[2 + 2], [4 + 1], and [4 + 2] cycloaddition reactions of silylated bisketenes

Article abstract of DOI:10.1021/jo961169r

Cycloaddition reactions of the bisketenes O=C=C(SiMe₃)CR=C=O (15, R = SiMe₃; 16, R = Ph) include BF₃-catalyzed [2+2] cycloaddition of 15 with CH₃CHO to form an isolable β-lactone 18 adduct which undergoes thermal decarboxylation to the vinylketene 19. Reactions of 15 and 16 with CH₂N₂ proceed by [4+1] cycloaddition to give mixtures of cyclopentene-1,3-diones 20 and methylenefuranones 21, while Me₃SiCHN₂ and PhCHN₂ give only 20. The reactions are interpreted in terms of a steric preference for nucleophilic attack by the substituted diazomethanes from the side of the ketene bearing the Me₃SiC=C=O substituent, leading to formation of 20. With the less bulky CH₂N₂, attack occurs from both sides and approach from the side of the R group leads to formation of lactones 21. Reaction of tetramethoxyethylene with 15 yields both a cyclopentene-1,3-dione 24 from net addition of dimethoxycarbene and a spirocyclopropylbutenolide 25. Free dimethoxycarbene generated by heating an oxadiazoline precursor also reacted with 15 to give dione 24. Various electrophilic dienophiles do not react with 15, but nucleophilic alkynes react with 16 in thermal reactions to give spirocyclopropenylfuranones 33-36, and Me₃SiC≡COEt and 16 react by net [4 + 2] cycloaddition to give the quinone 37 as the major product.

Full text of DOI:10.1021/jo961169r

9522
J . Org. Chem. 1996, 61, 9522-9527
[2 + 2], [4 + 1], a n d [4 + 2] Cycloa d d ition Rea ction s of Silyla ted
Bisk eten es
J im D. Colomvakos,† Ian Egle,† J ihai Ma,† David L. Pole,‡ Thomas T. Tidwell,*,† and
J ohn Warkentin‡
Departments of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6, and
McMaster University, Hamilton, Ontario, Canada L8S 4M1
Received J une 20, 1996X
Cycloaddition reactions of the bisketenes OdCdC(SiMe3)CRdCdO (15, R ) SiMe3; 16, R ) Ph)
include BF3-catalyzed [2+2] cycloaddition of 15 with CH3CHO to form an isolable â-lactone 18
adduct which undergoes thermal decarboxylation to the vinylketene 19. Reactions of 15 and 16
with CH2N2 proceed by [4+1] cycloaddition to give mixtures of cyclopentene-1,3-diones 20 and
methylenefuranones 21, while Me3SiCHN2 and PhCHN2 give only 20. The reactions are interpreted
in terms of a steric preference for nucleophilic attack by the substituted diazomethanes from the
side of the ketene bearing the Me3SiCdCdO substituent, leading to formation of 20. With the
less bulky CH2N2, attack occurs from both sides and approach from the side of the R group leads
to formation of lactones 21. Reaction of tetramethoxyethylene with 15 yields both a cyclopentene-
1,3-dione 24 from net addition of dimethoxycarbene and a spirocyclopropylbutenolide 25. Free
dimethoxycarbene generated by heating an oxadiazoline precursor also reacted with 15 to give
dione 24. Various electrophilic dienophiles do not react with 15, but nucleophilic alkynes react
with 16 in thermal reactions to give spirocyclopropenylfuranones 33-36, and Me3SiCtCOEt and
16 react by net [4 + 2] cycloaddition to give the quinone 37 as the major product.
The [2+2] cycloaddition reactions of ketenes 1, includ-
that they do not ordinarily give observed [4+2] cycload-
dition products except in unusual circumstances.4
Bisketenes have been rare species, and only a few
cycloaddition reactions of this family have been studied.
The metal complex 3 reacts by a net [4+2] cycloaddition
to form quinone complexes 4 (eq 2).5a Photolyses of
ing dimerization and reactions with alkenes, imines, and
aldehydes to form cyclobutenones, â-lactams, and â-lac-
tones 2, respectively (eq 1), are the most distinctive
reactions of these fascinating species.1,2 There have been
authoritative arguments for the concerted nature of these
reactions1f-h and also strong arguments for stepwise
processes.3 These reactions are also the subject of intense
theoretical study.2 It is characteristic of monoketenes
cyclobutene-1,2-diones such as 5 in the presence of
cyclopentadiene were proposed to form bisketenes 6
(3) (a) Gompper, R. Angew. Chem., Int. Ed. Engl. 1969, 8, 312-
327. (b) Wagner, H. U.; Gompper, R. Tetrahedron Lett. 1970, 2819-
2822. (c) Corey, E. J .; Arnold, Z.; Hutto, J . Tetrahedron Lett. 1970,
307-310. (d) Corey, E. J .; Desai, M. C. Tetrahedron Lett. 1985, 26,
3535-3538. (e) Holder, R. W.; Graf, N. A.; Nuesler, E.; Moss, J . C. J .
Am. Chem. Soc. 1983, 105, 2929-2931. (f) Raynolds, P. W.; DeLoach,
J . A. J . Am. Chem. Soc. 1984, 106, 4566-4570. (g) Moore, H. W.;
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1993, 115, 2165-2177. (c) Yamabe, S.; Dai, T.; Minato, T.; Machiguchi,
T.; Hasegawa, T. J . Am. Chem. Soc. 1996, 118, 6518-6519.
(5) (a) J ewell, C. F., J r.; Liebeskind, L. S.; Williamson, M. J . Am.
Chem. Soc. 1985, 107, 6715-6716. (b) Miller, R. D.; Kirchmeyer, S.
J . Org. Chem. 1993, 58, 90-94. (c) Forster, D. L.; Gilchrist, T. L.;
Rees, C. W.; Stanton, E. Chem. Commun. 1971, 695-696. (d) Staab,
H. A.; Ipaktschi, J . Tetrahedron Lett. 1966, 583-586. (e) Staab, H.
A.; Ipaktschi, J . Chem. Ber. 1968, 101, 1457-1472. (f) Krohn, K.;
Rieger, H.; Broser, E.; Schiess, P.; Chen, S.; Strubin, T. Liebigs Ann.
