Stereoselective [4 + 1] annulation reactions with silyl vinylketenes derived from fischer carbene complexes

Article abstract of DOI:10.1021/jo060994x

Stable silyl vinylketenes were prepared via the thermal reaction of Fischer carbene complexes with triisopropylsilyl- or tert-butyldimethylsilyl- substituted alkynes. The ability of these silyl vinylketenes to participate with carbenoid reagents in [4 + 1

Full text of DOI:10.1021/jo060994x

Stereoselective [4 + 1] Annulation Reactions with Silyl Vinylketenes
Derived from Fischer Carbene Complexes
William H. Moser,* Laura A. Feltes, Liangdong Sun, Matthew W. Giese, and
Ryan W. Farrell
Department of Chemistry, Indiana UniVersity-Purdue UniVersity Indianapolis,
Indianapolis, Indiana 46202
wmoser@mmm.com
ReceiVed May 12, 2006
Stable silyl vinylketenes were prepared via the thermal reaction of Fischer carbene complexes with
triisopropylsilyl- or tert-butyldimethylsilyl-substituted alkynes. The ability of these silyl vinylketenes to
participate with carbenoid reagents in [4 + 1] annulation reactions was investigated. The best results
were obtained with diazomethane and substituted diazomethane reagents, which provided cyclopentenone
products in excellent yields and essentially complete stereoselectivity.
Introduction
Cyclopentenones are common structural units in natural
products and useful synthons for the construction of more
complex compounds. As a result, a wide variety of strategies
for the synthesis of cyclopentenones have been developed.1 A
particularly appealing subset of these strategies consists of
cycloaddition reactions, in which the cyclopentenone ring is
formed in a single step from appropriate acyclic precursors. The
most widely established method of this type is the Pauson-
Khand reaction, involving a formal [2 + 2 + 1] cycloaddition
that joins alkene, alkyne, and carbon monoxide moieties together
(Figure 1, path a).2 However, since regio- and stereochemical
ambiguities exist with intermolecular versions of the Pauson-
Khand reaction, motivation to develop complementary methods
persists.
FIGURE 1. Cycloaddition strategies toward cyclopentenones.
ties are avoided.3 As depicted in Figure 1, path b, the
combination of a vinylketene and a carbenoid unit would appear
to represent a very direct approach to cyclopentenones. How-
ever, despite the ability of vinylketenes to serve as versatile
four-carbon building blocks,4 their use in [4 + 1] cycloaddition
reactions is rendered difficult by their characteristic instability
and a preference to undergo dimerization and/or [2 + 2]
cycloaddition reactions instead.5 In contrast, silyl vinylketenes
A [4 + 1] cycloaddition provides another attractive approach
to the cyclopentenone ring system since regiochemical difficul-
(1) For reviews, see: (a) Gibson, S. E.; Lewis, S. E.; Mainolfi, N. J.
Organomet. Chem. 2004, 689, 3873. (b) Tius, M. A. Acc. Chem. Res. 2003,
36, 284. (c) Mitsudo, T.; Kondo, T. Synlett 2001, 309. (d) Mikolajczyk,
M.; Mikina, M.; Zurawinski, R. Pure Appl. Chem. 1999, 71, 473. (e)
Iwasawa, N. Synlett 1999, 13. (f) Piancatelli, G.; D’Auria, M.; D’Onofrio,
F. Synthesis 1994, 867. (g) Hudlicky, T.; Price, J. D. Chem. ReV. 1989, 89,
1467. (h) Paquette, L. A. Top. Curr. Chem. 1984, 119, 1. (i) Ramaiah, M.
Synthesis 1984, 529.
(2) For recent reviews of the Pauson-Khand reaction, see: (a) Sugihara,
T.; Yamaguchi, M.; Nishizawa, M. Chem.-Eur. J. 2001, 7, 1589. (b)
Brummond, K. M.; Kent, J. L. Tetrahedron 2000, 56, 3263. (c) Keun Chung,
Y. Coord. Chem. ReV. 1999, 188, 297. (d) Oliver, G.; Schmalz, H. Angew.
Chem., Int. Ed. 1998, 37, 911. (e) Schore, N. E. Org. React. 1991, 40, 1.
