Generation of 1,2-bisketenes from cyclobutene-1,2-diones by flash photolysis and ring closure kinetics

Article abstract of DOI:10.1021/ja9722685

The interconversion of cyclobutene-1,2-diones (1) and 1,2-bisketenes (RC-C-O)₂ has been surveyed for different combinations of substituents R = H, Me, t-Bu, Ph, Me₃Si, CN, Cl, Br, R¹O, alkynyl, and PhS. The bisketenes 2 have been generated by flash photolysis, and the kinetics of their conversion to 1 have been studied by time-resolved infrared and ultraviolet spectroscopy. The rate constants of the ring closure of 2 are correlated by the ketene stabilization parameters (SE) and with calculated barriers. The rate constant of ring closure of the di-tert-butyl bisketene 2g to cyclobutenedione 1g is only 40 times smaller than for the dimethyl analogue, showing a rather modest steric barrier. The quinoketene 2s has a fast rate of ring closure, but not as fast as anticipated on the basis of calculated geometric and thermodynamic factors. A lag in the attainment of aromatic stabilization in the transition structure for ring closure is a possible cause of this diminished reactivity.

Full text of DOI:10.1021/ja9722685

J. Am. Chem. Soc. 1997, 119, 12125-12130
12125
Generation of 1,2-Bisketenes from Cyclobutene-1,2-diones by
Flash Photolysis and Ring Closure Kinetics1a
Annette D. Allen,† Jim D. Colomvakos,† Franc¸ois Diederich,‡ Ian Egle,†
Xiaokuai Hao, Ronghua Liu,† Janusz Lusztyk,*,§ Jihai Ma,†
Michael A. McAllister, Yves Rubin,| Kuangsen Sung,† Thomas T. Tidwell,*,† and
Brian D. Wagner#
Contribution from the Department of Chemistry, UniVersity of Toronto,
Toronto, Ontario, Canada M5S 3H6, Laboratorium fu¨r Organische Chemie, ETH-Zentrum,
UniVersita¨tstrasse 16, CH-8092 Zu¨rich, Switzerland, Steacie Institute for Molecular Sciences,
National Research Council of Canada, Ottawa, Canada K1A OR6, Department of Chemistry,
UniVersity of California, Los Angeles, California 90095-1569, Department of Chemistry,
UniVersity of North Texas, Denton, Texas 76203, and Department of Chemistry,
UniVersity of Prince Edward Island, Charlottetown, Canada C1A 4P3
ReceiVed July 8, 1997X
Abstract: The interconversion of cyclobutene-1,2-diones (1) and 1,2-bisketenes (RCdCdO)2 (2) has been surveyed
for different combinations of substituents R ) H, Me, t-Bu, Ph, Me3Si, CN, Cl, Br, R1O, alkynyl, and PhS. The
bisketenes 2 have been generated by flash photolysis, and the kinetics of their conversion to 1 have been studied by
time-resolved infrared and ultraviolet spectroscopy. The rate constants of the ring closure of 2 are correlated by the
ketene stabilization parameters (SE) and with calculated barriers. The rate constant of ring closure of the di-tert-
butyl bisketene 2g to cyclobutenedione 1g is only 40 times smaller than for the dimethyl analogue, showing a rather
modest steric barrier. The quinoketene 2s has a fast rate of ring closure, but not as fast as anticipated on the basis
of calculated geometric and thermodynamic factors. A lag in the attainment of aromatic stabilization in the transition
structure for ring closure is a possible cause of this diminished reactivity.
The photochemical ring opening of cyclobutene-1,2-diones
(1) to give 1,2-bisketenes (2) was discovered simultaneously
by Mallory and Roberts1b and by Blomquist and LaLancette1c
in 1961 (eq 1). The 1,2-bisketenes were not directly observed
ones 1, while those with one silyl substituent were less stable
than the corresponding cyclobutenediones but were relatively
long lived at room temperature.3 This opened the way for
extensive studies4 of the spectroscopic and chemical properties
of 1,2-bisketenes 2, including an X-ray crystallographic structure
determination,4f which showed a 119° angle between the
adjacent ketenyl groups in an analogue of 2, confirming the
twisted structure predicted3c,f on the basis of theoretical studies
and dipole moment measurements.
In a preliminary study,3d we have examined the formation of
several 1,2-bisketenes 2 by the photolysis of cyclobutenediones
1 and have used time-resolved infrared spectroscopy (TRIR)
for the observation of some of these species. This technique5
is well suited for the observation of such 1,2-ketenes because
of their strong IR bands near 2100 cm-1, a region that is free
from absorption by most other functional groups. This method
is also useful for the measurement of fast reaction kinetics and
but were inferred as reactive intermediates on the basis of the
products formed. Since that time, this technique has been
utilized to generate several 1,2-bisketenes, but usually, these
species have only been observed at low temperatures, often in
matrices because of their facile thermal ring closure back to 1.2
Recently, we found that for R ) R1 ) Me3Si the 1,2-bisketene
2 was thermodynamically stable relative to the cyclobutenedi-
(3) (a) Zhao, D.; Allen, A. D.; Tidwell, T. T. J. Am. Chem. Soc. 1993,
115, 10097-10103. (b) Allen, A. D.; Lai, W.-Y.; Ma, J.; Tidwell, T. T. J.
Am. Chem. Soc. 1994, 116, 2625-2626. (c) McAllister, M. A.; Tidwell, T.
T. J. Am. Chem. Soc. 1994, 116, 7233-7238. (d) Allen, A. D.; Colomvakos,
J. D.; Egle, I.; Lusztyk, J.; McAllister, M. A.; Tidwell, T. T.; Wagner, B.
D.; Zhao, D.-c. J. Am. Chem. Soc. 1995, 117, 7552-7553. (e) Tidwell, T.