Chem. 1988, 943-948. (g) Borrmann, D.; Wegler, R. Chem. Ber. 1966,
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(6) (a) Wasserman, H. H.; Berdahl, D. R.; Lu, T. The Chemistry of
Cyclopropanones. In The Chemistry of the Cyclopropyl Group; Rap-
poport, Z., Ed.; Wiley: New York, 1987; Chapter 23, pp 1455-1532.
(b) Turro, N. J . Acc. Chem. Res. 1969, 2, 25-32. (c) Zaitseva, G. S.;
Novikova, O. P.; Livantsova, L. I.; Kisin, A. V.; Baukov, Yu. I. Zh.
Obshch. Khim. 1990, 60, 1073-1077. (d) Zaitseva, G. S.; Bogdanova,
G. S.; Baukov, Yu. I.; Lutsenko, I. F. Zh. Obshch. Khim. 1978, 48, 131-
137. (e) Bohen, J . M.; J oullie, M. M. J . Org. Chem. 1973, 38, 2652-
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† University of Toronto.
‡ McMaster University.
X Abstract published in Advance ACS Abstracts, November 15, 1996.
(1) (a) Tidwell, T. T. Ketenes; Wiley, Inc.: New York, 1995. (b) Hyatt,
J . A.; Raynolds, P. W. Org. React. 1994, 45, 159-646. (c) Ghosez, L.;
O’Donnell, M. J . Pericyclic Reactions of Cumulenes. In Pericyclic
Reactions; Marchand, A. P., Lehr, R. E., Eds.; Academic Press: New
York, 1977; Chapter 2, pp 79-140. (d) Ulrich, H. Cycloaddition
Reactions of Heterocumulenes; Academic Press: New York, 1967;
Chapter 2, pp 38-109. (e) Ghosez, L.; Marchand-Brynaert, J . Forma-
tion of Four-membered Heterocycles: In Comprehensive Organic
Synthesis; Trost, B. M., Ed.; Pergamon: New York, 1991; Vol. 5,
Chapter 2.2, pp 85-122. (f) Woodward, R. B.; Hoffmann, R. Angew.
Chem., Int. Ed. Engl. 1969, 8, 781-853. (g) Huisgen, R. Acc. Chem.
Res. 1977, 10, 117-124. (h) Huisgen, R. Pure Appl. Chem. 1980, 52,
2283-2302. (i) Schaumann, E.; Scheiblich, S. Methoden der Organis-
chen Chemie; Thieme: Verlag, Germany, 1993; Vol. E15, Part 3,
Chapters 4, 6, 8.
(2) (a) Seidl, E. T.; Schaeffer, H. F., III. J . Am. Chem. Soc. 1991,
113, 5195-5200. (b) Schaad, L. J .; Gutman, I.; Hess, B. A., J r.; Hu, J .
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J .; Sordo, T. L. J . Am. Chem. Soc. 1992, 114, 6249-6251. (g) Yamabe,
S.; Minato, T.; Osamura, Y. J . Chem. Soc., Chem. Commun. 1993, 450-
452. (h) Cossio, F. P.; Ugalde, J . M.; Lopez, X.; Lecea, B.; Palomo, C.
J . Am. Chem. Soc. 1993, 115, 995-1004. (i) Salzner, U.; Bachrach, S.
M. J . Org. Chem. 1996, 61, 237-242.
S0022-3263(96)01169-3 CCC: $12.00 © 1996 American Chemical Society

Cycloaddition Reactions of Silylated Bisketenes
J . Org. Chem., Vol. 61, No. 26, 1996 9523
which underwent [2+2] cycloadditions to give the unob-
served monoketenes 7 which rearranged to the spirocy-
clopropyl ∆R,â-butenolide adducts 8 (eq 3).5b The bisketene
theoretical methods.7e We now report the study of the
cycloaddition reactivity of bisketenes 15 and 16.
Resu lts a n d Discu ssion
9 was reported to give a [4+2] cycloaddition to maleic
anhydride (eq 4),5c,dand derivatives of 9 add to substituted
Cycloaddition of ketenes with aldehydes is a major
preparative route to â-lactones, and these latter com-
pounds are prone to decarboxylation.8 Reaction of 15
with acetaldehyde, catalyzed by BF3‚OEt2, leads to net
[2+2] cycloaddition and formation and isolation as a solid
of the novel ketenic â-lactone 18 (eq 8). The structure of
benzoquinones.5f
Reaction of adipyl chloride with triethylamine in the
presence of chloral gave 12, and this could have involved
reaction of the bisketene 11, but stepwise routes seem
more likely (eq 5).5g
18 was established by its spectral properties, especially
the distinctive features for ketenes summarized
elsewhere,1a such as the IR band at 2084 cm-1, and the
13C signals for CR and Câ of the silylketene at δ 178.6
and -3.7, respectively. The stereochemistry of 18 was
determined by an NOE experiment in which irradiation
of the ketenyl Me3Si group resulted in 0.95% and 0.63%
enhancement of the CH3 and CH resonances, respec-
tively, while irradiation of the lactone Me3Si group
resulted in 1.01% and 13.6% enhancement of the CH3
and CH resonances, respectively.
