Influence of ortho substituents on 17O NMR chemical shifts in phenyl esters of substituted benzoic acids

Article abstract of DOI:10.1002/poc.1784

¹⁷O NMR spectra for 29 phenyl esters of ortho-, para,- and meta-substituted benzoic acids, X-C₆H₄CO₂C ₆H₅, at natural abundance in acetonitrile were recorded. The δ(¹⁷O) values o

Full text of DOI:10.1002/poc.1784

Research Article
Received: 23 April 2010,
Revised: 13 July 2010,
Accepted: 15 July 2010,
Published online in Wiley Online Library: 5 October 2010
(wileyonlinelibrary.com) DOI 10.1002/poc.1784
17
Influence of ortho substituents on O NMR
chemical shifts in phenyl esters of substituted
benzoic acids
Vilve Nummerta, Vahur Ma¨emetsa, Mare Piirsalua and Ilmar A. Koppela
*
17O NMR spectra for 29 phenyl esters of ortho-, para,- and meta-substituted benzoic acids, X-C6H4CO2C6H5, at natural
abundance in acetonitrile were recorded. The d(17O) values of carbonyl and the single-bonded oxygens for para
derivatives gave good correlation with the sR constants. The d(17O) values for meta derivatives correlated well with
the sm constants. The influence of ortho substituents on the d(17O) values of carbonyl oxygen and the single-bonded
oxygens was analyzed using the Charton equation containing the inductive, sI, resonance, sRR, and steric, EsB,
substituent constants. For ortho derivatives, excellent correlations with the Charton equation were obtained when the
data treatment was performed separately for derivatives containing electron-donating RR and electron-attracting SR
substituents. The electron-donating substituents in ortho-, meta-, and para-substituted esters resulted in shielding of
the 17O signal and the electron-withdrawing groups caused deshielding. In phenyl ortho-substituted benzoates, the
substituent-induced positive inductive (rI > 0), resonance (rR > 0), and steric (dorthoEBs > 0) effects were found. The steric
interaction of ortho substituents with ester group was found to produce a deshielding effect on the carbonyl and
single-bonded oxygens. For ortho derivatives with SR substituents, the resonance term was insignificant and the
steric term was ca. twice weaker as compared to that for derivatives with RR substituents. The d(17O) values for
ortho-substituted nitrobenzenes, acetophenones, and benzoyl chlorides showed a good correlation with the Charton
equation as well. In ortho-substituted nitrobenzenes the inductive, resonance and steric effect were found to be ca. 1.7
times stronger as compared to that for phenyl ortho-substituted benzoates. Copyright ß 2010 John Wiley & Sons, Ltd.
Keywords: NMR spectra of phenyl benzoates; 17O NMR chemical shifts; ortho effect; correlation equations; substituent
effects
The d(17O) values of carbonyl and methoxy oxygens in meta- and
para-substituted methyl benzoates gave good correlations with
INTRODUCTION
In our previous papers, the influence of substituent effects,
especially ortho effect, on the rates of the alkaline hydrolysis in
various media (as shown in References[1–7]), the carbonyl carbon
sm and sþ constants, respectively. In the hindered benzoates,
2,6-dimethyl-4-X-benzoates, the through-conjugation was found
to be essentially reduced due to the increased torsion angle
between the carbonyl group and the ring plane.[13,14] It was
found that in methyl benzoates reaction series an introduction of
electron-attracting substituents causes deshielding and electro-
n-donating substituents shielding of the carbonyl and methoxy O
atoms.[14–16] It has been shown[26–29] that in substituted carbonyl
derivatives, Y-C6H4CO-X, RCO-X, the shielding of the carbonyl
oxygen is increased and the sensitivity of the Y substituent is
diminished with an increase in the electron-donating power of
the X group.
In ortho-substituted compounds the d(17O) data have been
quite extensively studied for nitrobenzenes,[34,35] acetophe-
nones,[36–38] benzaldehydes,[36] and benzoyl chlorides.[39,40] But
to the best of our knowledge, up to now the correlation
equations were not applied to describe the influence of ortho
substituents on the 17O NMR chemical shifts. In ortho-substituted
13C NMR chemical shifts, dCO [8] and the carbonyl carbon infrared
,
[9]
,
stretching frequencies, nCO
in substituted phenyl benzoates,
(X-C6H4CO2C6H5, C6H5CO2C6H4-X) were studied. In the present
paper, we extend our study of the ortho substituent effect to the
17O NMR chemical shifts, d(17O), in substituted phenyl benzoates,
containing substituents in benzoyl moiety (X-C6H3CO2C6H5).
To the best of our knowledge, no study of the ortho, meta, and
para substituent effects on the d(17O) values for phenyl esters of
substituted benzoic acids, X-C6H4CO2C6H5, could be found in the
literature. There are also no d(17O) data for the phenyl esters of
substituted benzoic acids, except unsubstituted phenyl benzoate,
[10,11]
H,
—
X
available in the literature. The values of d(17O) in the
—
ortho-substituted esters of substituted benzoic acids are available
only for methyl benzoates, X-C6H4CO2CH3, with X ¼OCH3, Cl, NO2,
CH3, and 2,6-disubstituted methyl benzoates, X1X2-C6H3CO2CH3,
[12–14]
—
—
with X1
X
2
CH , C(CH )
—
—
3
3 3.
Although there are numerous investigations on the d(17O)
values for esters in literature,[10–33] the influence of the substituent
effects on the d(17O) of the carbonyl and methoxy oxygens in esters
of substituted benzoic acid, using correlation equations, has been
studied only for meta- and para-substituted methyl benzoates
and para-substituted methyl 2,6-dimethyl-4-X-benzoates.[14–16]
*
Correspondence to: I. A. Koppel, Institute of Chemistry, University of Tartu,
Ravila, 14a, 50411, Tartu, Estonia
E-mail: ilmar.koppel@ut.ee
a
V. Nummert, V. Ma¨emets, M. Piirsalu, I. A. Koppel
Institute of Chemistry, University of Tartu, Ravila, Tartu, Estonia
J. Phys. Org. Chem. 2011, 24 539–552
Copyright ß 2010 John Wiley & Sons, Ltd.

V. NUMMERT ET AL.
derivatives, the steric influence of ortho substituents on the d(17O)
values has been explained by steric inhibition of resonance and
van der Waals interactions.[12–14,17,39,41–44] It was suggested that
in systems in which the functional group is free to rotate, due to
sterical consequences the functional group is rotated out of the
plane of the aromatic ring increasing the double bond character
of the functional group. In the rigid systems, the steric influence
of ortho substituents was explained by the van der Waals
interactions between the ester group and the adjacent bulky
substituent.[41,42]
4-F, 4-Cl, 4-Br, 4-CH3, 4-OCH3, 4-NH2, 4-C(CH3)3, 3-NO2, 3-CN, 3-Cl,
3-F, 3-CH3, 3-OCH3, 3-N(CH3)2, 2-NO2, 2-CN, 2-F, 2-Cl, 2-Br, 2-I,
2-CF3, 2-CH3, 2-OCH3, 2-N(CH3)2, 2-NH2, 2-SO2CH3, 2-SCF3), were
recorded at natural abundance in acetonitrile (Table 1). The
17O NMR spectra were recorded on a Bruker Avance II 200
spectrometer equipped with 10 mm BBO probe at the resonance
frequency 27.13 MHz. Benzoates were dissolved in the mixture of
1 ml CD3CN and 2 ml CH3CN. Acetonitrile-d3 ‘Special HOH’ 99.8
R
¨
atom % D, Aldrich and Acetonitrile SPECTRANAL , Riedel-de Haen
were used. Depending on the type of the benzoate, the con-
centration varied between values from 0.3 molal to 1 molal.
Temperature of the measurements was 50 8C. Water at 50 8C was
used as an external reference (17O chemical shift was taken
0 ppm). Depending on the sample, about 0.5 to 1.8 million scans
(measurement time up to 70 h) were obtained at the spectral
width 23 148 Hz and its size 2048 data points. The 10 ms (at the
ꢂ11 ms 908 pulse) exitation pulse with pre-scan delay 64 ms and
the relaxation delay 50 ms was applied. Acquisition time was 44
ms. The data were processed with Bruker TopSpin 2.0 software
package. During the data processing an exponential multipli-
cation of the FID with LB ¼ 40 was applied. For the correction of
the baseline rolling caused by acoustic ringing effect, a backward
linear prediction for the first 10 data points of the FID was also
applied during the data processing. The line width of the 17O
spectral lines of the phenyl esters measured was within the range
250–400 Hz and up to 50–60 Hz for the water standard. The error
of the chemical shifts measured was estimated on the basis of
repetitive measurements and was ꢃ ꢄ0.4 ppm.
