Reactions of carbonyl compounds in basic solutions. Part 25. 1 the mechanism of the base-catalysed ring fission of 3,4-diphenylcyclobut-3-ene-1,2-diones

Article abstract of DOI:10.1039/a606412d

The rate coefficients for the base-catalysed ring fission of a series of substituted 3,4-diphenylcyclobut-3-ene-1,2-diones to give the corresponding (Z)-2-oxo-3,4-diphenylbut-3-enoic acids have been determined in 50% (v/v) aqueous dimethyl sulfoxide (DMSO

Full text of DOI:10.1039/a606412d

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1
Reactions of carbonyl compounds in basic solutions. Part 25. The
mechanism of the base-catalysed ring fission of 3,4-diphenylcyclo-
but-3-ene-1,2-diones
Abdulaziz Al-Najjar, Keith Bowden* and (in part) M. Vahid Horri
Department of Biological and Chemical Sciences, Central Campus, University of Essex,
Wivenhoe Park, Colchester, Essex, UK CO4 3SQ
The rate coefficients for the base-catalysed ring fission of a series of substituted 3,4-diphenylcyclobut-3-
ene-1,2-diones to give the corresponding (Z )-2-oxo-3,4-diphenylbut-3-enoic acids have been determined in
5
(
0% (v/v) aqueous dimethyl sulfoxide (D M SO) at 25.0 and 45.0 ؇C, as well as for a limited series in 70%
v/v) aqueous D M SO. The activation parameters have been calculated. The effect of both mono- and
di-substitution of the phenyl groups has been studied and gives a H ammett ñ value of ca. 1.3 in 50%
1
8
aqueous D M SO at 25.0 ؇C. The kinetic solvent isotope and solvent effect and enrichment in O-enriched
water have been studied. The product composition of the reaction of the monosubstituted compounds
gave a ñ value of ca. 0.9. All the evidence indicates a mechanistic pathway which proceeds by a rapid,
reversible addition of hydroxide anion to the dione, followed by a benzilic acid-type rearrangement to
form a 1-hydroxycyclopropene-1-carboxylic acid. The latter then suffers ring fission to give the final product.
The base-catalysed ring fission of cyclobutenediones has been
treated with sodium methoxide in methanol or a catalytic
2
–4
studied by several groups. However, no definitive reaction
amount of 1,5-diazabicyclo[4.3.0]non-5-ene in chloroform or
benzene, a quantitative reverse benzilic acid-type rearrange-
ment gives 1,2-diphenylcyclobutene-3,4-dione. The latter ring
did not undergo ring cleavage under these mild conditions. This
result gives support to the pathway proposed by Skattebøl and
2
mechanism has been established. Thus, Skattebøl and Roberts
suggested that the dione adds hydroxide anion to form the
anion of the mono-hydrate, which collapses to give a 1-hydroxy-
cyclopropene-1-carboxylic acid anion. The latter suffers ring
fission to give a 2-oxobut-3-enoic acid as the immediate reac-
2
Roberts.
3
tion product. However, Blomquist and LaLancette indicated a
We describe here the base-catalysed ring fission of a series
of substituted 3,4-diphenylcyclobut-3-ene-1,2-diones 1. The
products and their distribution, the effect of substituents on
the rates, activation parameters, solvent and solvent kinetic
pathway involving addition of water to the activated ethylenic
bond, catalysed by hydroxide anion, followed by addition of
hydroxide anion to carbonyl group β to the hydroxy substitu-
ent. Thus, these cyclobutenediones react by bond fission of the
carbonyl carbon–olefinic carbon. On the contrary, the base-
catalysed ring fission of benzocyclobutenediones results in the
formation of 2-formylbenzoic acids, which involves fission of
the carbonyl carbon–carbonyl carbon bond in the cyclobutene-
1
8
effects and hydrolysis in O-enriched water have been studied.
The detailed mechanistic pathway has been elucidated and is
discussed in relation to related reactions.