Ketenes react with diazomethanes by [2+1] cycload-
ditions to give cyclopropanones,6a-d and at -78 °C reac-
tion with sulfur dioxide gives the adduct assigned the
R-sultone structure 13 from [2+1] cycloaddition (eq 6).6e
The BF3-catalyzed cycloaddition of Me3SiC(Hx-n)dCdO
with an aldehyde is reported to give a mixture of E/ Z
isomers,8c and cycloadditions of Me3SiCHdCdO with
aldehydes give a predominance of E or Z isomers depend-
ing upon the particular situation.8b Recent theoretical
studies8d of the cycloaddition of chloroketene with ac-
etaldehyde favor a transition state for the uncatalyzed
reaction in which bond formation from the aldehyde
oxygen to the ketenyl carbonyl carbon is most advanced
and the trans cycloadduct is favored, whereas the transi-
tion state for the Lewis acid-catalyzed transition state
involves predominant C-C bond formation with high
zwitterionic character and favors cis products.8d
Upon heating of 18 decarboxylation occurred with
formation of the vinylketene 19 (eq 8). The structural
assignment of 19 is based on its spectral properties,
including the distinctive ketenyl IR band at 2080 cm-1
and the 13C NMR signals for CR and Câ of the ketene at
δ 176.6 and -1.3, respectively, and the stereochemical
assignment shown is based on the assumed loss of CO2
without change in the stereochemistry, as generally
found in â-lactone decarboxylations.8b Although vi-
nylketenes are ordinarily quite reactive,1a 19 is isolable
with no apparent tendency for dimerization, in common
with other silyl derivatives.1a,9
We have recently been engaged in the study of long-
lived 1,2-bisketenes stabilized by the presence of tri-
methylsilyl groups.7 These species are prepared by
thermolysis or photolysis of the corresponding cy-
clobutene diones 14, and the bisketene 15 (R ) Me3Si)
is thermodynamically stable relative to the cyclobutene-
dione, whereas for 16 and 17 (R ) Ph and Me, respec-
tively) the bisketenes re-form 14 at measurable rates, but
are sufficiently long-lived that their chemical reactivity
can be studied (eq 7).7c,d The factors influencing the
interconversion of these species have been studied by
(7) (a) Allen, A. D.; Ma, J .; McAllister, M. A.; Tidwell, T. T.; Zhao,
D.-c. Acc. Chem. Res. 1995, 28, 265-271. (b) Zhao, D.-c.; Allen, A. D.;
Tidwell, T. T. J . Am. Chem. Soc. 1993, 115, 10097-10103. (c) Allen,
A. D.; Lai, W.-Y.; Ma, J .; Tidwell, T. T. J . Am. Chem. Soc. 1994, 116,
2625-2626. (d) Allen, A. D.; J i, R.; Lai, W.-Y.; Ma, J .; Tidwell, T. T.
Heteroat. Chem. 1994, 5, 235-244. (e) McAllister, M. A.; Tidwell, T.
T. J . Am. Chem. Soc. 1994, 116, 7233-7238. (f) Allen, A. D.;
Colomvakos, J . D.; Egle, I.; Lusztyk, J .; Tidwell, T. T.; Wagner, B.;
Zhao, D.-c. J . Am. Chem. Soc. 1995, 117, 7552-7553.
(8) (a) Ghosez, L.; Marchand-Brynaert, J . In Comprehensive Organic
Synthesis; Trost, B. M., Ed.; Pergamon: New York, 1991; Vol. 5,
Chapter 2.2. (b) Pommier, A.; Pons, J .-M. Synthesis 1993, 441-459.
(c) Pons, J .-M.; Pommier, A.; Lerpiniere, J .; Kocienski, P. J . Chem.
Soc., Perkin Trans. 1 1993, 1549-1551. (d) Lecea, B.; Arrieta, A.;
Lopez, X.; Ugalde, J . M.; Cossio, F. P. J . Am. Chem. Soc. 1995, 117,
12314-12321.
Reaction of the bisketenes 15 and 16 (which preferen-
tially exist in the twisted conformations shown)7a,e with
(9) (a) Danheiser, R. L.; Sard, H. J . Org. Chem. 1980, 45, 4810-
4812. (b) Tang, P. C.; Wulff, W. D. J . Am. Chem. Soc. 1984, 106, 1132-
1133.

9524 J . Org. Chem., Vol. 61, No. 26, 1996
Colomvakos et al.
diazomethanes R1CHN2 led to two types of products,
namely 2,5-disubstituted 4-(trimethylsily)-4-cyclopen-
tene-1,3-diones (20) and 4-substituted 3-(trimethylsilyl)-
5-methylene-2(5H)-furanones (21) (eq 9). However, the
latter product was only observed for the reaction with
CH2N2, while reactions of the bisketenes with Me3-
SiCHN2 and PhCHN2 led only to 20. No reaction was
observed on treatment of the bisketenes with N2CHCO2-
Et, whereas n-C5H13CHN2 led to a mixture of products
from which no pure materials were isolated.
methylenefuranones 21, respectively. When the diazo-
methane bears a bulky Me3Si or Ph substituent; the size
of the reagent evidently permits only attack anti to the
group R (Me3Si or Ph) by the less hindered approach syn
to the Me3SiCdCdO substituent. Only for CH2N2 is
some attack syn to the R groups feasible, leading to some
formation of 21. Consistent with this interpretation, it
has been previously proposed5b that attack on 6 occurs
preferentially from the side of the PhCdCdO group.
Reaction of 15 with (MeO)2CdC(OMe)211b,c yielded the
cyclopentenedione 24 and the spirocyclopropylbutenolide
25. The structures of 24 and 25 follow from their
The derivatives 20c and 20d were not isolated, and
their presence in the crude reaction product was only
1
1
inferred from the initial H NMR spectra. These com-
distinctive spectral properties, including the simple H
and 13C NMR spectra of 24 and comparison of the data
for 25 to those of other 5-spirocyclopropylbutenolides.5b,12a
The formation of these products can be rationalized as
resulting from attack on 15 anti or syn, respectively, to
the Me3Si groups (eqs 12, 13). Attack anti to the Me3Si
pounds evidently undergo facile replacement of the group
R1 ) Me3Si by hydrogen to form the isolated 20a and
20b because of the activation of the Me3Si to nucleophilic
displacement by the two adjacent carbonyl groups.
The formation of these products may be explained by
a process involving initial attack of the diazomethane on
the more activated of the ketenyl groupings, which is the
phenyl-substituted moiety for 16 (R ) Ph),7c,d to form the
stereoisomeric zwitterions 22 or 23, depending upon
whether the diazomethane attacks the ketenyl group anti
or syn to the substituent R (eqs 10, 11). In previous
on the in-plane CdO π-system, illustrated for clarity by
15a , would give the zwitterion 26, and this could expel
(MeO)2C (or its equivalent) as shown, perhaps by transfer
to another molecule of 15. As shown in eq 13 the initial
zwitterion 27 formed from attack syn to the Me3Si group
can cyclize first to the zwitterion 28, which can form 25
studies, we have analyzed such nucleophilic additions to
ketenes as occurring in the ketene plane syn to the
smaller group.10 In the present case, inspection of the
structures suggests that approach of the nucleophilic
diazoalkane may be more facile syn to the ketenyl group
Me3SiCdCdO. Ring closure of the zwitterions 22 and
23 would lead to the cyclopentenediones 20 or the
(11) (a) Creary, X. Org. Syntheses; Wiley: New York, 1990; Collect.