(3) For a rhodium-catalyzed carbonylative [4 + 1] cycloaddition, see:
Murakami, M.; Itami, K.; Ito, Y. Angew. Chem., Int. Ed. Engl. 1995, 34,
2691.
(4) For a review, see: Tidwell, T. T. Ketenes; Wiley: New York, 1991.
(5) Vinylketenes stabilized by transition metals have also been utilized
as four-carbon synthons. See: Gibson, S. E. AdV. Organomet. Chem. 1999,
44, 275.
10.1021/jo060994x CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/26/2006
6542
J. Org. Chem. 2006, 71, 6542-6546

[4 + 1] Annulation Reactions with Silyl Vinylketenes
TABLE 1. Synthesis of Silyl Vinylketenes
TABLE 2. Silyl Vinylketenes from 1a and Alkyl- or
Alkenyl-Substituted Alkynes
carbene
products
entry
complex
alkyne
SiR3
(% yield)
1
2
3
4
5
6
7
1a
1b
1c
1d
1d
1e
1d
2
2
2
2
2
2
3
TIPS
TIPS
TIPS
TIPS
TBS
4a, R′ ) CH3 (58)
4b, R′ ) n-Bu (65)
4c, R′ ) c-C3H5 (32)
4d, R′ ) Ph (88)
4e, R′ ) Ph (55)
4f, R′ ) (p-OMe)Ph (87)
4g (76)
TIPS
TIPS
4a-f as the major products in good yields. The use of 3 to
generate silyl vinylketene 4g (entry 7) also demonstrates that
substitution on the aromatic alkyne precursor is tolerated.
Structural assignments of silyl vinylketenes 4, which were
readily purified by crystallization or flash chromatography on
silica gel, were initially based on characteristic IR ketene
absorbances (ca. 2085-2090 cm-1) and 13C NMR chemical
shifts for the ketene carbonyl carbon and silyl-bearing carbon
(ca. 179-180 ppm and 15-16 ppm, respectively).11 As previ-
ously reported,10 X-ray crystallographic analysis of 4e verified
the proposed structure and demonstrates the exclusive formation
of the (E)-alkene isomer in this process when carbene complexes
bearing aryl groups are employed. To verify the same stereo-
chemical result for carbene complexes bearing sterically and
electronically different alkyl groups, 4c was subjected to X-ray
crystallographic structure determination, and indeed the (E)-
alkene isomer was observed in this case as well.12 The exclusive
formation of the (E)-alkene isomers in this process is consistent
not only with previously reported examples, but also with the
“trans effect” hypothesis suggested by the Wulff group.13 In
support of this stereoelectronic hypothesis, Wulff and co-workers
demonstrated that formation of the corresponding (Z)-alkene
isomers in related transformations required the use of alkynes
bearing strongly electron-withdrawing substituents such as aryl
or alkyl ketones.13
have been demonstrated as viable precursors to cyclopentenones6
and other cycloadducts.7 The silyl substituent provides signifi-
cant stabilization to the ketene moiety and suppresses the
dimerization pathway,8 thereby facilitating their participation
in [4 + 1] cycloaddition reactions with carbenoid reagents.
The development of silyl vinylketenes in [4 + 1] cycload-
ditions has entailed not only explorations of the scope of the
transformation, but also the evolution of new methods to prepare
the silyl vinylketene precursors themselves.8,9 Previously we
have established an efficient protocol for the construction of
silyl vinylketenes from the thermal reaction of Fischer carbene
complexes and silyl-substituted alkynes.10 On the basis of the
unique silyl vinylketene substitution pattern available from this
method, we have chosen to further examine the process and
investigate their utility in [4 + 1] cycloaddition reactions with
carbenoid units. Herein we report a highly efficient and
stereoselective construction of functionalized cyclopentenones
that complements existing [4 + 1] strategies.
