Ring-chain tautomerism. Part 10 +. The reaction of oxocarboxylic acids with diazodiphenylmethane

Article abstract of DOI:10.1135/cccc19991601

The rate coefficients for the esterification of a series of oxocarboxylic acids with diazodiphenylmethane have been determined in ethanol or 2-methoxyethanol at 30.0°C. These and the rates of reaction with model compounds have been used to estimate the equilibrium constants for ring-chain tautomerism for the oxocarboxylic acids.

Full text of DOI:10.1135/cccc19991601

Ring–Chain Tautomerism
1601
RING–CHAIN TAUTOMERISM. PART 10+. THE REACTION OF
OXOCARBOXYLIC ACIDS WITH DIAZODIPHENYLMETHANE
b
a
Keith BOWDENa1,*, Milica M. MIŠIĆ-VUKOVIĆ and Richard J. RANSON
a
Department of Biological and Chemical Sciences, Central Campus, University of Essex, Wivenhoe
Park, Colchester, Essex CO4 3SQ, U.K.; e-mail: keithb@essex.ac.uk
Department of Organic Chemistry, Faculty of Technology and Metallurgy, University of Beograd,
Karnegijeva 4, POB 494, 11001 Beograd, Yugoslavia
1
b
Received Jun e 29, 1999
Accepted August 9, 1999
Dedicated to Professor Otto Exner on the occasion of his 75th birthday in recognition of his
outstanding contributions to physical organic chemistry.
Th e rate coefficien ts for th e esterification of
a series of oxocarboxylic acids with
diazodiph en ylm eth an e h ave been determ in ed in eth an ol or 2-m eth oxyeth an ol at 30.0 °C.
Th ese an d th e rates of reaction with m odel com poun ds h ave been used to estim ate th e equi-
librium con stan ts for rin g–ch ain tautom erism for th e oxocarboxylic acids.
Key words: Rin g–ch ain tautom erism ; Diazodiph en ylm eth an e; Esterification ; Reaction kin etics;
Equilibrium con stan ts.
Rin g–ch ain tautom erism h as been sh own to occur in a n um ber of
oxocarboxylic acids2. Th us, th is h as been sh own to give rise to both rin g 2
an d ch ain 1 form s (Eq. (1)) of th ese system s, with an equilibrium con stan t,
Keq, given by Eq. (2).
O
O
C
C
OH
O
Keq
O
(1)
(2)
C
R
C
R
OH
2
1
Keq = arin g ach ain
+ For Part 9 see ref.1
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