Results
1
dione ring. However, a benzilic acid-type of rearrangement
has been demonstrated in the base-catalysed reactions of cyclo-
butanediones to form 1-hydroxycyclopropane-1-carboxylic
acids. An important study by DeBoer has shown that, when
ethyl 3-hydroxy-1,2-diphenylcyclopropene-3-carboxylate† was
The base-catalysed ring fission of the substituted 3,4-diphenyl-
cyclobut-3-ene-1,2-diones 1 gives the corresponding (Z )-2-oxo-
3,4-diphenylbut-3-enoates 2 in quantitative yield. The reaction
5
6
†
More correct name: ethyl 1-hydroxy-3,4-diphenylcyclopropene-1-
carboxylate. IUPAC preferred names have been used throughout this
paper.
Table 1 Rate coefficients (k ) and product ratios (Ratio) for the base-catalysed ring fission of substituted 3,4-diphenylcyclobut-3-ene-1,2-diones in
2
Ϫ3
a
aqueous DMSO (µ = 0.1 mol dm
)
3
Ϫ1 Ϫ1
k /dm mol
s
2
in 50% (v/v)
aqueous DMSO
in 70% (v/v)
aqueous DMSO
b
Substituent(s)
c
X
Y
at 25.0 ЊC
at 45.0 ЊC
at 25.0 ЊC
at 45.0 ЊC
Ratio
λ/nm
H
H
H
H
H
70.7
40.6
22.0
123
117
74.0
40.6
190
211
47.9
22.4
339
373
311
178
88.8
363
229
129
1.0
269
274
282
274
276
280
306
280
283
CH3
OCH3
Cl
0.75 (±0.06)
0.67 (±0.06)
1.5 (±0.15)
1.6 (±0.1 )
Br
H
128
5
5
CH3
OCH3
Cl
CH3
OCH3
Cl
24.7
11.0
212
106
36.9
152
61.0
Br
Br
235
a
b
Rate coefficients were reproducible to ±3%. Substituent X is at the 4-position of one phenyl ring (shown as Ar in diagrams) and substituent Y at the
c
4
-position of the other. Wavelength used to monitor the reaction.
J. Chem. Soc., Perkin Trans. 2, 1997
993

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Table 2 Activation parameters for the base-catalysed ring fission of substituted 3,4-diphenylcyclobut-3-ene-1,2-diones in aqueous DMSO (µ = 0.1
Ϫ3
a
mol dm ) at 30 ЊC
b
Substituent(s)
in 50% (v/v) aqueous DMSO
in 70% (v/v) aqueous DMSO
‡
Ϫ1
‡
Ϫ1
Ϫ1
‡
Ϫ1
‡
Ϫ1
Ϫ1
X
Y
∆H /kcal mol
∆S /cal mol
K
∆H /kcal mol
∆S /cal mol
K
c
c
H
H
H
H
H
4.1 (8.2)
Ϫ36 (Ϫ26)
0.85
1.75
2.9
Ϫ44
Ϫ42
Ϫ40
5
CH3
OCH3
Cl
5.1
5.2
3.5
4.1
5.6
6.1
3.8
3.75
Ϫ34
Ϫ35
Ϫ37
Ϫ35
Ϫ33
Ϫ33
Ϫ35
Ϫ35
Br
H
CH3
OCH3
Cl
CH3
OCH3
Cl
2.8
4.15
Ϫ40
Ϫ37
Br
Br
a
‡
‡
Ϫ1
Ϫ1
Ϫ1
b
Values of ∆H and ∆S are considered accurate to within ±300 cal mol and ±2 cal mol
K , respectively (1 cal = 4.184 J). See footnote b in
c
Table 1. In water containing 0.33% (v/v) DMSO.
1
8
Table 3 Rate coefficients (k ) for the base-catalysed ring fission of 3,4-
When the base-catalysed ring fission was studied in O-
enriched water, the product of ring fission 3 showed three-
enrichments; whereas the exchange of the latter acid itself
studied under identical base-catalysed conditions gave only
one-enrichment.