In the previous papers, we found that in substituted phenyl
benzoates, (X-C6H4CO2C6H5, C6H5CO2C6H4-X) the influence of
ortho substituents on the log
k values in the alkaline
hydrolysis,[1–7] the carbonyl carbon 13C NMR chemical shifts,
[8]
,
dCO
and the carbonyl carbon infrared stretching frequencies,
was precisely described with the Charton equation[45]
[9]
nCO
,
using the inductive, sI,[8,46] resonance, s8R,[8,47] and steric, EBS,[6,48]
substituent constants. Nearly the same results were obtained
when for ortho substituents the Charton steric scale, y,[49] based
on van der Waals radii, was used. In the phenyl esters of ortho-
substituted benzoic acids (X-C6H4CO2C6H5), the contribution of
ortho inductive and ortho steric components were found to be
the dominant factors in the alkaline hydrolysis in water,[6] the
[1,9]
carbonyl carbon infrared stretching frequencies, nCO
,
for cis
[8]
:
conformers as well as in the 13C NMR chemical shifts, dCO
log korthoðH2O; 25 ꢀCÞ ¼ ꢁ0:333 þ 2:13sI þ 0:31sRꢀ þ 2:67EsB
R ¼ 0:992; s0 ¼ 0:126; n=n0 ¼ 11=11
The standard 1H and proton-decoupled 13C NMR spectra were
recorded in CDCl3 solution with 1% TMS added for the spectral
referencing at a room temperature and at 200.13 and 50.33 MHz,
correspondingly. 2D HSQC or HETCOR spectra were also recorded
(1)
ðnCOÞcis ¼ 1742:7 þ 16:4sIꢁ22:6EsB
R ¼ 0:993; s0 ¼ 0:117; n=n0 ¼ 9=9
(2)
(3)
1
for assignment of the H and 13C peaks.
ðdCOÞortho ¼ 165:32ꢁ5:08sI þ 1:58sꢀRꢁ4:33EsB
R ¼ 0:978; s0 ¼ 0:257; n=n0 ¼ 11=11
Synthesis of compounds
In reaction series considered (Eqns (1)–(2)) the resonance term
was negligible. In the 13C NMR chemical shifts, dCO [8] the value of
,
The preparation procedure and characteristics for the phenyl
esters of the ortho-, meta-, and para-substituted benzoic acids,
(X-C6H4CO2C6H5), has been described in Reference.[6] The phenyl
esters of 2-N(CH3)2-, 2-SO2CH3-, and 2-SCF3-substituted benzoic
acids were prepared by the addition of thionyl chloride to the
mixture of corresponding ortho-substituted benzoic acid and
phenol in pyridine at 0 8C with stirring.[50] Phenyl 2-N(CH3)2-
benzoate was recrystallized from aqueous 80% ethanol: yield
62%, m.p. 63–64 8C, Reference,[51] m.p. 67–698C. Phenyl 2-SO2CH3-
benzoate was recrystallized from aqueous 67% ethanol: yield
56%, m.p. 116–117 8C, Reference,[51] m.p. 108–110 8C. Phenyl
2-SCF3-benzoate was extracted from the reaction mixture
with diethyl ether. The ether layer was dried with unhydrous
magnesium sulfate. Diethyl ether was removed under the
reduced pressure. Phenyl 2-(trifluoromethylthio)benzoate: b.p.
145–147 8C/2.5 mbar, yield 40%. The phenyl esters of 3-CN-,
3-OCH3-, and 3-F-substituted benzoic acids were prepared
from corresponding substituted benzoyl chloride and phenol
in pyridine solution.[52] Phenyl 3-CN-benzoate was recrystallized
from aqueous ethanol: yield 27%, m.p. 95–968C, Reference, [53] m.p.
92–95 8C. Phenyl 3-OCH3-benzoate: yield 66%, m.p. 62–63 8C,
Reference,[53] m.p. 64–66 8C. Phenyl 3-F-benzoate: yield 64%, m.p.
55–56 8C, Reference,[54] m.p. 54–55 8C. Purity of synthesized
phenyl esters of 2-N(CH3)2-, 2-SO2CH3-, 2-SCF3-, 3-CN-, 3-OCH3-,
(rR)ortho has the opposite sign ((rR)ortho ¼ þ1.6) as compared to
that for para derivatives ((rR)para ¼ ꢁ1.00).
Recently we found[1,8] good correlations between the log k
values of the alkaline hydrolysis of ortho-substituted phenyl
benzoates and the infrared stretching frequencies of carbonyl
group, nCO, as well as the 13C NMR chemical shifts, dCO, in case the
additional resonance and steric scales were included.
The aim of the present paper was to check the applicability
of the Charton equation to describe the influence of ortho
substituents on the d(17O) values in phenyl esters of ortho-
substituted benzoic acids, (X-C6H4CO2C6H5), and to compare
the substituent effects in the d(17O) values with those in the alkaline
hydrolysis,[6] the infrared stretching frequencies, nCO [9] and the 13
, C
[8]
NMR chemical shifts, dCO
.
For the comparison, the d(17O) values for substituted nitroben-
zenes,[34,35] acetophenones,[36,38] and benzoyl chlorides,[39–40]
published in literature were correlated with the Charton equation
as well.
EXPERIMENTAL
NMR measurements
17O NMR spectra for 29 phenyl esters of ortho-, meta- and
and 3-F-substituted benzoic acids was confirmed by H and 13C
1
—
para-substituted benzoic acids, X-C6H4CO2C6H5 (X H, 4-NO ,
NMR spectroscopy in CDCl3 at 25 8C.
—
2
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 539–552

INFLUENCE OF ORTHO SUBSTITUENTS IN PHENYL ESTERS
Table 1. 17O NMR chemical shifts, d(17O), of the carbonyl oxygen and the single-bonded oxygen for phenyl esters of ortho-, para-
and meta-substituted benzoic acids, X-C6H4CO2C6H5, in CH3CN at 50 8C
Substituent
d(17O)C¼O
d(17O)OPh
Other functions
600.1
cestera(m)
d(13CO)
2-NO2
2-CN
2-F
2-Cl
2-Br
368.4
358.3
362.4
370.2
372.4
368.8
375.1
375.4
373.9
366.9
364.6
364.2
340.6
345.7
345.8
351.9
344.4
346.7
345.4
343.2
342.7
336.7
333.5
344.1
350.8
349.9
350.6
348.1
349.0
346.5
346.7
344.4
196.3
191.0
195.6
197.7
199.1
197.1
199.4
201.5
197.4
197.8
195.9
197.8
188.7
187.3
187.5
188.3
187.0
187.5
187.1
186.5
186.8
184.9
183.8
187.0
187.6
187.7
188.2
186.9
188.0
188.2
187.8
186.9
2-I
2-CF3
2-SO2CH3
2-SCF3
2-OCH3
2-CH3
2-N(CH3)2
2-NH2
H
161.1
54.0
165.75
164.74
166.27
165.17b
0.4
0.4
0.5
0.5
0.5
4-NO2
4-F
4-Cl
4-Br
4-CH3
574.6
60.8
0.3
4-OCH3
4-NH2
4-C(CH3)3
3-NO2
0.3
0.5
0.5
576.8
574.6
3-CN
3-F
3-Cl
3-CH3
3-OCH3
3-N(CH3)2
0.5
163.21
164.00
51.9
165.00
a Approximately 1 molal solutions of ortho, meta and para derivatives in mixture of CH3CN and CD3CN (2 ml þ 1 ml) were used, except
where shown otherwise.
b Reference.[2]
Phenyl 2-(methylsulfonyl)benzoate
(C-4); 131.65 (C-5); 130.05 (C-6); 165.75 (C-7); 150.59 (C-8); 121.56
(C-9.13); 129.69 (C-10,12); 126.50 (C-11); 45.01 (C-14).
Phenyl 2-(trifluoromethylthio)benzoate
1H NMR: 7.91 m, 1H (H-3); 7.68–7.82 m, 2H (H-4,5); 8.18 m, 1H
(H-6); 7.25-7.5 m, 3H (H-9,11,13); 7.45 m, 2H (H-10,12); 3.34 s, 3H
(H-14). 13C NMR: 132.67 (C-1); 139.60, (C-x2); 129.82 (C-3); 133.64
1H NMR: 7.76 d, 1H (H-3), 3J ¼ 7.8; 7.35–7.56 m, 4H (H-4,5,10,12);
3
4
8.09 dd, 1H (H-6), J ¼ 7.4, J ¼ 2.0; 7.19–7.27 m, 3H (H-9,11,13).
J. Phys. Org. Chem. 2011, 24 539–552
Copyright ß 2010 John Wiley & Sons, Ltd.
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V. NUMMERT ET AL.
13C NMR: 128.46 (C-1); 133.0 (C-2); 133.72 (C-3); 132.81 (C-4);
129.22 (C-5); 131.43 (C-6); 164.74 (C-7); 150.84 (C-8); 121.59
(C-9,13); 129.65 (C-10,12); 126.30 (C-11); 129.77 (C-14),
1JC-F ¼ 310.0.
