Rearrangement of substituted (E)-5,5′-diphenylbifuranylidenediones to 3,7-diphenylpyrano[4,3-c]pyran-1,5-diones

Article abstract of DOI:10.1002/(SICI)1099-1395(199704)10:4<191::AID-POC888>3.0.CO;2-5

The rate coefficients for the rearrangement of substituted (E)-5,5′-diphenylbifuranylidenediones to form the corresponding 3,7-diphenylpyrano[4,3-c]pyran-1,5-diones in ethane-1,2-diol at 30.0 °C, catalysed by sodium acetate, were determined. For the unsubstituted dione, the rate coefficients for there arrangement in various alcohols at 30.0 °C, and for three at 59.8 °C, were measured. The reactions were first order in both substrate and acetate anion. Rate coefficients were also measured for the catalysis of the rearrangement of the unsubstituted dione in ethanol and ethane-1,2-diol at 30.0 °C by a series of sodium meta/para-substituted benzoates. Bronsted-type correlations for the latter give α≈1.2. Substrate substitutent effects on the rates of rearrangement of the diones were not marked. A good correlation was found between the rates of rearrangement of the unsubstituted dione and the solvent effect parameter E _T(30). This and other evidence indicated a probable mechanism involving a rapid pre-equilibrium between the substrate diones and the carboxylate anion, followed by the formation of an s-trans-enolate anion. The latter rotates to the s-cis isomer, which then intramolecularly attacks the second lactone ring. The process is then repeated with the product dione resulting from loss of the carboxylate anion.

Full text of DOI:10.1002/(SICI)1099-1395(199704)10:4<191::AID-POC888>3.0.CO;2-5

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 191–195 (1997)
REARRANGEMENT OF SUBSTITUTED
(E)-5,5Ј-DIPHENYLBIFURANYLIDENEDIONES TO
3,7-DIPHENYLPYRANO[4,3-c]PYRAN-1,5-DIONES
KEITH BOWDEN* AND RICHARD J. RANSON
Department of Biological and Chemical Sciences, Central Campus, University of Essex, Wivenhoe Park, Colchester, Essex CO4 3SQ, UK
The rate coefficients for the rearrangement of substituted (E)-5,5Ј-diphenylbifuranylidenediones to form the
corresponding 3,7-diphenylpyrano[4,3-c]pyran-1,5-diones in ethane-1,2-diol at 30.0 °C, catalysed by sodium acetate,
were determined. For the unsubstituted dione, the rate coefficients for there arrangement in various alcohols at 30.0 °C,
and for three at 59.8 °C, were measured. The reactions were first order in both substrate and acetate anion. Rate
coefficients were also measured for the catalysis of the rearrangement of the unsubstituted dione in ethanol and ethane-
1,2-diol at 30.0 °C by a series of sodium meta/para-substituted benzoates. Brønsted-type correlations for the latter give
␣≈1.2. Substrate substitutent effects on the rates of rearrangement of the diones were not marked. A good correlation
was found between the rates of rearrangement of the unsubstituted dione and the solvent effect parameter E T(30). This
and other evidence indicated a probable mechanism involving a rapid pre-equilibrium between the substrate diones and
the carboxylate anion, followed by the formation of an s-trans-enolate anion. The latter rotates to the s-cis isomer, which
then intramolecularly attacks the second lactone ring. The process is then repeated with the product dione resulting
from loss of the carboxylate anion. © 1997 by John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 10, 191–195 (1997) No. of Figures: 1 No. of Tables: 5 No. of References: 17
Keywords: substituted (E)-5,5Ј-diphenylbifuranylidenediones; 3,7-diphenylpyrano[4,3-c]pyran-1,5-diones; rearrangement; rate
constants
Received 27 June 1996; revised 9 December 1996; accepted 17 December 1996
INTRODUCTION
c]pyran-1,5-diones. The kinetics, catalysis, substituent
effects, solvent and solvent isotope effects and activation
parameters, were investigated in formulating a mechanistic
pathway.
In 1959, it was reported1 that the pigments, substituted (E)-
5,5Ј-diphenylbifuranylidenediones (1), described as
‘Pechmann dyes,’2,3 rearrange into 3,7-diphenylpyrano[4,3-
c]pyran-1,5-diones (2) by heating in alkanediols and
alkanetriols, such as ethane-1,2-diol, propane-1,2- or
-1,3-diol and butane-1,2,4-triol. Owing to the low solubility
of the pigments, the latter solvents were used in combina-
tion with a co-solvent, such as nitrobenzene or benzyl
alcohol. This transformation had previously been accom-
plished by alkaline hydrolysis of the five-membered
dilactones (1), using alcoholic potassium hydroxide, fol-
lowed by ring closure in an acidic medium.3 More recently,
Rahmani and Crombie4 studied the rearrangement of two
five-membered dilactones to form the corresponding six-
membered dilactones and a pathway involving methanolysis
was suggested.
EXPERIMENT AND RESULTS
Materials
The methods used for the synthesis of the substituted (E)-
5,5Ј-diphenylbifuranylidenediones and 3,7-diphenylpyrano-
[4,3-c]pyran-1,5-diones have been described previously.5
Solvents, catalysts and other materials were carefully
purified by standard procedures.6–8
Kinetic measurements and product analysis
The present investigation involved a study of the
rearrangement of substituted (E)-5,5Ј-diphenylpyrano[4,3-
Rate coefficients for the rearrangement of the (E)-5,5Ј-diph-
enylbifuranylidenediones were determined spectro-
photometrically by use of a Perkin-Elmer Lambda 5 UV–
visible spectrophotometer. The cell temperature was
* Correspondence to: K. Bowden.
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CCC 0894–3230/97/040191–05 $17.50