2
diphenylcyclobut-3-ene-1,2-dione in aqueous DMSO and dioxane
Ϫ3
a
(
µ = 0.1 mol dm ) at 25.0 ЊC
3
Ϫ1 Ϫ1
k /dm mol
s
2
Volume (%)
non-aqueous solvent
DMSO
Dioxane
b
c
d
Discussion
0
15.1 (18.3) 15.1 (72.0)
3
5
6
7
0
0
0
0
28.7
70.7
115
311
11.9
8.64
8.13
7.60
The likely mechanism for the base-catalysed ring fission is
shown in Scheme 2. The tetrahedral adduct 6 is initially formed
Ar
Ar
O
Ar
Ar
k ′1
a,b
c
d
See Table 1. In deuterium oxide containing 0.33% (v/v) DMSO. At
OH –
+
6
0.0 ЊC.
k ′–1
OH
O
O
–
O
6
is of first order in both the diones and hydroxide anion. Rate
coefficients for the base-catalysed ring fission of the substituted
,4-diphenylcyclobut-3-ene-1,2-diones 1 in 50% (v/v) aqueous
1
k ′–2 k ′2
3
dimethyl sulfoxide (DMSO) at 25.0 and 45.0 ЊC are shown in
Table 1, as well as those for the five least reactive substrates in
Ar
Ar
7
0% (v/v) aqueous DMSO using a UV–VIS spectrophotometric
method. The activation parameters for the reactions of these
compounds are shown in Table 2. The effects of increasing con-
centrations of DMSO and dioxane on the rate of ring fission of
the parent dione are shown in Table 3, as well as the rate co-
efficients in both water and deuterium oxide. The rates of reac-
–
O
CO2H
7
k ′3
H
Ar
Ar
fast
tion are relatively fast and k for parent dione at 25 ЊC in water
2
3
Ϫ1 Ϫ1
is 15.1 dm mol s , but the rate is only ca. 0.05 of that of the
CO CO2–
Ar
Ar
CO CO2H
ring fission of benzocyclobutene-1,2-dione under the same
conditions. The isolated products of ring fission are the substi-
tuted (Z)-2-oxo-3,4-diphenylbut-3-enoic acids 3. For the
1
2
8
Scheme 2
in a relatively fast pre-equilibrium. This is confirmed by the
1
8
study of the hydrolysis in O-enriched water which indicates
rapid, reversible hydration with equilibration of the adduct.
Similar behaviour is shown in the base-catalysed ring fission of
1
unsubstituted and symmetrically disubstituted diones, only one
product was isolated, as expected. However, for the monosub-
stituted diones 4, the product can be either the (Z)-2-oxo-3-
benzocyclobutene-1,2-dione and for benzil in the benzilic acid
7
2
D O
rearrangement. The solvent kinetic isotope effect, k2
k2 , was found to be 1.2 at 25 ЊC, as shown in Table 3. This is
/
2
H O
1
(
substituted phenyl)-4-phenylbut-3-enoic acid 5a or the (Z)-2-
also in accord with the formation of the adduct in a rapid pre-
equilibrium, as also found for the base-catalysed ring fission of
benzocyclobutene-1,2-dione and benzilic acid rearrangement.
oxo-3-phenyl-4-(substituted phenyl)but-3-enoic acid 5b, as
shown in Scheme 1. The ratio of 4- to 3-(substituted phenyl)
products are given in Table 1 as Ratio.
1
8
The adduct 6 then collapses to form the anion of 1-hydroxy-3,4-
diphenylcyclopropene-1-carboxylic acid 7 in a benzilic acid-
9
type rearrangement. Unlike the ethyl ester of the latter
6
under other conditions, in excess aqueous base, the anion (or
dianion) 7 then undergoes ring fission. This could possibly be
via the carbanion 8. However, this fission step could appear
likely to be concerted with proton transfer to the backside of an
orbital that is involved in the carbon–carbon bond that is
1
0
undergoing cleavage. Thus this reaction involves inversion of
configuration at the alkene ring carbon 2. Recent theoretical
treatments of benzilic acid and related rearrangements have
1
1
Scheme 1
indicated alternative paths with similar energetics.