Phenyl 3-fluorobenzoate
Phenyl 2-(dimethylamino)benzoate
3
1H NMR: 7.87 dddd, 1H (H-2), JF-H ¼ 9.2, 4J ¼1.6 and 2.7,
5J ¼ 0.4; 7.16–7.52 m, 7H (H-4,5,9-13); 7.99 ddd, 1H (H-6), 3J ¼ 7.7,
4J ¼ 1.6 and 1.1.
3
2
13C NMR: 132.06 (C-1), JF-C ¼ 7.7; 117.07 (C-2), JF-C ¼ 23.3;
1
2
162.79 (C-3), JF-C ¼ 247.4; 120.62 (C-4), JF-C ¼ 21.5; 130.27 (C-5),
3JF-C ¼ 7.9; 125.94 (C-6), 4JF-C ¼ 3.3; 164.00 (C-7), 4JF-C ¼ 3.1; 151.02
(C-8); 121.62 (C-9,13); 129.56 (C-10,12); 126.07 (C-11).
1H NMR: 6.99 d, 1H (H-3), 3J ¼ 8.4; 7.37–7.45 m, 3H (H-4,10,12);
3
6.89 m, 1H (H-5), Jav ¼ 7.8; 7.94 dd, 1H (H-6), 3J ¼ 7.8, 4J ¼ 1.7;
7.20–7.27 m, 3H (H-9,11,13); 2.92 s, 6H (H-14). 13C NMR: 119.80
(C-1); 153.16 (C-2); 117.05 (C-3); 132.92 (C-4); 118.65 (C-5); 132.19
(C-6); 166.27 (C-7); 151.30 (C-8); 121.72 (C-9,1sI sI 3); 129.40
(C-10,12); 125.60 (C-11), 43.78 (C-14).
DATA PROCESSING AND RESULTS
The d(17O) values for the carbonyl and single-bonded oxygens in
the phenyl esters of para- and meta-substituted benzoic acids,
X-C6H4CO2C6H5, given in Table 1 were correlated with Eqns (4)
and (5). For meta substituents sm, sI, and s8R were used.
Phenyl 3-methoxybenzoate
17
17
dð OÞparaðmetaÞ ¼ dð OÞH þ ðrþÞparaðmetaÞsþðsmÞ
(4)
17
dð OÞparaðmetaÞ
¼ dð OÞH þ ðrIÞparaðmetaÞsI þ ðrþR ÞparaðmetaÞsRþðsRÞ
ꢀ
17
(5)
In the case of ortho-substituted derivatives, the influence of the
inductive, resonance, and steric factors on the d(17O) values was
separated using the following Charton equations[45]
:
17
17
dð OÞortho ¼ dð OÞH þ ðrIÞorthosI þ ðrþR ÞorthosþR þ dorthoEsB (6)
1H NMR: 7.70 dd, 1H (H-2), J ¼ 2.6 and 1.6; 7.13–7.30 m, 4H
4
17
17
dð OÞortho ¼ dð OÞH þ ðrIÞorthosI þ ðrþR ÞorthosRþ þ dortho
y
(7)
(H-4,9,11,13); 7.36–7.47 m, 3H (H-5,10,12); 7.81 ddd, 1H (H-6),
4
3J ¼ 7.6, J ¼ 1.6 and 1.1; 3.86 s, 3H (H-14).
The correlation of the d(17O) values with Eqns (6) and (7) was
performed separately for ortho-substituted derivatives contain-
ing the ortho electron-donating þR substituents (X ¼OCH3, CH3,
N(CH3)2, F, Cl, Br, H) and derivatives with the ortho electro-
n-withdrawing ꢁR substituents (X ¼ NO2, CN, CF3, SCF3, SO2CH3,
H). The Brown and Okamoto constants, sþ,[55,56] the polar
substituent constants, sm,[56] the Taft inductive sI,[8,46] resonance
sþ constants (sþ ¼ sþpꢁsI), and s8R (s8R ¼ s8pꢁsI)[8,47] were
13C NMR: 131.14 (C-1); 114.80 (C-2); 159.92 (C-3); 120.14 (C-4);
129.61 (C-5); 122.64 (C-6); 165.00 (C-7); 151.22 (C-8); 121.73
(C-9,13); 129.48 (C-10,12); 125.86 (C-11), 55.54 (C-14).
Phenyl 3-cyanobenzoate
R
R
used in the data processing. As the steric constants for ortho
substituents, two steric scales were employed: the EBs constants
and the Charton scale of y.[49,57,58] The steric constants, EBs ,
determined on the basis of the acid hydrolysis of ortho-sub-
stitued phenyl benzoates, C6H5CO2C6H4-X,[6,48] in the case of
substituents X ¼ H, F, Cl, Br, I, CH3, C(CH3)3, CF3, were found to be
nearly linear function of the y values, calculated on the bases of
van der Waals radii, rv.
The results of correlations of the carbonyl oxygen and the
single-bonded oxygen d(17O) values, for phenyl esters of para-,
meta-, and ortho-substituted benzoic acids, X-C6H4CO2C6H5, with
Eqns (4)–(7) are listed in Table 2. For comparison, the correlation
of the d(17O) values published in literature for ortho-, para-, and
meta-substituted nitrobenzenes,[17,34,35,59,60] X-C6H4NO2, aceto-
4
5
1H NMR: 8.50 dt, 1H (H-2), Javer ¼ 1.7, J ¼ 0.7; 7.92 ddd, 1H
(H-4), 3J ¼ 7.8, 4J ¼ 1.7 and 1.3; 7.67 dt, 1H (H-5), Javer ¼ 7.9,
3
5J ¼ 0.7; 8.43 ddd, 1H (H-6), 3J ¼ 7.9, 4J ¼ 1.3 and 1.7, 7.18–7.35 m,
3
3H (H-9,11,13); 7.46 m, 2H (H-10,12), Javer ¼ 7.9.
13C NMR: 131.28 (C-1); 133.79 (C-2), 113.53 (C-3), 136.50 (C-4),
129.69 (C-5), 134.11 (C-6), 163.21 (C-7), 150.77 (C-8), 121.47
(C-9,13), 129.67 (C-10,12), 126.35 (C-11), 117.67 (C-14).
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 539–552

INFLUENCE OF ORTHO SUBSTITUENTS IN PHENYL ESTERS
Table 2. Correlation of the 17O NMR chemical shifts, d(17O), of the carbonyl oxygen and the single-bonded oxygen (OPh) with Eqns
(4)–(7)
d
Scales
d(17O)H
rI or rþ(r)
rþR(r8R)
dortho
Ra
(R20)b
sc
n/n0
Phenyl esters of ortho-substituted benzoic acids, X-C6H4CO2C6H5
þR substituents included, acetonitrile at 50 8C
C ¼ 17O
346.2 ꢄ 1.8
347.4 ꢄ 3.5
17.3 ꢄ 1.8
14.3 ꢄ 4.3
9.68 ꢄ 1.13
4.62 ꢄ 2.08
ꢁ81.2 ꢄ 5.2
33.2 ꢄ 5.0
0.993
0.961
0.182
0.182
1.06
2.41
7/7e
sI, sþR, EBs
sI, sþR, y
7/7e,f
ꢁR substituents included, acetonitrile at 50 8C
346.9 ꢄ 3.1
346.5 ꢄ 2.0
16.0 ꢄ 5.1
0
ꢁ32.6 ꢄ 4.0
0.979
0.991
0.304
0.304
2.40
1.61
6/6g
6/6f
sI, sþR, EBs
sI, y
18.1 ꢄ 1.3
0
15.9 ꢄ 1.3
17OC6H5
þR substituents included, acetonitrile at 50 8C
187.7 ꢄ 1.1
188.2 ꢄ 1.1
7.12 ꢄ 1.13
6.04 ꢄ 1.85
2.48 ꢄ 0.70
ꢁ33.5 ꢄ 3.2
13.0 ꢄ 1.7
0.986
0.963
0.100