T. Ketenes; Wiley: New York, 1995. (f) Allen, A. D.; Ma, J.; McAllister,
M. A.; Tidwell, T. T.; Zhao, D.-c. Acc. Chem. Res. 1995, 28, 265-271.
(4) (a) Allen, A. D.; Egle, I.; Janoschek, R.; Liu, H. W.; Ma, J.; Marra,
R. M.; Tidwell, T. T. Chem. Lett. 1996, 45-46. (b) Liu, R.; Tidwell, T. T.
J. Chem. Soc., Perkin Trans. 2 1996, 2757-2762. (c) Colomvakos, J. D.;
Egle, I.; Ma, J.; Pole, D. L.; Tidwell, T. T.; Warkentin, J. J. Org. Chem.
1996, 61, 9522-9527. (d) Egle, I.; Lai, W.-Y.; Renton, P.; Tidwell, T. T.;
Zhao, D.-c. J. Org. Chem. 1997, 62, 18-25. (e) Allen, A. D.; Ma, J.;
McAllister, M. A.; Tidwell, T. T.; Zhao, D.-c. J. Chem. Soc., Perkin Trans.
2 1995, 847-851. (f) Allen, A. D.; Lough, A. J.; Tidwell, T. T. Chem.
Commun. 1996, 2171-2172.
† University of Toronto.
‡ ETH.
§ Steacie Institute of Molecular Sciences.
| UCLA.
University of North Texas.
# UPEI.
X Abstract published in AdVance ACS Abstracts, December 1, 1997.
(1) (a) Issued as NRC Publication No. 40850. (b) Mallory, F. B.; Roberts,
J. D. J. Am. Chem. Soc. 1961, 83, 393-397. (c) Blomquist, A. T.;
LaLancette, E. A. J. Am. Chem. Soc. 1961, 83, 1387-1391.
(2) (a) Staab, H. A.; Ipaktschi, J. Chem. Ber. 1968, 101, 1457-1472.
(b) Kasai, M.; Oda, M.; Kitahara, Y. Chem. Lett. 1978, 217-218. (c) Maier,
G.; Reisenauer, H. P.; Sayrac, T. Chem. Ber. 1982, 115, 2192-2201. (d)
Obata, N.; Takizawa, T. Bull. Chem. Soc. Jpn. 1977, 50, 2017-2020. (e)
Hochstrasser, R.; Wirz, J. Angew. Chem., Int. Ed. Engl. 1989, 28, 181-
183. (f) Diederich, F.; Rubin, Y.; Chapman, O. L.; Goroff, N. S. HelV.
Chim. Acta 1994, 77, 1441-1457. (g) Miller, R. D.; Kirchmeyer, S. J. Org.
Chem. 1993, 58, 90-94. (h) Adam, W.; Patterson, W. S. J. Org. Chem.
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S0002-7863(97)02268-3 CCC: $14.00 © 1997 American Chemical Society

12126 J. Am. Chem. Soc., Vol. 119, No. 50, 1997
Allen et al.
1
was applied to the determination of the rate constants for ring
closure in selected examples.3d We have also used UV
spectroscopy for the measurement of the rates of thermal
conversion of the 1,2-bisketenes 2 back to cyclobutenediones
1 in examples where the reactions are slower3b,d,4b and now
report a survey of this process for representative structural types
of 1,2-bisketenes. These include ketenes with CN,6 Br,7
alkynyl,8 thio,9 and alkoxy10 substituents and permit a quantita-
tive test of previously derived ketene substituent parameters
(SE)11a,b and studies of substituent effects on cyclobutene-
diones.3c,11c As a test of steric effects on the interconversion
of eq 1, the crowded di-tert-butylcyclobutene-1,2-dione (1g) has
also been prepared and its interconversion with the bisketene
(t-BuCdCdO)2 (2g) studied.
2101 cm-1 and the H NMR signal at δ 1.17. Hydration by
adventitious H2O evidently led to 8. As described below
photolysis of 1g also gave 2g, together with alkyne 3.
3,4-Di-tert-butoxycyclobutenone (1k)14a was prepared by
reaction of silver squarate prepared from 3,4-dihydroxycy-
clobutenone (squaric acid) with tert-butyl chloride (eq 5).
Results
The cyclobutenediones except for 1g,o are known com-
pounds.1-3,12 The bis(t-Bu) derivative 1g was made by two
independent routes, as shown in eqs 2 and 3. The route of eq
3-Ethoxy-4-(trimethylsilyl)cyclobutenedione (1o) was pre-
pared by reaction of the known 912h with AgOTf/AgOTs.
Hydrolysis of 9 using H2SO4 resulted in desilylation, and so
this was effected by silver salt induced hydrolysis as shown
(eq 6). This latter method has been used in related cases where
desilylation is a problem,4b and for 9 a 1/1 mixture of salts was
found to be most effective.
3 from meso-2,3-di-tert-butylsuccinic acid (6)13 presumably
involved the bisketene 2g as an intermediate. Evidence for the
formation of 2g was obtained by the reaction of 7 with DMAP
at room temperature, which gave spectral evidence for the
presence of 10% of 2g together with the known3a anhydride 8
(eq 4). The structure of 2g was indicated by the IR band at
After di-tert-butylcyclobutenedione (1g) was heated to 180
°C and cooled, no change in the 1H NMR spectrum was
detected. Upon photolysis in CDCl3 with 350 nm light, a new
1H NMR signal at δ 1.17 and a new IR band at 2101 cm-1
were observed. These were attributed to the bisketene 2g, with
an estimated conversion from 1g of 5%. Irradiation at lower
wavelengths or with benzophenone as a photosensitizer also
gave 15% 2g, along with 35% of the alkyne 3. Irradiation in
C6D6 with 350 nm light gave NMR and IR absorptions attributed
to 2g at δ 1.21 and 2099 cm-1, respectively, and an estimated
conversion of 14%.
