Steric effects in the base-catalyzed cyclization of 1-[2-(methoxycarbonyl)phenyl]-3-(2-substituted phenyl)triazenes

Article abstract of DOI:10.1135/cccc19982075

Eleven model 1-[2-(methoxycarbonyl)phenyl]-3-(2-substituted phenyl)triazenes were synthesized and their cyclization kinetics examined in aqueous-methanolic buffer solutions (51 wt.% methanol) at various pH values. 3(2-Substituted phenyl)benzo[d][1,2,3]tri

Full text of DOI:10.1135/cccc19982075

Steric Effects of Triazenes
2075
STERIC EFFECTS IN THE BASE-CATALYZED CYCLIZATION
OF 1-[2-(METHOXYCARBONYL)PHENYL]-3-(2-SUBSTITUTED
PHENYL)TRIAZENES
1
2
Miroslav LUDWIG and Ingrid BAUEROVA
Department of Organic Chemistry, University of Pardubice, 532 10 Pardubice, Czech Republic;
e-mail: 1 miroslav.ludwig@upce.cz, 2 ingrid.bauerova@upce.cz
Received May 21, 1998
Accepted September 29, 1998
Eleven model 1-[2-(methoxycarbonyl)phenyl]-3-(2-substituted phenyl)triazenes were synthesized and
their cyclization kinetics examined in aqueous-methanolic buffer solutions (51 wt.% methanol) at
various pH values. 3(2-Substituted phenyl)benzo[d][1,2,3]triazin-4(3H)-ones were identified as the
cyclization products. The log kobs vs pH plot was linear with a slope of unity. Investigation of the
steric and electronic effects of substituents in the ortho position revealed that substituents at the ring
which is bonded to the N(3) nitrogen affect the cyclization rate through their steric effect only, while
their electronic effects are statistically insignificant. This fact was explained in terms of the ring
being tilted from the plane in which the remaining part of the conjugate base anion of the model
substrate lies. The assumed and confirmed BAc2 mechanism involving specific base catalysis begins
by deprotonation of the triazene giving rise to the conjugate base, continues with formation of a te-
trahedral intermediate, and ends with elimination of the methanolate ion. Other mechanisms, such as
the elimination-addition mechanism via a ketene intermediate or the mechanism involving general
base catalysis, are unlikely.
Key words: Triazenes; Base catalyzed cyclization; Steric effect; Substituent effect; Reaction kinetics.
Although some triazene derivatives (1) have been described1,2 (R = CH3, X = H, CH3,
OCH3, Cl, NO2, COOCH3, NHCOCH3), only their physical constants and spectral proper-
ties have been published – UV-VIS spectra, vibrational spectra3, and 13C and 15N NMR
spectra4. As far as kinetic measurements are concerned, the acid-catalyzed decomposi-
tion and base-catalyzed cyclization have been studied for substances structurally related
to the triazenes (1) (R = Me, Et, Pr, i-Pr, Bu, s-Bu, Hex, allyl; X = H) and the reaction
has been modelled mathematically. The cyclization reaction has been observed to slow
down with increasing steric demands of the substituent5. Other kinetic studies of struc-
turally related 1,3-diaryltriazenes centered mainly on the elucidation of the mechanism
of the decomposition reactions occurring in acid solutions6–10. With the model 1-[2-
(methoxycarbonyl)phenyl]-3-(2-substituted phenyl)triazenes (1), intramolecular cyclization
takes place in basic solutions giving rise to 3-(2-substituted phenyl)benzo[d][1,2,3]triazin-
4(3H)-one3,5,11,12. In previous studies, the addition-elimination mechanism BAc2 associ-
ated with the formation of a tetrahedral intermediate as the rate-determining step was
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

2076
Ludwig, Bauerova:
assumed to be the most likely mechanism of formation of the cyclization product
(Scheme 1). Among features characteristic of this mechanism is a high susceptibility of
the reaction to the steric demands of substituents near the reaction centre.