0.100
0.648
1.06
7/7e
7/7e,f
sI, sþR, EBs
sI, sþR, y
0
ꢁR substituents included, acetonitrile 50 8C
187.3 ꢄ 1.2
187.1 ꢄ 1.7
5.52 ꢄ 1.90
6.76 ꢄ 2.80
—
—
ꢁ15.5 ꢄ 1.5
7.35 ꢄ 1.10
0.986
0.967
0.225
0.225
0.895
1.35
6/6g,h
6/6f
sI, EBs
sI, y
ortho-substituted nitrobenzenes, X-C6H4NO2
þR substituents included, acetonitrile at 75 8C
N(17O)2
578.1 ꢄ 3.5
26.8 ꢄ 5.2
11.7 ꢄ 2.4
ꢁ108.4 ꢄ 7.8
0.975
ꢁ0.072
3.47
3.12
11/12e,i,j,k
6/6l
sI, sþR, EBs
sI, EBs
ꢁR substituents included, acetonitrile
573.7 ꢄ 3.5
20.2 ꢄ 6.3 ꢁ58.4 ꢄ 6.0
0
0.981
0.072
ortho-substituted benzoyl chlorides, X-C6H4COCl
þR substituents included, acetonitrile at 75 8C
C ¼ 17O
486.0 ꢄ 2.2
35.0 ꢄ 2.9
16.3 ꢄ 2.5
ꢁ93.3 ꢄ 4.9
0.994
ꢁ0.188
ꢁ0.187
1.38
1.68
6/6k,m,n
sI, sþR, EBs
ortho-substituted acetophenones, X-C6H4COCH3
þR substituents included, dioxane at 30 8C
C ¼ 17O
550.6 ꢄ 2.3
26.2 ꢄ 2.9
40.8 ꢄ 2.7
ꢁ148.0 ꢄ 5.4
0.996
7/7k,o,p
sI, sþR, EBs
Phenyl esters of para-substituted benzoic acids, X-C6H4CO2C6H5
C ¼ 17O
Acetonitrile at 50 8C
sþ
345.2 ꢄ 0.3
345.9 ꢄ 0.5
17OC6H5
9.07 ꢄ 0.55
7.21 ꢄ 0.99
—
9.71 ꢄ 0.54
—
—
0.986
0.990
—
0.106
0.918
0.752
9/9
9/9
p
sI, sþ
R
sþ
187.1 ꢄ 0.2
2.26 ꢄ 0.26
1.26 ꢄ 0.41
—
2.60 ꢄ 0.22
—
—
0.952
0.976
—
0.004
0.428
0.307
9/9
9/9
p
sI, sþ
187.5 ꢄ 0.2
R
Methyl esters of para-substituted benzoic acids, X-C6H4CO2CH3
C ¼ 17O
Acetonitrile at 40 8C
sþ
337.8 ꢄ 0.2
337.6 ꢄ 0.5
17OCH3
9.66 ꢄ 0.29
10.0 ꢄ 1.1
—
9.56 ꢄ 0.41
—
—
0.990
0.990
—
0.437
0.970
0.991
23/23q
23/23
p
sI, sþ
R
sþ
127.8 ꢄ 0.2
127.4 ꢄ 0.5
C ¼ 17O
3.72 ꢄ 0.26
4.64 ꢄ 0.97
—
3.47 ꢄ 0.37
—
—
0.949
0.948
—
0.443
0.884
0.884
23/23
23/23
p
sI, sþ
R
Acetonitrile-d3 at 40 8C
sþ
335.8 ꢄ 0.4
336.8 ꢄ 0.6
17OCH3
11.5 ꢄ 0.6
—
12.2 ꢄ 0.6
—
—
0.990
0.994
—
0.086
1.12
0.885
8/8r
8/8
p
sI, sþ
8.72 ꢄ 1.38
R
sþ
125.8 ꢄ 0.3
126.2 ꢄ 0.7
C ¼ 17O
3.85 ꢄ 0.54
—
4.14 ꢄ 0.67
—
—
0.935
0.931
—
0.044
0.969
1.00
8/8
8/8
p
sI, sþ
2.70 ꢄ 1.56
R
Acetone at 40 8C
9.40 ꢄ 0.43
sþ
341.7 ꢄ 0.2
341.5 ꢄ 0.5
17OCH3
—
9.19 ꢄ 0.59
—
—
0.993
0.992
—
0.317
0.616
0.654
8/8s
8/8
p
sI, sþ
9.89 ꢄ 0.97
R
sþ
128.4 ꢄ 0.2
3.90 ꢄ 0.34
5.07 ꢄ 0.53
—
3.39 ꢄ 0.32
—
—
0.974
0.986
—
0.464
0.491
0.359
8/8
8/8
p
sI, sþ
127.9 ꢄ 0.3
R
(Continues)
J. Phys. Org. Chem. 2011, 24 539–552
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

V. NUMMERT ET AL.
Table 2. (Continued)
rI or rþ(r)
rþR(r8R)
dortho
Ra
(R20)b
sc
n/n0
d
Scales
d(17O)H
para-substituted nitrobenzenes, X-C6H4NO2
Acetonitrile at 75 8C
N(17O)2
sþ
573.3 ꢄ 0.7
14.9 ꢄ 0.8
—
15.3 ꢄ 1.0
—
—
0.980
0.979
—
0.158
2.57
2.61
14/14t
14/14
p
sI, sþ
574.3 ꢄ 1.6
12.1 ꢄ 3.6
R
para-substituted benzoyl chlorides, X-C6H4COCl
C ¼ 17O
Acetonitrile at 75 8C
sþ
sI, sþ
485.5 ꢄ 0.1
485.6 ꢄ 0.5
21.2 ꢄ 0.3
—
21.4 ꢄ 0.3
—
—
0.999
0.999
—
0.239
0.485
0.502
11/11m
11/11
p
20.9 ꢄ 0.6
R
R
C ¼ 17O
Tetrachloromethane at 40 8C
sþ
sI, sþ
490.8 ꢄ 0.3
491.3 ꢄ 0.5
21.0 ꢄ 0.5
—
21.5 ꢄ 0.7
—
—
0.998
0.998
—
0.365
0.706
0.696
8/8u
8/8
p
20.0 ꢄ 1.1
para-substituted acetophenones, X-C6H4COCH3
C ¼ 17O
Acetonitrile at 60 8C
sþ
sI, sþ
548.3 ꢄ 0.9
549.1 ꢄ 1.9
C ¼ 17O
22.8 ꢄ 1.4
—
23.6 ꢄ 1.8
—
—
0.986
0.985
—
0.230
2.65
2.75
9/9v
9/9
p
20.3 ꢄ 4.0
R
R
R
Dioxane at 30 8C
24.1 ꢄ 1.9
sþ
sI, sþ
547.9 ꢄ 1.2
550.1 ꢄ 2.7
C ¼ 17O
—
25.8 ꢄ 2.5
—
—
0.976
0.976
—
0.259
3.52
3.53
9/9o
9/9
p
19.4 ꢄ 5.1
Deuterochloroform at 25 8C
sþ
sI, sþ
542.6 ꢄ 0.4
542.3 ꢄ 0.8
24.9 ꢄ 0.5
—
24.7 ꢄ 0.6
—
—
0.998
0.998
—
0.336
1.25
1.30
12/12w
12/12
p
25.7 ꢄ 1.8
Phenyl esters of meta-substituted benzoic acids, X-C6H4CO2C6H5
C ¼ 17O
Acetonitrile at 50 8C
sm
sI, s0
346.1 ꢄ 0.3
346.5 ꢄ 0.4
17OC6H5
7.17 ꢄ 0.68
6.16 ꢄ 0.86
—
3.87 ꢄ 0.90
—
—
0.969
0.967
—
0.735
0.564
0.584
8/8
8/8
R
sm
sI, s0
187.4 ꢄ 0.1
1.71 ꢄ 0.33
1.82 ꢄ 0.63
—
0
—
—
0.932
0.783
—
—
0.187
0.309
5/5x
5/5y
187.2 ꢄ 0.2
R
Methyl esters of meta-substituted benzoic acids, X-C6H4CO2CH3
C ¼ 17O
Acetonitrile at 40 8C
sm
sI, s0
337.5 ꢄ 0.2
337.1 ꢄ 0.2
17OCH3
5.53 ꢄ 0.41
6.86 ꢄ 0.52
—
1.93 ꢄ 0.46
—
—
0.945
0.966
—
0.880
0.603
0.481
23/23q
23/23
R
R
sm
sI, s0
127.3 ꢄ 0.1
3.20 ꢄ 0.29
3.79 ꢄ 0.28
—
0
—
—
0.923
0.955
—
—
0.430
0.245
22/23q,z
0
127.0 ꢄ 0.1
19/23a
meta-substituted nitrobenzenes, X-C6H4NO2
N(17O)2
Acetonitrile at 75 8C
sm
sI, s0
575.7 ꢄ 0.4
5.28 ꢄ 0.82
—
0
—
—
0.904
0.960
—
—
0.778
0.512
10/10t
10/10
575.0 ꢄ 0.3
6.78 ꢄ 0.66
R
R
meta-substituted acetophenones, X-C6H4COCH3
C ¼ 17O
Dioxane at 30 8C
sm
sI, s0
549.8 ꢄ 0.6
548.6 ꢄ 0.9
10.6 ꢄ 1.8
—
0
—
—
0.931
0.910
—
—
1.34
1.52
6/6o
6/6
12.8 ꢄ 2.5
a R – correlation coefficient.
b R0 – zeroth correlation coefficient.
c s – standard deviation.
d n0 reflects the total number of data involved in the correlation; n – the number of points remaining after exclusion of significantly
deviating points.
e The d(17O) values for 2-NH2 and 2-I derivatives were omitted.
f Charton’s modified steric constants, y,[6,8,49] were used.