(6) (a) Bock, H.; Hirabayashi, T.; Mohmand, S. Chem. Ber. 1981, 114,
2595-2608. (b) Holmquist, B.; Bruice, T. C. J. Am. Chem. Soc. 1969, 91,
3003-3009. (c) Moore, H. W.; Gheorghiu, M. D. Chem. Soc. ReV. 1981,
10, 289-328. (d) Moore, H. W.; Decker, O. H. W. Chem. ReV. 1986, 86,
821-830.
(7) (a) Durst, T.; Koh, K. Tetrahedron Lett. 1992, 33, 6799-6802. (b)
Westwood, N. P. C.; Lewis-Bevan, W.; Gerry, M. C. L. J. Mol. Spectrosc.
1989, 136, 93-104.
(8) (a) Allen, A. D.; Gong, L.; Tidwell, T. T. J. Am. Chem. Soc. 1990,
112, 6396-6397. (b) Allen, A. D.; Andraos, J.; Kresge, A. J.: McAllister,
M. A.; Tidwell, T. T. J. Am Chem. Soc. 1992, 114, 1878-1879. (c) Rubin,
Y., Lin, S. S.; Knobler, C. B.; Anthony, J.; Boldi, A. M.; Diederich, F. J.
Am. Chem. Soc. 1991, 113, 6943-6949.
(9) (a) Jones, J., Jr.; Kresge, A. J. J. Org. Chem. 1992, 57, 6467-6469.
(b) Inoue, S.; Hori, T. Bull. Chem. Soc. Jpn. 1983, 56, 171-174. (c) Bock,
H.; Ried, W.; Stein, U. Chem. Ber. 1981, 114, 673-683.
Photolysis of 1g in MeOH gave the d,l-ester 1013b (eq 7),
while photolysis in CDCl3 containing H2O gave the known3a
cis-anhydride 8. The stereochemical assignment of 10 is based
(10) (a) Maier, G.; Rohr, C. Liebigs Ann. 1996, 307-309. (b) Chiang,
Y.; Kresge, A. J.; Pruszynski, P.; Schepp, N. P.; Wirz, J. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 792-794. (c) Dehmlow, E. V. Tetrahedron Lett.
1972, 1271-1274.
(11) (a) Gong, L.; McAllister, M. A.; Tidwell, T. T. J. Am. Chem. Soc.
1991, 114, 6021-6028. (b) McAllister, M. A.; Tidwell, T. T. J. Org. Chem.
1994, 59, 4506-4515. (c) Cerioni, G.; Janoschek, R.; Rappoport, Z.;
Tidwell, T. T. J. Org. Chem. 1996, 61, 6212-6117.
on the vicinal H,H coupling observed from the 13C-H satellites,
a technique we have recently used in other studies of succinates.4d
13C-H
For d,l-10, J
) 130.4 Hz and JH,H ) 1.2 Hz. The low
(12) (a) Liebeskind, L. S.; Fengl, R. W.; Wirtz, K. R.; Shawe, T. T. J.
Org. Chem. 1988, 53, 2482-2488. (b) Ooms, P. H. J.; Scheeren, J. W.;
Nivard, R. J. F. Synthesis 1975, 639-641. (c) Smutny, E. J.; Caserio, M.
C.; Roberts, J. D. J. Am. Chem. Soc. 1960, 82, 1793-1801. (d) De Selms,
R. C.; Fox, C. J.; Riordan, R. C. Tetrahedron Lett. 1970, 781-782. (e)
Ohno, M.; Yamamoto, Y.; Shirasaki, Y.; Eguchi, S. J. Chem. Soc., Perkin
Trans. 1 1993, 263-271. (f) Rubin, Y.; Knobler, C. B.; Diederich, F. J.
Am. Chem. Soc. 1990, 112, 1607-1617. (g) Schmidt, A. H.; Ku¨nz, C.;
Debo, M.; Mora-Ferber, J.-P. Synthesis 1990, 819-822. (h) Danheiser, R.
L.; Sard, H. Tetrahedron Lett. 1983, 24, 23-26.
magnitude of the latter coupling constant is consistent with a
near-90° dihedral angle, as expected for the d,l-ester.
Laser flash photolyses of cyclobutenediones 1 with time-
resolved infrared detection were carried out using a 308 nm
laser, either directly or, in the cases of 1f-j in the presence of
4-methoxyacetophenone as a sensitizer. The appearance of
strong infrared bands near 2100 cm-1, assigned to absorption
(13) (a) Eberson, L. Acta Chem. Scand. 1959, 13, 40-49. (b) Jacobsen,
E. N.; Totten, G. E.; Wenke, G.; Karydas, A. C.; Rhodes, Y. E. Synth.
Commun. 1985, 15, 301-306.
(14) (a) Dehmlow, E. V.; Schell, H. G. Chem. Ber. 1980, 113, 1-8. (b)
Eggerding, D.; West, R. J. Am. Chem. Soc. 1976, 98, 3641-3644. (c) Liu,
H.; Tomooka, C. S.; Moore, H. W. Synth. Commun. 1997, 27, 2177-2180.