Results and Discussion
Silyl vinylketenes were prepared by our previously reported
thermal reaction of Fischer carbene complexes 1a-e with
triisopropylsilyl (TIPS) or tert-butyldimethylsilyl (TBS) sub-
stituted alkynes.10 As indicated in Table 1 (entries 1-6), heating
benzene solutions of 1 and 2 at reflux under argon atmosphere
for 12-24 h afforded the yellow crystalline silyl vinylketenes
The use of the silyl-substituted phenylacetylenes is notable
in that it promotes capture of the chromium fragment by the
phenyl ring and affords access to the silyl vinylketenes as arene
complexes that display excellent thermal stability.10 However,
we have also observed that arene complex formation is not a
prerequisite for generation of silyl vinylketenes by this method.
As shown in Table 2, treatment of Fischer carbene complex 1a
with the TIPS-substituted alkynes 5-7 under the same reaction
conditions generates silyl vinylketenes 8-11, which are all
stable to purification via silica gel chromatography.
(6) (a) Davie, C. P.; Danheiser, R. L. Angew. Chem., Int. Ed. 2005, 44,
5867. (b) Rigby, J. H.; Wang, Z. Org. Lett. 2003, 5, 263. (c) Dalton, A.
M.; Zhang, Y.; Davie, C. P.; Danheiser, R. L. Org. Lett. 2002, 4, 2465. (d)
Loebach, J. L.; Bennett, D. M.; Danheiser, R. L. J. Am. Chem. Soc. 1998,
120, 9690. (e) Colomvakos, J. D.; Egle, I.; Ma, J.; Pole, D. L.; Tidwell, T.
T.; Warkentin, J. J. Org. Chem. 1996, 61, 9522.
Our initial cycloaddition studies entailed the reaction of arene
chromium tricarbonyl-containing silyl vinylketenes (12) with
(7) Bennett, D. M.; Okamoto, I.; Danheiser, R. L. Org. Lett. 1999, 1,
641.
(8) Huang, W.; Tidwell, T. T. Synthesis 2000, 457 and references therein.
(9) (a) Pommier, A.; Kocienski, P.; Pons, J. J. Chem. Soc., Perkin Trans.
1 1998, 2105. (b) Loebach, J. L.; Bennett, D. M.; Danheiser, R. L. J. Org.
Chem. 1998, 63, 8380.
(11) Do¨tz, K. H.; Fu¨gen-Ko¨ster, B. Chem. Ber. 1980, 113, 1449.
(12) See the Supporting Information for X-ray crystallographic structures.
(13) Waters, M. L.; Brandvold, T. A.; Isaacs, L.; Wulff, W. D.;
Rheingold, A. L. Organometallics 1998, 17, 4298.
(10) Moser, W. H.; Sun, L.; Huffman, J. C. Org. Lett. 2001, 3, 3389.
J. Org. Chem, Vol. 71, No. 17, 2006 6543

Moser et al.
TABLE 3. [4 + 1] Annulation Reactions
TABLE 5. Removal of Chromium Fragment
carbenoid
reagent
products
(% yield)
entry
substrate
SiR3
R′
R′′
product
entry
1
SiR3
R′
1
2
3
4
5
6
7
8
15a
15b
15c
15d
18a
18b
18c
18d
18e
18f
TIPS
TIPS
TIPS
TBS
TIPS
TIPS
TIPS
TBS
TIPS
TIPS
TIPS
TIPS
TBS
CH3
H
H
H
H
TMS
TMS
TMS
TMS
Ph
Ph
Ph
Ph
Ph
19a
19b
19c
19d
19e
19f
19g
19h
19i
+
-
TIPS
CH3
15a (49)
Me2S-CH2
n-Bu
Ph
13
13
2
3
TIPS
TIPS
n-Bu
CH3
15b (27)
15a (84)
Ph
CH2N2
14
14
14
14
CH3
n-Bu
Ph
4
5
6
TIPS
TIPS
TBS
n-Bu
Ph
Ph
15b (83)
15c (72)
15d (70)
Ph
9
CH3
n-Bu
Ph
(p-OMe)Ph
Ph
10
11
12
13
19j
19k
19l
18g
18h
18i
TABLE 4. Stereoselective [4 + 1] Annulation Reactions
19m
reaction mixtures. Notably, the cyclopentenone products 18
contain two contiguous stereogenic centers, and in all cases only
a single diastereomer could be observed by 1H NMR spectros-
copy of the crude reaction mixtures. X-ray crystallographic
analysis of cyclopentenone 18h established the cis relationship
between the two phenyl substituents, while the crystal structure
of 18a verifies that the same cis relationship is observed when
(trimethylsilyl)diazomethane is utilized, even with the sterically
smaller methyl group.12
In all cases, the chromium tricarbonyl group is readily
removed from the [4 + 1] annulation products by treatment
with ceric ammonium nitrate (CAN), providing cyclopentenones
19a-m in essentially quantitative yields (Table 5). For sub-
strates 15a-d and 18a-d, photolytic removal of the chromium
fragment is also successful; however, similar irradiation of
compounds 18e-i results in rapid product decomposition. This
decomposition may be due to a Norrish Type I cleavage of the
cyclopentenone ring, which should be rendered more facile by
the presence of the phenyl ring adjacent to the carbonyl moiety.