g For 2-SCF3 substituent, the isosteric constant EsB ¼ ꢁ0.582 was calculated with relation: y ¼ (0.026 ꢄ 0.049)ꢁ(2.05 ꢄ 0.15)EBs ,
(R ¼ 0.976, n ¼ 10) using steric constant, y ¼ 1.22[57] for OC(CH3)3 substituent.
h For 2-SO2CH3 substituent, the steric constant EBs ¼ ꢁ0.667 was calculated from relation[6] Dlog k ¼ 0.021 þ 2.284sI þ 0.862s0R þ
2.477EBs , using Dlog k ¼ ꢁ0.309 for alkaline hydrolysis of phenyl ester of 2-(methylsulfonyl)benzoic acid in aqueous 50% DMSO.[50]
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INFLUENCE OF ORTHO SUBSTITUENTS IN PHENYL ESTERS
i Data from References[34,35] were used.
j The 2-SCH3 derivative (EBs ¼ ꢁ0.366 was calculated using Dlog k ¼ ꢁ0.522 (Note h)), was excluded at confidence level t ¼ 0.99.
k For 2,6-(CH3)2 derivative, the duple values of inductive, resonance, and the steric constants were used.
l For 2-CO2CH3 substituent, the isosteric constant EBs ¼ ꢁ0.485 was calculated using y ¼ 1.02[58] for CH(CH3)CH2CH3 substituent (Note
g) and for 2-CONH2 substituent, the steric constants for 2-CH(CH3)2 substituent EBs ¼ ꢁ0.341 was used.
m Data from Reference[39] were used.
n Data from Reference.[40]
o Data from Reference.[36]
p The d(17O) ¼ 604 for 2,6-(CH3)2-acetophenone was calculated using DdX value (DdX ¼ dXꢁdH ¼ 54).[38]
q Data from Reference.[15]
r Data from Reference.[14]
s Data from Reference
[16]
t Data from References.[17,59,60]
u Data from Reference.[27]
v Data from Reference.[61]
w Data from Reference.[37]
x The d(17O) values for 3-NO2, 3-CH3 and 3-F derivatives were omitted. When these values were included, no correlation was found
(R2 ¼ ꢁ0.064).
y The d(17O) values for 3-NO2, 3-CH3 and 3-F derivatives were omitted. When these values were included, no correlation was found
(R2 ¼ 0.054).
z The 3-SO2NH2 derivative was excluded.
a0
The 3-SO2NH2, 3-SO2CH3, 3-SO2Cl, and 3-CH2Cl derivatives were excluded.
phenones,[36–38,61] X-C6H4COCH3, and benzoyl chlorides,[27,39,40]
X-C6H4COCl, with Eqns (4)–(6) are shown in Table 2 as well.
To compare the substituent effects on the d(17O) values of the
carbonyl oxygen and the single-bonded oxygen for phenyl
esters of ortho-, para-, and meta-substituted benzoic acids,
X-C6H4CO2C6H5, with those in the kinetic data of the alkaline
hydrolysis, log k, the carbonyl carbon 13C NMR chemical shifts,
dCO, and the infrared stretching frequencies of the carbonyl
group, nCO, in substituted phenyl benzoates, X-C6H4CO2C6H5, the
following relationships were used:
alkaline hydrolysis in water and aqueous 50% DMSO, the carbonyl
carbon 13C NMR chemical shifts, dCO, and the IR stretching
frequencies of the carbonyl group, nCO, for phenyl benzoates,
X-C6H4CO2C6H5, used in correlations with Eqns (8)–(13) are given
in References.[6,8,9,51,52] In the case of phenyl esters of
ortho-substituted benzoic acids, the nCO values for cis conformers
estimated by the carbonyl stretching frequencies deconvolution
were used. The results of data treatment with Eqns (8)–(13) are
listed in Table 3. For the data processing, a multiple-parameter
linear least-squares (LLSQ) procedure[62] was used.
17
17
dð OÞortho ¼ dð OÞH þ a1ðorthoÞDlog kX þ a2ðorthoÞsRþ
DISCUSSION
þ a3ðorthoÞEsB
(8)
(9)
The d(17O) values in phenyl esters of para-substituted benzoic
acids, X-C6H4CO2C6H5, (Table 1) gave excellent correlations,
so with the single parameter treatment using the Brown and
Okamoto sþ constants[55,56] (Eqn (4)) as the dual substituent
17
17
dð OÞortho ¼ dð OÞH þ a1ðorthoÞðDdCOÞX þ a2ðorthoÞsRþ
þ a3ðorthoÞEsB
17
17
dð OÞortho ¼ dð OÞH þ a1ðorthoÞðDnCOÞX þ a2ðorthoÞsRþ
parameter treatment with the Taft sI[8,46] and sþR (sþ ¼ sþꢁsI)
R
constants (Eqn (5), Table 2):
þ a3ðorthoÞEsB
(10)
(11)
17
dð OÞpara ¼ ð345:2 ꢄ 0:3Þ þ ð9:07 ꢄ 0:55Þsþ
17
dð OÞparaðmetaÞ
¼ dð OÞH þ a1paraðmetaÞDlog kX þ a2paraðmetaÞsþR ðsRÞ
(14)
R ¼ 0:986; s ¼ 0:918; n=n0 ¼ 9=9
ꢀ
17
17
dð OÞpara ¼ ð345:9 ꢄ 0:5Þ þ ð7:21 ꢄ 0:99ÞsI þ ð9:71 ꢄ 0:54ÞsRþ
17
dð OÞparaðmetaÞ
R ¼ 0:990; s ¼ 0:752; n=n0 ¼ 9=9
ꢀ
(15)
17
¼ dð OÞH þ a1ðparaÞðmetaÞðDdCOÞX þ a2ðparaÞðmetaÞsþR ðsRÞ (12)
In para-substituted phenyl benzoates, the calculated sensitivity
of the d(17O) values of the carbonyl oxygen towards the inductive
effect and through-conjugation shown by Eqns (14) and (15)
(Table 2) were approximately the same as earlier were found for
methyl benzoates in References[15,16] (rþ ¼ 9.76,[15] 9.03,[16] 9.66
(Table 2)). The excellent correlation of the d(17O) values obtained
17
dð OÞparaðmetaÞ
ꢀ
17
¼ dð OÞH þ a1ðparaÞðmetaÞðDnCOÞX þ a2ðparaÞðmetaÞsþR ðsRÞ (13)
In Eqns (8)–(13), Dlog kX ¼ log kXꢁlog kH, (DdCO)X ¼ (dCO)Xꢁ
(dCO)H, and (DnCO)X ¼ (nCO)Xꢁ(dCO)H. The log k values for the
J. Phys. Org. Chem. 2011, 24 539–552
Copyright ß 2010 John Wiley & Sons, Ltd.
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V. NUMMERT ET AL.
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 539–552

INFLUENCE OF ORTHO SUBSTITUENTS IN PHENYL ESTERS
J. Phys. Org. Chem. 2011, 24 539–552
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

V. NUMMERT ET AL.
with sþ, sI, and sþ constants (Eqns (14) and (15)) suggests
shifts could be expressed as follows:
R
that the d(17O) values for phenyl esters of para-substituted
benzoic acids are beside the inductive effect dependent on
the through-conjugation between substituent and the carbonyl
oxygen similar to that for para-substituted methyl benzo-
ates,[14–16] acetophenones,[36,37,61] benzoyl chlorides,[27,39,40]
and nitrobenzenes.[35,59,60] The d(17O) values of the single-
bonded phenoxy oxygen show good correlations with Eqns (4)
and (5) as well. The influence of para substituents from
the benzoyl part of esters on the d(17O) of the single-bonded
phenoxy oxygen was ca. one-half the size of substituent effects in
the d(17O) of carbonyl oxygen (Table 2) which is similar to that in
References.[14–16] The chemical shifts of carbonyl oxygen for meta
derivatives correlated well with the single parameter treatment
using the Taft sm constants as the dual substituent parameter
treatment with the Taft sI and s8R constants (Table 2).
ðd17OÞortho ¼ ð346:2 ꢄ 1:8Þ þ ð17:3 ꢄ 1:8ÞsI
þ ð9:68 ꢄ 1:13ÞsþR ꢁð81:2 ꢄ 5:2ÞEsB
R ¼ 0:993; s ¼ 1:06; n=n0 ¼ 7=7
(16)
Equation (16) was obtained in case the more significantly
deviating (d17O) values for 2-NH2 and 2-I derivatives were
excluded before data treatment. In the case of 2-NH2 ester
the deviation was attributed to the intramolecular hydrogen-
bonding shielding effect.[9,17] Thus, in the ortho-substituted
esters the carbonyl oxygen is deshielded by ꢁI and steric effects
and shielded by þR substituent effects.