Generation of 1,2-Bisketenes
J. Am. Chem. Soc., Vol. 119, No. 50, 1997 12127
Table 1. IR Bands and Rate Constants of Ring Closure of
1,2-Bisketenes 2 Generated from Cyclobutenediones 1
ν1, ν2 a
b
R1, R2
(cm-1
)
I1/I2
kobs (s-1
)
∑SE f
a
b
c
d
e
f
Ph, H
Ph, Ph
2103, 2127 0.55 (1.70 ( 0.13) × 10-4
2096, 2110 0.75 (3.89 ( 0.15) × 10-2
2093, 2112 0.60 (3.50 ( 0.09) × 10-2
2116, 2140 2.68 (8.51 ( 1.29) × 10-2
2105, 2132 0.98 0.137 ( 0.022
4.8
1.8
0.9
1.5
(-6.8)
0.0
Ph, Me
Ph, CN
Ph, Br
Me, Mec
t-Bu, t-Buc
Cl, Clc
2092, 2114 1.82 (3.60 ( 0.09) × 10-2
g
h
2087, 2113 2.63 (8.19 ( 0.20) × 10-4
0.0
2123, 2154 1.42 (3.20 ( 0.80) × 103 -15.4
(5.2 ( 0.5) × 103 a
i
Cl, MeOc
2098, 2142 1.38 (2.97 ( 0.90) × 104 -22.2
(1.0 ( 0.1) × 105 a
j
k
l
t-Bu, i-PrOc
t-BuO, t-BuO
2092, 2108 1.22 (3.2 ( 0.3) × 104 a
-14.5
d
(2.52 ( 0.40) × 104 -29.0
(t-BuMe2SiCtC) 2111, 2119 0.60 (4.23 ( 0.37) × 10-3
1.0
1.0
0.4
m (PhCtC)
d
2112
(1.59 ( 0.15) × 10-2
n
o
p
q
r
PhS, PhS
(5.62 ( 0.56) × 10-3
Me3Si, EtO
Me3Si, Ph
Me3Si, Me
Me3Si, Me3Si
2090, 2104 1.33 (1.77 ( 0.14) × 10-3 -3.0
2076
2101
2084
(2.03 ( 0.20) × 10-6 12.4
(4.37 ( 0.44) × 10-6 11.5
e
(10-10
)
23.0
s
(CHdCHCHdCH) 2073, 2128 0.81 (1.9 ( 0.2) × 104 a
a In CH3CN, measured by TRIR. b In isooctane measured by UV
unless noted. c Generation sensitized with 4-methoxyacetophenone.d Not
measured. e Estimated, see text. f Sum of substituent stabilization ener-
gies (kcal/mol) defined by eq 9.
Figure 2. Transient IR absorption traces from 308 nm laser flash
photolysis of 1j in CH3CN with 4-methoxyacetophenone as a sensitizer.
(a) Generation and decay of 2j. The inset shows the transient IR
spectrum of 2j. (b) Bleaching and almost complete recovery of 1j.
apparatus. For 2a,b,p,q, the decay of the ketenes was suf-
ficiently slow that these could be generated by conventional
UV photolysis, and the kinetics of ring closure were observed
by UV spectroscopy.
In the cases of cyclobutenediones 1i,j, recovery of the original
IR absorption after photolysis (monitored at 1804 cm-1) follows
the same kinetics as the decay of the corresponding bisketene
and is essentially complete (Figure 2). In the case of 1s, this
does not occur, and this bisketene is known2a to form dimers.
Decarbonylation of the bisketenes may occur upon photolysis,3a,e
as well as hydration by traces of H2O in the solvent, and these
may also contribute to any lack of recovery.
For 1c-g,l-o, photolyses were carried out by conventional
flash photolysis and rate constants for ring closure of the
resulting bisketenes were measured using conventional ultra-
violet spectroscopy. For 2h,i,k, the rate constants were obtained
using conventional flash photolysis with time-resolved ultra-
violet detection in isooctane solvent. For 2h,i, the rate constants
were measured both by TRIR in CH3CN and by UV in
isooctane, and those in CH3CN are larger by factors of 1.6 and
3.4, respectively. This is consistent with previous comparisons3b,4b
in which ring closure rate constants were larger by factors of
1.5-4.3 in the more polar CDCl3 compared to isooctane.
Decarbonylation of the substrates to form the corresponding
alkynes was observed with several of the substrates, and this
reaction has been previously observed upon photolysis of
bisketenes.3 This was particularly prominent with the bis-
(phenylethynyl)-substituted cyclobutenedione 1m, in which the
bisketene was not observed upon laser flash photolysis with IR
detection but was seen using lower photolysis power levels with
UV detection.
Figure 1. Infared spectrum of (t-BuMe2SiCtCCdCdO)2 (2l) from
308 nm laser flash photolysis of 1l in CH3CN.
by the ketenyl groups in bisketenes 2, was observed in most of
the examples, as noted in Table 1. Representative spectra, for
2l (R1 ) R2 ) t-BuMe2SiCtC) and 2j (R1 ) t-Bu, R2 ) i-PrO)
are shown in Figures 1 and 2, respectively. The bisketenes from
1k,m were observed only by UV.
Usually two well-resolved ketene bands were observed, and
the band positions of each are given, along with the ratios of
the maximum peak heights of the two bands (Table 1). Ab initio
calculations (MP2/6-31G*//MP2/6-31G*)3d for 2f (Me, Me)
predict the appearance of IR bands at 2098 and 2113 cm-1
(relative intensity (rel int) 1.41), compared to the observed bands
at 2092 and 2114 cm-1 (rel int 1.82), while HF/6-31G*//HF/
6-31G* calculations of 2s predict IR absorptions at 2062 and
2133 cm-1 (rel int 0.96), compared to the observed bands at
2073 and 2128 (rel int 0.81). In the case of 2r (SiMe3, SiMe3),
only a single IR band is observed, at 2084 cm-1. Calculation
for (SiH3, SiH3)3d predicts absorption bands at 2089 and 2090
cm-1, and these are not resolved experimentally. In the cases
of 2n,p-r, only single bands were observed, but these are also
attributed to unresolved doublet absorptions.
Upon flash photolysis of the bis(t-BuO) cyclobutenedione
1k,14a formation of the bisketene 2k which reformed 1k could
be observed by UV. Upon continuous photolysis, the cyclo-
propenone 1114b was formed and isolated (eq 8). The formation
of diethoxycyclopropenone has been reported from photolysis
In the cases of 2a-g,l,n-r, the ketenyl infrared bands were
long lived on the time scale of the laser flash photolysis

12128 J. Am. Chem. Soc., Vol. 119, No. 50, 1997
Allen et al.