O
C
O
C
OCH
OCH
3
3
B
fast
-BH
N
N
N
H
N
N N
X
X
1
O
CH O
3
O
N
N
N
N
N
-CH O
3
N
X
X
SCHEME 1
The term “steric effect” is usually defined as a hindrance to the access of the reagent
to the reaction centre13–18. Such effects can be considerable, reducing the reaction rate
and varying irregularly in dependence on the increasing steric demands. Steric effects
of substituents were first described quantitatively by Taft13,19, who characterized them
by the Es constants. Several corrections to the Es constants have been suggested; among
them are the ECs constants proposed by Hancock and coworkers20. An approach which
is theoretically different has been applied by Charton21,22, who defined steric constants
υ based on the van der Waals radii. Additional, principally new steric constants Ωs have
been proposed by Komantsuzaki and coworkers23: the constants represent the relative
area of the shadow cast by the substituent onto a spherical surface having a unit area.
The constants have been reported nearly solely for alkyl groups.
The main aim of this work is to take advantage of synthesized 1-[2-(methoxycarbonyl)-
phenyl]-3-(2-substituted phenyl)triazines (1) as model for studying of steric and ele-
X
H
1
a
b
c
d
e
f
CH
3
O
C
C H
2
5
OCH
CN
3
CF
3
N
N N
OCH
F
3
H
X
g
h
i
Cl
1
Br
NO
2
j
SO CH
k
2
3
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Steric Effects of Triazenes
2077
tronic effects of various substituents used on the rate of cyclization in basic media. This
paper is a continuation of works published by our group on area of various types of
triazenes reactions in various media and also works focused upon steric a ortho effects.
Substitution of the aromatic ring in the ortho position to the side chain with the reaction
centre exhibits very specific features compared with substitution in the remaining posi-
tions. Kinetic and mechanistic studies are conducted with the aim to describe discussed
effects on model substances.
EXPERIMENTAL
Synthesis of 1-[2-(Methoxycarbonyl)phenyl]-3-(2-substituted phenyl)triazenes 1
Methyl anthranilate (1.51 g, 0.01 mol) was mixed with 20 ml of hydrochloric acid diluted with water
in the 1 : 1 ratio. The mixture was cooled to –5 °C, and a solution of sodium nitrite (0.69 g, 0.01 mol)
in water (10 ml) was added dropwise to the stirred mixture while keeping the temperature below 5 °C.
The diazotization end point was indicated by using iodine-starch paper. The diazonium salt solution
was added dropwise to a vigorously stirred mixture of 2-substituted aniline (0.01 mol) and sodium
acetate (12.8 g, 0.4 mol) in water (40 ml). The triazenes formed were purified by column chromatography
(stationary phase: neutral aluminium oxide supplied by Woelm company; mobile phase: toluene–
ethyl acetate–hexane 1 : 5 : 1, the crystallization from diethyl ether was done after chromatography)
or by crystallization. The yields of the crude triazenes 1, their melting points and purification proce-
dures are given in Table I.
TABLE I
Yields, melting points and purification procedures of 1-[2-(methoxycarbonyl)phenyl]-3-(2-substituted
phenyl)triazenes 1
Melting point
Substrate
Substituent X
Yield, %
Purification procedure
° C
1a
1b
1c
1d
1e
1f
H
31.5
60.1
38.2
50.0
34.0
31.6
70.0
41.5
55.0
29.0
29.0
64–66a
70–71b
chromatography
CH3
C2H5
CN
crystallization from methanol
crystallization from methanol
crystallization from methanol
chromatography
62–64
150–152
130–132
95–97c
CF3
OCH3
F
crystallization from methanol
chromatography
1g
1h
1i
77–80
Cl
102–104d
105–107
164–166e
163–165
chromatography
Br
chromatography
1j
NO2
SO2CH3
crystallization from diethyl ether
crystallization from methanol
1k
a
b
c
d
e
Ref.25 69–71 °C; ref.25 69.5 °C; ref.2 105 °C; ref.2 109 °C; ref.3 165–167 °C.