Vol. VII, pp 438-443. (b) Bellus, D. J . Org. Chem. 1979, 44, 1208-
1211. (c) Scheeren, J . W.; Staps, R. J . F. M.; Nivard, R. J . F. Recl.
Trav. Chim. Pays-Bas 1973, 92, 11-19.
(12) (a) Miller, R. D.; Theis, W.; Heilig, G.; Kirchmeyer, S. J . Org.
Chem. 1991, 56, 1453-1463. (b) Mosandl, T.; Wentrup, C. J . Org.
Chem. 1993, 58, 747-749.
(10) (a) Baigrie, L. M.; Seikaly, H. R.; Tidwell, T. T. J . Am. Chem.
Soc. 1985, 107, 5391-5396. (b) Tidwell, T. T. Acc. Chem. Res. 1990,
23, 273-279.

Cycloaddition Reactions of Silylated Bisketenes
J . Org. Chem., Vol. 61, No. 26, 1996 9525
(eq 14), or alternatively [2+2] cycloaddition to form 29
be generated in a low equilibrium concentration by
heating of 14b (eq 7), and has been found to be much
more reactive than 15.7c,d Heating of 14b in the presence
of 2-butyne led to no observed reaction, but photolysis of
14b with 350 nm light at 5 °C for 1 week in the presence
of 2-butyne or 1-(trimethylsilyl)propyne led to formation
of 1:1 adducts, which on the basis of their spectral
properties were assigned the spirocyclopropenylbuteno-
lide structures 33 and 34 (eq 16). The formation of such
viaeither 26 or 27 can occur, and then 29 could form 25,
as has been proposed for other R-ketenylcyclobu-
tanones.5b,12a
Although the reaction of bisketenes with cyclopenta-
diene has been interpreted5b as occurring by an initial
concerted [2+2] cycloaddition, our review of the literature1a
suggests that stepwise processes also merit consideration.
The highly nucleophilic (MeO)2CdC(OMe)2 is expected
to favor zwitterionic pathways, and a zwitterionic path-
way has been proposed for the reaction of 9 with
methanol.12b
The cyclopentenedione 24 also forms from the reaction
of bisketene 15 with dimethoxycarbene (31) as generated
from the oxadiazoline precursor 30 (eq 15).13 The bis-
ketene 15 and the carbene were both generated in situ
from the thermolysis of the cyclobutenedione 14a and 30,
respectively, in benzene, and 24 was obtained in 62%
yield. Tetramethoxyethylene is formed from dimethoxy-
carbene under the conditions of eq 15,13 and this suggests
that the formation of 24 from bisketene 15 and (MeO)2-
CdC(OMe)2 described above does not involve a dissocia-
tion of the latter to form dimethoxycarbene, which reacts
with 15 as in eq 15.
spirocyclopropenylbutenolides was previously observed
in the reactions of 9.5e When 14b was heated with the
more reactive dienophile 1-phenyl-2-(trimethylsilyl)-
acetylene, a 1:1 adduct was formed and was assigned the
analogous structure 35. This was also obtained by
photolysis of 14b in the presence of the alkyne. Photo-
chemically generated bisketene 16 reacted upon addition
of the even more reactive 1-ethoxy-2-(trimethylsilyl)-
acetylene at -25 °C to give the adduct 36 and also the
unstable [4+2] cycloadduct 37 in yields of 8% and 64%,
respectively (eq 16). Upon chromatography, 37 tended
to undergo desilylation to 38, which was also rather
unstable.
The formation of the spirocyclopropenylbutenolides
33-36 could occur by a stepwise process with attack syn
to the phenyl group forming a zwitterionic intermediate
such as 39 as shown in eq 17 for the example forming
36. This mechanism is consistent with the observed
greater reactivity with increasing nucleophilicity of the
different alkynes in the formation of 33-36 and is also
in accord with the pattern of eqs 10, 12, and 15.
The nucleophilic dimethoxycarbene 31 generated from
30 would be expected to react with the bisketene 15 by
in-plane nucleophilic attack at the carbonyl carbon from
the face opposite the Me3Si group to form intermediate
32 (eq 15), analogous to the formation of 26 in eq 12. No
product analogous to 21, which would result from attack
on the opposite face of the ketene (eqs 10, 11), was
detected. A concerted reaction in which the in-plane
HOMO of the carbene attacks one ketenyl group while
the out-of-plane LUMO of the carbene undergoes simul-
taneous bonding to the other carbonyl carbon is also
conceivable, but the present evidence does not establish
this point.
Alternatively there could be initial attack anti to the
phenyl with formation of cyclobutenones such as 40
which rearrange to 33-36 in a process analogous to that
proposed5b for initial formation of cyclobutanones (eq 3).
Formation of the [4+2] cycloadduct 37 could also occur
from 40 or alternatively could involve the intermediate
41 formed from 16 by approach of the alkyne to the
PhCdCdO moiety anti to the phenyl. A twisted geom-
etry for 41 is shown, but the question of the electronic
nature of this reaction has not been examined in detail,
and alternatives may be envisaged.
To examine the reactivity of silyl-substituted bis-
ketenes with electron-deficient dieneophiles, 15 was
heated with maleic anhydride, dimethyl acetylenedicar-
boxylate, and 4-phenyl-1,2,4-triazoline-3,5-dione, but these
procedures uniformly gave no observed reaction. This
failure of the bisketene to react with electrophiles is
consistent with the known behavior of monoketenes.1
The bisketene 16 with one phenyl-substituted ketene
moiety may be generated photochemically from the
corresponding cyclobutenedione 14b in high yield, or may
The formation of butenolide-type products is com-
monplace in the reactions of 1,2-bisketenes5b,e,12 and, for
example in the case of bisketene 9, has been taken as
(13) (a) El-Saidi, M.; Kassam, K.; Pole, D. L.; Tadey, T.; Warkentin,
J . J . Am. Chem. Soc. 1992, 114, 8751-8752. (b) Pole, D. L.; Warkentin,
J . Liebigs Ann. Chem. 1995, 1907.