9
94
J. Chem. Soc., Perkin Trans. 2, 1997

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Table 4 Hammett reaction constants (ρ) for the base-catalysed ring
relation of log Ratio with σ is very successful, as shown in Table
p
fission of the substituted 3,4-diphenylcyclobutene-1,2-diones in 30%
4
. The ρ value found equals 0.87 and compares closely with the
Ϫ3
a
aqueous DMSO (µ = 0.1 mol dm ) at 25 ЊC
21
result found by Bird and Harmer for the base-catalysed ring
fission of mono-substituted 2,3-diphenylcyclopropenones in
methanol at room temperature with ρ equal to 0.75. Thus, ρa
can be estimated for the present study as ca. 2.6. This value
suggests that the carbanionic character of the alkene carbon
undergoing fission is partially developed, cf. ρ for the detri-
tiation of benzyl cyanides in methanol catalysed by methoxide
ρ
log ko
r
s
n
Mono-substituents with σp
Di-substituents with 2σp
Mono- and di-substituents with
σp and 2σp
1.414 1.794 0.986 0.139
1.269 1.786 0.996 0.066
1.298 1.783 0.994 0.056
5
5
9
Log ratio with σp
0.871 0.035 0.987 0.081
5
22
at 25 ЊC equals 3.9. The effects of substituents cannot be
3
a
accommodated by the alternative pathway, which involves add-
r is the correlation coefficient, s the standard deviation and n the
number of substituents.
ition to the ketovinyl double-bond and fission of a cyclobutane-
dione ring.
Activation parameters
The activation parameters for the base-catalysed ring fission of
Solvent effects
1
are shown in Table 2. They clearly indicate a bimolecular
The effects of solvent on the rates of base-catalysed ring fission
of the parent dione are shown in Table 3. The present results
can be compared with the effects of solvent composition on the
pathway. In general, electron-withdrawing groups decrease and
‡
‡
electron-releasing increase ∆H . The values of ∆S do not
show any significant variation in 50% aqueous DMSO. How-
ever, as the solvent is changed from water to have an increased
DMSO content, ∆H values decrease markedly; while ∆S
values become somewhat more negative, as has been noted for
2
0
benzilic acid rearrangement, the alkaline hydrolysis of methyl
1
2,13
benzoate
and base-catalysed ring fission of benzocyclo-
‡
‡
1
butene-1,2-dione. The rates of reaction increase markedly with
increasing DMSO content which is considered to arise mainly
from the increased activity of hydroxide. However, the effect of
increasing dioxane content on the rates of the base-catalysed
reactions has been found to be comparatively minor, but
appears to be discriminating with both increases and decreases
1
2
the alkaline hydrolysis of alkyl benzoates (see also later).
Kinetic substituent effects
The Hammett equation (1) is usually employed in the quanti-
2
3
in rates observed. The behaviour observed for the ring fission
of the dione studied here is similar to that of the alkaline
log(k/k ) = ρσ
(1)
o
2
3
tative assessment of metal/para-substituent effects in aromatic
systems. The rates of the mono- and di-substituted diones have
hydrolysis of methyl benzoate and very similar to that of the
1
3
1
base-catalysed ring fission of benzocyclobutene-1,2-dione.
been very successfully correlated separately with σ and 2σ ,
This result favours the formation of charge-localised inter-
p
p
respectively, as shown in Table 4. The two ρ values are, within
the uncertainties, identical and the two series can be combined
to give a ρ value of ca. 1.3, using σ and 2σ for the mono- and
mediates like 6 and 7.