For phenyl esters of ortho-substituted benzoic acids containing
electron-withdrawing ꢁR substituents (X ¼ H, NO2, CN, CF3,
SO2CH3, SCF3) we found:
In meta-substituted phenyl benzoates the calculated sensitivity
of the d(17O) values of carbonyl oxygen towards the sm constants
was nearly the same as earlier found for methyl benzoates in
Reference[15] (rm ¼ 6.44,[15] 7.17 Table 2). In the case of meta
derivatives, the d(17O) values for single-bonded phenoxy oxygen
showed very low sensitivity towards the substituent effects
in benzoyl moiety (rm ¼ 1.71 Table 2). In case, the d(17O) values
of single-bonded phenoxy oxygen for meta derivatives were
correlated with the Taft sI and s8R constants; the resonance term
was excluded as insignificant.
ðd17OÞortho ¼ ð346:9 ꢄ 3:1Þ þ ð16:0 ꢄ 5:1ÞsIꢁð32:6 ꢄ 4:0ÞEsB
R ¼ 0:979; s ¼ 2:40; n=n0 ¼ 6=6
(17)
The data treatment with Eqns (6) and (7) using two steric scales
for ortho substituents, EBs and the modified Charton y constants,
gave nearly identical results (Table 2). When the Charton steric y
constants were used, the susceptibility to the steric effect, dortho
,
The positive values of rþ, rI, and rþ calculated using the
was positive and approximately twice smaller as compared to the
dortho value obtained using the EBs scale. We found the influence of
ortho substituents to the d(17O) values in phenyl benzoates
containing substituents in benzoyl moiety to be influenced by
the positive inductive effect approximately by the same
extent for both þR substituents ((rI)ortho ¼ 17.3, Eqn (16)) and
ꢁR substituents ((rI)ortho ¼ 16.0, Eqn (17)). The influence of the
ortho inductive effect was found to be approximately twice as
strong as the corresponding influence from the para position
(Eqns (14) and (15)). For phenyl esters of substituted benzoic
acids, X-C6H4CO2C6H5, nearly the same ratio (rI)ortho/(rI)para ꢅ 2
was observed in the carbonyl carbon 13C NMR chemical shifts,
R
d(17O) values for carbonyl oxygen and single-bonded phenoxy
oxygen in phenyl esters of substituted benzoic acids,
X-C6H4CO2C6H5, (Table 2) prove that the electron-withdrawing
substituents cause high-frequency shift of the 17O signal showing
deshielding effect of the O atom. The electron-donating sub-
stituents have an opposite effect resulting in low-frequency shift
and shielding of the O atom. This is consistent with the increased
double bond character of the carbonyl group produced by an
electron-withdrawing groups and an increase in single bond
character resulting from the electron-donating substitu-
ents.[26,36,63–65] An increase in the negative charge on the carbonyl
oxygen indirectly causes the oxygen lone pair to be less tightly
bound. Earlier[8,9] the increased double bond character of the
carbonyl group produced by an electron-withdrawing groups
in phenyl esters of substituted benzoic acids was proved by the
carbonyl carbon 13C NMR chemical upfield shifts, dCO, and the
dCO, .
[8] and infrared stretching frequencies, nCO [9] For the alkaline
hydrolysis of substituted phenyl benzoates in water for both
series containing the substituents in benzoyl and in the phenyl
part of benzoates, (rI)ortho/(rI)para was approximately 1.5.[6]
In the case of ortho derivatives with þR substituents, the
contribution of the through-conjugation between substituent
and the carbonyl oxygen was observed by the same extent as
that for para substituents. The sensitivity of the d(17O) toward the
through-conjugation in the case of ortho þR substituents was
9.68 (Eqn (16), Table 3). The same value for para-substituted
derivatives was 9.71 (Eqn (16)). In the case of esters with ꢁR
substituents the resonance factor was excluded as insignificant.
Results of correlations in Eqns (16) and (17), and Table 2 show
that due to the steric positive requirements of ortho substituents,
the d(17O) values increase (high-frequency shift of the 17O signal)
showing deshielding effect of the O atom in the case of both the
ortho þR substituents and ortho ꢁR substituents. We found that
the steric requirement in the case of ortho þR substituents was
approximately twice as strong as in the case of ꢁR substituents.
In the case of the single-bonded phenoxy oxygen, similar to
the inductive and resonance terms, the influence of the steric
term appeared to be one-half the size of the steric term for
carbonyl oxygen (Table 2). The influence of ortho substituents to
the single-bonded d(17O) values in the case of þR substituents
increased infrared stretching frequencies of the carbonyl group, nCO
.
We determined the d(17O) values for thirteen phenyl esters of
ortho-substituted benzoic acids, X-C6H4CO2C6H5, (Table 1). All
measured ortho-substituted benzoates, except X¼ 2-NH2, showed
the high-frequency shift of the 17O signal. It could be mentioned
that the substituent-induced 17O NMR chemical shifts, Dd(17O)¼
d(17O)Xꢁd(17O)H, for phenyl esters of substituted benzoic acids
for X ¼OCH3, Cl, NO2 determined in the present work (Table 1)
were essentially larger (Dd(17O)OCH3 ¼ 21.2, Dd(17O)Cl ¼ 21.2,
Dd(17O)NO2 ¼ 22.7) as compared with those given earlier
in Reference[12] for ortho-substituted methyl benzoates
(Dd(17O)OCH3 ¼ ꢁ1.2, Dd(17O)Cl ¼ 2.9, Dd(17O)NO2 ¼ ꢁ1.7).
The d(17O) values for phenyl esters of ortho-substituted benzoic
acids, X-C6H4CO2C6H5, listed in Table 1 showed excellent corre-
lations with the Charton equation (Eqns (6) and (7)) in case
the data treatment was carried out separately for derivatives with
þR substituents and with ꢁR substituents. In the case of þR
substituents (X ¼ H, 2-F, 2-Cl, 2-Br, 2-CH3, 2-OCH3, 2-N(CH3)2) the
influence of ortho substituents to the carbonyl oxygen chemical
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 539–552

INFLUENCE OF ORTHO SUBSTITUENTS IN PHENYL ESTERS
could be expressed as follows:
chemical shifts, dCO, ((rI)ortho ¼ 5.00[8]). In the substituent-induced
chemical shifts of carbonyl oxygen for phenyl esters of ortho-
substituted benzoic acids with þR substituents, the contribution
of the steric effect (dortho ¼ ꢁ 81.2, Table 2) was ca. 18 times
stronger compared to the steric effect in the case of the carbonyl
carbon 13C NMR chemical shifts, dCO (dortho ¼ ꢁ4.40). Dahn and
Carrupt[26,64] have reported that the (de)shielding effect of the
carbonyl oxygen atom is mainly determined by the energy of the
nꢁp* excitation; donor–acceptor type interactions influence the
level of both orbitals. As the n orbital of this transition is nearly
ðd17OÞortho ¼ ð187:7 ꢄ 1:1Þ þ ð7:12 ꢄ 1:13ÞsI
þ ð2:48 ꢄ 0:70ÞsþR ꢁð33:5 ꢄ 3:2ÞEsB
R ¼ 0:986; s ¼ 0:648; n=n0 ¼ 7=7
(18)
Separation of the inductive, resonance and steric components
of substituent effects in the d(17O) values for phenyl esters of
ortho-substituted benzoic acids, X-C6H4CO2C6H5, with donor þR
substituents (Eqn (16)) show that the influence of the through-
conjugation between ortho þR substituents and the carbonyl
oxygen occurs by the same extent as that for para substituents.
This confirms that in esters considered with þR substituents the
carbonyl group is still coplanar with the ortho-substituted
aryl ring and due to the through-resonance effect of electro-
n-donating substituents the carbonyl oxygen shows a strong
shielding effect. Consequently, in the ortho-substituted esters
with þR substituents only one substituent in the ortho position
could not distort the planarity between the carbonyl group and
the ring plane essentially. Similar to esters considered, in the case
of the d(17O) values for nitrobenzenes[34], the resonance effect
remains important despite of the steric interactions between the
substituent and the nitro group. It was concluded[43,65,66] that a
small steric effect of ortho substituent in the case of 2-methyl-
benzoate does not necessarily distort planarity or result in steric
inhibition of resonance. In a series of substituted benzoate
anions, the ab initio calculations of the carbonyl group rotation
barriers indicated[67] that due to p-resonance effect, the planar
conformations are preferred.
exclusively located on the O atom, the excitation acts less on 13
C
and thus mainly responsible for the difference in the substituent
sensitivity between 13C and 17O shifts. Chao[40] concluded
that the 17O shift values in carbonyl compounds are strongly
influenced by the nꢁp* mixing, whereas the 13C values are
determined by multiple factors. The nꢁp* mixing does not affect
the 13C chemical shifts because the lone pair is located on the O
atom. Studying the 17O chemical shifts in benzamides derivatives,
De Rosa[69] suggested that in localized exited state the
through-conjugation is possible between the substituent and
the carbonyl oxygen and is not affected by the ground state
torsion angle.