9526 J . Org. Chem., Vol. 61, No. 26, 1996
Colomvakos et al.
Hz), 6.05 (q, 1, J 1,2 ) 6.6 Hz); 13C NMR (CDCl3) δ -1.26, 0.15,
17.2, 29.7, 129.3, 139.1, 176.6; IR (CDCl3) 2080 (s), 1793 (s)
cm-1; EIMS m/z 226 (M+, 24), 198 (M+ - CO, 12), 171 (36),
110 (M+ - Me3Si, CH3, CO, 76), 73 (Me3Si+, 100); HRMS m/z
calcd for C11H22OSi2 226.1209, found 226.1212.
4,5-Bis(tr im eth ylsilyl)-4-cyclop en ten e-1,3-d ion e (20a ).
Bisketene 15 (63.2 mg, 0.279 mol) was dissolved in 10 mL of
anhydrous ether in a 25-mL three-necked round bottomed
flask under N2 flow, Me3SiCHN2 (31.9 mg, 0.279 mmol) was
added dropwise, and the solution was stirred overnight. The
ether solution was washed twice with H2O (10 mL), dried over
anhydrous MgSO4, and evaporated to give a yellow liquid/
brownish-red solid mixture. This mixture was chromato-
graphed, using a 1/9 EtOAc/hexane solvent to give yellow
crystals of 20a (43 mg, 0.18 mmol, 64%); mp 38.0-38.5 °C; 1H
NMR δ 0.34 (s, 18), 2.76 (s, 2); 13C NMR (CDCl3) δ 0.51, 41.2,
175.5, 205.8; IR (CDCl3) 1730, 1690 (s) cm-1; UV λmax (hexanes)
434 nm (ꢀ ) 50), 250 nm (shoulder); EIMS m/z 240 (M+, 24),
225 (M+ - CH3, 50), 197 (M+ - CO, CH3, 51), 155 (M+ - CH2-
CO, CO, CH3, 100), 73 (Me3Si+, 74); HRMS m/z calcd for
C11H20O2Si2 240.1002, found 240.0994. Anal. Calcd: C, 54.95;
H, 8.38. Found: C, 54.58; H, 8.61.
4,5-Bis(t r im et h ylsily)-4-cyclop en t en e-1,3-d ion e (20a )
a n d 3,4-Bis(tr im eth ylsilyl)-5-m eth ylen e-2(5H)-fu r a n on e
(21a ). The bisketene 15 (27.8 mg, 0.123 mmol) was dissolved
in anhydrous ether (2 mL), diazomethane (0.63 mmol in 5 mL
of ether solution) was added, and the solution was stirred at
rt under nitrogen for 2 h. The solvent was then evaporated
to give crude product which was further purified by thin layer
chromatography (on silica gel eluted by 3% ethyl acetate in
hexane) to give 20a (vide supra) (6.5 mg, 0.027 mmol, 22%)
and the colorless oil 21a (18.5 mg, 0.077 mmol, 63%): 1H NMR
(CDCl3) 0.35 (s, 9), 0.39 (s, 9), 4.98 and 5.19 (ea 1, d, J 1,2 ) 2.5
Hz); 13C NMR (CDCl3) δ 0.53, 1.60, 97.0, 144.2, 159.6, 165.8,
173.0; IR (CDCl3) 1744, 1628 cm-1; EIMS m/z 240 (M+, 22),
225 (M+ - CH3, 65), 197 (M+ - CH3, CO, 46), 181 (M+ - Me,
CO2, 54), 155 (Me3SiCtCSiMe2+, 100), 73 (Me3Si+, 60); HRMS
m/z calcd for C11H20O2Si2 240.1002, found 240.1000.
4-(Tr im et h ylsilyl)-5-p h en yl-4-cyclop en t en e-1,3-d ion e
(20b). A solution of 3-(trimethylsilyl)-4-phenyl-3-cyclobutene-
1,2-dione (14b), (17.2 mg, 0.075 mmol) in CDCl3 (0.5 mL) was
photolyzed for 2 h using 350 nm light at 6 °C to give the
bisketene 2-(trimethylsilyl)-3-phenyl-1,3-butadiene-1,4-dione
(16)7c in 99% yield as measured by 1H NMR. The bisketene
was then added to Me3SiCHN2 (15 mg, 0.132 mmol) in hexane
(2 mL), and the solution was stirred under nitrogen for 16 h
at rt. The solvent was evaporated to give a crude solid product
which was further purified by thin layer chromatography (on
silica gel eluted by 5% ethyl acetate in hexane) to give 20b as
a yellow-green solid product (12.6 mg, 0.052 mmol, 69%): mp
98.5-99.0 °C; 1H NMR (CDCl3) δ 0.12 (s, 9), 3.02 (s, 2), 7.20-
7.50 (m, 5); 13C NMR (CDCl3) δ -0.72, 42.0, 128.1, 129.0, 129.8,
131.0, 161.5, 168.4, 200.7, 204.0; IR (CDCl3) 1733 (s), 1696 (s)
evidence for the intervention of the cyclic carbene struc-
ture 42 (eq 18).14
However, as noted above, it is not necessary to invoke
carbene-like structures to explain the reaction products
in these bisketene reactions, and the same conclusion has
been reached by others.12 Thus bisketene 16 is generated
in high yield and is directly observed by NMR, and no
direct evidence for the presence of carbene structures
analogous to 42 has been found. Similarly in our
previous study of the generation and ring closure of 9
using detection by time-resolved infrared spectroscopy,
no evidence for 42 was seen, and the data could be
explained without its intervention.7f Thus while the
existence of 42 and related carbenes cannot be disproven,
direct evidence for them is lacking, and these appear to
be unnecessary.