Mechanistic pathway
p
p
The conclusions indicate that the reaction proceeds by the
pathway shown in Scheme 2. The rate-determining step appears
to be the formation of the cyclopropene intermediate 7; where-
as the product-determining step is the breakdown of the latter.
di-substituted series, respectively. Treatments attempting allow-
ance for statistical factors, i.e. two identical carbonyl groups in
the disubstituted series, do not give improved correlations.
There are two important conclusions that can be made from
this study of substituent effects. First, the equality of the effect
of the first and second identical substitution clearly indicates
that the transition state must have a structure in which both
substituted phenyl groups are equivalent, e.g. close to 7. Sec-
ondly, the observed ρ value of 1.3 in 30% aqueous DMSO at
3
The evidence clearly excludes the alternative addition pathway.
In this study, the formation of a very strained cyclopropene
from a strained cyclobutene is apparently favoured by the
stereochemistry of the attack by the migrating vinyl group. This
is in stark contrast to the benzocyclobutene system in which
strain apparently becomes so excessive that the fission type
switches from carbonyl carbon–carbonyl carbon to carbonyl
2
5 ЊC can be compared to those for the equilibrium addition
of hydroxide anions to benzaldehydes in water at 25 ЊC (2.2
1
1
4
carbon–aryl carbon.
or 2.8) and the alkaline hydrolysis of ethyl benzoates in
1
5
water and 65% aqueous DMSO at 25 ЊC (1.33 and 2.40).
These comparisons would indicate that the negative charge
developed in the transition state for the present system
Experimental
Materials
A direct route
1
6
appears to reside one further ‘carbon atom’ away, cf. the
24–26
was employed for the synthesis of the 3,4-
1
7
transmission coefficient for Csp
2
which equals 0.67. This
result would also indicate a transition state structure close to
in structure.
diphenylcyclobutene-1,2-diones. 3,4-Dichlorocyclobutene-1,2-
24,25
dione (squaryl dichloride)
was reacted with excess of
7
benzene or a substituted benzene, in the presence of 3 equiv. of
aluminium chloride and under a blanket of dry nitrogen to give
the symmetrically di-(4-substituted phenyl) compounds. Use
Product substituent effects
26
For the mono-substituted diones 4, there are two possible
products, as shown in Scheme 1, i.e. the 4-(substituted phenyl)
or 3-(substituted phenyl) carboxylates, 5a and 5b, respectively.
of benzene and 1 equiv. of aluminium chloride gave 3-chloro-4-
24
phenylcyclobutene-1,2-dione. Reaction of the latter with a
substituted benzene as described above gave the 3-phenyl-4-
substituted phenyl)cyclobutene-1,2-diones. The purity of these
compounds was monitored by IR, mass and H and C NMR
spectroscopy, as well as by elemental analysis. Their mps, after
1
8
Using the Wegscheider’s principle for concurrent reactions of
(
1
9
the same order, Exner has related the amount of both prod-
1 13
ucts to the rate coefficients, k and k , and hence to the reaction
a
b
constants, ρ and ρ , for the relevant process in Scheme 1, as
a
b
repeated recrystallisation and drying under reduced pressure
shown in eqn. (2), cf. ref. 20. As shown in Scheme 2, the product
log(x /x ) = (ρ Ϫ ρ )σ (2)
determining step is kЈ . The mono-substituent will affect both
26
(
P O ), were in agreement with lit. values or are reported in
2
5
Table 5.
Inorganic salts were of analytical grade and were used with-
out further purification. The solvents for the kinetic studies
a
b
a
b
3
2
7
reaction constants, but the effect on ρ is transmitted via a fur-
b
were purified as described previously.