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

2078
Ludwig, Bauerova:
Identification of Triazenes 1
The structure of the triazenes 1 was confirmed by the UV-VIS spectra of their methanolic solutions,
1
measured on a Hewlett–Packard HP 8452 diode array spectrophotometer, and by the H NMR spectra
of their solutions in deuterated chloroform, scanned at 25 °C on a Bruker AMX spectrometer (360.14 MHz,
standard: hexamethylenedisiloxane, δ 0.05 ppm). Since the triazenes are sensitive to acids present
even in trace amounts, the solvent was freed from traces of HCl by extraction with the same volume
of 5% aqueous sodium carbonate and with the same volume of water, dried with anhydrous sodium
sulfate, and distilled.
Identification of the Product of Base-Catalyzed Cyclization of Triazenes 1
A small amount of the product was isolated from the reaction mixture of 1-[2-(methoxycarbonyl)phenyl]-
3-phenyltriazene (substrate 1a) and aqueous-methanolic buffer at pH 11.68. The UV-VIS spectrum of
this substance was measured in methanol, and the wavelengths of the observed absorption maxima
(λ1 292 nm, λ2 314 nm) were compared with published data (290 and 314 nm) pertaining to a differ-
ent reaction pathway starting from anthranilanilide26. The isolated product melted at 148–150 °C,
which also agrees well with published data (151–152 °C, ref.27).
Kinetic Measurements
Aqueous-methanolic buffers containing 51.1 wt.% methanol were used. Buffers for the range of pH
7.54–8.75 were obtained by mixing saturated solutions of disodium hydrogen phosphate and potas-
sium dihydrogen phosphate (both in 51.1 wt.% methanol) in various proportions, whereas buffers for
the range of pH 9.13–11.68 were obtained by mixing a saturated solution of disodium hydrogen
phosphate and a 0.2 M solution of sodium hydroxide in the same solvent (ref.28). The pH values were
read on a Radiometer TITRALAB 3 pH-meter using a glass electrode combined with a modified ca-
lomel electrode.
Kinetic measurements were performed on a Hewlett–Packard HP 8452A instrument; an HP DUR-
RUM D-150 instrument of the same manufacturer was employed for systems with half-lives below 3 s.
Prior to the measurement, the buffer (approximately 2 ml) was thermostatted to 25 ± 0.1 °C in a
quartz cell, and a solution of triazene 1 in dioxane (approximately 4 µl) was added with a Hamilton
701 N syringe so that its concentration in the cell was about 5 µmol l–1. The cell contents were
stirred thoroughly, and the decrease in absorbance was monitored in dependence on time for a period
no shorter than four half-lives. In the measurements on the DURRUM instrument, following thermo-
statting to 25 ± 0.1 °C, the buffer solution was mixed with an aqueous-methanolic solution of the
triazene 1 at a concentration of 2.5 µmol l–1 by means of the injecting device. The results of meas-
urement were evaluated by non-linear regression using an optimizing program. The observed rate
constants are given in Table II.
RESULTS AND DISCUSSION
Spectra of 1-[2-(Methoxycarbonyl)phenyl]-3-(2-substituted phenyl)triazenes
The UV-VIS spectra of methanolic solutions of the model substances 1 exhibited two
absorption peaks: one at 202 nm and the other, characteristic of triazenes, in the 360 nm
range. The absorption peak wavenumbers and molar absorptivities are given in Table III.
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Steric Effects of Triazenes
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Collect. Czech. Chem. Commun. (Vol. 63) (1998)

2080
Ludwig, Bauerova:
The 1H NMR spectra of the substances were measured in deuterated chloroform. The
chemical shifts δ(1H) are given in Table IV.