1602
Bowden, Mišić-Vuković, Ranson:
Quan titative m easurem en ts of th is beh aviour can be m ade directly, e.g. by
IR, UV-VIS an d 1H NMR spectroscopic m eth ods, or in directly, e.g. by pKa
m easurem en ts. For th e form er, th e m easurem en ts are lim ited to th ose sys-
tem s h avin g Keq in th e ran ge of ca 0.02 to 50, togeth er with h avin g observ-
able spectroscopic differen ce between 1 an d 2. For th e latter, th e observed
pKa can be related to th e true pKaT by Eq. (3) (ref.3). If a reliable estim ate of
pKaT can be m ade, Keq can be foun d by m easurin g pKa.
pKaT = pK – log K + 1
(3)
(
)
a
eq
In prin ciple, Keq > ca 2 could be estim ated with reason able certain ty usin g
th is m eth od. Th is m eth od depen ds on th e sign ifican tly greater acidity of
th e carboxylic acid th an th at of th e h ydroxylacton e.
The principal problem s in m easuring Keq values for ring–chain tautom erism
of oxocarboxylic acids are th ose wh ere Keq is very large or sm all, wh ere th e
spectral differen ce between tautom ers is sm all or wh ere solven t effect
m akes direct spectral m easurem en t im practical. A possible m eth od, wh ich
could overcom e som e of th ese problem s, is based on th e m easurem en t of
th e reactivity of th e rin g–ch ain tautom eric system in wh ich on ly on e tauto-
m er, n orm ally th e ch ain , is sign ifican tly m ore reactive an d wh ere th e reac-
tivity of th at tautom er alon e can be reliably estim ated. Th e observed rate
coefficien t k2 is related to th e true rate coefficien t kT as sh own in Eq. (4).
2
k2 = k2T ach ain
a
(
+ arin g
(4)
)
ch ain
Th is en ables th e calculation of Keq if both k2 an d kT are kn own , as sh own in
2
Eq. (5).
Keq = kT k2 – 1
(5)
(
)
2
Carboxylic acids react at con ven ien t rates with diazodiph en ylm eth an e
(DDM) an d th e reaction can be followed spectroscopically4. Th e m ech a-
n ism of th is reaction an d rate–reactivity relation s h ave been studied in de-
tail5. Bell et al.6 h ave developed a reactivity-based m eth od in wh ich th e
catalytic effect of th e an ion s of th e carboxylic acids in water on th e decom -
position of n itram ide an d on th e m utarotation of glucose was m easured.
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Ring–Chain Tautomerism
1603
Usin g th e relation between th e catalytic con stan ts of carboxylic acid an ion s
an d th eir pKa values to determ in e th e true pKaT , th e equilibrium con stan t
Keq can be evaluated from th e observed pKa usin g Eq. (3).
In th e presen t study, th e rates of reaction of a series of oxocarboxylic acids
with DDM h ave been m easured. Usin g m odel com poun ds to estim ate th e
true rate coefficien ts, Keq values for th ese acids h ave been determ in ed.
RESULTS AND DISCUSSION
In Table I th e rate coefficien ts for th e esterification of th e oxocarboxylic
acids with DDM at 30.0 °C are sh own . A reaction path way is sh own in
Sch em e 1. Th e oxocarboxylic acids selected are th ose with Keq values wh ich
are eith er kn own or expected7 to be >2. For th ese acids, k2 an d kT will be
2
sufficien tly differen t to en able determ in ation of Keq with reason able
O
O
O
C
C
C
Keq
Ph2CN2
OCHPh2
O
OH
O
N2
+
O
C
R
C
R
C
R
OH
SCHEME 1
certain ty. Th e solven t em ployed in th is study was eth an ol, except for th ose
acids too in soluble in th e latter for wh ich 2-m eth oxyeth an ol was used. Th e
rate coefficien ts were reproducible to ±3% an d are th e observed rate coeffi-
cien ts, k2, for th ese oxocarboxylic acids. Th e true rate coefficien ts, kT , are
2
also sh own in Table I. Th ese h ave been estim ated from m odel system s. Th e
rate coefficien ts for th e esterification of 2-substituted ben zoic acids with
DDM h ave been m easured by Ch apm an an d co-workers8. Th e k2T values for
th e 2-acylben zoic acids are estim ated from th e “ortho”-ρ value for th e DDM
esterification of 2-substituted ben zoic acids9 in eth an ol at 30 °C an d th e σp
values for th e acyl substituen ts10. Furth erm ore, 2-ben zoylben zoic an d
9-oxofluoren e-1-carboxylic acids h ave Keq values th at can be con fiden tly es-
tim ated to be <0.1 in eth an ol, cf. refs2,7. Th e form er acid h as been used as a
poin t of referen ce for th e estim ation of k2T for th e 2-acylben zoic acids. Th e
value for th e (Z)-3-ben zoyl-2,3-diph en ylacrylic acid h as been based on th e
values for (E)-2-ph en ylcin n am ic an d (Z)-3-substituted acrylic acids9,11 in
eth an ol at 30 °C an d for th e 8-acyl-1-n aph th oic acids, as well as 5-form yl-
ph en an th ren e-4-carboxylic acid, on th e values for th e 8-substituted
1-n aph th oic acids studied previously12 in 2-m eth oxyeth an ol at 30 °C. Th e
form er estim ate was based on th e correspon din g “ρ” value9 an d σp of th e
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