The construction of the cyclopentenone products 19 can be
considered a formal [2 + 1 + 1 + 1] annulation process in
which the five carbons of the cyclopentenone products are
derived from the two carbons of the alkyne, the carbene complex
carbon, carbon monoxide, and the carbon of the carbenoid
reagent. The wide range of substituents that can be incorporated
by variation of these four components suggests that the method
will provide broad access to cyclopentenone and/or cyclopen-
tanoid products.
carbenoid
reagent
products
(% yield)
entry
1
SiR3
R′
TIPS
CH3
TMSCHN2
18a, R′′ ) TMS (93)
16
16
16
2
3
4
5
TIPS
TIPS
TBS
TIPS
n-Bu
Ph
18b, R′′ ) TMS (79)
18c, R′′ ) TMS (62)
18d, R′′ ) TMS (86)
18e, R′′ ) Ph (80)
Ph
CH3
16
PhCHN2
17
6
7
8
9
TIPS
TIPS
TIPS
TBS
n-Bu
17
17
17
17
18f, R′′ ) Ph (83)
18g, R′′ ) Ph (73)
18h, R′′ ) Ph (77)
18i, R′′ ) Ph (76)
Ph
(p-OMe)Ph
Ph
dimethylsulfonium methylide 13, whose participation in
[4 + 1] cycloadditions was previously established by Danheiser
and co-workers.6d Addition of 1.1 equiv of 13 to a solution of
the silyl vinylketenes 12 in 1:1 THF/DMSO at 0-25 °C for
1.5 h provided moderate yields of the desired cyclopentenone
products 15a,b after chromatographic purification, along with
significant decomposition of the silyl vinylketene starting
material (Table 3, entries 1 and 2). A notable improvement was
observed upon switching to diazomethane as the carbenoid
reagent, which cleanly provided the cyclopentenone products
15a-d in good to excellent yields (Table 3, entries 3-6).14
We were also pleased to find that substituted diazomethane
reagents serve as suitable reagents in [4 + 1] annulations with
the silyl vinylketenes. Both phenyldiazomethane (17)15 and
commercially available (trimethylsilyl)diazomethane (16) pro-
vided the corresponding cyclopentenone products 18 in good
to excellent yields (Table 4).14 The cyclopentenone products
could be purified by silica gel chromatography, although the
trimethylsilyl group in cyclopentenones 18a-d was found to
be somewhat labile to these conditions.6e Alternatively, the
desired products could be crystallized directly from the crude
An important aspect of this [4 + 1] annulation reaction is
the apparent preservation of the (E)-alkene geometry in the
diastereoselective cyclopentenone formation. To further probe
this observation, we sought to obtain the corresponding silyl
vinylketenes bearing the (Z)-alkenyl geometry, with the hopes
that they would provide stereoselective access to the alternate
cyclopentenone diastereomers. It seemed feasible that isomer-
ization of the (E)-alkenyl silyl vinylketenes may be possible
through reversible thermal 4π electrocyclic ring closures, which
have been documented for vinylketenes,16 and silyl allenyl-
ketenes.8,17 Silyl vinylketene 4d bearing the chromium tricar-
(14) Silyl vinylketenes 8-11 were also suitable for [4 + 1] annulation
reactions. Results with these substrates will be reported elsewhere.