Table 2. Ab Initio Calculated Energies (hartrees), Relative
Energies (kcal/mol, parentheses), and Geometries of Benzocyclo-
butenedione (1s), 5,6-Bis(oxomethylene)-1,3-cyclohexadiene (2s),
and Transition State TSs
of 1 (R ) R1 ) OEt),10c and photolysis of dihydroxycy-
clobutenedione (1, R ) R1 ) OH) in an Ar matrix at 10 K led
to formation of dihydroxycyclopropenone in a process proposed
to involve a ketenylcarbene.10a An analogous ketenylcarbene
12 is depicted in eq 8. However, recent calculations15 at the
MP2/6-31G*//MP2/6-31G* level for 1 (R ) R1 ) OH) indicate
that the thermal conversion of dihydroxycyclobutenedione to
the cyclopropenone is a one-step process without the formation
of a discrete ketenylcarbene intermediate.
2s
TSs
1s
a
EHF
-454.90102
0.09249
-454.80853
(0.0)
-456.24074
-456.14825
(0.0)
-457.59952
-457.50703
(0.0)
-454.85494
0.09062
-454.76432
(27.7)
-456.22462
-456.13400
(8.9)
-457.58072
-457.49010
(10.6)
-454.95880a
0.09482
-454.86398
(-34.8)
-456.30178
-456.20696
(-36.8)
-457.65243
-457.55761
(-31.7)
1.381
ZPVE
EHF + ZPVE
b
EMP2
EMP2 + ZPVE
c
EB3LYP
EB3LYP + ZPVE
The structure of R,R′-dioxo-o-quinonemethide [5,6-bis(keten-
ylidene)-1,3-cyclohexadiene] (2s), benzocyclobutene-3,4-dione
(1s), and the transition structure (TSs) for their interconversion
were calculated at the HF/6-31G* level using Gaussian 94,16a
and energies for this geometry were calculated at the MP2/6-
31G* and B3LYP16b levels, as reported in Table 2. In previous
studies3c we have found good agreement between MP2/6-31G*/
/MP2/6-31G* and MP2/6-31G*//HF/6-31G* energies for similar
ring closure reactions of 1,2-bisketenes, and the good agreement
between the relative energy changes calculated at the MP2 and
B3LYP levels using the HF/6-31G* geometries also indicate
that these energies are reliable.
C1C2
1.493
1.427
C2C3
C3C4
C4C5
C1C7
C1O
C7C8
C1C2C3
C1C2C8
C2C3C4
C3C4C5
C6C1C7
C1C7O
C7C1C2C8
1.474
1.328
1.469
1.315
1.434
1.348
1.448
1.375
1.390
1.379
1.409
1.500
1.176
1.570
122.4
93.6
115.6
122.0
144.0
137.3
1.147
1.158
116.6
122.8
120.2
121.4
133.7
177.9
35.2
119.2
107.1
116.4
122.1
133.7
153.7
22.4
0.0
Discussion
a HF/6-31G*//HF/6-31G*. b MP2/6-31G*//HF/6-31G*. c B3LYP/6-
31G*//HF/6-31G*.
The measured rate constants of ring closure of the bisketenes
2 span a range of more than 1010. In addition, a rate constant
of 10-10 s-1 may be estimated for ring closure of the 2,3-(Me3-
Si)2 bisketene 2r from the rate constant of 10-6 s-1 at 25 °C in
CDCl3 for ring opening of 1r derived from measurements at
higher temperatures,3a and the equilibrium constant of 3.0 ×
104 for ring opening of the 3,4-(SiH3)2-substituted cyclobutene-
dione based on the calculated3c ∆G of -6.1 kcal/mol. This
extends the range of rates to more than 1014.
These rate constants are affected by the influence of the
substituents on both the reactants11a,b and the transition states,
and as discussed elsewhere,3c,4b the latter are influenced by the
properties of the product cyclobutenediones.11c We have
reported11a,b calculated stabilization energy (SE) parameters for
the effect of substituents on ketenes compared to alkenes defined
(in kcal/mol) by the isodesmic reaction in eq 9 and have found
good correlation suggests that the substituent effects on the
ketene stabilities have a significant influence on the bisketene
reactivity.
The SE parameters are correlated with group electronega-
tivities,17 and electropositive substituents stabilize ketenes.11a,b
The least reactive bisketenes are those with electropositive Me3-
Si groups, while those with electronegative substituents (e.g.,
t-BuO, t-BuO; MeO, Cl; Cl, Cl) are the most reactive. The
substituents Ph, Me, CN, SH, and HCtC have rather similar
SE values, and the substrates bearing only these groups are
clustered together on the correlation.
The deviations from the correlation indicate that other factors
are involved as well. Thus the 40-fold greater reactivity of the
Me, Me compared to the t-Bu, t-Bu bisketene 2g reflects the
steric crowding in the di-tert-butylcyclobutenedione. However,
this crowding does not lend steric protection comparable to that
found for t-Bu2CdCdO, which is indefinitely stable to dimer-
ization.18 The bisketene 2g could not be isolated in pure form
and appeared sensitive to air, moisture, and further photolysis.
The π-donor substituents (RO, PhS, halogen) are expected to
stabilize the electron-deficient cyclobutenedione and thus would
also be expected to accelerate the ring closure. However, the
electronegative character of these groups also destabilizes the
ketenyl groups and enhances ring closure, and the respective
role of the two effects cannot be separated.
RCHdCdO + CH3CHdCH2 SE ) ∆E8
CH3CHdCdO + RCHdCH2 (9)
that these can explain many observed properties of ketenes.
These give a correlation of the observed rate constants by the
relationship log k(bisketene) ) -(0.29 ( 0.02)∑SE - (2.02 (
0.26), r ) 0.96 (Figure 3, Supporting Information). This rather
(15) Sung, K.; Fang, D.; Glenn, D.; Tidwell, T. T. Submitted for
publication.