1604
Bowden, Mišić-Vuković, Ranson:
TABLE I
Rate coefficien ts, k2, for th e esterification of th e oxocarboxylic acids with diazodiph en yl-
m eth an e an d rin g–ch ain equilibrium con stan ts, Keq, at 30.0 °Ca
Acid
k2/dm 3 m ol–1 m in –1 k2T /dm 3 m ol–1 m in –1
Keq
2-Substituted ben zoic acids
In eth an ol
H
1.00 (0.990)b
1.17
–
–
CHO
7.9
7.7
7.7
7.7
7.7
7.8
5.8(4.6,14.8,5.6)c,d,e
4.0(2.4,3.2,3.0)c,d,e
4.1(3.5,2.6)c,d
4.2(2.2,1.9,5.6)c,d,e
5.4(4.6,3.2)c,d
3.0(2.6,2.5)d
2.6
COCH3
1.54
COCH2CH3
COCH(CH3)2
COC(CH3)3
COCH2Ph
COCOPh
COCCl3
1.50
1.49
1.21
1.96
16.5
60
0.370
0.360
7.70
36
36
96(78)f
99(100)f
COCBr3
g
COPh
–
–
g
9-Oxofluoren e-1-carboxylic
7.18
(Z)-3-Ben zoyl-2,3-
diph en ylacrylic
0.0263
28
1 100(1 600)i
In 2-m eth oxyeth an ol
H
0.650 (0.640)b
–
–
8-Substituted 1-n aph th oic acids
H
0.810(0.802)b
0.00470
–
–
CHO
COCH3
6.0
6.0
1 300(1 200)h
78(100,130)e,h
0.0764
5-Substituted ph en an th ren e-4-carboxylic acid
CHO 0.00482
2 700(3 000)h
13
a
Rate coefficien ts are reproducible to ±3% or better an d are th e m ean of at least two deter-
b
c
m in ation s.
m eth od)7.
Refs4,5,12
.
In 80% aqueous 2-m eth oxyeth an ol (1H NMR spectroscopic
d
e
In m eth an ol (1H NMR spectroscopic m eth od)7.
In water (catalytic/pKa
m eth od)6. In water (pKa m eth od)7. Ke estim ated to be <0.1, cf. refs3,7
2-m eth oxyeth an ol (pKa m eth od)7.
.
In 80% aqueous
f
g
h
Collect. Czech. Chem. Commun. (Vol. 64) (1999)

Ring–Chain Tautomerism
1605
ben zoyl group10, wh ile th e latter was based on th e correspon din g “ρ”
value12 an d σp for th e form yl an d acetyl groups10. Th e un certain ties in k2T
values can be estim ated to be ±5% for th e 2-acylben zoic acids an d to be
±10% for th e rem ain der. Th e calculated Keq values for eth an ol sh own in
Table I can be estim ated to h ave un certain ties of ±8% for th e first group
an d ±13% for th e secon d group. Th e literature Keq values are also sh own in
Table I for eith er m eth an ol an d 80% aqueous 2-m eth oxyeth an ol, usin g a di-
rect 1H NMR spectroscopic m eth od, or for water an d 80% aqueous
2-m eth oxyeth an ol, usin g in direct pKa m eth ods6,7. However, rin g–ch ain
tautom eric equilibrium con stan ts for oxocarboxylic acids are kn own to be
solven t-depen den t2,13. Th e literature Keq values sh own in Table I are th ose
for solven t system s of com parable solvatin g power to th at of eth an ol14.
Th ere is good agreem en t, in gen eral, between th e Keq values obtain ed by
th e presen t n ovel m eth od usin g th e DDM reaction an d th ose obtain ed usin g
both direct an d in direct m eth ods studied previously2,13. An advan tage of th e
DDM reaction m eth od is th at th is reaction can be studied in an extrem ely
wide ran ge of solven ts4.
EXPERIMENTAL
Materials
All th e oxocarboxylic acids h ad been prepared for use in previous studies7. Th e purity of th e
com poun ds were m on itored by m .p., 1H an d 13C NMR an d IR spectroscopy. Solven ts were
purified by stan dard m eth ods14. DDM was prepared by th e oxidation of th e correspon din g
h ydrazon e5.
Rate Measurem en ts
Th ese were determ in ed, usin g a Perkin –Elm er lam bda 16 UV-VIS spectroph otom eter, at 30.0
(±0.05) °C in a sim ilar m an n er to th at described previously5a,9,11,12 usin g, n orm ally, a
10-fold excess of acid (0.06 m ol dm –3). An altern ative procedure was em ployed for acids
available in very lim ited quan tity. Th e acid solution (3 m l, 0.06 m ol dm –3) was placed in th e
cuvette, th erm ostatted at 30.0 °C, an d a DDM solution (3 × 10–3 m l, 6 m ol dm –3) added to
in itiate th e reaction . Wh en required, th e secon d-order rate coefficien ts were ch ecked by th e
m eth od devised by Corbett15, wh ich can be applied if th e excess used is less th an 10-fold.
We thank D. Papadopoulou and A. Nikokavoura for carrying out some preliminary studies.
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Collect. Czech. Chem. Commun. (Vol. 64) (1999)

1606
Bowden, Mišić-Vuković, Ranson:
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Collect. Czech. Chem. Commun. (Vol. 64) (1999)