(15) Creary, X. In Organic Syntheses; Wiley & Sons: New York, 1990;
Collect. Vol. VII, pp 438-443.
(16) Moore, H. W.; Yerxa; B. R. AdV. Strain Org. Chem. 1995, 4, 81.
6544 J. Org. Chem., Vol. 71, No. 17, 2006

[4 + 1] Annulation Reactions with Silyl Vinylketenes
SCHEME 1. Equilibration of (E)-SVK and Cyclobutenone
SCHEME 2. Isomerization to (Z)-SVK and Resultant
[4 + 1] Annulation
mechanism of the transformation.6a,c,d One interpretation, as
outlined in Figure 2, involves initial stereoselective addition of
bonyl moiety did not display any propensity toward thermal
isomerization upon heating at reflux in toluene over extended
periods of time. In contrast, photolytic removal of the chromium
moiety quantitatively afforded silyl vinylketene (E)-20a, which
slowly converted to an equilibrium mixture of (E)-20a and
cyclobutenone 21a at room temperature (Scheme 1). Evidence
for this transformation is provided by IR spectroscopy, as the
initially generated silyl vinylketene (E)-20a displays a strong
absorption at 2086 cm-1, with subsequent emergence of a
characteristic cyclobutenone absorption at 1745 cm-1 over
several hours. This observation suggested that even when the
cyclobutenone form is thermally accessible, torquoselective
preference to have the electron-donating alkoxy group rotate
outward would preclude thermal ring opening to the desired
(Z)-silyl vinylketene isomer.18 Thermal isomerization to the (Z)-
silyl vinylketene was even unsuccessful with (E)-20b, in which
the phenyl substituent at the terminal position was replaced by
the more electron-rich para-methoxyphenyl substituent. Fortu-
nately, it has also been established that this stereoelectronic
effect can be overridden by photolytic ring opening of cyclo-
butenones.19 Indeed, we were pleased to find that photolysis of
the (E)-20/21 mixtures in degassed benzene for 12 h resulted
in complete conversion to silyl vinylketenes (Z)-20a,b (Scheme
2), as evidenced by a single carbonyl IR absorbance in the
FIGURE 2. Possible annulation mechanism.
carbenoid reagents 24 to provide the (Z)-enolate 25,20 followed
by ionization of the leaving group to generate 2-oxidopentadi-
enyl cation 26 as the sterically favored intermediate. A concerted
4π electrocyclic ring closure would preserve the original alkene
geometry and provide the observed diastereomer of the cyclo-
pentenone. Although we cannot discount any of the other
mechanisms suggested by Danheiser, we have observed rate
differences that may lend credence to the mechanism in Figure
2. Specifically, the [4 + 1] annulation with substrates 4d or 4f
bearing the chromium tricarbonyl fragment proceeded signifi-
cantly faster than the identical reactions with the chromium-
free silyl vinylketenes 20 (ca. 24 h vs 72 h, respectively). This
rate difference would be consistent with the proposed mecha-
nism, as the ability of the chromium fragment to stabilize
developing charge character in the benzylic position21 should
accelerate the rate-determining cationic 4π electrocyclic ring
closure.22
1
2085-2090 cm-1 range and H and 13C NMR spectral data
consistent with silyl vinylketenes but distinct from those
obtained for (E)-20a,b. Importantly, the (Z)-silyl vinylketene
isomers were found to be stable to silica gel chromatography
and displayed no tendency to isomerize at room temperature.
Following their purification, we were gratified to find that
exposure of silyl vinylketenes (Z)-20a,b to phenyldiazomethane
provided cyclopentenones 22a,b in good yield, with none of
the original diastereomers 19k or 19l visible by 1H NMR
spectroscopy. (Scheme 2) These experiments not only provided
further evidence that the geometry of the silyl vinylketene is
conserved in a concerted ring-closure process, but also dem-
onstrated that either diastereomer of the cyclopentenone products
should be generally accessible through our approach.