In summary bisketenes 15 and 16 have been shown
to undergo cycloaddition reactions by a diversity of
pathways to yield products resulting from net [2+2],
[4+1], and [4+2] processes and also tandem processes
forming spiro products. The pathways followed are
determined both by the structures of the bisketenes and
of the ketenophilic reagents.
Exp er im en ta l Section
Gen er a l P r oced u r es. All glassware was oven-dried at 120
°C overnight and allowed to cool in a dessicator before use.
Air and moisture sensitive reactions were carried out under
an atmosphere of nitrogen. DME and ether were distilled from
sodium/benzophenone directly before use. All other reagents
were used as received from commercial sources, unless noted.
(E )-2-(T r im e t h y ls ily l)-2-(1′-t r im e t h y ls ily l)-2′-o x o -
eth en yl)-3-m eth ylp r op iola cton e (18). Bisketene 15 (156
mg, 0.69 mmol) was dissolved in 1 mL of CHCl3, and freshly
distilled acetaldehyde (200 µL, 160 mg, 3.6 mmol) and BF3‚Et2O
(10 µL, 12 mg, 0.08 mmol) were added. The solution was left
for 1 h at 25 °C, the solvent evaporated, and the residue
recrystallized from ether to give 18 (153 mg, 0.57 mmol,
cm-1; EIMS m/z 244 (M+, 53), 229 (M+ - CH3, 58), 201 (M+
-
CH3, CO, 23), 159 (PhCtCTMS+ - CH3, 100), 73 (Me3Si+, 7);
HRMS m/z calcd for C14H16O2Si 244.0919, found 244.0915.
3-(Tr im et h ylsilyl)-4-p h en yl-5-m et h ylen e-2(5H )-fu r a -
n on e (21b). A solution of 14b (51.0 mg, 0.222 mmol) in CDCl3
(1 mL) was photolyzed as above for 1.5 h to give the bisketene
1
16 (87%, monitored by H NMR). The solution was cooled to
1
0 °C, diazomethane (0.3 mmol) in ether (1.1 mL) was added
over 5 min, and the solution was kept at 0 °C for 1 h and then
at rt overnight. The solvent was evaporated, and the crude
product was further purified by thin layer chromatography (on
silica gel eluted using 5% ethyl acetate in hexane) to give 20b
(vide supra) as a yellow-green solid (26.5 mg, 0.11 mmol, 49%)
and 21b as a colorless oil (8.7 mg, 0.036 mmol, 16%): 1H NMR
(CDCl3) δ 0.17 (s, 9), 5.01 and 5.31 (ea d, 1, J 1/2 ) 2.6 Hz),
7.25-7.50 (m, 5); 13C NMR (CDCl3) δ 0.22, 98.2, 128.1, 128.4,
129.2, 129.5, 131.4, 142.9, 157.6, 169.4; IR (CDCl3) 1746 (vs),
1630 (s) cm-1; EIMS m/z 244 (M+, 47), 229 (M+ - CH3, 26),
201 (M+ - CH3, CO, 18), 159 (PhCtCSiMe2+, 100), 73 (Me3-
Si+, 15); HRMS m/z calcd for C14H16O2Si 244.0919, found
244.0916.
82%): mp 64.5-65.0 °C; H NMR (CDCl3) δ 0.22 (s, 9), 0.29
(s, 9), 1.51 (d, 3, J ) 6.2 Hz), 4.55 (q, 1, J ) 6.2 Hz); 13C NMR
(CDCl3) δ -3.66, 0.35, 11.4, 18.1, 48.2, 73.6, 171.4, 178.6; IR
(CDCl3) 2084 (s), 1796 (s) cm-1; EIMS m/z 270 (M+, 31), 255
(M+ - CH3, 22), 227 (M+ - CO, CH3, 33), 199 (30), 171 (44),
155 (46), 147 (68), 73 (Me3Si+, 100); HRMS m/z calcd for
C12H22O3Si2 270.1108, found 270.1086.
2,3-Bis(tr im eth ylsilyl)-1,3(Z)-pen tadien -1-on e (19). The
ketenyl â-lactone 18 (30.0 mg, 0.111 mmol) dissolved in
anhydrous ether (200 µL) was injected into a gas chromato-
graph (column O.V. 17, injector 250 °C, column 100 °C, rt 22
min), to give 19 as a colorless oil (8.8 mg, 0.039 mmol, 35%):
1H NMR (CDCl3) δ 0.09 (s, 9), 0.18 (s, 9), 1.75 (d, 3, J 1,2 ) 6.6
4,5-Bis(tr im eth ylsilyl)-2-p h en yl-4-cyclop en ten e-1,3-d i-
on e (20e). The bisketene 15 (32.6 mg, 0.144 mmol) dissolved
in anhydrous ether (0.5 mL) was added to 2.5 equiv of
(14) (a) Tomioka, H.; Yamamoto, K. J . Chem. Soc., Chem. Commun.
1995, 1961-1962. (b) Boate, D. R.; J ohnston, L. J .; Kwong, P. C.; Lee-
Ruff, E.; Scaiano, J . C. J . Am. Chem. Soc. 1990, 112, 8858-8863.

Cycloaddition Reactions of Silylated Bisketenes
J . Org. Chem., Vol. 61, No. 26, 1996 9527
phenyldiazomethane11a (43 mg, 0.36 mmol) in anhydrous ether
under nitrogen, and the mixture was stirred 16 h at rt. The
solvent was evaporated to give the crude solid product which
was further purified by thin layer chromatography (on silica
gel eluted using 5% ethyl acetate in hexane) to give 20e as a
yellow solid (30.5 mg, 0.097 mmol, 67%): mp 87.6-88.0 °C;
1H NMR (CDCl3) δ 0.38 (s, 18), 3.76 (s, 1), 7.00-7.35 (m, 5);
13C NMR (CDCl3) δ 0.51, 56.4, 127.4, 128.4, 128.9, 133.5, 175.3,
205.8; IR (CDCl3) 1689 (s) cm-1; EIMS m/z 316 (M+, 37), 301
(M+ - CH3, 6), 183 (9), 155 (Me3SiCtCSiMe2+, 13), 118
(PhCHCO+, 100), 73 (Me3Si+, 42); HRMS m/z for C17H24O2Si2
316.1315, found 316.1315.
