobutane intermediate by

Full text of DOI:10.1021/om9703999

Organometallics 1997, 16, 4525-4526
4525
Tin - a n d Ger m a n iu m -Su bstitu ted En ol Eth er s fr om
F isch er Ca r ben e Com p lexes a n d Gr ou p 14 Vin yl
Der iva tives
J ose´ Barluenga,* Rosario Gonza´lez, and Francisco J . Fan˜ana´s
Instituto Universitario de Qu´ımica Organometa´lica “Enrique Moles”-Unidad Asociada
al CSIC, J ulia´n Claverı´a, 8, Universidad de Oviedo, 33071 Oviedo, Spain
Received May 13, 1997X
Sch em e 1
Summary: Trialkyltin- or trialkylgermanium-substitut-
ed enol ethers 5 and 2-alkoxy-1,3-butadienes 7 have been
prepared by the reaction of Fischer carbene complexes
1-3 and 6 with vinylstannanes and -germanes 4. The
opening of the metallacyclobutane intermediate by met-
alloid migration has been shown by using a 2,2-dideu-
teriated olefin.
Fischer carbene complexes have attracted much at-
tention as useful reagents in organic synthesis.1 Among
the great variety of reactions of group 6 metal carbene
complexes, cyclopropanation of electron-deficient and
electron-rich alkenes has been one of the most developed
transformations.2 However, only a few examples of
intramolecular cyclopropanation of unactivated olefins
have been reported.3 In some cases, in addition to the
expected cyclopropane, varying amounts of new prod-
ucts corresponding to C-H insertion into the starting
alkene have been detected.4 Moreover, the reaction of
iron carbene complexes with functional olefins leads
exclusively to C-H insertion products.5 Here we report
the preparation of tin- and germanium-substituted enol
ethers and 2-alkoxy-1,3-butadienes from group 6 metal
alkoxycarbene complexes and trialkylvinyltin and tri-
alkylvinylgermanium compounds.
Ta ble 1. P r ep a r a tion of P r od u cts 5 a n d 7 fr om
Ca r ben e Com p lexes 1-3 a n d 6 a n d
Vin ylm eta lloid s 4
starting
carbene
vinyl-
metalloid
t
pro-
yield
R
R′
M′
(h) duct Z/E (%)a
1a
2a
3a
1a
1b
1b
6a
6b
6b
6c
6d
6e
Ph
Me
Me
Me
Me
Me
Me
Me
4a
4a
4a
4b
4a
4b
4a
4a
4b
4a
4a
4b
SnBu3 1.5 5a a
SnBu3 0.25 5a a
b
b
b
b
b
b
62
60
27
72
64
69
Ph
Ph
Ph
Me
Me
Ph
SnBu3
GeEt3
SnBu3
GeEt3
3
1
3
3
5a a
5a b
5ba
5bb
SnBu3 0.5 7a a 1/3 60
2-Fuc Me
2-Fu Me
Ph
2-Fu 2-Ph-c-C6H11
2-Fu (-)-menthyl
SnBu3
GeEt3
1
3
7ba 1/5 68
7bb 1/5 80
(-)-menthyl
SnBu3 0.25 7ca 1/4 58
SnBu3 0.5 7d a 1/5 65
GeEt3
3
7eb 1/5 76
a
b
Isolated yield based on the starting carbene complex. Only
the Z isomer was detected. c Fu ) furyl.
The reaction of aliphatic and aromatic chromium
carbene complexes 1 with tributylvinyltin (4a ) in tolu-
ene at 80 °C gives rise to enol ethers 5 as a single
Sch em e 2
X Abstract published in Advance ACS Abstracts, September 1, 1997.
(1) (a) Schmaltz, H. G. Angew. Chem., Int. Ed. Engl. 1994, 33, 303.
(b) Reissig, H. U. Org. Synth. Highlights 1991, 186. (c) Wulff, W. D.