Reaction Order and Catalytic Rate Constants
The reaction order was calculated by applying the method of two different starting
concentrations29 to the fluoro (1g) and ethyl (1c) derivatives (pH 8.03 and 8.75, respec-
tively). The n values were 0.967 and 0.981, respectively; hence, the reaction is first
order with respect to substrate.
The catalytic rate constant logarithms (log kB) were calculated as the average dif-
ferences between the observed rate constant logarithms (log kobs) and pH. The log kB
values are given in Table V.
Substituent Effects and Mechanism of Base Catalyzed Cyclization
of 1-[2-(Methoxycarbonyl)phenyl]-3-(2-substituted phenyl]triazenes
To investigate the substituent effects, the log kobs values were subjected to analysis by
the PCA (ref.30) and CDA (ref.31) methods; a completely full matrix 11 × 11 was used.
(Note: for some substituents, the kobs values in three buffers at the lowest pH could not
be determined due to the long reaction half-life.) Although the F-test gave a value of 3
for the number of necessary latent variables, the 2nd and 3rd latent variables lay at the
TABLE III
Wavelengths of absorption maxima λ1, λ2, related molar absorptivities ε1, ε2 of methanolic solutions
and elemental analyses of triazenes 1
ε1
ε2
Calculated/Found
% N
Substrate Substituent X
λ1, nm
λ2, nm
l mol–1 cm–1
l mol–1 cm–1
1a
1b
1c
1d
1e
1f
H
202
202
202
202
202
202
202
202
202
202
202
1.206 . 105
1.370 . 105
1.424 . 105
1.285 . 105
1.695 . 105
1.377 . 105
1.266 . 105
1.679 . 105
1.508 . 105
1.513 . 105
1.611 . 105
358
362
360
368
356
370
360
360
360
362
362
2.638 . 104
2.587 . 104
2.830 . 104
2.512 . 104
3.055 . 104
2.422 . 104
2.606 . 104
2.937 . 104
2.244 . 104
2.219 . 104
2.367 . 104
16.47/16.62
15.61/15.45
14.84/14.72
20.00/19.96
13.00/12.95
14.73/14.68
16.09/15.88
14.51/14.62
13.05/13.02
18.66/18.62
12.61/12.49
CH3
C2H5
CN
CF3
OCH3
F
1g
1h
1i
Cl
Br
1j
NO2
SO2CH3
1k
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Steric Effects of Triazenes
2081
O
C
TABLE IV
5’
2’
3
Chemical shifts δ(1H) of triazenes 1
OCH
N
3
2
4
5
4’
3’
N
N
H
X
Protons
aromatic δ, ppm
Substrate
(Substituent)
aliphatic δ, ppm
other δ, ppm
1a
(H)
3.93 s, 3 H (–OCH3) 8.01 d, 1 H (H-5′)
7.94 d, 1 H (H-2′)
7.56 d, 2 H (H-2,6) 12.37 s, 1 H (–NH)
7.42 t, 2 H (H-3,5)
7.51 t, 1 H (H-3′)
7.29 t, 1 H (H-4)
7.01 t, 1 H (H-4′)
1b
(CH3)