EIMS m/z 404 (M+, 3), 389 (M+ - CH3, 58), 299 (M+ - CH3,
CO, 56), 174 (PhCtCSiMe3+, 65), 159 (PhCtCSiMe2+, 94), 73
(Me3Si+, 100); HRMS m/z calcd for C24H28O2Si2 404.1628, found
404.1624.
Th er m a l Gen er a tion of 16 a n d Cycloa d d ition . A solu-
tion of 3-phenyl-4-(trimethylsilyl)cyclobutenedione (14b) (46
mg, 0.2 mmol) and 1-phenyl-2-(trimethylsilyl)acetylene (5
equiv) in CHCl3 (2 mL) was degassed by bubbling with argon
for 0.5 h, and then the container was sealed and heated at
120 °C for 24 h. The solvent was evaporated, and the resulting
crude product was purified by thin layer chromatography (5/
95 ethyl acetate/hexane) to give 35 in 42% yield.
4,5-Bis(tr im eth ylsilyl)-2,2-d im eth oxy-4-cyclop en ten e-
1,3-d ion e (24) a n d 3′,4′-Bis(t r im et h ylsilyl)-2,2,3,3-t et -
r a m eth oxysp ir o(cyclop r op a n e-1,5′-2(5′H)-fu r a n on e) (25).
Cyclobutenedione 14a (252 mg, 1.11 mmol) and tetramethoxy-
ethylene11b-d (1 mL, 6.4 mmol) were heated for 4 days in a
5-mL round bottomed flask at 90 °C, and then the solution
was dissolved in ethyl acetate and washed three times with
brine before being dried over MgSO4. Filtration and evapora-
tion of the solvent yielded a heterogeneous orange solid/oil.
Radial chromatography (10% EtOAc/hexanes) yielded two
fractions. The first was recrystallized from MeOH/H2O to give
24 (orange prisms, 33.6 mg, 0.11 mmol, 10%): mp 48.5-51.9
P h otoch em ica l Gen er a tion of 16 a n d Cycloa d d ition .
A solution of 3-phenyl-4-(trimethylsilyl)cyclobutenedione (46
mg, 0.2 mmol) in 2 mL of CDCl3 was degassed by bubbling
with argon for 30 min and was then irradiated for 1 h with
350 nm light at 6 °C to give the bisketene 16 in 88% conversion
as determinated by 1H NMR. The reaction mixture was cooled
in a refrigerator at -25 °C, 1-ethoxy-2-(trimethylsilyl)acetylene
(71 mg, 0.5 mmol) was added in one portion, and the yellow
color of the reaction mixture turned immediately to orange.
The reaction mixture was left for 16 h, and the solvent
evaporated. Radial chromatography (5% EtOAc in hexane)
gave the furanone 36 (5.2 mg, 0.014 mmol, 8%) and the
desilylated 1,4-benzoquinone 38 (33.8 mg, 0.113 mmol, 64%).
In one experiment the nondesilylated quinone 37 was also
isolated by radial chromatography, but this compound was
rather unstable and tended to form 38.
3′-(Tr im eth ylsilyl)-4′-ph en yl-1-eth oxy-2-(tr im eth ylsilyl)-
sp ir o(cyclop r op en e-3,5′-2′(5′H)-fu r a n on e) (36): 1H NMR
(CDCl3) δ 0.00 (s, 9), 0.09 (s, 9), 1.48 (t, 3, J 1,2 ) 6.83 Hz),
4.15-4.20 (m, 2), 7.0-7.4 (m, 5); 13C NMR δ -0.95, -0.81, 14.6,
70.2, 84.4, 127.9, 128.3, 128.9, 131.1, 133.6, 146.4, 175.5, 178.0
(one C not observed); IR (film) 1809, 1730, 1702 cm-1; EIMS
m/z 372 (M+, 6), 343 (M+ - Et, 100), 255 (38), 159 (PhCCTMS+
- CH3, 34), 73 (Me3Si+, 8); HRMS m/z calcd for C20H28O3Si2
372.1577, found 372.1576.
1
°C; H NMR (CDCl3) δ 0.35 (s, 18, Me3Si), 3.54 (s, 6, OCH3);
13C NMR (CDCl3) δ 0.12, 51.3, 85.5, 173.4, 200.1; IR (vapor)
1714 cm-1 (CdO); EIMS m/z 300 (M+, 18), 285 (24), 272 (20),
257 (34), 226 (24), 155 (Me3SiCtCSiMe2+, 100), 125 (58), 73
(Me3Si+, 92); CIMS, NH3 m/z 318 (M + NH4+, 100); HRMS
m/z calcd for C13H24O4Si2 300.1213, found 300.1209. The
second fraction, following recrystallization from MeOH/water,
yielded 25 as bright yellow spars (49.4 mg, 0.13 mmol, 12%):
mp 78.1-81.2 °C; 1H NMR δ 0.28 (s, 9), 0.31 (s, 9), 3.37 (s, 6),
3.45 (s, 6); 13C NMR (CDCl3) δ 0.25, 0.84, 51.0, 52.0, 96.9, 108.4,
112.3, 156.5, 183.2; IR (KBr) 1722 cm-1; UV (CH3CN) λmax 212
(very strong, enone K band), 369 (weak, enone R band) nm;
EIMS m/z 374 (M+, 3), 359 (13), 346 (35), 331 (89), 315 (38),
227 (99), 199 (37), 171 (55), 125 (60), 89 (72), 73 (Me3Si+, 100);
HRMS m/z calcd for C16H30O4Si2 374.1581, found 374.1578.
Reaction of 14a (80 mg, 0.35 mmol) and oxadiazoline 3013a
(60 mg, 0.37 mmol) in 12.5 mL of dry benzene in a sealed
thermolysis tube at 110 °C for 20 h followed by evaporation of
the solvent and chromatography on silica gel (10% EtOAc/
hexanes) also gave 24, in 62% yield.