(16) (a) Gaussian 94, Revision D2: Frisch, M. J.; Trucks, G. W.;
Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman,
J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari,
K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.;
Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng,
C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E.
S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.;
Baker, J.; Stewart, J. P.; Head-Gordon, J.; Gonzales, C.; Pople, J. A.
Gaussian, Inc.: Pittsburgh, PA, 1995. (b) Becke, A. D. J. Chem. Phys.
1993, 98, 5648-5652.
A rapid rate of ring closure of the o-quinoketene 2s would
be expected on the basis of the constrained cisoid geometry
(17) (a) Boyd, R. J.; Edgecombe, K. E. J. Am. Chem. Soc. 1988, 110,
4182-4186. (b) Boyd, R. J.; Boyd, S. L. J. Am. Chem. Soc. 1992, 114,
1652-1655.
(18) (a) Newman, M. S.; Arkell, A.; Fukunaga, T. J. Am. Chem. Soc.
1960, 82, 2498-2501. (b) Allen, A. D.; Tidwell, T. T. J. Am. Chem. Soc.
1987, 109, 2774-2780.

Generation of 1,2-Bisketenes
J. Am. Chem. Soc., Vol. 119, No. 50, 1997 12129
Table 3. Calculated Energeticsa of Ring Closure of Bisketenes 2 to Cyclobutenediones 1
d
R, R1
E(2)b
E(TS)b
E(1)b
log kc
Eact
∆Ed
Me, Me
Cl, OH
Cl, Cl
-381.3533
-837.1304
-1221.1484
-883.3908
-381.3237
-837.1134
-1221.1238
-883.3397
-381.3772
-837.1696
-1221.1727
-883.3830
-1.44
4.47
3.50
18.6
10.7
15.5
32.1
8.9
-15.0
-24.6
-15.2
4.9
e
SiH3, SiH3
-10.0
(CHdCHCHdCH) (2s)f
4.28
-36.8
a MP2/6-31G*+ZPVE//HF/6-31G*, ref 19. b hartrees. c Table 1. d kcal/mol. e Reference 3c. f Table 2.
and also because of the benzene resonance gained on ring
closure (eq 10). It is, however, surprising that this compound
In summary, a group of 1,2-bisketenes 2 with various
combinations of representative substituents have been generated,
and the rate constants of their ring closures to the corresponding
cyclobutenediones 1 have been correlated with the ketene
stabilization parameters SE and with calculated barriers. The
40-fold lower reactivity in ring closure for the 1,2-di-tert-
butylbisketene (1g) compared to the dimethyl analogue 1f is a
measure of the steric crowding in the transition structure for
the former.
is less reactive than are the acyclic derivatives 2i,k, and this
illustrates the dominant influence of the ketene destabilization
by the strongly electronegative substituents on the rate of ring
closure in the latter two cases. The calculated geometry of
quinoketene 2s (Table 2) shows a 35.2° dihedral angle between
the two ketenyl units, and this non-coplanarity of the ketenyl
groups parallels that found in acyclic 1,2-bisketenes (eq 1).3c,f,4f
This phenomenon has been attributed3f to the mutual electronic
repulsions at the two electron-rich Câ positions of the ketenes
and at the two electropositive CR positions.
In 2s, the unfavorable electronic interaction between the two
Câ carbons could be enhanced by antiaromatic effects, as a
coplanar structure with excess electron density at the Câ carbons
would have more than 6π electrons. The six-membered ring
in 2s shows strong bond alternation (Table 2), consistent with
a non-delocalized structure.
The ring closure of 2s to 1s is calculated to be strongly
exothermic, by 31.7-36.8 kcal/mol at the different theoretical
levels, but with an appreciable barrier of 8.9 and 10.6 kcal/mol
at the MP2 and B3LYP levels, respectively. By contrast, ring
closure of 2 (R ) R1 ) F)3c at the MP2/6-31G*//HF-6-31G*
level is calculated to be exothermic by 20.4 kcal/mol, which is
16.4 kcal/mol less than for 2s, but with a similar barrier of 9.5
kcal/mol compared to that of 8.9 kcal/mol for 2s.
Even in the transition structure TSs, there is a rather large
C7C1C2C8 dihedral angle of 22.4° and significant bond alterna-
tion in the six-membered ring. The product benzocyclobutene-
dione 1s is however fully planar, with greatly reduced bond
alternation between the C-C bonds, with a maximum difference
of 0.030 Å. These results suggest that there is little gain of
aromatic stabilization in the transition structure, leading to the
high barrier for ring closure even though the reaction is strongly
exothermic.
Experimental Section
Previously reported cyclobutenediones were prepared by the reported
methods: 1a,1a 1b,1b 1c,12a 1d,12b 1e,12c 1f,12a 1h,12d 1i,12e 1j,12a 1l,12f
1m,12f 1n,12g 1p,3b 1q,3b 1r,3a and 1s.2a
3-Ethoxy-4-(trimethylsilyl)cyclobut-3-ene-1,2-dione (1o). 4,4-
Dichloro-2-(trimethylsilyl)-3-ethoxycyclobutenone (9)12h (0.55 g, 2.6
mmol) was dissolved in benzene (10 mL), silver trifluoromethane-
sulfonate (0.67 g, 2.6 mmol) and silver toluenesulfonate (0.73 g, 2.6
mmol) were added in one portion, and the reaction mixture was refluxed
for 3 h. The AgCl precipitate was filtered off, the solution was diluted
with ether (100 mL) and washed with water, and dried over MgSO4,
and the solvent was evaporated. The crude product was purified by
TLC (silica gel, 3% EtOAc/hexane) to give 1o as a yellow oil (0.30 g,
1.52 mmol, 74%): 1H NMR (CDCl3) δ 0.32 (s, 9), 1.49 (t, 3, J ) 7.2
Hz), 4.78 (q, 2, J ) 7.2 Hz); IR (CDCl3) 1776 (s), 1755 (s), 1557 (s)
cm-1; UV λmisoaoxctane 220 (ꢀ ) 5.4 × 104), 255 (ꢀ ) 4.2 × 104 nm); 13C
NMR (CDCl3) δ -2.3, 15.6, 70.5, 186.9, 194.9, 197.3, 204.8; EIMS
m/z 199 (M+ + 1, 0.5), 183 (M+ - CH3, 1), 170 (M+ - CO, 4.5), 141
(TMSCtCOEt+ - 1, 30), 73 (TMS+, 100); HRMS m/z calcd for
C8H11O3Si (M+ - CH3) 183.0477, found 183.0480.