Metal Carbene Cycloadditions. In Comprehensive Organic Synthesis;
Trost, B. M., Ed.; Pergamon Press: Oxford, U.K., 1991; Vol. 5, p 1065.
(d) Do¨tz, K. H. New J . Chem. 1990, 14, 433. (e) Wulff, W. D. In
Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F.
G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol.
diastereoisomer (Scheme 1 and Table 1). The Z config-
uration was established by NOE experiments. Molyb-
denum and tungsten complexes 2 and 3 also afford enol
ethers 5 under the same reaction conditions. The
relative reactivity of the three metal complexes is Mo
> Cr > W. While complex 1a (M ) Cr) is consumed
after 1.5 h when heated in the presence of 4a , complex
3a (M ) W) requires a longer time (3 h) for the reaction
to go to completion. The reaction of the molybdenum
complex 2a is the fastest (0.25 h). In order to study the
scope of this reaction, we tested different metal-
substituted olefins. When complexes 1 were heated in
toluene in the presence of triethylvinylgermanium (4b),
(Z)-germanium-substituted enol ethers 5 were isolated
as the only product (Scheme 1 and Table 1). However,
the formation of the corresponding enol ethers from
trialkylvinylsilanes was not observed; instead, mixtures
of unidentified compounds were obtained.
12,
p 469. (f) Hegedus, L. S. In Comprehensive Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon Press: Oxford, U.K., 1995; Vol. 12, p 549.
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Brookhart, M.; Studabaker, W. B. Chem. Rev. 1987, 87, 411. (c) Doyle,
M. P. In Comprehensive Organometallic Chemistry II; Abel, E. W.,
Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, U.K.,
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96, 271.
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Soc. 1984, 106, 3754. (b) Casey, C. P.; Shusterman, A. J . Organome-
tallics 1985, 4, 736. (c) Casey, C. P.; Hornung, N. L.; Kosar, W. P. J .
Am. Chem. Soc. 1987, 109, 4908. (d) Alvarez Toledano, C.; Rudler, H.;
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(e) So¨derberg, B. C.; Hegedus, L. S. Organometallics 1990, 9, 3113. (f)
Barluenga, J .; Montserrat, J . M.; Flo´rez, J . J . Chem. Soc., Chem.
Commun. 1993, 1068.
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(b) Wienand, A.; Reissig, H. U. Angew. Chem., Int. Ed. Engl. 1990, 29,
1129. (c) Wienand, A.; Reissig, H. U. Tetrahedron Lett. 1990, 29, 2315.
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(5) Semmelhack, M. F.; Tamura, R. J . Am. Chem. Soc. 1983, 105,
6750.
Chromium alkenylcarbene complexes 6 also react
with trialkylvinyltin or -germanium compounds, afford-
ing 2-alkoxy-1,3-butadienes 7 as a mixture of Z/ E
S0276-7333(97)00399-3 CCC: $14.00 © 1997 American Chemical Society

4526 Organometallics, Vol. 16, No. 21, 1997
Communications
Sch em e 3
diastereoisomers (Scheme 2 and Table 1). In all cases,
the E isomer is the major one, as shown by NOE
experiments. The reactions of chiral complexes 6c-e
are especially interesting, since they lead to chiral
2-alkoxy-1,3-butadienes 7ca , 7d a , and 7eb.
that the reaction of group 6 metal carbene complexes
and tin- or germanium-substituted alkenes involves the
migration of the trialkyltin or -germanium moieties. The
diastereoselectivity of the process can be explained by
assuming trans positions of the R group and the group
14 metal in the metallacyclobutane 9, which would lead
to Z enol ethers. However, in the case of R,â-unsatur-
ated carbenes, the interaction of the double bond with
the empty d orbitals of the tin or germanium atoms
would favor cis positions of both groups, affording the
E isomers as major compounds.