Gen er a l P r oced u r e for th e Cycloa d d ition of P h oto-
ch em ica lly Gen er a ted 16 w ith Alk yn es. A solution of
3-phenyl-4-(trimethylsilyl)cyclobutenedione (14b) (46 mg, 0.2
mmol) and the appropriate alkyne (5 equiv) in chloroform (2
mL) was degassed by bubbling through argon for 30 min and
was then irradiated with 350 nm light for 7 days at 5 °C. The
solvent was evaporated, and the residue was purified by thin
layer chromatography (5/95 ethyl acetate/hexane) to give the
products as yellow oils.
3′-(Tr im eth ylsilyl)-4′-p h en yl-1,2-d im eth ylsp ir o(cyclo-
p r op en e-3,5′-2′(5′H)-fu r a n on e) (33) (43%): 1H NMR (CDCl3)
δ 0.05 (s, 9), 2.01 (s, 6), 6.9-7.4 (m, 5); 13C NMR (CDCl3) δ
-0.92, 9.02, 115.5, 127.2, 127.9, 128.5, 131.2, 134.0, 174.6,
177.0; IR (CCl4) 1728 (vs), 1585 (s) cm-1; UV λmax (CH3CN)
248 nm (ꢀ ) 8800); EIMS m/z 284 (M+, 14), 269 (M+ - CH3,
57), 241 (M+ - CH3-CO, 60), 174 (Me3SiCtCPh+, 20), 159
(PhCtCSiMe2+, 92), 73 (Me3Si+, 100); HRMS m/z calcd for
C17H20O2Si 284.1233, found 284.1232.
3′-(Tr im et h ylsilyl)-4′-p h en yl-1-m et h yl-2-(t r im et h ylsi-
lyl)sp ir o(cyclop r op en e-3,5′-2′(5′H)-fu r a n on e) (34) (35%):
1H NMR (CDCl3) δ 0.06 (s, 9), 0.25 (s, 9), 2.20 (s, 3), 7.2-7.5
(m, 5); 13C NMR (CDCl3) δ -1.24, -0.75, 11.6, 121.5, 127.7,
127.8, 128.6, 130.0, 133.8, 133.9, 174.8, 178.4 (one C not
observed); IR (CCl4) 1730 (vs), 1587 (s) cm-1; EIMS m/z 342
(M+, 6), 327 (M+ - CH3, 58), 299 (M+ - CH3, CO, 56), 159
(PhCtCSiMe2+, 88), 73 (Me3Si+, 100); HRMS m/z calcd for
C19H26O2Si2 342.1471, found 342.1469.
2,5-Bis(t r im et h ylsilyl)-3-p h en yl-6-et h oxy-1,4-b en zo-
qu in on e (37): 1H NMR (CDCl3) δ 0.07 (s, 9), 0.07 (s, 9), 1.47
(t, 3, J 1,2 ) 7.0), 4.28 (q, 2, J 1,2 ) 7.0), 7.10-7.35 (m, 5); 13C
NMR δ 0.28, 2.21, 15.0, 69.2, 93.2, 111.9, 127.5, 128.1, 130.0,
132.8, 148.4, 148.5, 191.5, 204.0; IR (CDCl3) 1690, 1594 cm-1
;
UV λmax (CH3CN) 239 (ꢀ ) 3.0 × 105), 293 (sh, ꢀ ) 5.9 × 104),
346 nm (sh, ꢀ ) 2.0 × 104); EIMS m/z 372 (M+, 13), 344 (M+
-
CO, 33), 316 (M+ - 2CO, 40), 271 (53), 73 (Me3Si+, 100); HRMS
m/z calcd for C20H28O3Si2 372.1577, found 372.1573.
2-(Tr im e t h ylsilyl)-3-p h e n yl-6-e t h oxy-1,4-b e n zoq u i-
n on e (38): 1H NMR (CDCl3) δ 0.09 (s, 9), 1.47 (t, 3, J ) 6.9
Hz), 4.14 (q, 2, J ) 6.90 Hz), 4.69 (s, 1), 7.1-7.40 (m, 5); 13C
NMR δ 0.65, 14.7, 68.1, 86.7, 128.1, 128.8, 129.2, 130.4, 132.8,
145.7, 146.8, 187.3, 200.0; IR (CDCl3) 1700, 1594 cm-1; UV
λmax (CH3CN) 235 (ꢀ ) 1.3 × 105), 273 nm (sh, ꢀ ) 3.5 × 104);
EIMS m/z 272 (M+ - CO, 95), 244 (M+ - 2CO, 40), 229 (81),
159 (82), 73 (Me3Si+, 100); HRMS m/z calcd for C16H20O2Si (M+
- CO) 272.1232, found 272.1230.
In an NOE experiment with 38, saturation of the CH2 gave
a 15.7% increase of the 4.69 ppm proton with no NOE
enhancement of the phenyl and trimethylsilyl groups, and
saturation of the 4.69 ppm proton gave 4.1% NOE of the CH2
group with no change in the phenyl and trimethylsilyl groups.
Saturation of the TMS protons gave a 7.2% increase of the
two ortho Ph protons at 7.15-7.25, with no change of the 4.69
ppm H and ethyl groups.
Ack n ow led gm en t. Financial support by a Natural
Sciences and Engineering Research Council Collabora-
tive Grant and by the Ontario Centre for Materials
Research is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: NMR spectra for
compounds 18-20a ,b,e, 21a ,b, 24, 25, and 33-38 (15 pages).
This material is contained in libraries on microfiche, im-
mediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
3′-(Tr im et h ylsilyl)-4′-p h en yl-1-p h en yl-2-(t r im et h ylsi-
lyl)sp ir o(cyclop r op en e-3,5′-2′(5′H)-fu r a n on e) (35) (37%):
1H NMR (CDCl3) δ 0.12 (s, 9), 0.15 (s, 9), 6.9-7.6 (10, m); 13
C
NMR (CDCl3) δ -1.2, -0.7, 122.0, 126.9, 127.8, 128.0, 128.8,
129.1, 129.6, 130.7, 133.4, 133.7, 136.4, 174.8, 177.7 (one C
not observed); IR (CCl4) 1730 (vs), 1600 (s), 1587 (s) cm-1
;
J O961169R