Conclusion
Thermal reaction of Fischer carbene complexes with silyl-
substituted alkynes provides an efficient route to stable silyl
Stereoselective silyl vinylketene/carbenoid [4 + 1] annula-
tions have also been reported by the Danheiser group, who have
proposed several alternative pathways to account for the
(20) (a) Tidwell, T. T. Acc. Chem. Res. 1990, 23, 273. (b) Baigrie, L.
M.; Seikaly, H. R.; Tidwell, T. T. J. Am. Chem. Soc. 1985, 107, 5391.
(21) (a) Davies, S. G.; Dohohoe, T. J. Synlett 1993, 323. (b) Davies, S.
G.; Donohoe, T. J.; Williams, J. M. J. Pure Appl. Chem. 1992, 64, 379. (c)
Uemura, M.; Minami, T.; Hayashi, Y. J. Organomet. Chem. 1986, 299,
119.
(17) Huang, W.; Fang, D.; Temple, K.; Tidwell, T. T. J. Am. Chem.
Soc. 1997, 119, 2832.
(18) For a review on torquoselective electrocyclic reactions, see: Dolbier,
W. R., Jr.; Koroniak, H.; Houk, K. N.; Sheu, C. Acc. Chem. Res. 1996, 29,
471.
(19) Foland, L. D.; Karlsson, J. O.; Perri, S. T.; Schwabe, R.; Xu, S. L.;
Patil, S.; Moore, H. W. J. Am. Chem. Soc. 1989, 111, 975.
(22) Similar rate accelerations have been observed for Nazarov cycliza-
tions, which are also proposed to proceed through electrocyclic ring closures
of pentadienyl cations. See: Habermas, K. L.; Denmark, S. E. In Organic
Reactions; Paquette, L. A., Ed.; Wiley: New York, 1994; Vol. 45, pp
1-158.
J. Org. Chem, Vol. 71, No. 17, 2006 6545

Moser et al.
concentrated under reduced pressure, and the crude product was
purified by flash chromatography (SiO2, 19/1 hexane/Et2O), af-
fording cyclopentenone 15a (87 mg, 84% yield). IR (neat): ν 1971,
1892, 1704 cm-1. 1H NMR (CDCl3): δ 5.87 (d, J ) 6.6 Hz, 1H),
5.59 (dd, J ) 6.6, 5.9 Hz, 1H), 5.52 (d, J ) 6.6 Hz, 1H), 5.09 (dd,
J ) 7.4, 6.6 Hz, 1H), 5.04 (dd, J ) 6.6, 5.9 Hz, 1H), 3.29 (s, 3H),
2.74 (d, 16.9 Hz, 1H), 2.47 (d, J ) 16.9 Hz, 1H), 1.67 (s, 3H),
1.11 (sept, J ) 7.4 Hz, 3H), 0.96 (d, J ) 7.4 Hz, 9H), 0.91 (d,
J ) 7.4 Hz, 9H). 13C NMR (CDCl3): δ 232.2, 207.6, 179.4, 146.6,
103.5, 99.1, 96.7, 96.6, 85.9, 85.2, 83.8, 51.1, 48.9, 23.1, 19.2, 11.8.
Anal. Calcd for C25H34CrSiO5: C, 60.71; H, 6.93. Found: C, 60.76;
H, 6.85.
vinylketenes, with capture of the chromium fragment occurring
in cases where the alkyne precursor contains a phenyl substitu-
ent. We have demonstrated that these silyl vinylketenes
participate in [4 + 1] annulation reactions with carbenoid
reagents to afford cyclopentenones in excellent yields. Annu-
lations with substituted carbenoids and silyl vinylketenes bearing
(E)- or (Z)-alkenes each proceed with essentially complete
stereoselectivity to provide diastereomeric cyclopentenone
products. Further studies of the scope and limitations of this
transformation and application to the synthesis of natural
products are ongoing.