3.93 s, 3 H (–OCH3) 8.00 d, 1 H (H-5′)
2.48 s, 3 H (–CH3) 7.95 d, 1 H (H-2′)
7.50 t, 1 H (H-3′)
7.53 t, 1 H (H-3)
7.17–7.24 m, 3 H
(H-2,4,5)
12.35 s, 1 H (–NH)
12.36 s, 1 H (–NH)
12.72 s, 1 H (–NH)
12.58 s, 1 H (–NH)
12.58 s, 1 H (–NH)
–
6.99 t, 1 H (H-4′)
1c
(C2H5)
3.93 s, 3 H (–OCH3) 8.01 d, 1 H (H-5′)
2.92 k, 2 H (–CH2–) 7.96 d, 1 H (H-2′)
1.22 t, 3 H (–CH3) 7.49 m, 1 H (H-3′) (H-2,4,5)
7.01 t, 1 H (H-4′)
7.53 m, 1 H (H-3)
7.18–7.27 m, 3 H
1d
(CN)
3.93 s, 3 H (–OCH3) 8.04 d, 1 H (H-5′)
7.98 d, 1 H (H-2′)
7.69–7.73 m, 2 H
(H-2,5)
7.55 m, 1 H (H-3′) 7.59 m, 1 H (H-3)
7.09 t, 1 H (H-4′)
7.33 t, 1 H (H-4)
1e
(CF3)
3.96 s, 3 H (–OCH3) 8.02 d, 1 H (H-5′)
7.93 d, 1 H (H-2′)
7.67–7.72 m, 2 H
(H-2,5)
7.52 m, 1 H (H-3′) 7.54 m, 1 H (H-3)
7.05 t, 1 H (H-4′)
7.34 t, 1 H (H-4)
1f
(OCH3)
3.95 s, 3 H (–OCH3) 7.99 d, 1 H (H-5′)
7.57 d, 1 H (H-2)
7.29 d, 1 H (H-5)
6.97–7.01 m, 2 H
3.91 s, 3 H
(X = OCH3)
7.93 d, 1 H (H-2′)
7.50 t, 1 H (H-3′)
6.98 m, 1 H (H-4′) (H-3,4)
1g
(F)
3.92 s, 3 H (–OCH3) 7.99 d, 1 H (H-5′)
7.90 d, 1 H (H-2′)
7.63 t, 1 H (H-4)
7.22 m, 2 H (H-2,5)
7.13 t, 1 H (H-3)
7.51 t, 1 H (H-3′)
7.05 t, 1 H (H-4′)
1h
(Cl)
3.94 s, 3 H (–OCH3) 8.02 d, 1 H (H-5′)
7.93 d, 1 H (H-2′)
7.60 d, 1 H (H-5)
7.45 d, 1 H (H-2)
7.19–7.26 m, 2 H
(H-2,5)
12.57, 1 H (–NH)
12.57, 1 H (–NH)
7.52 t, 1 H (H-3′)
7.04 t, 1 H (H-4′)
1i
(Br)
3.94 s, 3 H (–OCH3) 8.02 d, 1 H (H-5′)
7.93 d, 1 H (H-2′)
7.64 d, 1 H (H-5)
7.56 d, 1 H (H-2)
7.31 t, 1 H (H-3)
7.12 t, 1 H (H-4)
7.52 t, 1 H (H-3′)
7.04 t, 1 H (H-4′)
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

2082
Ludwig, Bauerova:
TABLE IV
(Continued)
Protons
Substrate
(Substituent)
aliphatic δ, ppm
aromatic δ, ppm
other δ, ppm
1j
3.92 s, 3 H (–OCH3) 7.97 d, 1 H (H-5′)
7.77 m, 1 H (H-5)
12.57, 1 H (–NH)
(NO2)
7.80 m, 1 H (H-2′) 7.69 d, 1 H (H-2)
7.55 m, 1 H (H-3′) 7.58 m, 1 H (H-3)
7.14 t, 1 H (H-4′)
7.31 t, 1 H (H-4)
1k
(SO2CH3)
3.95 s, 3 H (–OCH3) 8.08 d, 1 H (H-5′)
8.18 d, 1 H (H-5)
7.76 d, 1 H (H-2)
7.64 t, 1 H (H-3)
7.43 t, 1 H (H-4)
12.78, 1 H (–NH)
3.42 s, 3 H
(–SO2CH3)
7.93 d, 1 H (H-2′)
7.58 t, 1 H (H-3′)
7.11 t, 1 H (H-4′)
limit of statistical significance, while the 1st latent variable accounted for 98.74% of
the source matrix variability. The values of the first score t1 vector calculated by the
CDA method of conjugate deviations with 2 latent variables are given in Table V.