3,4-Di-tert-butyl-2,4-dichlorocyclobutenone (4). Zinc dust (2.0 g,
0.03 mol) in a 100 mL three-neck flask was heated under N2 with a
Bunsen flame for 10 min with stirring. After cooling, di-tert-
butylacetylene (3)20 in 40 mL of ether was added, followed by dropwise
addition with stirring of CCl3COCl (1.4 mL, 0.013 mol) in 15 mL of
ether over 1 h at room temperature. The solution was stirred for 16 h
and filtered, and the collected solid was washed with 20 mL of ether.
The combined filtrate was washed with NaHCO3 and NaCl solutions,
dried over MgSO4, and evaporated, and the resulting solid was
chromatographed on silica gel (5% EtOAc in hexane) to give 4 (1.9 g,
7.7 mmol, 77%): mp 38.5 °C; 1H NMR (CDCl3) δ 1.08 (s, 9), 1.34 (s,
9); 13C NMR (CDCl3) δ 27.2, 28.6, 35.8, 37.9, 92.8, 128.6, 182.6, 186.8;
IR (CDCl3) 1785 (vs), 1568 (s) cm-1; EIMS m/z 248 (89, M+), 199
(100), 141 (52); HRMS m/z calcd for C12H18Cl2O 250.0580, found
250.0570.
Calculated barriers for ring closure at the MP2/6-31G* +
ZPVE//HF/6-31G* level are available for bisketene 2f (Me, Me)
and 2h (Cl, Cl),19 as well as for 2 (R ) Cl, R1 ) OH19 and R
) R1 ) SiH33c), which are models for 2i and 2r, respectively.
These are summarized in Table 3, along with the present results
for 2s, and give the correlation with the experimental rate
constants log k(bisketene) ) -(0.67 ( 0.04)Eact + (11.7 (
0.05), r ) 0.98 (Figure 4, Supporting Information). This
correlation is comparable to that with the ketene SE parameters
(Figure 3, Supporting Information) but also permits the inclusion
of 2s. Overall, the correlation of the theoretical and experi-
mental results is quite satisfactory.
3,4-Di-tert-butylcyclobut-3-ene-1,2-dione (1g). To 4 (0.5 g, 2.0
mmol) in 10 mL of benzene was added AgO2CCF3 (0.9 g, 4.1 mmol),
and the mixture was refluxed for 6 h. The AgCl precipitate was filtered
off, 100 mL of ether was added, and the solution was washed with
H2O, dried over MgSO4, and evaporated. The resulting solid was
chromatographed on silica gel (3% EtOAc/hexane) to give 1g (182
mg, 0.94 mmol, 47%) as a yellow solid [mp 57.5-58.0 °C, Rf 0.33
(10% EtOAc/hexane)] and 2-chloro-3,4-di-tert-butyl-4-(trifluoroac-
etoxy)cyclobutenone (5) (150 mg, 0.46 mmol, 23%) [Rf 0.44 (10%
EtOAc/hexane)]. 1g: 1H NMR (CDCl3) 1.44 (s, 18); 13C NMR (CDCl3)
δ 28.9, 35.4, 198.9, 206.4; IR (CDCl3) 1770 (vs), 1576 (s) cm-1; UV
isooctane
max
λ
217 (ꢀ ) 18 700), 353 (ꢀ ) 25), 370 (ꢀ ) 26) nm; EIMS m/z
194 (45, M+), 138 (10), 123 (100), 95 (26), 57 (66); HRMS m/z calcd
for C12H18O2 194.1307, found 194.1305. 5: 1H NMR (CDCl3) δ 1.17
(s, 9), 1.37 (s, 9); 13C NMR (CDCl3) δ 26.5, 27.9, 35.4, 36.6, 105.2,
(19) K. Sung and M. McAllister, unpublished results.
(20) Capozzi, G.; Romeo, G.; Marcuzzi, F. J. Chem. Soc., Chem.
Commun. 1982, 959-960.

12130 J. Am. Chem. Soc., Vol. 119, No. 50, 1997
Allen et al.
114.0 (q, J ) 285 Hz), 133.8, 155.8 (q, J ) 42.8 Hz), 182.1, 183.2; IR
(CDCl3) 1798 (vs), 1576 (s) cm-1; EIMS m/z 326 (3), 270 (18), 156
(10), 84 (18), 57 (100); HRMS m/z calcd for C14H18ClF3O3 326.0897,
found 326.0912.