Representative Procedure for Decomplexation of Cyclopen-
tenones: Cyclopentenone 19a. Cerium ammonium(IV) nitrate
(CAN, 439 mg, 0.800 mmol) was directly added to a 0 °C solution
of cyclopentenone 15a (198 mg, 0.400 mmol) in methanol (20 mL).
After 15 min, the solution was diluted with H2O and extracted with
Et2O. The combined organic layers were dried over MgSO4, filtered,
and concentrated under reduced pressure. The residue was purified
via flash chromatography (SiO2, hexane/Et2O 19/1) to afford
cyclopentenone 19a (138 mg, 97% yield) as a clear, colorless oil.
IR (neat): ν 1702 cm-1. 1H NMR (CDCl3): δ 7.33-7.31 (m, 3H),
7.21-7.20 (m, 2H), 3.24 (s, 3H), 2.78 (d, J ) 18.4 Hz, 1H), 2.37
(d, J ) 18.4 Hz, 1H), 1.28 (s, 3H), 1.03 (sept, J ) 7.4 Hz, 3H),
0.93 (d, J ) 7.4 Hz, 9H), 0.88 (d, J ) 7.4 Hz, 9H). 13C NMR
(CDCl3): δ 209.8, 184.6, 141.8, 135.8, 128.4, 127.8, 127.3,
83.8, 51.0, 45.4, 25.0, 19.1, 11.3. HRMS calcd for C19H27SiO2
(M+ - C3H7), 315.1781; found, 315.1777.
Experimental Section
Representative Procedure for the Reaction of Fischer Car-
bene Complexes with Alkynes: Silyl Vinylketene 4a. A solution
of carbene complex 1a (500 mg, 2.00 mmol) and TIPS-substituted
phenylacetylene (620 mg, 2.40 mmol) in benzene (20 mL) was
degassed using several freeze-pump-thaw cycles. The solution
was then heated at reflux for 12 h. Upon cooling to room
temperature, the solvents were removed under reduced pressure,
and the residue was purified via flash chromatography (SiO2, 19/1
hexane/ethyl acetate), affording silyl vinylketene 4a (560 mg, 58%
yield) as a yellow solid. IR (neat): ν 2084, 1960, 1885 cm-1. H
1
NMR (CDCl3): δ 5.69 (d, J ) 5.9 Hz, 2H), 5.30 (dd, J ) 6.6, 5.9
Hz, 2H), 5.22 (t, J ) 5.9 Hz, 1H), 3.72 (s, 3H), 2.19 (s, 3H), 1.02
(s, 21H). 13C NMR (CDCl3): δ 233.9, 179.3, 155.3, 110.1, 100.0,
96.5, 91.8, 91.2, 56.0, 18.5, 16.3, 14.0, 12.4. Anal. Calcd for C24H32-
CrSiO5: C, 59.98; H, 6.71. Found: C, 60.06; H, 6.65.
Acknowledgment. Financial support provided by the Ameri-
can Chemical Society Petroleum Research Fund (38338-AC1)
and the Indiana University-Purdue University Indianapolis
School of Science is gratefully acknowledged.
Representative Procedure for [4 + 1] Annulation Reac-
tions: Cyclopentenone 15a. Diazomethane, which was generated
in a 125-mL Erlenmeyer flask by addition of an aqueous NaOH
solution to a slurry of diazald (90 mg, 0.42 mmol) in EtOH, was
bubbled into a solution of silyl vinylketene 12 (SiR3 ) TIPS, R′ )
CH3; 100 mg, 0.21 mmol) in Et2O (20 mL). (Warning: diazo-
methane is an explosion hazard. Be sure that all glassware used in
this procedure is flame-polished and contains no rough edges.) After
12 h, argon was bubbled through the solution for 15 min to remove
any remaining diazomethane. The resultant solution was then
Supporting Information Available: Full experimental proce-
1
dures for all new compounds, copies of H NMR and 13C NMR
spectra for compounds 4, 8-11, 15, 18-20, and 22, and crystal
structure data for 4c, 18a, and 18h. This material is available free
of charge via the Internet at http://pubs.acs.org.
JO060994X
6546 J. Org. Chem., Vol. 71, No. 17, 2006