192
REARRANGEMENT OF BIFURANYLIDENEDIONES TO PYRANOPYRANDIONES
followed was similar to that described previously.9 The
substrates were studied at 0.5ϫ10Ϫ4–2ϫ10Ϫ4 mol dmϪ3
Table 1. Rate
coefficients
(k2)
for
the
rearrangement
of (E)-5,5Ј-diphenylbifuranylidenedione (1a) to 3,7-diphenylpyr-
.
ano[4,3-c]pyran-1,5-dione (2a) at 30.0 °Ca,b
The parent solution of the substrates were prepared in
dioxane and the kinetic solutions contained 1.7% (v/v)
dioxane. The products of the rearrangement of the sub-
strates studied here were confirmed both by isolation in
quantitative yield and, spectroscopically, by comparison of
the spectrum of the product with that of the isolated reaction
product in the solvent system under study. Only in alcoholic
solutions, containing a suitable catalyst, was the rearrange-
ment found to give reproducible kinetics and quantitative
results. However, if the alkoxide (as the sodium salt) was
present as a catalyst, the only process observed was the
formation of the diester of 2,3-dibenzoylfumaric acid.3 The
rearrangement itself in alcoholic solution was found to be
strongly catalysed by the salts of carboxylic acids and
pyridine (see Table 1), but not by n-butylamine and aniline.
The reaction was observed to be first order in substrate up to
80% of the reaction and was found to be strongly catalysed
by acetate anions. At or up to concentrations of acetate
anions of 0.5ϫ10Ϫ3 mol dmϪ3 in ethanol and ethane-
1,2-diol, the reaction was first order in acetate anion. Use of
lithium acetate or sodium acetate containing or not contain-
ing 18-crown-6 gave almost identical rates. At or up to
concentrations of benzoate and substituted benzoate anions
of 5ϫ10Ϫ3 mol dmϪ3, the reactions were first order in the
carboxylate anions for all substrates and solvents studied.
Furthermore, there is no significant fall in absorbance of the
substrates on initial mixing with the carboxylate anions,
which would have occurred if a relatively stable adduct has
been rapidly formed. Good isosbestic points were observed
for all the rearrangements studied. Tables 1–3 show the rate
coefficients for the rearrangements. The activation parame-
ters for the rearrangement of the parent compound at
30.0 °C in three alcohols are shown in Table 4.
k2 (dm3 molϪ1 sϪ1
)
Catalyst
In ethanol
In ethane-1,2-diol