H2O, acetone, and ether, and dried to give 6.0 g of silver squarate. The
dried salt (5.0 g, 0.015 mol) was stirred for 18 h with t-BuCl (30 mL)
in 50 mL of dry ether at 25 °C. The precipitated AgCl was removed
by filtration through anhydrous K2CO3. The solvent was evaporated,
and the solid product was chromatographed on silica gel (5% EtOAc
meso-2,3-Di-tert-butylsuccinyl Chloride (7). To a suspension of
meso-613 (0.1 g, 0.44 mmol) in 20 mL of CH2Cl2 cooled in a dry ice
bath was added PCl5 (3 equiv), and the solution was warmed to room
temperature over 3 h. The solvent was evaporated, the residue
redissolved in 30 mL of ether, and the residual solid removed by
filtration. The ether solution was washed five times with H2O (10 mL),
dried over MgSO4, and evaporated. The residue was dissolved in
hexane, and the mixture was filtered and evaporated to give 7 (93 mg,
35 mmol, 80%): 1H NMR (CDCl3) δ 1.18 (s, 18), 3.27 (s, 2); 13C NMR
(CDCl3) δ 29.1, 36.1, 67.2, 174.5; IR (CDCl3) 1801 cm-1; EIMS m/z
251 (2, M+ - CH3), 231 (6), 118 (10), 111 (31), 57 (100); HRMS m/z
calcd for C11H17Cl2O2 251.0606, found 251.0600.
A solution of 7 (80 mg, 0.30 mmol) in 6 mL of THF with Et3N (61
mg, 0.6 mmol) was heated in a sealed vial for 16 h at 120 °C. The
precipitate was removed by filtration and the solvent evaporated.
Chromatography on silica gel with elution by 5% EtOAc in hexane
gave 1g (41 mg, 0.21 mmol, 70%).
in hexane) to give 1k14a,c (2.0 g, 8.8 mmol, 59%) as colorless crystals:
hexane
max
1H NMR (CDCl3) δ 1.61, UV λ
250, 339 nm.
Di-tert-butoxycyclopropenone (11).14b A solution of 1k (100 mg,
0.44 mmol) in 50 mL of ether in a quartz tube was photolyzed for 18
h with 254 nm light. The 1H NMR spectrum indicated the presence of
11 (92%) with 2% residual 1k. Chromatography on silica gel (5%
EtOAc in hexane) gave 1114b (22.0 mg, 0.11 mmol, 25%): 1H NMR
(CDCl3) δ 1.47; 13C NMR (CDCl3) δ 28.5, 84.3, 127.2, 137.2; UV
λpentane 230 nm (sh, ꢀ ) 1200), 290 nm, (sh, ꢀ ) 100).
Kinetics. Photolyses of 1c-g,l-o were carried out using a
conventional flash photolysis apparatus, and the change in UV
absorption with time at a wavelength of maximum change was
monitored with a Cary 220 instrument. Photolysis of 1h-k was done
using conventional flash photolysis with time-resolved UV detection.
Several separate solutions were prepared for each compound, and
multiple kinetic runs were carried out for each solution.
For kinetic determinations on 2a,b 10-2 M solutions of 1a (CH3-
CN) or 1b (in pentane) were irradiated for 5 min at -80 °C with 350
nm light. Aliquots (3-10 µL) of these solutions were added to 1.2
mL of solvent in the UV cell, and the change in absorbance with time
was monitored [for 2a increase at 280 (isooctane) or 287 nm (CDCl3)
and decrease at 240 (isooctane) or 260 nm (CDCl3); for 2b decrease at
240 and increase at 320 nm].
Laser flash photolyses with time-resolved IR detection were carried
out using a 308 nm Lumonics Excimer-500 laser and a Mutek MPS-
1000 diode laser as a monitoring source. Details of the system are
provided elsewhere.5a
A solution of 7 (10 mg, 0.04 mmol) in 6 mL of THF with DMAP
(10 mg, 0.08 mmol) was stirred for 3 h at room temperature. The
solvent was evaporated, and the residue was dissolved in CDCl3. The
1
presence of bisketene 2g was indicated by the H NMR absorption at
δ 1.17 and by the IR absorption at 2101 cm-1, and the presence of 8
1
was indicated by the H NMR absorptions at δ 1.05 and 2.60.
Dimethyl d,l-2,3-Di-tert-butylsuccinate (dl-10).13b Dione 1g (52.0
mg, 0.268 mmol) in 0.5 mL of CDCl3 and 50 µL of MeOH was
irradiated for 12 h at 6 °C with 350 nm light. The solvent was
evaporated, leaving d,l-10 (69.2 mg, 0.268 mmol, 100%) as the sole
product: 1H NMR (CDCl3) δ 1.02 (s, 18), 2.50 (s, 2), 3.63 (s, 6); 13C
NMR (CDCl3) δ 28.0, 34.3, 51.1, 54.0, 174.2; IR (CDCl3) 1733 (vs),
1721 (vs) cm-1; EIMS m/z 227 (M+ - MeO, 14), 145 (90), 131 (58),
113 (65), 57 (t-Bu+, 100). The stereochemistry was confirmed by the
Acknowledgment. This article is dedicated to the memory
of Professor Paul D. Bartlett (1907-1997) in recognition of
his contributions to the study of organic reaction mechanisms.
Financial support by the Natural Sciences and Engineering
Research Council of Canada is gratefully acknowledged. We
thank Professor A. J. Kresge for the use of the flash photolysis
apparatus used for the rate measurements using UV detection.
13
1
13
C satellites of the H signal at δ 2.50: J C-H ) 130.4 Hz, JH,H ) 1.2
Hz.
Photolysis of 1g (28.0 mg, 0.144 mmol) in 0.5 mL of CDCl3
containing 20 µL of H2O with 350 nm light for 20 h at 5 °C followed
by evaporation of the solvent and recrystallization from THF/hexane
gave the anhydride (Z)-83a (24.2 mg, 0.114 mmol, 79%): 1H NMR
(CDCl3) δ 1.05 (s, 18), 2.60 (s, 2).
Supporting Information Available: 1H NMR spectra and
Figures 3 and 4 (9 pages). See any current masthead page for
ordering and Internet access instructions.
Di-tert-butyl Squarate (1k).14a,c Squaric acid (2.73 g, 0.024 mol)
in 125 mL of water was neutralized with 1 N NaOH, then AgNO3 (8.2
g, 0.048 mol) in 100 mL of H2O was added, and the mixture was stirred
for 10 min. The light yellow precipitate was filtered, washed with
JA9722685