The difference between the logarithms of the catalytic rate constants ∆ log kB = log kBX
–
log kBH was used to quantify and evaluate the substituent effects on the reaction kinetics
TABLE V
Logarithms of catalytic rate constants log kB, steric constants υeff, and 1st score vectors t1 calculated
by the CDA method for triazenes 1
Substrate
Substituent X
log kB
υeff
t1
1a
1b
1c
1d
1e
1f
H
–11.33
–11.59
–11.76
–11.71
–12.27
–12.02
–11.33
–11.61
–11.69
–11.35
–12.57
0.00
0.52
0.56
0.40
0.90
0.36
0.27
0.55
0.65
0.35
1.13
1.000
0.772
0.625
0.683
0.231
0.421
0.994
0.761
0.692
0.983
0.000
CH3
C2H5
CN
CF3
OCH3
F
1g
1h
1i
Cl
Br
1j
NO2
SO2CH3
1k
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Steric Effects of Triazenes
2083
and mechanism. The differences were processed by linear regression analysis with dif-
ferent independently variables characterizing the substituent, viz. σi0, σi, σI, σR, and σp
(refs32,33). Correlation with those substituent constants appeared invariably to be statis-
tically insignificant. A considerably better correlation was achieved with the par-
ameters characterizing the steric demands of the substituents, viz. with Charton’s υeff
constants22. The correlation of the ∆ log kB values with the υeff constants was made with
all points included, as well as with the exclusion of two outliers (triazenes 1a and 1f).
The correlation coefficient improved thereby from 0.845 (n = 11) to 0.962 (n = 9), and
the explained variability then was 92.49%. Logically, then, the best statistical charac-
teristics (r = 0.9995, explained variability 99.91%) were obtained if the ∆ log kB values
were correlated with the t1 vector calculated by the CDA method. The use of other
steric parameters such as σis (ref.32) did not lead to statistically significant results be-
cause the parameters did not cover the range of substituents used by us and thus were
unusable. The results were also processed by multiple linear regression using various
combinations of the substituent constants32. Not a single suitable multiparameter model
describing the variability of the experimental results was found.
Physical meaning can be attached to the 1st latent variable, characterized by the
score vector t1, if a vector of the substituent constants correlating well with the score
vector can be identified. The best correlation was observed with Charton’s constants υeff,
describing the steric demands of the substituents. After eliminating two outliers (H and
OCH3 substituents), 92.49 per cent of variability of the experimental data set was ex-
plained (r = 0.962). An attempt was also made to correlate the 2nd score vector t2 with
some constants accounting for the inductive and mesomeric substituent effects but all
of the correlations were statistically insignificant. The fact that no inductive or me-
someric substituent effect was observed can be due to the long distance between the
substituent and the reaction centre (for the inductive effect) or to the non-planarity of
the molecule (for the mesomeric effect). This can be explained by the ring at the N(3)
nitrogen being more or less tilted from the plane in which the remaining part of the
molecule lies. The degree of tilting is related with the substituent bulk: a bulky substi-
tuent hinders the attack of the negatively charged nitrogen on the carbon in the
COOCH3 group carbonyl, and its electronic effects do not manifest themselves.
On the basis of the slowing-down of the reaction of some related substrates observed
when a sterically more demanding substituent was present in the vicinity of the reaction
centre, it has been suggested recently3,5,11,12 that the BAc2 mechanism, with the second
step as the rate determining step, is the most likely mechanism of their cyclization. The
model 1-[2-methoxycarbonyl)phenyl]-3-(2-substituted phenyl)triazenes studied in this
work exhibit similar dependences, and so the above mechanism can be regarded as
confirmed for them as well.
This research was supported by the Grant Agency of the Czech Republic (grant No. 203/98/0602).