In conclusion, we have described the reaction of
Fischer carbene complexes and tin- and germanium-
substituted olefins, leading to enol ethers and 2-alkoxy-
1,3-butadienes bearing a trialkyltin or -germanium
moiety, in a process involving a tin or germanium
migration. We are currently investigating the synthetic
applications of the reaction products, in particular, of
the tin-substituted derivatives which can be further
derivatized by tin-lithium transmetalation followed by
treatment with electrophiles.8 The study of their be-
havior as allylic tin reagents in addition reactions with
carbonyl compounds9 and as dienes10 in [4 + 2] cycload-
ditions also is underway.
The possible mechanisms for the formation of com-
pounds 5 and 7 are depicted in Scheme 3. Two regio-
isomeric metallacyclobutanes 8 and 9 could be obtained
in the first step. Enol ethers 5 and 7 could be formed
from metallacyclobutane 8 by â-hydride elimination to
afford 10 followed by reductive elimination (pathway a).5
On the other hand, pathway b would involve the opening
of metallacyclobutane 9 by group 14 metal (Sn, Ge)
migration to the electrophilic center6 (M) and subse-
quent reductive elimination from intermediate 11.
Finally, there is a third possibility from both metalla-
cyclobutanes via reductive elimination to give cyclopro-
pane 12 (pathway c).4a Ring opening of this interme-
diate would lead to the final products.
In order to study the mechanism of this transforma-
tion, chromium carbene complex 1a was treated with
2,2-dideuteriated vinylgermanium derivative 13, giving
rise exclusively to compound 14 in 68% yield (Scheme
3). This result rules out pathways a and c, since they
would afford an enol ether bearing the deuterium atoms
at different positions. However, formation of 14 is in
agreement with pathway b.7 Therefore, we can conclude
In a typical procedure tributylvinyltin 4a (1.2 mmol,
0.35 mL) was added to a solution of chromium carbene
complex 1a (1 mmol, 0.31 g) in anhydrous toluene (10
mL) under nitrogen. The reaction mixture was heated
at 80 °C for 1.5 h, cooled, and then filtered through
Celite. The solvent was removed under vacuum, and
the product was purified by column chromatography on
silica gel using hexane as eluent. Yield: 0.27 g (62%).
(6) (a) Macomber, D. W.; Madhukar, P.; Rogers, R. D. Organome-
tallics 1991, 10, 2121. (b) Herndon, J . W.; Patel, P. P. J . Org. Chem.
1996, 61, 4500.
(7) One of the reviewers has suggested an alternative mechanism
involving cleavage of the cyclopropane to give a biradical intermediate.
However, this opening usually occurs at higher temperatures. For a
review, see: Wong, H. N. C.; Hon, M.-Y.; Tse, C.-W.; Yip, Y.-C.; Tanko,
J .; Hudlicky, T. Chem. Rev. 1989, 89, 165.
Ack n ow led gm en ts. Financial support from the
Direccio´n General de Investigacio´n Cient´ıfica y Te´cnica
(DGICYT, Grant No. PB92-1005) is gratefully acknowl-
edged. R.G. thanks the Ministerio de Educacio´n y
Ciencia for a fellowship.
Su p p or tin g In for m a tion Ava ila ble: Listings of spectral
data and figures giving 13C NMR spectra for 5, 7, and 14 (15
pages). Ordering information is given on any current mast-
head page.
(8) (a) Corey, E. J .; Wollenberg, R. H. J . Am. Chem. Soc. 1974, 96,
5881. (b) Seyferth, D.; Lambert, R. L. J . Organomet.Chem. 1975, 88,
287. (c) Seebach, D.; Bu¨rstinghaus, R. Angew. Chem. 1975, 87, 37.
(9) Yamamoto, Y.; Asao, N. Chem. Rev. 1993, 93, 2207.
(10) Barluenga, J .; Toma´s, M.; Sua´rez-Sobrino, A.; Lo´pez, L. A. J .
Chem. Soc., Chem. Commun. 1995, 1785 and references cited therein.
OM9703999