CORRELATION OF THE 17O NMR CHEMICAL
SHIFTS, d(17O), FOR ORTHO- AND
PARA-SUBSTITUTED NITROBENZENES,
BENZOYL CHLORIDES AND
ACETOPHENONES
We suggest that in the case of þR substituents the observed
steric effects in the d(17O) values similar to that for carbonyl
To compare the substituent effects in the d(17O) values for
ortho- and para-substituted nitrobenzenes, X-C6H4NO2, benzoyl
chlorides, X-C6H4COCl, and acetophenones, X-C6H4COCH3, with
those in d(17O) values for the phenyl esters of para- and
ortho-substituted benzoic acids, X-C6H4CO2C6H5, the correspond-
ing d(17O) values were correlated with Eqns (4)–(6).
Similarly to the phenyl esters of ortho-substituted benzoic
acids, X-C6H4CO2C6H5, (Eqns (16) and (17)), the d(17O) values for
ortho-substituted nitrobenzenes showed excellent correlation
with the Charton equation (6) in case the data treatment was
performed separately for nitrobenzenes with þR substituents
and with ꢁR substituents. For d(17O) values of ortho-substituted
nitrobenzenes, X-C6H4NO2, with þR substituents (X ¼ H, CH3,
CH2CH3, CH(CH3)2, C(CH3)3, OCH3, SCH3, N(CH3)2, F, Cl, Br,
2,6-(CH3)2) we obtained (Table 2):
carbon 13C NMR chemical shifts, dCO
,
are caused by the
[8]
repulsive van der Waals deshielding effect. Due to the bulky ortho
substituents, the p-electron density around the CO oxygen in an
ortho-substituted ester would be reduced due to the electrostatic
repulsion between the orbitals of the substituent and those of the
CO bond, with the effect increasing as substituent increases the
size (the van der Waals deshielding[17,41,42,44,68]). In the case of the
electron-donating substituents due to the repulsive van der
Waals steric effect the shielding effect of the O atom is reduced.
We found that in the case of the ꢁR electron-withdrawing
substituents (X ¼ H, NO2, CN, CF3, SO2CH3, SCF3) the steric
deshielding effect to be twice weaker as compared to that for the
electron-donating substituents and the resonance factor was
insignificant. We admit that in the case of ꢁR substituents the
carbonyl group is rotated out of the ortho-substituted aryl plane
and similar to that for electron-donating substituents, the steric
effect is caused by the repulsive van der Waals deshielding
effect.[17,41,42,44,68] Evidently, in the case of ꢁR substituents
the observed repulsive van der Waals deshielding effect was
approximately twice weaker as compared to that for planar esters
with þR substituents due to the increased torsion angle between
the carbonyl group and the ortho-substituted aryl plane.[13] In the
case of ortho electron-withdrawing substituents the deviation
from a coplanar configuration between the ring and the carbonyl
group is presumably due to prevention of the positive charge
delocalization.[36]
ðd17OÞortho ¼ ð578:1 ꢄ 3:5Þ þ ð26:8 ꢄ 5:2ÞsI
þ ð11:7 ꢄ 2:4ÞsþR ꢁð108:4 ꢄ 7:8ÞEsB
(19)
R ¼ 0:975; s ¼ 3:47; n=n0 ¼ 11=12
In the case of ortho-substituted nitrobenzenes, X-C6H4NO2,
with ꢁR substituents (X ¼ H, NO2, CN, CF3, CONH2, CO2CH3) the
substituent effect (d17O)ortho is expressed as follows (Table 2):
ðd17OÞortho ¼ ð573:7 ꢄ 3:5Þ þ ð20:2 ꢄ 6:3ÞsIꢁð58:4 ꢄ 6:0ÞEsB
R ¼ 0:981; s ¼ 3:12; n=n0 ¼ 6=6
(20)
In phenyl esters of ortho-substituted benzoic acids the
calculated sensitivities of the d(17O) values towards the
substituent inductive effect, (rI)ortho, were more than three times
stronger ((rI)ortho ꢅ 17, Table 2) as compared to the same
sensitivities, (rI)ortho, in the case of the carbonyl carbon 13C NMR
The d(17O) values of carbonyl oxygen for the phenyl esters of
ortho-substituted benzoic acids, X-C6H4CO2C6H5, showed excel-
lent correlation with d(17O) values for ortho-substituted nitro-
benzenes, X-C6H4NO2, including simultaneously derivatives with
J. Phys. Org. Chem. 2011, 24 539–552
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V. NUMMERT ET AL.
þR and ꢁR substituents.
nitrobenzenes coincide well with the similar rm values reported
earlier (for methyl benzoates rm ¼ 6.44[15], for nitrobenzenes
rI ¼ 6.5[60], rRꢁ ¼ 0.6[60]. In meta- substituted nitrobenzenes
and acetophenones the susceptibility to the inductive effect
was ca. one-half the size of that for the corresponding para
derivatives and the resonance term was excluded as insignificant
(Table 2).
dð17OÞ2-X-nitrobenzenes ¼ ð572:2 ꢄ 1:7Þþð1:63 ꢄ 0:08ÞDdð17OÞ2-X-benzoates
R ¼ 0:986; s ¼ 2:90; n=n0 ¼ 12=12
(21)
dð17OÞ4-X-nitrobenzenes ¼ ð575:1 ꢄ 1:4Þþð1:70 ꢄ 0:24ÞDdð17OÞ4-X-benzoates
R ¼ 0:935; s ¼ 3:73; n=n0 ¼ 8=12
(22)
We can see (Eqns (21) and (22), Table 2) that in substituted
nitrobenzenes the inductive, resonance, and steric substituent
effects appeared to be 1.7 times stronger as compared to that for
the phenyl esters substituted benzoic acids, X-C6H4CO2C6H5.
In the case of ortho-substituted benzoyl chlorides, X-C6H4COCl,
and acetophenones, X-C6H4COCH3, the correlations with the
Charton equation (Eqn (6)) were performed for derivatives with
þR ortho substituents only. Unfortunately in literature, only very
few values on d(17O) for ortho-substituted benzoyl chlorides and
acetophenones with ꢁR substituents are available to treat the
corresponding d(17O) values with Eqn (6).
CORRELATION OF THE 17O NMR CHEMICAL
SHIFTS, d(17O), WITH THE
SUBSTITUENT-INDUCED RATES OF THE
ALKALINE HYDROLYSIS, Dlog k, THE
CARBONYL CARBON 13C NMR CHEMICAL
SHIFTS, DdCO, AND IR CARBONYL
STRETCHING FREQUENCIES, DnCO
We correlated the d(17O) values of the carbonyl oxygen and
the single-bonded oxygen for phenyl esters of ortho-, para, and
meta-substituted benzoic acids, X-C6H4CO2C6H5, with the
corresponding rates of the alkaline hydrolysis, Dlog k, the
carbonyl carbon 13C NMR chemical shifts, DdCO, and IR carbonyl
stretching frequencies, DnCO, using Eqns (8)–(13) (Table 3). The
d(17O) values for ortho derivatives, show a good correlation with
the corresponding Dlog k values of the alkaline hydrolysis, the
carbonyl carbon 13C NMR chemical shifts, DdCO, and IR carbonyl
stretching frequencies, DnCO, (0.998 > R > 0.963, Table 3) in
case the additional resonance and steric terms were included
(Eqns (8)–(13)) and the data treatment was carried out separately
for esters with þR substituents and ꢁR substituents. The d(17O)
values for para derivatives were correlated well with the corres-
ponding values of Dlog k for the alkaline hydrolysis, the 13C NMR
For ortho-substituted benzoyl chlorides, X-C6H4COCl, contain-
ing þR substituents we found (Table 2):
ðd17OÞortho ¼ ð486:0 ꢄ 2:2Þ þ ð35:0 ꢄ 2:9ÞsI
þ ð16:3 ꢄ 2:5ÞsþR ꢁð93:3 ꢄ 4:9ÞEsB
(23)
R ¼ 0:994; s ¼ 1:38; n=n0 ¼ 6=6
In the case of ortho-substituted acetophenones, X-C6H4COCH3,
containing þR substituents, the substituent effect is described as
follows (Table 2):
ðd17OÞortho ¼ ð550:6 ꢄ 2:3Þ þ ð26:2 ꢄ 2:9ÞsI
þ ð40:8 ꢄ 2:7ÞsþR ꢁð148:0:3 ꢄ 5:4ÞEsB
(24)
R ¼ 0:996; s ¼ 1:68; n=n0 ¼ 7=7
The d(17O) values for para-substituted nitrobenzenes, benzoyl
chlorides, and acetophenones showed excellent correlation
with Eqn (5) an (6) using the Brown and Okamoto substituent
constants sþ.[55,56] The calculated sensitivities towards the
through-resonance, rþ, for para-substituted benzoyl chlorides
and acetophenones were respectively 21.2, and 24.1 (Table 2).