ther ‘carbon atom’. Using the transmission coefficient for C
2
sp
1
7
equal to 0.67, ρ can be estimated to be 0.67 ρ . Thus, the
Measurements
b
a
observed ρ for the product ratio, Ratio, equals 0.33 ρ . The cor-
Rate coefficients for the base-catalysed ring fission of the sub-
a
J. Chem. Soc., Perkin Trans. 2, 1997
995

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a
Table 5 The physical constants of the previously unreported 3,4-di(4-substituted phenyl)cyclobutene-1,2-diones
Found (%)
Calc. (%)
C
Substituent
Mp/ЊC
C
H
Other
Formula
C H O
H
Other
CH3
OCH3
Cl
173–176
184–185
143–145
167–171
88–90
81.9
72.5
62.6
48.9
76.0
5.2
4.6
3.0
2.1
4.7
82.45
73.45
63.35
49.0
5.35
4.75
2.85
2.05
4.8
18
14
2
4
C H O
18
14
22.4 (Cl)
40.0 (Br)
C H Cl O
23.4 (Cl)
40.0 (Br)
]
1
6
8
2
2
2
Br
C H Br O
1
6
8
2
3
b
[
(Z)-2-Oxo-3,4-diphenylbut-3-enoic acid
C H O
76.2
1
6
12
a
b
As yellow needles, recrystallised from hexane–chloroform. As colourless needles, recrystallised from dichloromethane–light petroleum (bp 40–
6
0 ЊC).
stituted 3,4-diphenylcyclobut-3-ene-1,2-diones were deter-
mined spectrophotometrically by use of a Perkin-Elmer lambda
using the acid product itself as substrate. The mass spectrum of
the samples and control (using dione and ordinary water) was
recorded on an AEI MS12 spectrometer. The enrichment was
5
UV–VIS spectrometer. A Haake thermostatted water circu-
lating bath was used to control the temperature of the cell to
0.05 ЊC. The reactions were followed at the wavelengths shown
ϩ
ϩ
calculated from the peak areas due to M and M ϩ 2.
±
2
7
in Table 1. The procedure was that described previously. Buf-
fers were prepared from AnalaR grade chemicals and distilled
Acknowledgements
One of us (A. Al-N.) thanks the State of Kuwait for the award
of a research studentship. We thank Dr A. G. Osborne for his
kind and detailed help in the interpretation of NMR spectra.
water (freshly boiled and cooled under N bubbling). The ionic
2
Ϫ3
strength was maintained at 0.1 mol dm with sodium chloride.
The pH of the buffer solutions was measured with a Pye-
Unicam model PW9409 direct reading pH meter with glass
combination electrode, calibrated with standard buffers at
References
2
5 ЊC. Buffer solution pH was measured using a thermostatted
1
Part 24. K. Bowden and M. V. Horri, J. Chem. Soc., Perkin Trans. 2,
997, p. 989.
glass vessel at the reaction temperatures. The pD values were
1
2
8
obtained by adding 0.40 to the pH meter readings. Both phos-
phate and borate buffers were used and no buffer catalysis was
2 L. Skattebøl and J. D. Roberts, J. Am. Chem. Soc., 1958, 80, 4085.
3 A. T. Blomquist and E. A. LaLancette, J. Am. Chem. Soc., 1962, 84,
220.
Ϫ3
observed. Buffer concentrations of up to 0.1 mol dm and
4
5
H. Knorr and W. Ried, Synthesis, 1978, 649.
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aqueous DMSO to hydroxide concentrations was confirmed by
studying the kinetics of the parent dione in aqueous DMSO
containing hydroxide. The pHЈ values in aqueous DMSO were
found linear with pOH. The aqueous DMSO solvent was
required due to low solubility of certain diones in aqueous
solutions.
1
968, 7, 215; J. J. Bloomfield, J. R. S. Irelan and A. P. Marchand,
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9
J. Hine and H. W. Haworth, J. Am. Chem. Soc., 1958, 80, 2274.
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0 C. H. DePuy, F. W. Breitbeil and K. R. DeBruin, J. Am. Chem. Soc.,
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1
Product studies
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to be 2. For the parent dione, the product was isolated in quan-
titative yield and was confirmed spectrophotometrically by
comparison of the spectrum of the acid in base with that of the
reaction product. The mp and elemental analysis of the product
1
1 I. Rajyaguru and H. S. Rzepa, J. Chem. Soc., Perkin Trans. 2, 1987,
1
819.