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

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Ludwig, Bauerova:
REFERENCES
1. Grammaticakis P.: Bull. Soc. Chim. Fr. 1957, 129.
2. Grammaticakis P.: Bull. Soc. Chim. Fr. 1957, 139.
3. Stevens H. N. E., Stevens M. F. G.: J. Chem. Soc. 1970, 765.
4. Axenrod T., Mangiaracina P.: Helv. Chim. Acta 1976, 59, 1655.
5. Pytela O., Halama A.: Collect. Czech. Chem. Commun. 1996, 61, 751.
6. Benes J., Beranek V., Zimpirich J., Vetesnik P.: Collect. Czech. Chem. Commun. 1977, 42, 702.
7. Svoboda P., Pytela O., Vecera M.: Collect. Czech. Chem. Commun. 1986, 51, 553.
8. Isaacs N. S., Rannala E.: J. Chem. Soc., Perkin Trans. 2 1974, 899.
9. Nevecna T., Pytela O., Ludwig M., Kavalek J.: Collect. Czech. Chem. Commun. 1990, 55, 147.
10. Pytela O., Nevecna T., Ludwig M.: Collect. Czech. Chem. Commun. 1990, 55, 156.
11. Pytela O., Dlouhy V.: Collect. Czech. Chem. Commun. 1990, 55, 2468.
12. Pytela O., Bahnik Z.: Collect. Czech. Chem. Commun. 1990, 55, 2692.
13. Taft R. W. in: Steric Effects in Organic Chemistry (M. S. Newman, Ed.). Wiley, New York
1956.
14. Unger N. R., Hansch C.: Prog. Phys. Org. Chem. 1976, 12, 91.
15. Shorter J.: Correlation Analysis of Organic Reactivity, Chap. 4. Wiley, New York 1982.
16. Gallo R.: Prog. Phys. Org. Chem. 1983, 14, 115.
17. Exner O.: Correlation Analysis of Chemical Data. Plenum, New York 1988.
18. Charton M. in: Similarity Models in Organic Chemistry, Biochemistry and Related Fields (R. I.
Zalewski, T. M. Krygowski and J. Shorter, Eds), p. 629. Elesevier, Amsterdam 1991.
19. Taft R. W.: J. Am. Chem. Soc. 1950, 72, 4783.
20. Hancock C. K., Meyers E. A., Yager B. J.: J. Am. Chem. Soc. 1961, 83, 4211.
21. Charton M.: J. Am. Chem. Soc. 1975, 97, 1522.
22. Charton M.: J. Am. Chem. Soc. 1975, 97, 3691.
23. Komantsuzaki T., Akai I., Sakakibara K., Hirota M.: Tetrahedron 1992, 48, 1539.
24. Campbell T. W., Day B. F.: Chem. Rev. (Washington, D. C.) 1951, 48, 299.
25. Mehner J.: J. Prakt. Chem. 1901, 63, 241.
26. Suschitzky H., Price D.: Justus Liebigs Ann. Chem. 1976, 946.
27. Mair A. C., Stevens M. F. G.: J. Chem. Soc. 1970, 2317.
28. Sykora V., Zatka V.: Prirucni tabulky pro chemiky, p. 64. SNTL, Praha 1967.
29. Tockstein A.: Zaklady fyzikalni chemie, Vol. 2. (Texts for students of the Institute of Chemical
Technology), Pardubice 1988.
30. Wold S., Esbensen K., Geladi P.: Chemom. Intell. Lab. Syst. 1987, 2, 37.
31. Pytela O.: Collect. Czech. Chem. Commun. 1990, 55, 42.
32. Pytela O., Liska J.: Collect. Czech. Chem. Commun. 1994, 59, 2005.
33. Exner O.: Korelacni vztahy v organicke chemii. SNTL/ALFA, Praha 1981.
Collect. Czech. Chem. Commun. (Vol. 63) (1998)