The corresponding values of rþ coincide well with the similar rþ
values reported earlier (for benzoyl chlorides rþ ¼ 21.7,[39] for
acetophenones rþ ¼ 22.5[27,61]). For para-substituted nitroben-
zenes we obtained rþ ¼ 14.9 (Table 2, rI ¼ 18.6[34] and
chemical shifts, DdCO, and IR the stretching frequencies, DnCO
,
when the additional resonance term, a2(para)sþR, (Eqns (11)–(13),
Table 3) was included. To correlate the d(17O) values for meta
derivatives, the additional a2(meta)s8R scale was included. The
slope a1 in Eqn (8)–(13) is the ratio of inductive effects in the 17
O
NMR chemical shifts and in the corresponding process compared.
So the slope a1 in Eqn (8) represents the ratio of the substituent-
dependent inductive effects in the d(17O) values and in Dlog k
values of the alkaline hydrolysis and the slope a1 in Eqn (9) is the
ratio of the susceptibility to the inductive effect in the d(17O)
values and in the carbonyl carbon 13C NMR chemical shifts, DdCO
The slope a1 in Eqn (10) corresponds to the ratio of the inductive
effects in d(17O) values and in the IR stretching frequencies, DnCO
rþ ¼ 24.0[34]).
R
.
CORRELATION OF THE 17O NMR CHEMICAL
SHIFTS, d(17O), FOR META-SUBSTITUTED
METHYL BENZOATES, NITROBENZENES,
AND ACETOPHENONES
,
for the ortho-substituted esters. Correlation equations obtained
in Table 3 could be used to estimate the rates of the alkaline
hydrolysis, the carbonyl carbon 13C NMR chemical shifts, DdCO
,
and IR carbonyl stretching frequencies, DnCO, using the d(17O)
values for esters considered.
For comparison the d(17O) values published in literature for
meta-substituted methyl benzoates, X-C6H4COCH3, nitroben-
zenes, X-C6H4NO2, and acetophenones, X-C6H4COCH3, were
correlated with Eqns (4)–(6) as well. The d(17O) values for
meta-substituted methyl benzoates showed good correlations, so
with the single parameter treatment using sm constants (Eqn (4))
as the dual substituent parameter treatment with the Taft sI and
s8R constants (Table 2). In methyl benzoates for the d(17O) values
of the carbonyl oxygen, the calculated rm value was 5.53
(rI ¼ 6.86, r8R ¼ 1.93) and for nitrobenzenes rm ¼ 5.28 (rI ¼ 6.78,
r8R ¼ 0). The calculated values of rm for methyl benzoates and
We found the carbonyl 17O NMR chemical shifts, d(17O), for
the phenyl esters of ortho-substituted benzoic acids with þR
substituents to be correlated well with the Dlog k values of the
alkaline hydrolysis in water at 25 8C, the carbonyl carbon 13C NMR
chemical shifts, DdCO, and the IR stretching frequencies, DnCO, as
follows (Eqns (25)–(27), Table 3):
17
dð OÞortho ¼ ð346:2 ꢄ 1:7Þ þ ð8:90 ꢄ 0:89ÞDlogk
þ ð7:93 ꢄ 1:10ÞsþR ꢁð98:3 ꢄ 5:0ÞEsB
(25)
R ¼ 0:993; s ¼ 0:990; n=n0 ¼ 7=7
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INFLUENCE OF ORTHO SUBSTITUENTS IN PHENYL ESTERS
17
dð OÞortho ¼ ð345:4 ꢄ 2:0Þꢁð3:62 ꢄ 0:43ÞDdCO
CONCLUSIONS
þ ð10:0 ꢄ 1:3ÞsþR ꢁð95:2 ꢄ 5:8ÞEsB
R ¼ 0:991; s ¼ 1:17; n=n0 ¼ 7=7
(26)
(27)
The 17O NMR spectra for 29 phenyl esters of ortho-, para-, and
meta-substituted benzoic acids, X-C6H4CO2C6H5, were recorded.
The electron-donating substituents in ortho-, para-, and meta-
substituted esters resulted in shielding of the 17O signal (upfield
shift) and electron-withdrawing groups caused deshielding
(downfield shift). The 17O NMR chemical shifts of the carbonyl
oxygen and the single-bonded oxygen for para derivatives
correlated well with the sþ constants. The d(17O) values for the
ortho derivatives showed excellent correlation with the inductive,
sI, resonance, sþR, and steric, EBS, substituent constants in case
the data treatment was performed separately for derivatives
containing the electron-donating þR substituents and electro-
n-attracting ꢁR substituents. The steric interaction of ortho
substituents with the ester group was found to produce
deshielding effect of the carbonyl and single-bonded oxygens.
We suggest that in phenyl esters of ortho-substituted benzoic
acids, the steric effect is caused by the repulsive van der Waals
deshielding effect. For ortho derivatives with ꢁR substituents, the
resonance term was insignificant and the steric term was ca.
twice weaker as compared to that for derivatives with þR
substituents. The d(17O) values for para- and ortho-substituted
nitrobenzenes correlated well with the sþ, inductive, sI,
resonance, sþR, and steric, EBS, substituent constants, respectively,
similar to that for phenyl esters of substituted benzoic acids. Only,
in d(17O), for nitrobenzenes (XC6H4NO2) the susceptibility to the
sþ, inductive sI, resonance, sþR, and steric, EBs , substituent
constants, was 1.7 times stronger as compared to that for phenyl
esters of substituted benzoic acids. The d(17O) values for
meta-derivatives correlated well with the sm substituent constants.
17
dð OÞortho ¼ ð346:3 ꢄ 1:6Þ þ ð0:64 ꢄ 0:08ÞDnCO
þ ð7:76 ꢄ 1:56ÞsþR ꢁð75:1 ꢄ 4:8ÞEsB
R ¼ 0:990; s ¼ 0:942; n=n0 ¼ 6=6
The same relationships for the phenyl esters of ortho-
substituted benzoic acids with ꢁR substituents are expressed
as follows:
17
dð OÞortho ¼ð347:0 ꢄ 2:9Þþð6:31 ꢄ 2:08ÞDlog kꢁð50:3 ꢄ 5:5ÞEsB
R ¼ 0:978; s ¼ 2:47; n=n0 ¼ 6=6
(28)
17
dð OÞortho ¼ ð346:0 ꢄ 1:1Þꢁð3:97 ꢄ 1:14ÞDdCOꢁð47:4 ꢄ 5:8ÞEsB
R ¼ 0:996; s ¼ 1:09; n=n0 ¼ 6=6
(29)
17
dð OÞortho ¼ ð345:4 ꢄ 1:9Þ þ ð1:32 ꢄ 0:13ÞDnCO
(30)
R ¼ 0:987; s ¼ 2:09; n=n0 ¼ 4=4
In Eqn (8) a1 ¼ rI(17O)/rI(AH), a2 ¼ rR(d17O)ꢁa1rR(AH) and
a3 ¼ dS(d17O)ꢁa1dS(AH). In Eqn (9) a1 ¼ rI(17O)/rI(dCO), a2 ¼
rR(d17O)ꢁa1rR(dCO) and a3 ¼ dS(d17O)ꢁa1dS(dCO). In Eqn (10)
a1 ¼ rI(d(17O))/rI(nCO), a2 ¼ rR(d17O)ꢁa1rR(nCO) and a3¼ dS(d17O)ꢁ
a1dS(nCO). In relations shown, the alkaline hydrolysis is denoted by
(AH), the susceptibility to the steric factor by dS, the carbonyl
carbon 13C chemical shifts by dCO, and the IR carbonyl stretching
frequencies by nCO
.
Using the alkaline hydrolysis of the phenyl esters of substituted
benzoic acids, X-C6H4CO2C6H5, rI(AH)ortho ¼ 2.13, rR(AH)ortho
¼
0.31, and d(AH)ortho ¼ 2.67 in water at 25 8C[6] as well as
rI(17O)ortho ¼ 17.3, rR(17O)ortho ¼ 9.68 and, dS(17O)ortho ¼ ꢁ81.2
for esters with þR substituents (Table 3), we obtained the values
of a1 ¼ 8.12, a2 ¼ 7.28 and a3 ¼ ꢁ102.8 which are approximately
the same as shown by Eqn (25) (Table 3).
Acknowledgements
This work was supported by the grants No. 8162 and No. 6705 of
the Estonian Science Foundation.
The positive values of the parameter a1 in Table 3 prove that
the substituent-induced d(17O) values, the Dlog k values of the
alkaline hydrolysis, and the IR carbonyl stretching frequencies,
(nCO), grow with increase of the inductive effects of substituent
included. In correlation of the d(17O) values with the carbonyl
carbon 13C NMR chemical shifts, DdCO, (Eqn (9)) the negative value
of the parameter a1 shows that in phenyl esters of substituted
benzoic acids, the influence of the substituent-induced inductive
effect on the d(17O) values is opposite to that in the 13C NMR
chemical shifts, DdCO. With increase in sI values of substituents,
the d(17O) values were found to increase but the carbonyl carbon
13C NMR chemical shifts, DdCO, were found to diminish when the
electron-attracting substituents are involved.
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