1
1
2 D. D. Roberts, J. Org. Chem., 1966, 31, 4037.
3 C. D. Johnson, The Hammett Equation, Cambridge University Press,
Cambridge, 1973.
1
13
is given in Table 5. H and C NMR, IR and mass spectra
14 P. Greenzaid, J. Org. Chem., 1973, 38, 3164; W. J. Bover and
P. Zuman, J. Chem. Soc., Perkin Trans. 2, 1973, 786.
3
confirmed the structure. In an earlier study, the structure of the
1
1
1
1
1
5 M. Hojo, M. Utaka and Z. Yoshida, Tetrahedron Lett., 1966, 19, 25.
6 K. Bowden, Org. React. (Tartu), 1995, 29, 19.
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9 O. Exner, Collect. Czech., Chem. Commun., 1961, 30, 593.
product was inferred from isolated products, which had arisen
from base-catalysed decomposition of the immediate product.
In general, the products of the kinetic runs after ten ‘half-lives’
were isolated by acidification and extraction with diethyl ether.
The extract was either evaporated or treated with diazomethane
in diethyl ether. The products were then isolated and examined
20 K. Bowden and K. D. Williams, J. Chem. Soc., Perkin Trans. 2, 1994,
77.
1
13
21 C. W. Bird and A. F. Harmer, J. Chem. Soc. (C), 1969, 959.
by H and C NMR, IR and mass spectral studies. For the
mono-substituted diones, the product is a mixture of two iso-
mers, i.e. as shown in Scheme 2. The relative compositions were
2
2 K. Bowden, N. S. Nadvi and R. J. Ranson, J. Chem. Res., 1990,
S)299, (M)2474.
(
2
2
3 K. Bowden and M. J. Price, J. Chem. Soc. (B) , 1971, 1748.
4 R. C. De Selms, C. J. Fox and R. C. Riordan, Tetrahedron Lett.,
1970, 781.
1
obtained by integrations of H NMR spectra of the isolated
product using a Jeol EX270, 270 MHz multinuclear FT instru-
ment and the values of ratio are shown in Table 1, together with
25 G. Maahs and P. Hegenberg, Angew. Chem., Int. Ed. Engl., 1966, 5,
1
888.
their uncertainties. The H NMR spectra show a vinyl proton(s)
2
2
6 W. Reid and D. P. Schafer, Chem. Ber., 1963, 102, 4193; A. H.
Schmidt and W. Ried, Synthesis, 1978, 1.
7 K. Bowden and A. M. Last, J. Chem. Soc., Perkin Trans. 2, 1973,
at 6.5 to 5.5 ppm in deuteriated DMSO. These were used to
2
9
establish the stereochemistry and estimate the composition.
3
45.
1
8
Hydrolysis using O-enriched water
28 L. Pentz and E. R. Thornton, J. Am. Chem. Soc., 1967, 89, 6931;
P. K. Glasoe and F. A. Long, J. Phys. Chem., 1960, 64, 188; B. Zerner
and M. L. Bender, J. Am. Chem. Soc., 1961, 83, 2267.
The base-catalysed fission of the parent dione was studied by
1
8
use of O-enriched water (4.5 atom%). The reaction was stud-
ied under kinetic conditions of excess base for approximately
ten ‘half-lives’ of the dione. After neutralisation and extraction
with diethyl ether, the extracts were treated with diazomethane
in diethyl ether. The methyl ester of (Z)-2-oxo-3,4-diphenylbut-
2
9 C. Pascual, J. Meier and W. Simon, Helv. Chim. Acta, 1966, 49, 164;
S. W. Tobey, J. Org. Chem., 1969, 34, 1281.
Paper 6/06412D
Received 17th September 1996
Accepted 23rd December 1996
3
-enoic acid was isolated. The same procedure was followed
9
96
J. Chem. Soc., Perkin Trans. 2, 1997