Na acetate
Na benzoate
Na m-toluate
Na m-chlorobenzoate
Na m-nitrobenzoate
Na p-methoxybenzoate
Pyridine
0·534
0·426
0·320
0·0640
0·00822
0·714
–
2·84
2·00
1·59
0·450
0·0612
2·62
0.260
a Rate coefficients were reproducible to <±3%.
b The wavelengths used were 500 and 510 nm for ethanol and ethane-
1,2-diol, respectively.
controlled to within ±0.05 °C by means of a Churchill
thermocirculator. The reactions were followed at suitable
wavelengths, which were at or close to ␭
for the
max
substrates and are shown in Tables 1–3. The procedure then
Table 2. Rate coefficients (k2) for the rearrangement of substituted
(E)-5,5Ј-diphenylbifuranylidenediones (1) to 3,7-diphenylpyr-
ano[4,3-c]pyran-1,5-diones (2) in ethane-1,2-diol containing
sodium acetate at 30.0 °Ca
Substituent
k2 (dm3 molϪ1 sϪ1
)
␭ (nm)
H
2·84
2·20
2·48
3·16
1·92
2·90
2·90
2·98
510
520
530
520
540
520
530
535
p-OMe
p-OPh
p-Me
p-Ph
p-Cl
p-Br
m-NO2
DISCUSSION
a Rate coefficients were reproducible to <±3%.
A probable mechanistic pathway is shown in Scheme 1 for
the rearrangement. The first step is the relatively rapid pre-
equilibrium formation of a mixed anhydride, 3, from the
reactive five-membered dilactone and the carboxylate
anion. The effect of substituents in the meta/para-substi-
tuted benzoate anions can be correlated using the Hammett
equation:
Table 3. Rate coefficients (k2) for the rearrangement
of (E)-5,5Ј-diphenylbifuranylidenediones (1a) to 3,7-diphenylpyr-
ano[4,3-c]pyran-1,5-dione (2a) in various alcohols containing
sodium acetate at 30.0 °Ca
Alcohol
102k2 (dm3 molϪ1 sϪ1
)
␭ (nm)
Table 4. Activation
parameters
for
the
rearrangement
of (E)-5,5Ј-diphenylbifuranylidenediones (1a) to 3,7-diphenylpyr-
ano[4,3-c]pyran-1,5-dione (2a) in ethane-1,2-diol, 2-methoxy
ethanol and ethanol at 30.0 °C
Methanol
Benzyl alcohol
Ethanol
Ethane-1,2-diol
2-Methoxyethanol
Propan-1-ol
Propan-2-ol
278 (212)b
65·6
520
510
500
510
525
495
495
53·4 (39·0)b (692)c
284 (3460)c
83·2 (1400)c
38·8
a
a
Alcohol
⌬H‡ (kcal molϪ1
)
⌬S‡ (cal molϪ1 KϪ1
)
Ethane-1,2-diol
2-Methoxyethanol
Ethanol
16·2
18·4
16·6
Ϫ3
2
Ϫ5
9·78
a See Table 1.
b [O-2H1]Alcohol.
c At 59.8 °C.
a Values of ⌬H‡ and ⌬S‡ are considered accurate to ±300 cal molϪ1 and ±1
cal molϪ1 KϪ1, respectively.
© 1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 191–195 (1997)

K. BOWDEN AND R. J. RANSON
193
Scheme 1
© 1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 191–195 (1997)

194
REARRANGEMENT OF BIFURANYLIDENEDIONES TO PYRANOPYRANDIONES
Table 5. Hammett reaction constants (␳) for the rearrangement of
the bifuranylidenediones (1) to the pyrandiones (2) at 30.0 Ca
state for the reaction is at or after the formation of the
intermediate 3. The effect of the di-meta-/para-substitution
in the substrate can also be assessed using the Hammett
equation (1). The result of the correlation is shown in Table
5. The latter is very poor and is typical for Hammett
relations where ␳ is close to zero.13 In the second step, the
enolate anion is formed by fission of the first strained five-
membered lactone ring which will then suffer rotation about
the single bond from the s-trans (3) to the s-cis (4) isomer.
The next step is the intramolecular nucleophilic attack of the
s-cis-enolate anion on the second five-membered lactone
ring to form the second tetrahedral intermediate 5. This
process is exactly analogous to several neighbouring group
participation reactions.14
System
␳
Log k0
r
s
n
m/p-Substituted benzoates as
catalysts in ethane-1,2-diol Ϫ1·677
0·119 0·972 0·236 5
As above in ethanol
5,5Ј-Di (m/p-substituted
phenyl) diones in
Ϫ1·991 Ϫ0·565 0·984 0·208 5
ethane-1,2-diol catalysed by
acetate
0·149
0·415 0·554 0·091 8
a s is the standard deviation, r the correlation coefficient and n the number of
substituents.
The intermediate 5 should be unstable as it has a fused
six- and five-membered bicyclic ring structure, which will
be strained. This should collapse rapidly to the s-trans
isomer (6). A passage over a rotational barrier then gives the
s-cis isomer (7). This is followed by an intramolecular
attack of the s-cis-enolate anion on the mixed anhydride.
The tetrahedral intermediate formed will rapidly lose the
original nucleophilic carboxylate anion to form the fused
six- and six-membered bicyclic structure or product 2.
Studies of the effect of solvent on the rates were very
illuminating. As shown in Table 3, the rate increases by
about a factor of about 30 on going from the less polar
propan-2-ol to the more polar ethane-1,2-diol. The linear
correlation between log k1 and ET(30)6 is good, with a
correlation coefficient of 0.954 and a slope of 0.17 (±0.02).
This clearly indicates the importance of solvation in
stabilizing the transition state in comparison with the initial
state. Enolates and similar anionic species require protic
solvation for stabilization.15 The kinetic solvent isotope
effect, kROH/kROD, for both methanol and ethanol is about 1.3.
This is in accord with the expected change in the hydrogen-
bonding solvating capacity of the solvents arising from
replacing OH and OD.16 Hence, this excludes any primary
isotope effects on the rate-determining step.
log (K/K0) or (k/k0)=␳
(1)
The correlation is satisfactory, as shown in Table 5, with a
␳ value of about Ϫ1.7 for ethane-1,2-diol and -2.0 for
ethanol. These results can be related to the ionization of
these acids measured in the same solvent systems,10 having
␳ values for about 1.5 for both ethane-1,2-diol and ethanol.
A Brønsted-type coefficient, ␣, of about 1.1 and 1.3 can be
calculated from the ratio Ϫ(␳cat/␳ioniz).11 Whereas Brønsted
coefficients for acid- and base-catalysed reactions are
generally between zero and unity, the slopes of the
relationship between nucleophilic rate coefficients and
ionization constants are not so limited.12 A relevant reaction
for comparison is the nucleophilic catalysis (pyridines and
other bases) of the hydrolysis of p-nitrophenyl acetate in
water at 25.2 °C.12 The value of ␣ for the latter reaction is
1.62 in water and the reaction gives first the reactive
intermediates, the N-acetylpyridinium cations. Such hydrol-
ysis reactions are also catalysed by acetate anions and then
can proceed via acetic anhydride.12 Hence the present
results indicate the likelihood of relatively rapid formation
of the adduct in a pre-equilibrium step. The Brønsted
coefficients of greater than unity suggest that the transition
The activation parameters for the rearrangement reaction
of the parent compound in the three alcoholic solvents
shown in Table 4 are consistent with a bimolecular
mechanistic pathway, i.e. a rate-determining unimolecular
process following a relatively rapid pre-equilibrium bimo-
lecular process. However, the values for the entropies of
activation are close to zero and strongly suggest a rate-
determining step involving a process releasing strain and
increasing freedom, i.e. kЈ or kЈ in Scheme 1.
2
3
The drive for the overall rearrangement of the five-
membered dilactone 1, to the six-membered dilactone, 2, is
the greater thermodynamic stability of the bicyclic six- and
six-membered fused ring system, 2, which is fully coplanar
and almost strain free.17
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JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 191–195 (1997)